Modern Electronics and Communication Engineering [1 ed.] 1032120886, 9781032120881

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Modern Electronics and

Communication Engineering

Modern Electronics and

Communication Engineering

M.L. Anand

Consultant Engineer

First published 2022 by CRC Press 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN and by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 © 2022 Manakin Press Pvt. Ltd.

CRC Press is an imprint of Informa UK Limited The right of M.L. Anand to be identified as author of this work has been asserted in accordance with sections 77 and 78 of the Copyright, Designs and Patents Act 1988. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. For permission to photocopy or use material electronically from this work, access www. copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact [email protected]

Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Print edition not for sale in South Asia (India, Sri Lanka, Nepal, Bangladesh, Pakistan or Bhutan).

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record has been requested ISBN: 978-1-032-12088-1 (hbk) ISBN: 978-1-003-22297-2 (ebk) DOI: 10.1201/9781003222972

Preface Is there any justification of adding one more book to the already large stock of books on the subject. Perhaps there is. This is the book, in which the subject matter is dealt from elementary to the advance level in a unique manner, which will certainly fascinate the readers. Three outstanding features can be claimed for the book viz. (i) style; the student, while going through the pages would feel as if he is attending a class room. (ii) language: that an average student can follow and (iii) approach: it takes the student from “known to unknown” and “simple to complex.” The book is reader friendly, thought provoking and stimulating. It helps in clearing cobwebs of the mind. The style is lucid and un-adulterated. Unnecessary mathematics has been avoided. Understandably, it has the language of an average student. What strands out is the stark simplicity, with which the ideas have been portrayed. Errors might have crept in, inspite of utmost care to avoid them. The author will be grateful if the same are pointed out along with suggestions for improvement of the book. The author thanks the Publishers for publishing the book and pricing it moderately inspite of heavy cost of paper and printing. M.L. ANAND

Brief Contents

PART I : ELECTRONICS ENGINEERING 1. Introduction

3–6

2. Semi-Conductors

7–16

3. Micro Electronics

17–32

4. Electronic Devices

33–94

5. Analysis of Bipolar Transistors at Low Frequencies

95–106

(h-parameters) 6. Analysis of Bipolar Transistor at High Frequencies

107–116

(h – pi Parameters) 7. Microphones & Loudspeakers

117–144

8. Surface Mount Boards

145–164

9. Electronic Hardware Components

165–182

10. Multimeter & CRO

183–202

11. Medical Electronics

203–230

PART II : COMMUNICATION ENGINEERING 12. Introduction

233–246

13. Amplitude Modulation (AM)

247–260

14. SSB-AM Modulation

261–270

15. AM Transmitters

271–278

16. AM Receivers

279–292

viii

Brief Contents

17. Frequency Modulation (FM)

293–306

18. FM Transmitters

307–320

19. FM Receivers

321–330

20. Phase Modulation (PM)

331–338

21. Digital Modulation Transmission & Reception

339–364

22. More About Transmitters and Receivers

365–384

23. Television Basics and Monochrome Television

385–420

24. Colour Televisions

421–448

25. Cable Television and DTH

449–454

26. Facsimile (FAX)

455–464

27. Modern Communication Techniques

465–478

Appendices

479–500

Detail Contents

PART I : ELECTRONICS ENGINEERING 1. Introduction 1.1 Electronics Engineering 1.2 Applications of Electronics Summary

2. Semi-Conductors 2.1 Properties of Semi Conductors 2.2 Factors Affecting Resistivity of Semi-Conductors 2.3 Study of Germanium and Silicon 2.4 Energy Band Theory 2.5 Classification of Semi Conductors 2.6 Semiconductor Components and Devices Summary

3. Micro Electronics 3.1 Field of Micro Electronics 3.2 Integrated Circuit (I.C.) 3.3 Discrete Circuits 3.4 Advantages and Disadvantages of I.Cs Over Discrete Circuits 3.5 Classification of I.Cs. 3.6 Special Features of I.Cs. 3.7 Applications of I.Cs. 3.8 Types of I.C.s 3.9 Fabrication of Monolithic I.Cs. 3.10 I.C. Timers 3.11 Microprocessor (mP) Summary

4. Electronic Devices 4.1 4.2 4.3 4.4 4.5

Semiconductor Diode Zener Diode Light Emitting Diode (LED) Laser Diode Liquid Crystal Diode (LCD)

3–6 3

4

5

7–16 7

8

8

10

11 14

16

17–32 17

17

17

18

18 19

20

20

20

25

27

31

33–94 33

43

45

48

50

x

Detail Contents

4.6 Schottky Diode 4.7 Charge Couple Devices (CCD) 4.8 Bipolar Transistors (at Low Frequencies) 4.9 Thyristors 4.10 Stepper Motors 4.11 Servomotors Summary

5. Analysis of Bipolar Transistors at Low Frequencies (h-parameters) 5.1 Hybrid Parameters (h parameters) 5.2 Two-Port Network 5.3 Hybrid (h) Models 5.4 Analysis of CE Amplifier by h Parameters 5.5 Limitations of h-Parameters Summary

6. Analysis of Bipolar Transistor at High Frequencies (h – pi Parameters) 6.1 Behaviour of Biploar transistor at High Frequencies 6.2 H.F. Hybrid pi Model 6.3 Transistor Cut off Frequencies 6.4 Miller Effect 6.5 C.E. Short Circuit Gain with Pi Model 6.6 C.E. Current Gain with Hybrid Pi Model with a Resistive Load 6.7 Relation Between Hybrid (h) and hybrid (Pi) Parameters 6.8 Gain-Bandwidth Product Numerical Problems Summary

7. Microphones & Loudspeakers 7.1 Microphone 7.2 Loud Speaker 7.3 P.A. System Summary

8. Surface Mount Boards 8.1 Surface Mount Technology (SMT) 8.2 Advantages & Disadvantages of SMT 8.3 SMT Components 8.4 Design of SMT Boards 8.5 General Fabrication Process for SMT Boards 8.6 Commercial Fabrication of SMT Boards 8.7 SMT-Equipment

51

53

56

69

79

86

93

95–106 95

95

98

100

101

105

107–116 107

107

108

109

110

112

113

114

114

116

117–144 117

132

142

144

145–164 145

146

148

149

149

150

153

Detail Contents

8.8 SMT Measurements 8.9 Terms Used in SMT Process Summary

9. Electronic Hardware Components 9.1 Switches 9.2 Connectors 9.3 Computer Hardware Components Summary

10. Multimeter & CRO 10.1 Multimeter 10.2 Cathode Rays Oscilloscope (CRO) 10.3 Storage CROs Summary

11. Medical Electronics 11.1 Role of Electronics in Medicine 11.2 Human Cell 11.3 Medical Electrodes 11.4 Heart and Cardio Vascular System 11.5 Human Brain 11.6 Mascular Strength 11.7 Blood Pressure (BP) 11.8 Diabetes 11.9 Safety of Operators and Patients Summary

xi

161

163

164

165–182 165

174

177

180

183–202 183

189

196

201

203–230 203

204

205

208

216

222

224

228

229

230

PART II : COMMUNICATION ENGINEERING 12. Introduction 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 12.10

Communication Methods of Communication Process of Communication Brief History of Communication Electronic Communication Structure of an Electronic Communication System Bandwidth Requirement Types of Electronic Communication Systems Transmission Mediums or Channels Important Facts About Sound and Light

233–246 233

233

234

234

235

235

236

237

237

238

xii

12.11 Modulation 12.12 Need for Modulation 12.13 Types of Modulations 12.14 Radio (Wireless) Broadcasting, Transmission and Reception Summary

13. Amplitude Modulation (AM) 13.1 Amplitude Modulation (AM) 13.2 Expression for Amplitude Modulated Wave 13.3 Frequency Spectrum of A.M. Wave 13.4 Modulation Factor/Index (m) 13.5 Significance of m 13.6 Power Distribution in A.M. Wave 13.7 Calculation for Current 13.8 Limitations of Amplitude Modulation Summary

14. SSB-AM Modulation 14.1 Different Forms of Amplitude Modulation 14.2 Single Sideband Amplitude Modulation (SSB-AM) 14.3 Various Single Sideband (SSB AM) Techniques Summary

15. AM Transmitters 15.1 Transmitter 15.2 Types of A.M. Transmitters 15.3 Negative Feedback In A.M. Transmitters 15.4 A.M. Modulators 15.5 Linear Modulators 15.6 Block Diagram of A.m. Transmitter Summary

16. AM Receivers 16.1 Demodulation or Detection 16.2 AM Detectors/Receivers 16.3 Linear/Diode Envelope Detector 16.4 Types of AM Receivers 16.5 Automatic Gain Control (AGC) 16.6 Automatic Frequency Control (AFC) 16.7 General Qualities of Receivers Summary

17. Frequency Modulation (FM) 17.1 Frequency Modulation

Detail Contents

240

240

241

243

245

247–260 247

248

249

251

252 253

255

259

260

261–270 261

262

263

269

271–278 271

273

273

274

274

276

277

279–292 279

279

279

281

285

287

287

290

293–306 293

Detail Contents

17.2 Expression of FM Wave in Time Domain 17.3 Power of FM Wave 17.4 Calculation of BW (Carson Rule) 17.5 Plotting Frequency Spectra for FM 17.6 Pre-Emphasis and De-Emphasis 17.7 FM Versus AM Summary 18. FM Transmitters 18.1 FM Generation 18.2 Direct Methods of FM Wave Generation 18.3 Indirect Methods of FM Generation 18.3 FM Transmitters Summary

19. FM Receivers 19.1 Demodulation (Detection) of FM Waves 19.2 Frequency Discriminators/Detectors 19.3 FM Receivers Summary

20. Phase Modulation (PM) 20.1 Phase Modulation (PM) 20.2 Comparison of AM, FM and PM 20.3 Expression for PM Wave 20.4 Generation, Transmission and Reception of PM/FM Wave 20.5 (a) Generation of PM Signal From Frequency Modulator 20.5 (b) Generation of FM Signal From Phase Modulator 20.6 FM vs PM Summary

21. Digital Modulation Transmission & Reception 21.1 21.2 21.3 21.4 21.5 21.6 21.7 21.8 21.9 21.10

Pulse Digital (Pulse) Modulation Sampling Sampling Theorem for Low Pass Signals: Nyquist Theorem Effects of Sampling Rate on a Frequency Spectrum Sampling Techniques Analog and Digital Signals Advantages and Disadvantages of Digital Communication Logic System Binary Number System

xiii

294

298

298

300

301

303

304

307–320 307

308

313

317

319

321–330 321

321

329

330

331–338 331

331

332

334

335

336

336

337

339–364 339

340

340

341

342

343

344

344

345

345

xiv

Detail Contents

21.11 Logic Gates 21.12 Principle of Digital Communication 21.13 Communication Speed 21.14 Quantizing 21.15 Types of Digital (Pulse) Modulation 21.16 Pulse Code Modulation (PCM) 21.17 Multiplexing 21.18 (a) Transmission and Reception of TDM 21.18 (b) Transmission and Reception of FDM 21.19 TDM vs FDM 21.20 TDM is Superior to FDM Summary 22. More About Transmitters and Receivers 22.1 Basic Requirement of AM Transmitter: Flywheel Effect 22.2 AM Radio Transmitter 22.3 Privacy Devices in Radio Telephony 22.4 Image Frequency Rejection 22.5 Tracking and Alignment of Receivers 22.6 Stereo FM Transmitter and Receiver 22.7 SSB Receivers 22.8 Coherent and Non Coherent SSB Detection Summary

23. Television Basics and Monochrome Television 23.1 23.2 23.3 23.4 23.5 23.6 23.7 23.8 23.9 23.10 23.11 23.12 23.13 23.14

Television TV Applications Broadcasting, Transmission and Reception of Monochrome TV TV Camera Various B&W-TV Cameras Picture Tube Scanning Synchronizing Pulses Blanking Pulses The TV Standards Composite Video Signal TV Signal Transmission TV Transmission Techniques Types of TV Receivers

23.15 Monochrome TV Receiver

347

348

350

350

352

352

358

359

359

361

363

364

365–384 365

365

367

370

371

374

376

383

384

385–420 385

385

388

389

392

397

400

406

407

407

409

411

412

414

415

Detail Contents

Summary

24. Colour Televisions

xv

418

421–448

24.1 Colour Television

421

24.2 Primary, Secondary and Complementary Colours

421

24.3 Additive And Subtractive Mixing of Colours

422

24.4 Types of Colour Video Signals

423

24.5 Chrominance and Luminance Signals

424

24.6 Important Terms

424

24.7 Visibility Curve

425

24.8 Sub Carrier and Multiplexing

427

24.9 Popular TV Systems

427

24.10 Colour TV Camera

434

24.11 Colour Picture Tubes

435

24.12 Colour TV Receivers

437

24.13 Special TVs

441

Summary

25. Cable Television and DTH

447

449–454

25.1 Cable TV (CATV)

449

25.2 DTH (Direct To Home) Service

452

Summary

26. Facsimile (FAX)

453

455–464

26.1 Facsimile (Fax)

455

26.2 Basic Fax System

457

26.3 Principle of Operation of Fax

457

26.4 Transmission and Reception

458

26.5 Types of Fax Machines

459

26.6 Conversion of Optical Signal into Electrical Signal

460

26.7 (a) Fax Transmitter

461

26.7 (b) Fax Receiver 26.8 Synchronisation of the Signal Summary

463

463

464

27. Modern Communication Techniques 27.1 Radio Telephony 27.2 (a) Mobile/Cellular Phone/Cell Phone 27.2 (b) Wire Less Loop (WLL) 27.3 TV Remote Control 27.4 E-mail

465–478 465

466

471

471

472

xvi

Detail Contents

27.5 (a) Internet 27.5 (b) World Wide Web (www) Summary

Appendices

474

477

477

479–500

PART I

ELECTRONICS ENGINEERING

1 Introduction It is a well known fact that, we cannot exist in this modern world without the knowledge of Electronics.

1.1 ELECTRONICS ENGINEERING The term ‘electronics’ is derived from ‘electron.’ All matter is made up of atoms and each atom consists of a nucleus (having protons and neutrons) and one or more electrons. The protons are positively charged particles, the neutrons carry no charge and the electrons are negatively charged particles, revolving around the nucleus. Electronics engineering can be defined as the branch of engineering which deals with the flow of electrons (i.e., current) through the vacuum, gases and semiconducting materials. It may also be defined as the branch of engineering which deals with electronic devices and circuits.

Electrical vs Electronics Engineering Remember, Electrical engineering is the science which deals with the flow of electrons through the conductors (metals) only whereas, the Electronics Engineering deals with the flow of electrons through semiconductors and insulators. Note: We know that materials can be classified as 1. Conducting materials 2. Semiconducting materials 3. Insulating materials

4

Chapter 1

Introduction

Table. 1.1: Classification of Materials on the Basis of Atomic Theory S. No.

Particulars

Conducting Material

Semiconducting Material

Insulating Material

1.

Resistivity

Low

Medium

High

2.

Electrons in the last orbit

1, 2 or 3

4

More than 4

3.

Free electrons

Last orbit electrons May be made free are free

4.

Bonding

Metallic bond

Covalent bond

Not free —

1.2 APPLICATIONS OF ELECTRONICS We will study the applications of electronics in different fields. 1. We have electronic rectifiers which can convert AC into DC and have also ‘inverters’ which can do the reverse.

Now the control in industry is through electronic and computerized

operations. We are controlling our machines in our workshops

through computers using computerized numerically controlled (CNC)

machines.

2. It may be a wire communication like telephone and telegraph. Or, a wireless communication which is used in radio and TV broadcasting. We have photo phones in which we can see the person making the call. We have Fax system through which we can send written messages from one place to another. We have satellite communication, through which we can see TV programmes, telecast from any part of the world. 3. We have electrocardiograph (ECG) and X-ray machines. Now sitting outside we can see on CCTV what is happening inside an operation theatre. We also have the electron microscope, computerized axial tomography (CAT), magnetic resonance imaging (MRI) and many other devices. 4. We know a variety of electronic devices which provide us entertainment at our home, e.g., LCD/LED TV, etc. Now we have remote control in our hand and we can control all devices like TV, fan, etc., from a distance. 5. The invention of integrated circuits (ICs) has made electronic devices smaller and handy. We are going towards miniaturization. Now a single IC chip can work as a complete microprocessor.

Summary

5

6. We have precision and sophisticated instruments which give accurate measurements. The most important electronic instrument is Cathode Ray Oscilloscope (CRO) which can show wave shapes of input and output signals.

SUMMARY 1. Electronics deals with the current flow through insulators and semiconductors. 2. Electrical Engineering deals with the flow of current through metals only. 3. Electronics has captured all fields: entertainment, defence, medicine etc. qqq

2

Semi-Conductors Semiconductor is half (semi means half) conductor and half insulator. Its properties lie in between conductors and insulators. Its resistivity is less than insulator and more than a conductor. Thus, “A semi-conductor can be defined as a substance which has properties in between a conductor and an insulator”. Examples of Semi Conductors are: 1. Silicon

(Si)

2. Germanium

(Ge)

3. Salenium

(Se)

4. Carbon

(C)

5. Sulphur

(S)

2.1 PROPERTIES OF SEMI CONDUCTORS (i) The resistivity of a semi-conductor lies in between conductor and an insulator. The resistivity of copper (conductor) is 1.7 × 10–8 Ω m and of glass (an insulator) is 9 × 1011 Ω m., whereas resistivity of Germanium (a semi-conductor) is 0.6 Ω m. (ii) Resistance of conductors increases with the increase of temperature that is, they have a positive temperature co-efficient of resistance. In case of semi-conductor the case is reverse. The resistance of a semi­ conductor decreases with temperature and vice versa. That is, semi­ conductors have negative temperature co-efficient of resistance. In other words, semi-conductors behave as insulators at room temperature but on heating to a particular temperature, they behave as conductors.

8

Chapter 2

Semi-Conductors

(iii) When controlled quantity of a foreign material (impurity) is added to a semi-conductor its properties are further changed and it shows good conductivity. The process is called doping.

2.2 FACTORS AFFECTING RESISTIVITY OF SEMI-CONDUCTORS The following are the factors affecting resistivity of semi-conductors: 1. Temperature 2. Addition of impurity 3. Electric field 4. Light The first two have been discussed above. Effect or Electric Field: The resistance of semi-conductor greatly varies with the magnitude of electric field. It is surprising that current in a semi-conductor does not follow ohm’s law and increases more rapidly than the applied voltage. Effect of Light: The resistance of semi-conductor increases in darkness. This property has been used in photo diodes.

2.3 STUDY OF GERMANIUM AND SILICON The most commonly used semi-conductors are Germanium and Silicon. (i) Germanium: Germanium is an earth element. Pure Germanium is obtained from coal ash in the form of Germanium dioxide. Germanium is a tretravalent element and crystalline in nature, i.e., its shape has definite geometrical figure. The atomic number of Germanium is 32. It has therefore 32 protons and 32 electrons, its atomic structure can be shown as in fig. 2.1. –

–

– –

– –

32 +

– –

– –

–

– –

– –

– –

Fig. 2.1

–

–

–

–

–

–

– –

– – – –

–

–

–

2.3 Study of Germanium and Silicon

9

Germanium is formed by covalent bonding. In a bid to gain stability, an atom of Germanium shares its valence electrons with 4 neighbouring atoms. Fig. 2.2 shows covalent bonding in Germanium atom.

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Ge

Fig. 2.2

(ii) Silicon: Silicon is an element found in most of the rocks. Silicon is obtained by reducing its oxide. Silicon is also a tetravalent element having crystalline structure. Its atomic number is 14 and its atomic structure can be drawn as shown in Fig. 2.3.

+14

Fig. 2.3

Electron Volt: It can be seen from atomic structures that no electron is free in a Germanium or Silicon atom at room temperature. So additional energy is required to break the strong covalent bonds for liberation of electrons This energy is measured in “Electron Volt” (eV) An electron volt is a small unit of energy (The bigger unit is Joule) and one electron volt is the amount of energy acquired by an electron in moving through a potential difference of 1 V. For example : If an electron moves from 0 potential to a potential of 5 V, the energy acquired by the electron is 5 eV.

10

Chapter 2

Semi-Conductors

Table 2.1: Comparison of Germanium and Silicon S. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

Particulars Availability No. of electrons in an atom Valence electrons Shape

Germanium (Ge) It is found in earth crest 32

Silicon (Si) It is found in rocks 14

4 Crystalline

4 Crystalline

Type of bonding Density M.P. Working temp. Co-eff. of Thermal expansion Energy required to break bonding Working voltage Cost

Covalent bonding 53 gm/cm3 930°C 100°C 6 × 10–6 per deg. °C

Covalent bonding 25 gm/cm3 1400°C 200°C 4 × 10–6 per deg. °C

0.7 eV

1.0 eV

32 V Cheap

60 V Costly Comparatively

Note : From the table, it can be seen that Si has superior properties.

2.4 ENERGY BAND THEORY Range of energies (in electron volts, eV) possessed by electrons in an atom is called Energy Band. Important energy bands found in solids are as under (See fig. 2.4) (i) Valence Band: The electrons in the outermost orbit of an atom are called valence electrons and the range of energies possessed by these electrons is known as Valence Band. (ii) Conduction Band: In certain solids (like copper), electrons in the last orbit leave the valence band and form conduction band, the range of energies possessed by these electrons is known as conduction band. Conduction Band Forbidden Gap Energy

Last Orbit (Valence Band)

2nd Orbit or band 1st Orbit or band

Fig. 2.4

2.5 Classification of Semi Conductors

11

(iii) Forbidden Gap: The separation between the above two bands is known as forbidden gap. The materials can be classified also on the basis of the forbidden gap. (a)

In conductors, forbidden gap is more or less than zero.

(b)

In semi conductors, forbidden gap is about 1.0 eV.

(c) In insulators, forbidden gap is very high i.e., 15 eV. (See Table 2.2) Table 2.2: Forbidden energy gap of some semi-conductors Materials

Forbidden Energy gap in eV

1.

Germanium (Ge)

0.7

2.

Silicon (Si)

1.0

3.

Tin (Sn)

0.1

4.

Lead Selenide (Pb Se)

0.2

5.

Cadmium Sulphide (Cds)

2.4

6.

Gallium Arsenide (Ga As)

1.35

7.

Indium Arsenide (In As)

0.18

8.

Lead Sulphide (Pb S)

0.40

2.5 CLASSIFICATION OF SEMI CONDUCTORS Following chart shows the classification of semi conductors. Semi Conductors

Intrinsic Semi Conductors

Extrinsic Semi Conductors

N type Semi Conductors

P type Semi Conductors

(a) Intrinsic or Pure Semi Conductors. A pure semi conductor is called an intrinsic semi conductor. As told earlier, their covalent bonding is very strong and no free electron is available at room temperature, therefore a semi conductor material in pure state behaves almost as an insulator.

12

Chapter 2

Semi-Conductors

(b) Extrinsic or Impure Semi-Conductors. To make use of semi-conductor materials, some impurity is added in its pure form. Addition of impurity changes its properties effectively and it attains the conducting properties, the semi conductor so obtained after adding impurity is called as an extrinsic or impure semi-conductor. It should be kept in mind that impurity always is “that which is in small percentage”. For example if in a bucket of milk, a jug of water is added the water is called an impurity. On the other hand if in a jug of milk a bucket of water is added in this case, milk will be called as impurity. Doping is the process of mixing impurity to get an impure or extrinsic semiconductor. By doping we get two types of extrinsic semi conductors. (a) N-type, Electron or Donor type Semi Conductor. They are the semi conductors which conduct current with electrons (negative charges).

To get this type of conductivity we add pentavalent (which has 5

electrons in the last orbit) impurity into pure germanium or silicon

(which are tetravalent crystals).

The pentavalent metals which are commonly used for this purpose are - Arsenic (Atomic Number 33) and Antinomy (Atomic No.51).These materials provide or give (donate) extra free electrons to the pure semi conductor crystal hence called donor semi conductor. Explanation: Germanium (or silicon) has four and Arsenic (As) has five electrons in the last orbit. If an arsenic atom is added into a germanium (or silicon), 4 electrons of Arsenic form covalent bonding with Germanium electrons. The fifth electron of Arsenic finds no electron to make bond and hence it is a FREE electron (See fig. 2.5). As each impurity atom donates one free electron, thus a small amount of arsenic will provide millions of free electrons for conduction. Impurity

Ge

Ge

Ge

As

Ge Free electron

Ge

Ge

Ge

Fig. 2.5

2.5 Classification of Semi Conductors + vely Charged Impurity Atom

13 Free Electron

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Fig. 2.6

Now if this N type semi-conductor, so obtained is connected with a battery, all the free electrons will be attracted towards the positive terminal of the battery (see fig. 2.6) and current with free electrons (negative charges) will be obtained in the circuit. (b) P type or Hole or Acceptor type semi conductor. These are the semi conductors which conduct current through holes

(positive charges)

A hole can be taken as a positive charge with opposite polarity.

A hole is a physical charge as much as an electron. Hole represents

a missing electron in a covalent bond. Further, movement of a hole

(positive charge) can be taken in a direction opposite to the direction of

movement of electron.

To get this type of conductivity, we add Trivalent (which has 3 electrons in

the last orbit) impurity into pure Germanium or Silicon crystal. The

trivalent metals which are commonly used for this purpose are Gallium

(At. No. 31) and indium (At. No. 49). These metals can accept electrons

hence they are also called Acceptor semi conductors. Explanation: Germanium (Ge) has 4 electrons and Gallium (Ga) has 3

electrons in the last orbit. If a Gallium atom is added into Germanium

atom, the three electrons of gallium can form only three covalent bonds

with 3 germanium electrons and the fourth bond is incomplete being

short of one electron. This missing electron in the fourth bond is called

hole, that can accept an electron (See fig. 2.7) Ge

Ge

Ge

Ge

Ga

Ge

Impurity

Hole Ge

Fig. 2.7

14

Chapter 2

Semi-Conductors

Thus each atom of Gallium will create one hole and thus a small amount of gallium can create millions of holes. Now if so obtained P type semi conductor crystal is connected with a battery, all the holes are shifted from one impurity atom to next and thus towards the negative terminal of the battery. We can also say otherwise, that electrons from the battery move in the opposite direction and jumps into the holes of Gallium atoms. More electrons leave their bonds to occupy the holes and current conduction starts (through electrons). See fig. 2.8 Hole Electron

Fig. 2.8

2.6 SEMICONDUCTOR COMPONENTS AND DEVICES Here we are giving brief introduction of few semi conductor components and devices. 1. Diode: When p and n type semi conductors are kept touching each other, this gives p-n Junction. A pn junction is called a semiconductor diode. It is made of germanium or silicon. It has two terminals: anode and cathode. It conducts only when it is forward biased i.e., its anode (A) is connected to positive and the cathode (K) to negative supply. (Fig. 2.9) Arrow

(+)

A

Bar

K

(–)

Fig. 2.9

When it is reverse biased, it does not conduct. Diodes are used as rectifiers 2. Transistor: It is a device consisting of two PN junctions formed by sandwitching either P or N type semiconductor material between a pair of opposite types. Accordingly, they are PNP, (Fig. 2.10 a) and NPN types. (Fig. 2.10 b)

2.6 Semiconductor Components and Devices

15

B E

P

N

B P

C

E

N

P

C

N

C C

B

B

E

(a)

(b)

E

Fig. 2.10

A transistor has three terminals: Emitter (E), Base (B) and collector (C). A transistor conducts, only when it is forward biased. Transistors are used as amplifiers. 3. Field Effect Transistor (FET): A field effect transistor is a 3 terminal semiconductor device in which current conduction is by one type of carriers (electron or hole) and is controlled by the effect of an external field. D

D D

P

G

D

P

n Channel S

N

G

G

N G

p Channel

S

S

(a)

S (b)

Fig. 2.11

There are two types of FETs, 1.

N channel FET [Fig. 2.11(a)]

2.

P channel FET [Fig. 2.11(b)]

Each of the two FETs has 3 terminals— Drain (D), source (S) and

Gate (G)

The FET is used as a Buffer amplifier.

16

Chapter 2

Semi-Conductors

4. Silicon controlled Rectifier (SCR): A silicon controlled rectifier is a 3 junction, 3 terminal device which acts as a controlled switch to perform various functions such as controlled rectifier or as inverter etc. This is made of silicon necessarily. See Fig. 2.12. G

G A

P

N

P

N

K

A

K

Fig. 2.12

It has 3 terminals called Anode (A), Cathode (K) and the Gate (G). Note: There are many other semiconductor components/devices discussed in coming pages.

SUMMARY 1. The semi-conductors are half conductors. Their properties lie in between conductors & insulators. 2. The examples of semi conductors are- germanium, silicon, selenium, carbon etc. 3. All semi conductors have 4 electrons in their last orbit. They have resistance more than conductors but less than insulators. They have a negative temperature coefficient of resistance. When a small impurity is added, their properties are changed. 4. The important semi conductors used for making electronic devices are: germanium and silicon. Silicon has superior properties than Germanium 5. The semi conductors may be- (i) Intrinsic type and (ii) Extrinsic type. The extrinsic type may be further sub classified as (a) P-type and (b) N-type. 6. The intrinsic semi conductor is the pure semi conductor obtained from earth crust. An extrinsic semi conductor is obtained by adding an impurity in intrinsic semi conductors. 7. The important semiconductor devices are : diode, transistor, FET and SCR. qqq

3 Micro Electronics Micro electronics is the science that deals with very small devices and components such as Integrated Circuits (I.C.s)

3.1 FIELD OF MICRO ELECTRONICS The chart below shows the field of micro electronics. Microelectronics

Mini-dircrete devices

I.Cs

Functional devices

Hybrid circuits

Silicon monolithic circuits

Film circuits

Silicon thin film circuits

Discrete thin film circuits

Thin film circuits

Thick film circuits

3.2 INTEGRATED CIRCUIT (I.C.) Fabrication of thousands of components/circuits in a small chip gives an integrated circuit (I.C.)

3.3 DISCRETE CIRCUITS In these circuits different components are clearly visible to the naked eyes. The example of discrete circuit is a Printed Circuit Board (P.C.B.)

18

Chapter 3

Micro Electronics

3.4 ADVANTAGES AND DISADVANTAGES OF I.Cs. OVER DISCRETE CIRCUITS (a) Advantages of I.Cs: 1. They have superior reliability due to absence of solder joints. The components are connected with each other through special techniques. 2. They have very small size; hence need lesser space. 3. Their weight is also very less. 4. They can be operated at comparatively high temperatures. 5. They have extremely low power consumption. 6. They have low cost, when processed in large numbers at a time. 7. Temperature differences between different parts of an integrated circuits is small. 8. They have close matching of components and their temperature coefficients.

(b) Disadvantages of I.Cs: 1. If any component does not work, the component can’t be replaced. The whole I.C. chip has to be thrown out, whereas in case of discrete circuits (PCBs), a particular component can be replaced by a new one. 2. The heavy components like inductors, transformers, etc., cannot be fabricated within the chip. These are to be connected externally in the circuit. In discrete circuits, however, the heavy components can be soldered onto the board itself: 3. Maximum power ratings for which I.Cs can be produced in 10 W, where a discrete circuit has no such limitation. 4. In I.C., parasitic capacitance is formed due to isolation of components from each other, which restricts high frequency response. 5. In I.C., it is difficult to achieve low noise and high voltage.

3.5 CLASSIFICATION OF I.Cs. The I.Cs are broadly classified into two classes: 1. Linear I.Cs: These I.Cs deal with linear (analog) signals. A linear signal is that which varies in direct proportion to its input. 2. Non-linear/digital I.Cs: These I.Cs deal with non-linear/digital signals. A digital signal is that in which two discrete voltage levels (0 and 1) are used. The logical circuits are known as ‘Gate’, which

3.6 Special Features of I.Cs.

19

are made of both passive and active components. A Gate is a piece of hardware or an electronic circuit, which has more than one input but only one output. It is used to perform logic operations in computers. Digital I.Cs may be sub-classified as: (i) VLSI (Very Large Scale Integration): This contains 1000 or more logic gates or components.

Now, Very Large Scale Integration (VVLSI) and Extra Large

Scale Integration (Ex. LSI) are also available.

(ii) LSI (Large Scale Integration): This contains 100 – 1000 logic gates or components. (iii)

MSI (Medium Scale Integration): This contains from 12 to 100 logic gates or components.

(iv) SSI (Small Scale Integration): This contains less than 12 gates or components.

3.6 SPECIAL FEATURES OF I.Cs. 1. No component of an I.C. is visible on the surface of the chip. 2. For looking into the connections in the chip, we need a high power microscope. 3. The most important characteristic of an I.C. is its size. It is thousand times smaller than a discrete circuit. Now a complete microprocessor can be built in a single chip of size 9/32″ × 1/4″, which contains more than 257 × 103 components. 4. If a component fails, the whole I.C. should be replaced as a more economical approach. 5. Automation is becoming most important in the manufacture of I.Cs: The automation reduces the amount of handling by the people and thus the possibility of contamination. 6. The ‘I.C. density’ is also an important factor. By this we mean the number of components which can be formed per unit length of the chip. Now the manufacturers use ‘microns’ (micrometer) to specify the size of an I.C. Recall that 1 micron = 10–6 m. 7. Cleanliness is of utmost importance in the I.C. production room. The workers before entering the room have to wear special apron. Even a smallest dust particle can ruin the quality of the product.

20

Chapter 3

Micro Electronics

3.7 APPLICATIONS OF I.Cs. The I.Cs. are used in: 1. Digital watches 2. Electronic calculators 3. Personal computer (PC) 4. MP-3 players 5. Digital camera 6. Mobile phone 7. VCD and DVD players 8. Electronic games 9. Radio, TV 10. Computers, etc

3.8 TYPES OF I.C.s According to construction, I.Cs. may be classified as: 1. Monolithic I.Cs: In this, all components are formed such that they become part of a single p-type thin ‘wafer’. They are called monolithic integrated circuits as they are formed on a single silicon chip and the components automatically become an integral part of the chip. The word monolithic is derived from Greek words ‘monos’ meaning ‘single’ and ‘lithos’ meaning ‘stone’. Thus, a monolithic circuit is built into a single stone or a single crystal (chip). 2. Thin Film I.Cs: In this, components are formed on an insulated base (glass or ceramic). All components are evaporated on this base.

3. Thick film I.Cs: In this, resistors and capacitors are formed on a base (called substrate), but transistors are added from outside. 4. Hybrid I.Cs: In this, ‘monolithic’ and ‘thin film’, both are combined on a single platform.

3.9 FABRICATION OF MONOLITHIC I.Cs. Note that monolithic I.C.s are most popularly used.

Here we shall study the various steps involved in the fabrication of the ICs by

planar technology:

1. Wafer: The first step towards fabrication of monolithic I.C.s is to prepare a wafer (i.e., thin slice) of p-type semiconductor. For this, a cylinder of p-type silicon crystal is grown having dimensions of 25 cm length and 2.5 cm diameter (Fig 3.1)

21

2. 5c m

3.9 Fabrication of Monolithic I.Cs.

25cm

Fig. 3.1

This crystal is then cut by a diamond saw into thin slices called wafers The size of the wafers is about 2.5 cm length. Fig. 3.2 [(a) and (b)] shows two views of a p-type silicon wafer.

Flattened edge

(a)

200 mm 2.5 cm (b)

Fig. 3.2

The thickness of these wafers is about 200 mm.(=1/1000 inch = one fifth of the thickness of this page). The cutting or slicing process is shown in Fig. 3.3 Flattened edge

Diamond blade

Supporting block

Rotating cutting knife

Fig. 3.3

These wafers later on are polished on one of its surfaces to obtain mirror smooth surface, on which subsequent operations are carried out. This silicon wafer acts as a base (substrate) for fabrication of various components of the circuit.

22

Chapter 3

Micro Electronics

Note: Silicon has been the main material for making I. Cs but as the density (no. of components per unit area) is increasing, silicon is being replaced by materials like GaAs (Gallium Arsenide). 2. Epitaxial Layer: The epitaxial layer is a thin film of silicon (of the same material). The epitaxial layer is of n-type usually, but it can be of

p-type also.

The epitaxial (means arranged upon) layer is usually 25 mm thick

(Fig. 3.4) All the components are built within this layer (later on) by

‘diffusion’.

Epitaxial layer 25 mm P-substrate

Fig. 3.4

3. SiO2 Layer: In order to prevent contamination of epitaxial layer, a thin layer of SiO2 is formed over the entire surface. This is also called oxidation layer (See fig. 3.5). SiO2 layer (1mm) n type epitaxial layer

25 mm P-substrate 200 mm

Fig. 3.5

The SiO2 layer is grown by exposing the epitaxial layer in an atmosphere of oxygen to about 1000°C. The SiO2 layer prevents any impurities from entering the n type epitaxial layer. However selective etching of this SiO2 layer will allow the diffusion of proper impurities into the designated areas of n-type epitaxial region of the silicon wafer. 4. Masking, Photolithography and Photo-etching: The wafer with SiO2 layer is coated with a uniform layer of some photosensitive emulsion (such as photo resist). An enlarged black and white pattern

3.9 Fabrication of Monolithic I.Cs.

23

of the desired opening is first made and then reduced photographically to the desired smaller size to form a stencil (negative). This stencil or mask is then placed over the photo resist as shown in Fig. 3.6 (a). Stencil or mask (negative)

U.V. Light

Polymerized photo resist

photo resist SiO2 layer

Silicon Chip

SiO2 layer

(a)

Silicon Chip

(b)

Fig. 3.6

The ultraviolet (u.v.) rays are made to fall on the mask. The photo resist under the transparent region of the stencil gets polymerised. The stencil is then removed and the wafer is then developed using some chemical (such as trichloro-ethylene) which dissolves the unexposed (or unpolymerised) part of the photo resist film leaving the photo resist pattern as shown in Fig. 3.6 (b).

The unremoved parts of the photo resist are then cured, making them

resistant to the corrosive etching process.

The chip is now immersed in an etching solution (such as hydrofloric acid). The acid dissolves the SiO2 from the parts unprotected by the photo resist pattern. After photo¬etching and diffusion, the resist mask and the under neath SiO2 layer is removed using some chemical (such as H2SO4) and by mechanical abrasion. 5. Diffusion of impurities: The next process is the diffusion of impurities to fabricate different components in the chip. Usually phosphorous (pentavalent material) is used as impurity. In order to keep diffusion time small, diffusion temperature is kept above 1000°C. For fabrication of I.Cs, high temperature diffusion furnace is used with rigorous temperature control. The diffused impurities may be in solid, liquid or gaseous form. 6. Aluminium metallisation: Interconnection of various components is done through metallisation.

24

Chapter 3

Micro Electronics

This involves the following steps: (i) Forming a new layer of SiO2 at the top. (ii) Forming a new pattern of windows in the SiO2 layer at positions, where contacts are to be made. (iii) Making interconnection through vacuum deposition of thin uniform layer of aluminium over the entire wafer surface. (iv) Etching away all unwanted aluminum using photo resist technique. 7. Scribing and mounting: Several similar ICs are manufactured simultaneously on a single wafer. After aluminium metallization of the sample wafer, it is scribed with a diamond tipped tool and cut into individual I.C. chips. Each I.C. is then mounted on a ceramic wafer and attached to a header. The package leads are connected to the I.C. chips by thick gold or aluminium wire running from the terminal pad on the chip to the package lead. At the end, a monolithic I.C. is obtained as shown is Fig. 3.7. resistor

diode

Transistor B

E

p

p

p

n

n

n

C Aluminium metallisation SiO2 layer

Fig. 3.7

Now we are in a position to brief the various processes involved in the fabrication of monolithic I.Cs in a flow diagram (Fig. 3.8). Preparing wafer

Cleaning & Polishing

Expitaxial layer

Individual I.C.

Scribing & mounting

Aluminium metalization

Leads connection

Testing

Packing

Fig. 3.8

SiO2 layer

Diffusion of impurities and Components formation

Masking, photo litho graphy & etching

3.10 I.C. Timers

25

3.10 I.C. TIMERS The IC timer is the most often used general purpose linear integrated circuit. This versatile IC has so many applications that it has become an industrial standard. These ICs can give a good time delay and are, therefore more useful in timer applications. This also justifies their name as ‘timer’. The important Timer I.C.s are: IC 555, 556 and 566. Timer I.C. 555 The I.C. 555 is the most popular timer I.C. It is an analog-digital IC having variety of applications. It is a versatile general purpose IC. (a) Block diagram of IC 555 The IC 555 is a combination of linear comparators and digital RS (Reset-set) flip flop as shown in Fig. 3.9. The series resistances R1 and

R2 set the reference voltage for the comparators. The output of these

comparators are fed to the flip flop.

The output of the flip flop is brought out through an output amplifier

stage. It also operates a discharge transistor inside the I.C. The transistor

collector discharges a timing capacitor.

The above structure is housed in a package and 8 pins are brought out.

(b) Pin Configuration (of IC 555) The following are the details of the pins of an IC 555: (Fig. 3.9) Pin 1: It is the ground terminal. All voltages are measured with respect to this terminal. Pin 2: It is the trigger terminal. If the voltage at this terminal is greater than VCC/3, the output is LOW. If the voltage is less than VCC/3, the output is HIGH.

Pin 3: It is the output terminal. There may be two output states, viz,

LOW and HIGH.

Pin 4: It is the reset terminal. When not in use, to avoid any false

triggering it is connected to VCC (Pin 8). The timer can be reset by

applying a negative pulse at this pin.

26

Chapter 3

Micro Electronics

8 (VCC) 7 R1 5

6

Discharge Transistor Upper Comp R

R2 S

2

R.S. Flip Flop

Output Stage

3

Output

Lower Comp

1 1-Ground 5-Control Voltage

4 2-Trigger 6-Threshold

3-Output 7-Discharge

4-Reset 8-VCC

Fig. 3.9

Pin 5: It is the control voltage terminal. The trigger voltage can be changed through this pin. This is used to control the reference voltage of the comparators. A capacitors C2 (0.01 mf) (Fig. 3.10) is connected to this pin to bypass the supply noise, if any. An external voltage applied at this pin changes the threshold voltage (which is 2/3 VCC).

VCC 8 Discharge Transistor

R1 7 R2

R.S. Flip Flop

Pin 6: It is the threshold terminal. 6 5 4 A capacitor (C1 = 0.1 mF) is connected from this pin to ground. C1 0.1 mf C2 0.01 mf This terminal controls the voltage across the capacitor C2 when the Fig. 3.10 same is charged from the supply (Fig. 3.10). This is the non-inverting input of the upper comparator. When the voltage at this pin is equal to or greater than the threshold voltage of 2/3 VCC, the output of the upper comparator goes HIGH which in turn switches the output of the timer to LOW. Pin 7: It is the discharge terminal. It allows discharging of the capacitor C1 (when the output is LOW) through the external resistor R2.

Pin 8: It is the supply terminal (VCC). The I.C. 555 can be operated at

any voltage between + 3 and + 18 volts.

3.11 Microprocessor (mP) IC

27

(c) Salient Features of IC 555 1. The IC is available in 8 or 14 pins. 2. It operates at + 3 V to + 18 V supply in both a-stable and monostable modes. 3. It is reliable, easy to use and low in cost. 4. Theoretically, an IC 555 can give a time delay up to 1 hour, but practically up to half-an-hour. (d) Applications of IC 555 There is a long list of applications of this timer. These are listed below: (i) As astable multivibrator (ii) As monostable multivibrator (iii) D.C./D.C. converter (iv) Other applications of IC 555: • As timer for electrical appliances • To generate negative voltage from positive voltage. • As pulse width modulator (PWM) • As pulse position modulator (PPM)

3.11 MICROPROCESSOR (mP) IC The control and arithmetic logic units (ALUs) of a computer together are known as the central processing unit (CPU). By the use of integrated circuits, the CPU can be fabricated on a single semiconductor chip known as a microprocessor. Thus, a microprocessor is a CPU on a chip. A microprocessor (mP) is a LSI (Large Scale Integrated) chip that does almost all the functions of a CPU. It is also called as a “CPU in a single chip”. A variety of microprocessors are available in the market and they vary in their processing capability, addressing capacity and the instruction set. The popular microprocessors are: 8080 A (8 bit, 40 pins), 8085 (8 bit, 40 pins), 8086 (16 bit, 40 pins), 6800 (16 bit, 64 pins), Z-8000 (16 bit, 48 pins) and TMS 9900 (16 bit, 64 pins). Here 8085 microprocessor will be discussed. I.C. Microprocessor 8085 The 8085 is one of the most commonly used microprocessors. It is an 8-bit n-MOS microprocessor which forms the basis of all the microcomputer systems. (a) Features of 8085 I.C. 1. It is 8-bit general purpose microprocessor. 2. It is capable of addressing 64 K memory.

28

Chapter 3

Micro Electronics

3. 4. 5. 6.

It has 40 pins. It requires 5 V power supply. It operates on 3 MHz single phase clock. Its all signals are classified under six groups: (i) Address bus (ii) Data bus (iii) Control and status signals (iv) Power supply and frequency signals (v) Interrupts (vi) Peripheral initiated signals 7. It has TTL (Transistor-Transistor Logic) compatible 8. It is developed by using NMOS technology 9. It can interface with almost all peripherals (b) Pin diagram of I.C 8085 Figure 3.11 shows the pin diagram of 8085. It is available in 40 pin DIL (dual in line) package fabricated using n-MOS technology. X1 Crystal OSC input X2 Reset out SID Serial SOD I/O TRAP RST 7.5 RST 6.5 Interrupts RST 5.5 INTR INTA AD7 AD6 AD5 AD4 AD3 AD0 – AD7 AD2 Address Bus AD1 Data AD0 GND

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

8085

40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21

VCC = 5V HLDA HOLD Clock OUT RESET IN READY Time and IO/M Control S1 Signal RD WR ALE S0 A15 A14 A13 A12 A11 A8 – A15 A10 Address Bus A9 A8

Fig. 3.11

Pins 1 and 2: These are also called crystal oscillator input pins. The 8085 operates on a clock pulse, that is generated internally. The oscillator circuit is wired around an external crystal with same internal circuit within the chip. The Crystal oscillator is preferred because of its high stability and constant frequency.

3.11 Microprocessor (mP) IC

29

Pin 3: This pin carries the RESET OUT signal. When RESET OUT is high, it indicates that mP is being reset and program counter is reset to zero. Pins 4 and 5: Pin no 4 is called serial input data (SID) pin and pin no. 5 is called serial output data (SOD) pin. Any 8 bit parallel data can be taken out serially from pin no. 4 and the serial data gets converted into 8-bit parallel input when applied at pin 5. Pins 6 to 11: These pins are the part of interrupt controller. The 8085 have five inputs for interrupt requests and one output for interrupt acknowledge (INTA). The highest priority interrupt is applied through TRAP (pin 6). The RST 7.5 has next highest priority and so on. Whenever an interrupt is being serviced by the mP, an high interrupt acknowledge (INTA) signal comes out from pin 11. Pins 12 to 19: The address/data output (ADO–AD7) is available through these pins, i.e., they carry the 8 bit data of address, both then being multiplied. The data is multiplied with the lower address bus to limit the total number of pins to 40. Pins 20: It is the chip ground pin. It is connected to ground. Pins 21 to 28: The upper 8 bits of the address (A8 to A15) is available through these pins. This address along with lower byte of address in the Address/data bus specifies the complete address of the peripheral device on which the mP has to perform. Pins 29 and 33: These pins carry the output signals from the Timing and Control unit. These signals are known as status S0 and S1 and indicate whether an instruction read, write or other operation is taking place in the mP at present. Pin 30: This pin carries the address latch enable (ALE) signal. The ALE signal is used to regulate the address/data bus, making it to carry either address or data at a time. When ALE is high, the bus carries the address. The bus latches its contents in the memory address register when ALE goes low and is then ready to carry the data. Pins 31, 32 and 34: These three pins function together and help in the data transfer through the data bus. Pin 34 carries IO/M signal. When this signal is low, the mP has to communicate the data with the memory and when this signal is high the mP has to communicate with input/ output unit. Pin 32 caries RD signal and Pin 31 carries the WR signal. One of these signals has to go low for Read or Write operation.

30

Chapter 3

Micro Electronics

Pin 35: The way to slow down the speed of 8085 is READY signal. If the peripheral device is not ready; it will send a low READY signal to the mP. The mP then generates a number of NO OPERATION states called WAIT states. When the peripheral device is ready, it will send a high READY signal to the mP enabling it to complete the data transfer. Pin 36: This pin is used to reset the mP. When a low signal is supplied through this pin, the CPU will reset its program counter, instruction register and other devices. It also sends a high RESET OUT signal. The CPU remains reset until the RESET IN signal is high. Pin 37: This carries the clock output signal generated in the timing and control unit. This clock is the system clock and helps in synchronization of devices with the mP. Pins 38 and 39: These pins help the Direct Memory Address (DMA) data transfer. The pin 38 carries HOLD signal which indicates for a DMA request by the peripheral device. Pin 39 carries HLDA signal which indicates that the mP is ready. For the DMA data transfer it must be noted, that the mP is not involved, it gives up the control of address and data buses to the peripheral device, resulting in high speed data transfer. Pin 40: It is the last pin and is connected to + 5 V ± 5 % power supply. The power dissipation of 8085 is less than 1.5 watts. (c) Applications of Microprocessors (i) Microprocessor control has advantages of flexibility, accuracy, reliability and economy. Since the control strategy is through software, changes can be made very easily to cope with new requirements. Speed regulation and time regulation are much better as compared to other methods. Human errors in measurements and control are eliminated. (ii) A microprocessor is a very suitable device for control circuits. Some examples are: automatic on-off mains power supply, speed control, temperature control, timing control, switching ON/OFF protection and trip circuits, fault finding and diagnostic circuits, etc. An electronic system which is centred around a microprocessor is known as a microprocessor-based controlled system. Like any other digital computer, a system designed around a microprocessor needs to be programmed.

Summary

31

Few mP controlled systems are given below: • Measuring instruments such as oscilloscope, multi-meter and spectrum analyser. • Household items such as the microwave oven, door, washing machine and television. • Defence equipment such as fighter, missiles and radar. • Medical equipment, such as blood pressure monitor (manometer), blood analysers and monitoring systems (such as MRI). • Commercial such as banking. • Consumers such as games. • Smart scale • Industrial process control • Desk top publishing.

SUMMARY 1. In micro electronics, we study very small devices such as I.C.s. 2. Fabrication of thousands of components/circuits in a small chip gives Integrated circuits. This is done by a special technique. 3. The I.C.s may be of following types: Monolithic I.C.s, Thin film I.C.s, Thick film I.C.s and Hybrid I.C.s. Monolithic I.C.s are more popular. 4. I.C.s are also classified as linear and digital I.C.s. 5. According to no. of components on them they are classified as very large, large, medium and small scale I.C.s. 6. We have I.C. Timers, I.C. microprocessors etc. as a chip. qqq

4 Electronic Devices In this chapter, following devices have been discussed. 1. Semiconductor diode and rectifiers. 2. Zener diode 3. Light Emitting diode 4. Laser diode 5. Liquid crystal diode 6. Schottky diode 7. Charge couple device 8. Bipolar Transistors (at low frequencies) 9. Thyristors-SCR 10. Stepper motor 11. Servomotor

4.1 SEMICONDUCTOR DIODE When p and n types of semi conductor materials are kept touching each other, “p-n junction” is obtained and a “depletion layer” is formed at their junction. (Fig. 4.1 a)

Depletion layer

+

P

N

–

Fig. 4.1 (a)

The PN junction is commercially known as semiconductor diode. It is also called Junction diode. As mostly used for rectification, this is also called “Rectifier Diode”. The fig 4.1 (b) shows symbol of the diode. It has a Bar and Arrow. The arrow represents the conventional direction of current, which is opposite to the flow of electrons.

Arrow

Bar –

+

Fig. 4.1 (b)

34

Chapter 4

Electronic Devices

1. Operation of diode (a) Mechanism of Current Flow in Junction Diode Due to depletion layer, a diode does not conduct of its own. For this, an external e.m.f is to be applied across the diode. This is known as biasing. A diode can be biased in two ways. 1. Forward Biasing: When the diode is connected with an external battery such that its P side (arrow) is connected with positive terminal of the battery and N side (bar) is connected with negative terminal of the battery, This is known as forward biasing. This bias removes the depletion layer and the diode conducts in the forward direction. (Fig. 4.2 a) Current

No current

Forward Biasing + –

Reverse Biasing – +

(a)

(b)

Fig. 4.2

2. Reverse Biasing: When diode is connected with the battery such that its arrow is connected to negative terminal of the battery and bar to positive terminal, this is known as reverse biasing. This increases the depletion layer and the diode does not conduct, But if reverse bias increases beyond limits, the diode conducts in the reverse direction, which is harmful for the diode. (Fig. 4.2 b) (b) Volt Ampere Characteristic of Diode The curve between voltage across the diode and current through the

diode is called Volt Ampere characteristic.

Fig. 4.3 (a) shows circuit for the purpose and Fig. (b) shows the Volt

Ampere characteristic.

B

I S

R

mA A

+

V

–

diode Reverse characteristic

(a)

O Breakdown voltage

Fig. 4.3

Knee Voltage (b)

Forward characteristic V

4.1 Semiconductor Diode

35

AC INPUT

+

+ –

+

+

DC OUTPUT

Fig. 4.5

DC output

AC input v = V sin q

AC supply

The Characteristic has three parts 1. When Switch S is open, no voltage across the diode and no current. See point O on the curve. 2. When a forward bias is applied on the diode, depletion layer reduces and at point A it vanishes, OA is called knee voltage. After point A, current rises following Ohm’s law and we get forward characteristic OB. 3. If reverse bias is applied to the diode, it does not conduct. If the reverse bias is increased, a reverse current flows through the diode, and we get Reverse Curve. At some reverse voltage the diode is broken down and is damaged. This is called “Break down Voltage”. 2.  Diode as Rectifier A rectifier is a device that converts AC into DC. A diode can act as a rectifier, because it conducts only in one direction i.e., when it is forward biased. It does not conduct in other direction. The rectifiers are of following types: rd i (a) Half Wave Rectifier (Fig. 4.4) This rectifier only rectifies half cycle T (positive cycle) of the input A.C.

RL This circuit uses one diode.

The AC to be rectified is given to the diode through a step down

transformer (T). When positive

cycle appears across the diode, it is

Fig. 4.4 forward biased and conducts, the voltage across diode is zero, the whole positive cycle appears at the load. When the diode is reverse biased it does not conduct, the negative cycle is absorbed by the diode itself and no output appears at the load. Hence only positive cycle is rectified. The Fig. 4.5 shows input and output wave shapes.

36

Chapter 4

Electronic Devices

AC supply

(b) Centre-tap Full Wave Rectifier (Fig. 4.6) D1 This rectifier rectifies both cycles of ac A (+) input and uses two diodes. The ac is given to the diodes through step Vm T down transformer T, with a center tapped secondary at point P. When positive P cycle of the ac input appears, the diode RL D1 is forward biased and D2 is reverse Vm biased (as end A is positive and end B is D2 negative). The D1 conducts and D2 does B (–) rd not. As a result positive cycle is rectified Fig. 4.6 and appears across RL. When negative cycle of ac input appears, end A becomes negative, the diode D2 becomes forward biased and D1 reverse biased, D2 conducts and D1 does not, the negative cycle is rectified and obtained across RL. Thus we get full wave rectified output. The Fig. 4.7 shows input and output wave shapes

+

+ –

+ D1

Input (ac)

–

– D2

+ D1

– D2

Output (dc)

Fig. 4.7

(c) Bridge Rectifier (Fig. 4.8) The bridge rectifier is also a full wave rectifier. It uses 4 diodes in a “bridge” form as shown. The output voltage of this circuit is more than centre tap circuit and also there is no need to find the centre tap point of the secondary. A(+) D4

D1 Vm

RL rd D2

B(–)

Fig. 4.8

iL D3

4.1 Semiconductor Diode

37

When positive cycle appears, the end A becomes positive and B becomes negative. As a result diode D1 and D3 conduct. The current flows from D1 via load RL and D2. When negative cycle appears, diodes D2 and D4 act, as end B becomes positive and the current flows from diode D2 via load RL and Diode D4 and we get full rectified output as shown. The Fig. 4.9 shows input and output wave shapes.

+

+ AC (input) –

–

iL + D 1 D3

– D2 D4 p

+ D1 D3 2p

– D2 D4 3p

4p

DC (output)

Fig. 4.9

3. Ripple Factor (RF) The output obtained from rectifiers is not pure dc and it contains a.c. components called ripples.

The ratio of Irms and Idc components found in the rectified output is called

ripple factor (RF) Irms or RF = Idc Where Irms are ac components and Idc are dc components. rms value of ac components in the rectified output (Fig. 4.8 and 4.9.) Irms =

1 2π

2π

∫ ( iL − I dc )

2

d .wt

0

[where iL is the current through load RL] =

2 2 I rms + I d2c − 2I dc

=

2 2 I rms − I dc

Now ripple factor RF =

I rms = I dc

2 2 I rm s − I dc

I dc

38

Chapter 4

Electronic Devices

2

⎛ I rms ⎞

= ⎜ ⎟ − 1

⎝ I dc ⎠

The a.c. ripples are removed by “filter circuits” and pure dc is obtained.

4.  Analysis of Rectifiers (i) H.W. Rectifiers (a)

P.I.V This is peak or max. voltage, a diode can take in reverse direction. The peak inverse voltage (PIV) for half wave rectifier, = Vm i.e.,

voltage across diode during negative half cycle. (b)

.

.. (i)

Output DC Voltage The waveform (Fig. 4.10) shows the voltage across the load resistor RL. iL Im Idc p

0

2p

3p wt

Fig. 4.10

Hence, Im is the peak value of the load current. Now

Im =

Vm RL

To find dc or average value of current, net area under the curve over one complete cycle i.e., from 0 to 2p is calculated and dividing this area by the base 2p gives average d.c. current.(Iav or Idc) Area =

2π

∫ iL d ( wt ) 0

(where, iL is the load current)

π

2π

0

π

= ∫ I m sin wt d ( wt ) +

∫ 0.d ( wt )

4.1 Semiconductor Diode

39

= I m [ − cos wt ]0 + 0 π

= Im[–cos p – (– cos 0)] = 2Im Iav = Idc = or

Idc =

Im

area base

=

2Im 2p

=

Im p

(dc component)

p

(ii)

Hence output DC voltage Vdc

=Idc × RL =

Im p

× RL

If diode resistance rd in forward bias is also considered

Im =

VmRL

p(RL + rd)

So dc voltage across load resistor RL is written as

Vdc = or

≅

VmRL p(RL + rd) Vm p

=

V m p(1 + rd /RL)

(if rd < < RL).

(iii)

(c) RMS value of current: The rms or effective value of the current flowing through load is given as Irms =

1 2π

2π

∫ iL d ( wt ) 2

0

(Fig. 4.10)

2π

=

1 ⎡ 2 2 ⎢ ∫ I m sin ( wt ) d ( wt ) + 2π ⎢⎣ 0

=

I m2 (1 − cos 2wt ) d ( wt ) 2π ∫0 2 π

2π

⎤ t 0.d w ( ) ⎥ ∫ ⎥⎦ π

40

Chapter 4

I m2 sin 2 wt wt − 2π × 2 2

= =

Im

Electronic Devices

π 0

(a.c. component)

2

(iv)

(d) Ripple factor, (R.F) 2

⎛ I rms ⎞

= ⎜ ⎟ − 1

⎝ I dc ⎠

For half wave rectifier Irms Im/2 = = 1.57 Idc Im/p \

R.F. =

(1.57 )2 − 1 = 1.21

(v)

See that, Ripple (a.c.) current (or voltage) exceeds the dc current (or voltage). This shows half wave rectifier is a poor converter of ac into dc. (e) Rectification efficiency (h) The dc power delivered to the load, Pdc =

2

⎛ I ⎞

= ⎜ m ⎟ RL ⎝ π ⎠

2 I dc RL

and total input ac power is 2

I 2 Pac = I rms ( rd + RL ) = ⎜⎛ m ⎞⎟ ( rd + RL ) ⎝ 2 ⎠ Pdc

\

h=

If rd < < RL ⇒

h = 40.6%

Pac

=

(Im/p)2RL (Im/2)2(rd + RL)

× 100% =

40.6 1 + rd/RL

% (vi)

It means that under best conditions, only 40.6% of ac input power is converted into dc power.

(ii) F.W. Rectifiers (a) P.I.V. The voltage Vm is the maximum voltage across half of the secondary winding i.e., when D1 is conducting it has almost zero resistance. Therefore, the sum of voltage across the lower half winding and voltage

4.1 Semiconductor Diode

41

across the load resistor RL is the reverse voltage that appears across the non-conducting diode. P.I.V. = Vm + Vm = 2Vm

(i)

(b) Output dc voltage (Vdc) See Fig. 4.11 Full wave rectifier uses both cycles so, the dc or average voltage available in a full wave rectifier will be double of the dc voltage which was available in half wave rectifier. Mathematically, v0 = Vm sin wt ; for 0 ≤ wt ≤ p = –Vm sin wt ; for p ≤ wt ≤ 2p iL Im Idc 0

p

2p

4p

3p

wt

FIG. 4.11

\

1 Vdc = 2π

2π

∫ v0 d ( wt ) 0

π

1 ⎡ ⎢ Vm sin wt d ( wt ) + = 2π ⎢⎣ ∫0

= = Vdc =

2π

⎤

π

⎦

∫ ( −Vm sin wt ) d ( wt )⎥⎥

1 ⎡ π 2π −Vm cos wt 0 + Vm cos wt π ⎤ ⎦ 2π ⎣ Vm 2p

[–cos p + cos 0 + cos 2p – cos p]

2Vm

(ii)

p

(c) rms value of current (Irms = a.c. component) For a full wave rectified current wave, time period is p.

iL = Im sin wt

π

Irms =

π

1 2 1 2 iL d ( wt ) = I m sin 2 wt d ( wt ) π ∫0 π ∫0

(

)

42

Chapter 4

=

=

= or

Irms =

I m2 π

π

∫

(1− cos 2wt ) d 2

0

I m2 wt sin 2 wt − π 2 4

Electronic Devices

( wt )

π 0

I m2 π × π 2 Im

(iii)

2

(d) The dc or average value of the current (dc component, Idc) π

Idc =

2I 1 π 1 iL d ( wt ) = ∫ I m sin wtd ( wt ) = m ∫ π0 π 0 π

(iv)

2

(e) Ripple factor:

=

⎛ I rms ⎞

⎟ − 1

⎜ ⎝ I dc ⎠

2

R.F. =

⎛ Im / 2 ⎞ − 1 = 0.482 ⎜⎜ 2I / π ⎟⎟ ⎠ ⎝ m

(v)

The ac component is less, so the F.W. rectifier is superior. (f) Rectification efficiency: The dc power delivered to load is 2

⎛ 2 I ⎞

2 Pdc = I dc RL = ⎜ m ⎟ RL and the total input ac power is ⎝ π ⎠

2

⎛I ⎞ 2 Pac = I rms ( rd + RL ) = ⎜ m ⎟ ( rd + RL ) ⎝ 2⎠ \ Rectification efficiency, 2

( 2I m / π ) P × 100% h = dc = 2 Pac I m / 2 ( rd + RL )

(

=

81.2 rd + RL

)

%

4.2 Zener Diode

43

when rd < < RL ⇒ h = 81.2%.

(vi)

Rectification efficiency of a full wave rectifier is twice that of a half wave rectifier under identical conditions. • P.I.V. For Rectifiers The P.I.V. stands for peak inverse voltage. This is the maximum reverse voltage, a diode can withstand without destroying the junction. It has special significance for rectifiers. It is to be ensured that the reverse voltage across the diode should not exceed the P.I.V. otherwise it will be damaged. (i) Half wave rectifier. In this case during –Ve cycle the diode is reverse biased and maximum value of reverse voltage (P.I.V.) is Vm. (ii) Centre tap rectifier. In this case the value of P.I.V. = 2 Vm which acts as reverse bias across each diode. (iii) Bridge rectifier. In case of bridge rectifier, the value of P.I.V. is Vm for each diode.

4.2 ZENER DIODE Zener diode is known by the name of its scientist. It is always used in reverse biased mode and invariably made of silicon. In reverse bias, its breakdown occurs at exact voltage as marked on the diode. When it breaks down, it becomes equivalent to d.c. source of that value hence its most important application is as d.c. voltage regulator or stabilizer. Its symbol is similar to a semiconductor diode with Z shaped bar (Fig. 4.12) Z shaped bar

Fig. 4.12

(a) Zener Characteristics (Fig. 4.13) The VI characteristic of Zener is similar to the ordinary diode except that. (i) It has a sharp breakdown Voltage (Vz). Forword characteristic

If

Vz Vf

Vr

Reverse

characteristic

Ir

Fig. 4.13

44

Chapter 4

Electronic Devices

(ii) After breakdown, the voltage across zener diode is almost constant. (iii) During reverse breakdown, the Zener does not burn as long as the external circuit limits the current through it.

(b) Applications of Zener There are two important applications of Zener :

Constant voltage

R

Ir

IZ Unregulated supply

IL Load

1. As Voltage Regulator: (Fig. 4.14) A voltage regulator is a device which provides constant voltage at the load irrespective of any change in the input voltage.

Vz

VZ

Fig. 4.14

When Zener breaks down the voltage across it (Vz) remains constant, though current through Zener IZ may change, extra voltage is dropped across R and extra current passes through Zener. 2. As Wave Shaper: Zener diodes are used to get various shapes. The Fig 4.15 shows a circuit to convert sine wave into square wave. R IZ

Vz Square wave

Sine wave Vz

Fig. 4.15

When input voltage reaches equal to Zener Voltage (Vz), the two Zeners are broken down and a heavy current passes through Zeners (Through R).

This causes a large voltage drop in R, and a nearly square wave output

is obtained.

(c) Breakdown in Semiconductor Devices Two types of breakdowns occur in semiconductor devices when reverse biased. 1. Zener breakdown This type of breakdown occurs in semiconductor devices with thin depletion layer (like zener diode). These devices have heavy doping

4.3 Light Emitting Diode (LED)

45

with thin junction. When small reverse voltage is applied to the junction, electric field becomes very high in the depletion layer. As a result, few electrons jump across the barrier. The phenomenon is known as Zener breakdown. The device does not damage and regains original position, when the reverse bias is removed. 2. Avalanche breakdown This is the next stage of Zener breakdown. When the device is reverse biased, the number of electrons jumping across the junction becomes too large and due to these electrons, current reaches to the “Burn Value” of the device. The phenomenon is called Avalanche Breakdown, which destroys the device permanently. This breakdown occurs in the thicker junctions with light doping, e.g., ordinary semi conductor diode. The device cannot regain its original position and is damaged permanently.

4.3 LIGHT EMITTING DIODE (LED) (a) The L.E.D. (light emitting diode) is a diode, that emits visible light, when energised (or biased) properly. In all PN junctions near the junction, recombination of electrons and holes takes place. In this process, the energy possessed by free electrons is converted into heat and light (photons). In germanium and silicon the greater percentage is given up in the form of heat and the emitted light is insignificant. In other materials such as “gallium arsenide phosphate” (Ga As P), or gallium phosphate (Ga P), the no. of photons emitted is sufficient to create a visible light. The phenomenon of giving off light (by applying

an electrical source) is known as “Electro luminescence”.

Anode

Cathode (a)

Light Brightness

The Fig. 4.16 (a) shows symbol and (b) shows characteristic of LED.

(b)

V

Fig. 4.16

(b) Process of emission (Fig. 4.17): The conducting surface connected to the p-type material is smaller than N type material to allow emergence of maximum number of photons (light). This occurs near the junction.

46

Chapter 4

+– N +–

+– P Light

+–

+

+–

Electronic Devices

–

+–

Conducting surface

Fig. 4.17

There is no doubt some photons are absorbed by the diode itself, but a very large percentage of photon energy is able to come out. The Fig. 4.17 shows the process of emission of light. (c) Materials used for LED: The table 4.2 shows the list of materials and other details. Table 4.1 S. No. 1. 2.

Materials

Colour of light

Wave length (nm)

Luminous efficiency lm/w

Ga P. Zn. O

red

690

20

Ga. P.N

green

575

610

Ga. As. P

red

540

72

Ga. As. P.N

yellow

650

450

Gallium phosphide (Ga.P) is used for emissions in the visible spectrum. It can be doped with “Zinc” and “Oxygen” to give out red light or nitrogen (N) to give out green light. Gallium Arsenide and gallium phosphide combine to give a solid solution of “gallium arsenide phosphate” (Ga.As P), doping with nitrogen increases the conversion efficiency and also the wavelength of emission. The lumen per watt (lm/w) is the luminous efficiency which takes into account the sensitivity of human eye, which is most sensitive to green and hence increased luminous efficiency is needed in this region.

4.3 Light Emitting Diode (LED)

47

(d) Construction: The Fig. 4.18 shows construction of a Ga.As.P-LED. It has a mesh type construction. It uses an N type alloy of Ga.As.P, which is grown epitaxically on a Ga-As substrate. Into this, the P region is diffused and covered with a “comb” type metal electrode (anode) to complete the PN junction. This distributes the diode current uniformly. Comb type metal anode

Light P region

Junction

N region

Metal cathode

Fig. 4.18

The emission of photons is the result of recombination of electrons and holes, which is only possible when both “energy” and “momentum” are conserved. A photon has a considerable energy but its momentum is very small, therefore the simplest and most probable recombination process will be that, where the electrons and holes have the same value of momentum. This condition exists in many of the group III and IV compound semiconductors with minimum conduction band and maximum valence band at zero momentum position. (e) Advantages: The advantages of LEDs as electronic displays are given below : (i) These are small in size and can be stacked together to form alphanumeric (alphabets as well as numerals) displays in high density matrix. (ii) The light output from LED is a function of current flowing through it, hence intensity of emitted light can be controlled. (iii)

These have high efficiency and need little power for operation. A voltage drop of 1.2 V and current of 20 mA is sufficient for full brightness.

(iv) These can emit radiations of many colours, such as red, green, yellow etc.

48

Chapter 4

Electronic Devices

(v) The switching time is less than 1 ns. (vi) The LEDs are manufactured by the same technology as used for I.Cs., hence are economical and highly reliable. (vii)

These are rugged and can withstand shocks and vibrations.

(viii)

These can be operated for a temperature range of 0°C to 70°C.

(f ) Applications of LEDs (i) Solid state video displays, which are replacing cathode ray tubes. (in LED – TVs) (ii) Picture phones (in image sensing circuits). (iii)

Optical fibre communications.

(iv) Data links and remote controls. (v) Arrays for displaying alpha-numeric or for entering information into computer memories.

4.4 LASER DIODE The LASER stands for “light amplification stimulated emission of radiation”. These are special kind of diodes and emit coherent light by stimulated emission. The Laser diode is also called as injection diode. It consists of a P-N junction inside a slab of semiconductor which is less than a millimeter (Fig. 4.19). The excitation is provided by current flowing through the device. The laser diode is thus a diode within a resonator cavity that is formed on the surfaces of the diode. A current passing through the diode produces light emission (beam), when electrons and holes recombine at the P-N junction. Because of the small size of active medium (light emission), we need special optics (lens or minor) to get a focused beam. l

Metal contact Oxide N-region Active region P-region

Fig. 4.19

Beam

4.4 Laser Diode

49

The semiconductor lasers are fabricated from Gallium Arsenide (Ga-As), Aluminium Gallium Arsenide (Al-Ga-As) or Indium Gallium Arsenide Phosphate (In-Ga-As-P), etc. Figure 4.20 shows a comparison between the light emitting diode (LED) and the laser diode. Figure 4.20 (a) shows their curves between output power and injection current. Figure 4.20 (b) shows their curves between output power and the wavelength (spectral widths) and Fig. 4.20 (c) shows their emission pattern.

Output Power (mW)

Output power Laser LED LED Threshold current

Laser

Injection Current (mA)

Wavelength (b)

(a)

LED (i)

LASER (ii) (c)

Fig. 4.20

If a current flowing through a Ga-As P-N junction is increased in magnitude beyond a certain “critical value,” a lasing action takes place, i.e., the spontaneous emission “stimulates” an increase in the radiant power. The LASER phenomenon is similar to the avalanche breakdown. The radiated output from a solid state laser lies in the invisible spectrum and is less coherent compared with those produced by “Gas laser”. However, it makes the laser diode very compact and makes it possible to provide an output power upto 60 watts from a single laser diode. Application: The laser diodes are used in optical (Fibre cable) communication. An LED functions on the principle of spontaneous emission”, where as a laser

50

Chapter 4

Electronic Devices

diode works on the principle of “stimulated emission”. The two processes are compared below: S. No. Spontaneous emission (LED)

Stimulated emission (Laser diode)

1.

In this emission, a light source (atom, In this emission, the emitted photon molecule, nucleus) in excited state strikes the excited atom etc. and emits undergoes a transition to a lower one more photon. energy state; and emits a photon.

2.

The spontaneous emission occurs A photon created in this manner without regard to the ambient has the same phase, frequency electromagnetic field. polarization and direction of travel, as the incident photon.

3.

The phenomenon is found in LEDs This phenomenon is key process of and fluorescent tubes. formation of laser beam.

4.5 LIQUID CRYSTAL DIODE (LCD) Unlike LED, the liquid crystal diode does not generate light by it self, but simply alters or controls existing light to make selected areas appear bright or dark. (a) Construction: Basic construction is as shown in Fig. 4.21. It has a thin layer of a liquid crystal between two glass sheets Glass sheet +

Transparent electrodes

Spacer – Liquid crystal

Fig. 4.21

For the application of electric potential across the crystal, a transparent conducting material is deposited on the inner surface of each glass sheets which form the electrodes. Materials used to form the electrodes include stanic oxide SnO2 and indium oxide ln2O3. The LCDs consume very little power in comparison to LED display. (b) Working principle: It is based on the fact that chemically produced organic compounds having unique properties that make the crystals which seem to behave like ordinary liquids and at the same time as crystal solids also. Many organic chemicals exhibit this property. Sumetic liquid and chalestic liquid crystals are generally used for this purpose. The liquid crystals are shaped like a microscopic rod and these rods have two poles positive and negative, means they have dipole

4.6 SChottky Diode

51

characteristics. As like poles repel and opposite poles attract each other therefore, by applying electric charge, we can align or twist the liquid crystals in any way or in any direction as per the required display. LCD assembly appears like a clear glass, the segments are not visible. These are visible only on application of external field. (c) Application: LCDs are used in wrist watches, calculators, TV, digital instruments etc. (d) Merits & Demerits: LCDs can be made in large sizes and shapes, cost is low and consume very low power. But their disadvantages are that their operating speed is low, frequency range is low (25Hz to l KHz) and have limited operating temperature (–10°C to 50°C).

4.6 SCHOTTKY DIODE (a) Construction: Schottky diode is a special purpose device with no depletion layer, eliminating the stored charges at the junction. Its construction is very different from the normal p-n junction as in this, a metal semiconductor junction is developed. On one side of the junction, a metal (such as gold, silver, platinum, molybdenum, chrome or tungsten) is used and on other side of the junction, n-type doped Si is used. It increases frequency range and lowers forward bias. In this diode, electrons (e–s) are the majority carriers Fig. 4.22 shows construction and Fig. 4.23 shows two symbols of Schottky diode. Gold leaf metal contact Cathode

–

Fig. 4.22

Metal contact

N Metal Anode

+

Metal – semiconductor Cathode junction

+

N-type silicon

–

Metal Anode SiO2 screen

Cathode

(a)

Anode

(b)

Fig. 4.23

(b) Operation: In metal, minority carriers are insignificant. When diode is unbiased, e–s (electrons) on n-side have low energy levels than e–s in metal and so the e–s cannot cross the junction barrier called schottky barrier. When it is forward biased, e–s on n-side gain enough energy to cross the junction and enter metal. Since e–s plunge into metal with very large energy, the electrons are called hot carriers and the device is called a hot carrier diode.

52

Chapter 4

Electronic Devices

jun ctio n

PN

Sc hot tky

dio de

I

dio de

(c) Merits and VI Characteristic: Schottky diode is a unipolar device and there is no depletion region. It is faster in switching. It has lower barrier potential (0.2 – 0.25V) whereas normal Si diode has 0.7V. It is more efficient for higher power applications than Si rectifiers. Schottky diode is having higher leakage currents and lower reverse breakdown voltage.

The VI characteristic of schottky diode is shown in comparison to PN

junction diode in Fig. 4.24.

V

–V

–I

Fig. 4.24

(d) Schottky vs conventional diode (i) Schottky diode is a special purpose device with no depletion layer to eliminate the stored charges at the junction. Its construction is very different from the normal p-n junction, as explained above. (ii) The reverse recovery time is very short in small signal schottky diodes. It is significant above 10 MHz. (iii) Because of lack of charge storage, the schottky diode is switched on and off faster than the ordinary diode. (iv) It is a unipolar device, where as the conventional diode is a bipolar device. (v) It is almost a noise free device. (vi) It has lower barrier potential (0.25V) compared to 0.7V in case of conventional diode. (e) Applications: (i) Used for fast switching applications. (ii) Schottky diode can easily rectify frequencies above 300 MHz. (iii) Due to its fast switching, Its most important application is in digital computers.

4.7 Charge Couple Devices (CCD)

53

Due to absence of charge storage, the low power schottky TTLS (Transistor-transistor logic) are widely used. (iv) Suitable for high power applications.

4.7 CHARGE COUPLE DEVICES (CCD) The CCD is a member of broader class of charge transfer devices. The charge transfer devices are the dynamic devices that move charge along a predetermined path under the control of clock pulses. They find application in memories, logic functions, signal processing and imaging. (a) Principle The basis of CCD is the dynamic storage and withdrawl of charge in a series of MOS capacitors. A depletion region exists and the surface potential increases considerably under the gate. As a result, the surface potential forms a potential well which can be used for storage of the charge. The potential well should not be confused with the depletion region, which extends into the semiconductor. The depth of the well is measured by electrostatic potential and not by distance. The electrons stored in the well are located very near the semiconductor surface. The Fig. 4.25 (a) shows a MOS capacitor with a p type substrate and depletion region with surface charge and Fig. 4.25 (b) shows potential well partially filled with electrons corresponding to the surface charge (shown in Fig. (a)). Gate (G) SiO2 Depletion region with surface charge (a) MOS capacitor

p substrate

Potential well partially filled with electrons (b) Potential well

Fig. 4.25

If the positive pulse at the gate is applied for a sufficiently long time, electrons accumulate at the surface. The source of these electrons is thermal generation at or near the surface. The time required to fill the well thermally is called thermal relaxation time. For a good semiconductor material, “the relaxation time” is much longer than “charge storage time” involved in CCD operation.

54

Chapter 4

Electronic Devices

If we can inject electrons into this “well” electrically or optically, they will be stored there. The storage is temporary, because the electrons must be moved to another storage location before the thermal generation becomes very high. Now we need a simple method to allow charge to flow from one well to an adjacent well quickly without losing any charge in the transfer. If it is done, we can inject, move and collect “charge packets” dynamically to a variety of electronic functions. (b) Basic CCD A basic CCD consists of a series of metal electrodes (G1, G2 ...) forming an array of mos capacitors as shown in Fig. 4.26 which uses 3 phase system. The voltage pulses are supplied in three lines 1,2,3. The G1 is connected to line 1, G2 is connected to line 2 and G3 is connected to line 3. The voltages are clocked to provide potential wells, which vary with time. At time t1, the G1 is positive and the charge packet is stored in “G1 potential well”. At time t2, both (G1 and G2) are positive and the charge is distributed between the two wells (between G1 and G2). The charge may be assumed as a fluid, which has a tendency to equate the level. At t3 the potential (E1) at G1 is reduced and the charge flows to the second well. At t4, the transfer of charge to G2 well is complete, when E1 is reduced to zero. 1 2 3

G1

G2

G3

G1

G2

G3

Array of MOS capacitors

Potential wells Potential wells

t1

E1(+)

t2 t3

E1 < E2

Potential wells

Potential wells

t4

E2(+)

Fig. 4.26

4.7 Charge Couple Devices (CCD)

55

The process is continuous and with time, charge moves to G3 and so on. In this way, charge can be injected using an input diode, transported down the line and detected at the output. (c) Buried channel CCD The following improvements are done in a basic CCD. (i) The separation between electrodes should be very small to allow coupling between the electrodes. This can be done by overlapping gate structure with alternating poly crystalline silicon and metal (like aluminium). (ii) Some charge is inevitably lost during transfer along the CCD. The situation can be improved by providing enough bias to the zero state. (iii)

The efficiency of transfer can be improved by moving the charge transfer layer below the semiconductor-insulator interface. This can be done by using an ion (donor) implantation to create an opposite type of layer than the substrate.

This gives a Buried Channel CCD. See Fig. 4.27 (a).

It is a two phase system. The voltages are sequentially applied to the

alternating gate electrodes from two lines. A two level poly Si gate

structure is used in, while the gate electrodes overlap and an Ion (donor)

implant near the Si surface creates a built in extra potential well under

right half of each electrode.

When both the gates are turned off (see Fig. 4.27 (b)), the potential

wells exist only under the implanted regions and the charge can be stored in any of these wells. G2

G1

Aluminum G2

(a) SiO2 layer

G1

Poly crystalline S1

Ion (donor) implant

(b) G1, G2 off

(c) G2 on G1 off

Fig. 4.27

56

Chapter 4

Electronic Devices

When electrode G2 is given a positive pulse, the charge packet shown in Fig. (b) is transferred to the deepest well under G2, which is its implanted region. This has been shown in Fig. 4.27 (c). The next sequence will be to apply positive pulse to G1 so that the charge moves to the implanted region towards right under G1

(d) applications of CCD. The CCDs are used (i) For signal processing functions such as delay, Filtering, multiplexing (sending several signals simultaneously). (ii) For imaging in solid state TV cameras. An area image sensor can be made, which scans the image electronically in both dimensions. (iii) They find application in memories and logic functions.

4.8 BIPOLAR TRANSISTORS (AT LOW FREQUENCIES) A transistor is a 3 semiconductor layers, 3 terminal, 2 junction device. It is called Bipolar, as conduction takes place by holes as well as by electrons. A bipolar transistor has two junctions so, it is called Bi Junction transistor (BJT). The one junction is forward biased and has low resistance. The second junction is reverse biased and has high resistance. Signal is transferred from low resistance to high resistance. The property of the device to transfer signal through resistances has given it the name of transistor.

1. Types of Transistors A transistor is obtained by sandwitching a different material between two same materials. Accordingly there are two types of transistors. (a) PNP transistor: (Fig. 4.28 a) This is obtained by keeping a N type layer between two P type layers. E

E

B

B Symbol PNP

E

P

NPN

C

N

P

C

E

B (a)

N

C P B (b)

Fig. 4.28

N

C

Structure

4.8 Bipolar Transistors (at Low Frequencies)

57

(b) NPN transistor: (Fig. 4.28 b) This is obtained by keeping a P type layer between two N type layers. The transistors have three terminals and three currents. (Fig. 4.29) (i) Emitter (E): When the transistor is properly biased, emitter emits electrons if it is N type, and emits holes if it is P type. It constitutes emitter current (IE) (ii) Base (B): The electrons or holes pass through base and recombination takes place in base region, as a result a small base current (IB) is produced. E

IE

IC

C

IE

E

IC

IB

IB

B (a) PNP

B (b) NPN

C

Fig. 4.29

(iii) Collector (C): Through the base, electrons (or holes) reach into collector region and collector current (IC) is produced, which is the output current.

2. Mechanism of Current Flow in Transistors This will be studied: (i) When a transistor is unbiased and (ii) When it is biased. (a) When Transistor is Unbiased (Fig. 4.30) When a transistor is not biased, the electrons and holes diffuse from one region to another region, as a result depletion layers are produced at the junctions. Their width at emitter base junction (dep. I) is thin and width at collector base junction (dep. II) is wider. E

E

B

N

Dep I

hole

P

B

Dep II

Fig. 4.30

C

electron

N

C

58

Chapter 4

Electronic Devices

(b) When Transistor is Biased (i)

NPN Transistor The fig. 4.31 shows a NPN transistor in biased condition. The emitter base junction is forward biased by VEB. The collector base junction a reverse biased by VCB. The N type emitter has electrons as majority carriers. It emits electrons. This constitutes emitter current IE. In base, few electrons (say 5%) combine with holes and a base current IB is produced. After travelling through base, electrons enter into collector region and constitute collector current IC. The following equation is satisfied. IE = IB + IC This current relation holds good in all configurations of both transistors whether PNP or NPN.

E

IE

N

P

E

IC

N

B

C

C

Recombination IB VEB

VCB

Fig. 4.31

(ii) PNP transistor. (Fig. 4.32) When input junction of PNP transistor is forward biased (FB) and output junction is reverse biased (RB) the holes from P type emitter constituting emitter current (IE) enters into base. Few holes (5%) recombine with electrons of the base, constituting base current (IB) and remaining holes enter into collector and constitute collector current (IC). The fig. 4.32 shows conventional direction of currents, which satisfy the following equation

IE = IB + IC

4.8 Bipolar Transistors (at Low Frequencies)

59

Recombination E

B P

IE

C

N

P

IE

IC

IC

IB

VEB

VCB

Fig. 4.32

3.  Transistor Configurations Transistor has 3 terminals. One terminal is kept common and thus transistor may be connected in 3 ways. These are called configurations. These are: (a) Common base configuration (b) Common emitter configuration (c) Common collector configuration (a) Common Base Configuration (Fig. 4.33) In this configuration, the base is made common. The emitter base junction is forward biased and collector base junction is reverse biased by battery VEB and VCB respectively. Input is given between emitter and base and output is taken across collector resistance RC.

Input

RC – + VEB

IB

IE

– + VCB

IC

RC VEB + –

(a) NPN

PNP

IB (b) PNP

Output

IC

Input

NPN

Output

IE

VCB + –

Fig. 4.33

(i) Current amplification factor or current gain. This is the ratio of output collector current (IC) and the input emitter current (IE). It is written as alpha (a).

60

Chapter 4

Electronic Devices

In CB configuration, IC a = (the value of a is less than 1) IE or

IC = a IE

This is called a(dc) which is amplification factor when only biasing is provided. When transistor is given a.c. signal for amplification in that case, we get a (a.c.) which is the ratio of change in collector current with respect to change in the base current: a(a.c.) =

D IC D IB

The value of both alphas (a) is less than 1 (ii) Collector Current, (IC): The IC has two parts (i) IC = a. IE (ii) When collector base Junction is reverse biased and emitter base Junction is kept open (Fig. 4.34) a current flows from “collector to base with open emitter (ICBO)” This is due to minority carriers. If collector is N type, it is due to holes and if collector is P type, it is due to electrons. This is called “Leakage current”. ICBO NPN

– + Open

Fig. 4.34

\ The total collector current IC = a. IE + ICBO

•  Characteristics of Common Base Configuration A common base transistor has input and output characteristics. The Fig. 4.33 shows circuit for drawing both the characteristics. 1. Input Characteristics: Input characteristic is the curve between input current (IE) and Input voltage (VEB) at constant output voltage (VCB).

4.8 Bipolar Transistors (at Low Frequencies)

mA

– VEB +

IE

61

NPN

V1

IC

mA

V2

IB

–

+ VCB

Fig. 4.35

The Fig. 4.36 shows two Input curves at VCB = 0 V and VCB = 10 V. IE 3.0

0V

V 10 V

CB

=

B

VC

=

0

2

VEB

Fig. 4.36

Input resistance: This is the resistance offered by Input Junction. This is the ratio of Input Voltage (VEB) and input Current (IE).

DIE

VEB IE

For finding input resistance at point P,

make a small change in VEB and find corresponding change in input current

IE.

Now

P

0

VEB

DVEB

ri =

IE

Fig. 4.37

ri = D VEB /D IE (Fig. 4.37)

2. Output Characteristics: This is a curve between output voltage (VCB) and output current (IC) at a particular input current (IE). The circuit for drawing this curve is same as for input curve. The Fig. 4.38 shows four curves at IE = 0, IE = 2 mA, IE = 4mA and IE = 6mA. The curve has three regions :

IC Ac

Saturation region

on

egi

R tive

IE = 6mA IE = 4mA IE = 2mA IE = 0

Cut off region

Fig. 4.38

VCB

62

Chapter 4

(i)

Electronic Devices

Saturation region: In this region, both the junctions are forward biased. In fact, VCB is slightly negative in this case and Ic changes quite rapidly.

(ii) Cut off region: In this region, both junctions are reverse biased. This is the leakage current, that flows in collector region which is due to minority carriers. (iii) Active region: In this region, the input junction is forward biased and output junction is reverse biased. The IC entirely depends upon IE. Output resistance: This is the resistance of the output side. This is equal to the ratio of change in output voltage (DVCB) and change in output current (DIC). VCB DVCB r0 = = at constant IE IC DIC (b) Common Emitter Configuration (Fig. 4.39 a, b)

or IC = bdc.IB

RC

Output

NPN

IE + – VBE

–+ VCC

(a) For NPN transistor IE PNP

IB

RC IC – + VBE

+– VCE

(b) For PNP transistor

When signal applied, we Fig. 4.39 get bac, which is the ratio of change in IC with respect to change in IB. D IC ba.c. = ; D IC = ba.c. D IB D IB

Output

(i) Current amplification factor or Current gain is the ratio of output current (IC) and input current (IB). It is represented by b (beta). IC bdc = IB

IB

Input

In this configuration, emitter is made common. The input (base emitter) junction is forward biased and output (collector emitter) junction is reverse biased.

IC

4.8 Bipolar Transistors (at Low Frequencies)

63

(ii) Collector Current (IC). When input circuit is open and output circuit is reverse biased. (Fig. 4.40) some leakage current flows from collector to emitter when input is open (ICEO). Thus total collector current IC = b IB + ICEO

ICEO

NPN

• Relation between a and b.

IB

We know

That IE = IB + IC

IE

IB = IE – IC b=

IC IB

IC

Open

= IE – IC

–+

Fig. 4.40

IC /IE (dividing by IE) = IE /IE – IC /IE a

\

b=

If

a = 1

b=

1–a

1 1–1

–=

1

0

= ∞, so if a approaches to unity, b may be infinite, practically b may be upto 400. • Relation between ICEO and ICBO b= b+1= or,

=

So, ICEO =

a 1 – a a 1 – a

+ 1 (Adding 1 both sides)

a+1–a 1–a ICBO 1–a

=

1

1–a

64

Chapter 4

Electronic Devices

• Characteristics of CE Configuration The CE configuration has also two (input and output) characteristics: 1. Input characteristics of common emitter configuration is the curve between input voltage (VBE) and input current (IB) at particular output voltage (VCE). The Fig(4.41(a)) shows the characteristic at VCE = 0, and VCE = 10 V and Fig. (b) shows the circuit arrangement for drawing the characteristics. IC (0-100mA) IB mA =1 0V V

(0-15V) NPN

+ –

CE

V

CE

=0 V

IB

VBE

(a)

VCE IE

(0-3V) 0

(0-100mA) mA

VBE

+ –

(b)

Fig. 4.41

Input Resistance. This is the resistance of input junction. This is the ratio of small change in the input voltage (DVBE) and the corresponding change in the input current (DIB). (Fig. 4.42) IE P

DIa

0

DVBE

VBE

Fig. 4.42

ri (at point P) =

VBE IB

DVBE =

DIB

2. Output Characteristic: (Fig. 4.43) This is the characteristic of output side. This is the curve between output voltage (VCE) and output current (IC); at particular input current (IB). It has 3 regions.

4.8 Bipolar Transistors (at Low Frequencies)

65

(i) Active region: When VCE is increased (output junction is reverse bias.) the transistor opIC IB = 30mA erates in active region IB = 20mA and IC increases with Saturation IB = 10mA Active IB = 5mA VCE for a constant value region Region of IB. In this region the input junction is in forVCE ward bias and output Cut off region junction is in reverse Fig. 4.43 bias. The value of IC can

be changed by changing the value of IB.

(ii) Saturation region: In this region, when VCE increases, IC increases from zero to a near saturation value for fixed value of IB. But when VCE is reduced, IC does not reduce. In this region, input as well as output both junctions are in forward bias. (iii)

Cut off region: If IB = 0, still some IC = ICEO flows in the collector. This is independent of IB or VCE. In this region, both the junctions are reverse biased.

Output resistance: This is the resistance of the output junction of the transistor. This is the ratio of a small change in the output voltage (DVCE) to the corresponding change in output current (DIC) at a particular input current IB. VCE DVCE = r0 = IC DIC (c) Common Collector Configuration (Fig. 4.44) In common collector configuration, input is supplied through the base and output taken through the emitter, while the collector is common. IE

IC + – VBC

(a)

Output

P

Input

RC

Output

IB

PN

N

Input

IB

NP

IE

–+ VCE

IC – + VBE

Fig. 4.44

(b)

+– VCE

66

Chapter 4

Electronic Devices

(i) Current amplification factor or current gain is the ratio of output current (IE) and input current (IB). This is represented by gamma (g) IE g= IB or

IE = g.IB

The output current is IE = gIB + leakage current (ii) Relation between g and a g= or,

g=

IE IB

=

IE IE – IC

=

IE/IE IE/IE – IC/IE

(dividing by IE)

a 1–a

If a approaches unity, then g =

1

1

= ∞. 1–1 0 The g can be infinite, but practically it is upto 400. (iii)

=

Relation between a, b and g

a a

g= = +1 1–a 1–a g=b+1

Table 4.2: Comparison of three configurations CB configuration

CE configuration

CC configuration

1.

Input impedance

Low (up to 100 Ω)

Medium (1k)

Very High (100k)

2.

Output impedance

Very High (500k)

High (50k)

Low (50 ohm)

3.

Current gain

Less than unity

High (up to 400)

High (up to 400)

High (about 450)

Less than 1

S. No.

Particulars

4.

Voltage gain

Small (150)

5.

Leakage current

Between 2 mA and Between 20 mA 5 mA and 450 mA

6.

Application

For high frequency amplification and impedance matching

For audio frequency ampliflication

Between 20mA and 450mA For impedance matching

4.8 Bipolar Transistors (at Low Frequencies)

67

Input

IC 4.  Transistor as Amplifier NPN A transistor has low input resistance IB and high output resistance because input is forward biased and output is reverse biased. A transistor basically RC = 5K transfers signal from low resistance to IE high resistance and in this process, it increases strength of the signal. In other + – –+ words, it amplifies the signal. This can be Fig. 4.45 explained by the following example. The Fig. 4.45 shows a transistor in common emitter configuration. Assume, input resistance is 20 Ω and output resistance = 5K and b = 500. Let Input voltage a signal of 200 mv is given to the input i.e., IB = Input resistance 200mv So, IB = = 10mA 20 Ω IC = b . IB = 500 × 10 = 5000 mA = 5A

Output Voltage = 5A × 5 KΩ = 5 × 5000 = 25000V Thus 200 mV is amplified as 25000V through amplifying action.

5.  Various Gains of Amplifier  There are three gains (Fig 4.46) 1. Current gain. When transistor is amplifying, the ratio of small change in collector current (DIC) to the corresponding change in base current (DIB) is called current gain. DIC C.G. = b = DIB +VCC

RC IC RB

C IB

VO = VCE

B NPN E RE IE

Fig. 4.46

68

Chapter 4

Electronic Devices

2. Voltage gain. In an amplifier, the ratio of change in output voltage (DVCE) obtained from a change in the input voltage (DVBE) is called voltage gain. DVCE V.G. = AV = DVBE 3. Power gain. In an amplifier the ratio of output power and the input power is called power gain.

P.G. =

IC.VCE IB.VBE

=

IC IB

.

VCE

VBE

= b × AV = C.G. × V.G.

The power gain is the product of current gain and voltage gain.

6.  Identification of Transistor Terminals In order to identify terminals of transistor, the following methods maybe adopted. (i) When the terminals are in the same plane, then centre

terminal is “Base” (B) the terminal nearer to base is

emitter (E) and third terminal is Collector (C). (Fig. 4.47)

(ii) In some transistors, a “dot” is engraved on the collector. Then the central terminal is “Base” and the third terminal is “Emitter” (See Fig. 4.48).

C

B

E

B

C

Fig. 4.47

E

Fig. 4.48

(iii) When the terminals are on the circumference of a circle, a tab is provided at Emitter; and going clockwise, we get Base and then Collector. (Fig. 4.49) TAB

C E B

Fig. 4.49

4.9 Thyristors

69

7. Nomenclature of Transistors This is explained below: (i) First letter shows the material of the transistor.

A = Germanium

B = Silicon

C = Gallium Arsenide

(ii) Second letter shows application of the transistor.

C = AF Transistor

D = AF Power transistor

G = HF transistor

S = Switching transistor

(iii) The number given in the end helps to find parameters of the transistor from the data book such as 125 or 194. Illustration: (i) AC 125 is an audio frequency low power germanium transistor. (ii) BF 194 is a high frequency, high power silicon transistor.

4.9 THYRISTORS A thyristor is a solid equivalent of thyratron gas tube. It is a large family of devices having 2, 3, 4, terminals, 4, 5 P-N semiconductor layers and three or more P-N junctions. The thyristors have compact form, and short on/off timings. Due to these unique characteristics, these have replaced thyratrons, mercury arc rectifiers, ignitrons in applications like: DC/AC motors control, induction heating, power transmission, special power supplies needed for aircraft and computers and other applications. There is a family of Thyristors, Few important members of the family are: 1. Silicon Controlled Rectifier (SCR) 2. Triac 3. Diac 4. Silicon Controlled Switch (SCS) 5. Silicon unilateral switch (SUS) 6. Silicon Bilateral switch (SBS) 7. Light Activated SCR (LASCR) etc. The SCR (silicon control rectifier) is its most important member. Generally by thyristor, we mean an SCR. Hence SCR has become ‘synonym’ for thyristors.

70

Chapter 4

Electronic Devices

1.  Silicon Controlled Rectifier (SCR) Among the thyristors, SCR is of greatest interest today. The device is used as controlled rectifiers, relay control circuits, time delay circuits, regulated power supplies, switching circuits, motor controls, choppers, inverters, cycloconverters, battery chargers etc. SCRs are available which can handle power as high as 10 MW and a current of 2000 A at 1800V. They can handle 50 KHz frequencies and therefore also find use in ultrasonic cleaning and H.F. heating. (a) Construction An SCR is a P-N-P-N device and is basically a rectifier constructed of silicon with a third terminal as “Gate” to control rectification. Here this device makes a difference from the ordinary diode. It is not enough to make anode to cathode forward bias but a pulse of sufficient magnitude must also be applied also at the gate to make the device ON. The forward resistance of the device is about 0.1 Ω and reverse resistance of 100 K or more. The Figure 4.50 (a) shows construction and Figure (b) shows its symbol. It has 4 semiconductor layers, 3 P-N junctions (J1, J2, J3) and 3 terminals (Anode, Cathode and Gate). Gate (G) + Anode (A)

P

N J1

P J2

(a)

N J3

– Cathode (K)

A

K G (b)

Fig. 4.50

(b) Working of an SCR The circuit diagram for the SCR is given in Figure. 4.51

The working will be discussed in two steps.

Case 1: The SCR is biased but gate circuit being open

(i) When SCR is forward biased, i.e., anode of the SCR is made positive w.r.t. cathode and no voltage is applied at the gate i.e., the switch S is open. At this, the junction J1 and J3 become forward biased but the junction J2 remains reverse biased, due to which current cannot flow from anode to cathode and SCR remains OFF.

4.9 Thyristors

71 S Ig

P

A(+)

N J1

RL

G

P J2

VG

N

K(–)

J3 VF

Fig. 4.51

If the forward bias (VF) between anode and cathode is increased, at certain high voltage, the junction, J2 breaks down the SCR becomes ON and reaches in a highly conducting state. The voltage at this point is known as “Break Over Voltage (VBO)” The value of forward voltage (VF) which changes the SCR suddenly from its OFF state to ON state is called its Break over voltage (Forward). (ii) If an SCR is reverse biased, i.e., anode is made negative w.r.t. cathode, the junctions J1 and J3 become reverse biased and J2 forward biased. The SCR will not conduct. However, if the reverse voltage is increased, at a certain value of this voltage the junctions J1 and J3 will breakdown and the SCR again will turn ON into high conduction stage. This voltage is called “break over voltage” (Reverse). The value of voltage in reverse bias (VR) which changes an SCR suddenly from its OFF state to ON state is called its Breakover voltage (reverse). Case 2 : The SCR as well as the gate is given forward bias If the SCR is forward biased across anode and cathode and also, a forward bias is applied at its gate by closing the switch S, such that a gate current Ig flows and makes the junction J2 forward biased, the SCR can be turned ON at a voltage, which is very less than “Break Over Voltage” of the device. The SCR will now again in the high conducting state. (c) V-I Characteristics of SCR An SCR has forward as well as reverse characteristics : The Fig. 4.52 shows the circuit and Fig. 4.53 shows the characteristics.

72

Chapter 4

Electronic Devices

1. Forward Characteristic: This is the curve between forward voltage (VF) across SCR and the anode current (IA) flowing through the load. (i) When gate current is zero: At small forward voltage (gate circuit is open) there is a very small current through the SCR, which is the leakage current due to leakage of minority carriers through junction J2 (which is reverse biased). The device is said to be OFF. VG

Ig G IA

A

RL

K

VF

Fig. 4.52

(ii) When the forward voltage (VF) is increased, at a value called “Break over voltage (VBO),” the SCR reaches in the highly conducting (Short circuit) stage and is said to be ON, the whole supply voltage appears across the load (RL) and voltage across the SCR drops to a very small value ( ~ 1V). The device remains ON until the anode current is maintained to the holding current (IH). The SCR may be turned OFF, if supply voltage (hence current) reduces to zero. (Fig. 4.53) (iii) If gate circuit is also closed voltage VG, the SCR can be turned ON at a voltage which is very less than VBO.Note that more is the gate current Ig, lesser is the voltage, at which SCR starts, Note that Ig < Ig1 < Ig2 and VBO > VB01 > VB02. Also note that a large gate current can give a curve like OBC.,

4.9 Thyristors

73

VR Avalanche breakdown

C Ig2

IA/IF

Reverse breakdown voltage

B

IH Leakage current O Reverse Blocking Region

VH

VBO – 2

Ig1

Forward characteristics

Ig E A D

VF VBO VBO – 1

Rev. Ch.

Fig. 4.53

2. Reverse characteristic: This curve can be obtained if SCR is reverse biased. It causes breakdown of junctions and again the SCR is turned ON. But VR >>> VF, the method is however not safe and not used to start the SCR. (d) Salient Features of SCR 1. (a) An SCR can be switched on in forward bias as well as in reverse bias conditions. But the value of Breakover voltage (VBO) is very high. The gate circuit may be kept open. (b) If the gate as well as the device both are forward biased, the device can be switched ON at a very low supply voltage. The second method (b) is usually adopted. 2. Once the SCR is switched ON, it reaches into a highly conducting state and the gate has no control on the working of the device therefore, only way to OFF the device is to reduce the supply voltage. 3. An SCR has only two states-either it is ON or it is OFF. There is no in between state. It makes it most suitable for use as a high quality “latch” (switch). 4. Biasing the gate is technically known as “firing” or “triggering.” (e) SCR Terms The important terms related to SCR are defined below : 1. Forward Break Over Voltage (VBO): This is the voltage above which SCR enters the conduction region. Its value depends upon the gate current, and goes on reducing with increase of the later. The typical value is from 50 V to 2500 V.

74

Chapter 4

Electronic Devices

2. Holding Current (IH): This is the value of current, below which the SCR switches from conduction state to forward blocking (off) region. Its value is in few mA. The corresponding voltage is called holding voltage (VH), the typical value being 1V. 3. Forward and reverse blocking regions: These are the regions corresponding to the open circuit in forward and reverse conditions for SCR, which blocks the flow of current from anode to cathode. 4. Reverse breakdown voltage: This is the reverse voltage which causes breakdown of the SCR. This is also called avalanche breakdown voltage. The typical value of reverse breakdown voltage for an SCR is in few kilovolts. 5. Forward current rating: (IF) This is the maximum anode current, an SCR is capable of handling without destruction. The typical value is 30 A to 2400 A. 6. Peak inverse voltage (PIV): This is the maximum reverse voltage, an SCR can handle without conducting in the reverse direction. A typical value is 2.5 kV. (f) Application of SCR The SCR has many applications, few are described below: 1. Single phase half-wave controlled rectifier. An SCR can act as a half-wave rectifier similar to a diode with the difference that it can give controlled rectification ; in other words, the voltage to the load can be varied by changing the firing angle of the device. The Fig 4.54 shows an SCR Half-Wave rectifier circuit. The A.C. voltage to be rectified is stepped down by a transformer (T) and is given to the SCR as shown. RL T

VF

OUTPUT

A

V m

G K

Fig. 4.54

r lg VG

4.9 Thyristors

75

Suppose gate circuit is open (Fig. 4.54) then if the A.C. peak voltage (Vm) is less than the forward break over voltage (VBO), the SCR will not conduct during any part of the positive cycle. During negative cycle also, the SCR will not conduct (We have to select an SCR with reverse break over voltage more than the applied A.C. peak voltage). If the gate circuit is closed and gate current (Ig) flows, the SCR will start at a voltage below its break over value. Let us suppose it starts at V1 which is less than peak voltage. As and when the input to the SCR from the transformer reaches the value V1, (Fig. 4.55) the SCR will start conducting. Once it starts, gate current may be removed and it will remain ON till the voltage across it reduces to zero. During the negative cycle it will become OFF. In the next positive cycle, it will again become ON when the voltage reaches V1. Figure 4.55(a) shows input to SCR and Fig. (b) Show the output of SCR. From O to a°, the SCR is OFF, from a° to 180° it is ON in each positive cycle, alpha (a) is known as its ‘firing angle’ and (180 – a) is known as its ‘conduction angle.’ We can choose any firing angle between 0° and 180° by changing Ig and and can get a variable output to the load. V (a) V m

AC input

(b)

Vm V1 O

V1 SCR off

a 180 Firing angle

SCR on 360

720

DC output across RL

Fig. 4.55



2. Single phase full-wave controlled rectifier Figure 4.56 shows a centre-tap full wave (F.W.) rectifier circuit. This is similar to the centre tap (C.T.) ordinary diode rectifier circuit with the difference that the diodes have been replaced by SCRs. Both the SCRs are given gate current through different sources.

76

Chapter 4

Electronic Devices

For positive cycle of the A.C. supply, the SCR-1 is forward biased and conducts and for the negative cycle, the SCR-2 is forward biased and conducts. The output across the load is variable. Fig. 4.57 (a) shows input and Fig. (b) shows output of the circuit. The firing angle a can be selected depending upon requirement by changing gate current (Ig). V

AC input

Vm

r VG1 V

SCR-1 VF

RL

Vm

V1 0

3.

DC output across RL

Vm

Output

SCR-2

(a)

lg

VG2

r

a

Firing angle

Fig. 4.56

180 a

360

SCR-1 SCR-2 conducts conducts(SCR-1 off) (SCR-2 off) (b)

Fig. 4.57

Speed control of D.C. motors: As explained already, two SCRs can provide F.W. controlled rectified D.C. output. This circuit can be used to control speed of DC motors. Figures 4.58 shows the necessary circuit for speed control of a D C. motor. The diodes D1, D2, D3 and D4 form a bridge rectifier circuit which does full-wave (uncontrolled) rectification of the A.C. supply. The output of the bridge rectifier circuit is given to the field (f) of the D.C. motor to be controlled. SCR-1 and SCR2 control or regulate this rectified output. The gate current to the SCRs is provided through variable resistor r. D4

D1

SCR-1

r1 f

D2

A D3

Fig. 4.58

Ig1

r

Ig2

r

SCR-2

4.9 Thyristors

77

The SCR-1 conducts during positive cycle of the supply, whereas SCR-2 remains OFF. During negative cycle SCR-2 conducts and SCR-1 remains OFF; This provides a controlled D.C. input to the armature (A) of the motor. Required controlled input to the armature can be had by controlling the gate current and firing the SCRs at the required angle; in this way speed of the armature can be increased or decreased (controlled) as required. The method of control by SCRs is superior to the conventional method of changing the field excitation, as in the later method, the armature torque is also changed,

Note: An SCR cannot control AC motors (i.e. ac supplier).

4. Automatic Battery charger: Figure 4.59 shows a battery charger. The diodes D1 and D2 form center tapped F.W. rectifier circuit and provides input to the battery to be charged. When the F.W. rectified input is sufficiently large to produce the required gate current, SCR-1 will turn ON and the battery charging starts. A zener diode (z) is put in the circuit for maintaining SCR-2 in OFF state during this period. As the charging is continued, at a point the battery voltage reaches a value which turns ON the SCR-2 (as well as the Zener diode). Now the SCR-1 will turn OFF. When the battery is fully charged, the charging current will be cut off automatically. Thus the circuit recharges the battery whenever the voltage drops and as well as prevents overcharging of the battery. D1 SCR1

R

D2

Battery

230V AC SCR2 Z

Fig. 4.59

5. Emergency light: Figure 4.60 shows an emergency light which will maintain the charge on the battery and also provide D.C. energy to the lamp (L) in emergency. The diode D1 and D2 form an F.W. rectifier circuit, the output of this circuit appears across the lamp. The battery is being charged through diode D3 at a rate determined by resistance (R).

78

Chapter 4 D3

Electronic Devices

R SCR

D1 C

230V AC

Battery

L D2

Fig. 4.60

6. Inverter: Inverters are D.C. to A.C. converters. Figure 4.61 shows an inverter circuit using SCRs. The input to this circuit will be D.C., while the output across load will be A.C. The magnitude of the output will depend upon the turn ratio of the transformer (T) and frequency of the output will depend upon the frequency of triggering (gate) circuit for the SCRs. AC output RL

+

T

Choke A1

DC input

A2

SCR1 K1

G1

Triggering CKT

SCR2 G2

K2

–

Fig. 4.61

7. Cyclo converter: Cyclo converters are A.C. to A.C. converters. The output frequency is different from the input frequency, which can also be varied. Cyclo converters are used for low speed A.C. motors. Figure 4.62 (a) shown a single phase cyclo converter with four SCRs. The transformer has a centre tapped secondary. The output of the circuit is shown in Fig. (b). Note that frequency of A.C. output is three times to that of the A.C. input.

4.10 Stepper Motors

79

AC input

SCR-1

SCR-2

V AC input

SCR-3

q SCR-4 RL AC output (a)

AC output

Fig. 4.62

(b)

4.10 STEPPER MOTORS These are digital motors which can be controlled by computers and microprocessors. These motors are also called as ‘Stepping” or “Step” motors, because these rotate through a fixed angular step in response to each input pulse. Their wide demand is due to excessive growth of computer industry. They have an advantage that they can be controlled directly by computers, micro processors and programmable controllers. The stepper motors are ideally suited for precise positioning of a job/tool or precise speed control or both (without feedback) in automation systems. The conventional motors cannot be used for this purpose. The torque developed by these motors ranges from 1 mN-m (in wrist watch application) to 40 Nm (in machine tool application), their power output ranges from 1 watt to 2500 watt. The only moving part in these motors is rotor, which has no winding, no commutator and no brushes, this makes the motor quite robust and more reliable than the conventional motor. 1. Important terms: The terms are described below: (i) Step angle: The angle, through which the shaft of the stepper motor moves for each command pulse is called “step angle”. Smaller the step angle, greater is the number of steps per revolution and higher the resolution or accuracy. The step angle may be from 0.72° to 90° but the most common steps used are 1.8°, 2.5°, 7.5° and 15°. The step angle can be expressed in two ways: N – Nr (i) b= s × 360° Ns × N r 360° (ii) or b= m.Nr

80

Chapter 4

where

Electronic Devices

Ns = stator poles or teeth Nr = rotor poles or teeth m = No. of stator phases.

(ii) Resolution: This is given by the number of steps required to complete one revolution of the rotor shaft.

Number of steps

Resolution = No. of Revolutions (iii) Shaft speed: The speed of a stepper motor in “steps per second” is called its shaft speed. The stepper motor can operate at as high as 20,000 steps per second while remaining in synchronism with the command pulses. If f is the stepping frequency or pulse rate and b is the step angle, the shaft speed is given as b.f n= Revolutions per second (RPS) 360°

or, = Pulse frequency resolution.

If the stepping rate is increased too quickly, the motor loses synchronism and stops. Same thing happens, if the motor is skewing (running at very high speed). In this case, the command pulses are suddenly stopped, instead of being progressively slowed down.

2. Types of Stepper Motors The stepper motors are of the following types : (a) Variable reluctance (VR) stepper motors : (b) Permanent magnet (PM) stepper motors. (c) Hybrid stepper motors. (a) Variable Reluctance (VR) Stepper Motor (Fig. 4.63) It has wound stator poles but the rotor poles are made of a ferromagnetic material. This is called “variable reluctance” motor, because reluctance of the magnetic field (circuit) formed by the rotor and stator poles (teeth) varies with the angular position of the rotor.

Here we shall describe “single stack single phase or full step motor” Which is a widely used VR motor. The stator is made from stack

of steel laminations and has 6 projecting poles each wound with an

exciting coil. There are 3 independent stator circuits and each can be

energized by direct current pulse.

4.10 Stepper Motors

81

The fig. 4.63 (a) shows the stator and a circuit arrangement for supplying current to its coils in proper sequence. The 6 stator coils are connected in 2 coil groups to form 3 separate circuits called “phases”. Each phase has its own independent solid state switch. Diametrically opposite stator coils are connected in series, such that, when one pole (tooth) of the stator becomes N pole, the other becomes S pole. When there is no current in stator coils, the rotor is free to rotate. Energising one or more stator coils causes the rotor to take a “step” forward (or backward) to a position that forms a path of least reluctance with the magnetising stator teeth. The step angle of 3 phase stator, 4 teeth rotor motor is given by b =

360° m.Nr

=

360° 3×4

= 30°

The fig. 4.63 (b) shows position of the motor having 6 stator poles and 4 rotor poles when switch S1 [Fig. 4.63(a)] has been closed for energising phase A, and a magnetic field with its axis along the stator poles of phase A is created. The rotor is therefore attracted to a position of minimum reluctance with diametrically opposite rotor teeth 1 and 3 aligning with stator teeth 1 and 4 respectively. Closing S2 and opening S1 energises phase B, causing rotor teeth 2 and 4 to align with stator teeth 3 and 6 respectively. The rotor rotates through a step of 30° clockwise. See fig. 4.63 (c) Similarly when S3 is closed after opening S2, phase C is energised which causes rotor teeth 1 and 3 to line up with stator teeth 2 and 5 respectively and the rotor rotates through next 30° clockwise (total q = 60°) [See Fig. 4.63 (d)] Now, if S3 is opened and S1 is closed again, the rotor teeth 2 and 4 will align with stator teeth 1 and 4 respectively, therefore the rotor moves further 30° (Fig. d), thus the total angle is (q = 90°) A

A′

S1

B

B′

S2

C

C′

S3

+ – (a)

Fig. 4.63 (a)

Solid state switch

82

Chapter 4

Electronic Devices q=3

q = 0° Rotor B Rotor poles (teeth)

0°

Winding A 1 6

1

4

3

C 5

2

B

C′

A 1 6 4

2 3

4 A′

Stator projected poles (teeth) Stator

B′

C 5

q=

A 1 6

4 3 2

C 5

3

2

C′

2 3

4 A′

B′

(c)

(b)

B

1

60

°

C′ 1

4 A′

B

2 3

6

C 5

B′

q = 90°

A 1

(d)

4 3 21 4 A′

C′ 2 3

B′

(e)

Fig. 4.63

So as each switch is closed and the preceeding switch opened, each time the rotor moves through 30°. By repeatedly closing the switches in sequence of 1-2-3-1 and thus energising stator phases in sequence A-B-C-A, the rotor moves clockwise in 30° steps. This is most widely used mode for variable reactance motor operation. The stator phase switching “Truth Table” is shown below. The + sign shows that the phase A, B or C is energised with positive currents A

B

C

q

+

0

0

0°

0

+

0

30°

0

0

+

60°

+

0

0

90°

If the operation of switches is made in sequence 3-2-1-3, which will make phase sequence C-A-B-C or (A-C-B-A), the rotor will rotate anticlockwise.

4.10 Stepper Motors

83

Note that direction of the stator magnetising current has no significance as a stator pole of either magnetic polarity will always attract the rotor pole by inducing opposite polarity. (b) Permanent Magnet (PM) Stepper Motor Its stator construction is similar to a single stack variable reluctance (VR) motor described above. It also has wound stator poles, but its rotor is made of a permanent magnetic material like hard ferrites. While the stator has projecting poles, the rotor is cylindrical in shape. The fig. 4.64(a) shows the PM stepper motor with stator having four poles and the rotor having two poles. Since two stator poles have been energised by one winding, the motor has two windings AA′ and BB′ (or two “phases” marked A and B as they are called.) The step angle of this motor. 360° 360° b = = = 90° m × Nr 2 × 2 m = No. of phases Nr = No. of rotor poles or teeth.

where

The direction of rotation depends on the polarity of the stator currents. When a particular stator phase is energised, the rotor poles move into alignment with the stator poles. The stator windings (or phases) A and B can be excited with either positive or negative currents, + IA represents a positive current and – IA represents a negative current. The fig. 4.64 (a) also shows that when the phase A is excited with positive current (+ IA), q = 0°. If the excitation is now switched to phase B (see Fig. b), the rotor rotates by a full (step) angle of 90° (q = 90°) in clockwise direction. +IA

A

A

q = 0° Rotor Stator

q = 90°

S

S

S N

B

N

A′

+IB B′

N

S

B′

B

N

A′

(a)

Fig. 4.64

N

(b)

84

Chapter 4

Electronic Devices

A

A N

S

q = 270°

N

B′

B

S

N B′

B

S –IA A′ q = 180° (c)

A′

(d)

Fig. 4.64

The fig. 4.64 (c) shows when the phase A is excited with negative current (–IA), the rotor turns through another 90° (total angle, q = 180°). Similarly, excitation of phase B with negative current (–IB), further turns the rotor through another 90° (in clockwise) as shown in Fig. 4.64 (d). (q = 270°) After this, the excitation of phase A with +IA makes the rotor run through one complete revolution of 360°. (q = 360°) The Truth Table below, shows the operation of PM stepper motor. It shows current sequence for providing clockwise motion when only one phase is energised at a time in “1 phase ON mode” giving step angle of 90° as described above. A

B

q

+

0

o

0

+

90°

-

0

180°

0

-

270°

+

0

360° = 0°

The phase A or B can be energised by positive (+) as well by negative (–) currents. (c) Hybrid Motor The hybrid stepper motor combines the features of VR and PM stepper motors. It has wound stator similar to single stack VR motor but the rotor is a cylindrical like PM motor. The direction of the torque depends on the polarity of the stator currents. The Fig. 4.65 shows a typical hybrid stepper motor. The stator consists of 4 poles, which are excited by two windings in pairs. The cylindrical

4.10 Stepper Motors

85

rotor has five N-poles on one side (Fig. 4.61 a) and five S-poles at the other side (Fig. 4.65 b). The step angle of this motor 360° b = m × Nr Where

m = No. of stator poles or phases

Nr = No. of rotor poles

360°

b = = 18° 4×5 These motors are suited when small step angles of 1.8°, 2.5° etc. are required. Stator

(+) A S N (–)

N

S B′

N

N N

Rotor pole

S

N

B

Rotor

A

Stator pole

S

S B′

B S

Pole winding

S N A′

A′

(b)

(a)

Fig. 4.65

In fig. 4.65 (a), the phase A is energised positively such that the top stator pole becomes south pole and it attracts north pole of the rotor and brings it in line with axis A-A’. To turn the rotor, the phase A is de-energised and phase B is energised positively. The rotor turns in anticlockwise by a step of 18°. Now phases A and B are energised negatively, one after the other to produce further movement of 18° in the same (anticlockwise) direction. The Truth Table for the motor is given below: A + 0 – 0 +

B 0 + 0 – 0

q 0° 18° 36° 54° 72°

For producing clockwise motion, the phase sequence would be A+, B–, A–, B+, A+ etc.

86

Chapter 4

Electronic Devices

In practice, hybrid motors are made with more rotor poles so that higher angular resolution may be obtained. The stator poles are also “slotted” to increase number of stator poles (teeth). By having more than two stacks on the rotor, the step angle can be decreased up to 1.8° or so, this also improves resolution. Merits of hybrid motors Vis-a-Vis VR and PM motors (i) The hybrid motor gives small steps (1.8°) with a simple magnetic structure whereas a P.M. motor requires a multiple (complex) structure. (ii) The hybrid motors require less excitation to give the same torque as compared to a VR motor. (iii) The hybrid motor produces a torque similar to PM motor, moreover, if the power is switched OFF, this torque holds the rotor stationary, and there is no risk of motor itself being shifted to the new position.

• Applications of stepper motors (i) For operation control in computer peripherals, textile industry, I.C. fabrication, and robotics. (ii) In typewriters, line printers, tape drives, floppy disk drives, CNC (Computerised Numerically Controlled) machines, X-Y plotters etc. (iii) For process like mixing, cutting, striking, metering, blending etc. (iv) In packed food stuffs. (v) For production of science fiction movies. (vi) As output device for microprocessor controlled systems such as “paper drive” for printers and x-y plotters. (vii) In servo mechanism to position machine tools. (viii) As a stepper motor is given a digital signal (0, 1), so the motor can also work as digital to analog converter (DAC) (ix) For precise speed control in automation system.

4.11 SERVOMOTOR

These motors respond to an error signal obtained by comparing input and output signals. The principle is called as “servomechanism”. (Fig. 4.66) Input

Error signal

Amplifier

Output

Feedback

Fig. 4.66

Basically, a servomotor is a rotary or linear actuator. It consists of a suitable motor coupled to a sensor for precise control.

4.11 Servomotor

87

1. Types of Servomotors The servomotors may be: (i) D.C. Servomotors (ii) A.C. Servomotors Both are available from a fractional H.P. to 1200 HP (i) D.C. Servomotors (Fig. 4.67): In these motors, the armature current is kept constant and the field is excited by a D.C. (directly coupled) amplifier; which amplifies the error signal.

Error signal

A

Field

+

Ra

D.C. Servomotor

–

D.C. Amplifier

Fig. 4.67

The torque of the motor is proportional to the field current. The motor can be reversed by reversing the field polarity. (ii) A.C. Servomotors (Fig. 4.68): These are generally two phase motors. The stator has two windings placed at quadrature to each other. The first winding (I) is excited by a.c. supply and the second winding (II) is fed by the amplified error signal. The rotor of the motor is usually a squirrel cage type. Rotor

Error signal

A

AC Servomotor

II

Amp I

Stator windings displaced at quadrature, i.e., at 90°

~ a.c. supply

Fig. 4.68

88

Chapter 4

Electronic Devices

2. Applications of Servomotors The servomotors are used for precision position control in industries and in robotic applications.

Numerical Problems Problem 4.1: Determine the following for the rectifier circuit shown. (Fig. 4.69) (i) Im, Id.c.and Ir.m.s (ii) a.c. output power (iii) Rectifier efficiency rL = 20Ω 230 V

RL = 1KΩ 23V

Fig. 4.69

Sol.

Here, \

(i)

Vr.m.s. = 23 V Vm = Im = Id.c. = Ir.m.s. =

2 Vr.m.s. = Vm RL + rL Im p Im 2

= =

=

2 × 23 = 32.53 V 32.53

1000 + 20

31.89 p 31.89 2

= 31.89 mA (1 KΩ = 1000Ω)

= 10.15 mA = 15.94 mA

(ii) d.c. output power

Pd.c. = I 2d.c. × RL = (10.15)2 × (1000 × 10–6)

= 103.02 mW (iii) Rectifier efficiency, Now Pa.c. = I 2r.m.s. (RL + rL) = (15.34 × 10–3)2 ×(1000 + 20) = 256.16 mW \ Rectifier efficiency, P 103.02 h = d.c. = × 100% = 39.75% Ans. Pa.c. 256.16

4.11 Servomotor

89

Problem 4.2: In Fig. 4.70 maximum secondary voltage is 136 V. Find the following: (a) V d.c. (b) Peak inverse voltage (c) Rectifier efficiency. rL = 20Ω 136 V

220 V 50 Hz

RL = 1K

Fig. 4.70

Sol.

Vm = 136V rL = 20 Ω RL = 1K Ω Vd.c. =

(a)

2Vm p

=

2 × 136 3.14

= 81.6V

(b) Peak inverse voltage (P.I.V.) = Vm = 136 V (c) Efficiency,

h=

81.6 81.6 81.6 ×1000 %= = 2r 40 1040 1+ 1+ L 1000 RL

= 78.25% Ans. Problem 4.3: (Fig. 4.71) Calculate the value of R. The maximum value of forward current of a diode is 100 mA.

R

100 mA

1. When the diode is of Ge.

1.5 V

2. When it is of Si.

+ –

Sol. 1. The Ge diode can bear a voltage of 0.3 V across it, Voltage across R

Fig. 4.71

= 1.5 – 0.3 = 1.2V \

R=

1.2V If

=

1.2V 100 mA

= 12Ω Ans.

[If = 100 mA]

90

Chapter 4

Electronic Devices

2. If the diode is of silicon, 1.5 – 0.7V R= = 8Ω Ans. 100 mA Problem 4.4: (Fig. 4.72) Find maximum voltage across AB in the circuit shown. Assume the diode as ideal.

A 5K A

30 V

Sol. The resistance 5K is in series with 10 K and this combination is in parallel with 15 K. Hence the total circuit resistance:

10 K

15 K

B

= (10 + 5) || 15K

Fig. 4.72

= 15 || 15K = 7.5K Hence, voltage across AB will be equal to the voltage drop 30V across the diode, i.e., = (15) = 50V Ans. 7.5K Problem 4.5: The input A.C. power to a half-wave rectifier is 140 W and D.C. power output obtained is 60 W. Calculate the efficiency of rectification. D.C. output power Sol. Efficiency = A.C. input power 60 h= = 42.8% Ans. 140 Problem 4.6: For the circuit (Fig. 4.73), find the output voltage, voltage drop across series resistance and current through zener diode. 10kΩ R 120 V

48 V

15 K

Fig. 4.73

Sol.

(i)

Output voltage = Vz = 48V Ans.

(ii) Voltage across series resistance R = 120 – 48 = 72V Ans. 72 (iii) Current through series resistance = = 7.2mA Ans. 10 × 103 Problem 4.7: In a transistor, a = 0.98, the emitter current IE is 2mA. Calculate the values of collector current and base current. Sol.

Given a = 0.98 and IE = 2 mA

4.11 Servomotor

91

We know that a =

IC IE

\

IC = a × IE = 0.98 × 2 = 1.96 mA Ans.

Now,

IE = IC + IB

\

IB = IE – IC = 2 – 1.96 = 0.04 mA Ans.

Problem 4.8: In a NPN transistor, a = 0.995, IE = 10 mA and ICBO = ICO = 0.5 mA. Find the values of IC, IB, b and ICEO. Sol.

IC = a IE + ICO IC = 0.995 × 10 + (0.5 × 10–3) mA = 9.9505 mA

\ and Further \ and

IB = IE – IC = 10 – 9.9505 = 0.0945 mA = 49.5 mA a b= 1–a 0.995 0.995 b= = = 199 1 – 0.995 0.005 I 0.5 × 10–3 ICEO = CO = mA = 100 mA Ans. 1 – a 1 – 0.995

Problem 4.9: (Fig. 4.74) The transistor has a = 0.98 and a base current of 30 mA. Find the values of: (a) b;

IC

30mA

VCE

(b) Zero signal base current; (c) Zero signal emitter current. Sol.

(a)

a = 0.98 a b= 1 –a 0.98 = = 49 Ans. 1 – 0.98

(b)

IB = 30 mA (Fig. 4.74) Ans.

(c)

IC = bIB

IE

Fig. 4.74

\

IC = 49 × 30 × 10–6 = 1.47 mA

\

IE = IB + IC = (30 × 10–3) + 1.47 = 1.5 mA Ans.

Problem 4.10: The CB connection has the value of a = 0.97. A voltage drop of 5V is obtained across a resistor of 5K in collector circuit. Calculate the currents IC, IE and IB.

92

Chapter 4

Sol.

IC = 5.0V/5K = 1 mA Ans.

(a) (b)

Electronic Devices

As

(c)

a = IC/IE

I 1 mA

= 1.03 mA Ans. IE = C = a 0.97 IB = IE – IC = 1.03 – 1 mA = 0.04 mA Ans.

Problem 4.11: Find the value of b, if a = 0.97 a b= Sol. 1–a 0.97 b= = 32.33 Ans. 1 – 0.97 Problem 4.12: Find IE, if b = 50 and IB = 30 mA. Sol.

IC = bIB = 50 × 30 = 1500 mA. IE = IB + IC = 30 + 1500 = 1530 mA. Ans.

Problem 4.13: In amplifier, DVCE = 2.2V and DVBE = 20 mV. Calculate voltage gain. Sol. V.G. = DVCE /DVBE =

2.2

20 × 10–3

= 110 Ans.

Problem 4.14: In an SCR half wave rectifier, what peak load current will occur at a firing angle of 30°, if we measure an average (dc) current of 1 amp. Sol. Let Im is the peak load current. Vm Iav = (1 + cos a) 2p.RL Iav = Im =

Im 2p

(1 + cos a)

Vm   Im = R  L 

2p.Iav 1 + cos a 2p × 1

= 3.36 Amp. Ans. 1 + cos 30° Problem 4.15: An SCR full wave rectifier supplies to a load of 100 Ω. If peak ac voltage between center tap and one end of secondary is 200 V. Find for a firing angle 60°. =

(a) dc output voltage (b) Load current

Summary

93

Sol. (a) D.C. output voltage Vdc =

Vm

(1 + cos a) p 200 = (1 + cos 60°) = 95.5 V Ans. p (b) Load current (D.C.)

V 95.5

= 0.955 Amp. Ans. Iav = av = RL 100

SUMMARY 1. By keeping P and N type materials together we get PN junction, known as semiconductor diode commercially. 2. PN Junction diode works only when forward biased. Due to this property this is used as rectifier. 3. We have (1)

Half wave rectifiers.

(2)

Full wave rectifiers. They are sub classified as: (i) Centre tap rectifier (ii) Bridge rectifier.

4. The filter circuits are used to get pure d.c. 5. The Zener diode is a special diode used for voltage stabilization. 6. A bipolar transistor is a three terminal device. It has 2 Junctions and 3 semiconductor layers. 7. A bipolar transistor may be of NPN and PNP type. It works only when its input Junction is forward biased and output Junction is reverse biased. The conduction takes place by electrons as well as by holes. 8. Transistor can be connected in a circuit in 3 ways. i.e. keeping base, emitter or collector, one of the three as common. 9. The current gain in common base is a = 0.98, in common emitter, it is b = 500 and in common collector it is g = 450. 10. The common emitter is most important configuration used for amplification. The common base and common collector configurations are used for impedence matching. 11. The output characteristic of common emitter configuration has 3 regions: active, staturation and cut off.

94

Chapter 4

Electronic Devices

12. The three gains of a transistor are current gain, voltage gain and power gain. The last is the product of the first two. 13. Thyristor is a family of multilayer devices. An SCR is its most important member. 14. An SCR (Silicon controlled rectifier) is made of silicon and gives controlled rectification. 15. An SCR is used for speed control of d.c. motors, for making inverters etc. 16. A stepper motor moves in steps. 17. The stepper motor is used as a drive for printers etc. 18. A servomotor works on the principle of servomechanism. qqq

5 Analysis of Bipolar Transistors at Low Frequencies (h-parameters) Usually, a transistor amplifier is analysed with the help of b and other parameters. Though this method is simple, but very accurate results are not obtained. The reason is that for the analysis, the input and output circuits of an amplifier are considered to be completely independent but, in practice, it is not so. Therefore, for analysing behaviour of amplifiers, hybrid method is used which gives the most accurate results at low frequencies. Quite often the manufacturers also specify characteristics of transistors in hybrid or h-parameters, as they can be measured easily. They are called hybrid parameters as they are mixture of constants having different units and dimensions.

5.1 HYBRID PARAMETERS (h parameters) The parameters, which have mixed (i.e., different) units and dimensions are called hybrid or h-parameters. Any linear circuit with input and output terminals, can be analysed accurately using h-parameters. These parameters are four in number—one is measured in ohms, second in mho and the remaining two are dimensionless. Advantages of Using h-parameters: • They give accurate results as at low frequencies the interaction of input and output circuits of the amplifier have been taken into account. • These parameters can be measured easily.

5.2 TWO-PORT NETWORK A transistor is a three-terminal (Emitter E, Base B, Collector C) device. In all the three configurations, one of the three terminals is kept common to input and output circuits (say emitter in Fig. 5.1), so there are two-ports (pair of terminals) in a transistor circuit. Therefore, it can be considered as a two-port network.

96

Chapter 5 Analysis of Bipolar Transistors at Low Frequencies

1 Input Port (+) (–)

i1

B

C

v1

i2

2

v2

1

2

E

Output (+) Port (–)

Transistor as Port

Fig. 5.1

The circuit has input voltage v1 and input current i1; in the same way, output voltage is v2 and output current i2. The upper end has been considered positive and the low end negative as shown. The reader should note that these are just the standard conventions and may not correspond to the actual direction. The voltage and current of the above port can be related by the following equations: v1 = h11 i1 + h12v2

...(i)

i2 = h21 i1 + h22v2

...(ii)

These relations can be proved by advance circuit theory, which is beyond the scope of this book. Here h11, h21, h12 and h22 are constants and are known as hybrid parameters. These parameters relate four variables v1, v2, i1 and i2 by the above equations.

Determination of h-parameters Proceed as follows: 1. Short Circuit Output Terminals (Fig. 5.2) Now the output voltage v2 = 0 (as the output is shorted), putting the value in Eq. (i)

(a)

we get, v1 = h11.i1 + h12.0 v1 h11 = i1

...(iii)

h11 is called input impedance. (+)

1

i1

Port

i2

2

v1 (–)

(+) v2 = 0

1

2

Fig. 5.2

(–)

5.2 Two-Port Network

(b)

97

Putting v2 = 0 in Eq. (ii) i2 = h21.i1 + h22.0

...(iv)

h21 = i2/i1 h21 is called current gain or forward current ratio. 2. Open the input terminals (Fig. 5.3) (a)

This will reduce input current i1 = 0 Putting i1 = 0 in Eq. (i) v1 = h11.0 + h12.v2

...(v)

h12 = v1/v2 h12 is called reverse voltage ratio or feedback voltage ratio. (+)

1

Port

2

(+)

V2

Open (–)

i2

1

2

(–)

Fig. 5.3

(b)

Putting i1 = 0 in Eq. (ii) i2 = h21.0 + h22.v2

...(vi)

h22 = i1/v2 h22 is called output admittance (reverse of resistance) Now we can define the various h parameters as below: v 1. h11 = 1 v2 = 0: Called Input Impedence and is the ratio i1 of input voltage and input current, when output is short circuited (unit is ohms). v 2. h12 = 1 i1 = 0: Called reverse transfer voltage ratio and v2 is the ratio of input voltage and output voltage with zero input current (no units). i 3. h21 = 2 r2 = 0: Called Forward transfer current gain and i1

is the ratio of output and input currents with short circuited output (no units).

98

Chapter 5 Analysis of Bipolar Transistors at Low Frequencies

i 4. h22 = 2 i1 = 0 : Called output admittance and is the ratio v2 of output current and output voltage at zero input current (unit is siemens) Table 5.1

Typical Values for h-parameters in three configuration

S. No. 1.

Parameter

CE

hi (input impedance) = h11

CB

CC

21.5 Ω

1.0 K –4

1.0 K

3.0 × 10

–4

2.

hr (reverse voltage ratio) = h12 2.5 × 10

1

3.

hf (forward current ratio) = h21

50

–0.98

–50

4.

ho (output admittance) = h22

25 micro siemen or mho

0.5 micro siemen or mho

25 micro siemen or mho

5.3 HYBRID (h) MODELS Figures 5.4 shows the circuit arrangements, hybrid models and v-i equations

for the three configurations for an N-P-N transistor.

The circuits and equations shown are valid either for N-P-N or P-N-P transistors

and independent of the type of load or method of biasing.

S. No. 1.

Configuration

Circuit

Common

C

Emitter

B vb

– 2.

vc

ib

E

E

Common

ie

vb

hieib

E

ib

E + –

vc hoe E

hic B

+ vb hrcve –

ib

– ic C

+

C

vb = hieib + hrevc ic = hfeib + hoevc

–

–

ve

+

ic

hie

+ vb hrevc – –

ie

Collector

–

ib

+ + B

ic

+

v-i Equations

Hybrid Model

+

Fig. 5.4

C

ie +

E hfcib

ve hoc C

–

vb = hicib + hrcve ie = hfcib + hocve

5.3 Hybrid (h) Models 3.

99

Common Base

ie

+

C ic

ve

ib

ie

++

B

+

C

+ ve hrbvc –

vc

B

ic

E

– –

hib

hfbic vc hcb

B

B

vc = hibie + hrbvc ic = hfbic + hobvc

–

–

Fig. 5.4

• CE Transistor h-model Figures 5.5 (a) shows the hybrid model of a transistor in CE configurations. This model is most popularly used for analysis of a transistor at low frequencies. Figure 5.5 (b) shows the non-hybrid model for the same for comparison. Comparing the Fig. (a) and Fig. (b), we see that: hie = ri : the dynamic input resistance hfe = b: the current amplification factor hoe = r0: the dynamic output resistance B ib

hie(h11)

ic

hrevc

vb

hfe ib

hoe

E

C

vc

E (a)

B

vb

ib

ri

b.ib

E

ic

C

ro

vc

E (b)

Fig. 5.5

100

Chapter 5 Analysis of Bipolar Transistors at Low Frequencies

5.4 ANALYSIS OF CE AMPLIFIER BY h PARAMETERS The Fig. 5.6 shows the transistor port in CE configuration.

(+) (–)

1

i1

B

i2 2

C

v1

Port

1

E

v2

(+)

rL 2

(–)

Fig. 5.6

We shall find (a) Current gain (b) Voltage gain (c) Power gain

(a) Current Gain (Ai) Current gain is the ratio of the output current to the input current. Ai = i2/i1 i2 = h21.i1 + h22.v2 (in terms of h-parameters)

Now, or

= h21i + h22(–i2rL) (v2 = –i2rL)

or

= h21i1 – h22.rL.i2

or

i2(1 + h22 × rL) = h21.i1 i2

or or

i1

=

C.G., Ai =

h21 1 + h22 × rL −h fe ⎛ i ⎞ Ai = 2 ⎟ ⎜ i1 ⎠ 1+ hoe .rL ⎝

If hoe.rL > fb, the graph is a straight line having a slope of 6dB/octave or 20 dB/decade.

6.6 C.E. CURRENT GAIN WITH HYBRID Pi MODEL WITH A RESISTIVE LOAD The Fig. 6.7 shows simplified hybrid pi model with resistive load RL. B

Iin

rbb′

C IL Vb′e

Z

E

gm.Vb′e

RL

E

Fig. 6.7

6.7 Relations Between Hybrid (h) and hybrid (Pi) Parameters

113

The impedance Z is the parallel combination of rb′e and Xc. [where XC is reactance due to capacitance C(= Cc + Ce), not shown] Z =

\

rb′e.Xc rb′e + Xc

Xc = 1 + jwC

Keeping and solving, we get:

rb′e Z = 1 + jw rb′e.C

...(i)

The current gain (C.G.) is given by Output current Input current

=

IL Iin

=

–gm.Vb′e Vb′e/Z

= –gm.Z

...(ii)

Putting value of Z from eq. (i) C.G. =

–gm.rb′e 1 + jw rb′e.C

gm.rb′e = hfe

Now putting

C.G. =

–hfe 1 + jw rb′e.C

6.7 RELATIONS BETWEEN HYBRID (h) AND HYBRID (Pi) PARAMETERS These are related as following 1.

rb′e =

hfe gm

2. rbb′ = hie – rb′e

3.

rb′c =

4.

gce =

5.

fT =

rb′e

hre 1 rce gm

2p(Ce + Cc)

114

Chapter 6 Analysis of Bipolar Transistors at High Frequencies

6.8 GAIN-BANDWIDTH PRODUCT For any amplifier, when the bandwidth is multiplied by gain at mid frequencies, it is known as gain-bandwidth product. For any amplifier, gain bandwidth product is constant. For example If b = 1 at a frequency of 6 MHz, then gain band-width product is 6 MHz.

NUMERICAL PROBLEMS Problem 6.1: A transistor has the following h parameters at Ic = 10 mA. hie = 1K, hre = 2 × 10–4; hoe = 4 × 10–5 A/V Calculate all resistive components of the hybrid pi model. Solution: gm =

(i) \

10

= 0.384 Q 26 100

rb′e = = 258.5 Ω Ans.

0.384 rbb′ = hie – rb′e = 1000 – 258.5 = 741.5 Ω Ans.

(ii)

[hie = 1 K = 1000 Ω] rb′e 258.5 = = 1.29 MΩ Ans. hre 2 × 10−4 100 gce = hoe – hfe.gb′c = 4 × 10–5 – 1.29 × 106 [1.29 MΩ = 1.29 × 106Ω]

rb′c =

(iii) (iv)

= 3.98 × 10–5 Q rce =

(v)

1 1 = = 25116.8 Ω Ans. gce 3.98 × 10−5

Problem 6.2: A transistor has hie = 1 K, hfe = 100, short circuit current gain = 10 at frequency 10 MHz. Calculate beta and alpha cut off frequencies. Solution: (i)

C.G. =

Putting value, we get

h fe 2

⎛ f ⎞

1+ ⎜ ⎟ ⎝ fβ ⎠

Numerical Problems

115

10 =

100 6 ⎞2

⎛ 10 1+ ⎜10 × fβ ⎟⎠ ⎝

(f = 10 MHz = 10 × 106 Hz)

Solving,

Beta cut off frequency, fb = 1 MHz Ans.

(ii) Alpha cut off frequency:

fa = (1 + hfe).fb

= (1 + 100) × (1 × 106) = 101 MHz Ans. Problem 6.3: The short circuit C.E. gain of a transistor is 20 at 2 MHz. The beta cut off frequency = 150 KHz. Calculate hfe and C.G. at 5 MHz. Solution: (i)

fT = C.G. × f = 20 × (2 × 106) = 40 MHz

= 40 × 106 Hz

fT 40 ×106 ⎡ f = h . f ⎤ fe β ⎦ hfe = f = ⎣ T β 150 ×103 = 266.67 Ans.

(ii)

C.G. =

h fe ⎛ f ⎞ 1+ ⎜ ⎟ ⎝ fβ ⎠

2

=

266.67 ⎛ 5 × 106 ⎞ 1+ ⎜ 3⎟ ⎝ 150 ×10 ⎠

2

(f = 5 MHz = 5 × 106 Hz) = 8 Ans Problem 6.4: A h.f. emitter follower uses a transistor having, gm = 80 m siemens and a load RL with CC = 30 pf. Calculate (i) The 3 dB frequency (fH) (ii) Time constant (t0) for the output circuit for h.f. hybrid Pi equivalent circuit (iii) If Ce = 120 pF, find fT (iv) Voltage gain at 250 MHz

116

Chapter 6 Analysis of Bipolar Transistors at High Frequencies

Solution: fH =

(i)

gm 80 ×10−3 = 2πCC 2π × 30 × 10−12

= 424.4 MHz Ans. (ii)

t0 = = fT =

(iii)

1 2pf H 1 2p × (424.4 × 106)

= 0.375 nano seconds Ans.

f H .Cc 424.4 × 106 × 30 × 10−12 = Ce 120 ×10−12

= 106 MHz Ans. Av =

(iv)

=

1 1 + ( f / fH )

2

1 1 + ( 250 / 424.4 )

2

= 0.86 Ans.

SUMMARY 1. Behaviour of a bipolar transistor at H.F. is peculiar due to interelectrode capacitances. 2. Analysis of the transistor at H.F. cannot be carried out by its hybrid model, instead its “hybrid Pie model” is used. 3. Miller effect comes into play at high frequencies. 4. Transistor’s cut off frequencies at HF are (i) Alpha cut off frequency (fa) = 0.707 of its low frequency value. (ii) Beta cut off frequency (fb) = 0.707 of its low frequency value. qqq

7 Microphones & Loudspeakers A microphone converts sound energy into electrical energy and loudspeaker converts electrical energy back into sound. Both are essential components of almost all audio video systems.

7.1 MICROPHONE A microphone is a device that converts sound into electricity.

1. Characteristics of Microphones The important characteristics of microphones are described below. 1. Sensitivity. Sensitivity of a microphone is its ability to pick up the weakest sound. This can be measured by the voltage obtained at output of the mike at a given sound pressure. It is usually expressed in milli volts per microbar or in decibels (db) referred to 1 volt. The common sound pressure used for measurement is 94 dB (10 m bar). Hence sensitivity is measured as: (i) Open circuit voltage at 0 dB = 1 volt/m bar. (ii) Max. power output at 0 dB = 1 mW/10 m bar. 2. Signal/Noise Ratio (SNR). This is defined as a ratio in dB output to the output in absence of noise at a sound pressure of 1 m bar It is to be kept in mind that some noise is produced in the mike itself in its transformer and in the resistance of its circuitary. Signal Noise Ratio, Output in presence of a noise S.N.R. = Output when noise is not present e.g. the output of a microphone (at a sound pressure of 1 m bar) is 1000

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Microphones & Loudspeaker

mV. In absence of noise, the output is 100 mV. The S.N.R. of the mike will be = 20 log

) ) 1000 100

= 20 × 1 = 20 dB. 3. Directivity. By directivity, we mean the sensitivity of a microphone towards sounds coming from different directions. The directivity of mikes is described by polar diagrams. The polar diagram shows output of a mike at different angles of incidence of sound on its diaphragm. According to this characteristic, the mikes can be classified as: (a) Uni-directional mikes: Some mikes can pick up sound from one direction only. The person must stand in front of the microphone when speaking e.g. moving coil microphones are unidirectional. The shape of polar diagram of uni-directional mike is like a heart (or cardiod). This is shown in Fig. 7.1 Polar diagram

Mike

Fig. 7.1

In these mikes, diaphragm is left open at the front. Such an arrangement produces a uni-directional response of the mike towards sound. (b)

Bi-directional mikes: These mikes can pick up

sound from two directions i.e. from front as well as

from the back. A Ribbon microphone is an example.

Mike

Whele using these microphones the source of the sound are to be positioned carefully. To obtain bi directional (two directional) response, Fig. 7.2 the diaphragm is kept open on both sides, thus it operates on the principle of pressure gradient on the two surfaces of the diaphragm. Its polar diagram gives shape of Eight (8). See Fig. 7.2

7.1 Microphone

119

(c) Non-directional or Omni directional or all

directional. These mikes can pick up sound

from all directions. In other words they are

without any direction. Crystal microphone is

one example. These mikes pick up sound

from all sides with same intensity. The source

of sound is no concern.

Mike

Fig. 7.3

The polar diagram of these mikes is a simple circle with mike at

its centre. See Fig. 7.3.

These mikes have their diaphragm open at front but completely

closed at back side.

The performance of this mike is quite good at low and at mid

frequency of the sound signal, but at higher frequencies (HF),

it produces a “shadowing effect” and does not pick sound from

sides and back with same intensity. To reduce the shadowing

effect at H.F., the size of the mike may be reduced.

4. Output impedance of microphones. A microphone has a certain output impedance. It is given in ohms. It helps to provide suitable impedance matching between microphone and the transformer (TF) for maximum transfer of power to amplifier and then to the loud speaker (L.S.). (Fig. 7.4). Mike

Amp. L.S. Output T/F

Fig. 7.4

5. Frequency response. How a microphone responds to various audio frequencies will be known by its frequency response. In other words, the output voltage of a mike in response to the different sound frequencies falling on its diaphragm is called its frequency response. Naturally, it will be a graph obtained between output voltage of the mike versus incident frequencies. A mike with flat curve between 50 Hz to 16000 Hz will be considered as Hi-Fi mike. However for normal use, a mike with a bandwidth of 70 Hz to 8000 Hz may be acceptable. Frequency response of few microphones are shown in Fig. 7.5.

120

Chapter 7

Microphones & Loudspeaker

Carbon mike

0

Crystal mike

Output in volts

30

Capacitive mike

60

Moving coil mike

90

Ribbon mike 120

1

5

10

15

20

Frequency (KHz)

Fig. 7.5

6. Distortion. The various distortions produced by microphones are: (i) Amplitude distortion. A mike produces amplitude distortion which results in the production of harmonics, which may not be present in the original sound. This distortion should not be more than 5% for a normal quality mike and not more than 1% for a hi-fi mike. (ii) Phase distortion. The mike produces phase distortion between different components of a sound input. (iii) Frequency distortion. The mike produces frequency distortion between different components of frequency.

2.  Classification of Mikes Each microphone has a “diaphragm” which is a very sensitive part. This intercepts the sound waves and vibrates, thus converting sound into electrical pulses. (i) When sound strikes on only one surface of the diaphragm, the mike is said to be “pressure microphone”. (ii) When sound strikes on both surfaces of the diaphragm, the mike is said to be “pressure gradient microphone”.

7.1 Microphone

121

Mikes can be classified as under: 1. Pressure Microphones. Examples are: (i) Carbon Microphone (ii) Capacitive or Condenser Microphone (iii)

Moving Coil Microphone

(iv) Crystal Microphone 2. Pressure gradient microphones. Example is: (i) Ribbon/velocity microphone. 3. Other microphones are (i) Radio/wireless microphones (ii) Lavalier or mobile microphones (iii)

Noise cancelling microphones

(iv) Electret microphones (v) Tie clip microphone.

(i) Pressure microphones The working of these microphones is based on the sound pressure incident upon the diaphragm (membrane) of the microphone. Important pressure microphones are described below: (a) Carbon Microphone—Construction In this microphone there is a carbon block and also a diaphragm of carbon. The space between the two contains “carbon granules”. When a person speaks before it, the diaphragm begins to vibrate depending upon the sound pressure on it. By the inward and outward movement of the diaphragm, the granules are ‘compressed’ or ‘rarified’ and their resistance changes accordingly. [See Fig. 7.6 (a)] The connection of the microphone with amplifier and speaker are shown in Fig. 7.6 (b). With change of resistance of the granules, primary of the transformer carries a varying current and so the secondary current. The output of transformer is given to amplifier and then to the speaker. Merits (1)

This is cheaper and durable.

(2)

It is mechanically rigid.

(3)

It is very cheap and small in size.

(4)

It is very sensitive and can pick up even weak signals.

122

Chapter 7

Microphones & Loudspeaker

Carbon granules

Carbon diaphragm

Carbon block

(a) Output transformer Mike

+ – Battery

Amp. L.S. (b)

Fig. 7.6

Demerits (1) Due to local heating of granules, it sometimes produces noise (2) It has poor frequency response hence not suitable for hi-fi work. (3)

It is affected by heat and moisture.

Characteristics (i) Sensitivity of carbon mike is high. Its value is about 100 mV. (ii) The signal/noise ratio is poor. The mike creates self noise due to change of resistance of carbon granules. (iii) It has a bandwidth from 150 Hz to 5 KHz, hence not suitable for hi-fi work. (iv) Distortion is about 10% due to variation in resistance and sticking of carbon granules. (v) It is omni-directional i.e. it can pick up signal from all directions. (vi) Output impedance is less (upto 400 Ω). Working of a Carbon Mike When a person speaks before the mike, the diaphragm moves. This changes the resistance of the carbon granules due to the pressure on the diaphragm. This produces a change in the current flowing from battery. The change in the current depends upon the sound pressure

7.1 Microphone

123

incident on the mike. This change of current is amplified by the step up transformer and then by amplifier. When carbon granules release back, their resistance increases and current decreases. Applications (i) Due to rigidness, high output, low cost, carbon mikes are very much used in telephone and radio communication systems. (ii) As mentioned earlier, it is not suitable for hi-fi applications due to more distortion, low bandwidth and more self noise. (b) Capacitive or condenser microphone It basically consists of a (parallel plate) air capacitor of which the diaphragm makes one plate and other plate is a back plate. The dielectric between the two is air. The diaphragm is movable depending upon sound pressure. The back plate is fixed and has grooves. (Fig. 7.7 (a)]. Air gap Back plate Diaphragm

(a) Condenser mike

RL +

Amp. L.S.

– (b)

Fig. 7.7

When sound waves strike its diaphragm, it starts vibrations depending upon the intensity of sound pressure. With the inward and outward motion of the diaphragm, the distance of air gap changes and thus capacity of the parallel plate capacitor also changes accordingly.

124

Chapter 7

Microphones & Loudspeaker

The connections of capacitor microphone is shown in Fig. 7.7 (b). The potential drop (p.d.) across output resistance RL varies with capacity of the microphone. This p.d. is fed to amplifier and then to the speaker. The principle of capacitive microphone is based on the fact that when capacity of a capacitor changes the voltage changes accordingly. Q

We have V = C Where V is the voltage, Q is charge and C is capacitance of the capacitor. It is clear that on changing C, voltage (V) will change, provided Q is kept constant. Merits (1)

It has almost a constant frequency response.

(2)

It can be used for hi-fi (quality) work.

(3)

It is almost free from noise.

Demerits (1)

Its sensitivity is very low.

(2)

The amplifier needs to be kept very near to the mike.

(3)

The mike is to be protected from dust and dirt.

(4)

It is also affected by heat and moisture.

(5)

It needs a separate dc bias, hence costly.

(6)

It is a delicate mike, hence needs careful handling.

Characteristics (i) The output is very low and therefore a amplifier is “inbuilt” in the mike. (ii) They have a high signal-noise ratio. (iii)

They have excellent frequency response and have a flat band width between 20 Hz to 12 kHz.

(iv) They have very low distortion (about 2%). (v) They are omni directional. (vi) They have high input impedance of about 100 M Ω. (vii)

Sensitivity is about 50 dB.

Working of a Capacitive Mike The diaphragm acts as one plate and the fixed back plate acts as second plate. The two plates are 25 mm apart and form a parallel plate capacitor of about 40 pF. A 200 V dc is applied as a bias between the two plates.

7.1 Microphone

125

When sound waves strike its diaphragm, it moves. During compression the distance between plates decreases, it increases the capacitance. During rarefaction, the distance increases, it decreases the capacitance. This change in capacitance gives a varying voltage across the two plates depending upon the sound pressure. Applications (i) Due to excellent frequency response, a capacitive mike is used as a standard mike to calibrate other mikes and testing of loud speakers. (ii) Due to above reason, It is used in sound level meters which can measure sound level of a source. (iii)

It is used in hi-fi recording work.

(iv) It is useful for accurate laboratory measurements. (v) It is also used for theater and studio work. (vi) Due to more cost, they are not used for ordinary work. (c) Moving Coil or Dynamic Microphone (See Fig. 7.8) In this microphone, a coil having large number of turns moves between the magnetic poles of a “Pot magnet”. When sound waves strike the diaphragm, it starts moving forward and backward in accordance to the sound pressure on it. As the coil is fixed with the diaphragm, the coil also starts moving accordingly. There is a relative motion between coil and the magnet, hence an emf is induced in the coil (according to the Faraday’s laws of electro magnetic induction). Diaphragm N

S Coil

Pot magnet

Fig. 7.8.

This induced emf is fed to the amplifier which is in turn coupled with the loudspeaker. The connections are shown in Fig. 7.9.

126

Chapter 7

Microphones & Loudspeaker

N Amp.

L.S.

S O/P Transformer

Moving

coil mike

Fig. 7.9

Merits (1) Its impedance is low, in between 25 to 40 ohms. (2) It is not effected much by atmospheric changes and hence it is more durable. (3) The output of this mike is about 50 db. (4) It is unidirectional. Demerits (1) This microphone is to be kept very near to the source of sound. (2) The magnet is large and is to be protected in a heavy cover. Working The magnet is POT type as a result, a uniform magnetic field is produced. The diaphragm is of non magnetic material. A protective cover is used to save diaphragm and coil assembly against mishandling. As mentioned earlier when sound strikes the diaphragm, it moves and hence the coil moves in the magnetic field. Due to change of flux linking, an emf (e) is induced in the coil given by the following: dφ

e = N.

dt Where, N = No. of turns in the coil. dφ

= Rate of change of flux. dt As a result of this emf, a current is produced which later on is amplified. Characteristics (i) Sensitivity is about 35 mV, which is stepped up to 100 mV by a step up transformer. (ii) S/N ratio is about 35 dB. (iii)

It gives a flat response from 50 Hz to 8 kHz.

7.1 Microphone

127

(iv) Its distortion is about 4%. (v) The mike is omni-directional. (vi) The output impedance is about 30 Ω hence a step up transformer is built within the microphone to increase impedence for matching purposes. However modern mikes use many turns of thin wire which makes their resistance upto 200 Ω and then an “inbuilt” transformer is not needed. Applications (i) It is uneffected by atmospheric problems, hence used in public address (P.A.) system. (ii) It is also used for broadcasting work. (iii)

If this mike is connected in series with a ribbon velocity mike, a very good directivity pattern (cardioid) is obtained. This makes the system very suitable to be used for theatres. This assembly is available in the market.

(d) Crystal Microphone The crystal microphone works on the principle of piezo-electric effect. According to this effect, crystals of few materials, when applied pressure, start producing “vibrations” or “oscillations”. Crystal microphone makes use of crystals of Quartz, Roschelle salt, or of Ammonium dihydrogen phosphate (ADP), as these materials give better results. It has a diaphragm, when sound waves strike it, it starts vibrating. The frequency of vibrations and their amplitude depends upon the pressure of sound incident on it. This diaphragm in turn exerts pressure on the crystal (Fig. 7.10). As a result we obtain a varying electrical output which is amplified and fed to the speaker.

Crystal

Diaphragm

Fig. 7.10

128

Chapter 7

Microphones & Loudspeaker

Merits (1) This microphone has a very high impedance such that it does not need any output transformer and the microphone can directly be connected with the amplifier coupled to the speaker. (2) The output is about 50 db. (3) Its frequency response is in between 30 Hz to 10 KHz. (4) It is omni-directional. Demerits (1) The microphone cannot bear high temperature. Their use is recommended for below 100°C. (2) They cannot bear direct sunlight. Construction and Working We know that few crystals like quartz, rochelle salt, ceramic and touramaline show piezo effect, thus when opposite faces of these crystals are subjected to a pressure, a potential difference is produced between the faces. Out of the above materials ceramic and quartz are widely used, as they show good piezo effect and also they can work in unfavourable atmospheric conditions. Two thin slices of quartz, cut along suitable axis are placed with air spaces between them. An aluminium diaphragm is also attached with the crystal such that when diaphragm moves with sound pressure, it exerts a mechanical pressure on the faces of the crystal. Usually, a crystal is formed by joining two quartz slices of opposite polarity. Hence when sound waves strike the diaphragm, it pushes one slice in tension and other in compression and opposite charges are developed on their surfaces. The emf so produced is fed to the amplifier and then to loudspeaker. Characteristics (i) It has a good sensitivity of 50 mV. (ii) Its S/N ratio is quite high, a typical value is 45 dB. (iii) It responds to a bandwidth of 100 Hz to 8200 Hz. (iv) Distortion is very less, about 2%. (v) It is omni directional. (vi) Output impedance is about 1 M Ω. Applications (i) They are extensively used in public adress system due to their robust construction and cheapness. (ii) They are used in sound level meters. (iii) Sometimes they are also used in hearing aids.

7.1 Microphone

129

(ii) Pressure Gradient Microphones In this category, we will discuss Ribbon Velocity microphone. Ribbon Velocity Microphone The ribbon velocity microphone has a thin ribbon shaped aluminium foil suspended between the two poles of a magnet. The ribbon is free to move. When sound waves strike the microphone, the foil moves according to the velocity of sound waves. The net effect is proportional to the difference in the sound pressures on the two sides of the ribbon foil. [See Fig. 7.11 (a) and (b)] From the ribbon, the output is amplified and sent to the speaker. [See Fig. (c)] Ribbon N Parmanent magnet

S (b)

Ribbon

Amplifier Ribbon mike

(c)

L.S.

(a)

Fig. 7.11

Merits (1) The output of the microphone is about 60 dB. (2) The mike is bi-directional. For best results, the speaker should stand at least 1.5 feet away from the mike. Demerits (1) The mike should be protected from the high velocity winds. (2) Its sensitivity is less than moving coil mike. Construction and Working The moving coil microphone studied previously, is heavy due to its diaphragm, coil and magnet. This leads to a poor response for high frequencies. For this reason, the diaphragm has been replaced by a ribbon, which acts as a diaphragm as well as the coil. The mike therefore becomes light and has a better response for high frequencies.

130

Chapter 7

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When the aluminium ribbon placed in the field of a permanent magnet moves due to sound pressure on it, due to change in the flux linking with the ribbon, an emf is induced in the ribbon. This emf is fed to amplifier and then to the loudspeaker. The mechanical force working on the ribbon is proportional to the difference of sound pressures acting on its both surfaces. Hence the mike is called pressure gradient microphone. Further the emf induced is dependent on the velocity of the ribbon, hence it is also known as a “velocity mike’’. The ribbon is a single thin conductor, the emf induced is very small, hence a transformer is used to step up the voltage before feeding it to the amplifier. The transformer steps up the induced voltage to about 50 times. Characteristics (i) It has a very good frequency response for low as well as for high frequencies. It has a bandwidth of 15 Hz to 13 KHz. This is due to the very small size (few microns) of the ribbon. (ii) These mikes have “EIGHT”(8) shape directivity. These mikes are available such that their one part is pressure operated and other part is pressure gradient operated. Such a mikes give a very good “hyper cardioid” directivity pattern. (iii) Their impedance is very small about 0.25 Ω which is to be raised to 200 Ω by a step up transformer for impedance matching with the line. (iv) Their sensitivity as mentioned above is very less. It is 3 mV. With the help of transformer, the voltage may be raised to 100 mV. (v) Signal/Noise ratio is higher. Its value is about 45 dB. This mike therefore is more suitable for hi-fi work. (vi) Its distortion is low about 2%. Applications (i) The ribbon mike is suitable for broadcasting purposes. (ii) It is very suitable for theatre work. (iii) It is not suitable for close talking as the source of sound should be kept minimum 1.5 feet away from the mike. (iv) It is not suitable for outdoor work as the wind can vibrate the diaphragm and this will produce unwanted noise within the mike.

S.No.

Particulars

Carbon mike

Capacitive mike

Moving coil (Dynoamic) mike

Crystal mike

Ribbon velocity

1.

Sensitivity

100 mV

4 mV

3 mV

50 mV

3 mV

2

Frequency response

150 Hz – 5 kHz

40 Hz – 14 kHz

50 Hz – 8kHz

100 Hz – 8 kHz

20 Hz – 13 kHz

3.

Distortion

10%

1%

4%

1%

2%

4.

Directivity

Omni directional

Omni directional

Omni directional

Omni directional

bi directional

5.

Impedence

100 Ω

100 M

20 Ω

1M

0.3 Ω

6.

External Basing

Required

Not required

Required

Not required

Not required

7.

Distance of the source of sound from mike

Close (3 inch)

1.5 feet

(6 inch)

1.5 ft.

5 ft.

8.

Cost

Cheapest

High

Medium

Cheap

High

9.

Self noise

Highest

Lowest

High

Lower

Low

10.

Remarks

Used in telephone

Used in recording and calibration

Used in P.A. system Used in recording and broadcasting and mobile communications

7.1 Microphone

Table 7.1: Microphones—a comparison

Used in theatre, broadcasting and orchestra work.

131

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7.2 LOUD SPEAKER A loudspeaker is an electro-acoustic transducer which converts electrical input obtained from microphone into the acoustic (sound) output of same frequency as of the input. In this way, it is “reverse” of the microphone. The diaphragm of the speaker is made to vibrate, this in turn sets the molecules of the surrounding medium into motion, such that the sound can be heard at a distance.

1. Characteristics of Loudspeakers The following characteristics of loud speakers determine their performance. (i) Efficiency: It is defined as the ratio of output sound energy obtained from a loud speaker to the amount of electrical energy as input to the loudspeaker from the mike. Naturally the output power will depend upon the impedence matching between the source (loud speaker’s coil) and the load (air to be disturbed by the speaker). The efficiency of commercial (horn) type loud speakers is about 10% and that of domestic (cone) type loud speaker is only 1%. (ii) Sensitivity: It is equal to the input signal which can provide an output sound pressure of 1 microbar at a distance of 1 m from the loud speaker. (iii) Signal/Noise ratio: The loudspeaker creates “self noise” as its mechanical parts also produce vibrations at their natural frequency. The S/N ratio of loud speakers is equal to Signal output SNR = Signal output without noise

Response in db

(iv) Frequency response: This is the range of audio frequency to which a loudspeaker responds. Ideally, a loudspeaker’s frequency response should be for the entire range of audio frequency i.e., from 20 Hz to 20 kHz, but practically, this is affected by many factors e.g., mass of the vibrating components of the loud speaker. Practical loud speakers have a response from 100 Hz to 16 kHz. See Fig. 7.12

10 Hz

100 Hz

1 KHz Frequency

Fig. 7.12

16 KHz

7.2 Loud Speaker

133

Frequency distortion is more important in

loud speakers as it produces second and third

harmomics of frequencies. See Fig. 7.13

Truly speaking, any of the distortions is not

desirable in loudspeakers.

Amplitude

(v) Distortion: The output of the loud speaker may not be exact replica of the sound input at the mike. This is called distortion. This may be an amplitude, frequency or phase distortion.

(vi) Directivity: Loud speakers are always 2f2 f1 3f1 directional i.e., they produce maximum f intensity in a particular direction. Ratio Fig. 7.13 of sound intensity in the direction of maximum intensity to the sound intensity that would be available if the loud speaker would have been omni (all) directional, is called its “Directivity”. (vii) Impedance: (Fig. 7.14) The input impedance (of speaker coil) is given in ohms. It helps in its matching with the amplifier e.g., If a loud speaker has been marked as 16 ohm, the output impedance of the amplifier should be 16 ohm in order to get maximum transfer of power from amplifier to the loudspeaker. L.S. 16 Ω Amp. 16 Ω

Fig. 7.14

At low frequencies its impedance is higher and at high frequencies its impedance is low. Usually loud speaker is available in 4, 8 and 16 ohms. Knowledge of impedance is also helpful, when loud speakers are to be connected in series or parallel. (viii) Power Handling capacity: Power in watts, which can be handled by a loudspeaker without distortion is known as its power handling capacity. If a loudspeaker is marked as 1 W, it means maximum power the loud speaker can handle is 1 W. If it is used to handle power more than 1 W, it may damage the loudspeaker.

2.  Classification of Loudspeakers A loud speaker has a vibrating surface which radiates sound into the surrounding air. An arrangement within the speaker sets the vibrating surface in motion. In addition, the loud speaker has an enclosure to couple sound energy with the surroundings.

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Accordingly, loudspeakers are classified into: (i) Direct radiation (or cone) type: This has a vibrating surface (the diaphragm) directly coupled with air. This is compact in size, cheaper and have a frequency response for a wide audio band. But, they have low output, narrow directivity and poor response at higher frequencies.

In a direct radiation type loudspeaker, whole cone acts as a diaphragm

and radiates sound directly into the surrounding.

(ii) Indirect radiation (or Horn) type: The indirect radiation type loud speaker does not radiate sound directly into the surroundings, but the sound is first delivered to a fixed non vibrating horn and from there, it is thrown into the surroundings. Table 7.2 S.No.

Comparison between the two types of speakers Direct radiating (or cone type) speakers

Indirect radiating (or horn type) speakers

1.

It works on motor principle

It also works on motor principle.

2.

The cone is made of paper.

There is no paper cone. A metal diaphragm is used.

3.

The paper radiates sound directly into the air.

The diaphragm radiates sound in air through a horn.

4.

There exists no matching.

Some impedance matching exists between diaphragm (Horn) and the air.

5.

Its efficiency is 5%

Its efficiency is 40%.

6.

It has a frequency response from 500 Hz to 5 kHz.

Its frequency response is from 20 Hz to 12 kHz.

7.

Distortion is 10%

Distortion is 2%

8.

It can handle power upto 80 W.

It can handle power upto 400 W.

9.

Its input impedance is 16 Ω.

Its input impedance is 16 Ω.

It is used in Radio, TV, VCR etc.

It is generally used in P.A. system

10.

(i) Direct Radiating (Cone) Speakers They may be: 1. Moving coil with permanent magnet type 2. Moving coil with temporary magnet type.

7.2 Loud Speaker

135

1. Moving Coil (with Permanent magnet) or dynamic cone speakers: It is the most popular loudspeaker, because of its simplicity in construction, precision, easy to coordinate with other equipment and freedom from electrical troubles. It is widely used in all kinds of domestic reproducing systems. The other major factor of its wide use is its “own power house”. It requires no external power for operation. Its power house is its permanent magnet. It does not lose its magnetism for the whole of its life. (a)

Construction: The major parts of this loud speaker are: (i) Voice coil

(ii) Permanent magnet

(iii) Spider

(iv) Diaphragm (cone)

(v) Basket. The Fig. 7.15 shows the loud speaker and its various components. Diaphragm Dust cap

Former

Basket Spider Permanent magnet

Voice coil

Yoke

Fig. 7.15

(i) Voice coil: The voice coil is the only thing in a speaker, that carries the electrical signal. As the name indicates the voice coil is the part of the loud speaker which “talks” when energised by the signal (or speech) currents coming from the amplifier. (ii) Permanent magnet: The magnet is pot type and made of Alnico (A1 = 10%, Ni = 2% and cobalt 10%).

The magnet is the heart of the loud speaker. The current

flowing through the voice coil sets up a magnetic field around the coil. The interaction between this field and the magnetic field produced by the permanent magnet sets up a torque on the voice coil, which forces the coil to move. Thus the working of the loud speaker depends upon the “motor action”.

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Due to the interaction of the two magnetic fields (one stationary field of magnet and other movable field of the voice coil) the voice coil will move up or down. The direction of the voice coil field will be determined by the direction of the signal currents through the coil. The direction of motion of the voice coil is generally parallel to the length of the gap, in which it is balanced. (iii) Spider: The spider is a device, which balances the voice coil in the air gap. It holds the coil centred in the gap hence it is also called “centering spider”. The coi1, while in motion vibrates in and out of the gap. The spider does not allow any lateral movement of the coil at the same time it does not obstruct the movement of the cone. The coil also should not strike the metal walls of the gap. In this way, spider is an important role to play in the proper functioning of the loudspeaker. (iv) Diaphragm (Cone): The voice coil is attached to the diaphragm, which actually puts the surrounding air into motion. The cone is made of special impregnated paper or of cloth. Cone moves back and forth along with the voice coil depending upon the strength of the signal currents flowing through the coil. The cone causes the surrounding air to vibrate exactly in the same manner as the signal, this creates sound waves in the air.

(b)

(v) Basket: All the components are housed on a sheet metal basket Impedance of the Speaker This loud speaker is “current operated”, as it is actuated by the signal currents through the voice coil. For being current sensitive, the device should have minimum impedance. Therefore this speaker has an impedance range of 2, 4, 8 and 16 ohms. The 8 ohm loud speaker being the most popular.

(c) Working As mentioned earlier, the moving coil loud speaker works on the motor principle. When the signal current passes through the voice coil placed in the magnetic field of a permanent magnet, the coil experiences a torque and moves. This makes the paper cone to vibrate, which produces sound oscillations in the air.

7.2 Loud Speaker

137

2. Moving coil with electromagnet (Electrodynamic) cone speaker: The dynamic type moving coil speakers studied earlier, have low wattage therefore electrodynamic moving coil type speakers with an electromagnet (in place of permanent magnet) are used. In these speakers, power handling capacity may be upto few hundred watts. The magnet here is also pot type and has E shape, the South pole being at the centre surrounded by two north poles. The voice coil wound on an aluminium former is placed in the annular gap as shown in Fig. 7.16 Cone

Voice Coil Supply N

Spider

S

Dust cover

N Electromagnet Basket

Fig. 7.16

Working The working is also similar to that of the dynamic speaker. The audio signal obtained from amplifier is made to flow through the voice coil and produces a varying magnetic field around the coil according to the signal. The interaction of magnetic fields of the coil and of the electromagnet produces a torque in the coil which starts vibrating. These vibrations are transmitted to the cone, which produces vibrations in the surrounding air. Electrodynamic speaker has high power handling capacity and has better frequency response. But, on the other hand, external supply is needed for the electromagnet. More over it is heavier and costlier than the dynamic speaker. • Efficiency of Moving Coil Speakers The efficiency of a moving coil speakers is less than 5%. The reason of low efficiency of speakers is that the electrical energy fed to the loud speaker is not converted directly into sound but it has following stages:

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(i) First of all, electrical energy is converted into magnetic energy. (ii) This magnetic energy is then converted into mechanical energy. (iii)

Now this mechanical energy is converted finally into sound energy.

Further, during the stages, the following losses also take place. (i) When electric energy flows through the coil, power loss takes place in the coil, which will depend upon the resistance of the coil. (ii) During conversion into magnetic energy, a loss will occur again, which will depend upon flux linkage between voice coil and the flux in the gap. (iii)

During conversion from mechanical energy to the sound, losses will occur in the mechanical drive system of the loud speaker.

(ii) Indirect Radiation (or Horn) Type Speaker There is no major difference between cone and horn type speakers, except that the cone has been replaced by a horn to get more audio output, as a horn provides better impedance matching with the surrounding air. There is also a very little difference in working of the two. The cone speaker radiates sound directly into air but in horn type, the sound is radiated into air through the horn. The horn does this job acoustically which was been done by the cone mechanically. Moreover horn acts as a sound transformer. All this give it a better efficiency. The horn type speakers can give upto 40% efficiency whereas, cone type speaker has an efficiency of 5%. • General Construction of horn A horn may be defined as a tapered tube such that, its diameter increases from a small value at one end called throat to a large value at another end called mouth (Fig. 7.17). Due to its special shape, it can work as sound transformer. The shape of the horn is major factor, which determines its performance. Diaphragm Throat

Unit

Fig. 7.17

Mouth

7.2 Loud Speaker

139

• Types of Horn Speakers They may be of the following types: (a) Moving coil dynamic speakers (b) Moving coil electrodynamic speakers (c) Armature speakers (d) Induction speakers (e) Capacitive (condenser) speaker. (a) Moving Coil dynamic speaker (with permanent magnet): This speaker is very popular due to its versatile properties and is widely used. They have a permanent magnet and do not require any external source of supply. It consists of a coil, moving in the field of a permanent magnet. Thus basically it is an electric motor. With the coil, a diaphragm is connected which is also movable. The output current of the amplifier is given to the coil which is called “Voice coil”. As the current carrying voice coil is suspended in a magnetic field, it starts rotating (motor rule). This sets the diaphragm into vibrations. The diaphragm (i.e. Horn) puts the surrounding air into motion and sound can be heard at a distance. The Fig. 7.18 shows this speaker. (diaphragm or horn) Parmanent magnet

Suspended voice coil

Mouth Throat

Unit

Horn

Fig. 7.18

The “Horn” which acts as a diaphragm is a type of sound transformer. The sound while travelling from unit to the horn becomes louder due to the transformer action of the horn. Actually it establishes a type of “impedance matching” with the driven air and thus output power is improved. The horn’s shape may be: conical, exponential or flat. A conical is that in which cross-section of the horn is increased in proportional to its length whereas in exponential horn, the cross section is an exponent of the length.

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The ratio of diameters of throat to mouth is 1 : 7 which means due to narrow throat, the pressure of sound is very high at the throat (about 7 times) than that at mouth. As the sound moves from “throat to mouth”, it goes on expanding thus its pressure goes on decreasing and the sound waves are spread over to a large surroundings. This is the reason that the sound can be heard at a large distance. The speaker may be with the unit resistance of 2, 8 or 16 ohm and wattage of 10, 20 and 80 W etc. Efficiency The efficiency of a speaker is the ratio between the useful acoustic power radiated to the electrical power supplied. In case of these speakers, efficiency is very low and maximum upto 35%. (b) Moving coil Electrodynamic Speaker (with temporary magnet): Moving coil electrodynamic speaker contains an electromagnet and hence needs a separate source of supply. It has a three pole magnet. The central limb contains a coil which is to be energised by a separate d.c. source. (Fig. 7.19) Three-pole magnet

S

Voice coil Horn

N Mechanical link

S

+ Separate – D.C. source

Fig. 7.19

There is a voice coil which is connected with the output of the amplifier. With the same principle of moving coil speaker, the voice coil starts rotating; the torque acting on the coil is in accordance to the current following through the voice coil. With oscillations of the voice coil, the diaphragm (horn) starts oscillating due to its mechanical coupling with the voice coil.

7.2 Loud Speaker

141

(c) Armature Speaker: In this speaker, an armature is made to rotate between the poles of a permanent magnet as shown in Fig. 7.20. From amplifier

N

N

Armature Horn

Voice

coil

S

S

Permanent magnet

Fig. 7.20

On the armature, a voice coil is wound which is connected with the output of the amplifier. When signal current obtained from amplifier flows through this coil, it produces its own magnetic field. By the intersection between this magnetic field and the magnetic field of the permanent magnet, a torque is resulted which tends the armature to rotate. The armature being mechanically linked with the horn, makes horn to oscillate accordingly. (d) Induction speakers: These speakers work on the principle of electromagnetic induction. Which says “that emf (hence current and torque) is produced whenever there is a charge in the flux linking with a coil”. This speaker has a diaphragm (Horn) which is placed between two co-centric coils. The direct current is supplied to the coils in opposite direction. The signal current also flows through the coils. With the principle of induction, a torque acts on the horn and it starts vibrating. (See Fig. 7.21) Permanent magnet

Coil I Horn Coil II

Mechanical link

Fig. 7.21

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(e) Capacitive or Condenser Speakers (Fig. 7.22): This speaker contains two plates separated by some dielectric, thus forming a capacitor or condenser. The stationary (fixed) plate is of copper or aluminium. The movable plate or diaphragm is made of some light material. The dielectric between the plates is thin, flexible with high dielectric constant and high breakdown voltage. dielectric Copper plate (fixed)

Movable plate (diaphragm or horn)

Transformer

– +

Fig. 7.22

When the plates are given voltage, an electric field is built up. On this field, the field of the signal is superimposed. This causes attraction and repulsion between the plates, as a result the movable plate (diaphragm) starts vibrating corresponding to the audio signal.

7.3 P.A. SYSTEM The P.A. system stands for “Public Address System” This is an electro-acoustic system to send an audio signal to a distance. It is a common experience that intensity of sound goes on decreasing with increase in distance. If a sound is to be transmitted to a distance (e.g., a public meeting), the sound should be suitably amplified so that it may reach to every corner and a person may listen comfortably. For this, amplifiers are to be used. The electronic circuit, which amplifies the sound and processes the same so that it may reach to the audience at a distance is called a P.A. system. This system is used in public speeches, sports meet, cultural functions, auditoriums and also at railway stations and aerodromes for giving necessary information to the passengers.

7.3 P.A. System

143

1. Components of a P.A. System The important components of a P.A. system are: (Fig. 7.23) (1) Microphone: The person speaks before a microphone, the function of which is to convert the sound (speech) of the person into electrical pulses; (as it is easy to transmit a sound in the form of electrical energy). The selection of suitable microphone and its positioning in the P.A. system is an important factor. The mike used in P.A. system is generally dynamic (moving coil) type or sometime ribbon velocity type. (2) Mixer: The output of microphone is fed to a mixer circuit which isolates different frequencies from each other. Sound

Mixer Mike

2 or 3 voltage amplifiers

Driver Amplifier

Power Amplifier

Sound L.S.

Amplification

Fig. 7.23



(3) Amplifier: The output of the mixer stage is now amplified. Generally amplification is done in following stages: (i) Voltage amplification: The signal obtained from the mike has extremely low voltage (2 mV) therefore the voltage level of weak signal is raised by two or more voltage amplifiers. Generally RC coupled amplifiers are used for voltage amplification. (ii) Driver stage: The output of the last voltage amplifier is given to a driver amplifier. It supplies necessary power to the output stage. The driver amplifier is a class A transformer coupled power amplifier. Here concentration is given on the maximum power gain. (iii) Output power stage: The output of the driver amplifier is given to the output power amplifier. It is the final stage which delivers power to the loud speaker. This stage necessarily uses a class B push pull amplifier. Concentration is given on the maximum power output to the speaker. The amplifier used for P.A. system has a rating ranging from 20 W to 130 W. (4) Loudspeaker: A loud speaker is the last stage of P A system. It converts electrical signal into the original sound produced at the mike. The output of the power amplifier is connected to the speaker through a impedence matching device. Impedance matching delivers maximum power from amplifier to the speaker.

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For PA system, horn type loud speakers are used.

Note: For maximum power transfer, a series parallel combination of

the speakers should be arranged.

SUMMARY 1. Microphone converts sound energy into electrical energy. 2. Loudspeaker converts the electrical energy back into sound energy. 3. Important m icrophones are carbon microphone, moving coil microphone and crystal microphone. 4. The speakers may be cone type used in radio/T.V. or horn type used in public gatherings. 5. The PA system is a “public address system” used in all public places, and in radio, T.V., radar etc. for reception of sound. 6. The components of a P.A. systems are: microphone, amplifier and loudspeaker. qqq

8 Surface Mount Boards In a conventional printed circuit board (PCB), the components are inserted into holes and soldered, but in surface mount technology (SMT), the boards are manufactured by soldering the components on the board itself.

8.1 SURFACE MOUNT TECHNOLOGY (SMT) The Surface Mount Technology (SMT) is an advanced technology, which has brought revolution in PCB manufacturing. In the conventional PCB assembly method of “through hole” technology, the components—both discrete as well as ICs have leads for insertion into the holes. These leads are soldered on opposite side of the board, as shown in Fig. 8.1 (a). Through hole components Through holes

Board Solder

Solder

(a) Conventioned PCB SMT component

Board (b) Surface mount board

Fig. 8.1

But in SMT, the components called surface mount components (SMCs)

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have leads smaller than in the conventional components. These leads can be soldered onto the board directly as shown in Fig. (b). In other words, in SMT, holes are not drilled and components (specially manufactured) are soldered on the copper side of the board. Moreover SMT components may be sometimes without any lead. Note: The SMT boards are prepared generally by microprocessor controlled machines.

8.2 ADVANTAGES & DISADVANTAGES OF SM TECHNOLOGY (a) Advantages (1) The component’s size is smaller and occupy a smaller board area and hence the size of the board is reduced. (2) The package density is more in case of surface mount boards, as more number of components can be accommodated. (3) The surface mount devices can be used at very high frequency upto giga hertz. (=109 Hz.) (4) The parasitic effects are reduced due to lesser length of leads of the surface mount components. This increases speed and efficiency of operation and reduces electrical noise. (5) Since the package height is small, the board occupies lesser space. (6) Since the number of holes are reduced, the board becomes stronger and more reliable. (7) SMT-based designs enable the components to be mounted on both sides of the board. This further reduces the size of boards to one-third of the conventional size. (8) The SMT components allow their automatic placement and soldering. (9) The technology is better suited for large volume production. (10) More connections can be realized per unit area of the board. This is known as ‘interconnectivity’ which is directly related with the size of the board. (11) Number of holes is reduced to 50 per cent as holes are not required for soldering leads but they are required only for interconnections. This reduces the cost of the board. (12) There is a significant saving in the weight of the board. The SMT components weigh one- tenth of their through hole counterparts. This makes the SM technology popular in mobile applications.

8.2 Advantages & Disadvantages of SM Technology

147

(13) Since SMT components have smaller (or even without) leads and the leads may act as antenna, their Electro Magnetic Interference (EMI) is significantly reduced. (14) The SMT also results in the reduction of radiation area.

(b) Disadvantages (1) The SMT demands ‘zero defect’ manufacturing, as repairs are extremely difficult. This demands rigorous process control. (2) The cost of the SMT components is more and hence initial cost of SMT boards are higher than through hole boards. (3) The components are mounted closely. This reduces their heat dissipation and increases possibility of their getting overheated and damaged. (4) The SMT design calls for high precision. (5) For the same reason, automatic high precision "placement and soldering techniques" become a must. (6) The capital investment is very high and thus the technology is not suitable for small scale production. (7) SMT demands for highly skilled personnel. Note: (1) The base materials to be used for SMT should have an accurate coefficient of thermal expansion, higher thermal and chemical resistance, low electrical losses and low moisture absorption. Generally high class thermoplastics (like teflon) and thermosets (likes epoxies) are used for the substrate of these boards. The epoxies are modified by adding some percentage of polyamides to them. This improves their electrical properties, i.e., the modified form has low dielectric losses, low moisture absorption and better adhesive properties. (2) The coefficient of thermal expansion of copper foil and of the board is different. This may peel off the copper foil from the board. To increase the ‘peel strength’, copper is passed through an oxide treatment. (3) Micro drills are used for drilling SMT boards. The holes are about the size of 0.004". Tungsten carbide is invariably used for the tip of these drills.

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(4) Special attention is required while soldering surface mount components, like: (a)

Chip components

(b)

Discrete components

(c) Chip carriers (5) The modern practice is to adopt mixed technology, i.e., a mixture of surface mount components and through hole components.

8.3 SMT COMPONENTS The SMT components have a special shape and size. They can be differentiated among themselves by their (a) shape, and (b) type of pins/leads.

(a) Shape Normally there are 2 major shapes: (i) Rectangular shape—in which pins are provided on both sides of the package. On its longer side, number of pins may go upto 28. (ii) Square shape—in which, pins are provided on all sides.

The Fig. 8.2 shown few SMT components/devices

Rectangular shape

Square shape

Fig. 8.2

(b) Type of Pins or Leads The pins or leads may be of the following types (See Fig. 8.3) (i) J-type. Fig. 8.3 (i)

8.4 Design of SMT Boards

149

(ii) Gull-wing type. Fig. 8.3 (ii) (iii) Butt type. Fig. 8.3 (iii) J-type lead

(i)

Gull-wing lead

(ii)

(iii)

Butt type lead

Fig. 8.3

8.4 DESIGN OF SMT BOARDS The design should be produceable at low cost, should be demanding lesser time and should not demand any modification in the existing process. Since the technology uses automation process, the components should facilitate auto placement. Designing Rules (i) A space of 0.12 cm to 0.24 cm from the edge of the board should be provided. (ii) All the components should be oriented in the same direction, as it makes automatic placement easy. (iii) Wider traces connecting to solder pads should be avoided. (iv) Exposed metal should not be left below SMT resistors, inductors and capacitors, which are the heat producing components.

8.5 GENERAL FABRICATION PROCESS FOR SMT BOARDS The Fig. 8.4 shows the steps generally involved in the fabrication of SMT boards. Clean the board

Apply adhesive

Place the components on the board

Fig. 8.4

Apply solder paste

Setting of paste

Again clean the board

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(i) The board should be thoroughly cleaned. (ii) Apply adhesive on the board—The adhesive retains the components temporarily on the substrate till the soldering operation is done. The adhesive can be applied by pressure or by screen printing method. The adhesives generally used are: epoxy, urethane and acrylics, which have good adhesion properties. (iv) Put the components at their proper place on the board. (iv) Then solder paste of thickness 0.004" is applied. The paste is made of a solder alloy (60 per cent tin and 40 per cent lead) incorporated with a flux (which acts as binder). A resin can also be used as a flux. The solder paste can also be applied by pressure or by screen printing process. For mass production, wave soldering is invariable used. (v) The setting of paste takes about half an hour. (vi) The cleaning of the board after soldering assures reliability of the service.

8.6 COMMERCIAL FABRICATION OF SMT BOARDS Commercially available major types of SMT boards and their soldering processes are described in the following; Type I: in which the board contains only SMT components on both sides. The board, which contains only SMT components on both sides of the board is shown in Fig. 8.5(a). The flow diagram for soldering technique for this board has been shown in Fig. 8.5 (b) SMT component

Solder

Solder SMT component

Fig. 8.5 (a)

8.6 Commercial Fabrication of SMT Boards

151

Apply solder paste by screen printing on one side

Place SMT components

Dry paste

Reflow solder

Invert the board

Apply solder paste by screen printing

Place SMT components

Dry paste and Reflow solder

Clean both the sides of the board

Fig. 8.5 (b)

Type II: which contains I.C. chips and active components on one side and passive components on the other side. The board, which contains I.C. chips and active SMT components on one side and passive SMT components on other side is shown in Fig. 8.6(a). Figure 8.6(b) shows flow diagram for the technique for such boards. SMT passive component

Solder

Chip component

SMT active component

Fig. 8.6 (a)

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Apply solder paste by screen printing on both sides

Place SMT active and chip components on one side

Dry paste

Reflow solder

Invert the board

Place SMT passive components on other side

Cure adhesive

Invert the board

Wave solder

Clean the soldered board

Fig. 8.6 (b)

Note: Diode, transistors, I.Cs. etc. are called active components, while resistors capacitors etc. are called passive components. Type III: Hybrid board: containing SMT as well as through hole components The board, which contains the SMT components mounted on both sides and through hole components only on one side of the board is shown in Fig. 8.7 (a). The flow diagram for soldering such a board is shown in Fig. (b)

8.7 SMT-Equipment

153 SMT component Solder

Solder

Through hole

Solder SMT component

Through hole component

Fig. 8.7 (a) Insert the through hole components

Invert the board

Apply adhesive

Place SMT components

Allow setting of adhesive

Invert the board

Wave solder

Clean the soldered board

Fig. 8.7 (b)

Note: This is the most popularly used board today.

8.7 SMT-EQUIPMENT The various facilities/machines needed in an SMT plant are as under: (1) PCB computer aided designing facility. (2) PCB fabrication facility.

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(3) Screen printing. (4) Glue (adhesive) applicator machine. (5) Pick and place machine. (6) Soldering machine. (1) PCB Computer Aided Designing (CAD) Facility: It includes; (a) A computer with a PCB-CAD software. (b) A laser printer (for documentation). (c) Plotter. (2) PCB Fabrication Facility Holes are drilled in the board by Computerised Numerically Controlled (CNC) machines. The holes will also be required on the board for connecting the layers of multilayer board as well as to insert ‘through hole components’ on the SMT board. (3) Screen Printing The SMT components like chip resistors, chip capacitors and ICs are attached to the board by applying a solder paste in the required place by screen printing process. On the solder paste, IC (or other components) will be placed by a “pick and place’ machine. The assembly will be passed through an infra-red (IR) machine (oven) where the solder paste will melt and join the copper track with the components. For screen printing, first a stencil is made which is basically a metallic sheet in which holes are etched making patterns corresponding to the places where soldering is required. The screen printing machine will mark this stencil as well as the PCB automatically and a uniform layer of solder paste is applied on the board through the holes in the stencil (board kept below the stencil). After that a ‘squeegee’ will be called to slide over the stencil applying a uniform layer of solder paste producing a fine ‘screen print’ on the board. Typical technical specifications of screen printing machine are: Power supply

400 V – 50 Hz – 3 phase ac

Power rating

3 kW

Dimensions

1.7 m × 1.5 m × 1.0 m

Weight

710 kg

Printing speed

160 mm/s.

Squeegee pressure

400 N.

8.7 SMT-Equipment

155

(4) Glue (Adhesive) Applicator Machine This machine is required only if through hole components along with SMT components are to be used. The SMT components preferably are mounted on the bottom side of the board and require wave soldering; therefore SMT components are not attached with solder paste but are directly soldered with the board. The hole components are to be kept in their proper place on the board by passing the board through a glue applicator machine which puts 2 or 3 drops of glue (adhesive) at the place for proper adhesion of the components so that they do not fall, during the soldering process. It is important that if through hole components are not to be used and only SMT components are used, the glue applicator and wave soldering machine are not required at all. Then the SMT components can be soldered on the bottom side of the board using only solder paste process as already described. (5) Pick and Place Machines (a) A ‘pick and place’ machine places the hole components on top of the glue. When this assembly is passed through an ultraviolet oven, the glue gets set and holds the components firmly with the board. The following two types of ‘pick and place’ machine are popular in SMT boards: (1) Chip shooter and (2) Fine pitch placer. • Chip Shooter It can place chip (hole) components at the rate of 30,000 components per hour; but it is not very accurate. This machine is used when smaller chip resistors and capacitors are used. • Fine Pitch Placer Machine This machine is very accurate and is used when a fine ‘pitch’ is involved. It is used for placing PLCC (Plastic leads chip carrier) package ICs. As this machine has a camera and an image processing software, it can do the job with the same speed as the chip shooter. It also takes care of fine pitch (distance between pins) of the components. (b) A pick and place machine is generally computer controlled and has an auto laser movement guide for error free pick up and placement. It can also be equipped with a camera and a video monitor for accurate placement of fine pitch components. Figure 8.8 (a) shows a pick and place tool for very small components. It is like a tweezer with a bulb which can be squeezed. It is used for two purposes:

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(i) For picking components Steps: (i) Position the tool on the components (ii) Squeeze bulb (iii) Lower the tool on the component (iv) Release bulb (v) Pick up the component. (ii) For placing the components over the board Steps: (i) Position the tool (with the component) on the board where it is to be placed (ii) Lower the tool on the point (iii) Align position on the board (iv) Squeeze the bulb to release the component. Figure 8.8 (b) shows surface mounted components’ removal tweezers. Bulb

(a)

Fig. 8.8

(b)

(c) Pick and place machines (PPMs) can be classified as: (i) Fully automatic which can place about 35,000 components per hour and are used for large volume production. (ii) Semi automatic which can place about 6,000 components per hour and are used for medium volume production. (iii)

Manual which can place about 1000 components per hour and are used for small volume productions.

Fully automatic PPMs are now being used commercially.

8.7 SMT-Equipment

157

Important specifications and features of fully automatic PPMs are: (i) Speed: Machines with a speed of 50,000 components per hour are available. (ii) Capacity: Today machines are available for handling all types of components. (iii)

Pitch handling: Now machines are available to handle upto 12 mil (1 mil = 25 mm) pitch.

(iv) Alignment technique: The machines are available with mechanical alignment (chucks), camera or laser alignment techniques. (v) Variety of components to be handled: Now-a-days, machines are available which can handle 100 varieties of components. (vi) Accuracy: The of ± 0.1 mm.

machine

have

placement

accuracy

(vii)

Mounting speed: The time taken by a machine to mount one component on the board is known as its mounting speed. It varies from 0.1-0.5 s.

(viii)

Dimensions handling: Machines are available to handle boards from 10 cm2 to 50 cm2 size.

(ix) Software: The PPM should be loaded with the required software for picking and placing components. (x) No. of placement heads: In case of chip shooters, number of heads may be upto 30, whereas in fine pitch place machines, they are two in number. The heads may be arranged in linear or rotary motion. Figure 8.9 shows rotary motion mechanism. Placement heads

Rotary head

Fig. 8.9

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(6) Soldering machines Three types of machines are used: Vapour reflow machine, Infrared reflow machine and wave soldering machine. (a) Vapour Reflow Soldering machine: In this machine the latent heat of the vapours of an inert liquid, such as fluorocarbon (whose vapours condense on the parts to be soldered) is used for soldering (See Fig. 8.10) Input

Reflow soldering

Output and cooling

Fluorocarbon vapours

Board in

Soldered Board out

Condensing surfaces Boiling fluorocarbon

+ –

Heating element

Fig. 8.10

Advantages (i) The temperature for soldering remains constant and is independent of the size and shape of the board. (ii) All the processes are automatic. Disadvantages Improper soldering due to the movement of the components during the soldering process. (b) Infrared Reflow Soldering Machines In these machines, the board is soldered by pre-heating it through top and bottom heating panels (zones). After the appropriate pre-heating, the assembly is passed through the reflow temperature for soldering and then through cooling. The sequence of processes in the reflow process can be summarized as under:

8.7 SMT-Equipment

159

(i) Solvent evaporation: This process is violent, there are explosions resulting in the formation of solder balls which can cause short circuits if they fall between two metal conductors. (ii) Flux melting: In the next step, the flux melts and starts the chemical reaction with the metal oxides, preparing a clean metal surface for the solder. (iii) Wetting: In this process, solder spheres (balls) melt and wetting begins. (iv) Melting completes: Solder melting completes and the surface tension of this volume of molten solder prevents any further wetting. (v) Cooling: In this step cooling starts. Figure 8.11(a) shows a typical Infrared (IR) machine and Fig. 8.11(b) shows IR reflow Time vs Temperature graph. Flue gases Board in

Cooling fan Board out

Conveyor

Stop timer Heaters

Last heating section

Heating zone (a) First heating

Subsequent

heating

Reflow

Cooling

230°

Temp (°C)

Solder paste melts

Solder paste solidfies

180°

125°

22°

60

180

300 (b)

Fig. 8.11

400

Time (s)

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The machine hardens the solder paste and also reflows it. The machine is useful for larger boards for large scale production. The heaters are installed on the upper and lower areas to provide uniform heating. The infrared sources are quartz lamps, whose range of temperature is 400°– 1000°C. An Infrared reflow machine is a furnace, which is used to melt the solder paste and to make soldering of the components. The machine has a maximum temperature of 300°C, which is divided into different heating zones (called preheating and reflow zones). It is to be remembered that the melting temperature of the solder paste

is about 185°C.

The sequence of steps taking place in an IR furnace is:

(1) The board is received from the pick and place machine (2) Soldering paste is hardened in the first heating section (3) The board is preheated in the second heating section (4) The board is kept hot in the next heating section (5) The solder paste is reflowed in the last heating section (6) The board is cooled down by the cooling fan (7) The board is taken out from the furnace (c) Wave Soldering Machine This machine uses two solder waves and each wave has specific properties and function.

The first wave is turbulent and narrow with great pressure. It penetrates

between the components such that all pads are sufficient wet with the

solder.

The second wave eliminates “solder bridges” formed by the first wave.

In this way, the second wave cleans and forms the “fillet”. The Fig.

8.12 shows a line diagram of the dual wave soldering machine. C

ts

en

on

p om

° wa ve I

45

ve II

wa

Fig. 8.12

8.8 SMT Measurements

161

The dual wave soldering allows double sided soldering to be performed, as it is possible to pass both sides of the board through the soldering process. Figure 8.13 (a) shows hot air convection currents and Fig (b) shows how the nozzle penetrates deep on the components on both sides of the board. Air discharge

Diffuser box

Manifold Air in-take

Perforated emitter panel Panel heaters (a) Wave nozzle I

Component Board

Wave nozzle II (b)

Fig. 8.13

8.8 SMT MEASUREMENTS The SMT is growing fast due do its several merits as explained already. But the technology demands ‘zero defect’ manufacturing and therefore it needs rigorous controls for which every accurate measurements are very essential in respect of: (1) Solder paste (2) Print thickness

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(3) Screen printing of paste (4) Solderability (5) Temperature (6) Ionic contaminations on the soldered boards (1) Solder paste: The following measurements are required: (a) Composition: The most commonly used composition of solder is 62 per cent tin, 36 per cent lead and 2 per cent silver with a melting point of 185°C. This is atomized in an inert atmosphere as fine balls of 25 to 45 microns. Then it is powdered and mixed with a flux to make the paste. This paste can be stored in a fridge for about a year. (b) Contamination of the paste: As per specifications, the various acceptable contaminants and their percentage in the paste is given below: (i) Antimony

0.63 percent

(ii) Copper

0.32 pre cent

(iii)

0.25 per cent

Bismuth

(iv) Gold

0.22 per cent

(v) silver

0.12 per cents

The other contaminants which may be present in the paste are:

Aluminium, cadmium, iron, zinc and nickel.

If the percentage of contaminants is more, the reliability of the paste is

badly affected. (c) Adhesiveness of the paste: The solder paste should have sufficient adhesiveness (tackiness) to keep the components in their place on the board till soldering operation is over. Hence correct measurements of its tackiness is also essential. (d) Viscosity: The viscosity of the solder paste is desirable for ‘printing’ of the paste. It should have sufficient viscosity which is measured by a viscometer. If viscosity is low, ‘smearing’ will occur and if it is high, there will be ‘skips’ during printing. (2) Print thickness: Generally a print thickness of 200-240 microns is acceptable. If the thickness is less, it will result in insufficient solder and if it more, it will result in excess solder and sometimes "short circuit".

8.9 Terms Used In SMT Process

163

(3) Screen printing of paste: Following are the factors which will decide the level of acceptable printing of the paste. (a)

Squeegee pressure: The squeegee pressure should be about 26 lb/in2 for stencil and 55 lb/in2 for the screening. If the pressure is low it will result in ‘skips’ and if it is high, it will cause ‘smears’.

(b)

Print speed: A print speed of 5-25 cm/s is preferred. If the speed is low, it leads to ‘smears’ and if it is high, it causes ‘skips’.

(c) Mesh-screen: The size and tension of the screen should be as per requirement. (4) Solderability: Solderability means that the components and the boards should have an ability to get ‘wetted’ by the solder. More than 75 per cent soldering problems are due to poor wetting. (5) Temperature: The soldering temperature is very important factor to be controlled for getting good results. Uniform heating of all the points of the board should also be insured, for which thermocouples may be used. (6) Ionic contaminations on the soldered board: Ionic contaminates present on the soldered board may cause failure of the assembly. The ionic contaminations may be present due to soldering flux, solder paste and even by human hands and perspirations. As per standards, these contaminations should not be more that 15 mg/in2 of the board.

8.9 TERMS USED IN SMT PROCESS The important SMT terms are defined below: (1) Rheology: The study of flow properties of viscous materials is called Rheology. (2) Thixotropy: The quality of certain materials which are paste like at rest, but fluid like when under stress is termed as Thixotropy. (3) Slump: The ability of viscous materials to spread is called Slump. (4) Tackiness: The ability of viscous materials to make contacts is called Tackiness. (5) Viscosity: The measure of a material resistance against flow or change of shape is called its viscosity. It is defined as the ratio of shear stress to the shear rate. The widely used unit of viscosity is kilo centipoise (Kc P).

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SUMMARY 1. In surface mount boards, the components are soldered onto the surface itself. There is no need to drill holes in the board, and the components may have a smaller lead, or it may have no lead at all. 2. The SMT boards have smaller area and can be used at giga hertz frequencies. 3. The SMT boards demand high precision and highly skilled personnel. 4. The SMT equipment include CAD facility, screen printing machine, soldering machine etc. qqq

9 Electronic Hardware Components In this chapter we shall study switches, relays, connectors and computer hardware components.

9.1 SWITCHES A switch is a device to make, break or to change connections in an electric circuit. All the switches therefore, have fixed and moving contacts. The moving contact moves and makes or breaks the connection. The switches may be (a) Manual Switches (b) Sensing Switches (c) Electrically operated switches (or relays).

(a) Manual Switches Their operation is all manual. Few examples of these switches are. 1. Push Buttons : These are momentary contact switches such that they make contact and operate a circuit so long remain pressed. They have a spring action. They find application in alarm and motor circuits. These may be (i) Normally opened (NO) type Fig. 9.1 (a): They are open in normal condition, but when operated they get closed and make the circuit. (ii) Normally closed (NC) type Fig. 9.1(b): They are closed in normal condition, but when operated they become open and break the circuit. They are always “break before make” type. The Fig. 9.1(c) shows outlook of the push buttons.

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Or Or (a) NO type

(b) NC type

(c) Out look

Fig. 9.1

2. Toggle Switch: This switch is used for making ON/Off a supply. The switch has a moving contact and has lever operation. The switch may be locking or non locking type. They may have 2 or 3 positions. They are also “break before make” type. They can be of following types: (i) Single Pole single throw (SPST)

Fig. 9.2 (a)

(ii) Single Pole double throw (SPDT)

Fig. 9.2 (b)

( )

Douole Pole double throw (DPDT)

Fig. 9.2 (c)

The Fig. 9.2 (d) shows a toggle’s outlook.

The toggle switches are also available

with centre off position.

Note: The pole means moving contact

and throw means “ways”. An DPDT

switch means, it has 2 contacts and 2

ways and it can make or break “one by

one” or both simultaneously.

(a)

(b)

(c)

(d) Fig. 9.2

9.1 Switches

167

The Fig. 9.3 shows internal construction of a toggle switch.

Handle

Terminals Fig. 9.3

3. Rocker switch : The Fig. 9.4 (a) shows outside and (b) inside view of a rocker switch. In these switches, actuating button is pivoted such that it rocks to either of the two terminals. (positions). It has also a neutral (mid) terminal position. The Fig. 9.4 (c) shows outlook of rocker switches.

(a)

(b)

Terminal

Neutral Terminal

Terminal Terminals

(c) Fig. 9.4

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4. Rotary switches : Rotary switches are available with many poles and many positions. Both shorting (make before break) and non shorting (break before make) types are available, moreover, both types can be put in a single unit. The shorting type is useful to prevent any open circuit between switch positions and the non-shorting types are useful, when the separate lines being switched to one common line but must never be connected to each other. The Fig. 9.5 shows a break before make (non-shorting) type rotary switch. If rotary arm is at A, it means contact A is closed and all other contacts being open. If the arm is rotated to B, contact B will be closed and all other contacts will open. Note that rotary switches can also be used as selector switch. D

C A

Rotary arm B

Fig. 9.5

5. Slide switches: These switches have a sliding arm which can be moved and number of contacts can be operated simultaneously. They may be single or gang type. The Fig. 9.6 (a) shows construction and (b) shows its operation. Sliding arm

Fixed contacts (a)

(b)

Fig. 9.6

9.1 Switches

169

(b) Sensing Switches/Sensers These are operated neither manually nor electrically. Their operation is usually automatic by sensing. The examples of sensing switches are float switches, limit switches, proximity switches etc.

(a)

(b)

Fig. 9.7

For example, if an overhead tank is to be filled with water by a motor, we can use a float switch or a limit switch, so that the motor is automatically switched On and Off, when the tank is empty and full respectively.

The Fig. 9.7(a) shows a normally open (NO) and (b) a normally closed (NC)

sensor switch.

• Snap switches : These switches are used as a limit switch or safety switch etc. The Fig. 9.8 (a) and (b) shows a snap switch. Moving contact

Blade Fixed contacts

(a)

(b)

Fig. 9.8

It has a blade of beryllium copper with a spring. It operates, when tension is created on one edge of the blade. According to the tension, the moving contact of the blade moves towards upper or lower fixed contacts. They are available from 2 to 10 A and 100 to 600 V with snapping time of 5 ms. These are also called micro switches and are used in digital circuits.

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(c) Electrically Operated Switches (Or Relays) A relay is a device that opens or closes a circuit, It is also a type of switch with specific working conditions. Usually a relay is a part of an auxilliary circuit (and not of the main circuit). A relay is like a switch that operates electrically and also sometimes

automatically.

Types of Relays: They are of following types:

1. Electromagnetic relay: The fig. 9.9 shows an electromagnetic relay. It has a coil and a plunger (shaft) on which number of make (normally opened) and break (normally closed) contacts are provided. When the coil gets supply, it becomes a magnet and pulls the plunger, as a result NO contacts are closed to “make” a circuit and simultaneously NC contacts are opened, thus breaking the another circuit. Soft Iron piece

Plunger (shaft)

NC

NO Contacts

Coil

Magnet

Fig. 9.9

2. Latching relays: They are like electromagnetic relays discussed above. The only difference is that these do not need continued excitation to keep in ON or OFF position, where as the above relay needs continuous flow of current through its coil to .keep the relay excited. The Fig. 9.10 (a) shows its construction and (b) shows its symbol.

These are used in alarm and motor control circuits.

In this relay, contacts remain in the last energised position even

after removing the signal. They can be released to normal position

electrically or mechanically.

9.1 Switches

171

Spring

Armature

Coil Spring

Coil Armature

(a)

(b)

Fig. 9.10

The thyristors such as silicon controlled rectifiers (SCRs) act as automatic latching relay, 3. Solenoid relay: These relays employ a solenoid and can handle large currents and powers. These involve large movement of the armature. They are used in cassette recorders. 4. Dry reed relay: If a coil is wound on a glass tube, it gives a reed relay. It is a very small and compact relay. When a direct current flows through the coil, the contacts may open or close.

The Fig. 9.11 shows a dry reed relay.

Glass tube

Contacts

Coil

Fig. 9.11

These relay are available in open frame or in dual in line package

(DIP). The whole assembly forms a very compact magnet.

These relays are very susceptible to the external noise, hence they need

a magnetic shield or a metal casing.

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5. Wet reed relay (Fig. 9.12): These relays are like dry reed relays except that these are not dry but in the glass tube there is some mercury. This makes the relay more efficient and can be used at large power requirements. Moreover, mercury reduces the contact resistance and provides noise free operation. However, they are to keep always in vertical position for their proper functioning.

Filled with Mercury Contacts Coil

Fig. 9.12

Advantages : The wet reed relays have the following advantages : 1. Mercury allows higher voltage and current rating. 2. They have clean make and break contacts. 3. They have noise free operation. Disadvantages: These relays have the following disadvantages : 1. They must be placed in vertical position for operation. 2. They cannot function at low temperatures. 6. Stepping relay (Fig. 9.13): It is like a selector switch. One or more input circuits can be connected with one output circuit with the help of a wiper. In the relay, there are many contacts and the wiper steps from one to another and connects any of the contacts, when the relay coil gets energised. Ratchet

Coil Wiper

Armature

Fig. 9.13

Contacts

9.1 Switches

173

7. Thermal Relay Commercially this is called “thermostat” and is used in geysers, refrigerators, A.C.s etc. to control the temperature. This is made of two strips of different thermal expansion coefficient. (Fig. 9.14). Fixed strip + Contacts Movable strip

–

Fig. 9.14

When temperature rises beyond the limit, the movable strip expands and opens the circuit. This is a type of temperature sensing switch. A Anode 8. Electronic or solid state automatic relays: The devices like diode, transistor, silicon control rectifier (SCR), triac, diac etc. can SCR act as relays. The SCR (Fig. 9.15) can handle hundreds of amperes and its operation is like a “latch relay” described above. This is Gate Cathode the reason that SCR is sometimes called as a G K “latch”. Fig. 9.15

The electronic relays are used for speed control of motors, fans etc.

Table 9.1 Comparison between Electronic and Mechanical Switches S. No.

Particulars

Electronic (Solid State) Switches

Mechanical Switches/ Relays

1.

Moving Parts

No Moving parts hence less wear and tear, noise less operation, needs little maintenance and operation without sparking.

It has moving parts hence more wear and tear, the operation is noisy and with sparking. They need more maintenance.

2.

Service

Trouble free service

Frequent breakdown

3.

Size and weight Small and light weight

Large and bulky

4.

Cost

Cheap

Costly

5.

Life

Unlimited

Very small

174

Chapter 9 S. No.

Particulars

6.

Speed

7.

Voltage power

Electronic Hardware Components

Electronic (Solid State) Switches 108 operations second

Mechanical Switches/ Relays

per 5 operations per second

and Can handle small Can handle large voltages and power (5 A voltages and power at 50 V) (several amperes at several kilo volts)

9.2 CONNECTORS A connector is an essential but perhaps the most unreliable component of any electronic equipment. The connectors help in: (a) maintaining connectivity and continuity. (b) bringing signals in and out of an instrument. (c) routing signal and dc power between various components of an instrument. (d) providing flexibility by permitting circuit boards of the instrument to be unplugged and replaced. Connectors have been one of the major causes for poor reliability of electronic equipment. It is not because the connector quality is bad but because not enough thought is given in the selection and application of these components.

1. Types of Connectors (a) The connectors are of the following types. 1. Commercial Connectors: These connector simply act as a device for maintaining electrical continuity. A less sophisticated connector may be used. 2. Industrial Connectors: They can perform in more rugged and detrimental environments including thermal shock, vibration, sand and dust. 3. Military Connectors: They are extremely reliable and can withstand extreme environmental conditions. 4. Hermetically Sealed Connectors: These are used in chemical plants, refrigeration units, underwater and also in aerospace. 5. Rectangular Connectors: Some of the examples are: heavy duty rectangular, miniature rectangular, environment resistant, rack and panel coaxial, jones connectors, mini-Jones connectors, D subminiature series connector, blue ribbon connector, micro ribbon connectors etc.

9.2 Connectors

175

6. Circular Connectors: Wall mounting receptacles, cable receptacles, box mounting receptacles are some of the circular connectors. They with 104 contacts are available. 7. Edge Board Connectors: A wide range of performance capabilities can be achieved by proper selection of contact design. 8. Two Piece Connectors: Here the plug is soldered to the board and the matching part(the receptacle) is mounted on a chassis or on another board. Pin and socket is the oldest design but blade and punched fork (and hermaphroditic forks) are also used. Their pin misalignment causes the maximum failures. 9. Coaxial Board Connectors: These are used for maximum signal isolation between boards. 10. Zero Insertion Force (ZIF) Connectors: This is used to provide maximum force during operating mode and minimum force for insertion and removal. Cam devices are used which open and close a split receptacle body. Backplane connectors and surface mount connectors are the other types available. The IC testers usually use ZIF connectors. (b) The important connectors used at radio frequencies (RF) are described below. 1. BNC connector: The bayonet coupling connectors (BNC) are quick disconnect type connectors. They are found at the front panel of electronic instruments and aerospace equipment: They have normal impedance of 50 ohms and working voltage of 500 V. 2. TNC (Threaded Neill Concelman) Connectors: These are like BNC connector but with a threaded coupling. They are suited for air craft and missile applications. They have 50 ohm impedance and 10 GHz frequency range. 3. UHF connectors: They can be used at ultra high frequencies (UHF) upto 300 MHz. They are suitable for radar, television and other such applications. 4. SMA (Sub Miniature-version–A) connectors: They give best performance at frequency of 20 GHz and that too with low VSWR (Voltage Standing Wave Ratio). They have an impedence of 50 ohm and a working voltage of 1000 V. They are used in radars and micro communication.

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2. Important Characteristics of Connectors Few important desirable characteristics of connectors are given below: (i) Contact Resistance: This is tested by millivolt tester and should not be more than 0.001 ohms. There are many factors both mechanical and electrical that affect the contact resistance. (ii) Breakdown Voltage: This depends on the contact spacing, geometry, shell spacing and seal materials. (iii) Insolation Resistance: This is a measure of the leakage current that flows not only through the material but also along its surface and can be measured to see if the connector is functioning properly. If the input to a circuit is not adequate, the current may be leaking due to metallic chip, moisture etc. (iv) Current rating: Each of the connector pin is rated for a specific current. The total current handled by the connector equals the sum of currents carried by the pins. (v) Durability: This specifies the number of insertions and withdrawals, or the number of mating cycles : typical being greater than or equal to 100 cycles. Table 9.2

Various types of connectors

S. No.

Types/Application

1.

Printed Circuit Standard Size Miniature Connectors

2.

Coaxial Connectors

3.

Triaxial Connectors

4.

Cabling and Harness Connectors

5.

Aircraft Cabling and Harness Connectors

Sketch of the Connector

9.3 Computer Hardware Components 6.

Missile Cabling and Harness Connectors

7.

Replaceable Assemblies Connectors

8.

Modular Circuits Connectors

9.

Chassis Plug-in Connectors

10.

Chassis Connectors

177

9.3 COMPUTER HARDWARE COMPONENTS Hardware are those parts of computer which can be physically touched and

seen.

All the computer hardware components work together to complete a task. The

monitor, keyboard, printer, hard disk, mouse etc. all are attached to each other

through the CPU (central processing unit) using cables.

Hardware components are classified into three categories: (a) Input Devices (b) Output Devices (c) Storage Devices

(a) Input Devices All the input devices are used to provide data to the computer. Common input devices are: 1. Keyboard (Fig. 9.16) The keyboard is the most

commonly used input

device. It has many buttons

which are called keys.

Fig. 9.16

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Different types of keys present on the keyboard are: (Fig. 9.17) •

Alphabetical keys Q

W A

E S

Z

R D

X

T F

C

Y G

V

U H

I J

O K

B

N

M

6

7

8

P L

(a) •

Numeric keys 1

2

3

4

5

9

10

(b) Enter •

Enter key

•

Backspace key

Backspace

(c) •

Delete key

(d) •

Delete

CAPS lock Key

(e)

•

Caps Lock

A

(f)

Arrow keys

•

Space bar

(g)

Space bar (h)

Fig. 9.17

2. Mouse (Fig. 9.18) A mouse is a pointing device. It has two buttons and a scroll wheel. The mouse pointer can be

moved in order to select an item on the screen.

Fig. 9.18

3. Microphone (Fig. 9.19) A microphone is an input device which is used for

recording sounds.

4. Web camera (Fig. 9.20) A web camera is also known as a webcam. It is

used to capture images and store them digitally

on the storage devices such as pen drive, hard

disk, CD etc. It is also used for live chats or video

conferencing.

Fig. 9.19

Fig. 9.20

9.3 Computer Hardware Components

179

5. Lightpen (Fig. 9.21) A light pen is an input device which allows the

user to point to words, displayed objects or to

draw on the screen.

Fig. 9.21

6. Scanner (Fig. 9.22)

A scanner is somewhat like a photocopier

machine. While the photocopier makes a copy

of the document or image and prints it out on

a sheet of paper, the scanner saves the copy as a digital image on the computer. Fig. 9.22 7. Joystick (Fig. 9.23)

Joystick is often used to play video games and has one

push-botton on it along with a handle which can be

turned to control the game.

Fig. 9.23 (b) Output Device Output devices receive the result or the matter from the computer to convey to the user. Common output devices are:

1. Monitor (Fig. 9.24) The Monitor is also called the Visual Display Unit (VDU). It displays the output which can be text or pictures. You can also watch movies on a monitor. Fig. 9.24 2. Printer A printer helps us to get our work on paper. Printouts are the printed version of material on the computer which can be a text or an image. There are two types of printers: • Impact Printers In these printers, the printing mechanism uses tiny hammers or pins. These pins (or hammers) strike an inked ribbon. The sheet of paper is placed right after the ribbon. When the pins hit the ribbon it leaves a mark on the sheet of paper. Characters and images are formed by carefully controlling this process. Following are some examples of impact printers: (Fig. 9.25)

(a) Dot Matrix Printer

(b) Drum Printer

Fig. 9.25

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(d) Chain Printer (c) Line Printer

•

Fig. 9.25

Non Impact Printers In these printers, there is no contact with the paper. They are the most popular printers being used in schools, office etc. Some non-impact printers are: • Inkjet printer (Fig. 9.26 (a)) • Laser printer (Fig. 9.26 (b))

(a)

(b)

Fig. 9.26

3. Speakers (Fig. 9.27) Speakers let you hear music and other sounds stored on the computer.

SUMMARY

Fig. 9.27

1. A switch is a device, that makes/breaks a circuit. 2. Switch may be (a)

Manually operated switches: e.g. Push buttons, toggle switches, rocker switches, slide switches, rotary switches etc.

(b)

Sensing switches: e.g. snap switches, proximity switches etc. they work on mechanical effect, pressure etc.

(c) Electrically operated switches: e.g. relays, which work on electric effect.

The relays may be:

(i) Monostable: which remain energised so long they get electric supply.

Summary

181

(ii) Bistable: Which remain energised even if electric supply once given to them is switched off. 3. Mercury wetted relays should always be operated in vertical position. 4. Relays consisting of electronic components are called solid state relays. 5. Connectors may be : one piece edge connector, two piece edge connector, co-axial connectors etc. 6. Connectors may also be screwed type or unscrewed type. 7. The computer hardware components are: key board, mouse, printer etc. qqq

10 Multimeter & CRO A multimeter is a basic instrument, which is used for measuring Amperes, Volts and Ohms and so sometimes called as AVO meter. A CRO (cathode ray oscilloscope) is a versatile instrument, which is used for many purposes but it is mainly used for viewing wave shapes.

10.1 MULTIMETER Multimeter can measure more than one quantities hence the name. It can measure currents (in amperes), voltage (in volts) and resistance (in ohms). It can also be used to find continuity in a circuit and for other testing purposes. • Types of Multimeters The multimeters may be 1.

Analog multimeter

2.

Digital multimeter.

1. Analog Multimeter An analog multimeter has two selectors on its face panel. 1. Function selector: It is a switch to select the function to be measured, i.e., voltage (D.C./A.C.), current (D.C./A.C.) and resistance. 2. Range selector: It is a switch to select the approximate range of the function under measurement. (a)

Construction (Fig. 10.1) An analog multimeter basically is a permanent magnet moving coil D′ Arsonval meter. It has a coil wound on an aluminium former which can freely move in the field of a magnet. With the coil, an aluminium pointer is attached which moves on a calibrated scale. (See Fig. 10.1)

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Scale

a

0 Pointer

N

S Coil

Permanent magnet

Iron core

Fig. 10.1

The coil is mounted on a fixed iron core, which makes the magnetic field of the permanent magnet as radial within the air gap, in which the coil is to move. This ensures a uniform magnetic field. (b)

(c)

Working When the coil rotates, the pointer moves on the scale. The pointer stops at a position where defecting torque and controlling torque become equal. The measurement is taken on the particular scale according to the function selected. Measurements with Analog Multimeter 1. Measurement of direct currents When the multimeter is to be used to measure direct currents, the function selector is turned on to current (D.C.). The multimeter can be used to measure any range of current (in amperes) with the help of universal shunt AB (See Fig. 10.2) provided within the instrument. In other words, a number of low resistors are connected in parallel with the meter through a range selector switch. The required range can be selected by moving the selector switch on to a particular position. Actually by doing this, a ‘designed value’ of shunt is automatically connected in parallel to the meter. 2. Measurement of direct voltages A multimeter contains universal multiplier AB within the instrument, i.e., a number of high resistors are connected in series with the meter (Fig. 10.3). By setting the range selecter at the particular position, a particular value of the multiplier is automatically connected in series with the meter.

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185

When the multimeter is used to measure d.c. voltages, the ‘function selector’ is set at ‘voltage’ (D.C.) and the range selector is set at the particular range. Meter A

Meter

r1 A

1m

r3 r2 r4 10 mA 100 mA 50 0m A

B

r1 10 V r2 50 V r3

Universal multiplier

100 V r4 B 500 V

Range selector +

–

–

+

Fig. 10.2

Fig. 10.3

3. Measurement of alternating currents and voltages The multimeter can also measure alternating currents and voltages. The method is same as explained above with the difference that the A.C. supply to be measured is given to the meter through a rectifier circuit (See Fig. 10.4). For this purpose, a full-wave bridge rectifier is provided inside the multimeter. The function selector is to be set on current (A.C.) or voltage (A.C.) respectively. The range is also to be selected accordingly.

AC supply to be measured

A

Universal shunt

Range selector

Bridge rectifier

To meter

Fig. 10.4

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4. Measurement of resistance A multimeter can also measure a resistance. We should remember, that a resistor eats power; for this purpose a battery is provided with the multimeter. This battery is connected in series with the meter through different values of control resistances, which controls the value of current through the meter within the desired range (See Fig. 10.5). The multimeter, when out of circuit should read zero. So, before every measurement the pointer is set at zero by varying a variable resistor called ‘zero adjuster’. It is worthwhile to mention that actually the meter reads the value of current through the resistor but the scale is calibrated in terms of ‘ohms’. Control resistors

Range selector Zero adj.

Meter

R

A

Resistance to be measured

Multimeter battery B

Fig. 10.5

Before measuring a resistance, the range selector is set at a particular range, this connects one of the control resistors in the circuit. Now connecting the resistance (R) to be measured across AB, the meter gives the value of the R directly in “ohms” on its scale.

2. Digital Multimeter (DMM) A digital multimeter in its construction employs digital components, among them, “Analog to Digital Converter” (ADC) and “Digital to Analog Converter” (DAC) are main components. (a) Advantages of digital instruments 1. They display directly into decimal, so human errors are eliminated. 2. The reading may be taken to any number of significant figures by shifting the decimal point, thus it enhances the accuracy.

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187

3. The digital output of the digital instrument can be fed into computer memories, or into any data storage device. 4. The power consumption of the digital instruments is negligible. (b) Disadvantages of digital Instruments 1. The digital instruments are very costly. 2. The digital instruments are very complex in construction and their repair if not impossible, is very costly. Table: 10.1 Digital vs Analog Instruments S.No. Digital Instruments Analog Instruments 1. These instruments display a quantity These display a quantity in terms of in decimal number. deflection of a pointer on a calibrated scale. 2. They have much greater accuracy. They have comparatively poor accuracy. 3. Their resolution is very high. They resolution is comparatively poor. 4. They consume negligible power They consume larger power. during measurement. 5. They do not load the circuit. They load the circuit under measurement. 6. They are of complex construction. They are simple in construction. 7. They are affected greatly by They are little affected. environment. 8. They are free from human errors. They suffer from parallax and other human errors.

(c) Digital Instrument terms (i) Resolution (r): It is defined as number of digits used in a digital instrument e.g., a three digit display on a digital instrument for 0.1 volt range will be able to show from 0 to 999 mV with smallest resolution of 1 mV. But in practice, a fourth digit capable of indicating either 0 or 1 only is placed to the left of the active digits. This permits going above 999 to 1999 to give overlap between ranges for convenience. This is known as “over ranging”. This type of display is called a three and half i.e., 31 2 display. If x is the number of full digits, Resolution, r = 1/10x. (ii) Sensitivity (s): This is defined as the smallest change in the input signal, which can be detected by a digital instrument. It is denoted by s = v × r Where, v = lowest full scale value and r = resolution of the meter.

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(d) Basic digital meter. All digit instruments have “Analog to Digital converter” (ADC) at the input. The signal obtained in analog form is processed. At output side we have “Digital to Analog converter” (DAC). Analog display is obtained at the final output. (Fig. 10.6) analog Input

ADC

Signal processer

DAC

analog output

Fig. 10.6

(e) Construction and Measurements by DMM The DMM is basically a digital voltmeter (DVM). All quantities other than voltage are first converted into equivalent d.c. voltage (Fig. 10.7) As compared to analog MM, DMM gives reading in decimal display, hence human errors are eliminated. It can measure resistance, a.c. voltage and current, d.c. voltage and current. 1. Measurement of resistance: The resistance is measured by measuring voltage across resistance under measurement. The resistance to the measured is connected externally. This voltage results from current flow through the resistance from an internal current source. The accuracy of the measurement depends upon accuracy of the current source. The efficiency is of the order of 0.1 percent. Ohm

Current source

AC(V) AC(mA) Function selector

DC(mA) DC(V)

Buffer AMP Attenuator

F.W. Rectifier

Shunt A/D converter

Display

Shunt Attenuator

Fig. 10.7

(i) For a.c. voltage measurement the voltage is given to an attenuator then to rectifier. The output d.c. voltage is passed through AD converter. The voltage is displayed on the output. (ii) For alternating current measurement, the ac is first rectified, and given to the converter and is displayed on the monitor.

10.2 Cathode Rays Oscilloscope (CRO)

189

(iii) For direct current measurement the voltage drop across a suitable shunt (which gets connected across the meter) is measured, converted through A/D converter and displayed. (iv) For d.c. voltage measurement, it is given to AD converter through attenuator. The output of the converter is displayed on the monitor of the MM. The accuracy of a.c. measurement is poor due to a.c. –d.c. rectification. The meter gives r.m.s. value of a.c. currents and voltages.

10.2 CATHODE RAYS OSCILLOSCOPE (CRO) Cathode Ray Oscilloscope (CRO) is a versatile electronic instrument which performs multifarious jobs. The main function being display of waveshapes. The CRO can show what is happening inside a circuit. So, it is an “electronic eye”.

1. Construction of a CRT. Cathode Ray Tube (CRT) see Fig. 10.8 (a) is the heart of a CRO. The Fig. 10.8 (b) shows its vertical and horizontal deflection plates. 1

10 2

3 4 5 6

7

8

8 9

9

12 11

(a)

(b)

Fig. 10.8

It has the following important parts. 1. Evacuated glass tube: The evacuated glass tube contains the different parts of the CRT. 2. Electron gun: T his is tungsten coated cathode, which emits high velocity electrons. It is indirectly heated by 6.3 V a.c. supply. 3. Grid: This is placed at negative potential and controls flow of electrons. 4. Pre accelerating anode: It accelerates the beam. 5. Accelerating anode: It accelerates the beam further. 6. Focussing anode: It focusses the beam. 7. Final anode: It finally accelerates the beam towards the screen.

8. Vertical deflection plates: These provide vertical deflection to the electron beam.

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9. Horizontal deflection plates: These provide horizontal deflection to the electron beam. 10. Aquadag coating: This is graphite coating at the neck of the tube. This protects the tube against high negative charge. 11. Screen: This is the surface at which the beam is focussed finally. 12. Phosphor coating: The screen is coated with a phosphor material. When light falls on it, it produces luminance.

Note: This CRT is known as mono accelerator tube as it has no acceleration system after deflection. This can be used below 10 MHz. Above 10 MHz, this tube can not be used.

2. Block Diagram of CRO The CRT and the associated circuitry forms the CRO. The fig. [10.9 (a) & (b)] show the block diagrams. 1. CRT: The cathode ray tube is the heart of a CRO. Its cathode emits beam of electrons which is accelerated, focussed and deflected vertically as well as horizontally and finally strikes the screen; as an electron spot. 2. Horizontal and vertical amplifiers: The deflection plates require about one thousand volts for producting horizontal and vertical deflections. The signal is in mV, so also requires suitable amplification. The horizontal and vertical amplifiers are used for the purpose. 3. Attenuators: The attenuators are used to control gain of horizontal and vertical amplifiers. Sync. gen Vert

Input

Attenuator

Cathode Grid

Delay Ckt.

V.A. Acc Anode

Focus Anode

Horz def plates

Aquadag coating

Screen Vert Final Acc. Anode def plates Hor. Input

Attenuator Time Base Gen

HA Blanking circuit Fig. 10.9 (a)

10.2 Cathode Rays Oscilloscope (CRO) Vertical trigger

External trigger

191

Vertical amplifier

CRT

Sweep trigger

Sweep generator

Horizontal amplifier

A.C. line signal Horizontal input

Fig. 10.9 (b)

4. Supplies: The L.T. supply (6.3 V) is required for the cathode, whereas H.T. supply (+2000 V) is required for the anodes.

Final anode is kept at ground potential

5. Synchronising generator: This generates sync. voltage for synchronizing the sweep voltage and the signal. The sync. voltage may be provided internally or externally. If the signal is of 1 kHz, the sweep frequency will be also 1 kHz and so on. 6. Time base generator: It generates time base voltage for horizontal deflection plates (after amplified in horizontal amplifier). 7. Delay circuit: This is used in vertical amplifiers to delay the vertical signal, so that horizontal signal reaches prior to vertical signal. This enables to observe the leading edge of the signal waveform.

3. Various Functions/Applications of CRO 1.

The most important function of CRO is to display wave shapes.

2.

It can measure voltage, current, frequency of a supply.

3.

It can measure phase difference of two supplies.

4.

It is used in testing radio, T.V. and other electronic equipment.

5.

It is used for calibration of instruments.

6.

For testing of components.

7.

Measurement of modulation.

8.

Study of lissajous figures.

9.

Tracing of hysteresis loop for a magnetic material.

10.

Examination of wave shapes etc.

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Here we will discuss important functions: (a) Display of signal on a CRO For displaying a signal, the signal is given to vertical plates (a, a) through vertical amplifier. A sweep signal is given to horizontal plates (b, b) through horizontal amplifier (Fig. 10.10). (a)

Vert. AMP

(b) (b)

Signal

CRO Screen

Hor. AMP

(a)

Sweep signal

Fig. 10.10

By a synchronising circuit, the frequency of the signal and of the sweep voltage is kept equal. Otherwise we will get a distorted waveshape on the CRO. (i) When a sinusoidal signal to be observed is given to vertical plates, the spot on the screen feels movement up and down periodically depending upon the frequency of the signal as the plates get positive and negative alternately (Fig. 10.11). a 2 1

2 3 5

1,3,5 4

4

a

Fig. 10.11

(ii) When the sweep (saw tooth) signal is given to horizontal plates, the spot on the screen feels a movement on horizontal line left to right, right to left and so, the motion of the spot is uniform, as saw tooth is a linear curve (Fig. 10.12).

10.2 Cathode Rays Oscilloscope (CRO)

2

b

193

b

4 1,3,5

1

4

3

5

2

Sweep (saw tooth) signal

Fig. 10.12

(iii)

When both the above are applied simultaneously and properly synchronised, the wave shape is displayed on the screen (Fig. 10.13). a 2 1

3 5

2

b

1

3 5

b

4

4

a

1 2

4 3

5

Fig. 10.13

(b) Various Measurements on CRO The various measurements which can be done on CRO are as under: (i) Voltage measurement: To measure voltage, the knob volt/div on the CRO panel is set at a particular position. Consider the circuit in Fig. 10.14(a). The signal generator is used to produce a 1000 hertz sine wave. The AC voltmeter and leads to the vertical input of the oscilloscope are connected across the generator’s output. By adjusting the horizontal sweep time/cm and trigger knobs on the CRO panel, a steady trace of the sine wave may be displayed on the panel about the line of symmetry CD, which is proportional to the magnitude of the voltage at any instant of time.

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screen Voltmeter ac

Vm

C

Signal generator

D

Vpp

Vert. Input Oscilloscope (a)

(b)

Fig. 10.14

To determine the size of the voltage signal appearing at the output of terminals of the signal generator, an AC voltmeter is connected in parallel across these terminals (Fig. 10.14 (a)). This voltmeter is designed to read the “effective value” of the voltage. This effective value is also known as the “Root Mean Square” (RMS) value. The maximum voltage seen on the scope (CRO) face (Fig. 10.14 (b)) is Vm volts and is represented by the distance from the symmetry line CD to the maximum deflection. The relationship between the magnitude of the maximum voltage displayed on the scope and the effective or RMS voltage (VRMS) on the AC voltmeter is:

VRMS = 0.707 Vm (for a sine or cosine wave)

Thus

Vm =

VRMS 0.707

A compromise is expected between the voltage reading of the voltmeter and that of the oscilloscope. For a symmetrical (sine or cosine) wave, the value of Vm may be taken as 1/2 the peak to peak signal VPP. (Fig. 10.14 (b)) (ii) Frequency measurement: There are two methods. 1. When the CRO screen scale is calibrated for measurement of frequency, in this case the time per division knob is set at a mark. If time/div is set at 10 div. sec and waveform covers 4 divisions per cycle on X-axis (Fig. 10.15).

Y









0

1

2

3

Fig. 10.15

4

X

10.2 Cathode Rays Oscilloscope (CRO)

195

(i) Time period, T = 10 × 4 = 40 m sec (ii) Frequency of the input wave 1 1 106 f = = = = 2.5 kHz. T 40 × 10–6 40 2.

When the CRO screen is not calibrated, the frequency is measured by obtaining lissajous figures. The frequency to be measured is applied at vertical input and a known frequency is applied at horizontal input. A pattern is generated on screen. This pattern is called “lissajous pattern or figure”. The fig. 10.16 shows few lissajous patterns. Y(fV)

Y(fV) X(fH) X(fH) (b)

(a) Y

Y

Y(fV) X

X

X(fH) (d)

(c)

Fig. 10.16

The ratio of vertical frequency and horizontal frequency is called frequency ratio i.e., fV /fH. To vertical plates, the signal of unknown frequency (fV) is given, and a signal of known frequency (fH) is given to the horizontal plates from a variable frequency oscillator. The frequency of the oscillator is varied, till a suitable lissajous pattern is obtained. As the frequency of the oscillator (fH) is known, unknown frequency (fV) can be found, as frequency ratio (fV /fH) is also known.

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(iii)

Current measurement: For measurement of current (I), a known resistance (R) may be connected in the circuit. V I = [V is voltage drop across R] R (iv) Time period measurement: For this “ Time per div” on the CRO is set at a particular position say at 10 ms per div. Now the cycle covers total 20 div. on X axis (say). Time period = 10 × 20 = 200 ms (v)

Phase measurement (Fig. 10.17): For measuring phase angle between two signals of equal amplitude (V1 = V2) and equal frequency are applied at X and Y inputs and of the CRO, Y V2 b

a

X φ

V1 (a)

(b)

Fig. 10.17

An elliptical lissajous pattern is obtained on the screen (Fig. 10.17 b). The values a and b for the ellipse are measured on the screen and phase angle between two signals (V1 = V2) can be obtained as a sin φ = , φ = sin–1(a/b) b

10.3 STORAGE CROS The storage CROs are special CROs, capable to retain an image on the screen for longer time (10 – 150 hours). They have many applications such as storage of transients and of very low frequency signals. Two techniques are used to store signals, these are called: analog and digital storage. Analog storage is capable of higher speeds but it is less versatile than the digital technique. The analog storage technique uses the principle of secondary electron emission or “variable persistence” to build and store the signal on the surface of an insulated target.

10.3 Storage CROs

197

In digital technique, the waveform to be stored is digitized, stored in a digital memory and retrieved for the display. For this, conventional CRT can be employed. The display can be stored for an infinitie time. In a conventional CRT, the persistance of the phosphor varies from a few ms to several seconds, as a result, the persistence of the screen is smaller than the rate at which the signal sweeps across the screen. The “start” of the display would fade before the “end is written”. Accordingly, the storage CROs are of two types: (a) Analog storage CRO (b) Digital storage CRO (a) Analog storage CRO Analog storage CRO works on the principle of “secondary electron emission” to build up and store electrostatic charges on the surface of an insulated target. These CROs are used for : (i) for real time observation of events that occur only once. (ii) for displaying the waveform of a very low frequency signal. The construction of the special CRT using “variable persistence storage” technique, called half tone or mesh storage CRT is shown in Fig. 10.18. With the variable persistence, the slow swept trace can be stored or displayed continuously by adjusting persistence of the CRT screen to match the sweep time. Vertical def. plates

Collimation electrode

Flood gun Flood gun Writing gun

Horizontal def. plates

Collimation electrode

Collection mesh Face plate

Storage mesh

Fig. 10.18

Analog or mesh storage CRT contains a storage mesh, flood gun, and collimators; in addition to all the components of a conventional CRT. The storage mesh which is the storage target behind the phosphor screen is a conducting mesh covered with an insulating material such as magnesium fluoride (MgF2).

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The is a high energy electron gun similar to the conventional gun giving a narrow focussed beam which can be deflected and can be used to write information to be stored. The writing gun etches a +vely charged pattern on the storage mesh (target) by knocking off secondary electrons. In order to make the pattern visible even after several hours, the flood guns are switched ON, which emit low velocity electrons towards the screen. The electron path is adjusted by the collimation electrodes, which are biased so as to distribute the flood gun electrons evenly on the target surface. Most of the “flood gun electrons” are stopped/collected by the collector mesh, therefore never reach the screen.

The CRT display is the exact replica of the pattern which was initially

stored on the target and will be visible so long as the flood guns operate. For erasing the pattern, a negative charge is applied to neutralize the

stored positive charge.

For getting variable persistence, an “erase voltage” is applied in the

form of pulses instead of a steady dc voltage. By varying width of the

pulses, rate of erasing can be controlled.

An analog storage CRO can work on “secondary emission” as well as

on “variable persistence” principles.

•

Advantages of analog storage CRO. 1. This has a higher bandwidth and speed. 2. This can operate on “secondary emission” as well as on “variable persistence” modes.

•

Disadvantages of analog storage CRO. There are number of drawbacks of analog storage CRO, few are enlisted below: 1. There is a definite duration of time for which it can preserve the stored signal and then it is lost. 2. Power to the CRT must be ON, as long as the signal is to be stored. 3. The CRT used is a “special tube”. The trace is not fine as of a conventional CRT. 4. The “writing speed” of the storage tube is less than the conventional CRT but higher than a digital storage CRT.

10.3 Storage CROs

199

5. The storage tube is expensive as compared to the conventional CRT as the former needs additional power supplies. 6. Only one signal at a time can be stored. (b) Digital storage CRO. The best method of signal storage is the digital storage oscilloscope. In this technique, the waveform to be stored is digitized, stored in digital memory and is retrieved for display on the storage CRO. (Fig. 10.19) Data acquisition Buffer Input signal amplifier

Sample & hold circuit

Data display

ADC

Storage Memory

Vertical

Amp.

DAC

Vertical deflection plate

Address counter

CRT

Internal trigger External trigger

Time base counter

Control circuits

DAC

Horizontal deflection plate

Horizontal Amp.

Sample clock

Fig. 10.19

The input is amplified and attenuated with input amplifier as in any CRO. The data acquisition system contains a sample and hold (S/H) circuit and analog to digital converter (ADC) that repetitively samples and digitizes the input signal at the rate determined by sample clock. Then digital data is transmitted to memory for storage. The overall operation is controlled and synchronized by the control circuit, which usually has a microprocessor (executing a program stored in read only memory). The control circuits make sure that successive data points are stored in successive memory locations by continuously updating the memory’s ADDRESS counter. When memory is full, the next data from the ADC is stored in first memory location writing over the old data and so on. Here, the data acquisition and the storage process continues until the control circuits receive a trigger signal internally or externally. When triggering occurs or starts, the system stops acquiring data further and enters the display mode of operation.

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In display operation, two DACs (digital to analog converters) are used for providing the vertical and horizontal deflection voltage for CRT. The data from the memory produces the vertical deflection of electron beam; while the time base counter provides the horizontal deflection. The control synchronizes the display operation by incrementing the memory ADDRESS counter and time base counter at the same time so that each horizontal step of electron beam is accompanied by new data value from memory to the vertical DAC. The screen display consists of discrete dots representing the various data point but the number of dots are large (1000 or more). They blend together and appear to be a smooth continuous waveform. One very important property of digital storage CRO is its ability to provide a mode of operation called ‘pretrigger view’ (previous recording). This means that the oscilloscope can display what happened before a trigger input is applied. This mode of operation is very useful when a failure occurs and is marked by the appearance of signal. • Advantages of Digital storage CRO: 1. Its storage capability is very high as compared to analog storage CRO. 2. The stored signal can be displayed for an infinite time. • Disadvantages of Digital storage CRO: Its bandwidth is less and depends upon speed of analog to digital converter. (ADC) • Comparison between digital and analog storage oscilloscopes. The two have been compared below: 1. The digital storage oscilloscope has a CRT which is much cheaper than of an analog storage oscilloscope, as conventional CRT can be used in the digital CRO, so it becomes economical. 2. A digital storage oscilloscope is capable of an infinite storage time using its digital memory, where as in analog storage oscilloscope, the signal is lost after sometime. 3. Digital storage CRO gives a bright image at a fast speed as compared to the analog storage CRO. 4. As compared to analog storage CRO, a digital storage CRO is more accurate and stable, it also gives higher “resolution”. 5. A digital storage CRO can print out the stored information onto a hard copy (or disc storage) and becomes ready for another reading. 6. Digital storage CRO can not function in “variable persistence mode”, whereas an analog storage CRO can function on “secondary emission” as well as “variable persistence modes.

Summary

201

7. A digital storage CRO is capable of operating in a “look back” mode. An analog storage CRO collects data after it has been triggered, a digital storage CRO is always collecting data and the trigger tells it, when to stop. 8. An analog storage CRO has a higher bandwidth and higher writing speed than a digital storage CRO. An analog storage CRO is capable of operating at the speeds of 15 GHz/sec. The digital storage CRO has a lesser speed depending upon digitising capability of analog to digital converter (ADC). Problem 10.1: If in a CRT, the anode voltage is 3000 V, while parallel deflecting plates are 2 cm long kept 5 mm apart and the screen is 0.25 m from the centre of plates. Determine: Deflection sensitivity and deflection factor. Ans: l = 2 cm = 0.02 m d = 5 mm = 0.005 m S = 0.25 m Va = 3000 V l.S. (a) Deflection sensitivity = 2d.Va =

(b)

0.02 × 0.25

2 × 0.005 × 3000 = 0.1667 mm/v Ans. 1 Deflection factor = = 6 V/mm Ans. 0.1667

SUMMARY 1. A multimeter is a device which can measure more than one quantity. It can measure amperes, volts and ohms and so also called AVO meter. 2. The multimeter may be an “analog” type on a “digital” type. 3. Analog multimeter is basically a “moving coil permanent magnet” instrument while digital multimeter uses digital components for its contruction. 4. Cathode ray oscilloscope (CRO) is basically used to see waveshapes. 5. By CRO, we can measure voltage, frequency, phase etc .of a signal. 6. We have analog and digital storage scopes, which can “store” a signal to be viewed later on. qqq

11 Medical Electronics Almost all medical instruments work on electronic principles. In this chapter we describe basic human anatomy and physiology with common medical instrumentation.

11.1 ROLE OF ELECTRONICS IN MEDICINE The bio-medical instruments work on the principles of electronics. These instruments use electronic components/equipments : such as amplifiers, strip chart recorders, “analog to digital” and “digital to analog” converters,

computers, CROs, transducers etc.

Few medical instruments are enlisted below :

1. ECG (Electro Cardio gram) Machine: This is used to record electrical activity of human heart. It gives a graph called “Electro-cardio graph”. 2. Pace maker: This is a device to reproduce or regulate the rhythm (heart beat) of a slow or lazy human heart. 3. EEG (Electro Encephalo gram) Machine: This is used to record electrical activity of human brain. 4. MRI (Magnetic Resonance Imaging) Machine : This is a diagnostic machine to learn how brain (or any other part) of the human body is working in its normal, diseased or injured condition. 5. EMG (Electro Myo gram) Machine: This machine measures muscular strength of human body. 6. Blood Pressure Instrument (Sphygmo manometer): This instrument measures blood pressure in human body. The upper limit is called “systolic” and the lower limit is called “diastolic”.

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The normal blood pressure in a healthy person has been specified as : Less than 120 mm Hg : systolic pressure.

Less than 80 mm Hg : diastolic pressure.

7. Blood Sugar measuring Instrument: (Glyco-meter) This instrument measures blood sugar of a person. If its level is more than 160 mg%, the person is said to be a diabetic. 8. X-Ray equipment: The X-rays are electromagnetic radiations with a wave length in the range of 10 nanometers to 100 picometers. These rays can penetrate human body and give an image of the inside on a photographic plate or film. 9. Computer Tomography (CT) : Previously it was known as computed axial tomography (CAT). It is a medical imaging method employing tomography, where digital processing is used to generate a three dimensional image of an interior organ of human body. For accurate diagnose, MRI is preferred to X-ray or CT scan.

11.2 HUMAN CELL A human body is made up of numerous cells. A cell consists of following parts See Fig. 11.1. (i) Cell membrane: The cell is enclosed in a membrane. (ii) Nucleus and Nucleous: The nucleus is formed by a nuclear membrane and is the control center of the cell. The threads of chromatin in the nucleus contain Deoxyribo nucleic acid (DNA), which is the genetic material of the cell. Lysosome Cytoplasm Membrane Nucleus Mitochondiron

Fig. 11.1

(iii) Cytoplasm: Between membrane and the nucleus is Cytoplasm which is a gel like fluid. It is the medium for chemical reactions. All functions of human body are carried out in this fluid. In cytoplasm, the

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205

materials move by the process of diffusion. The little organs such as : Mitochondiron, Ribosomes, Vacuate, Lysosome, Endoplasmic, Golgi apparatus etc. are suspensions in the cytoplasm. The DNA in the nucleus directs protein synthesis in the cytoplasm. A gene is the portion of a DNA molecule, that controls the synthesis of one “specific” protein molecule. The proteins that are synthesised in the cytoplasm function as structural materials.

The RNA acts as a messenger and carries the genetic information from

DNA to the site of protein synthesis in the cytoplasm.

The nucleous is a dense region of Ribo nucleic acid (RNA).

The important bio medical terms are:

( ) Cell: This is the simplest unit of a human body. Its size may be from 1 mm to 100 mm. (ii)

Tissue: A tissue is an organization, i.e., made up of many similar cells.

(iii)

Organ: An organ is also an organization, i.e., made up of several different kinds of tissues, e.g. Heart, Brain, Stomach etc. are organs.

(iv) System: A system is also an organization i.e., made up of many organs put together such as: Respiratory system, digestive system, nervous system etc.

11.3 MEDICAL ELECTRODES 1. Electrodes: The electrodes are metal contacts, which are fixed at various parts of human body, while performing various tests (such as ECG, EEG, EMG etc.) The electrodes basically act as Transducers, and convert the ionic flow in the body into an electrical current through a wire. These are usually made up of metal. The two important characteristics of electrodes are Electrode potential and contact resistance. A good electrode should have low and stable value for both of the above characteristics. When a metal electrode comes in contact with the electrolyte (human body fluid), it forms a half electric cell. In the electrode, a potential is developed whose value depends upon the metal in use, the ions in the human body’s fluid and conditions of contact. Its value may be about 100 mV.

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The most widely used electrodes for bio-medical applications are silver electrodes coated with silver chloride (AgCl2). The fig. 11.2 shows equivalent circuit of an electrode placed against an human skin through a gelly. E Ce

Re

Electrode

Rg

Gelly

E Skin CS

RS

R

(a)

Skin of human body

(b) Ce = Electrode capacitance Re = Electrode resistance Rg = Gelly resistance Cs = Skin capacitance RS = Skin resistance

Fig. 11.2

Problems: There are two problems in the use of electrodes. (i) All electrodes suffer variation in “contact resistance” due to movement and drying out of any coupling medium. The contact resistance may be improved by setting the electrode slightly away from the surface of the skin by “floating” the electrode on a quantity of “Coupling Jelly” (electrolyte paste). (ii) Another problem arises if there is any direct current (dc) in microamperes flowing through the electrode due to faulty equipment or from small “biasing current” in the amplifier circuit. Over a period of time, this current causes chemical changes at the surface of the electrode, causing polarization with consequent increase in the electrode potential and damage to the human skin due to chemical reaction.

2. Types of Medical Electrodes The medical electrodes are of the following types. (1) Body surface electrodes: They are metal plate types (Fig. 11.3 (a)), Metal disc type (Fig. b) and disposable type (in two views) Fig. (c).

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207

Metal Disk and Electrolyte (a)

Adhesive on the surface of foam pad

(Bottom)

Snap Foam pad

(Top)

(c)

(b)

Fig. 11.3

(2) Floating electrodes: (Fig. 11.4) For picking up potential from the skin for ECG, EEG and EMG tests, even the best electrodes cause “artifacts”, whenever there is a movement of the electrode. The movement changes the quality of the contact between electrode and the skin, thus affecting the electrode potential and the contact resistance. The problem can be reduced by mounting the electrode at a short distance from the skin on a plastic washer and filling the space in between with an electrolyte type jelly. These are called floating electrodes. The Fig. 11.4 shows two types of floating electrodes. Metal Disk

Insulating package

Electrolyte Jelly in space (a)

Double sided Adhesive tape ring

(b)

Fig. 11.4

(3) Microelectrodes: (Fig. 11.5) These may be formed of metal and glass. These are very small electrodes with the tip of size ranging from 0.05 micron to 10 micron. The tip is moved through the human skin. Insulation

Metal film

Fig. 11.5

Glass

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3. Other types of electrodes are (i) Suction electrodes (ii) Enzyme electrodes (iii)

Needle electrodes

(iii)

Series active electrodes

11.4 HEART AND CARDIO VASCULAR SYSTEM (a) Heart: (Fig 11.6 (a)) The central organ of our cardio vascular (or circulatory) system is Heart which consists of four channels two of which are “thin walled” called right and left atrium. The other two are “thick walled” and are called right and left ventricles. There is a valve between right atrium and right ventricle, similarly there is another “valve” between left atrium and left ventricle called Tricuspid valve and Mitral valve respectively. The right ventricle is connected to pulmonary artery by another valve and left ventricle is connected to Arota. The heart is surrounded by pericardium, whose inner lining produces fluid to lubricate the heart motion.

Ar

ot

a

Pulmonary artery

Right atrium Left atrium Tri id p cus e valv

Mitral valve

Left ventricle

Right

ventricle

Fig. 11.6 (a)

The Fig. 11.6(b) shows electrical conduction system and voltage waves from various parts of a human heart. The Fig. 11.6 (c) shows outer look of the heart.

11.4 Heart and Cardio Vascular System

209 Voltage waves

Upper ventricles

Sinus node Atrial muscle A-V mode Common bundle Purkinje fibres Ventricular muscle

Lower ventricles (b)

P

R

T

Q S

U (c)

Fig. 11.6

The heart acts as a “double pump” to keep blood circulating through blood vessels (arteries). The blood enters the heart via the right atrium. The right atrium fills the right ventricle, which pumps blood to Arota and systemic arteries. Here Atrioventricular and “semilunar” valves open and allow the heart to pump blood in an unidirectional flow. Electromagnetic emission at a frequency around 7.83 Hz in the ELF (extermely low frequency) band from the human brain has been reported. It is natural to enquire whether similar emission in the ELF band takes place due to heart. It is now well-established from the recording of an ECG machine that the rhythmic expansion and contraction of the heart of a living human takes place at a rate approximately 60 per min to 90 per min (corresponding to frequencies of 1 Hz to 1.5 Hz) causes separation of positive and negative charges across the heart. This in turn, produces a time-varying voltage, rich in harmonics across the two regions (upper ventricle and lower ventricle) causing electromagnetic emission. (b) Cardio Vascular (Blood circulation) System Fig. 11.7: The cardio vascular or simply “blood circulation” system consists of central organ i.e., heart. (which is a mascular pumping device) and a closed system of vessels (called arteries, veins and capillaries). It is a closed cycle and blood circulates again and against through the various organs of the

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body. The blood circulation is a continous and controlled movement of blood through the thousands of miles of capillaries, that “permeate” every tissue and reach every cell and surrounding of the cell and waste products are removed and thrown out of the body. The heart is the mascular pump, that provides the force necessary to circulate the blood to all the tissues in the body. Its function is vital because to survive, the tissues need a continuous supply of oxygen for nutrients and also, the metabolic waste products to be removed. Deprived of these necessities, the cell will soon undergo “irreversible” changes that leads to death. The blood is the transport medium. The heart is the organ that keeps the blood moving through the vessels. Head

Lung

Lung

Right Atriium

Heart

Right Ventricle

Liver Intestine

Kidneys

Legs

Fig. 11.7

The normal adult heart pumps about 5 litres of blood every minute throughout our life. If it loses its pumping effectiveness for even a few moments, the individual’s life is in danger.

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1. Electro Cardio Gram (ECG) The ECG stands for “Electro cardio gram”. It is a basic test done on a person for measuring heart’s activity. (Fig. 11.8)

ECG machine

Electrodes

Fig. 11.8

It is simply a measurement of voltage changes in the heart with which any electrical event can be detected. It involves taping electrodes at various parts of the body and take readings of the electrical voltage of the heart. These activities are printed out onto a piece of graph paper for examination. The electrical active tissues in the human body are “muscles” and “nerves”. Small changes in the voltage can be detected, when these tissues are fired electrically. As the heart is also a muscle with well coordinated electrical

activity, any happening within the heart can easily be detected.

The electrical changes occurring as the heart beats can be detected by attaching

electrodes and plotting voltages between them versus time. For this purpose,

the ECG machine uses a roll of paper, that moves at a specific speed to represent “passage of time” and has a stylus (pen) that moves up and down w.r.t voltage to draw the electrical events. (Fig. 11.9) Graph paper

ECG curve

To patient

+ direction of paper movement

– Stylus

Fig. 11.9

The heated “stylus” moves with change in voltage as the graph paper moves. The stylus moves upward with positive voltage and down-ward with negative voltage, tracing out curves “voltage versus time” on the moving “heat sensitive” graph paper.

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In order to have electrical activity in different directions, it is necessary to have many electrodes fixed on the patient’s body. Electrical activity causes a voltage change only, if it is moving towards or away from an electrode. But if the electrical wave is moving at 90° between the electrodes it can not be detected. (i) Block diagram of an ECG Machine and Its Working The ECG (Electro cardio graph) machine is used to record electrical activity of the heart. The heart signals are picked up by the electrodes (which act as transducers) and convert them into electrical signals to obtain information about heart disorders. The block diagram of an ECG machine is given in Fig. 11.10. The function of each block is explained below: 1. Lead selector switch: The potentials picked up by the electrodes are taken to the lead selector switch. Each electrode connected to the patient is attached to the lead selector which determines which electrodes are necessary for a particular lead and connect them to the remaining circuit. The lead selector can be operated mechanically by an operator or by a micro computer in automatic mode. The computerised lead selector selects the electrodes “two by two” according to the “lead program”. Lead selector

Pre amplifier

Power amplifier

Frequency selector circuit

Bridge output circuit Feedback

Electrodes

Auxiliary circuit

Paper transport motor

Fig. 11.10

Pen motor

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213



2. Pre amplifier: The output of lead selector is given to a pre amplifier which is basically a differential operational amplifier consisting of 3-4 stages. The signals picked up by electrodes are of very low amplitude, they are pre amplified by this block. This stage should have very high input impedance and high CMRR (common mode rejection ratio). The preamplifier gets sufficient negative (current) feedback which gives it stability.



3. Power amplifier: The pre amplifier provides initial amplification and power amplifier provides final amplification to the signal. The power amplifier is generally a push pull power amplifier. It receives signal from pre amplifier, as well as from feedback circuitry. 4. Bridge output circuit: This is for feedback signal as well as it controls the pen motor. 5. Frequency selector feedback circuit: This provides feedback signal obtained from bridge output circuit to the power amplifier. This feedback signal is necessary for stabilizing the power amplifier. The circuit is a RC network which provides the necessary feedback. 6. Pen motor: This motor drives the pen/stylus, which moves on the graph paper to sketch the waveshape. A direct writing recorder is usually adequate, since the ECG signal of interest has a limited bandwidth. 7. Auxiliary circuit: This circuit is used basically for calibration purposes. It provides calibration signal of 1 mV. It also helps to block the pre amplifier during change in the position of the lead selector switch. 8. Paper transport motor: This motor moves the graph paper at specific speed. (ii) ECG Grid The graph paper (called ECG grid) on which the ECG is drawn is divided into 1 mm lines horizontally as well as vertically. The horizontal lines represent time. The paper moves at the rate of 25 mm/second, each 1 mm line represents 0.04 seconds, Every 5th line is dark (thick). The time between dark lines is 0.2 second and 5 dark lines equal 1 sec. (See Fig. 11.11)

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Voltage

0.1 mV 0.2 seconds 1 millivolt 0.04 sec

ECG curve

Time

Fig. 11.11

The vertical lines represent the strength (voltage) of electrical signal. The +ve voltage moves the stylus upward, while the –ve voltage moves it downward. Each mm vertically represents 0.1 mV. Ten vertical dark lines equal to 1 mV. (iii) Specifications, Advantages and Disadvantages of ECG Machine (a) Specifications: Few important specifications of ECG machines are given below: (i) Sensitivity

: 20 mm/mV

(ii) Input Impedance

: 5mΩ

(iii) Output Impedance

: 100 Ω

(iv) Standard Signal

: 1 mV

(v) CMRR (Common mode

: 10000 : 1

rejection ratio) (vi) Recording techniques

: Heated stylus moving on heat sensitive paper

(vii) Paper speed

: 25 to 50 mm/s

(viii) Frequency response

: 0.1 Hz to 60 Hz

11.4 Heart and Cardio Vascular System

(b)

215

Advantages: (i) It is non invasive, safe, inexpensive and easy to perform. (ii) The necessary equipment is easily available.

(c) Disadvantages: (i) It does not give correct diagnose always. (ii) It can be normal, inspite of a heart problem. (iii) The ECG reveals the heart rate and rythm only during the time, when ECG is being taken. (iv) Interpretation of ECG Heart beat

Diagnosis

Slower heart beat

Bradycardia

Higher heart beat

Tachycardia

Basic features of ECG missing

Heart block

Note: Normal heart beat is (60 to 100) per minute.

(v) Holter ECG For ailments, continuous ECG recording is needed. The 24 hrs ECG monitoring is called “Holter ECG recording”. In this, the cardiac activity for long time is recorded by a magnetic recorder. For this, a small device is placed on the chest (or breast) of the patient. The recorded tape is later on played back to diagnose the disease.

2. Cardiac Pacemaker This is a device needed when the heart is not stimulating properly on its own. It produces uniform stimulation (i.e., fixed heart rate) regardless of cardiac activity. Whenever the heart slows down, the pacemaker circuit provides a pulse and the heartbeat becomes regular. The output pulse of a pacemaker is shown is the Fig. 11.12 (a) and (b).

(a) Ideal

(b) Practical

Fig. 11.12

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(i) Block diagram The Fig. 11.13 shows block diagram of a cardiac pacer. Power source

Sense amplifier

Timing control

Output driver

Electrode

Pulse generator

Fig. 11.13

It has following blocks. • Power source: Lithium iodide cells with open circuit voltage of 2.8 V provide energy. (The cell is rechargable) • Pulse generator: It controls the pulse rate. • Sense amplifier. • Electrodes • Lead wires etc. (ii) Implantation: As told above the pacemaker is designed to regulate the rhythm of the heart. It is worn or implanted in the body of the patient near the heart. It is usually triggered to modify the output by sensing the intra cardiac potential in one or more cardiac chambers. It may also have anti-tachycardia function, i.e., it does not allow the heart rate to increase.

11.5 HUMAN BRAIN (a) Structure The human brain consists of four parts. (Fig. 11.14) 1. Cerebrum: The cerebrum (1) consists of two well demarcated hemispheres right and left. The outer layer of the cerebrum is called cerebral cortex. All sensory inputs from various parts of the body eventually reach the cortex. Various areas of cerebrum are responsible for hearing, sight, touch and control of muscles etc.

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217

The “cortex” is also the centre of “intellectual functions”. Large quantities of information can be stored temporarily and may be corelated, thus making basis of higher mental functions. The cerebrum is the most important part of the brain. The “cerebral cortex” which is the main subdivision of cerebrum contains about to 10 to 12 billions “neurons”. 2. Cerebellum (2): It lies below cerebrum just above the brain stem. It coordinates the body movements with information it receives from the cerebral vortex. It acts as a physiological microcomputer; which controls various sensory and motor nerves to smooth out the muscle motion which could be otherwise “jerky”. It also consists of two “hemispheres”, which regulate the coordination of muscular movements. 3. Brain Stem (3): It connects the spinal cord to the center of the brain just below the cerebral. (1) (3)

(2) (4)

Fig. 11.14

4. Modulla oblongata (spine cord) (4): is the most important part at the lower section of the brain stem, which is associated with control of some of the basic functions such as breathing, heart rate, kidney functions etc. For this purpose, the modulla seems to contain a number of “timing mechanism” as well as important “neuronal connections”. Note: (i) When one region of the brain is damaged another region seems to take over the function of the damaged region. (ii) The whole brain is protected inside a tough bony structure called skull.

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1. ELectro Encephalo Gram (EEG) The EEG stands for electro encephatogram The electroencephalography is a neurophysiologic measurement of electrical activity of the brain by recording through electrodes placed on the scalp or in special cases in cerebral cortex. The resulting traces (curves) are known as an EEG and represent electrical signals from a large number of “neurons.” (i) Block Diagram of EEG Machine An EEG (Electro encephalo gram) machine is a device used to create a picture of the electrical activity of the brain. The machine is used for medical diagnose as well as for neurological research. The fig. 11.15 shows a simple block diagram of EEG machine. The basic EEG system includes the components such as: electrodes, amplifier, data collection, storage and display. Micro

electrodes

Electrodes selector Amplifier

Human scalp

Filters

Writer Chart unit drive

ADC

Computer

CRO

Display

Fig. 11.15

1. Electrodes: The “micro electrodes” are attached to a person’s scalp, (Fig. 11.16) which transmit electrical signal to the EEG machine. The micro electrodes act as transducers as well as a galvanometers. Recall that galvanometers are instruments, which detect and measure small currents.









Electrodes

Fig. 11.16

11.5 Human Brain

219

2. Electrode selector: This selects the particular electrodes suitable for the particular job.

3. Amplifier: The amplifier converts the weak signal obtained from the brain into powerful signal. A differential preamplifier is useful when measuring relatively low level signals. The output from the preamplifier, after going through final amplifiers, become hundred or thousand times stronger. The optical isolators are used to isolate the main power circuitary from the patient. The separation prevents the possibility of the accidental electric shock to the patient or to the operator. 4. High and low pass filters: The signal is filtered by high pass and low pass filters. The “high pass filter” typically filters out low frequency signals whereas the “low pass filter” filters out the high frequency signals. 5. A.D.C.: The “analog to digital” converter converts the analog signal into digital signal, which is more suitable for the output. 6. Writer unit / chart drive: The recorders are hooked up with a pen which traces the electrical signal on the graph paper moving continously. The EEG recordings of a patient are made with his eyes open and closed respectively. A flashing light is used to assess, whether the patient is photosensitive i.e. if the patient will have a seizure in response to a flashing light. (ii) Various Type of Brain Waves The human brain radiates few waves in different mental conditions. The brain waves are oscillating electrical voltages in the brain measuring just a few millionths of a volt ( ~ 10–6 V). There are 4 widely recognized brain waves: 1. Alpha waves (calm relaxation): The alpha waves oscillate in the range of 8 – 13 Hz. (cycles per second). These waves are associated with calm and relaxed situation see Fig. 11.17 (a).

Fig. 11.17 (a)

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2. Beta Waves (awake): Their frequency range is 13 – 40 Hz (cycles/ second). These waves are associated with active or anxious thinking and active concentrations See Fig. 11.17 (b).

Fig. 11.17 (b)

3. Delta Waves (deep sleep): Their frequency range is upto 4 Hz (cycles per second). These are seen in combination with other waves during deep sleep. See Fig. 11.17 (c)

Fig. 11.17 (c)

4. Theta waves (light sleep): These have the frequency range of 4 to 8 Hz (cycles per second). These are the characteristics of childhood, adolescence (13 years to 19 years age) and drowsiness in adults. These are also induced by meditation, deep day dreams, light sleep, immediately prior to falling asleep and before waking up. (Fig. 11.17 (d))

Fig. 11.17 (d)

Since alpha waves are commonly observed during relaxation, the researcher thought patients could get relief from anxiety, insomnia and epilepsy by learning their alpha waves activity. But further research showed that alpha waves study is only useful if it is combined with other therapies. These days, insomnia are thought to control theta waves and epileptics have found relief by monitoring the waves produced during seizures. (iii) Uses and limitations of EEG (a)

Uses: The EEG allows researchers to record electrical impulses travelling across the surface of the brain and observe changes under those impulses. The EEG recording has following uses. (i) The EEG makes continuous recording with split second accuracy.

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221

(ii) The EEG can indicate the general conscious state of a person e.g. asleep, awake, anaestheized (anaesthesia is given to the patient for surgeries etc. to make him unconscious), since each state is corelated with a particular EEG pattern. (iii) The EEG recordings are used to measure the time, it takes the brain to process various stimuli. (iv) The EEGs are also used to assess the brain damage, coma, mental retardation, encephalitis, cognitive impairment, brain tumours, epilepsy etc. (v) The EEGs are also used in sleep research. (b)

Limitations : (i) The EEG can not reveal the brain structure or its anatomy, nor it indicates functioning of various parts of the brain. (ii) The scalp electrodes are not sensitive enough to pick up the individual’s action, potentials or electric signalling in the brain. (iii) The EEG picks up the activity of large groups of neurons which produce a greater voltage than by firing of an individual neuron.

(iv) The EEG gives limited information compared with other brain mapping techniques such as “magnetic resonance imaging” (MRI). (iv) Typical Specifications and Features of EEG Machines. (a)

The specifications of EEG machines are given below (i) Input impedance 12 M ohms at 10 Hz. (ii) Sensitivity : 0.5 mV/mm maximum. (iii) Calibration voltage : 5 mV to 1000 mV (iv) CMRR (common mode rejection ratio): 2000 or 66 dB. Its value is maximum at 60 Hz (minimum at 10 Hz). (v) Chart speed 10-60 mm/s.

(b) Special features: (i)

The EEG must have a large number of channels.

(ii) Some EEG machines have a provision of connecting an automatic “brain wave analyzer” with the machine. (iii)

The EEG electrodes are smaller in size than the ECG electrodes.

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2. Magnetic Resonance Imaging (MRI): Fig. 11.18 Shows the block diagram. The MRI is a method used to visualize the inside of living organisms. In clinical practice, MRI is used to distinguish pathologic tissue from the normal tissue. Popularly this is used for brain mapping but can be used for any human organ such as heart, nose etc.

One of the advantages is that this is harmless to the patient. It uses strong

magnetic fields (usually 1.5 Tesla) and non ionizing radiations in radio frequency range. This gives accurate results, as compared to E.C.G. or EEG or X-rays.

R.F. Amp.

Patient

Amp. I

Amp. II

Amp. III

RF Coil

RMI Machine

Gradient Coil

Spectrometer

Computer

Fig. 11.18

The MRI is a non-invasive diagnostic technique that produces physiological images based on the use of magnetic and radio frequency (R.F.) fields. The MRI system uses powerful magnets to create a magnetic field which forces hydrogen atoms in the human body in a particular alignment (resonance). The RF energy is then distributed over the patient, which is disrupted by the body tissues. The disruptions corresponding to varying return signals are then processed and displayed on the computer.

11.6 MASCULAR STRENGTH Out mascular strength helps us to perform various mechanical functions. When a person is affected by paralysis or polio etc, his mascular strength is decreased. We have techniques to measure mascular strength of person, which is called Electro Myo Graphy (EMG)

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1. Electro Myo Graphy (EMG) The EMG stands for “electro myography”. The “myo” means muscle, so EMG may be taken as an “electrical muscle writing”, where the electrical signal recorded from a muscle is made to write a “pattern” on a video screen. An Integrator is used for integration of the activity of a muscle. A linear relation exists between the integrated EMG signal and the tension produced by a muscle. In other words the EMG voltage is developed, when integrated over time gives the total voluntary tension in the muscle. By rectifying the EMG signal i.e., by converting all negative potentials to identical positive potentials, we get an EMG pattern which consists of positive deflections only. The output curve is a measure of total electrical activity per second recorded from a muscle during voluntary contraction. This shows change in muscle activity due to a neuro-mascular disease such as polio. The Fig. 11.19 shows block diagram of a typical set up for EMG and as well as the recording set up.. The surface electrodes or needle electrodes are inserted into the muscles or fixed with the skin with the help of a paste. Signal from muscle

Senser

Instrumentation amplifier

High Pass Filter

Micro controller

Full wave rectifier

Low Pass Filter Oscilloscope (CRO)

EMG signal

EMG Amplifier

Tape recorder

AF AMP

Fig. 11.19

Loud speaker

The EMG signals range from 0.1 mV to 0.5 mV. They may contain frequency components from 20 Hz to 10 kHz (audio frequency) but using low pass filter, the frequency is restricted to 20 Hz to 200 Hz for clinical purposes. The normal frequency of EMG signal is 60 Hz. Such high frequency signals cannot be recorded on the conventional pen recorders, therefore signals are displayed on an oscilloscope and as well as magnetic tape recording is also made. It is also applied to an audio amplifier connected to a loudspeaker. The EMG interpretor

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can diagnose various muscular disorders by listening the sound produced by the loudspeaker. Normally there are two CROs, one for visual display and other is a storage CRO for permanent recording. For this, an ultraviolet (U.V) sensitive paper moves, over which an image is produced. For continuous recording the paper speed is kept at 5 cm to 25 cm/sec. After developing, one can see a visible image. The stimulators used in EMG machines may be single pulse, double pulse or train of pulses. The amplitude, duration, repetition and delay of the stimulus all are adjustable and facilities are also provided for external triggering. The output of the stimulator is either “constant voltage” or “constant current” type. The constant voltage stimulator provides square wave pulses with amplitude in the range of 0 – 500 V, pulse duration 0.1 to 3 ms and frequency 0 to 100 Hz. The output of the constant current stimulator can be adjusted between 0 – 100 mV. The EMG voltage waveform has a peak amplitude in the range of 50 mV to 1 mV which depends upon the position of electrodes. It looks like a noise, whose frequency ranges from 100 Hz to 3 kHz. Table 11.1: Comparison of ECG, EEG and EMG signals. Signals

Frequency (Hz)

1. ECG

0.05 to 100

10 to 5000

Voltage (mV)

Surface electrodes

ELectrodes uses

heart muscles

Activity

2. EEG

0.1 to 100

2 to 200

Surface and needle electrodes

brain

3. EMG

5 to 2000

20 to 5000

Surface or needle electrodes

muscles

11.7 BLOOD PRESSURE (BP) (a) Blood pressure (BP) is the force of the blood pushing against the artery walls. Each time the heart beats, it pumps blood into arteries resulting in the highest blood pressure as the heart contracts. The BP is lowest when the heart expands. Therefore, two numbers are recorded when measuring BP. The highest number called systolic BP refers to the pressure inside the artery, when the heart contracts and pumps blood through the body. The lowest number called diastolic BP, refers to the pressure inside the artery when the heart is at rest i.e. it is expanding and taking the blood into it. Both the “systolic” as well as “diastolic” numbers are recorded as “millimeters of mercury” (mm of Hg). This represents, how high the mercury column will be raised by the pressure of the blood.

11.7 Blood Pressure (BP)

225

(b) Standard values of BP (i) According to the “National Heart, Lung and Blood Institute (NHBLI)”. Blood pressures in adults are defined as • 140 mm Hg : systolic • 90 mm Hg : diastolic It is specified as 140/90. (ii) The latest guidelines issued by NHBLI are : • 120 mm Hg : systolic • 80 mm Hg : diastolic It is specified as 120/80 More BP than the standard values given above come in the category of hypertension (high BP) which increases the risk of coronary heart diseases (heart attack) and brain stroke (brain attack). With high BP, the arteries may have an increased resistance against the flow of blood, causing the heart to pump harder to circulate the blood. So one should maintain his BP within

limits (120/80).

A low BP (< 80 mm Hg) can cause fatigue, tiredness etc. Truly speaking, a low

BP is more fatal than a higher BP.

We have medicines to control high BP, but unfortunately no medicine to raise

a low BP.

However, intake of salt (sodium), coffee (cafene) can raise the B.P.

(c) B.P. Measuring Techniques The BP can be measured in two ways. 1.

Indirect measurement.

2.

Direct measurement.

1. Indirect Measurement of B.P. In this method, an instrument called sphymo-manometer is used in which application of pressure on the upper arm causes a spurt in the blood flow in the artery (situated in the upper arm) and a typical sound is heard from the stethoscope. In sphymo-manometer, [Fig. 11.20 (a) and (b)], there is an inflatable CUFF (hard cloth) containing bladder, which can be inflated by a rubber bulb and the pressure in the cuff is measured by the manometer. The inflated cuff can be deflated slowly by a needle valve.

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Hg column

Medical Electronics

Manometer

140

Meter

Rubber bulb

0

Cuff containing bladder

Stethoscope

Cuff

Rubber bulb (a)

(b)

(c)

Fig. 11.20

The cuff is fastened around upper arm and with pumping of a rubber bulb, the pressure in the cuff is increased upto 220 mm Hg much above the B.P. of the “brachial” artery (situated in the upper arm). The slow release of pressure in the bulb, causes a “spurt” of blood due to constriction and a typical sound is heard, now the pressure read by the manometer gives systolic pressure. The sound continues to be heard, till diastolic pressure is reached when the sound stops. The other sphymo-manometers are : (a) Electric Sphymo Manometer: It is an automatic instrument and uses a programmable device. In this, a pressure transducer is placed in between the artery and the cuff and a microphone is put over it. The “increases” and “decreases” of the pressure by the instrument can be programmed with a specific rate. The systolic and diastolic pressure points can be recorded and played back. (b) Electronic Sphymo Manometer: By the manometer described above, one cannot measure his own ‘B.P’. The electronic manometer enables us to measure our own B.P. The electronic manometer instrument can also measure heart rate or pulse. The Fig. 11.21 (a) shows the panel of the electronic manometer and fig (b) shows its use Systolic mm Hg

Pulse per min

Diastolic mm Hg Digital display

Select

Memory

Start

Power

(b) (a) Cuff containing bladder (Auto inflation)

Fig. 11.21

11.7 Blood Pressure (BP)

227

The pulse refers to the rhythmic expansion of an artery, that is caused by ejection of blood from the “ventricle”. It can be felt, where the artery is close to the surface and rests on something firm. Normal pulse rate is equal to the heart beat. It is 72 (or between 60 to 100) per minute. The Electronic BP machine does not have rubber bulb (pump). The cuff has automatic inflation. The stethoscope is also not required. (c) Advantage: The indirect method is simple and easy to use. (d) Disadvantages: The indirect method has the following disadvantages. (i) It does not provide a continuous recording of the blood pressure variation. (ii) Its practical repetition rate is limited. (iii)

It can not generate waveform of the blood pressure.

(iv) The method is subjective and does not give proper result, if the B.P. is very low.

2. Direct Measurement of B.P. In this method, a surgical operation is needed to place a catheter within the artery. The two methods of direct measurement are : 1. A sterile saline solution is introduced in the catheter so that the blood pressure is transmitted to a transducer outside the body. 2. In the second method, a pressure transducer is used within the catheter so that a continuous recording of the BP may be achieved. In the catheter, the following types of pressure transducers may be used. (i) capacitive type transducer: In this transducer, one plate of the capacitor is fixed and other plate is movable. With the change in the pressure due to blood flow, the distance between the plates changes, this brings a corresponding change in the capacitance of the capacitor. The change in the capacitance changes the frequency of a voltage controlled oscillator (VCO) and the measurement of the change in the frequency is the direct measurement of the blood pressure. The advantage of this method is that it is easy to be used in the artery. But the capacitive transducer is prone to change with the change in temperature and this introduces error in the measurement.

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(ii) linear variable differential transformer (LVDT): The core of the LVDT changes its position with change in the blood pressure. The change in the core position causes a change in the flux linkage to the secondary winding of the LVDT and this changes the secondary output voltage. A high frequency low voltage source is used to supply the current in the primary. Recall that an LVDT is an inductive transducer. • Advantages (i) The direct method is advantageous for its higher sensitivity and linearity. (ii) The direct method provides a continuous recording of BP waveform. (iii) Reading is accurate than the indirect method. • Disadvantages: The direct method requires blood vessel (artery) to be punctured by surgery to introduce the pressure sensor for a proper output. Due to above. the use of direct method is limited.

11.8 DIABETES When the quantity of sugar (glucose) in the blood of a person increases a limit, the person is known as suffering from “diabetes” and himself as “diabetic”. The safe limit of blood sugar is given as: 1. Fasting: When the person is empty stomach: the blood sugar should be less than 100 mg/dl. 2. Two hours after taking meals: The blood sugar may be upto 140 mg/dl . Any value higher than this declares the person as a “diabetic”. In layman’s language it is called as “sugar disease”. If the glucose level is higher than 200 mg/dl, a urine test becomes necessary.

Higher glucose level than the permissible values is very harmful. It has a very

bad effect on the health of various human organs such as kidneys, heart, lungs,

eyes etc.

Diabetese is the result of the malfunctioning of an organ in human body. This

important organ is called Pancreas. This organ has an important function to perform. It converts glucose into Insulin, which is required by the human body.

11.9 Safety of Operators and Patients

229

The diabetes may be of two types: (i) Type I diabetes: This occurs in children. (ii) Type II diabetes: This occurs in adults For a diabetic person, sugar is totally prohibited to consume and he/she is to take medicine in the form of tablets or “Insulin” injection, or both. The blood sugar can be measured by a simple lab test or by an instrument, called glycometer.

• Block diagram of Glucometer Fig. 11.22 shows block diagram of a glucometer. AMP

ADC

AMP

DAC

AMP

DAC

CPU

DAC

Sensor

LCD driver

Test strip

soaked with

blood

LCD display

Fig. 11.22

11.9 SAFETY OF OPERATORS AND PATIENTS The most important safety aspect of medical instruments is electric shock to the operator or to the patient. (a) Electric shock When a person comes in contact with naked electric wire and a current passes through him, he is said to be getting electric shock or “electrocuted”. The table 11.2 gives effect of electric current on human beings. Table 11.2:

Effect of Current

Current 0 to 0.5 mA 0 to 3.5 mA upto 10 mA upto 50 mA

Effects Perception, no effect Starts reaction, able to tolerate Muscles contract, unable to let go Heart Fibrillation (broken into pieces), damage of the cells start.

230

Chapter 11 Current

upto 100 mA greater than 100 mA

Medical Electronics

Effects Heart action does not recover, even when circuit is broken. Internal body burn.

First aid to a electric shock patient: If some one is struck by an electric shock, do the following: (i) Interrupt the currents (ii) If breathing is stopped, apply “mouth to mouth” breathing. (iii)

If pulse is absent in the person, apply the technique of “cardio pulmonary resuscitation”.

(iv) Call the doctor. (b) Microshock: In cardiac “intensive care units” (I.C.U.), the patients may have “catheters” and electrodes inserted into (or near) the heart. The patient may get microshock in case of a fault in the instrument/ machine. For this isolating transformers are used, so that no current carrying conductor is directly connected to the earth.

SUMMARY 1. Most of the medical instruments are based on electronic principles. 2. Some of the electronic medical instruments are: ECG machine, EEG machine, MRI machine etc. 3. The medical electrodes are the metal contacts, which are fixed on the body for a test. 4. Performance of heart is measured by ECG (electro cardio gram) and of brain by EEG (Electro encephalo gram). 5. The blood pressure is measured by a manometer. qqq

PART II

COMMUNICATION ENGINEERING

12 Introduction

A significant point about communication is that it involves a sender (transmitter) and a receiver. Only a receiver can complete the process of communication. Therefore dual process of “transmitting and receiving” or “coding and decoding” an information can be called as communication; thus, this is a two way process.

12.1 COMMUNICATION As a general concept, we can say that transfer of information from one place

to another is communication. The important elements of a communication system are:

1. Message or information 2. Sender, transmitter or coder 3. Receiver or decoder 4. Code 5. Channel (transmission path)

12.2 METHODS OF COMMUNICATION The communication may be: (a) Oral Communication: In this type of communication, the message is sent or transmitted from the sender to the receiver through spoken words e.g., direct talk or through telephone. The communication through hints or face expression also come under this category.

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(b) Written Communication: When the message is sent to the receiver in writing, it is called written communication, e.g., communication through letter, FAX etc.

12.3 PROCESS OF COMMUNICATION (SEE FIG. 12.1) The process of communication involves the following steps: 1. Encoding the message: The encoder for transmission encodes the message into suitable words, symbols etc. 2. Transmission: After developing the message into suitable code, it may be transmitted through a proper channel. 3. Reception: The information is received on the other side by the receiver. 4. Decoding: The coded message is decoded into the original form, so that it is easily understood by the person on the receiver side. Sender Message

Encode

Transmit

Receive

Decode

Use

Fig. 12.1

5. Use: The final stage of the communication process is to use the information for the purpose, it has been transmitted.

12.4 BRIEF HISTORY OF COMMUNICATION In earlier days pigeons were used to send message (information, signal) from one place to another. For this, pigeons were trained so that they could travel hundreds of miles to reach the destination. The message was tied round their neck or fastened in the beak and was flown towards the destination. See Fig. 12.2 (a).

(a)

Men were also engaged for this job. They were known as Harkara, they were collecting dak from one place and carried to the destination. Later on, they were provided with horse to speed up the work. See Fig. 12.2 (b). In 1830, letter boxes were installed in Britain and as India was under British rule, many such red painted letter boxes were installed in the localities in India, where the English people resided. (b) Fig. 12.2

12.5 Electronic Communication

235

We’ve had postal services of some kind or the other since times beyond memory. Every ruler employed dak runners to carry information Letters Letters Packets to and fro from the outposts of his kingdom to the palace. It was during British rule that postal services were linked to the police. A regular police force was set up in l832; and the first Indian postage stamp was issued in 1840. To start Fig. 12.3 with, post offices were located in the same buildings as police stations.

Later on, postal services outstripped the police and had to have large buildings like

General Post Offices to handle mail, telegrams, money orders, fixed deposits, etc. Now postal services are on the decline. People use telephones, courier services, e-mail and fax. In near future, post offices may become a relic of the past. Typical red painted PO Boxes used all over the country are shown in Fig. 12.3.

12.5 ELECTRONIC COMMUNICATION The function of an electronic communication system is to convey or send a message from one place to another using electronic equipment. The message may be an information or a signal. The information or a signal is obtained from a source, and through an electronic network it is sent to the receiver. Various communication systems are employed to transmit A.V. (audio-video) signals of telephone, radio, T.V., radar, etc.

12.6 STRUCTURE OF AN ELECTRONIC COMMUNICATION SYSTEM A communication system consists of following parts [Fig. 12.4]: 1. Information Source 2. Transmitter 3. Receiver 1. The information source: The information source is the source which generates or produces information or signal. Information source

Transmitter

Receiver

Coding and modulation

Decoding and demodulation

Fig. 12.4

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2. Transmitter: Transmitter is the device which transmits the generated information or signal. It has the following components: (a)

Coding: The transformation of the signal into a suitable form in which it can be transmitted.

(b)

Modulation: To superimpose the signal on an H.F. carrier, so that the signal can travel long distances.

3. Receiver: It receives the information, e.g., radio receiver, T. V. receiver, telephone receiver, etc. It consists of: (a)

Decoding: It is the reverse of coding, e.g., to regain the original form of the signal.

(b)

Demodulation: It is the reverse of modulation, i.e., to separate the original signal from the carrier.

12.7 BANDWIDTH REQUIREMENT By limiting the bandwidth for a signal, more channels can be accommodated. The bandwidth requirement for a signal mainly depends upon the modulating signals. The audio signal occupies a bandwidth upto 15 kHz but when a carrier is modulated by the audio signal, the modulated signal will certainly need more bandwidth. The Fig. 12.5 shows B.W. of few signals. Video Signal Audio Telephone 20 10

300 102

3300 103

15,000 104

105

4 × 106 106

107

f

Frequency in Hz in log scale

Fig. 12.5

1. The audio signal needs B.W. of about 20 Hz to 15 kHz for transmission. 2. The video signal needs a B.W. of about 4 MHz, while a fax signal needs a B.W. of 1 kHz only. 3. For a telephone, a B.W. of 300 to 3300 Hz is required.

12.8 Types of Electronic Communication Systems

237

12.8 TYPES OF ELECTRONIC COMMUNICATION SYSTEMS Basically, communication systems are of two types: 1. Wire communication, i.e., where communication is done through wires, e.g., cable T.V., wire telephony, etc. 2. Wireless or carrier communication, i.e., where communication is done without wires. In this system, a carrier wave is used and modulation is carried out, e.g., radio, T.V., radar, radio telephony, etc. [A carrier is a high frequency wave which ‘carries’ the signal, i.e., the signal is ‘superimposed’ on the carrier.]

Details of frequency band for different types of communications are given in

Table 12.1.

Table 12.1 SI No.

The Frequency Band for Various Communication Systems

Frequency band

Type of communication

1.

Very low frequency (5–30 kHz)

Long distance communication

2.

Low frequency (30–300 kHz)

Radio navigation

3.

Medium frequency (0.3–3 MHz)

Broadcasting marine

4.

High frequency (3–30 MHz)

FM broadcasting television

5.

Ultra high frequency (0.3–3 GHz)

Radar

6.

Super high frequency (3–30 GHz)

Satellite communication

12.9 TRANSMISSION MEDIUMS OR CHANNELS The various transmission mediums used in various range of the electro magnetic spectrum are shown in Fig. 12.6. Twin Wire

101

Co-axial Cables

104 105

Wave guide 108

1010

Optical fibre cables

Frequency in Hz

1020

Fig. 12.6

These mediums are twin wire lines, co-axial cables, wave guides and optical fibre cables. The various communication systems operating in different range of spectrum are: telephone, A.M. and F.M. broadcasting, T.V., satellite etc.

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The telephone channels need a bandwidth (B.W.) of 300–3300 Hz, the medium wave broadcasting needs a B.W. of about 10 kHz, the VHF stations need a B.W. of about 150 kHz and T.V. channels need a B.W. of about 8 MHz and so on.

12.10 IMPORTANT FACTS ABOUT SOUND AND LIGHT (a) Radio receivers and other audio devices/circuits deal with sound. Hence we should know important facts about sound. 1.

Sound is a form of energy.

2. Newton’s law for the velocity of sound in a medium is v = E / p , where E is the elasticity and p density of the medium. Velocity of sound in air is 330 m/s, in water 1500 m/s. Velocity of electricity is 105 times more than that of sound. 3. A woman’s voice is sweet, has more frequency, less wavelength and low pitch. A child can produce sound of even more high frequency than a woman can. The frequency of an adult’s voice is low. 4. An adult’s voice is hoase. He generates a sound of 20 Hz to 20 kHz. Note that a 20 Hz sound has a wavelength of 17 m and a 20 kHz sound has 0.017 m (or 1.7 cm) wavelength. 5. As mosquito cannot tolerate more than 25 kHz, the electronic mosquito repellers generate a frequency of more than 40 kHz. 6.

A bat produces ultrasonic waves (above 100 kHz). By echo the bat finds the presence of an obstacle. By phase difference of echoes coming from different directions, it finds its way. Note that a bat cannot see.

7. Best echo is heard from 11 m distance. In cinema halls, absorbers on walls and ceiling do not reflect sound; echo is not produced and we hear the sound clearly. 8. A man has two ears. We can therefore find also the direction of a particular sound from phase difference between the sounds coming from different directions of two ears. 9. A man can hear a sound of frequency between 20 Hz and 20 kHz (called audio frequency range). 10. On moon, we cannot hear as there is no air. The astronauts talk in hints. Note that sound cannot travel without a medium. 11. Sound effect remains for 1/10 second and light effect remains for 1/16 second.

12.10 Important Facts About Sound and Light

239

12. The pitch of buzzing of mosquito is much more than the roaring of a lion; but loudness of mosquito sound is much less than that of a lion. Loudness does not depend on pitch. 13. Earthquakes produce ultrasonics. Dogs can hear ultrasonic sounds and hence they start barking. 14. The relation between velocity, frequency and wavelength of sound is V = fl.

If a radio station has 1350 kHz frequency (f) and 250 m

wavelength (l), you can calculate the velocity of the radio

waves.

15. Music is a sound with simple harmonic motion and regular waves, whereas noise is a sound of combination of irregular waves. 16. All wireless communications occur through ozone layer in the atmosphere. The ozone layer is only 3 mm thick. 17. Transducers are devices which convert one form of energy into another. A loudspeaker is a transducer which converts sound into electrical waves. A camera converts light (picture) into electrical waves. A picture tube converts electrical waves into picture. Human eye, brain, etc., are also typical transducers. 18. The response of our ears to sound is ‘logarithmic’ and not linear. If sound becomes 100 times high, our ears feel it as double (log 100 = 2), not 100 times otherwise ear drum will tear off. This is the reason, that we use ‘decibel’ as a unit for sound. 19. When sound of all frequencies are processed through only one loudspeaker, we get ‘monophonic’ sound. When low and high frequencies are processed by separate loudspeakers we get a ‘stereophonic’ sound, which is a quality sound. This is also called a HiFi (high fidelity) sound. 20. Sound waves are longitudinal waves. 21. The graph between frequency and time period of a sound is a parabola. 22. Strength of sound is decided by its amplitude. Pitch of sound is decided by frequency. Tight membranes (of loudspeakers) produce more frequency. 23. Sound is produced by vibrations. All musical instruments vibrate.

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(b) Facts about light The T.V. receivers etc., deal with light. Few facts about light are: 1. Light travels in a straight line. 2. Light energy can be converted into electrical energy by video camera. 3. The electrical energy can be converted back to light energy by a picture tube. 4. The light waves are transversal waves.

12.11 MODULATION The process of changing some characteristic (amplitude, frequency, phase, etc.) of a carrier by the signal (audio or video) is called ‘modulation’. ‘Modulation’ means modification, variation or change. We modify the carrier according to the signal and hence the name. This is a basic process of wireless communication.

12.12 NEED FOR MODULATION In carrier (wireless) transmission, modulation is a necessity. This is explained below: (a) The first and the foremost reason is that the original sound produced by microphone (or video camera in case of video signal) is very weak and it has a very low frequency. The energy contained by the signal is proportional to its frequency. Thus due to losses in energy, the signal will die after some distance. So, it cannot travel long distance. Therefore, the low frequency signal is made to sit on high frequency ‘carrier’. Such an arrangement enables the signal to travel long distances before it dies out. At the receiver the signal is separated out and the carrier is grounded. The phenomenon can be illustrated by the following analogy. Suppose a man travels ‘on foot’ to deliver a message. Naturally he will take a long time to reach the destination; moreover, he cannot travel long distance. But if the man is provided a horse, the message can reach longer distance in shorter time. At the destination, the ‘receiver’ will take out the message and will leave the horse. Assume here, the message as a signal, horse as a carrier and the receiver as the radio or TV receiver. This explains the principle of radio transmission and reception.

12.13 Types of Modulations

241

(b) The next reason describes the height of the antenna needed. The transmitting antenna should have a height equal to the wavelength. This condition gives best results. We know that V = fl where

V = velocity of radio waves = 3 × 108 m/s f = frequency l = wave length (i) If the frequency of the signal is 20 kHz, the length of the antenna V 3 × 108 m/s = 15000 m = 15 km = f 20 × 103 i.e., if the sound produced at mike is to be transmitted as such, we need an antenna of 15 km height, which is totally impractical.

l = l=

(ii) If f = 1 MHz 3 × 108 = 300 m 1 × 106 i.e., if the frequency of the signal is raised to 1 MHz, it can be transmitted through a 300 m high antenna. This is a practical height. Therefore we can ‘modulate’ the signal according to the requirement. In other words, the signal is superimposed on a high frequency carrier.

now length of the antenna l = l =

(c) The last and the most important reason is that modulation permits the transmission without wire. We can receive audio/video signals from any corner of the world through wireless communication. We can witness a match being played at France, sitting in our bedroom. Imagine the length of wire needed if wireless communication were not possible.

12.13 TYPES OF MODULATIONS Modulation is a basic process in all wireless (carrier) communications. In this, the signal is superimposed on a high frequency carrier wave. Some characteristic (amplitude, frequency, phase; etc.) of the carrier wave is changed in accordance with the instantaneous value of the signal. A sine wave may be represented by e = Em sin (wt + j) where

e = instantaneous value of modulated wave Em = maximum amplitude w = angular velocity j = phase relation

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Introduction

Accordingly, modulation is of three types (see the above equation) 1. Amplitude modulation: By changing amplitude of the carrier. 2. Frequency modulation: By changing frequency of the carrier. 3. Phase modulation: By changing phase of the carrier. However, complete classification of modulation processes are given below: 1. Amplitude modulation (AM) (a)

Single sideband AM (SSBAM)

(b)

Double sideband AM (DSBAM)

In India, for sound, amplitude modulation is used 2. Frequency modulation (FM)

In India, for television signals, frequency modulation is used.

3. Phase modulation. Other modulation processes are: (1) Analog (pulse) modulation (used in telephone and telegraphy)–these may be: (a)

Pulse amplitude modulation (PAM)

(b)

Pulse time modulation (PTM)

(2) Digital (pulse) modulation (DM)–They may be: (a)

Pulse code modulation (PCM)

(b)

Differential PCM (DPCM)

(c) Adoptive PCM (ADPCM) (d)

Delta modulation (DM)

(e) Adoptive Delta modulation (ADM) 2. The amplitude modulation is often referred as linear modulation. The frequency and phase modulations are known as non linear, angular or exponential modulation. While there may be many forms of exponential modulations but only two i.e., frequency and phase modulations are important. In particular, both linear as well as non-linear modulations are continuous wave (CW) type modulations.

12.14 Radio (Wireless) Broadcasting, Transmission and Reception

243

12.14 RADIO (WIRELESS) BROADCASTING, TRANSMISSION AND RECEPTION The process of sending radio or T.V. signals by an antenna to multiple receivers which can simultaneously pick up the signal is called ‘broadcasting’. In simple words ‘to radiate radio waves from a station into space’ is called broadcasting or, to send signal in all directions (broad) is called broadcasting. After the waves are thrown into the space, the transmission starts and all the receivers in ‘the range’ can simultaneously pick up the signal. This is called ‘reception’. There is a little difference between broadcasting and transmission. However, the process of reception is quite different. Important components of a typical network are under: See Fig. 12.7. Modulated (radio) waves

Transmitting antenna Transmission

Receiving antenna

Elect. energy Sound

Mike

R.F Amp. Modulator Amp. Transmission Broadcasting

Original signal

Detector

R.F Amp.

L.S. Sound

Reception

Fig. 12.7

(a) Broadcasting 1. Microphone: At the broadcasting station, the person speaks before mike. The mike is a transducer and converts sound energy into electrical energy. The speaker generates a sound of frequency between 20 Hz and 20 kHz (i.e., audio frequency). 2. Amplifier: The electrical signal obtained from microphone (mike) is weak and the same is amplified through an amplifier(s) to the required strength. 3. Modulator: Here the modulation of the signal occurs. A local oscillator generates high frequency waves called ‘carrier’. The signal modulates the carrier (the signal is superimposed on the carrier). The resultant waves are called radio waves or modulated waves.

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4. Transmitting antenna: Through the transmitting antenna, the radio waves are propagated into the space. (b) Transmission: After broadcasting, the transmission starts. These radio waves travel in space at a speed of 3 × 108 m/s, as they are electromagnetic waves. (c) Reception: The picking of these radio waves by radio (or T.V.) receiver is called reception. A receiver has the following important parts: 1. Receiving antenna: The radio waves induced an e.m.f. on the antenna. 2. RF amplifier: The radio waves are of radio frequency (R.F.) range. The e.m.f. induced is amplified through R.F. amplifier(s). 3. Detector: Now the original signal is detected (separated) from the carrier by the detector circuit. The signal starts its forward journey while the carrier is grounded. 4. A.F. amplifier: The signal is now passed through the amplifier. Note that now the signal is of audio frequency range. It should have sufficient energy to strike the loudspeaker. 5. Loudspeaker (L.S.): This is the final stage. The electric signal is again converted into the original sound signal which was produced in the broadcasting station: Note: (i) Here ‘Radio’ does not mean radio receiver. The ‘radio’ means wireless. (ii) The principle of radio broadcasting, transmission and reception described above is same for radio, T.V. signals and also for all such wireless devices. (iii) Broadcasting means to ‘send out’ in all directions. It may be: (a)

A.M. radio broadcast band: Its range is 540–1600 kHz. The stations are assigned every 10 Hz in the above band.

(b)

F.M. radio broadcast band: Its range is 88–108 MHz. The stations are assigned every 200 Hz in the above band.

(c) T.V. broadcasting band: The T.V. channel is 6 MHz wide to include picture and sound signals for each broadcast station.

Summary

245

SUMMARY 1. The transfer of message from one place to another is called communication. 2. The communication may be oral like talking or telephone and written like a letter or FAX. 3. The steps of communication are encoding, transmission, reception, decoding and use. 4. At the beginning, communication was carried out through pigeons and then through men. The first post box started in India was around 1930. 5. The electronic communication is carried out through telephony, telegraphy, FAX, radio, T.V., radar etc. 6. The electronic communication may be wire communication e.g ., telephony and wireless communication e.g., T.V., mobile etc. 7. In wireless communication, the one of the parameters (amplitude, frequency, phase) of a wave (called carrier) is changed according to that of the information (called signal) before transmission. The process is called modulation. 8. Proper bandwidth is required for carrying out communications. 9. The signals may be continous/discrete and periodic/non-periodic etc. 10. The signals may be analysed by Fourier series. 11. The Hartley Shannon theorem is used to find channel capacity. qqq

13 Amplitude Modulation (AM)

In amplitude modulation, amplitude of the carrier wave is changed according to the amplitude of the signal. The technique is very much used in transmission of radio signals.

13.1 AMPLITUDE MODULATION (AM) This may be defined as the process of modulation in which ‘amplitude of the carrier is varied according to the signal’.

Figure 13.1 shows the process of amplitude modulation.

Voltage Em

Signal

Time

Ec Carrier

(Upper envelope)

Em

Ec + E m Modulated wave

Ec

Ec – Em

Lower envelope Unmodulated carrier

Am wave AM

Fig. 13.1

248

Chapter 13 Amplitude Modulation (AM)

(i) The signal is superimposed on a high frequency carrier and a modulated (radio) wave is obtained. (ii) Only amplitude of the carrier is varied, while its frequency and phase remain unchanged. (iii) When there is no signal, the amplitude of the carrier is equal to the unmodulated amplitude. When signal is present, the amplitude of the carrier changes in accordance with the instantaneous value of the signal. (iv) During positive cycle of the signal, the amplitude of the carrier increases to the sum of the amplitudes of the carrier and signal (Ec + Em). (v) During negative cycle of the signal, the amplitude of the carrier decreases and becomes equal to the difference of the amplitude of the carrier and the signal (Ec – Em).

13.2 EXPRESSION FOR AMPLITUDE MODULATED WAVE Let carrier representation be ec = Ec sin wct (See Fig. 13.2)

and let signal be represented by

em = Em sin wmt

Let A be the amplitude of the modulated radio wave. Then A = Ec + em

= Ec + Em sin wmt

= Ec + mEc sin wmt (Em = mEc ) = Ec (1 + m sin wmt)

Let the voltage equation of the output modulated wave be: e = A . sin wc t

em = Em sin w �m m..tt Signal (a)

ec = Ec sin w �cc..tt

Modulated wave (c)

Carrier (b)

Fig. 13.2

...(i)

13.3 Frequency Spectrum of A.M. Wave

249

Putting the value from Eq. (i) e = Ec (1 + m sin wmt). sin wct

...(ii)

This is the standard equation for the A.M. radio wave. Solving further

e = Ec sin wct + mEc sin wct sin wmt

Note that Em = mEc, where m is the modulating factor. Solving again, this comes to be mEc (2 sin ωc .t. sin ωm t) 2 [For the bracketed part, use the formulae = 2 sin A sin B = cos (A – B) – cos (A + B)] e = Ec sin ωc .t +

mEc mEc cos(ωc + ωm )t ...(iii) cos(ωc − ωm )t − 2 2 This is the expression for the equation of the modulated wave.

Then, we get e = Ec sin ωc .t +

13.3 FREQUENCY SPECTRUM OF A.M. WAVE The equation of A.M. wave is given by mEc mEc cos (wc – wm) t – cos (wc + wm) t 2 2 Note that the equation has three parts: Ec sin wct +

(i) First part is an unmodulated carrier wave, which remains unchanged in the process. The maximum amplitude is Ec.

(ii) Second part has a maximum amplitude of mEc/2 and its frequency is equal to the difference of carrier and the signal frequencies. This is called lower side band (L.S.B.). Recall that angular velocity of the carrier wc = 2pfc, where fc is the frequency of the carrier. Similarly wm = 2pfm.

Carrier

(iii) Third part has also max. amplitude of mEc/2 and frequency equal to the sum of carrier and signal frequencies. This is called upper side band (U.S.B.) (See Fig. 13.3)

Fig. 13.3

250

Chapter 13 Amplitude Modulation (AM)

Bandwidth (B.W.) of an A.M. wave: B.W. = (fc + fm) – (fc – fm) = 2fm e.g., if fc = 100 kHz, and fm = 1 kHz, B.W. = (100 + 1) – (100 – 1) = 101 – 99 = 2 kHz [= 2fm] Hence, in amplitude modulation, the bandwidth is twice the signal frequency. Problem 13.1. If the modulating signal is represented by em = Em cos wmt + (Em/2) cos 2 wt + (Em/3) cos 3 wmt + Em/4 cos 4 wmt and the carrier is represented by ec = Ec cos wct Draw the frequency spectrum of the A.M. wave.

mEc/16 fc + 4fm

fc + 3fm

fc + 2fm

fc + fm

fc

fc – fm

mEc/8

mEc/4

mEc/2

Ec

mEc/12

mEc/14 fc – 2fm

mEc/18 fc – 3fm

fc – 4fm

mEc/16

Solution. Figure. 13.4 shows the frequency spectrum of the A.M. wave. Note that each modulating frequency component generates two sideband frequencies, the amplitudes of which depend upon m.

Fig. 13.4

Problem 13.2. If one of the sidebands is removed from the modulated output, will the signal transmission be affected? Solution. Both sidebands carry intelligence (message/information) equally; therefore, if one of the sidebands is removed, the intelligence is not affected. In fact, in practice, only one of the sidebands is transmitted and the other is suppressed to save transmission power. Problem 13.3. What do you know about the following A.M. transmission systems: A3, A3J, A3H, A5C, A3B. Solution. A3—In this, both sidebands (SBs) are transmitted along with the carrier, i.e., (S.B. – 1 + S.B. – 2 + C), where C represents carrier.

13.4 Modulation Factor/Index (m)

251

A3J—In this, only one of the sidebands is transmitted and the carrier is

suppressed: (SB – 1)

A3H—In this, one of the sidebands is transmitted along with the carrier:

(SB – 1 + C)

A5C—In this, two sidebands are transmitted without the carrier: (S.B. – l +

S.B. – 2) A3A—In this, one side band is transmitted along with reduced (l/3rd) carrier: (SB – 1 + C/3)

13.4 MODULATION FACTOR/INDEX (m) The modulation factor/index (m) can be defined in one of the following ways:

Ec

Emin

Em = mEc

Emin = Ec – mEc

Fig. 13.5

The modulation index m can be calculated as follow: E − Emin m = max Emax + Emin From Fig. 13.5

Emax = Ec + mEc Emin = Ec – mEc

Emax = Ec + mEc

1. It is the ratio of maximum value of the signal to the maximum value of the carrier, i.e.,

m = Em/Ec, or Em = mE c 2. It is the ratio of the change in the amplitude of the carrier to its original amplitude, i.e.,

m = DEc/E c 3. It is the percentage change in the amplitude of the carrier, i.e., m = DEc/Ec × 100 4. It is the ratio of minimum amplitude to the maximum amplitude of the modulated (ratio) wave. If the modulation curve is displayed on a cathode ray oscilloscope (CRO), we get the curve as shown in Fig. 13.5.

252

Chapter 13 Amplitude Modulation (AM)

Adding,

Emax + Emin = 2 Ec Ec =

or

Emax + Emin 2

...(i)

Subtracting: Emax – Emin = 2 Em Emax − Emin 2 Em m =E c

Em =

or Now: or

m =

Emax − Emin / 2 Emax + Emin / 2

or

m =

Emax − Emin Emax + Emin

...(ii)

Hence m can also be expressed as the ratio of minimum amplitude to the maximum amplitude of the radio wave. Note: 1. The value of m lies between 0 and 1. 2. The value of m depends upon the amplitude of the signal as well as of the carrier.

13.5 SIGNIFICANCE OF m The modulation factor m plays a very important role in the modulation process. This will be made clear by calculating the value of m for different amplitude of signal and the carrier. 1. Let the amplitude of signal be zero (i.e., signal is not present) and amplitude of carrier is Ec. In this case, the amplitude of modulated wave = 0 + Ec = Ec. ∴ Change in the carrier amplitude Ec – Ec = 0. Modulation index = 0/Ec = 0 (No modulation) 2. Let the amplitude of carrier = Ec and amplitude of signal = Ec/2. Then, amplitude of modulated wave = Ec + Ec/2 = 3Ec/2 3 ∴ Change in the carrier amplitude: Ec – Ec = Ec/2 2 Ec / 2 and m= = 0.5 = 50% (Under modulation) Ec

13.6 Power Distribution in A.M. Wave

253

3. Let the amplitude of the signal as well as carrier be equal to Ec. Amplitude of modulated wave = Ec + Ec = 2Ec

Change in carrier amplitude 2 Ec – Ec = Ec

m = Ec/Ec= 1 = 100%

and

(Ideal modulation)

4. Let the amplitude of the carrier be Ec and that of the signal be 3/2 Ec ∴ Amplitude of modulated wave = Ec + 3/2 Ec = 5/2 Ec

Then, change in carrier amplitude = 5/2 Ec – Ec = 3/2 Ec m=

and

3 / 2 Ec = 3/2 = 7.5 = 150% Ec (Over modulation)

Table 13.1:  Significance of m Signal

Carrier

1.

No (zero) signal

Ec

2.

Ec/2

Ec

Modulated wave Signal + Carrier

Value of m

Remarks

0 = 0 Ec

No modulation

Ec / 2 = 50% Ec

Under modulation

= m

= m

3.

Ec

Ec

= m

4.

3/2 Ec

Ec = 100% Ideal Ec modulation

Ec = m

Clipping

3 / 2 Ec = 150% Ec

Over modulation (the wave is distorted )

Hence m depends on the amplitude of both the signal and the carrier. The value of m decides the strength of the modulated wave and hence that of the signal. When m = 1 (100%) the signal will be strongest, perfect and clear. In the case of over modulation (m = 150%), the modulated wave will be clipped off and huge distortion will occur in the reception. Hence, the ideal value of modulation is 1 or 100%.

13.6 POWER DISTRIBUTION IN A.M. WAVE We know that the power contained in a voltage wave is proportional to the square of its amplitude (∝ V2). Note that an A.M. wave is a voltage wave. The total power contained in an A.M. wave will be the sum of the powers contained in the three parts of the wave.

254

Chapter 13 Amplitude Modulation (AM)

Considering root mean square (R.M.S.) values: max.value ⎞

⎛

⎜ Recall that R.M.S. Value = ⎟ 2 ⎝

⎠

Power contained in the carrier: 2

⎡ E ⎤ E 2 Pc α ⎢ c ⎥ = c (Ec is the maximum value of the voltage of the carrier) 2 ⎣ 2 ⎦

Power contained in the lower sideband: 2

E 2 m 2 Pc m 2 ⎡ mE ⎤ PSB−1 = ⎢ c ⎥ = c = [Pc = Ec2 / 2] 8 4 ⎣2 2 ⎦ m is the modulated index. Power contained in the upper sideband: 2

PSB−2

⎡ mE 2 ⎤ P m2 = ⎢ c ⎥ =  c 4 ⎣2 2 ⎦

The total power contained in both the bands: PSB = PSB1 + PSB2

Pc m 2 Pc m 2 Pc m 2 + = 4 4 2 The total power in the A.M. wave PSB =

⎛ m2 ⎞ Pc m 2 = Pc ⎜1+ ⎟ 2 2 ⎠ ⎝ m2 PT ./PC = 1+ ∴ 2 Ratio of sideband power to the total power PT = Pc +

(where, Pc = Ec2/2)

Pc m 2 PSB m2 m2 / 2 2 = = = PT ⎛ m 2 ⎞ 1+ m 2 / 2 m 2 / 2 PC ⎜1 + ⎟ 2 ⎠ ⎝ Note that power in both the sidebands is equal at m = 1, the sidebands contain 1/3rd (33%) power and the carrier contains 66% of the total power, hence bands carry half the carrier power of the wave. As the signal is contained only in the sidebands, useful power is contained in sidebands. This is the reason, that we are interested only in the sidebands. The power in the sidebands go on increasing with the increase in the modulating index (m).

13.7 Calculation for Current

255

13.7 CALCULATION FOR CURRENT IC = Unmodulated carrier current

Let

lT = Modulated current of an A.M. wave (both in R.M.S. values) R = Resistance through which current flows. Assume it to be same in both the cases.

and Now,

PT IT2 .R m2 = 2 = 1+ (R is same) power = (current)2 × resistance PC I C .R 2

Note that PT is the total power of the A.M. wave, PC is the power contained in the carrier wave 2

From above

⎛ I T ⎞ m2 ⎜ ⎟ =1+ 2

⎝ I C ⎠

or

IT m2 = 1+ 2 IC

or

IT = I C 1+

m2 2

Problem 13.4. A transmitter supplies 10 kW power to an aerial, when unmodulated. Determine the power radiated, when modulated to 30%. Solution.

⎛ m 2 ⎞

Total power = carrier power × ⎜1 + ⎟ 2 ⎠

⎝

⎛ (0.3) 2 ⎞ ⎛ m2 ⎞ PT = PC ⎜1+ ⎟ = 10 ⎜1 + ⎟ ( m = 30% = 0.3) 2 ⎠ 2 ⎠ ⎝ ⎝ = 10.45 kW Ans.

Problem 13.5. A wireless transmitter radiates 4 kW with an unmodulated carrier and 4.8 kW when the carrier undergoes modulation. Calculate the percentage modulation employed. Solution.

⎛ m 2 ⎞

P PT = C ⎜1 + ⎟ 2 ⎠

⎝

⎡ m 2 ⎤

4.8 = 4 ⎢1 + ⎥ 2 ⎦

⎣ m = 62% Ans.

256

Chapter 13 Amplitude Modulation (AM)

Problem 13.6. The R.M.S. value of an aerial current is 10 A and 12 A before and after modulation. Calculate % modulation employed. Solution.

IT = I C 1+

m2 2

12 = 10 1 +

m2 2

m = 93.8% Problem 13.7. The unmodulated carrier current to the aerial of a transmitter is 100 A. Determine increase in the currents which will result from the application of 80% modulation. Solution.

2

IT = I 1+ m C 2

(0.8) 2 = 114.9 A 2 Increase in the current due to modulation IT = 100 1 +

= 114.9 - 100 = 14.9 Amp. Ans. Problem 13.8. (a) A radio transmitter using amplitude modulation has unmodulated carrier power output of 10 kW and can be modulated to a maximum depth of 90% by a sinusoidal modulating voltage. Find power of the modulated wave. Find the value of which the unmodulated carrier power may be increased, if the maximum permitted modulation is 40%. Solution.

⎛ (0.9) 2 ⎞ ⎛ m2 ⎞ = 10 PT = PC ⎜1+ ⎟ ⎜1 + ⎟ (m = 90% = 0.9) 2 ⎠ 2 ⎠ ⎝ ⎝ = 14 kW. Ans.

(b) This is the maximum power, which may be handled by the transmitter. The increased unmodulated carrier power is given by: ⎡ (0.4) 2 ⎤

14 = PC ⎢1 +

⎥ 2 ⎦

⎣ Pc = 12.96 kW Ans.

(now m = 40% = 0.4)

Note that the original power of the carrier is 10 kW. Problem 13.9. A 100 V, 10 kHz carrier is modulated with the help of a 5 V, 50 Hz signal. Calculate: (a) Modulation factor (m)

(b) Amplitude of each sideband

(c) Frequency of each sideband

(d) Bandwidth of modulated wave.

13.7 Calculation for Current

257

Solution. Em 5 1 (a) m = = = = 5% = 0.05 Ans. Ec 100 20 mEc 0.05 ×100 = = 2.5 Ans. (b) Amplitude of each sideband = 2 2 (c) Frequency of sideband

USB = fc + fm = 50 + 10000 = 10050 Hz Ans.

LSB = fc – fm = 10000 – 50 = 9950 Hz (d) Bandwidth of the modulated wave B.W. = 2fs = 2 × 50 = 100 Hz Ans. Problem 13.10 Calculate (a) Upper and lower sideband frequencies of an amplitude modulated wave when a carrier of 900 kHz is modulated by a 15 kHz signal. (b) Also find the range of frequencies contained in the modulated wave. Solution.

fc = 900 kHz fs = 15 kHz

(a) Frequency of USB = fc + fs = 900 + 15 = 915 kHz Frequency of LSB

= fc – fs = 900 – 15 = 855 kHz.

(b) Range of frequencies contained in the wave = from 885 kHz to 915 kHz. Bandwidth = 915 – 885 = 30 kHz. Problem 13.11. An A.M. wave has peak-to-peak voltage of 600 V and valley to valley voltage of 100 V. Find the percentage depth of modulation.

600 V

100 V

Solution. See Fig. 13.6 m=

Vmax −Vmin 600 −100 500 = = = 70% Ans. Vmax + Vmin 600 +100 700

Fig. 13.6

Problem 13.12. A 100 V, 100 kHz carrier is modulated with the help of a 10V, 1 kHz signal to the extent of 50%. Write down equation for the A.M. wave. Solution.

m = 50% = 0.5 Ec = 100 V; Em = 10 V fc = 100 kHz fm = 1 kHZ

258

Chapter 13 Amplitude Modulation (AM)

wc = 2pfc = 2 × 3.14 × 100 × 103 = 628000 wm = 2pfc = 2 × 3.14 × 1 × 103 = 6280 Putting these values in the standard equation for the modulated voltage wave ; e = Ec sin wct –

mEc mEc cos (wc – wm) t – cos (wc + wm) t 2 2

e = 100 sin 628000 t +

0.5 ×100 cos (628000 – 6280) t 2

0.5 ×100 cos (628000 + 6280) t 2 e = 100 sin 628000 t + 25 cos 621720 t – 25 cos 634280 t –

This is the equation for the A.M. wave. Ans. Problem 13.13. An amplitude modulated wave is represented by the equation e = 20 (1 + 0.7 sin 6280 t) sin 628000 t Determine: (i) modulation factor

(ii) carrier amplitude

(iii) signal frequency

(iv) carrier frequency

(v) maximum amplitude of A.M. wave (vi) minimum amplitude of A.M. wave (vii) bandwidth. Solution. The equation of A.M. wave is given by e = Ec (1 + m sin wmt). sin wct Comparing with the given equation e = 20 (1 + 0.7 × sin 6280 t). sin 628000 t We get (i) Modulation factor m = 0.7 (ii) Carrier amplitude Ec = 20V (iii) wm = 6280 ∴ Signal frequency,

fm =

ωm 6280 = = 1 kHz [wm = 2pfm] Ans. 2π 2π

fm =

ωc 628000 = = 100 kHz Ans. 2π 2π

(iv) wc = 628,000 Carrier frequency,

13.8 Limitations of Amplitude Modulation

259

(v) Maximum amplitude of AM wave

Emax = E + mEc = 20 + (0.7 × 20) = 34V Ans.

(vi) Minimum amplitude of AM wave

Emin = Ec – mE c Emin = 20 – mEc = 20 – (0.7 × 20) = 6V Ans. (vii) Bandwidth = 2fm = 2 × 1 = 2 kHz.

Problem 13.14. Calculate the percentage of the total power contained in the sidebands when m = 50%, 75%, 100%, 150% and 200%. Solution. Percentage of total power contained in the sidebands is given by m2 × 100 PSB = 2 m +2 (i) If m = 50% = 0.5 PSB = (ii) If

m = 75% = 0.75 PSB =

(iii) If

12 = 0.33 = 33% Ans. 12 + 2

m = 150% = 1.5 PSB

(v) If

(0.75) 2 = 0.22 = 22% Ans. (0.75) 2 + 2

m = 100% = 1 PSB =

(iv) If

(0.5) 2 = 0.11 = 11% Ans. (0.5) 2 + 2

(1.5) 2 m2 = = m 2 + 2 (1.5) 2 + 2 = 0.53 = 53% Ans.

m = 200% = 2 PSB =

4 22 = = 0.66 = 66% Ans. 2 2 +2 6

13.8 LIMITATIONS OF AMPLITUDE MODULATION The amplitude modulation suffers from the following limitations: 1. The useful power is contained in the sidebands and even at 100% modulation, the bands contain only 33% of the total power, hence the modulation efficiency is poor. 2. Due to poor efficiency, the transmitters employing amplitude modulation have very poor range. 3. The reception in this modulation is noisy. The radio receiver picks up all the surrounding noise along with the signal.

260

Chapter 13 Amplitude Modulation (AM)

SUMMARY 1. In amplitude modulation, amplitude of a carrier wave is changed according to the amplitude of the signal.

Change in amplitude of carrier

2. Modulation factor m = Original amplitude of carrier 3. The value of m decides the distortion, m = 100% is ideal. 4. The AM wave has one carrier and two sidebands. 5. The sidebands contain 33% of the total power. 6. The efficiency of amplitude modulation is poor.

qqq

14 SSB-Modulation In the previous chapter, we have studied the standard amplitude modulation technique. This is called double sidebands with full carrier (DSBFC) modulation as in this, both the sidebands along with the carrier are transmitted. This has been found as an uneconomical technique, hence other advanced AM techniques have been developed, which are more economical and have many other advantages. The two sidebands are exact “image” of each other, hence it is not necessary to transmit both the sidebands. Usually one sideband with or without the carrier is transmitted. This is called single side band amplitude modulation. In this chapter. We shall discuss single sideband (SSB) AM techniques.

14.1 DIFFERENT FORMS OF AMPLITUDE MODULATION The different forms of amplitude modulation are: (Fig. 14.1) (a) Double sideband with full carrier (DSBFC) modulation. (b) Double sideband with suppressed carrier modulation. (c) Single sideband modulation. Carrier

(a)

(b)

(c)

Lower sideband fc – fm

Upper sideband fc

Lower sideband

fc + fm

Upper sideband

fc – fm

fc + fm Upper sideband fc + fm

Fig. 14.1

262

Chapter 14

SSB-Modulation

For comparison, the Fig. 14.1 (a) shows double sideband with full carrier (DSBFC) and (b) shows double sideband with suppressed carrier (DSBSC) and (c) shows single sideband transmission with suppressed carrier (SSBSC). It can be noted that (c) requires only half the bandwidth (BW) as compared to (a) and (b). Note: The techniques (a) & (b) have already been discussed.

14.2 SINGLE SIDEBAND AMPLITUDE MODULATION (SSB-AM) In the theory of amplitude modulation (AM), we have seen that a carrier and two sidebands (SBs) are required for AM transmission. But it is not necessary to transmit all the three signals (1 carrier and 2 sidebands). The carrier and one of the sidebands may be removed (or attenuated). The SSB modulation is the fastest spreading form of analog modulation. The greatest advantage is its ability to transmit signals by using a very narrow band width and very low power for the distances involved. For 100% modulation (m = 1), only l/3rd of the total power is present in one of the sidebands, while 2/3rd power is carried by the carrier, which contains no information. Thus if the carrier and one of the sidebands is eliminated from the signal, the transmission will need only l/6th of the total power. Problem 14.1. Calculate the percentage power saved when the carrier and one of the sidebands is removed in an AM wave, when (i) m = 1

(ii) m = 0.5

Solution. (i) When m = 1 total power ⎛ m 2 ⎞

⎛ 12 ⎞

PT = PC ⎜1 + ⎟ = PC ⎜1 + ⎟ = 1.5 PC 2 ⎠

⎝

⎝ 2 ⎠

One sideband power Saving in power

PSB = PC . =

m2 12 = PC . = 0.25 PC 4 4

1.5 − 0.25 = 83.3% Ans. 1.5

14.3 Various Single Sideband (SSB AM) Techniques

263

(ii) When m = 0.5 total power ⎛ (0.5) 2 ⎞

⎛ m 2 ⎞

PT = PC ⎜1 + ⎟ = PC ⎜1 +

⎟ = 1.125 PC 2 ⎠

2 ⎠

⎝

⎝

One sideband power Saving in power

PSB = PC . =

m2 0.52 = PC . = 0.0625 PC 4 4

1.125 − 0.0625 = 94.4% Ans. 1.125

14.3 VARIOUS SINGLE SIDEBAND (SSB AM) TECHNIQUES The AM system, in which only one sideband is transmitted is the most popular system. The system in addition to many other advantages, needs only half the bandwidth as compared to the DSB system. The following are the various sideband techniques: 1. Single sideband with suppressed carrier (SSBSC) or A 3 J 2. Single sideband with reduced carrier (SSBRC) or A 3 A 3. Single sideband with full carrier (SSBSC) or A 3 H 4. Vestigial sideband with full carrier (VSBFC) 5. Independent sideband (ISB).

1. Single Side Band with Suppressed Carrier (SSBSC) In SSB-SC, power is saved by eliminating the carrier component. Further increase in the efficiency of transmission is possible by eliminating one more sideband, since the two side bands are images of each other, each is affected by changes in the modulating voltage amplitude and each is equally affected by changes in modulating frequency which further changes the frequency of side band itself. It is seen that all the information can be conveyed by the use of single side band only. The carrier is superfluous and the other side band is rebundant. Suppose a lower side band is required, say : eSSBSC (t) = cos (wc – wm) = cos wmt + sin wct sin wmt. If cos wmt and cos wct are signal and carrier respectively, then the required signal can be produced by a balanced modulator, provided that both the signal and carrier are shifted in phase by +p/2. This method is called phase shift method of producing single side band suppressed carrier signals.

264

Chapter 14

SSB-Modulation

The Fig. 14.2 shows the block diagram showing the complete process. Crystal Osc. Crystal Osc.

USB filter

Buffer

Balanced modulator

AF

Balanced mixer

LSB filter

Amp.

Pilot Carrier

SSB Signal AFC

Fig. 14.2

The Fig. 14.3 shows the modulating signal, carrier, SSBSC output signal and frequency spectrum of the output signal.

Modulating signal

Carrier

Phase Reversal

SSB-SC Output

Frequency spectrum of SSB-SC signal

(fc – fm)

fc

(fc + fm)

t

Fig. 14.3

Advantages: The SSB-BC system has the following advantages. 1. Bandwidth required for the system is half of that required for DSB system.

14.3 Various Single Sideband (SSB AM) Techniques

265

2. The effect of selective fading is minimum as only one side band exists. In long range high frequency communication, particularly in audiorange, SSB technique is employed. The quality of communication is better in this system. 3. It requires relatively low power for communication and efficiency of transmission is increased. Application: The system is used in point to point radio telephony and in marine mobile communication, specially at distress call frequencies. Achieving frequency stability: In SSBSC system, the carrier should be suppressed atleast by 45 db at the transmitter. Earlier this system was not successful because highly stable oscillators are required but with introduction of “Frequency synthesisers”, this system is now improved a lot. If a 100 Hz “frequency shift” occurs in a system through which signals of 300, 500 and 800 Hz are passed, all these signals are shifted to 200, 300 and 600 Hz just deterioting the performance of the system. So this system is not suitable for music and speech etc. The frequency stability can be obtained by using temperature controlled crystal oscillators with the transmitter which give highest transmitting stability. As told earlier, introduction of “frequency sythesisers” with the receiver, frequency stability also improves a lot at the reception side. As the SSBSC system does not transmit the carrier, it may cause a “frequency shift”, if highly stable oscillators are not used at the transmitter as well at the receiver. The frequency stability of this system is 10 PPM (parts per million) which can be said as satisfactory.

2. Single Side Band with Reduced Carrier (SSBRC) This is an old system and was used before invention of frequency synthesisors. In SSBRC or “Pilot carrier” system, a pilot carrier is transmitted along with the SSB signal. Block diagram of this system (See Fig. 14.4) is just an addition of “Pilot Fig. 14.4 carrier” to the SSB

266

Chapter 14

SSB-Modulation

system. An attenuated (reduced amplitude) carrier is added to the final SSB signal output. The inserted carrier level is of the order of 15 to 25 db below the unsuppressed carrier level. This pilot carrier is used at the receiver for demodulation and tuning. The frequency of pilot carrier is same as that of the original carrier. This system is identical to the SSB systems studied earlier. The reduced carrier and SSB signal are added in the adder to get SSBRC signal. This system is used in transmarine point to point radio telephony and mobile communication.

3. Vestigial Single Sideband (VSB) System We know that more is the information sent per second, larger is the BW required. In TV, where larger BW is required, SSB system is very important for reducing the B.W. The BW occupied by TV video signal is atleast 4 MHz. If we use DSBFC system, minimum B W of 9 MHz will be required. If SSB system can be used, considerable BW can be saved. Therefore a compromise between SSBSC and DSBFC has been found which is known as Vestigial sideband system. In VSB system, the desired sideband is allowed to pass completely but also a portion (called vestige) of the undesired sideband is also allowed to pass through. The vestige of the undesired sideband compensates for the loss of the desired sideband (Fig. 14.5). Moreover, the VSB system does not need a filter. Picture carrier

Sound carrier

Guard Total BW = 7 MHz

Full USB = 5.0 MHz

5.5 5.75

Slope = 0.5 MHz

5

Guard 0.25 MHz

0

Slope = 0.5 MHz LSB = 0.75 MHz

1.25 0.75

Fig. 14.5

In this system, 0.75 MHz of the lower sideband (along with the complete upper side band) is also transmitted to ensure that the lowest frequencies of

14.3 Various Single Sideband (SSB AM) Techniques

267

the desired USB will not be distorted. As only 0.75 MHz of LSB is transmitted saving of 3 MHz of VHF spectrum results with every TV channel, thus making possible to allow more number of channels in the same BW. Total BW needed is 7 MHz (Fig. 14.5) The sound occupies band near the video because it is required with the picture and it is not feasible to have a separate receiver for sound operating at some distant frequency i.e., much away from the video frequency. The VSB signals are easy, whereas SSBSC signals are relatively difficult to generate. In SSB signal generation using filtering technique, the filter must have very sharp characteristics. Basically such filters must have a flat passband and extremely high attenuation outside the passband.

This system contains advantages of SSBSC as well as of DSBSC systems.

4. Independent Sideband (ISB) Technique (Fig. 14.6) This system is usually used for medium density traffic. It is mostly a four channel transmission system.

Channel A

ISB unit (a)

Balanced mod. Crystal OSC

Channel B

Balanced mod.

3 MHz crystal OSC

USB filter Carrier attenuator

Adder

LSB filter

Bal mixer

3 MHz crystal OSC

Ant Buffer and Freq. multiplier

Balanced mixer

Amiplifier

fc

Transmitter (b)

Transmitted signal

Freq. Synthesizer

Fig. 14.6

268

Chapter 14

SSB-Modulation

The system carries two independent channels simultaneously as two side bands with carrier reduced. Each sideband is independent of the other and different transmissions can be made on them. Each channel has a BW of 6 kHz and is fed to separate “Balanced modulators” alongwith a 100 kHz signal from a crystal oscillator. The balanced modulator suppresses the carrier by about 45 dB. The USB and LSB channels are selected by Filters and “Added” together with a carrier attenuated by 26 dB. The output of the “Adder” is mixed in a Balanced mixer with 3 MHz oscillator’s output. See Fig. 14.6(a). The proper frequency is selected from the Balanced mixer output and amplified. The signal is then given to the transmitter section, where it is again mixed in a mixer with the output of a “frequency synthesizer” and frequency multiplier to raise its frequency. The usual transmitting frequency is between 3 to 30 MHz. The resulting ISB signal is amplified to a power level of about 60 kW; and then fed to the antenna. (See Fig. 14.6 b) For high density point to point communication, multiplexing techniques are used such as frequency division multiplexing (FDM). However, for low or medium density traffic, ISB transmission is often employed. The ISB essentially consists of two SSB channels added to form two sidebands around the reduced carrier. However, each sideband is quite independent of each other. It can simultaneously convey a totally different transmission to the extent, what the upper sideband could. It is not advisable to mix telephone and telegraph channels in one sideband since “key clicks” may be heard in the voice circuit. However such hybrid arrangements are sometime unavoidable since the demand almost invariably tries to outstrip existing facilities. Table 14.1 S. No.

Standard AM vs SSB AM Systems Standard AM system (DSBFC)

SSB–AM systems

1.

The detection or demodulation is easy The detection or demodulation is and inexpensive in standard AM system. complex and expensive. The receivers Therefore, the standard AM system is need additional synchronizing circuits. preferred for public communication in which a transmitter is associated with large no. of receivers.

2.

The standard AM system needs large and expensive power transmitters.

3.

The standard AM signals are easy to SSB signals are generated by balance generate. modulator and their generation is complex.

The suppressed carrier AM system needs low power and less expensive transmitters. The system is suitable for point to point communication, where many transmitters but a few receivers are used.

Summary S. No.

269 Standard AM system (DSBFC)

SSB–AM systems

4.

The transmission bandwidth needed is In this system the bandwidth needed is more in this system. about half than the standard AM system. Therefore, the SSB systems are used for long distance communication, e.g. VSB is used for picture signal transmission.

5.

At unity modulation index; the transmission efficiency of standard AM system is 33%.

At unity modulation index, the transmission efficiency of SSB AM systems may be achieved upto 100%.

6.

While considering noise, this AM system is inferior. The Figure of merit (which is the ratio of signal noise ratio at output to input) is 1/3.

The SSB AM systems are superior than standard AM system, when noise is considered. In this case, the Figure of merit is unity.

7.

The non linear distortion is maximum in standard AM system.

The non linear distortion is minimum in these systems.

Table 14.2 S. No.

Comparison of Various AM Systems Particulars

AM systems DSBFC

DSBSC

SSBSC

VSBSC

1.

Bandwidth

2f

2f

f

Between f and 2f

2.

Power saving (sinusoidal)

—

66.5%

83.25%

Between 66.5% and 83.25%

3.

Power saving (Non sinusoidal)

—

—

—

4.

Generation and Detection

Not difficult

Difficult

Difficult

Between to 74%

52%

Very difficult

SUMMARY 1. The conventional amplitude modulation (DSPFC) is uneconomical. Only single sideband (SSB) without carrier is the advance technique of communication. 2. Different forms of amplitude modulations are: DSBFC, DSBSC, SSBSC, SSBRC, SSBFC, VSBFC and ISB. 3. The sideband is suppressed by filter method, phase shift method and weaver method. qqq

15 AM Transmitters

Radio broadcasting employs both amplitude as well as frequency modulation. The broadcasting in medium wave (mw) and short wave (sw) frequency bands is accomplished by means of A.M. transmitters. The signal power to be transmitted in A.M. broadcast transmitters for regional coverage is of the order of one kilowatt and may go upto megawatts.

15.1 TRANSMITTER To remind the readers, a transmitter is a device in which the signal (AM/FM etc.) is modulated, amplified and transmitted. The modulator carries out the process of modulation. It generates the modulated (AM/FM etc.) wave. Radio transmitters may be classified on the following basis: 1. Type of modulation used: On this basis, transmitters are of the following type: (a)

A.M. transmitters: In these transmitters, carrier is ‘‘amplitude modulated’’ by the signal. These are employed for: (i) Radio broadcasting—on medium and short waves (ii) Radio telephony—on short waves (iii) T.V. picture broadcasting—on very short waves.

(b)

F.M. transmitters: In these transmitters, carrier is ‘‘frequency modulated’’ by the signal. These are used for. (i) Radio broadcasting—in V.H.F. and U.H.F. range. (ii) T.V. sound broadcasting—in V.H.F. and U.H.F. range. (iii) Radio telephony on V.H.F. and U.H.F. range.

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Chapter 15 AM Transmitters

(c) P.M. transmitters: In these transmitters, the carrier is ‘‘pulse modulated’’ and these are used in telephony and telegraphy. 2. Type of service: On the type of service, transmitters are of the following types: (a)

Radio broadcasting transmitters: These transmitters are employed for transmission of sound signals for public recreation. They have low distortion and less noise. They may be A.M. or F.M. transmitters. The A.M. transmitters operate on medium and short waves and F.M. transmitters operate on very short waves.

(b)

Radio telephone and telegraph transmitters: These transmitters are employed for radio telephony, i.e., to transmit telephone and telegraph signals over long distances by radio means. These transmitters are also equipped with special devices such as volume compressors, privacy devices, etc. Both the antennas are also specially designed. These may be A.M. and F.M. transmitters. The A.M. transmitters work on short waves, whereas F.M. transmitters work on very short waves.

(c) T.V. transmitters: Two transmitters are used for T.V. broadcasting —one for transmitting picture and other for sound. Both the transmitters operate in V.H.F./U.H.F. range. The picture transmitters are amplitude modulated, whereas the sound transmitters are frequency modulated. If vestigial sideband transmission is used, the total BW occupied by one T.V. channel is about 6 MHz. (d)

Radar transmitters: Radar transmitters may be pulse transmitters or a continuous wave (CW) transmitters. They are pulse or frequency modulated. They operate on microwave (3cm wavelength) and frequency of 3,000 to 10,000 MHz.

3. Frequency range: On the basis of frequency range, the transmitters may be: (a)

Medium wave transmitters: They operate over a range of 550 kHz to 1650 kHz frequency.

(b)

VHF/UHF transmitters: They operate in V.H.F. (30 MHz–300 MHz) or U.H.F. (300 MHz to 3000 MHz) range of frequencies. They are used for F.M. radio, T.V., radio telephony, etc.

(c) Microwave transmitters: These transmitters are used on frequencies above 1000 MHz. Their application is in radars. (d)

Short wave transmitters: They operate over a range of 3 MHz to 30 MHz. For these, ionosphere propagation is employed.

15.2 Types of A.M. Transmitters

273

15.2 TYPES OF A.M. TRANSMITTERS The A.M. transmitters (on the basis of modulation) may be of the following types: 1. Low Level A.M. Transmitter (Fig. 15.1): This employs Low Level Wideband Low level Modulation (LLM) Signal power amp modulator Source scheme. If the modulated signal is generated at a Fig. 15.1 power level, which is lower than the final transmitter power required, the scheme is known as low level modulation. In this scheme, the modulated signal is amplified subsequently. 2. High Level A.M.

Transmitter: (Fig. 15.2) Signal source This employs High Level

Modulation (HLM)

scheme. In this scheme,

the modulation is carried

out at the last stage,

and no further power RF carrier Osc amplification is required. In this case, the modulating signal should have power

higher than that in low level modulation.

Wideband power Amp High level modulator

Wideband power Amp

Fig. 15.2

Table 15.1 LLM Vs. HLM S. No.

Low Level Modulation

High Level Modulation

1.

This modulation is carried out at low power level.

This modulation is carried out at high power level.

2.

Needs lesser amplifier stages

Needs more amplifier stages

3.

After modulation, linear amplifiers can Non linear amplifiers can also be used. only be used. This gives lower power This leads to higher power efficiency. efficiency.

4.

Power loss in amplifiers is higher, the cooling problem is severe.

The power loss is less, the cooling problem is not severe.

15.3 NEGATIVE FEEDBACK IN A.M. TRANSMITTERS Generally, negative feedback is provided in A.M. transmitters in relation to the modulating signal. The radiated signal is picked up from the antenna, demodulated and combined with the modulated signal and used as negative

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Chapter 15 AM Transmitters

feedback at the appropriate point. The negative feedback be used with low level modulators, as well as with high level modulators. Advantages of using negative feedback with transmitters are same as with amplifiers. The negative feedback reduces amplitude as well as frequency distortion thus increases faithfulness of the transmitter. The negative feedback also reduces the undesirable hum and noise in the transmitter.

15.4 A.M. MODULATORS In A.M. transmitters ‘‘amplitude modulation’’ is carried out. The A.M. modulators may be classified as (i) Linear Modulators: These make use of linear part of the V – I curve of transistors (or diodes). The examples of these are collector modulators and base modulators etc. (ii) Non Linear or Square Law Modulators: These make use of non linear part of the VI characteristic of the transistors (or diodes). The Fig. 15.3 and 15.4 show VI characteristic of transistor and diode respectively, where OA is the linear part and beyond A is non linear part of the curve. IC

Id

A

A

O VCE

Vd

O

Fig. 15.3

Fig. 15.4

Here linear modulators will be discussed.

15.5 LINEAR MODULATORS As mentioned above, these make use of linear part of the characteristic of transistor or diode, below we describe linear modulators using transistors, these are: (i) Collector Modulator (ii) Base Modulator (i) Collector Modulator. The Fig. 15.5 shows the collector modulator. The modulating signal is fed to the collector in series with VCC of the transistor amplifier. The carrier is fed to the base through a tuned circuit and the modulated output is obtained from the collector as shown. The modulating signal is used to vary the collector voltage. The capacitor C provides a low impedance path to the carrier currents, in other words the capacitor should not bypass the modulating frequencies.

15.5 Linear Modulators

275 VCC Modulating signal

Turning ckt.

R1 Modulating output

Carrier signal R2

C

Fig. 15.5

The power requirement of the modulator and its load can be determined from the basic voltage and current relationship of a transistor power amplifier. To obtain 100% modulation, the maximum value of the modulating voltage Vmax must be equal to the VCC. i.e., Vmax = VCC In this condition the output of the modulation amplifier is zero at the negative peak of the modulating signal. Also, the average collector current IC of the modulator amplifier reduces to zero during negative cycle of the modulating signal, therefore, IC = Imax .∴ The modulator power output Vmax. Imax P= 2 VCC. IC or P= [Vmax = VCC , Imax = IC] 2 This shows that power output of the modulator is equal to one half of power supplied to the amplifier. Therefore power from the modulator provides power for sidebands also. This has been shown already that the power in sidebands is l/3rd of the total power at 100% modulation. Moreover, the modulator load RL =

Vmax Imax

=

VCC IC

Note that modulator may use any power amplifier circuit. Push pull amplifiers are mostly used for maximum power output.

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Chapter 15 AM Transmitters

(ii) Base Modulator. In base modulator, the modulating signal is fed into the base and the output is obtained from the collector of the amplifier. See Fig. 15.6 VCC

iC b�

R1

Modulated output

a� c� iB

Carrier

c Modulating signal

iB

a b

R2

(Vm) VBB

Fig. 15.6

VCB

Fig. 15.7

Considering the voltage current relationship (Fig. 15.7) in a base modulated amplifier, power requirement for the modulator is iB – Vm P= 2 where iB is the base current and Vm is the maximum amplitude of the modulating signal. Note that: 1. The power requirement for base modulator is less than the collector modulator. 2. The power output and efficiency of base modulator is comparatively low. 3. This modulator has also poor linearity and its adjustment is more critical. The circuit is used in TV transmission as it needs little power and can meet the power requirement of larger bandwidth.

15.6 BLOCK DIAGRAM OF A.M. TRANSMITTER The function of a transmitter is to perform process of modulation and to raise power level of the modulated signal to the desired extent for effective radiation into space. They may use low level or high level modulation. The Fig. 15.8 shows block diagram of a typical A.M. transmitter. A crystal oscillator generates the carrier frequency or its multiple. It is followed by a Buffer amplifier and a tuned driver amplifier. After this, a class C amplifier is

Summary

277

used which is generally a collector modulator. The audio signal is amplified by a chain of amplifiers and a power amplifier. Usually transformer coupled class B push pull power amplifier is used for power amplification. Now, the output of the final class C amplifier is passed through an impedance matching network which includes the tank circuit of the final amplifier. The Q-factor of this circuit should be low enough so that all the sidebands of the

signal are passed without any type of distortion.

The negative feedback is often used to reduce distortion in class C modulator

system. The feedback is introduced as shown in Fig. 15.8. A sample of RF signal

sent to the antenna is extracted and demodulated to produce the feedback.

F.B. De mod.

Fig. 15.8

The low power transmitters with output power upto 1 kW or so may be transistorized, but the higher power transmitters use vacuum tubes.

SUMMARY 1. The transmitter is the apparatus in which modulation is carried out before transmission of the signal. The apparatus which carry out the amplitude modulation and amplification are known as A.M. transmitters. 2. The A.M. transmitters may be of low or of high level modulation. 3. The A.M. transmitter is provided with negative feedback, which reduces noise and increases faithfulness. 4. The A.M. modulators may be linear or non linear. 5. The examples of linear modulators are Collector modulator and Base modulator. The examples of non-linear modulators are Balanced modulator and Ring modulator. qqq

16 AM Receivers The function of a receiver is to recover the original signal. There are number of signals floating in the space, the receiver should be able to select the signal of the desired frequency. It should then be able to demodulate the received signal to recover the original signal. It should be noted that the signal received is of very low level i.e., of a few pecowatts, which needs to be amplified.

16.1 DEMODULATION OR DETECTION The process of separating the original signal from the (AM) radio wave and grounding (rejecting) the carrier is called demodulation or detection. The demodulation/detection is a process taking place in receivers. After detection, the AF signal is fed to loudspeaker, which converts the electrical signal into sound. If the AF signal is not detected and the radio wave (signal + carrier) is directly given to the speaker, the diaphragm of the speaker cannot respond to such a high frequency and will not be able to perform its function. Therefore, it is necessary to detect (separate) the signal which will strike the speaker’s diaphragm and not to allow the H.F. carrier to reach the speaker.

16.2 AM DETECTORS/RECEIVERS The AM detectors are basically of two types. (i) Linear/diode envelope detector (ii) Synchronous/square law detector The linear detectors are discussed below:

16.3 LINEAR/DIODE ENVELOPE DETECTOR This detector is very much used in commercial receivers as it is cheap, simple and provides satisfactory performance.

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Chapter 16 AM Receivers

A diode operating in the linear region of its characteristic can extract the

modulating signal from the AM wave. The Fig. 16.1 shows the circuit for the linear diode detector. The circuit

basically consists of a diode and a RC net work. D

AM Wave

R

C

Output

Fig. 16.1

Operation

AM Wave

I The A.M. wave is applied at the Linear characteristic of diode input terminals of the circuit. As Detected the diode is operated in the linear envelope I region of its characteristic, during positive cycle of the A.M. wave, the output is proportional to the Output V input signal voltage. During the negative cycle of the input, the diode does not conduct and output Input is theoretically zero. If the time constant (RC) is correctly chosen the output will follow exactly the envelope of the A.M. wave, but Fig. 16.2 spikes are introduced by charging and discharging of the capacitor, which can be reduced by taking a large RC constant.

The Fig. 16.2 shows the linear characteristic of diode along with input and output wave shapes. The output may contain ripples which are later on filtered out. The diode detector basically performs two functions: 1. The diode rectifies the A.M. wave, i.e., eliminates the negative cycle of the wave (Fig. 16.3). We know that average of both cycles of an A.C. wave is zero. In such case if both cycles of wave is fed to the speaker without rectification, it will have no impact on the speaker’s diaphragm (due to its zero average value). This job is done by the diode.

16.4 Types of AM Receivers

281

Now, the positive cycle of the A.M. wave (containing carrier + signal) starts its journey towards speaker. 2. The positive cycle of the A.M. wave is passed through a capacitor filter, which suppresses the H.F. carrier and original signal reaches to the speaker. Diode AM wave To speaker

C

Half-wave rectifier and filter

Original signal

Fig. 16.3

16.4 TYPES OF AM RECEIVERS Recall that the AM receivers contain detector and amplifiers. Two types of AM receivers are popular and are of commercial importance.

They are: 1. Tuned radio frequency receivers, and 2. Superheterodyne receivers. These are described below:

1. Tuned Radio Frequency (T.R.F.) Receivers These are also called ‘‘straight radio receivers’’. In these receivers, two or more R.F. amplifiers (tuned together) are employed to select and amplify the incoming frequency and rejecting all others. After this, the A.F. signal is detected and amplified suitably. See Fig. 16.4 Power Amp.

Antenna

1st stage

IInd stage

R.F. Amp.

R.F. Amp.

Detector

Fig. 16.4

Audio Amp.

L.S.

282

Chapter 16 AM Receivers

The important blocks are described below: 1. Antenna or Aerial: The radio signals are picked up by the antenna or aerial. The signal floating in space induce small voltage (in peco volts) at the antenna. 2. R.F. Amplifiers: As mentioned above, there are one or more stages of R.F. amplifiers. They select the particular radio frequency (as they are tuned amplifiers) and amplify to the desired level. 3. Detector: The diode detector detects (separates) the audio signal. As described already, this block performs two functions, viz., half-wave rectification of the radio wave and detection of the AF signal. 4. Audio Amplifier: The detected (AF) signal is amplified. It is also done in stages— first voltage is amplified and the last stage is of power amplification. 5. Loudspeaker: The amplified signal is fed to the loudspeaker (L.S.), which converts the electrical signal into original sound. The major advantage of TRF receiver is its high sensitivity and low

cost. But the receiver suffers from poor selectivity, instability in gain and variation of BW over the band. The receiver works satisfactorily at low and medium frequencies but at higher frequencies it has poor reception or no reception. The receiver also suffers from a variation in Q factor and BW of the tuned circuit employed in RF amplifier.

2. Superheterodyne Receiver (Superhet) All modem radio receivers are essentially the superheterodyne receivers. They are the most superior (super) circuits which utilize the principle of ‘heterodyning’ (mixing or beating) two frequencies.

(a) Principle of Heterodyning The selected radio frequency and a high frequency (produced by a local oscillator provided in the receiver) are fed into a ‘mixer’ in which both frequencies are heterodyned, as a result of which lower and higher beats are produced. The mixer circuit is so designed that lower beats are accepted as ‘output’ and upper beats are rejected. For example, f1, is the selected radio frequency and f2 is the frequency produced by the local oscillator. When both are mixed, upper beats (f2 + f1) and lower beats (f2 – f1) are produced. The lower beats are obtained as output whereas upper beats are grounded (See Fig. 16.5).

16.4 Types of AM Receivers Selector circuit

283

f1

f2 – f1 Mixer

Local OSC

f2

Output

f2 + f1

Fig. 16.5

Suppose the selector circuit selects a frequency of 1000 kHz and the local oscillator generates 1455 kHz. These two are mixed (beated) in the mixer and (1000 + 1455 = 2455) kHz and (1455 – 1000 = 455) kHz are produced. The circuit will give an output as 455 kHz and the 2455 kHz will be grounded. This output (455 kHz) is called the Intermediate frequency (I.F.). It is interesting to note that standard value of I.F. is 455 kHz and it is nationally accepted. Note that the value of I.F. is very much less than the selected frequency. Now we have to design our amplifiers, detectors and other equipment ahead at very low frequency (455 kHz) and the system becomes economical; thus the concept of I.F. is advantageous. Whatever may be the selected frequency, the local oscillator will always produce a frequency 455 kHz more than the selected frequency. Remember that in case of T.R.F. receivers, the equipment were to be designed on selected frequency itself.

(b) Block Diagram of A Superheterodyne Receiver The important blocks of a superheterodyne receiver have been shown in Fig. 16.6. 1. R.F. amplifier: This selects and amplifies the particular radio frequency. It has parallel LC circuit. By changing the value of C, the LC tuned circuit produces a resonant frequency (fr = l/2p LC ); therefore the signal of a particular frequency out of many frequencies floating in space is accepted by R.F. amplifier and all others are rejected. This signal is then amplified to the desired level.

284

Chapter 16 AM Receivers

Antenna

(f2 – f1 = 455 kHz)

R.F. Amp. L C

Mixer f1 IF

I.F. Amp.

f2 L C

Detector

AF Amp.

LS

AGC

L.O

Shaft

Fig. 16.6

2. Local oscillator (L.O.): The local oscillator has its own LC circuit, which by varying C produces a frequency equal to the selected (R.F. + 455) kHz as explained above. 3. Mixer: The mixer is also an LC circuit. It mixes the selected R.F. and the frequency generated by the local oscillator. The output of the mixer is always equal to 455 kHz (called intermediate frequency). Note that variable capacitors of R.F. amplifier, local oscillator and the mixer have a common ‘shaft’ and operated simultaneously. Whenever we ‘tune’ our radio receiver in fact, we vary three capacitors simultaneously to get the I.F. (intermediate frequency) of 455 kHz. 4. I.F. amplifier: It amplifies the I.F. output of the mixer to the desired value. The amplification is done in stages. 5. Detector (demodulator): The detector detects (separates) the original A.F. signal out of I.F. output which contains signal as well as the carrier. The signal starts its journey onward, whereas the carrier is grounded. An A.G.C. voltage is applied between I.F. amplifier and the detector. A.G.C. stands for ‘Automatic gain control’. 6. A.F. amplifier: The original (A.F.) signal is amplified here. 7. Loudspeaker: It converts the signal into the original sound. Superheterodyne (superhets) are superior in quality than T.R.F. receivers. The superhets have superior selectivity, audio quality and are cheaper in cost. Note: For its sound section, a television also employs a superheterodyne receiver. Problem 16.1. A 10 kHz signal modulates a 1100 kHz carrier. Find the frequency to be generated by the local oscillator (L.O.) of the receiver to demodulate the signal. Solution. We know I.F. = 455 kHz Frequency of L.O. = I.F. + carrier frequency = 455 + 1100 = 1655 kHz Ans.

16.5 Automatic Gain Control (AGC)

285

(c) Double Heterodyne Receiver It is not practical to heterodyne a 30 MHz signal down to an intermediate frequency (I.F.) of 455 kHz. The local oscillator (L.O.) and input signal are too close in frequency. In such cases the heterodyning is done in stages. A more practical answer to the problem is a receiver as shown in Fig. 16.7.

RF Amp.

Mixer

I.F. Stage I

L.O.

Mixer

I.F. Stage II

Detector

L.O. L.S.

AF Amp.

Fig. 16.7

(d) Advantages of Superheterodyne Receivers The superheterodyne receiver has the following advantages on TRF receivers: (i) Improved selectivity (ii) Improved stability (iii) higher gain per stage (iv) uniform band width These advantages make them suitable for most of the radio receiver applications such as AM, FM, SSB communication TV and radar receivers.

16.5 AUTOMATIC GAIN CONTROL (AGC)

Receiver output

AGC is an electronic device by III I which gain of a radio receiver IV changes automatically with the

M II changing strength of the signal I. Without AGC II. Ideal AGC so that the output remains

III. Simple AGC constant. A dc bias voltage is

IV. Delayed AGC applied to a selected number of Signal strength R.F., I.F., and mixer stages. The Fig. 16.8 overall result on the receiver output has been shown in Fig. 16.8, which shows 4 curves for comparison.

286

Chapter 16 AM Receivers

The AGC filters out input signal amplitude variations and the gain control is not required to be adjusted everytime, the receiver is thus tuned automatically from one station to another. In a simple AGC, (explained above) gain of the receiver is reduced, when strength of signal increases. Figure 16.19 also shows two other AGC curves. The second is ideal AGC curve, here no AGC will be applied till the strength of the signal remains within limits after point M, a constant output will be obtained independent of signal strength. The fourth is a delay AGC curve. This shows that AGC in not applied till the signal strength is within limits and afterwards, the AGC is applied ‘strongly’. Now, the output rises with rise in the signal strength but ‘slowly’. Figure 16.9 shows a delayed AGC circuit. There are two separate diodes, one for detector and the other for AGC. A positive bias is applied to the AGC diode to prevent its conduction till the signal strength is within limits. A ‘delay control’ as shown is provided to allow manual adjustments of AGC and diode bias and hence on the signal level, at which AGC is applied. If weak stations are to be tuned, delay control setting may be quite high to keep AGC out. Delay control +VCC

C

AGC diode AGC Output

Detector diode

IF

R

C

R

IF AMP. C

C

R

IF AMP.

AF

Fig. 16.9

Note: The AGC is sometimes also called as AVC (Automatic Voltage Control).

16.6 Automatic Frequency Control (AFC)

287

16.6 AUTOMATIC FREQUENCY CONTROL (AFC) The heart of an AFC circuit is a frequency sensitive device such as phase discriminator, which produces a DC voltage whose amplitude and polarity are proportional to the amount of the local oscillator’s frequency error. This DC controlled voltage is RF then used to vary bias on a Stage variable reactance device First whose output capacitance is Mixer thus changed. This variable To Detector IF IF Second capacitance appears across Amp. Amp. Mixer the AFC, so that voltage Local frequency oscillator (VFO) is VFO Osc. automatically free from any AFC variation with temperature, Variable DC Phase Amp. voltage, etc. (See Fig. 16.10) It is to be noted that all receivers do not require an AFC.

Reactance

Discriminator

Limiter

Fig. 16.10

16.7 GENERAL QUALITIES OF RECEIVERS In general, the receiver should have the following qualities so that it can render the optimum performance. (i) It should be capable to receive the weakest signal and should provide a sufficient output. (ii) There may be hundreds of signals floating in space but the receiver should be capable to catch the signal which is desired and should reject all other signals. (iii) The output should be an exact replica of the original modulating signal i.e., the output should be free of any distortions. (iv) The receivers should not pick up any noise or interference.

Accordingly, qualities of a receiver can be listed as below: 1.

Sensitivity

2.

Selectivity

3.

Fidelity

4.

Signal/Noise ratio

5.

Image frequency rejection

6.

Double spotting.

288

Chapter 16 AM Receivers

1.

Sensitivity: Sensitivity of a receiver is the measure of its ability to receive weak signals. The receiver receives the signal, which is amplified by the inbuilt amplifiers, therefore the measurement of a receiver sensitivity is actually the measurement of performance (gain) of its amplifiers. The sensitivity may be defined as:

(a)

In case of AM receiver, this is defined as the input carrier amplitude modulated to 30% with a modulating signal of 400 Hz which when applied to the antenna of the receiver produces a standard output (500 mW), when all the receiver controls are adjusted at maximum output.

(b)

In case of FM receivers this is defined as the input carrier frequency modulated by a 400 Hz signal so as to produce a deviation of 22.5 kHz (which is 30% of the maximum permitted deviation of 75 kHz) required to produce a standard output (500 mW) at the receiver when all the controls are adjusted at maximum output. The sensitivity of receivers lies in the range of microvolts, (Fig. 16.11) i.e., receivers can pick up microvolt signals. As the gain of receiver is not constant over a frequency band, the sensitivity is also different at different frequencies. The most important factor determining the sensitivity of a receiver is the gain of the IF amplifier.

Sensitivity (µV)

25

0 Frequency (kHz)

1600

Fig. 16.11

2. Selectivity: This is the ability of a receiver to select a particular signal out of many signals floating in the space. The selection of the signal is done by resonant circuits so the selectivity of a receiver is the measure

16.7 General Qualities of Receivers

289

of performance of the resonant circuit of the receiver. In other words, it depends on the Q factor of the circuit. A resonant circuit with high Q factor has more selectivity and vice versa. The selectivity may also be expressed in terms of sensitivity as a ratio of Sensitivity of the receiver when it is mistuned Sensitivity of the receiver when is tuned correctly The selectivity of receivers is determined by the characteristics of the RF systems.

Output

3. Fidelity: By fidelity, we mean the ability of a receiver to reproduce correctly the different modulating frequency components present in an input signal. The fidelity curve is drawn between modulating frequencies and corresponding output of the receiver (Fig. 16.12)

Modulating frequency

Fig. 16.12

The fidelity at lower modulating frequencies is determined by the low frequency characteristic of the audio/video frequency amplifiers. At higher modulating frequencies, the fidelity is determined by higher frequency characteristic of the amplifier. 4. Signal Noise Ratio (SNR): The Signal Noise ratio is defined as the ratio between signal power and noise power received at the output, i.e.: Signal power SNR = Noise power It is easy to understand that for a good receiver this ratio should be more than one. Sometimes noise generation of a receiver is specified by a term “noise figure” (N.F.) which is the measure of the extent to which noise appearing in the receiver output in the absence of signal is greater than the noise that could be present if the receiver were a perfect one from the point of view of generating minimum possible noise. SNR of ideal receiver output NF = SNR of the receiver output under test

290

Chapter 16 AM Receivers

5. Image frequency rejection. Image frequency is that received undesired carrier frequency, which after mixing with the local oscillator frequency, produces a “difference frequency” equal to the Intermediate frequency (IF). Illustration. Let us assume that we have tuned the receiver at 1000 kHz. The local oscillator frequency for this would be 1455 kHz, as IF = 455 kHz. For this local oscillator frequency, a received frequency of 1910 kHz would also produce the same IF (1910 kHz - 1455 kHz = 455 kHz) and thus processed in different stages of the receiver. The 1910 kHz will be Image frequency of 1000 kHz and is highly undesireable. In general, Image frequency corresponding to a received signal frequency of fs is (fs + 2fi), where fi is the IF. The rejection of image frequency is achieved in RF section, which is always tuned to the frequency intended to be received and frequency component of 910 kHz away from the desired frequency gets eliminated. The image frequency rejection ratio can be defined as a ratio of the gain at the signal frequency to the gain at the image frequency. This gives the degree of image frequency rejection. 6. Double spotting. When a receiver picks up the same short wave station at two nearby points on the dial, it is called “double spotting.” The main reason for this, is its poor “front end selectivity” i.e., inadequate “image frequency rejection”. The effect of double spotting is that a weak station may be marked by the reception of a nearby strong station at the spurious point on the dial. However, double spotting may be used to determine value of I.F. of a receiver since the spurious point on the dial is precisely below the correct frequency.

SUMMARY 1. The receiver recovers the original signal, amplfies and feeds to the loudspeaker or other output device. 2. The process of separating original signal from radio waves is known as demodulation or detection. 3. The detectors are of two types: linear detector and square detector. 4. A simple diode detector is the most basic linear detector.

Summary

291

5. The AM receivers are: tuned radio frequency receiver and superheterodyne receiver. 6. Superheterodynes are superior and commercially used. Double heterodyne receivers are also available 7. We have receivers fabricated of ICs which have many qualities over the conventional receivers. 8. The other components used in receivers are: noise limiter, AGC and AFC. 9. The quality of receivers are given in terms of sensitivity, selectivity, fidelity and image frequency rejection etc. 10. The ratio of signal power to noise power is called signal noise ratio. qqq

17 Frequency Modulation (FM) As described already, it is possible to convey or transmit an information by varying its frequency as well as angle of phase. These are known as frequency and phase modulation respectively and both collectively are known as “Angle Modulation”. The frequency and phase modulation systems have similar characteristics with minor difference. In this chapter, we will describe “Frequency modulation”.

17.1 FREQUENCY MODULATION The process by which the frequency of the carrier wave is changed according to the amplitude of the signal is known as “frequency modulation”. Figure 17.1 (a) shows the signal, (b) shows the carrier and (c) shows the frequency modulated wave. A

C

Signal O

(a) B

D

Carrier

(b)

Freq.

Modulated

Wave

(c)

Fig. 17.1

294

Chapter 17

Frequency Modulation (FM)

Note that only the frequency of the carrier changes according to the instantaneous amplitude of the signal. Other characteristics (phase, amplitude) of the carrier remain unchanged. When the signal voltage is zero at O, (Fig. 17.1) the frequency of the carrier does not change, the modulation is zero. During positive cycle of the signal, frequency of the carrier increases and at the peak value of the positive signal (A and C), frequency of the carrier becomes maximum as shown by closely spaced frequency cycles in the modulated wave. However, during negative cycle, the frequency of the carrier decreases and at the peak value of the negative cycle (B and D), the frequency becomes minimum as shown by rarefied frequency cycles of the modulated wave.

17.2 EXPRESSION OF FM WAVE IN TIME DOMAIN If the modulating signal is em = Em cos wmt and the carrier is

ec = Ec cos wct (Fig. 17.2)

From the definition of FM, instantaneous value of frequency will be f = fc (1 + kEm cos wmt) Where k is the constant of proportionality and fc is unmodulated carrier frequency. Now if the FM signal may be given as e = Ec sin q where q = fw dt = fc (1 + k Em cos wmt) dt ⎡ kEm f c sin ωm t ⎤ = ⎢ωc t + ⎥ fm ⎦ ⎣ If d is the frequency deviation where,d = Max. instantaneous frequency – carrier frequency = fc(1 + k Em) – fc = k Em fc Now the FM signal

+V 0 –V

t Modulating Signal (em = Em cos wmt)

+V 0 –V

t Carrier Wave (ec = Ec cos wct)

+V

e = Ec sin q ⎡ kE f sin ωm t ⎤ = Ecsin ⎢ωc t + m c ⎥ fm ⎣ ⎦

t

–V FM Wave

Fig. 17.2

17.2 Expression of FM Wave in Time Domain

295

⎡ ⎤ δ = Ec sin ⎢ωc t + f sin ωm t ⎥ m ⎣ ⎦

( k Em fc = d)

= Ec sin (wct + mf sin wmt) d where mf= modulation index = . fm This is the equation of FM wave. Note: 1. The modulation index for FM is defined as

Maximum frequency deviation d

= mf = Modulating frequency fm

As the modulating frequency (fm) decreases and the modulating voltage amplitude remains constant, the frequency modulation index increases. This is the basis for distinguishing frequency modulation from phase modulation.

2. The mf is the ratio of two frequencies. 3. The max. change in the instantaneous frequency from the average frequency, wc is called max frequency deviation. Max frequency deviation = modulating index × modulating frequency (dm = mf × fm) The max frequency deviation is a useful parameter for determining bandwidth of FM signal. This may be a positive deviation or a negative deviation. (Fig. 17.3) – dm

+ dm

wc

Fig. 17.3

4. Unlike AM, the modulation index for FM may be greater than unity. 5. The above derived equation for FM signal is for single tone modulation as the modulating wave is single wave. In multitone modulation the modulating wave is usually of multitone nature i.e., it consists of a group of sine waves of different frequencies, which may be harmonically unrelated.

296

Chapter 17

Frequency Modulation (FM)

6. Depending upon the value of modulation index mf, the FM may be of two types: (a)

Narrow band FM : Which has small value of mf.

(b)

Wide band FM: Which has a large value of mf.

The BW for a narrow band FM is closely equal to twice of the BW for standard AM (DSBFC), where as in case of wide band FM, its value is quite more. Table 17.1

Narrow Band vs Wide Band FM Narrow band FM

Wide band FM

1. In this system modulating index is near unity.

1. In this system modulating index is more than unity and ranges between 5 to 2500.

2. The maximum modulating frequency is 3 kHz.

2. The modulating frequency ranges between 20 Hz to 20 kHz and maximum deviation allowed is 75 kHz.

3. In this system, the noise suppression is not good but occupies much less BW than wideband system.

3. With large deviation, noise is better suppressed, this system occupies 15 times more bandwith than occupied by narrow band system.

4. The narrow band system is more suitable 4. The wideband system is suitable for for communication systems. This is also entertainment broadcasting. used in FM mobile services used for police, ambulance etc.

Problem 17.1. In a frequency modulation system the audio frequency is 400 Hz and the audio frequency voltage is 2.5 V. The deviation is d = 5.0 kHz. If the audio frequency voltage is 7.5V; Find the new deviation, and the modulating index. Solution. As d ∝ Em, we can write as d 5.0 = = 2 kHz per volt Ans. Em 2.5 (i) When Em = 7.5 V, the new deviation. d = 2 × 7.5= 15.0 kHz. (ii) Hence the modulating index d 5.0 kHz mf = = = 12.5 Ans. [400 Hz = 0.4 kHz] fm 0.4 kHz Problem 17.2. A speech signal in a telephone system occupies frequency range of 300–3400 Hz (considered on a band). In a carrier system, it is transmitted as SSB signal. Calculate saving in the bandwidth, as compared to AM transmission.

17.2 Expression of FM Wave in Time Domain

297

Solution. Bandwidth of AM signal = 2 × 3400 = 6800 Hz Bandwidth of SSB signal = 3400 Hz Saving in Bandwidth = 6800 – 3400 = 3400 Hz Ans. Problem 17.3. An FM signal is represented by e = 10 sin (108t + 15 sin 2000 t)

..(i)

Find parameters of the FM wave. Solution. The standard equation of FM wave is: e = Ec sin (wct + mf sin wmt)

..(ii)

Comparing the given equation (i) with the standard equation (ii), we have (a) The carrier amplitude (b) The Carrier frequency (c) The modulating index

Ec = 10 V Ans. w 108 fc = c = = 16 MHz Ans. 2p 2p mf = 15 Ans.

(d) The modulating frequency

fm = (e) The max. frequency deviation

2000

= 318 Hz Ans. 2p

d = mf.fm = 15 × 318 = 4.7 kHz Ans.

Problem 17.4. An FM wave is represented by the voltage equation. v = 16 sin [6 × 108 t + 5 sin 1200 t] where t is the time in second. (a) Find (i) carrier voltage (iii)

(ii) modulating frequency

modulation index

(iv) max. deviation of the FM wave

(b) What power will the FM wave dissipate in a 20 W resistor. Solution. Comparing the given equation with the following standard equation of the FM wave. e = Ec sin [wct + mf sin wmt] Now, (i) carrier voltage = 16 V Ans.

(ii) modulating frequency (iii) modulation index (iv) deviation

1200

= 191 Hz Ans. 2p mf = 5 Ans. fm =

d = fm.mf = 191 × 5 = 955 Hz Ans.

298

Chapter 17

Frequency Modulation (FM)

(b) The power dissipated by FM wave in a 20 W resistor (Vrms ) 2 (16 2) 2 = =22.54 .54WW P == R 20

[Vrms = Vmax /

2 ] Ans.

17.3 POWER OF FM WAVE Though the frequency of FM wave varies with time, the carrier amplitude remains constant. Therefore, it can be shown that average power of a FM wave remains always equal to the carrier power. When modulation applied, the total power of the carrier is redistributed among all the components of the spectrum. At certain values of mf, when the carrier component becomes zero, all the

power is carried by the side frequencies.

17.4 CALCULATION OF BW (CARSON RULE) The Carson’s rule (a thumb rule) states that the BW required to pass an FM wave is twice the sum of deviation and highest modulating frequency, i.e., BW = 2 (d + fm) Further,

BW = 2 (d + fm) = 2d + 2fm ⎛ f ⎞

= 2d ⎜1+ m ⎟ ⎝ δ ⎠

⎛ 1 = 2d ⎜1+ ⎜ m f ⎝

⎞ ⎟⎟ ⎠

= 2d (fm.mf + fm)

[As mf = d/fm] [multiplying by fm.mf]

= 2dfm(l + mf). Note: (i) When d fm (wide band FM): i.e., mf >> 1 BW = 2d [Neglecting 1/mf] Note that this gives an approximate value only but gives sufficient good results if modulation index is more than 6.

17.4 Calculation of BW (Carson Rule)

299

Problem 17.5. What is the frequency deviation of an FM transmitter; if its modulating index is 6 in a bandwidth of 150 kHz.

Solution. According to a thumb rule (Carson rule) the BW of an FM wave is

twice the sum of deviation and the modulating frequency.

i.e.,

BW = 2(d + fm) mf =

but,

...(i)

d d d or fm = = fm mf 6

Now putting in Eq. (i) 150 = 2(d + d/6); d = 64.2 kHz Ans. Problem 17.6. The FM radio link having a deviation ratio of 10 is to transmit a speech band upto 5 kHz. What RF bandwidth should be used. Solution.

10 =

d fm

i.e.,

10 =

d or d = 50 kHz. 5 kHz

Now BW to be used

= 2(d + fm) (Carson rule) = 2(50 + 5) = 110 kHz. Ans.

. Table 17.2  Significant side Frequencies

Modulating Index (mf)

No. of significant side Frequencies

0.1

2

0.2

2

0.3

4

0.5

4

1.0

6

2.0

8

5.0

16

10.0

28

20.0

50

30.0

70

300

Chapter 17

Frequency Modulation (FM)

17.5 PLOTTING FREQUENCY SPECTRA FOR FM Using Table 17.2, we can find the size of carrier and each side band for a specific modulation index and can also draw frequency spectrum for the FM wave. (Fig. 17.4) EC mf = 1.0

fc – 3fm

fc + 3fm

(a) EC mf = 2.4

fc – 4fm

fc + 4fm

(b) EC mf = 5.0 fc – 9fm

fc + 9fm

(c) Fig. 17.4

Note that as modulation depth (mf) increases, the bandwidth also increases and also a reduction in the modulating frequency (fm) increases the number of sidebands (though not the bandwidth). Also note that though the no. of sidebands are theoretically infinite, in practice a lot of higher side bands have no significance and can be ignored. Notes: 1. The unmodulated carrier frequency in an FM signal is known as “centre frequency”.

17.6 Pre-Emphasis and De-Emphasis

301

2. In the modulated FM signal, the intelligence contained depends on the frequency variations and also the rate at which these variations take place. 3. The frequencies, which are having amplitude equal to or greater than 1% of the unmodulated carrier are considered to be “significant” and other as an “Insignificant”. The range of all significant sideband frequencies is called the Bandwidth. The number of significant frequencies depends upon the maximum frequency deviation. 4. Unlike AM system, where the power or the amplitude of the carrier remains same after modulation, in FM, the carrier amplitude falls after modulation because the sidebands derive their power from the carrier. So this is possible to make carrier amplitude zero and inject whole of the power into sidebands. This is highly desirable as carrier does not have any intelligence. 5. More the number of significant sidebands, more is the bandwidth, also if the modulating frequency is higher, the sidebands are widely spaced and the band width is increased. 6. The broad band FM has less distortion, but the adjacent channel interference is more, in other words, less number of FM signals can be transmitted on a particular radio spectrum simultaneously. The FM signal having band width comparable with an AM signal is called narrow band FM. A narrow band FM has heavy distortion, but adjacent channel interference is less. 7. The carrier frequencies allotted to FM broadcast are higher than the AM broadcast, as the FM has comparatively larger bandwidth, thus to accommodate more FM signals on a particular frequency spectrum without any adjacent channel interference, higher carrier frequencies are must.

17.6 PRE-EMPHASIS AND DE-EMPHASIS The “pre-emphasis” is the process of emphasing or increasing the amplitude of high frequency components of modulating signals before modulation. This is done to increase the signal-noise ratio of the high frequency components. This makes the reception noise free. By changing the amplitude of high frequency components some distortion is introduced in the signal. This is avoided by doing de-emphasis (i.e., decreasing the amplitude) during demodulation.

302

Chapter 17

Frequency Modulation (FM)

It can be seen that the noise has a greater effect on higher modulating frequencies than on the low modulating frequencies. Therefore, if higher modulating frequencies are amplified at the transmitter and accordingly cut at the receiver, an improvement in the noise immunity can be obtained. The process carried out in the transmitter is known as “Pre-emphasis’’ and the process carried out in the receiver is called de-emphasis. If two modulating signals have same initial amplitude and if the high frequency is amplified thrice of its amplitude and the low frequency remains uneffected, the receiver has to de-emphasis the first signal to one third so that both the signals have the same amplitude at the output of the receiver. When a signal is de-emphasised, any noise sideband voltages along with the signal are also de-emphasised and thus the effect of the noise is reduced. The Fig. 17.5 (a) shows a circuit for pre-emphasis, and (b) a circuit for deemphasis. In pre-emphasis, RL circuit is connected at the collector of transistor amplifier, the values of R and L are such that the time constant 0.75 L 0.75 H = = × 106 = 75ms. 10 × 103 ohm R 10 K +V 0.75 H

L

Pre-emphasised

input

10 K

Mod frequency

R

R 75 K

Pre-emphasised output

(a) Pre-emphasis

1 nf De-emphasised output C

(b) De-emphasis

Fig. 17.5

Similarly in de-emphasis, there is a RC circuit, where R = 75K and capacitor C is of 1 nano farad such that the time constant, RC= 75K × 1 nf = 75 × 103 W × 1 × 10– 9f

[1 nf = 10–9 f]

= 75 × 10–6 s = 75 ms. Therefore these will be called as 75 ms pre-emphasis and 75 ms de-emphasis which is the standard for these processes.

17.7 FM Versus AM

303

The Fig. 17.6 shows pre-emphasis and de¬emphasis curves for 75 ms. Note that de-emphasis curve corresponds to the curve which is 3 dB down at the frequency whose RC constant is 75 ms. and that frequency is given by: 1 f= 2pRC 1 = 2p × 75 ms 1 Hz = 2p × 75 × 10–6 = 2123 Hz.

+17 dB

e-

+3 dB

Pr

O –3 dB

–17 dB

De

-e

2123 Hz

Fig. 17.6

is

as

ph

em

m

ph

as

is

15 kHz AF

17.7 FM VERSUS AM 1. In AM, there are only three frequencies (one carrier and two sidebands), but in FM, there may be infinite number of carriers as well as the sidebands—separated from the carrier by fm , 2fm, 3fm and so on. 2. In AM, the modulating index (m) decides the power of the modulated wave but in FM, total transmitted power remains constant however, with higher value of m, ‘bandwidth’ increases. 3. In AM, the amplitude of the carrier remains constant. But in FM the amplitude of the carrier does not remain constant. Sometime in FM the carrier may disappear completely. (a) Advantages of FM over AM 1. The amplitude of FM signal is constant. In FM the total power remains constant but increased depth and modulation increases the total BW required for the transmission. Whereas in AM, increased depth of modulation increases the sideband power, i.e., total transmitted power. In other words in AM transmission, low level modulation may be used thereby giving higher efficiency. 2. The most important and unique feature of FM is that it is noiseless. The reason being that there happens to be less noise at frequencies at which FM is used, further the FM receivers can be fitted with Amplitude limiters, which eliminate the amplitude variations caused by noise. Moreover, it is also possible to reduce noise by increasing deviation. In AM, the noise is also carried by the modulated wave. 3. The adjacent channel interference is less in FM as compared in AM.

304

Chapter 17

Frequency Modulation (FM)

(b) Disadvantages 1. In FM, a B.W. of 10 times as in AM is required. 2. The area of reception for FM is much smaller than in A.M. 3. The equipment required in FM are more complex and costly than in A.M. Table 17.3 FM Versus AM S. No.

1.

FM

AM

The amplitude of FM wave is constant. It is The amplitude of AM wave is changing with independent of the modulating index.

2.

the modulating index.

The transmitted power remains constant. It The transmitted power does not remain is independent of modulating index.

constant

and

is

dependant

on

the

modulating index. 3.

All the transmitted power is useful.

Power of carrier and of one sideband are

4.

The BW depends on modulating index. The BW is not dependant on the modulating

useless. The BW is quite large.

index. The BW is much less than that of FM.

5.

The FM receivers are immune to noise.

6.

The FM transmitters and receivers are The AM transmitters and receivers are complex.

7.

The AM receivers are noisy. simpler comparatively.

The number of sidebands are many and The number of sideband is fix i.e., only two. depend upon the modulating index.

8.

The space wave propagation is used for Ground and sky wave propagation is used, FM, so the radius of transmission is limited for AM, so a larger area is covered than in to the line of sight.

9.

FM.

It is possible to operate several channels It is not possible to operate more channels on the same frequency.

10.

(transmitters) on the same frequency.

It is used in Radio, TV broadcasting, It is used in Radio and TV broadcasting police wireless and in point to point communication.

SUMMARY 1. When frequency of the carrier is changed according to the signal and other parameters remain constant, the process is known as frequency modulation. 2. Equation for FM wave is e = Ec sin (wct + mf sinwmt) 3. The frequency spectrum of FM wave is given by Bessel functions.

SUMMARY

305

4. The B.W. required to pass an FM wave is twice the sum of deviation and highest modulating frequency. This is known as Curson thumb rule. 5. The FM is much immune to noise. Amplitude limiter removes the amplitude variations caused by noise signal. 6. Higher modulating frequencies are amplified at the transmitter. This is known as pre-emphasis. The reverse process is done in receiver, known as de-emphasis. 7. In narrow band FM, the modulating index is nearly unity and in wideband FM, its value may be 2500. 8. The FM requires very large bandwidth than in AM. qqq

18 FM Transmitters

The FM transmitters have a very large bandwidth as compared to AM transmitters. In FM transmission, ground wave and sky wave propagation is not possible, and signals in VHF and UHF bands are propagated by line of sight propagation, which restricts the range upto 50 km. The FM transmitters generate FM wave and propagates it through antenna.

18.1 FM GENERATION We can generate FM signals by using various FM modulators. The basic requirement of FM modulator (FM signal generator) is to provide a variable output frequency with varying proportions to the instantaneous amplitude of the modulating voltage. The FM signals can be generated by two methods: 1. Direct Methods: The FM signals can be generated directly by frequency modulating the carrier. 2. Indirect Methods: The FM signals can be generated indirectly by integrating the modulating signal and then allowing it to phase modulate the carrier.

Or by using the modulating signal first to produce a narrow band FM signal and then by frequency multiplication, the frequency deviation may be increased to the desired level.

Table 18.1

Comparison of AM and FM broadcasting. AM Broadcasting

FM Broadcasting

1.

It requires smaller transmission bandwidth. It requires larger bandwidth.

2.

It can be operated in low, medium and high frequency bands.

It needs to be operated in very high and ultra high frequency bands.

3.

It has wider coverage.

Its range is restricted to 50 km.

308

Chapter 18 AM Broadcasting

FM Transmitters

FM Broadcasting

4.

The demodulation is simple.

5.

The stereophonic transmission is not In this, stereophonic possible. possible.

The process of demodulation is complex.

6.

The system has poor noise performance.

It has an improved noise performance.

7.

The AM signal reception does not have any threshold in the useful range of signal noise ratio (SNR).

The FM signal reception exhibits a threshold in the useful range of signal noise ratio (SNR). The SNR value should be higher than the threshold.

transmission

is

18.2 DIRECT METHODS OF FM WAVE GENERATION As mentioned above, in these methods, the modulating signal varies the carrier frequency directly. In general, the oscillators/multivibrators are used to generate the carrier signal. An oscillator has a tuned LC circuit, which determines the frequency of the carrier signal. If any of the two reactive elements (inductance or capacitance) is changed in accordance to the modulating voltage, the carrier frequency is changed accordingly. An oscillator, whose frequency is changed (modulated) by the modulating voltage is called a voltage controlled oscillator (VCO). A voltage variable reactance device is placed across the tuned or tank circuit of an oscillator which is tuned to the carrier frequency (in absence of modulation). Now if the modulating voltage is increased, the L or C will change accordingly. Larger is the departure of the modulating voltage from its normal value, larger will be the reactance variation and hence the variation in the frequency. There are number of devices, whose reactance can be varied by the application of voltage. The 3 terminal devices are bipolar transistor (BPT), field effect transistor (FET) and vaccum tube. The most suitable 2 terminal device is a varactor diode.

The following modulators using direct methods of FM generation are discussed

here:

(a) Reactance modulator (b) Varactor diode modulator (c) VCO modulator (d) Stablized reactance modulator

(a) Reactance Modulator The modulator makes use of a BJT (Fig. 18.1) or FET (Fig. 18.2), which exhibits a variable reactance with change in the modulating signal. The device

18.2 Direct Methods of FM Wave Generation

309

is connected across the tank or tuned circuit of the oscillator. The reactance may be made inductive or capacitive by some change in the component. The value of this reactance is proportional to the transconductance (gm) of the BJT (or of FET), which can be made to depend on the biasing of the base (or gate) of the device. Ig C

IC

XC

Modulating signal

Z

BJT

C

Vg

R

Tank circuit of OSC

L

V

Fig. 18.1 Ig C

IC

XC d

Modulating signal

g

Z FET

R

C

L

s

Vg

Tank circuit of OSC

V

Fig. 18.2

The Table 18.2 shows four different arrangements of reactance modulators. The two give a capacitive reactance and the other two give an inductive reactance. Table 18.2 S.No.

FET Reactance Modulators (Fig. 18.2) Type of the modulator

Zgd

Zgs

Condition

Reactance formulae

1.

RC Capacitive

C

R

XC >> R

Ceq = gm RC

2.

RL Capacitive

R

L

R >> XL

Ceq = gm L/R

3.

RC Inductive

R

C

R >> XC

Leq = RC/gm

4.

RL Inductive

L

R

XL >> R

Leq = L/gm .R

In the Table, R = Resistive, L = Inductive and C = Capacitive.

310

Chapter 18

FM Transmitters

Note: Any reactance modulator can be connected across the tank or tuned circuit of any LC oscillator provided that oscillator does not require two tuned circuits for its operation. The Hartley and Colpitt oscillators are most commonly used. Problem 18.1. Determine the value of the capacitive reactance of the FET modulator, whose transconductance is 10 millisiemens. The gate to source impedence (Zgs) is one-tenth of the gate to drain impedence (Zgd). Assume frequency as 5 MHz.

Solution. i.e.,

XC = 10 R

Z n = gd = 10 Zgs

gm = 10 mS = 10 × 10–3 S g The Ceq = m 2pfn and capacitive reactance 1 1 2pfn n XC = = × = 2pf.Ceq 2pf gm gm eq and

XC

eq

=

10 n = = 1000 ohm Ans. gm 10 × 10–3

Problem 18.2. Determine the capacitive reactance which can be obtained from a FET reactance modulator, whose gm = 12 mS. Assume Zgd = 8 Zgs. The frequency may be taken as 3 MHz. Solution.

Zgd = 8 Zgs, n = 8, gm = 12 mS = 12 × 10–3 S 8 8 × 1000 n XC = = = –3 gm 12 × 10 12 eq = 667 ohms

Ans.

(b) Varactor Diode Modulator The varactor (or capacitor) diode is a two terminal device whose capacitance varies with applied voltage. Here applied voltage is a combination of bias voltage (V0) and the modulating voltage m (t). The diode capacitance forms a part of the tuning capacitance which determines the frequency of oscillations. The capacitance varies with the change in the modulating signal and so does the frequency. The Fig. 18.3 shows a varactor diode modulator. The voltage V0 is the reverse bias voltage across the varactor diode. The value of C is kept smaller than

18.2 Direct Methods of FM Wave Generation

311

Modulating signal m (t)

C Tank circuit of OSC m(t)

V0

Cd Veractor diode

C1

L1

Output FM signal

Fig. 18.3

the diode capacitance Cd to keep the RF voltage from oscillator across the diode smaller than V0. The capacitance Cd comes in parallel to the LC circuit of the oscillator, thus total capacitance Ct is the sum of tank capacitance and the diode capacitance. Ct = C1 + Cd Hence the frequency of oscillation is given by 1 f = 2π L1 (C1 + Cd ) Since frequency of oscillations depends on modulating signal, therefore frequency modulated signal is generated. Though this is the simplest modulator, its applications are limited to automatic frequency control (AFC) and remote tuning.

(c) Voltage Controlled Oscillator (VCO) modulator The voltage controlled oscillator is a sinusoidal oscillator. Its frequency is controlled by an external voltage, thus it is a kind of frequency modulator. It is also available in the form of an IC chip. The Fig. 18.4 shows a block diagram of the modulator showing voltage controlled oscillator (VCO), followed by a series of frequency multiplier and mixer. The characteristics of this modulator are good stability and a wide band FM output.

312

Chapter 18

FM Transmitters

Fixed OSC

Modulating signal

VCO

Wide band FM wave output

Frequency multiplier

Band pass filter

Mixer

Band pass filter

Frequency multiplier

Fig. 18.4

(d) Stabilized Reactance Modulator In a commercial transmitter, the oscillator on which a reactance modulator operates can not be a crystal oscillator because the latter provides stable but fixed frequency. Therefore it is used to perform the frequency stabilisation of the reactance modulator. The process is similar to the automatic frequency control (AFC). The Fig. 18.5 shows its block diagram. The reactance modulator operates on the tank circuit of a LC master oscillator. It is isolated by a buffer, whose output goes to the amplitude limiter. A fraction of the limiter’s output (fs) is fed to a mixer which also receives a signal (fo) from a crystal oscillator. The output from the mixer which is the difference of the frequencies (fs – f0) and is about l/20th of the master oscillator’s frequency is amplified and fed to a LC master OSC

AF

input

Amp. Limiter

Buffer

FM signal

Phase discriminator

Reactance modulator

Crystal OSC

Fig. 18.5

fs Mixer f0

fs – f0

IF Amplifier

18.2 Direct Methods of FM Wave Generation

313

phase discriminator. The output of the discriminator is given to the reactance modulator which provides a dc voltage to correct automatically any drift in the average frequency of the master oscillator. Operation: The discriminator should be connected to give a positive output if the input frequency is higher than the discriminator’s tuned frequency, and it should give a negative output if input frequency is lower than the discriminator’s frequency. We consider the case when the frequency of the master oscillator drifts higher. A high frequency (fs) therefore will be fed to the mixer. The mixer output (fs – f0) will be bit higher which will be given to the discriminator. As explained above, the output of the discriminator will be a positive dc output. This positive dc output will be fed to the Reactance modulator, which will increase the transconductance (gm) of the modulator. As a result the equivalent capacitance of the reactance modulator will also increase as Ceq = gm RC. This will lower the oscillator’s central frequency as f 0 = 1 / 2π L.Ceq . Thus the frequency rise of the master oscillator is compensated.

Similarly, when the master oscillator drifts lower, a negative correcting voltage

obtained from the discriminator will be used to increase the oscillator’s

frequency.

• Limitations of Direct Methods The direct methods of FM generation suffer from the following limitations: (i) In direct methods of FM generation, it is difficult to obtain a high order of stability in carrier frequency. This is because the modulating signal directly controls the tank circuit which is generating the carrier. The crystal oscillator cannot be used as it provides a stable but fixed frequency. (ii) The non linearity produces a frequency variation due to harmonics of the modulating signal hence there are distortions in the output FM signal.

18.3 INDIRECT METHODS OF FM GENERATION The indirect method removes the limitations of direct method. The indirect method employs the phase modulation to obtain FM signal. It is only necessary to integrate the modulating signal prior to applying it to the phase modulator. A general block diagram for such a method is shown in Fig. 18.6. This is used in VHF and UHF radio telephony.

314

Chapter 18 Modulating signal

Modulating amplifier

FM Transmitters

Integrator

Freq multiplier

Phase modulator

PA

Crystal OSC

Fig. 18.6

Here Armstrong and phase shift indirect methods will be discussed.

(a) Armstrong Method : Principle This is the widely used indirect method for FM generation. This is known by the name of its inventor. The Fig. 18.7 explains the principle of Armstrong method. The modulating signal m(t) is first integrated and then the integrated output is used to phase modulate a carrier obtained from a crystal oscillator. The modulating index is kept small and a narrow band FM wave is generated. The narrow band signal is then multiplied in the frequency multiplier and a wide band FM wave in obtained. Note that frequency multiplier is a non linear device, which multiplies the frequency of the input signal. In general a nth law frequency multiplier can multiply the frequency and the modulating index by ‘n’. Integrated output

m(t)

Integrator

Narrow band FM wave

Phase modulator

Wide band FM wave

Freq multiplier ×n

Crystal Osc.

Fig. 18.7

The narrow band FM output of the phase modulator for a sinusoidal modulating signal is given by: e0 (t) = Ec cos [2pfc t + b1 sin (2pfm t)] Where fc is the frequency of the carrier obtained from crystal oscillator, b1 is the modulating index and fm is the frequency of the modulating signal.

18.2 Direct Methods of FM Wave Generation

315

The wide band FM wave is obtained after it is multiplied by n in the nth law frequency multiplier and can be given as, e(t) = Ec cos [2pnfc t × b2 sin 2pfm.t] where

b2 = n. b1

• Frequency Stabilized Armstrong FM Modulator/Transmitter (i) The crystal oscillator generates a carrier frequency of 200 kHz. The output of this crystal oscillator and the modulating signal are fed to a balanced modulator. The crystal oscillator output is also fed to phase shifting net work to produce a 90° phase shift. Both the outputs are now fed to a combining net work and FM output is obtained. The frequency deviation obtained in this method is very small (less than 50 Hz) and therefore tremendous frequency multiplication is desired which is carried in two sections to obtain the standard deviation of 75 kHz. (ii) The Fig. 18.8 shows a frequency stabilized indirect Armstrong FM generation in which a crystal oscillator generates 200 kHz carrier signal with a deviation of 24.4 Hz. The output is multiplied in first section by 64 (2 × 2 × 2 × 2 × 2) i.e., by frequency doubler which raises the FM signal to 200 kHz × 64 = 12.8 MHz and deviation to 24.4 Hz × 64 = 1.56 kHz. Now another crystal oscillator generates 10.925 kHz carrier which is fed to mixer, which reduces the FM signal to 12.800 – 10.925 = 1.875 MHz, the deviation remains unaltered i.e., 1.56 kHz. Now this output of the mixer is passed through the second frequency deviation multiplier to multiply it by 48 (3 × 2 × 2 × 2 × 2) i.e., one triplet and four doublers to get a standard FM output = 1.875 × 48 = 90 MHz and of deviation = 1.56 kHz × 48 = 75 kHz. Modulating signal Crystal Osc. 200 kHz

Crystal Osc. 10.925 MHz

Balanced modulator

Combining Network

200 kHz 24.4 Hz

Freq. multiplier × 64

12.8 MHz 1.56 kHz

Mixer

90° phase shifter

PA

90 MHz 75 KHz

Fig. 18.8

Freq. multiplier × 48

1.875 MHz

316

Chapter 18

FM Transmitters

Phasor diagrams. The Fig. 18.9 shows phasor diagram, which illustrates how PM wave is generated using Armstrong method. The Fig. 18.9 (a) shows an amplitude modulated signal. See that the resultant of two sideband phasors are always in phase with the unmodulated carrier phasor, so that there is only amplitude variation and phase or frequency variation is absent. But for frequency variation, an amplitude modulated signal is added to an unmodulated carrier of same frequency and both these signals are kept 90° out of phase. The resultant of these two is a complex form of phase modulation. [(See Fig 18.9 (b)]. Resultant of side bands

Side band (SB)

SB

Carrier

(a)

SB

(b)

Side band (SB)

Carrier Output

f

er

rri

Ca SB SB

(c)

Fig. 18.9

If the carrier of the AM wave is suppressed, so that only two sidebands are left which are added to an unmodulated carrier and phase modulation is obtained [Fig. 18.9 (c)]. After this, the carrier is suppressed by a “Balanced modulator.” The addition of the signal is carried out in a ‘combining net work’. Now we have obtained PM signal and FM signal can be obtained by “bass boosting” of the modulating signal. For this a simple RL network (called RL Equalizer) is used as shown in Fig. 18.10. AF Input

L

RL network

Fig. 18.10

Equalized AF output R

18.4 FM Transmitters

317

For FM broadcasting wL should be = R (at 30 Hz). With increase in frequency, the output of the equalizer falls at the rate of 6 dB/octave.

The method possesses a better frequency stability but suffers from excess noise

due to tremendous multiplication. The output also suffers from distortion.

(b) RC Phase Shift Method An RC phase shift modulator is shown in Fig. 18.11. The transistor is used as an amplitude sensitive resistor. The maximum phase modulation which can be obtained is 90°. Generally, a phase modulator is operational only for a phase change of 10°. A modulating signal, which creates a phase change results a change in the carrier amplitude, which is very small and tolerable. The output of the modulator is narrow band FM, which is converted into wideband FM by frequency multipliers.

VCC

R C

R

R C

R C

R

R

C

R

C

Fig. 18.11

18.4 FM TRANSMITTERS Depending upon the method of FM generation employed, the FM transmitters may be classified as: (a) Direct FM transmitters (b) Indirect FM transmitters.

(a) Direct FM Transmitters (Employing Reactance Method) These generally produce sufficient frequency deviation and need little frequency multiplication, but they have poor frequency stability. The frequency stability in reactance modulators may be achieved by using automatic frequency control (AFC). This system is called a cross by system. The Fig. 18.12 shows a block diagram for a cross-by direct FM transmitter for 96 MHz which is the standard frequency for broadcasting. The modulating signal after amplification is passed through a pre-emphasis to reduce the noise effect at higher audio frequencies. The reactance modulator can produce a maximum frequency deviation of 5 kHz. The carrier oscillator generates a carrier frequency of 4 MHz which is raised to 96 MHz by using frequency multipliers.

318

Chapter 18

FM Transmitters

Here a frequency multiplication of 24 (96/4 = 24) is required which can be done by using three “frequency doublers” and one “frequency tripler” (24 = 2 × 2 × 2 × 3). The initial deviation is selected in such a way that the frequency deviation attains the value of 75 kHz which is the prescribed frequency deviation for FM transmission. The maximum deviation of the reactance modulation is therefore kept equal to 75/24 = 3.12 kHz. Modulating signal

4 × 24 = 96 MHz Mixer

Audio Amp.

Pre emphasis

Frequency doubler and tripler circuit 2 MHz

4 MHz

Discriminator

Reactance modulator

Carrier Osc.

Zero d.c. voltage PA

Stabilized output

94 MHz Crystal Osc.

Voltage stabilization circuit

Fig. 18.12

The generated FM signal is fed to the frequency stabilization circuit. The function of this circuit is to provide a controlled dc voltage to the reactance modulator, whenever it drifts from the desired value of 4 MHz. The generated FM signal of 96 MHz and 94 MHz output of a crystal oscillator are fed to a mixer. The output of the mixer is the difference of the two inputs i.e., 96 – 94 = 2 MHz, which is given to the discriminator circuit. The discriminator provides zero dc voltage when its input is exactly 2 MHz. This is possible when the transmitter operates exactly at 96 MHz. The discriminator output will not be zero if the transmitter frequency differs than 96 Hz. Thus the output of the discriminator is utilized to adjust the frequency of transmitter output. The stabilized FM signal finally goes to power amplifiers. (PA).

(b) Indirect FM Transmitter It is also called Armstrong FM transmitter after the name of its originator and is based on the indirect method of FM generation. The advantage of this method is that a crystal oscillator can be used more efficiently. The crystal oscillator generates a carrier frequency of 200 kHz. The output of the crystal oscillator and the modulating signal are fed to a balanced modulator. The crystal oscillator output is also fed to a phase shifting net work to produce a 90° phase shift. Both the outputs are now fed to a combining network and FM output is obtained.

Summary

319

The frequency deviation obtained in this method is very small (less than 50 Hz) and therefore tremendous frequency multiplication is desired which is carried in two sections to obtain the standard deviation of 75 kHz. The Fig. 18.13 shows a typical method of indirect FM generation in which a crystal oscillator generates 200 kHz carrier signal with a deviation of 24.4 Hz. Modulating

signal

Crystal Osc. 200 kHz

Crystal Osc. 10.925 MHz

Balanced modulator

Combining Network

200 kHz 24.4 Hz

Freq. multiplier × 64

12.8 MHz 1.56 kHz

Mixer

90° phase shifter

PA

90 MHz 75 kHz

Freq. multiplier × 48

1.875 MHz 1.56 KHz

Fig. 18.13

The output is multiplied in first section by 64 (2 × 2 × 2 × 2 × 2 × 2) i.e., by frequency doublers which raises the FM signal to 200 kHz × 64 = 12.8 MHz and deviation to 24.4 Hz × 64 = 1.56 kHz. Now another crystal oscillator generates 10.925 MHz carrier which is fed to mixer, which reduces the FM signal to 12.800 – 10.925 = 1.875 MHz, the deviation remains unaltered is i.e., 1.56 kHz. Now this output of the mixer is passed through the second section of multipliers to multiply it by 48 ((3 × 2 × 2 × 2 × 2) i.e., one tripler and four doublers) to get the standard FM output = 1.875 × 48 = 90 MHz and of deviation = 1.56 kHz × 48 = 75 kHz. The method possesses a better frequency stability but suffers from excess noise due to tremendous multiplication. The output also suffers from distortion.

SUMMARY 1. The FM transmitters have a very large bandwidth as compared to AM transmitters. 2. The FM signals can be generated by direct and indirect methods. 3. The direct method is used in reactance modulators. The indirect method is used in phase shift modulator. 4. Accordingly, the transmitters may be of two types direct transmitters and indirect transmitters. qqq

19 FM Receivers The FM receiver also uses the superheterodyne principle and is quite similar to AM receiver. The basic differences, FM receiver has: much higher operating range, need for limiting action, need for de-emphasis, methods of detection and methods of obtaining AGC etc. The FM receivers are used (in the range of 40 MHz to 1000 MHz) for sound broadcasting, TV, police, radio, and military systems.

A low noise mixer is used, where as colpitt configuration is used as local

oscillator.

19.1 DEMODULATION (DETECTION) OF FM WAVES The process of extracting the original signal from a FM wave is known as frequency demodulation and the circuits that perform this job are called demodulators or detectors. The basic function of demodulation is inverse of that of the modulation. A frequency demodulator produces an output voltage, whose instantaneous amplitude is directly proportional to the instantaneous frequency of the FM wave. The FM detection or demodulation takes place in two stages. (i) The conversion of the FM wave into the corresponding AM wave by using frequency dependent circuits i.e., the circuits whose output voltage depends on input frequency (called discriminators). (ii) Feeding this AM wave to a linear diode detector to recover the original modulating signal (called detector).

19.2 FREQUENCY DISCRIMINATORS/DETECTORS As mentioned, the Frequency discriminators convert the FM signal into the corresponding AM signal to be fed to a diode detector to get the modulating signal as output.

322

Chapter 19

FM Receivers

These can be classified as (i) Slope Discriminator/Detector: The operation of this discriminator depends upon the slope of the frequency response curve of the frequency selector circuit. The FM discriminator using this principle are (a)

Single tuned discriminator or simple slope detector.

(b)

Staggered tuned discriminator or balanced slope detector.

(ii) Phase Difference Discriminator/Detector: These detectors operation depend on measurement of the phase difference. These are: (a)

Foster seely discriminator/detector.

(b)

Ratio detector.

1. Slope Discriminator/Detector We shall describe two such discriminators. (a) Single tuned (b) Staggered tuned. (a) Single tuned discriminator/single slope detector (Fig. 19.1) It consists of a resonant circuit which is tuned to a frequency slightly above the carrier frequency. This circuit converts the FM signal into the corresponding AM signal. This AM signal is then fed to the diode detector for detection.

C

Detector

R

R

C

Output

FM Input

Discriminator

Fig. 19.1

By tuning the circuit to receive the signal on the slope of the frequency response curve (Fig. 19.2), the carrier amplitude is made to vary the frequency. The circuit is tuned so that its resonant frequency (fc + Df) is higher than the carrier frequency (fc).

323

Output Voltage

19.2 Frequency Discriminators/Detectors

fc

fc + Df

Frequency

Fig. 19.2

When the signal frequency increases above fc, the amplitude of the carrier voltage rises and when the signal frequency decreases below fc the carrier voltage falls. If the circuit is detuned so that the carrier frequency lies on the positive slope of the curve, amplitude of the resulting voltage across the tuned circuit will vary with the frequency of the input circuit. Advantages: (i) It is simple. (ii) It is economical and hence suitable, when the cost is more important. Disadvantages: (i) The characteristic of the circuit is non linear. (ii) To reduce the distortions, the frequency deviation has to be less. (iii) The amplitude variation may rise due to noise and other factors. (b) Staggered Tuned Discriminator/Balance Slope Detector (Fig. 19.3) The Non linearity of the simple slope detector has been removed in this detector circuit. This detector uses two slope detectors connected back to back to the opposite ends of a centre tapped transformer, so that they

are 180° out of phase.

In this detector, the output is the difference of two outputs V1 and V2.

The resonant circuit No. 1 is tuned to a frequency f1 = (fc + Df) slightly higher than the carrier frequency and the resonant circuit No. 2 is tuned to a frequency f2 (= fc – Df) slightly lower than the carrier frequency. Then as the output is the difference of V1 and V2. The output will be zero as V1 and V2 are equal at central frequency. Thus output V0 = V1 – V2

324

Chapter 19

C1

+

V1

1 AM signal

fc C f2 = fc – Df

FM Input

f1 = fc + Df

D1

C2

2

FM Receivers

– +

V0

v2

–

D2

Fig. 19.3

The output curve will have a shape of the English alphabet ‘S’ (Fig. 19.4). The output is balanced to zero hence, the name as balanced detector. When the carrier deviates towards f1, the V1 increases while V2 decreases and the output V0 goes positive. When the carrier deviates towards f2, the V1 decreases while V2 increases and output V0 goes negative.

V1 f2 (fc – Df)

V2

o

f1 (fc + Df) Output (V0)

Fig. 19.4

The output of the device is almost linear so this is superior than the simple slope detector but this detector suffers from a disadvantage that the output is sensitive to the amplitude variation of the input signal. Note: As can be seen (Fig. 19.3) the secondary is a back to back connection of two tuned circuits. One tuned to one side of the FM signal center frequency (intermediate frequency) and other tuned to other side of it. In commercial FM broadcast, the FM signal center frequency (IF) is 10.7 MHz and the transformer primary will be tuned to this frequency. Maximum frequency deviation (Df) in FM broadcast is 75 kHz and the two secondaries are off tuned by 100 kHz i.e.,

19.2 Frequency Discriminators/Detectors

325

the center frequencies for these off tuned circuits would be 10.6 MHz and 10.8 MHz. When the instantaneous frequency equals to 10.7 MHz, the two tuned circuits produce equal amplitude outputs and thus cancel each other. When the instantaneous frequency equals fc + Df = 10.8 MHz (Fig. 19.5) in the present case, the output is positive maximum and it is negative maximum when the instantaneous frequency of the input signal is fc – Df = 10.6 MHz. For all other frequencies between 10.6 MHz and 10.8 MHz, the output amplitude lies between negative and positive maximum, thus giving rise to standard shaped pattern of FM detectors. (See Fig. 19.5).

L2 C2

L1 C1

fc – Df

fc

fc + Df

Fig. 19.5

2. Phase Difference Discriminators/Detectors As mentioned earlier, these depend for their operation on the phase difference. Here, we shall discuss the following two discriminators: (a) Foster seeley discriminator/center tapped detector. (b) Ratio detector. The Foster seeley and ratio detectors both belong to the category of “Quadrature Detectors’’. The principle of operation of a Quadrature detector can be briefly stated as below: The FM signal is fed to a tuned circuit whose center frequency is same as the unmodulated carrier frequency of the FM signal. The FM signal is shifted 90° in phase and vectorially added to the signal appearing at the output of the tuned circuit. The amplitude of the resultant change as the instantaneous frequency deviates from the center frequency due to change in the phase difference between the two signals being added vectorially. The resultant amplitude increases or decreases, whether the magnitude of the phase difference becomes more or less than 90°.

326

Chapter 19

FM Receivers

(a) Foster Seely Discriminator/Center Tapped Detector (Fig. 19.6) The circuit consists of two tuned circuits L1 C1 and L2 C2 which are inductively coupled and both the circuits are tuned to the carrier frequency. The center of secondary inductance L2 is connected with the primary tuned circuit through a capacitor C as shown. It acts as a coupling capacitor, it couples the signal frequency from primary to the center of L2. In other words, this capacitor blocks dc from primary to secondary circuit and the entire voltage applied across primary appears across the inductance L3 which acts as RFC (Radio Frequency Choke). Each half of the secondary tuned circuit has envelope detector and both the detectors (D1 and D2) are identical. C Discriminator

Detector

Vin

C1 L1

C2

Va

VA

Vb

VB

V3

L2

L3 V2

V0 = VA – VB

D1 V1

D2

Fig. 19.6

The center tapping of L2 has equal and opposite voltages V1 and V2. The voltages applied to the detectors are Va = V3 + V1 and Vb = V3 – V2. The voltage Va and Vb depend upon phase relations among V1, V2 and V3. The voltages Vl and V2 are always equal and are in phase opposition, but the phase position of V1, V2 and V3 depend on the input frequency. Thus, if relative magnitudes of Va and Vb are made to depend upon the instantaneous frequency of the input signal, the output voltage will also vary

as per the frequency variation of the input signal.

Three conditions may exist:

(i) When Va = Vb : When input voltages applied to the detectors are equal, the rectified currents will be equal and output VA – VB = 0.

19.2 Frequency Discriminators/Detectors

327

(ii) When Va > Vb : When input to upper detector is more than to the lower detector, the output VA – VB will be positive, and VA > VB. (iii) When Va < Vb : When the input to the lower detector is more, then the output VA – VB will be negative and VB > VA. Characteristic (Frequency response) (Fig. 19.7): The variation of output voltage V0 = (VA – VB) to the instantaneous frequency is called as characteristic. It is zero at resonance, positive above resonance and negative below resonance. The characteristic is linear for the region between the peaks of Va and Vb. This range is called Peak Saturation Range (MN). Note that the discriminator works satisfactorily over the peak saturation range (MN). This range should be twice the frequency deviation (Df), Where f is input frequency. Curve at Increased amplitude

V0

f

Curve at decreased amplitude

M

Zero at resonance

Df

Df

N

Fig. 19.7

Advantages : The Foster seeley discriminator provides almost a linear characteristic. Disadvantages : (i) Any variation in the input FM signal due to noise or otherwise changes the characteristic and the output gets distorted. (ii) To reduce the distortion, the receiver with this discriminator requires an “amplitude limiter”. (b) Ratio Detector The one important disadvantage of the foster seeley discriminator is that it needs an amplitude limiter. This limitation has been removed in the improved version called ratio detector. Here the ratio of voltage change depends on the signal frequency, hence the name as ratio detector (Fig. 19.8).

328

Chapter 19 C

FM Receivers

D1

V1

C1 L1

L2

C2 V3

Va

VA V0

R

C0

L3 V2

Vb

VB

D2 Reversed diode

Fig. 19.8

The following improvements have been made to obtain the ratio discriminator: (i) One of the diodes have been reversed. (ii) An additional capacitor C0 is connected at the output to limit the variations of amplitude. This is the reason that a ratio detector does not need an amplitude limiter. (iii) The place of taking output is changed. Now, the output is taken from the center tap of a resistor R. The output of the ratio detector is half of that of the Foster seeley discriminator i.e., V – VB V0 = A 2 The operation of ratio detector is almost same as that of foster seeley detector except that the output is reduced to half. The output voltage varies with the input signal exactly in the same way. At the center frequency both diodes conduct equally (one diode conducts in reverse direction due to its reversed connections). But when the input frequency shifts to either side, one of the diodes conduct heavily, as a result, VA or VB increases but their sum VA + VB remains constant. The ratio of voltages therefore changes depending upon the signal frequency. Now current flows in the circuit and charges the capacitor C0 to the peak value of voltage across L2. The amplitude variations due to noise or other factors have little effect on the charge on the capacitor C0 and the output remains constant. Since the output voltage is not affected by amplitude variations, amplitude limiter is not required with a ratio detector.

19.3 FM Receivers

329

19.3 FM RECEIVERS The FM receiver receives the FM signal coming from FM transmitter and then recovers the original modulating signal. These are superheterodyne type receivers with double frequency conversion. Their RF, IF and mixer stages are also similar to AM receivers. Since the FM receivers are to operate on UHF/ VHF range, their circuits are designed accordingly, moreover these receivers do not require AGC circuits and their RF and IF stages are to be designed to have adequate bandwidth (150 kHz) to accommodate the FM signals. The Fig. 19.9 shows the block diagram of an FM receiver. The RF amplifier receives and amplifies the FM signal. The IF amplifiers amplify the intermediate frequency signals. Note that IF for FM broadcasting is l 07 MHz (as compared to 455 kHz for AM receivers). The IF amplification is done in stages. Note that local oscillators and mixers are also to be designed at UHF/VHF ranges.

L.O II

L.O. I R.F. Amp. L.S.

AF Amp

IF Amp I

Mixer I

de-emphasis

Mixer II

FM discriminator

IF Amp II

Limiter

Fig. 19.9

The amplified IF signals are now passed through limiter stages. The limiter keeps the output voltage constant and removes all amplitude fluctuations produced due to noise etc. This is necessary, the reason being that the discriminator which is the next stage needs a constant amplitude FM voltage as its input for its satisfactory operation. The limiter stage may be deleted if a ratio detector is called to replace the discriminator. The discriminator or the detector removes the original modulating signal from the IF signal. Now the signal may be passed through de-emphasis to restore the original level of the signal. The A.F. amplifier now amplifies the signal, which is fed to the loud speaker, which converts this electrical signal into the original sound signal. An FM receiver always has an RF amplifier. It reduces noise level as it has to handle large bandwidth. The input impedance of the receiver should be properly matched with impedance of the antenna.

330

Chapter 19

FM Receivers

Alimiter is a type of ‘clipper circuit’, which checks that the input to the demodulator does not contain any ‘spurious’ signals which may be source of distortion. If need arises, two limiter circuits in cascade arrangement may be used. The FM demodulator is a bit different from the AM demodulator. An FM demodulator is frequency-to-amplitude converter which converts the frequency deviation of the carrier into AF amplitude variations identical to the original signal. In FM receivers, there is undesired amount of noise in the output in the absence of transmission on a given channel or between channels. To overcome this problem, muting circuits are some times included in FM receivers. Figure 19.10 shows another superheterodyne FM radio receiver which is similar to AM receiver with the difference that an FM receiver has much more operating frequency. It has limiter, de-emphasize network and A.G.C. A typical FM receiver operating at 87 MHz to 107 MHz may have an IF stage of 107 MHz and bandwidth of 200 kHz. RF Amp.

Mixer

IF Amp.

Discriminator

Limiter

De-emphasizer

AGC Local osc.

Amp.

LS

Fig. 19.10

SUMMARY 1. FM reception is almost noise free. 2. The FM receivers also use Heterodyning principle. Their range is 400 MHz to 1000 MHz. 3. The FM detection takes place in two stages: (i) The convertion of FM signal into corresponding AM signal by “discriminator” (ii) Feeding this AM wave into a “diode detector”. 4. The types of frequency discriminator/detector are: tuned discriminator, Foster seeley discriminator and ratio detector. qqq

20 Phase Modulation (PM)

The phase modulation is similar to frequency modulation, as a change in phase of the carrier also causes a change in frequency of the carrier at the same instant. Thus, changes in the frequency are proportional to the changes in phase producing an FM signal. The FM signal which is produced indirectly from a phase modulated signal is called “Equivalent FM”.

20.1 PHASE MODULATION (PM) In phase modulation, phase angle of the carrier voltage is changed in accordance with the instantaneous value of the modulating signal voltage, the amplitude and frequency remaining the same. The FM and PM both belong to the general class of Angle or Exponential Modulation. The FM and PM are not much different in the sense that variation in the phase of a carrier is accompanied by a corresponding change in the frequency. This is due to the relationship between phase angle f and frequency w of the carrier, w = d f/dt.

20.2 COMPARISON OF AM, FM AND PM It will be interesting here to compare AM, FM and PM. The Fig. 20.1 shows a sinusodial carrier being modulated by a step voltage.

In case of AM [Fig. 20.1 (a)], the amplitude follows the step change, but

frequency and phase remain constant. The change in the amplitude can be

observed on a CRO.

In case of FM [Fig. 20.1 (b)], the amplitude and phase remain constant, where as, the frequency follows the step change. The change in the frequency can be observed on a frequency counter.

332 E

Chapter 20 AM

f fc

E

FM

f fc

t

f

f

fc

fc

t

Phase Modulation (PM) E

t

f fc f

t (b)

(a)

PM

fc (c)

Fig. 20.1

In case of PM [Fig. 20.1 (c)], the phase angle follows the step change, while the amplitude remains constant. The change in the phase may be observed on a phase meter but not as direct as the measurement of amplitude or frequency. After the step change in the phase, the sinusoidal carrier appears as though it is a continuation of the dotted curve as shown. As the step changes in phase, the abrupt displacement of the waveform on the time axis makes it appear as though the frequency undergoes an abrupt change. This has been shown by the spike. The spike change in frequency can be directly measured by a frequency counter. Here it is also to be informed that for the same value of modulating index, spectrum for the PM wave will be same as for a FM wave.

20.3 EXPRESSION FOR PM WAVE Let the carrier be expressed as ec = Ec sin (wct + f0) and the modulating signal as em = Em sin wm.t The phase angle of the carrier before phase modulation is fc = wct + f0 After phase modulation, the instantaneous phase angle of carrier is given by fc(t) = wct + f0 + kp. em where kp is a constant called phase deviation constant and defined as the phase deviation per unit amplitude of the modulating signal. Its unit is rad/volt. The phase angle f0 is constant and plays no role in the process of phase modulation, thus may be omitted. Therefore, the equation of a PM wave may be expressed as: ePM = Ec sin (wct + kp.Em sin wmt)

20.3 Expression for PM Wave

333

Putting kp. Em = mp, the above equation can be written as: ePM = Ec sin (wct + mp sin wmt) Where, mp is the modulation index for phase modulation, which may be defined as maximum phase angle (fm) produced by the modulating signal and is proportional to Em, the maximum amplitude of the modulating voltage (but independent of fm). Notes : 1. The maximum frequency deviation produced by PM is (Dw)PM = kp wm Em

and depends on modulating frequency wm.

The students can recall that frequency deviation in FM = kf . Em, where kf is a constant. Therefore for an equal bandwidth in FM and PM kf = kp.wm 2. In FM, the modulation index mf is inversely proportional to the modulating frequency but in PM, the modulation index mp is directly proportional to the modulating voltage but independent of frequency. 3. If we integrate the modulating signal and then allow it to phase modulation, we obtain an FM wave. 4. The bandwidth in PM is given by Carson rule, (BW)PM = 2(Dw)PM = 2 kp Em. fm where kp. Em = mp = fm, which is the phase modulation index. 5. It is possible to obtain FM from PM by the Armstrong system. 6. Strictly speaking, there are two types of analog modulation systems — amplitude modulation and angle modulation. The angle modulation may be subdivided into two distinct types–FM and PM, which are closely allied. Problem 20.1. A 25 MHz carrier is modulated by a 400 kHz signal. If carrier voltage is 4V and the maximum frequency deviation is 10 kHz. Write down the equation for the PM wave. Solution. The carrier frequency wc = 2pfc = 2p × 25 × 106 = 1.57 × 108 rad/sec. and,

wm = 2p × 400 = 2513 rad/sec.

Frequency deviation

Df = 10 kHz = 10000 Hz

Carrier voltage

Ec = 4V

334

Chapter 20

Phase Modulation (PM)

The modulating index for PM: mp =

Df 10,000 = = 25 400 fm

The standard equation for PM wave ePM = Ec sin (wct + mp sin wmt) ePM = 4 sin (1.57 × 108 t + 25 sin 2513 t) Ans.

or

Problem 20.2. If a carrier Ec cos wct is phase modulated by a signal 5 sin 2p (15 × 103)t. Assume kp = 15 kHz/Volt, calculate modulation index and bandwidth. Solution. Modulating signal = 5 sin 2p (15 × 103)t Comparing this with

em = Em2p fm t

We get

Em = 5V and fm = 15 × 103 Hz kp = 15 kHz = 15000 Hz/V

Given (a) Modulation Index

mp = kp.Em

= 15,000 × 5 = 75,000 Ans.

(b) Bandwidth

= 2 kp. Em. fm

= 2 × 15000 × 5 × (15 × 103)

= 2250 × 106 Hz

= 2250 MHz Ans.

20.4 GENERATION, TRANSMISSION AND RECEPTION OF PM/FM WAVE (a) Generation: The same circuit can be employed for generation of PM as well as of FM wave. The Fig. 20.2 shows a basic PM/FM system. PM em(t) FM

dem/dt

Reactance tube

Osc

Low pass filter Frequency discriminator Crystal Osc

Fig. 20.2

Frequency multilplier

Amp

20.5 (a) Generation of PM Signal From Frequency Modulator

335

(b) Transmission (Fig. 20.2): Change in the oscillator frequency is accomplished by a reactance tube. The tube offers a reactance to an oscillator, the reactance being a function of input voltage. As the input voltage changes, the reactance also changes and thus the oscillator frequency. If the voltage supplied to the reactance tube is modulating signal em(t), the result is an FM wave, where as if the input voltage is a derivative of the modulating signal (dem/dt) the result is a PM wave. The output of the oscillator is now given to a frequency multiplier to obtain the desired frequency. The output of the multiplier is amplified and transmitted through an antenna.

At some point in the transmitter, the carrier frequency is sampled and compared with a frequency standard, preferably a crystal oscillator is used for this purpose. The output of the mixer is then given to a frequency discriminator. (c) Reception (Fig. 20.3): The signal is received and amplified by RF amplifiers. The output of RF amplifier is given to frequency converter and intermediate frequency is obtained which is then amplified and fed

RF Amp

Frequency Converter

R.F. Amp

Integrator

A.F. L.S.

I.F.

Discriminator

PM

FM

Fig. 20.3

to discriminator. The obtained PM/FM signal is amplified and fed to a loudspeaker which gives the original sound signal. Note that the PM signal before giving to audio amplifier is passed through an integrator net work.

20.5 (a) GENERATION OF PM SIGNAL FROM FREQUENCY MODULATOR It can be mathematically verified that, if the modulating signal is differentiated before it is applied to a frequency modulator, the output is a PM signal (Fig. 20.4). Modulating signal

Differentiator

Fig. 20.4

Frequency modulator

PM signal

336

Chapter 20

Phase Modulation (PM)

In a frequency modulator, the instantaneous frequency of the modulating signal varies according to the signal applied at its input. In this case, the instantaneous frequency will vary as per differential of the modulating signal that is, the integral of instantaneous frequency will vary according to the modulating signal. In other words, the instantaneous phase is modulated in accordance to the modulating signal, the output, therefore, is a PM signal.

20.5 (b) GENERATION OF FM SIGNAL FROM PHASE MODULATION It can be mathematically verified that if the modulating signal is integrated, before it is fed to a phase modulator the output is an FM signal. Modulating signal

Integrator

Phase modulator

FM signal

Fig. 20.5

In Fig. 20.5 the input to the phase modulator is the integral of the modulating signal. The instantaneous phase of the modulated signal varies in accordance with the modulating signal. In other words, derivative of the instantaneous phase varies as per the modulating signal. Since the rate of change of phase is frequency, we can say that the instantaneous frequency varies as per the modulating signal, which implies that output is an FM signal.

20.6 FM vs PM The FM and PM are compared below : 1. If frequency modulating factor mf and phase modulating factor mp are equal, the FM and PM have identical spectra. However, instantaneous phase of the FM and PM waves for a given modulating signal will be different. 2. The frequency deviation in PM is directly proportional to the frequency of the modulating signal, whereas in FM it is independent of the modulating frequency. In both the cases however, the frequency deviation is proportional to the amplitude of the modulating signal. 3. For a given modulating signal, modulation index for PM is independent of the modulating signal frequency, while for FM it is inversely proportional to the frequency of the modulating signal. 4. In PM, bandwidth is extremely large i.e., the side bands do not cover the increase in frequency, whereas in FM, side bands cover it repidly. 5. In PM, the phase deviation is proportional to the amplitude of the modulating signal but independent of frequency.

In FM also the frequency deviation is proportional to the amplitude of

the modulating signal. Hence FM must be a form of PM.

6. The difference between FM and PM is not apparent for the same

Summary

337

modulating frequency but it reveals when the modulating frequency is changed. 7. If a FM transmission is received on a PM receiver, the bass (very low) frequencies would have considerably more deviation of phase than a PM transmission. Since the output of a PM receiver would be proportional to the phase deviation, the signal would appear bass boosted. Alternately, PM transmission received on an FM receiver would be lacking in bass. This is a practical difference between FM and PM, but it is clear that one can be obtained from the other very easily. Keeping above in mind for transmission and reception of analog signals, FM is preferred over PM. Another reason for this decision is the difficulty of providing correct reference phase in the detection stage of the PM receiver. The PM thus is not suitable for commercial radio broadcasting and radio telephony. Table 20.1 :

FM Versus PM

S.No.

FM

PM

1.

Modulation index mf is proportional to The modulating index mp is proportional the modulating voltage as well as to the only to the modulating voltage. modulating frequency.

2.

The frequency deviation is proportional to the modulating voltage. Associated with the change in fc, there is some phase change. The FM can be received on a PM receiver.

3. 4. 5. 6.

The SN ratio is better than PM. The noise immunity is better than AM and PM.

The phase deviation is also proportional to the modulating voltage. Associated with the change in phase, there is some change in fc (carrier frequency). It is also possible to receive PM on an FM receiver The SN ratio is poor than FM. The noise immunity is better than AM but not than FM.

SUMMARY 1. In phase modulation, phase angle of the carrier is changed in accordance to the modulating signal, amplitude and frequency remains some. 2. The phase modulation can be obtained from frequency modulation and vice versa. 3. The equation for PM wave is

ePM = Ec sin (wc.t + mp.sin wmt)

4. The important components of a PM system are: PM Transmitter, receiver and output. 5. The stages of obtaining PM signal from FM are: differentiation, frequency modulation and output. 6. The stages of obtaining FM signal from a PM signal are: Integration, phase modulation and output. qqq

21

Digital Modulation Transmission & Reception All types of modulations discussed so for have been analog modulations.

In analog modulation the modulated parameter varies continuously and can

take any value according to the range of the message. When the modulated

wave is mixed with noise, there is no way for the receiver to detect the exact

transmitted value of the signal.

In digital modulation, the modulating signal is converted into a train of pulses,

which modulate an R.F. carrier.

21.1 PULSE A pulse is an abruptly changing voltage or current wave which may or may not repeat itself. The simplest non repetitive pulse is a stepped up voltage or current shown in Fig. 21.1 (a) which can be obtained by connecting a voltmeter across a battery through a switch and then suddenly closing the switch. The voltmeter will read zero upto a time when the switch is closed, where upon the voltage will suddenly rise to its maximum value and will stay there. The Fig. 21.1 (b) shows a repetitive pulse train and Fig. 21.1 (c) shows a pulse with its trailing and leading edge.

Ampliltude

(b)

Width

(a)

t Trailing Leading edge edge (c)

Fig. 21.1

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21.2 DIGITAL (PULSE) MODULATION The modulating signal wave is sliced into small units, the process is called quantizing or quantization (Fig. 21.2). These quantum points are then converted into digital binary codes, which represents amplitude of the wave at that point. Quantized waveform

Modulating signal Sampling

Fig. 21.2

An analog signal can be converted into digital signal by means of “sampling” and “quantizing”. Basically the process of analog to digital conversion is referred as digital (pulse) modulation. The random noise can be virtually eliminated, this is the whole idea of digital modulation.

21.3 SAMPLING Sampling is the process of taking periodic samples of the wave form to be transmitted and then transmitting these samples. If enough samples are sent, the complete wave form can be received at the receiver. The concept of sampling can be explained by the following example (Fig. 21.3 ). Suppose a factory has four processing vats, each having a thermometer which is to be carefully monitored. This can be done in two ways. (a) To receive continuous data of thermometers, four workers are required. Since any change in the temperatures will be gradual. [Fig 21.3(a)]. This will not be economical. (b) The sampling of data will be more economical, if only one worker can monitor the data of all the four thermometers. If this single worker can take the data samples faster before the thermometers reading can change, the same effect of continuous sampling can be achieved at a reduced cost [Fig 21.3(b) (In this case one fourth)].

21.4 Sampling Theorem For Low Pass Signals: Nyquist Theorem Receiving Continuous Data

341

Sampling Data

Thermometer

3

4

1

2 (a)

3

4

2

1 (b)

Fig. 21.3

The situation is similar to transmitting and receiving in pulse (digital) communication. Clearly, sampling data, rather than continuous monitoring produces more efficiency and makes it possible to send more than one information on one single carrier. Note that only one transmitter and one receiver shall be needed.

This sampling is known as Time Division Multiplexing (TDM), because

multiple signals are sent by sampling them at different times.

21.4 SAMPLING THEOREM FOR LOW PASS SIGNALS: NYQUIST THEOREM (a) If more no. of samples are taken, the information can be reproduced correctly. The other side is also correct, i.e., if fewer samples of one information are taken and in between other information can also be sent. This is similar as in our example in which one person is reading temperature of several thermometers, lesser the time the person spends reading one thermometer, more time he has left to read other thermometers, or we can say to get other information. Here, Nyquist theorem is to help, it says: In order to convey an information completely, The minimum sampling frequency of a pulse modulated system, should be equal to (or more than twice) the highest signal frequency. Mathematically. fs ≥ 2fm Where fs = Minimum sampling frequency to convey complete information. fm = Maximum frequency component present in the information signal.

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e.g. The minimum sampling frequency to transmit a pure sine wave of 2 kHz should be

= fs = 2fm = 2 × 2 kHz = 4 kHz

21.5 EFFECTS OF SAMPLING RATE ON A FREQUENCY SPECTRUM The Fig. 21.4 shows frequency spectrum of a modulating wave. Note that higher order harmonics are smaller in amplitude. V

fm

f

Fig. 21.4

(a) When fs = 2fm: The Fig. 21.5 (a) shows what happens when the sampling frequency fs is twice the maximum frequency component (fm) present in the modulating wave. Theoretically the harmonics of the sampling extend to infinity, but practically the resulting spectrum needs only be passed through a low pass filter. (b) When fs > 2fm: If the sampling rate is made slightly larger than 2fm, a practical low pass filter can be used to pass only the maximum frequency component (fm) and not any other frequency component. The high sampling rate creates a guard band between fm and the lowest frequency components (fs – fm) of the sampling harmonics. See Fig. 21.5 (b). V

fm

2fm

3fm

4fm

5fm

(a) Sampling at fs = 2 fm Fig. 21.5

6fm

7 fm

21.6 Sampling Techniques

343

Practical Low pass Filter Response Curve Guardband (fS–fm)

V

fm

fS + fm

fS

V

Aliasing Distortion (fS–fm)

2fS

(b) Sampling at fs > 2 fm

2fS

fS

fm

(c) Sampling at fs < 2 fm

Fig. 21.5

(c) When fs < 2fm : If the sampling rate is lower than 2fm, a distorted sampling spectrum is obtained. The distortion is known as Aliasing distortion. See Fig. 21.5 (c).

21.6 SAMPLING TECHNIQUES There are three sampling techniques: 1. Ideal, Instantaneous or Impulse sampling. 2. Natural sampling. 3. Flat top sampling. Table 21.1 S. No. 1.

Comparison of sampling techniques

Parameters Generation

Ideal sampling It uses multiplication

Natural sampling It uses multiplication or chopping circuit.

Flat top sampling It uses a sample hold circuit.

c(t) f (t)

f (t)

s (t)

Waveform

f (s)

s (t) 3.

Feasibility

This method is not practically feasible.

S2

C

s (t)

f (t)

2.

S1

f (s)

s (t) This is used practically.

f (s)

s (t) This is the most popular method.

s (t)

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S. No.

Parameters

Digital Modulation Transmission & Reception

Ideal sampling

Natural sampling

Flat top sampling

4.

Sampling rate

Sampling rate may be infinity.

The sampling satisfies the Nyquist theorem.

The sampling satisfies the Nyguist theorem

5.

Noise

Maximum.

Minimum.

Minimum.

21.7 ANALOG AND DIGITAL SIGNALS An analog signal can be best illustrated by a sinewave. Note that like sine wave, an analog signal is continuous and its value at any instant can be anywhere within the range of its extremes (Fig. 21.6 a). 1

1

1

1

1

1

0

0

0

0

(a)

0

0

0

0

(b)

Fig. 21.6

The digital signal (Fig. b) is not a continuous representation of the original signal. Instead, the digital signal represents the data as a series of digits such as a number. This digital representation is considered as a code, which approximates the actual value.

21.8 ADVANTAGES AND DISADVANTAGES OF DIGITAL COMMUNICATION (a) The use of digital communication has many advantages over analog communication, some are given below: (i) Noise immunity of digital signals, (ii) Digital hardware implementation is flexible, (iii)

Ease to multiplex several digital signals with more efficiency,

(iv) Reliable reproduction of digital message, (v) Easy and economic storage of digital signals, (vi) Privacy of information during transmission.

21.9 Logic System

345

(b) The disadvantage of digital communication are as follows: (i) The transmission medium (channel) band width required by the digital communication system is much more than the analog communication system. (ii) The digital communication systems are more complex than the analog communication systems. (iii)

A precise time synchronization is required between transmitter and receiver.

21.9 LOGIC SYSTEM The digital communication employs logic circuits, which usually work on binary number system. A digital circuit has two states, the output is either low (0) or high (1). In positive logic system in general, 0 represents zero volts and 1 represents +5 volts. Its reverse case is known as negative logic system (0, —5V).

21.10 BINARY NUMBER SYSTEM The word binary means two. The binary number system uses only two digits, 0 and 1. Thus, a binary number is a string of zeros and ones. In binary system, the base is 2. The abbreviation for binary digit is bit. The binary number 1111 has 4 bits, 110011 has 6 bits and 11001100 has 8 bits. A string of 8 bits is known as a byte. A byte is the basic unit of data in computers. In most of the computers, the data is processed in strings of 8 bits or its some multiples, (i.e., 16, 24, 32, etc.). The memory also stores data in strings of 8 bits or some multiples of 8 bits. Table 21.2 shows binary and decimal equivalence. Table 21.2 Binary and Decimal Equivalence. Binary

Decimal

0

0

0

0

0

0

0

0

I

1

0

0

1

0

2

0

0

1

1

3

0

1

0

0

4

0

1

0

1

5

0

1

1

0

6

0

1

1

1

7

1

0

0

0

8

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Binary

Decimal

1

0

0

1

9

1

0

1

0

10

1

0

1

1

11

1

1

0

0

12

1

1

0

1

13

1

1

1

0

14

1

1

1

1

15

The decimal number 435 can be written as = (4 × 102) + (3 × 101) +(5 × 100) [in binary system] Similarly binary number 1011 is = (1 × 23) + (0 × 22) + (1 × 21) + (1 × 20) = 8 + 2 + 1 = 11 in decimal system. Conversion: (i) The method to convert a decimal number into a binary number is known as double dabble. This method involves successive division by 2 and recording the remainder (the remainder will always be 0 or 1). The division is stopped when we get a quotient of 0 with a remainder of 1. The remainder, when read upwards, gives the binary number. Problem 21.1. Convert decimal number 14 into a binary number. Solution:

2

14

2

7

remainder 0

2

3

remainder 1

2

1

remainder 1

0

remainder 1

The binary number is 1110. Ans. (ii) A systematic way to convert a binary to a decimal number is as given below: 1. Write the binary number. 2. Write the weights: 22, 21, 22, 23 ; i.e. 1, 2, 4, 8, etc., under the binary digit. 3. Cross out any weight under 0. 4. Add the remaining weights.

21.11 Logic Gates

347

Problem 21.2. Convert binary number 1010 into decimal. Solution:

1

0

1

0 Binary number

× 8 × 4 × 2 × 1 Add weights 8

f

2

f Cross out weights under 0.

8 + 0 + 2 + 8 = 10 The decimal number is 10. Ans.

21.11 LOGIC GATES The elements required for performing logic functions or operations (addition, multiplication etc.) are called logic gates. These logic gates also work on binary number system. Logic gates are the electronic circuits with many inputs but only one output. Logic gates are the most important building blocks of any digital system. They do all sort of logic (i.e., boolean) operations i.e., addition, multiplications etc. Truth table: The truth table lists all possible combinations of input and the corresponding output of a logic gate. Table 21.3

Logic Diagram, Function and Truths Table of few gates

SI. No.

Gate

1.

AND

Logic diagram

= A. B Y

OR

A B

Y

NOT (Inverter)

Input A

B

Y

0

0

0

1

0

1

0

0

Input

Y

=A

Output

A

B

Y

0

0

0

0

1

1

1

0

1

1

1

1

Y = NOT A A

Output

0

Y = A or B =A+B

3.

Truth table

Y = A and B A B

2.

Function

Input

Output

A

Y

1

0

0

1

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Chapter 21

SI. No.

Gate

4.

NAND

Logic diagram

Digital Modulation Transmission & Reception Function Y = A NOT AND B = A NAND B

A B

5.

Y

NOR

= A.B

Input

A B

Y

EX-OR

=A+B

B

Y

0

0

1

0

1

1

1

0

1

1

1

0

Input

A B

Y

= AB + AB

Output

A

B

Y

0

0

1

0

1

0

1

0

0

1

1

0

Input

Y = A EX–OR =A⊗B

Output

A

Y = A NOT OR B = A NOR B

6.

Truth table

Output

A

B

Y

0

0

0

0

1

1

1

0

1

1

1

0

21.12 PRINCIPLE OF DIGITAL COMMUNICATION Figure 21.7 shows the functional elements and block diagram of a digital communication system. The input may be an analog signal, such as voice or picture or music. If it is in analog form, it must be converted into digital pulses prior to transmission and converted back to analog form at the receiver end (or destination). This can be done by A/D converter or input transducer at the input and by D/A converter or output transducer at destination (or receiver). The function of the encoder is to convert the digital output of the A/D converter into a sequence of binary digits. This process is called as data compression (or encoding). After encoding, the sequence of binary digits (message sequence) is given to the channel encoder. The function of channel encoder is to introduce some rebundancy (surplus data) in the binary message sequence which is used at the destination (receiver) to overcome the interference and noise effects on the transmission of signal through the channel.

21.12 Principle of Digital Communication Analog Information Source

A/D Converter Or Input Transducer Digital Information Source

D/A Converter or Output Transducer

349

Encoder

Channel Encoder

Digital Modulator

Communication Channel or Transmission Channel Output or Destination

Decoder

Channel Decoder

Digital Demodulator

Analog Information

Fig. 21.7

The output of the channel encoder then goes to the digital modulator which modulates [converts the binary message (information) sequence into sinusoidal form.] The communication channel is a medium used to send the signal from the transmitter to the receiver. During transmission through communication channel, the signal is affected by the noise (such as man made noise, thermal noise, atmosphere noise etc). This noise affected signal is processed by the digital demodulator at the receiver end and converts this sinusoidal signal into binary sequence. This binary sequence is passed to the channel decoder which recovers the original information or message sequence from the code used by the channel encoder at transmitter, and rebundancy contained in the received data sequence. It is not easy to recover the original message (or information sequence) at the decoder output because the noise and disturbances are already mixed in the communication channel. Because of this, the signal recovered/regenerated by the decoder is an approximation of the original information at its output (or destination). Then this digital message sequence is converted into analog form by the D-A converter at the receiver end.

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21.13 COMMUNICATION SPEED A Baud is the unit of signalling speed of communication channel. It is named after the name of French scientist JME Baudat, whose 5 bit code was adopted by the French telegraph system in 1877. A ‘‘Baud’’ is defined as the no. of code elements per second. If each signal element represents one bit information, the baud is one bit per second.

Now-a-days in digital communication instead of Baud, ‘‘Bit Per Second’’

(BPS) is used as the unit.

At present in standard digital voice communication, amplitude of the voice signal is sampled at the rate of 8000 samples per second. An 8 bit value for the amplitude of the frequency is transmitted digitally. The encoding scheme is known as Pulse Code Modulation and requires a bandwidth of 64 kilo bits per second (8000 samples per second at 8 bit per sample). This, digital transmission is widely used and much of the world utilises digital transmission links operating at 2.048 mega bits per second.

21.14 QUANTIZING In digital modulation, the pulses result from sampling the modulating signal wave. In other words, the modulating wave is sliced into small units, the process is called Quantizing or Quantization. These quantum points are then converted into binary code, which represents the amplitude of the waveform at the point. V (Volts)

Sampling Points

7

6

Resultant Quantized Waveform

5

4

3

2

1

0

Sampling Pulses

Fig. 21.8

In Fig. 21.8 the amplitude of modulating wave is 7 volts. Sampling points are taken at equal intervals and quantized waveform is obtained as shown in Fig. 21.9. Note that, if the modulating signal is not an exact value of the

21.14 Quantizing

351

resulting binary code, distortion or error is introduced which produces a noise called Quantizing noise. The noise can be reduced by increasing sampling points, but this also increases the required bandwidth. Each portion of the wave is then converted into a binary number that represents the final wave. 111

7V 110 110

6V 101

5V

110 101

101

4V 3V

011

011

010

010

2V

001 001 000 000

1V 0

000

000

Fig. 21.9

The resulting pulse code wave forms are shown in the table below along with the binary code. Quantizing level (Volts)

Binary Code

0

000

1

001

Pulse Code Wave 0 1 0

2

010

1 0

3

0

011

1

1 0

4

100

1

0

5

101

6

110

1

1

7

111

1

1

1

0

1

0

0

0

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21.15 TYPES OF DIGITAL (PULSE) MODULATION The popular types of digital modulations are: 1. Pulse Code Modulation (PCM) 2. Differential PCM 3. Delta Modulation (DM) 4. Adaptive delta modulation (ADM). Here we shall discuss Pluse code modulation only.

21.16 PULSE CODE MODULATION (PCM) This is a digital modulation process, in which signal to be transmitted is sampled at various instants. This results in a sequence of pulses each time, the signal is sampled depending upon magnitude of the signal at the sampling instant (Fig. 21.10). These pulses correspond to a certain code (usually binary). These pulses modulate an RF carrier and are transmitted. V (Volts)

Sampling Points

7

6

5

4

Resultant Quantized Waveform

3

2

1

0

Sampling Pulses

Fig. 21.10

At starting, a new code group, reference pulses are transmitted along the each code group. These reference pulses are different in amplitude or duration from the pulses carrying signal so that these can be easily detected at the receiver. In simple words, amplitude of the modulating signal is converted into a digital code (which is generally binary number) at the transmitter. The process is similar to an analog to digital conversion where the amplitude of an analog signal is converted into a digital code. At the receiver, the signal is decoded and the original signal can be obtained.

21.16 Pulse Code Modulation (PCM)

353

1. Block Diagram of PCM System The Fig. 21.11 shows block diagram of the PCM system. A PCM system has three parts: (a) Transmitter (b) Transmission Path (c) Receiver (a) Transmitter: High frequency components are first attenuated by low pass filler. The essential operations in a PCM transmitter are: sampling, quantizing and encoding as shown in Fig. 21.11(a). The quantizing and encoding both are performed in the same circuit called Analog to Digital converter. (ADC) ADC

Analog signal

Low Pass Filter

Sampling

Quantizing

Encoding

PCM Wave

(a) Transmitter

PCM Wave

Regenerative Repeater

Regenerative Repeater

Regenerated PCM Wave

(b) Transmission path Regenerated PCM Wave

Regenerative Repeater

Decoder

Reconstructed Low Pass Filter

Analog signal Output

(c) Receiver

Fig. 21.11

(b) Transmission Path: At intermediate points along the transmission route from transmitter to receiver, Regenerative Repeaters are used to reconstruct (regenerate) the transmitted sequence to coded pulses to combat the effects of signal distortion and noise as shown in Fig. 21.11 (b).

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• Regenerative Repeater: (Fig 21.12) The most important feature of a PCM system is its ability to control distortion and noise produced in the transmission of the PCM wave through the channel. This is accomplished by reproducing the PCM wave by means of a chain of regenerative repeaters located all over the route. Regenerated PCM Wave

PCM Wave

Transmitting medium

Distorted PCM Wave

Equalizer

Decision Making Device

Timing Circuitary

Noise

Fig. 21.12

A regenerative repeater performs the following functions: (i) Reshaping the incoming pulses by means of Equalizer to compensate the effect of distortion in the transmitting channel. (ii) The “Timing Circuitry” facilitates in sampling the equalizer pulses at the instants, where Signal-Noise ratio is maximum. (iii)

“The decision making device” is enabled, whenever the amplitude of the equalizer pulses plus noise exceeds a predetermincd voltage level at the time of sampling.

(c) Receiver: At receiver, regeneration of impaired signals and decoding of the train of quantized pulses is carried out as shown in Fig. 21.11. (c). These operations are usually performed in the same circuit called Digital to Analog converter (DAC).

2. PCM Transmitter Circuit A PCM transmitter is an encoder, which converts a signal into a specified code. This is basically an Analog to Digital converter, which converts input analog signal to a binary code. The encoders are electronic circuits. The Fig. 21.13 shows a typical encoder circuit consisting of three differential amplifiers. There are also one exclusive NOR gate and two AND gates.

21.16 Pulse Code Modulation (PCM)

355

Three Differential

Amps.

V + –

Analog Input

+ –

+

Ex-NOR

AND

V

D1 Digital Output D2

V AND

–

Fig. 21.13

3. PCM Receiver Circuit The PCM demodulator (Receiver) is just reverse of modulator, it is basically a decoder, which converts the digital output (D1D2) obtained from transmitter into analog form. In other words it is a digital to analog convertor. The resulting waveform is then passed through a low pass filter (LPF) and original signal is obtained (see Fig 21.14). Filter Receiver PCM wave (D1D2)

Serial to Parallel Converter

Synchronisation

Analog Output

AMP

LPF

Sample Hold Circuit

Fig. 21.14

4. Advantages of PCM (i) The major advantage of PCM is that it is much more immune to noise or interference. The intelligence (original signal) gets distorted, when some characteristic of the intelligence is affected. As in PCM, no characteristic of signal is affected, this has a noiseless reception.

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(ii) The PCM permits repeating (or amplifying) the encoded signal without significant distortion being introduced. (iii) The output SN ratio increases exponentially with bandwidth. (iv) A PCM system designed for analog data can be readily adopted to other inputs such as digital data, thus promoting flexibility. (v) Due to regeneration capacity, the PCM is beneficial for a system having many repeater stations.

5. Applications of PCM The applications of PCM are in the following fields: (i) The PCM is used for multi channel communication over wires i.e., long distance telephony. (ii) The pulse demodulated signals may be used to modulate an RF carrier and can be transmitted as radio signals.

6. Companding in PCM In PCM, there are two major problems: Large Signal (i) The uniform step size in quantizing means weak signals will have more quantizing noise. Sampling Points (ii) The signals with large dynamic range require many Small encoding bits, which may not Large Step Signal be practically feasible. The process to overcome these problems is known Companding. In this technique number of quantisation Fig. 21.15 steps are increased for small signals and decreased for large signals. The Fig. 21.15 shows the technique. 7. Bandwidth of PCM Refer the single pulse shown in the Fig. 21.16 (a). The Fig. 21.16 (b) shows frequency spectrum of the pulse. t t

Fig. 21.16 (a)

21.16 Pulse Code Modulation (PCM)

357

f

B.W. = 1/t

Fig. 21.16 (b)

The B.W. is equal to the distance of the first zero crossing i.e., B.W. = 1/t Hz

...(i)

The PCM wave train is composed of sequence of 1s and and 0s. Regardless of the sequence, the B.W. can not be greater than, that required for a single pulse,

which has been expressed in eq. (i).

Suppose an analog signal is sampled at the “Nyquist rate”, the time T between

pulses is given by : 1 T = 2f where fm is the highest frequency in the signal. Expanding this to n m signals (i.e., multiplexing), the width of the sample pulse will be : 1 tn = T/n = 2f n m. If each pulse is digitized, each code word can last only for T/n seconds. For an m bit PCM word, the width of each bit pulse will be T/ mn (Where m is number of bits in the PCM word and n is number of signals). Using eq. (i), the PCM bandwidth will be given B.W. (PCM) =

nm 1 = T Hz t

⎛ w ⎞

= n ⎜ m ⎟ Hz [wm = 2pfm] ...(ii) ⎝ π ⎠

For the binary process, the number of levels L in the quantization process is equal to 2m. Solving for m and substituting in eq. (ii), we get n.wm p log2 (L) Hz (For binary, the base is 2) = (2n.fm).m. [wm = 2pfm]

B.W. (PCM) =

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21.17 MULTIPLEXING This is a process used for transmitting telephonic messages or signals. It is a process of transmission of more than one signal together and simultaneously through the same line. This increases the handling capacity of the line. Multiplexing may be of following types: 1. Time division multiplexing 2. Frequency division multiplexing 1. Time Division Multiplexing (TDM): The pulses are generally narrow in width and separation between them is larger. This fact is utilized in this multiplexing. The space in between two pulses can be utilized by other signals. One line is assigned to channels turn by turn. In low speed TDM, rotating switches (mechanical in nature) are used in transmitter as well as in receiver synchronized with one another. Number of channels are fed to the transmitter switch which are separated by the receiver switch. In high speed TDM, electronic switches replace the mechanical switches. If the speech wave is sampled at a frequency greater than the highest frequency present in the speech (fs > 2fm), we get an output sample wave as shown (Fig. 21.17 (b)) in which the speech signal is present and same may be obtained at the receiver.

MV

Input Signal

RL

Output

Output TDM Sample Wave (a)

(b)

Fig. 21.17

For generating such a wave at the transmitter, (Fig. 2.17 (a)) a Multivibrator (MV) producing 10 kHz carrier wave with a bridge rectifier circuit is used. Across the load RL the sample wave is obtained. At the receiver, the same circuit detects the sample wave across its load, provided both the circuits (at transmitter as well as at receiver) work in synchronism. If two persons are provided with such a circuit

21.18 (a) Transmission and Reception of TDM

359

they can hear each other. Moreover, a number of signals can pass through the same transmission line. 2. Frequency Division

Modulation Multiplexing (FDM):

This is also known as 1st Signal Carrier electronic telephone

system. In this system,

Demodulation carrier frequency is

To Lines modulated by the

speech signal, and as

a modulated wave,

Modulation we get original carrier

along with side bands. 2nd Signal Carrier The side bands which

carry the signal are

Demodulation sent. At the receiver, the

Fig. 21.18 original speech signal is detected or demodulated. By using different carrier frequencies we get different side bands of different frequencies and therefore any number of signals can be transmitted simultaneously through the same line. Similarly the signals can be received at the receiver by using different filter circuits. The Fig. 21.18 shows simplified diagram of an electronic telephone

exchange using FDM. Only two signals have been shown but any number of signals can be sent through a pair of lines. In this system, each channel is assigned a carrier frequency which is modulated by the channel signal. The modulated carriers then travel through the line simultaneously.

21.18 (a) TRANSMISSION AND RECEPTION OF TDM The TDM is the process of utilizing the time scale for simultaneous transmission of more than one intelligence signal on the same carrier. In a pulse communication, if the unmodulated pulse train is having a low duty cycle, the interspace between the pulses can be utilized for transmission of another intelligence. A synchronizing pulse may be added for each group of pulses (consisting of one pulse from each signal) so that there is no problem in separating the signals at the receiver.

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The Fig. 21.19 shows transmission of two signals by TDM. Ref pulse

1 2 1

Modulator 1′ 2′

2

Modulator

TDM Circuit

1 1′ 2 2′

1 2 Detector

De Multi plexing circuit 1′2′

Detector Reception

Transmission

Fig. 21.19

At the receiver, these signals can be separated by proper synchronous detector. With the help of this method, we recover the various signals in the time domain.

21.18 (b) TRANSMISSION AND RECEPTION OF FDM (FIG. 21.20) The process of utilizing frequency scale for simultaneous transmission of more than one signal in the same carrier is called FDM. If two signals having same frequency modulate a carrier simultaneously, they will interact. Antenna

Sub carrier I

Signal I

Signa lI

10 kHz Sub carrier

Common RF Carrier

20 kHz Sub carrier

Composite carrier

Sub carrier II (a) Transmission Sub-Carrier I

Composite carrier

9–11 kHz

Detector Sub-Carrier II

19–21 kHz (b) Reception

Fig. 21.20

Detector

21.19 TDM Vs FDM

361

In order to avoid interaction, the signals first modulate the same carrier (different for different signals) called “sub carrier” then the modulated signals which differ in frequency, modulate a common RF carrier, without any interaction. The output is a composite carrier signal, which is transmitted through an antenna. At the receiver, this composite carrier signal is split into the individual carriers. These carriers are passed through detectors and original signals are obtained.

21.19 TDM VS FDM 1. Time division multiplexing (TDM): In this, complete channel width is allotted to one user for a fixed time slot. This technique is suitable for digital signals as these signals are transmitted intermittently and the time between two successive signals can be utilized for other signals. The TDM suffers from inter symbol interference (I.S.I). Note: The transmitted digits interfere with each other and cause distortion. This is called I.S.I. In case of TDM (Time division multiplexing), the signals are mixed (jumbled) in frequency domain, but separated in “time domain” such that the frequency spectra of the various signals occupy the same frequency and their wave shape identity is maintained. 2. Frequency division multiplexing (FDM): The signals are separately modulated and transmitted. Any type of modulation can be used, however SSB (Single Side Band) modulation is most widely used. At the receiver, the signals are separated by band pass filters and then demodulated. The FDM is used in telephony, telemetry and TV communications. The FDM suffers from the problem of “cross talk”. Note: Unwanted transfer of signals between communication channels is called crosstalk. A signal is completely specified either by “frequency domain” or “time domain”. In FDM (Frequency division multiplexing), all the signals to be transmitted are continuous and are mixed (jumbled) in the “Time domain” but separated (maintain their identity) in frequency domain, as spectra of the various modulated signals occupy different bands in frequency domain, i.e. their frequency spectra identity is maintained and can be separated by filters. The difference between TDM and FDM can be represented graphically on a communication space, which is used to transmit information.

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The Fig. 21.21 (a) shows that for TDM, each signal occupies a distinct time interval, not occupied by any other signal, but spectra of the signals have components in the same frequency interval.

Signal 5

Signal 4

Guard Time

Signal 3

Signal 2

Guard Time

Signal 1

f

t

Fig. 21.21 (a)

The Fig. 21.21 (b) shows that for FDM, each signal is present on the channel all the time and all signals are mixed, but each of the signal occupies a finite and distinct frequency interval not occupied by any other channel. f Guard Band

Signal 5 Signal 4 Signal 3 Signal 2

Guard Band

Signal 1

t

Fig. 21.21 (b)

21.20 TDM is Superior to FDM Table 21.4 S. No.

363

TDM vs FDM TDM

FDM

1.

The signals which are to be multiplexed The signals which are to be multiplexed can occupy the entire bandwidth but they are added in the time domain. But they are isolated in time domain. occupy different slots in the frequency domain.

2.

TDM is preferred for digital signals.

FDM is preferred for analog signals.

3.

Synchronization is required.

Synchronization is not required.

4.

The TDM circuitry is simple

The FDM requires a complex circuitry at the transmitter and receiver.

5.

The TDM suffers from the problem of inter symbol interference (I.S.I).

The FDM suffers from the problem of crosstalk due to imperfect band pass filters.

6.

Due to fading, only a few channels are affected.

Due to wideband fading in the transmission medium, all FDM channels are affected.

7.

Due to slow narrow band fading, all the Due to slow narrow band fading taking TDM channels may get wiped out. place in transmission channel, only a single channel may be affected in FDM.

21.20 TDM IS SUPERIOR TO FDM The TDM is supposed to be superior in the following aspects: (i) TDM instrumentation is simpler, whereas FDM requires modulators, filters and demodulators. But TDM synchronisation is slightly more demanding than that of FDM with suppressed carrier Modulation. (ii) TDM is invulnerable to the usual sources of FDM inter channel crosstalk. In fact, there is no crosstalk in TDM, if the pulses are completely isolated and non-overlapping. (Since message, information or modulating signal separation is achieved by decommutation, rather by filtering). (iii) In order to reduce crosstalk, the TDM system is provided with guard time between pulses, analogous to the guard band in FDM (Fig. 21.21). Thus, a practical TDM system will have both guard times and guard bands, the former to suppress crosstalk, the latter to facilitate message reconstruction with practical filters. (iv) One more point regarding the bandwidth requirement for TDM system to multiplexing ‘n’ signals, each band limited to ‘fm’ requires a bandwidth of ‘nfm’ Hz and if modulated by the carrier, the bandwidth becomes ‘2nfm’ Hz. Now if the ‘n’ signals are multiplexed in FDM, using a single side band (SSB) technique, the bandwidth is nfm.

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TDM systems are being used more commonly in long distance telephone communications. The TDM and FDM techniques are together used in radio telemetry. Note that Radio telemetry is used for measurements on distant objects.

SUMMARY 1. The process of analog to digital conversion is referred as digital modulation. This is done by means of “sampling” and “Quantizing”. 2. Digital modulation is noise free and privacy of information is possible. 3. Binary code is usually adopted for digital transmission. 4. The speed of communication is given in “Bauds” or Bits per second, the signalling speed may be of the order of 2.3 mega bits per second. 5. The examples of digital modulation are: pulse code modulation, differential modulation and delta modulation. 6. In PCM at transmitting end, the analog signal is converted into digital (binary) code and transmitted. At the receiving end the digital signal is decoded into analog signal again. 7. In PCM, the modulating signal is “sliced” into small units, the process is called quantizing. The quantum points are then converted into binary code, which represents the amplitude of the wave at that point. 8. In PCM, the no. of quantisation steps are increased for small signals and decreased for large signals. The process is called “companding”. This is done to reduce quantising noise. qqq

22 More About Transmitters and Receivers Transmitters and receivers are very important components of all communication systems. This chapter discusses few more topics on AM, SSB, FM and PM transmitters and receivers.

22.1 BASIC REQUIREMENT OF AM TRANSMITTER: FLYWHEEL EFFECT To generate AM waves, we supply current pulses to a tuned (tank) circuit. Each pulse is made proportional in amplitude to the size of the modulating sine wave. This will be followed by the next sine wave proportional to the next applied pulse and so on. At least 10 times pulses per cycle should be fed to the circuit; and if the current pulses are made proportional to the modulating voltage, we get a good AM wave. This is called “Flywheel Effect” of the tuned circuits and holds good for a tuned circuit whose Q factor is of moderate value.

22.2 AM RADIO TRANSMITTER The Fig. 22.1 shows block diagram of an A.M. Radio transmitter. The function of each block is given below: 1. Master oscillator: It generates oscillations of desired frequency. The generated frequency is required to remain constant within limits, inspite of variations in the supply voltage, ambient temperature etc. The frequency variation with time and age are also to be avoided.

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2. Buffer (or Isolating) amplifier: A buffer or isolating amplifier is placed between master oscillator and harmonic generator. The buffer amplifier does not draw any input current hence does not cause any loading on the master oscillator; therefore changes in carrier frequency due to variation in loading are avoided. Master Oscillator

Harmonic generator (class C amplifier)

Buffer AMP

Trans. Ant. Modulated AMP

Modulating signal

Modulating AMP

Fig. 22.1

3. Harmonic generator/Class C amplifier: The master oscillator generates voltage at a frequency, which is a submultiple of the carrier frequency. The harmonic generators basicially are class C amplifiers, in which the tuned circuit selects the desired harmonic frequency signal and amplifies the same. There is a chain of class C amplifiers which raise the power to the required level. 4. Modulated amplifier: This is a class B push pull power amplifier and is modulated by a modulating audio amplifier as shown. In small transistorized radio transmitters, collector modulation, base modulation or both may be used. In high power radio broadcast and radio telephone transmitters “vacuum tube plate modulation” may be used. 5. Modulating amplifier: This modulates the modulated amplifier. This is a class B push pull amplifier. In low power transmitters, class A amplifier may also be used. The low power transmitters with output power upto 1 kW or so may be transistorized, but the higher power transmitters use vacuum tubes.

22.3 Privacy Devices in Radio Telephony

367

22.3 PRIVACY DEVICES IN RADIO TELEPHONY (a) Some communication systems such as telephony utilizes radio waves that are radiated in space. In the route towards the receiver these waves may be intercepted causing encroachments in private conversation. So to avoid this encroachment, some sort of privacy is introduced. That is, signals are delivered unintelligible (not understandable i.e., not clear) at receivers of the unauthorised persons. These are used in wireless telephones of police and of other investigating agencies. (b) Requirements of privacy devices: The privacy devices should fulfil the following requirements: 1. Conversation should be delivered unintelligible to unauthorised persons, even if they employ special receiving equipment. 2. Performance of the circuit should not be affected by the use of a privacy device. 3. Bandwidth of the channel must be maintained same. 4. Time delay of privacy device is maintained as low as possible. (c) Types of privacy devices: The four different types of privacy devices are: 1. Speech inversion privacy device: In this method, the “speech band” is inverted before modulation. This inverted speech is unintelligible and can be used to modulate carrier, then fed to antenna and radiated. In receiver, the signal is demodulated and the original signal is collected by adopting the reverse procedure to that was used for inverting the speech. Example: If speech band contains frequencies of 200 Hz – 2800 Hz and is used to modulate a carrier of frequency 3000 Hz. This signal contains lower sidebands of frequency 2800 – 200 Hz and upper sidebands from 3200 – 5800 Hz. The lower sideband frequency is an inversion of the original speech. That is 200 Hz is represented by 2800 Hz and 2800 Hz is represented by 200 Hz. Now this signal is used to modulate the carrier of 3000 Hz and radiated through antenna, but the upper sidebands are filtered. In receiver, it is demodulated and corrected. 2. Split band privacy device (or scrambler system) (See Fig. 22.2): This system provides an ideal system of privacy which is sufficient for most of the applications. Here the whole speech (0–2800 Hz) is divided into four equal bands and the bandwidth of each band is 2800/4 = 700 Hz.

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The first band 1 extends from 0–700 Hz. The second band 2 extends from 700–1400 Hz. The third band 3 extends from 1400–2100 Hz. The fourth band 4 extends from 2100–2800 Hz. A1 10 kHz 1

M1

S1 10 kHz 1

F1 10–10.7 kHz

1’,

D1

F5 0 – 700 kHz

B1 10.7 kHz

I/P speech 0–2800 Hz

2

M2

F2 2 10–10.7 kHz

2’,

D2

F6 700 – 1400 kHz

B2 11.4 kHz S3 8.6 kHz

A3 8.6 kHz 3

M3

F3

10–10.7 kHz

3

3’,

D3

F7 1400–2100 kHz

Scrambled O/P 0–2800 Hz

S2 9.3 kHz

A2 9.3 kHz

B3 12.1 kHz S4 7.9 kHz

A4 7.9 kHz 4

M4

F4 10–10.7 kHz

4

4’

D4

F8 2100–2800 kHz

B4 12.8 kHz

Fig. 22.2

With the help of modulators and demodulators, each of these bands is displaced from its original position to a predetermined new position. In Fig. 22.2, the bands 1 and 2 are transposed, similarly the bands 3 and 4 are transposed. In this process of displacement, the bands may be inverted, if necessary.

22.3 Privacy Devices in Radio Telephony

369

Consider the block diagram shown in Fig. 22.2 in which the speech band is applied parallel to four modulators M1, M2 and M3 and M4 each of which combines with this signal with one of the two associated oscillators A (A1, A2 .........) or B (B1, B2 .........). The output of each modulator is applied to filter (F1, F2...) which has a pass band of 10 kHz-10.7 kHz in every case.

If oscillator A1 is used, the sidebands of 10 kHz to 12.8 kHz a 10 to

7.2 kHz will be produced at the output of modulator M1 and filter F1 except of 10–10.7 kHz, which is the OH of filter. That is, the speech band 0–700 Hz is converted into 10–10.7 kHz. Similarly M2, M3 and M4 convert 700–1400 Hz, 1400–2100 kHz and 2100–2800 Hz into 10–10.7 kHz respectively. By using A1, A2, A3 and A4, modulating band is contained in upper sidebands. If oscillator B1 is used, the output produces sidebands of 10.7–13.5 kHz and 7.9–0.7 kHz. In this case also filter F1 passes a band of 10–10.7 kHz, but the band is inverted. The second stage of this system consists of four detectors D1, D2, D3 and D4 and four corresponding oscillators S1, S2, S3 and S4. The frequencies of these oscillators are so chosen that output of the four detectors occupy frequency bands 1', 2', 3' and 4' respectively. Filters F5 to F8 select these bands and output of all the four filters are added up to produce the complete speech band of 0–2800 Hz. The connection between points 1, 2, 3, 4 and 1’, 2’, 3’, 4’ are made randomly. This system gives excellent privacy, does not extend the frequency band and involves no time lag. But the system produces poor quality of speech. Also, an elaborate equipment is required and a knowledge of code jumping is needed. 3. Wobbling speech privacy system: The interception of speech can be prevented by wobbling (moving to and fro) the carrier of the radio transmitter at about ±500 Hz at a very low rate, usually 2 or 3 times per second. Wobbling can be done by placing a rotating condenser at a low rate across tank circuit of the master oscillator. A simple detector cannot receive such wobbled signals. The signal is received by proper method, the carrier is eliminated and the privacy equipment is used to reconstruct the speech. 4. Displaced speech privacy system: In this system, the speech band is displaced in frequency by an amount equal to the highest audio frequency in the speech band and then to modulate the carrier frequency. If speech band is from 200 Hz–2800 Hz, this band is displaced by 3 kHz, which results the displaced band from 3.2 to 5.8 kHz.

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This displacement is achieved by modulating 3 kHz sub-carrier with the speech band of 200 Hz-2800 Hz, selecting only the upper sideband (USB) and rejecting the lower sideband (LSB) with a suitable filter. The system provides good degree of privacy, but bandwidth requirement is doubled.

22.4 IMAGE FREQUENCY REJECTION (a) In radio receivers, frequency of the local oscillator is kept equal to the signal frequency plus the intermediate frequency (IF) at all times. The value of IF is taken as 455 kHz. (Fig. 22.3) fIF = fO – fS fS

IF AMP

Mixer fO L.O.

Fig. 22.3

If fs is the signal frequency, fIF is the intermediate frequency, the frequency of the local oscillator (L.O.) should be

fO = fs + fIF

In the mixer, fo and fs are mixed resulting in various frequencies in which fIF is one. Only fIF is passed through the I.F. amplifier and all others are rejected. If somehow a spurious intermediate frequency fSIF = fO + fIF manages to enter the mixer, then this frequency will also mix with fO to produce another fIF and will also be amplified by the I.F. amplifier, resulting in an “interference”; i.e., two neighbouring stations will be received simultaneously. The fSIF is called IMAGE FREQUENCY (spurious) and is equal to the signal frequency plus twice the intermediate frequency i.e., fSIF = fS + 2fIF (b) Rejection of image frequency. The image frequency rejection is one of the most important characteristics of RF amplifiers. The rejection of an image frequency (by a single tuned RF circuit) is defined as the ratio of the gain at the signal frequency to the gain at the image frequency. This ratio is represented by a and given as

22.5 Tracking and Alignment of Receivers

371

a = 1 + Q 2ρ2 where,

r=

fSIF fS

–

fS fSIF

fs = signal frequency fSIF = Spurious Image Frequency = fs + 2fIF Q = Q factor of the tuned circuit/ antenna If there are two tuned circuits, in that case the rejection of each circuit is calculated and the total rejection is found by multiplying the both. The image rejection is governed by the “front end selectivity” of the receiver and the image frequency must be rejected before the IF stage. If the spurious (image) frequency manages to enter the first stage IF amplifier, it becomes impossible to remove it afterwards. If fSIF/fS is kept large (as in broadband), the RF section can be eliminated, however, the RF section is indispensable in short wave range and above. The image frequency rejection is not a problem in receivers without RF section, but care must be taken at L.F.

22.5 TRACKING AND ALIGNMENT OF RECEIVERS (a) Tracking. This is defined with reference to the tuned circuits. Two tuned circuits are said to be “tracking” each other when their resonant frequencies can be made to vary in the same proportion by rotating a common shaft. In a communication receiver, the tuned circuits of RF section and of the local oscillator “track” together. (b) Alignment. This is also defined with reference to tuned circuits. Different tuned circuits are said to be aligned together, when they are resonant at the same frequency. Different tuned circuits in the IF section of a communication receiver are aligned.

(a) Procedure for Tracking A superheterodyne receiver has a number of tuned circuits, which have to be properly tuned so as to have a perfect reception. This results in a mechanically coupled system which requires only one knob for tuning. Irrespective of the received frequency, the tuned circuits of RF section and of the mixer / local oscillator must be tuned to the incoming frequency. The local oscillator has to

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be tuned to a frequency equal to the sum of received frequency and intermediate

frequency. If there is any error in frequency difference, a wrong l.F. will be

generated. This will give “Tracking error.”

In fact, it is impossible tuning of RF oscillator

and of the local oscillator exactly same.

In practice, they are designed to be in step at

either end at the centre of the broadcast band.

Such alignment is accomplished with padder

capacitor associated with local oscillator

or trimmer capacitor mounted on the gang

capacitor or a combination of both may be

used.

(a)

The Fig. 22.4 shows padder and trimmer

respectively.

The Fig. 22.5 (a) shows Padder tracking (b)

(b)

shows Trimmer tracking and (c) shows the combination method. The associated tracking

Fig. 22.4 error is also shown with each method. Here fs is RF amplifier frequency and fo is local oscillator frequency. The P stands for Padder and T stands for Trimmer. + fO

fS

fO

P O

min

–

max Tracking error

fS + IF

Fig. 22.5 (a)

+

fS

fO

T O min –

Fig. 22.5 (b)

max fO Tracking error

fS + IF

22.5 Tracking and Alignment of Receivers

373 +

fS

P

fO

T

fO O min

mid

fS + IF max

– Tracking error

Fig. 22.5 (c)

The combination method can be adjusted to give error at three points across the band at each end and at the middle. It is also called “Three point Tracking.” The Fig. 22.6 shows curves for correct, misaligned and badly misaligned situations with correct tracking. A tracking error as low as 2 kHz can be obtained which is acceptable. To obtain correct tracking, coil of the local oscillator should be correctly adjusted, as the capacitor (connected in its series) has a fixed value and cannot be varied.

Tracking error (kHz)

+6

Badly misaligned

+4 +2

Correctly aligned

0

Misaligned

–2 –4 –6 600

1000 1200 1400 Tuned frequency (kHz)

800

1600

Fig. 22.6

(b) Procedure for Alignment For expeditious and simple tuning, all the tuned circuits must be tuned by movement of a single dial. Such a “single dial tuning” requires that rotor of all these tuning capacitors should be mounted on the same shaft, i.e., the tuned circuits of RF amplifiers and oscillators must be aligned.

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1. Alignment of Tuned circuits of RF amplifier: Proceed in the following steps: (i) Feeding at the receiver input, a signal of selected frequency at the higher frequency end of the tuning dial and tuning the receiver to this frequency and adjusting the RF circuit trimmer to get maximum power at the receiver’s output. (ii) Repeating the above procedure by feeding a signal of another selected frequency at the lower frequency end of the tuning dial. (iii)

Repeating the above two steps alternately to get the maximum output power at the receiver simultaneously at both these frequencies. For M.W. band (extending from 550 kHz – 1650 kHz) we may choose 600 kHz and 1600 kHz as the two signal frequencies for RF tuned circuits alignment.

2. Procedure for tuned circuits of oscillator: Proceed in the following steps: (i) Feeding at the receiver input, a signal of a selected frequency at the upper end of the tuning band and adjusting the oscillator circuit trimmer to get the maximum power at the receiver’s output. (ii) Feeding at the receiver input, a signal of another selected frequency at the lower end of the tuning band and adjusting the oscillator circuit padder to get maximum power at the receiver output. (iii)

Repeating the above two steps alternately to get maximum receiver power at both the frequencies.

22.6 STEREO FM TRANSMITTER AND RECEIVER (a) Stereo FM transmitter: Stereo FM transmission is a modulation system, in which sufficient information is sent to the receiver to enable it to reproduce original stereo material. The audio is picked up by two microphones located to emphasize different sections (left and right). The microphone output are combined in two distinctly different manners. The monaural (mono phonic) mixer is a straight forward mixer of the two channels. This results in sum of the channels (L + R) in the frequency range 0 to 15 kHz.

22.6 Stereo FM Transmitter and Receiver

375

The second channel receives a more sophisticated treatment. The output of the stereo mixer is the difference of the two microphones (L — R) in the frequency range 0 to 15 kHz . This is fed to the balanced modulator. Also fed to the balanced modulator is a 38 kHz carrier provided by a sub carrier generator. The balance modulator mixes the audio and 38 kHz carrier and creates ± sidebands mixing 0 to 15 kHz with 38 kHz resulting in a lower sideband of 23 to 38 kHz and upper sideband of 38 to 53 kHz. At the same time, the original 38 kHz carrier is eliminated (Fig. 22.7). Left Channel Microphone

Right Channel

Microphone

L

R

Monophonic mixer

Stereo Mixer

To RF Section

Balanced Modulator 38 KHz Sub carrier generator

Fig. 22.7

The final frequency groups modulating the FM stereo transmitter 0 to 15 kHz representing the sum of the two channels; 23 to 53 kHz, representing the difference of the two channels; and the 19 kHz signal derived from the 38 kHz sub carrier and used as a synchronizing or local oscillator carrier at the receiver. The above stereo system permits compatible monophonic reception by FM receivers that do not have stereo capability. (b) Stereo FM receiver: The stereo receiver reverses the process of transmitter. The (L + R) signals confined in 0 – 15 kHz frequency range is directed to a low pass filter (Fig. 22.8).

Ant.

Low pass filter FM tuner, IF Amplifier and limiter

L+R Audio

Demodulator

Matrix and de-emphasis NW (L–R) Audio

Band pass filter 23 to 53 kHz

(L–R) DSB

AM Demodulator 38 KHz

19 KHz filter

Fig. 22.8

19 kHz

Frequency doubler

L

R

To Audio Amplifier

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The L – R signal contained in the 23 to 53 kHz frequency range is extracted by an appropriate band pass filter. The 19 kHz pilot carrier is extracted by appropriate filter and converted to 38 kHz in a frequency doubler. The 38 kHz carrier is mixed with L – R signal in an AM demodulator. Here it functions as the carrier for the L – R double sideband. The output of the AM demodulator is L – R audio signal. The output combined with the L + R audio signals is mixed in a de-emphasis network. The output of this stage are the original L + R channel information. .

22.7 SSB RECEIVERS The SSB receivers demodulate the SSB signals and process them. The SSB receivers are not used as broadcast receivers. They are used to receive signal in crowded frequency bands such as short wave bands. So, these are usually made double (or multi) conversion type. Their special qualities are: (i) high reliability. (ii) simple maintenance. (iii) ability to demodulate SSB signals. (iv) suppression of adjacent channel signals. (v) high SN ratio. (vi) In case of independent sideband (ISD) receivers, it should be capable to separate independent sidebands. We will discuss various SSB receivers below:

1. General SSB Receiver The Fig. 22.9 shows block diagram of SSB receiver. The input sideband is received from receiving antenna and fed to RF amplifier. The RF amplifier amplifies the SSB signal with frequency band of 15.100–15.105 MHz (USB). The output is fed to first mixer where the signal is hetrodyned with the input voltage of the first local oscillator (L.O.) of frequency 12 MHz and produces the output with a frequency of 3.100–3.105 MHz. The output of the first mixer is amplified in the first I.F. amplifier and fed to second mixer.

22.7 SSB Receivers

377

The second mixer is also fed with output of second local oscillator (L.O.) of frequency 3 MHz, so that it produces an IF signal of 100–105 kHz. This 100–105 kHz IF signal is amplified by second IF amplifier and the signal is fed to final detector and crystal filter. Ant. 15.100 – 15.105 MHz 3.100 – 3.105 MHz First Mixer

First IF Amp

Final Detector

Audio Amp

L.S.

3 MHz

12 MHz

Crystal Filter

Second L.O.

First L.O.

Amplifier

AFC control voltage

AVC control voltage

Second IF Amp

Second Mixer

Reinserted carrier

RF Amplifier

100 – 105 kHz

AVC detector

AFC

Crystal Osc.

Fig. 22.9

The crystal filter filters out the carrier frequency of 100 kHz from upper sideband (USB) and is amplified by amplifier and fed to automatic frequency control (AFC) system. The AFC system produces AFC control voltage with frequency difference between the reinserted carrier and local oscillator signal. The AFC control voltage changes the second local oscillator frequency to maintain the synchronism between the crystal oscillator frequency and received carrier frequency. When this condition is achieved, the received signal is fed to final detector.

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The final detector reconstructs the original signal and fed to audio amplifier. The audio amplifier raises the strength of the detected signal to the required level which is fed to the loudspeaker.

2. Pilot Carrier SSB Demodulator/Receiver (a) Principle. In this modulator (Fig. 22.10), ‘a pilot carrier’ is used at the receiver to synchronise the local oscillator used for demodulation to the original carrier, this improves operation of the demodulator. Ant

Demodulator

USB Filter

Product demodulator

Pilot carrier filter

Phase Locked Osc

AF out

LPF

Fig. 22.10

At the receiver 3 MHz signal is picked up, amplified and converted to I.F. at 100 ± 5 kHz, producing pilot carrier as 100 kHz and upper sideband (USB) in the range of 100 — 104 kHz. The output of receiver is passed through USB filter and then to the product demodulator. A “phase lock oscillator” produces 100 kHz carrier for the demodulator. A final low pass filter (LPF) removes the sum component of the demodulator, leaving 0 to 4 kHz AF signal. (b) Block diagram. This receiver is a double conversion type with a squelch (mute) circuit (Fig. 22.11). It also has an AFC system which provides a good frequency stability. The local oscillator is used with a multiplier.

22.7 SSB Receivers

379

AF O/P RFA

H.F Mixer

L.F Mixer

Side Band filter

L.O. with Multiplier

Pilot carrier filter +Amp.

HIFA

RFA = Radio Frequency Amplifier BPF = Band Pass Filter HIFA = High IF Amplilfier LIFA = Low IF Amplifier

VFO Varactor diode

Phase comparator

Amp

Product Detector

Amp

Squelch circuit

Crystal Osc.

Fig. 22.11

The output of L.F. mixer contains the wanted sideband and the pilot carrier. The sideband goes to filter then to product detector and then to the amplifier. The pilot carrier is filtered and amplified separately. A phase comparator is also provided which has two inputs: one from pilot carrier amplifier and other from the crystal oscillator. The comparator compares these two input frequencies, and gives an output, which will be zero, if the frequencies are equal. This ideal condition gives very good frequency stability. The output of the comparator is given to a varactor diode and then to VFO (Variable Frequency Oscillator) and then to the H.F. mixer.

3. Suppressed Carrier/ISB Receiver The Fig. 22.12 shows block diagram of a suppressed carrier SSB receiver which is used for ISB (Independent sideband) reception. Its RF amplifier is a wideband amplifier covering a range of 15 kHz to 30 MHz. The first intermediate frequency is very high say 35 MHz. A high IF provides much higher image frequency rejection, which is very important in SSB receivers. Frequency synthesiser provides a high frequency stability. Upto low frequency IF stage, this receiver is similar to an ordinary double conversion type receiver, after this, difference arises due to the presence of two independent sidebands (ISBs), which are separated after the LF mixer. The upper ISB is filtered by a bandpass filter and IF is amplified and after the product detector stage, audio frequency (AF) output (say channel A) is obtained.

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RFA

BPF

Band Pass filter

HIFA

High IF Amplifier

BPF + HIFA

H.F. Mixer

More About Transmitters and Receivers

LISB Frequency synthesiser

LIFA

Low IF Ampllifier

UISB

Upper ISB

LISB BPF + LIFA

Prod Det. Product detector LISB

UISB BPF UISB Prod Channel A Det (USB output) + LIFA

L.F Mixer

Low ISB

Prod Channel B Det. (LSB output)

Fig. 22.12

The lower ISB is filtered through a separate band pass filter, amplified and after the product detector stage, AF output (say channel B) is obtained. In this way, both independent sidebands are obtained, note that carrier is absent.

4. SSB Receiver with Squelch and BFO (A Double Conversion System) (a) A double conversion system (Fig. 22.13) has two mixers. When a communication receiver has two I.F.s, it is said to have a double conversion system. The communication receivers which require a very high quality performance, use double conversion system. The first l.F. is quite high (in the range of several MHz) and the other is quite low (in the range of few kHz). The output of RF amplifier is mixed into HF mixer with the frequency of the local oscillator I. The output of HF mixer is higher than 455 kHz. This frequency is amplified by a high intermediate frequency amplifier (HIFA) and its output is given to LF mixer, where it is mixed with the frequency of local oscillator II. The output of the LF mixer is a low IF signal, which is detected as usual. Ant. Low IF

High IF RFA : RF Amplifier

RFA

HF mixer

HIFA

LF mixer Off

HIFA : High IF Amplifier LIFA : Low IF Amplifier BFO : Beat Frequency

Oscillator

LIFA

L.O. I

L.O. II

L.S.

AMP

Fig. 22.13

Squelch circuit

Detector

On BFO

22.7 SSB Receivers

381

The high IF gives image frequency rejection and allows its better attenuation. The low IF gives a good selectivity and adjacent channel rejection. Note that high IF should come first, otherwise image frequency rejection will not be sufficient and will be mixed with the signal. Any number of IF stages after this, will not be able to reject the image frequency.

The method is not used for domestic receivers or receivers working on

medium frequency band. The short wave band receivers however can

use this system.

The additional systems used in the double conversion receivers are discussed below: (i) Squelch (Mute or Quiet) System: When a signal is absent i.e., no transmission, a sensitive receiver will produce high noise. The reason is that in the absence of a signal, AGC (automatic gain control) disappears, causing the receiver to operate at its maximum sensitivity. The low level of the noise at the input of the receiver gets amplified and which is very much uncomfortable specially in case of police or ambulance receivers. To solve this problem, a squelch or mute or quiet circuit is used in the receiver; which enables the receiver’s output to remain cut off unless the signal is present. (ii) BFO (Beat Frequency Oscillator): The communication receivers should be able to receive transmission in morse code. In order to make dots, dash and spaces in morse code audiable, we use a BFO. This is a simple hartley type oscillator which operates at a frequency of 1 kHz above or below the last intermediate frequency. When the IF is present, a whistle is heard in the loud speaker and dots or dashes can be heard. To avoid interference, the BFO is switched off otherwise. The Fig. 22.14 shows block diagram of B.F.O. separately. It gives a very large frequency range with a single dial rotation. Fixed Frequency RF. Osc.

f1

RF Filter

Mixer

RF Filter

f2 Variable frequency RF Osc.

O/P

Amp

Fig. 22.14

Dial

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At the output of the mixer, we get sum and difference of f1 and f2. The BFO is usually affected by spurious beat notes called “whistles”. These whistles appear, when IF is obtained.

5. SSB-Transreceivers This circuit can be used as a modulator at transmitter side and as a demodulator at the receiver side (as one unit), and can be switched on to either side as per requirement, so it is preferred to a product demodulator. This is referred as “Transmitter plus receiver.” See Fig. 22.15. When used as a demodulator, SSB signal is fed at terminals (1, 2). The circuit behaves as a ‘non linear resistance’ giving sum and difference frequencies. The transformer blocks the RF frequencies and allows only AF frequencies at terminals (3, 4). D1 3

1

4

2

5

6

D2

Fig. 22.15

When used as a modulator input is given at terminals (5, 6). The modulated output is obtained across terminals (1, 2). When used as demodulator, the input is given at 1, 2 and output is obtained at 3, 4. modulator

Input at 5, 6

Output at 1, 2

demodulator

Input at 1, 2

Output at 3, 4

22.8 Coherent and Non Coherent SSB Detection

383

22.8 COHERENT AND NON COHERENT SSB DETECTION In coherent detection method, phase of the carrier generated by the local oscillator of the receiver should be identical (coherent or synchronised) with the carrier generated by the local oscillator of the transmitter. Thus the original signal is obtained; therefore, this method is called “coherent or synchronous” method of detection. In non coherent (non synchronous) detection, the frequency and phase of the local oscillators on the two sides is not identical and the detected signal is distorted. (a) Coherent Detection of Single Tone DSBSC Wave The Fig. 22.16 shows a Coherent SSB detector. The modulated SSB signal is first multiplied with the locally generated carrier and then passed through a low pass filter (LPF). The output is the original modulating signal.

m(t) Modulated

SSB signal

Multiplier

S(t)

LPF

O/P

C(t) L.O.

Fig. 22.16

The multiplier output is given by m(t) = S(t) × c(t) Where

S (t) = SSB wave 1

V V [cos 2p (fc + fm)t + cos 2p (fc – fm)t] 2 m c c(t) = local carrier output =

and

...(1)

= cos (2pfct) Substituting the expressions for S (t) and c (t) in eq. (1) we obtain m(t) = cos (2p fct) [

1 2

VmVc cos {2p(fc + fm)t} + cos 2p (fc – fm)t]

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Solving above we get, m(t) =

The output

1 2

VmVc [cos {2p(2fc + fm)t} + cos (2pfmt) 1 + VmVc [cos 2p (2fc – fm)t] + cos (2pfmt)] 2

1

V V cos(2pfm)t 2 m c Note that frequencies (2fc + fm) and (2fc–fm) are removed by low pass filter. V0(t) =

(b) Non Coherent detection: The disadvantage of non coherent method is that, it requires an additional system at the receiver to ensure the synchronisation. This makes the receiver complex and costly. So this method is not used.

SUMMARY 1. The AM transmitters are provided with negative feedback. 2. The frequency of modulator may vary, this is called “Frequency drift”. 3. In radio telephony, privacy devices are provided so that the conversation is not reached to authorised persons. 4. The important types of privacy devices are: (i) Speech inversion privacy devices (ii) Split band privacy devices. 5. Image frequency rejection is one of the most important characteristics of RF amplifiers. 6. The mixers may be (i) Additive mixers (ii) Multiplicative mixers 7. The SSB receivers may be (i) Coherent receivers (ii) Non coherent receivers qqq

23 Television Basics and Monochrome Television Television is a very popular audio video device. It is now a basic need of every house. It is a source of entertainment and education.

23.1 TELEVISION “Tele-Vision” means to “see at a distance”. The visual information in the picture is converted into electrical signal for transmission. At receiver it is reconverted into its original form. In monochrome TV, the picture is reproduced in black and white. In colour TV, it is reproduced in its original colours. The TV was invented by J.L. Baird. J.L. BAIRD

23.2 TV APPLICATIONS

Below important TV applications are described in brief: (a) Cable TV (CATV): When signal is to be sent to shorter distances, a cable is connected from transmitter to the receiver. Generally modulation is not required. The signals are distributed by co-axial cable to the consumers. (See Fig. 23.1) Antenna Signal

Distribution amplifier

Pre-amplifier

1

2 3 4 Consumers

5

1 2 3

Fig. 23.1

1 2 3 Consumers

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Chapter 23 Television Basics and Monochrome Television

(b) Close circuit TV (CCTV): In this system, video signal output of camera is connected by cable directly to the monitors placed at distance. Thus picture is reproduced on each monitor. (A monitor is a video display device. It is a TV receiver without RF and IF circuits). The CCTV equipment is available for monochrome (B & W), as well as for colour TV. A network of co-axial cable is used for connection. [See Fig. 23.2 (a), (b), (c)] Camera

Video amplifier

Monitor

(a) Camera directly linked with one monitor Monitor

Monitor

Distribution amp.

Camera

Monitor Monitor

Monitor

(b) Camera linked with several monitors

Monitor Camera

Transmitter

Monitor Monitor Reception

(c) Wireless link Fig. 23.2

(c) Picture phone: This is a “telephone plus TV”. We can see as well as hear each other by this phone. (d) Fax (Facsimile): This is electronic transmission of visual information over telephone lines. It is also called “slow scan TV” as scanning in this case is slow. Only still picture can be transmitted on fax. (e) Satellite TV: By installing a satellite in space we can transmit signals on TV receivers over large areas. Satellites are used as relay stations to provide world wide TV broadcasting. The Fig. 23.3 shows block diagram of a typical satellite transmission. A high power satellite is installed at a height of about 36000 km. The TV programs from an earth station are transmitted to the satellite at 6 GHz (FM carrier) with the help of a dish antenna.

23.2 TV Applications

387

The downward transmission from the satellite is done at 850 MHz. The earth transmitting station may be equipped with a dish antenna and the receiving station may also have a dish antenna to receive back the signals reflected from the satellite. Satellite

Transmission from earth Station (6 GHz) 6 GHz Earth station with transmitting dish antenna

Downward transmission from the satellite (850 MHz) Receiving dish antenna

P.T. RF Amp.

Mixer

IF Amp.

Discriminator

Video Amp.

Local Osc.

Fig. 23.3

(f) TV Games: The earlier TV games were using TTL (Transistor-Transistor Logic) circuits. Now with the development of microprocessors, more sophistication has been obtained. Basically all TV games are logic circuits. A TV game has two important parts. The first part is the “game unit” (which further includes control, logic circuit and RF oscillator circuit) and second part is the TV receiver. The system has been shown through a block diagram in Fig. 23.4. T.V. receiver

Game unit User’s controls

Game and control logic ckt.

Fig. 23.4

RF Osc. and Mod.

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23.3 BROADCASTING, TRANSMISSION AND RECEPTION OF MONOCHROME TV (i) “Broadcasting” means to “cast broad’ i.e., to send a massage in all directions. The TV signal has audio as well video information. The transmitting antenna radiates TV signals in the form of electro magnetic waves that can be picked up by a TV receiver placed in the range. The amplitude modulation is used for the picture signal and frequency modulation for the sound signal. (ii) Transmission (Fig. 23.5a): In a TV studio, audio signal produced at mike is converted into corresponding electrical pulses and fed to amplifier. For picture transmission, the picture signal (light energy) is converted through camera into electrical pulses. This video signal (electrical pulses) are given to a video amplifier. The both i.e., sound as well as the picture are sent to the transmitting antenna for transmission. (iii) Reception (Fig. 23.5 b): Separate carrier waves are used for sound and picture signals but they are radiated by one transmitting antenna. At TV receiver also, the same antenna is used to pick up both signals. The signals in the receiver are amplified and separated. The picture signal is given to picture tube, which by its transducing action converts it into original picture produced in the camera. The sound signal is fed to the loudspeaker which gives the original sound produced before mike. Transmitting antenna Picture

Camera

Video signal

Video amp.

Transmitter

Audio signal

Audio amp.

Transmitter

Scan and Sync. circuits

Sound

Mike

Fig. 23.5 (a)

23.4 TV Camera

Receiving antenna

389

Video signal

Video amp.

Syn. and Scanning circuits

P.T.

Video and Audio signals L.S.

Audio signal

Audio amp.

Fig. 23.5 (b)

Note that during transmission, to convert picture into video signal (electrical pulses), the camera scans the picture into horizontal and vertical lines (the picture is divided into 625 lines). This process is known as scanning. Similarly during reception, the picture tube re-assembles these lines to obtain the original picture. It is important that both the scanning processes taking place in camera as well as in the picture tube should be property synchronized. This is done by “scanning and synchronizing circuits”.

23.4 TV CAMERA Camera is the first and basic equipment in a TV. The input to a camera is the light from the picture or scene to be televized and output obtained from camera is the electrical pulses corresponding to the information contained in the picture. The TV camera is just analogous to human eye. The basic principle of all TV cameras is based on the fact that each picture may be assumed to be composed of small elements with different light intensity. The camera picks up each element (during transmission) and by transducing action converts it into “electrical signal” proportional to its brightness. There is a photosensitive layer called target or image plate in each camera which performs this job. At the same time simultaneous pick up of this information is also necessary. For this purpose (in the receiver), there is an electron gun (which produces an electron beam) which scans the image plate at a fast speed. Thus opto-electric conversion as well as pick up of the signal takes place simultaneously and at a fast speed.

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1. Working Principle (Function) of TV Camera (a) The camera works on one of the two principles: (i) Photo emessivity: Certain metals emit electrons when light falls on them. The property is called photo emission. The emitted electrons are called photo electrons and the surface is called a photo cathode. Generally, Cesium or Bismuth are used for making photo cathodes of cameras. (ii) Photo conductivity: is the property, according to which a metal loses its resistance when light is incident upon. Generally selenium and lead make good photo conductive surfaces in cameras. (b) A video camera performs the following functions (Fig. 23.6): (i) If converts the picture signal into electrical pulses of equivalent brightness. The picture is composed of small elements of different brightness. There is a photosensitive plate called “Image plate or Target plate”, which picks up each element of the picture and converts into electrical pulse of varying brightness.

Electron gun

Object

Camera lens

Image/Target plate

V0

Fig. 23.6

(ii) The camera has also an “electron gun” which “scans” the image plate and picks up these pulses simultaneously at a very fast speed.

2. Basic Construction of Camera The following are the components of cameras: (i) Target/Image Plate/Signal Plate: Construction of target is different in different cameras, but the basic is same. The plate has a photo emissive/ conductive coating which is scanned by the electron gun of the camera (tube). By means of photo electric effect, the picture (visual) signal is converted into electric signal. The target is also called signal plate or image plate.

23.4 TV Camera

391

(ii) Electron multiplier: Electron multipliers are like amplifiers which amplify the photo electric current obtained from picture. They work on the principle of secondary emission. The electron multiplier has a series of electrodes called dynodes each at a progressively higher positive potential. The Fig. 23.7 shows five such dynodes. From photo cathode, which is at zero potential, the primary electrons are made to bombard the first dynode which gives secondary electrons. These electrons bombard to each dynode as shown and finally we get a high anode current. The device is noise free, whereas the conventional amplifiers produce much noise. Cover

00

+4 0 60

+

V 4

+300 V

V

00

Anode

+2

V Dynode

3

2 1

5

0V

+50

+100 V

Cathode(0V)

Light

Fig. 23.7

3. Resolution of a Camera By resolution of a camera we mean, how efficiently a camera can scan the picture element. If resolution of camera is high, we can see finer details of the picture e.g., wrinkles on the face, or a on the forehead etc. Resolution is given in percentage. 4. Persistence of Vision and aspect Ratio (a) Our eye can retain a picture only for 50 ms after the picture is removed. This property of eye is known as persistence of vision. Video systems’ (TV etc.) work on this principle.

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Chapter 23 Television Basics and Monochrome Television

(b) It has been found that the scanned picture should have a rectangular format with width to height ratio of 4 : 3 called Aspect ratio. This size is most pleasing to eyes with least fatigue.

23.5 VARIOUS B&W TV CAMERAS The various TV cameras are: (1) Image Orthicon

(2) Vidicon

(3) Plumbicon

(1) Image Orithicon (I.O.) Camera (See Fig. 23.8) This camera lube can be divided into 3 sections—image section, scanning section and the multiplier section. (See Fig. 23.8) Electron gun and multiplier section

Image section

Scanning section

Deflecting coil

Photo cathode

Multiplier

Picture

Scanning beam

Electron gun Target

Focussing coil

Fig. 23.8

Working Light from the picture (scene) is focussed on the photo cathode. This gives an electron image corresponding to the picture. Needless to tell, that the cathode is made of photo emissive material like cesium oxide. This electron image is accelerated to the target to produce secondary emission. The secondary emission produces on the target, a pattern of positive charges corresponding to the picture, the white colour being most positive. The scanning beam from the electron gun neutralises these positive charges and the excess electrons of the beam return back to the gun. As the beam scans the target, the electrons turning back provide a signal current in accordance with the picture. The signal current is maximum for the

23.5 Various B&W TV Cameras

393

black colour of the picture. Now the signal current passes through the multiplier section, where it is amplified many fold. The amplified current passes through the load and produces an output voltage. If signal current is assumed as 5 mA and load as 20 K, the output voltage is 5 mA × 20K = 0.1 V. SnO2 (2) Vidicon Camera Vidicon is a very small camera tube of about 8″ Glass Sb2S3 long and l.5″ dia. It is also simple in construction as it has only target plate and the gun. (a) The target (See Fig. 23.9) has two layers, one layer is of transparent glass coated with some conducting material usually Fig. 23.9 Tin oxide. The second layer is coated with thin photo conductive material like Antimony compound (Sb2S3). The Tin oxide (SnO2) acts as positive terminal for the target. (b) Electron Gun: The electron gun has a heated cathode, a control grid and a focussing grid. The deflection of the beam produced by the gun is used for scanning, and the direction of the beam is controlled by deflecting coils. [See Fig. 23.10 (a)] Accelerating grid

Control grid

Target

Glass plate

Deflecting coil

Focussing coil Signal plate

(a) C

Photo sensitive layer

RL

Scanning beam Cathode of gun

(b) Fig. 23.10

Camera signal output

Heated cathode

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Chapter 23 Television Basics and Monochrome Television

Working With the optical image focussed on the target (or signal plate), it produces a charge image which is scanned by the electron gun. Each point in the charge image has a different positive potential on the side of the target facing the gun. The beam from the gun deposits its electrons on the photo layer surface of the target reducing the positive potential to zero. Excess electrons, are returned back but they are not used in vidicon, though they were used in the orithicon. This change in potential of the target plate causes signal current to flow in the circuit. This has been shown in Fig. (b) producing voltage across RL. For black in the picture where photo layer is less positive than the white, the deposited electrons cause a small change in signal current, hence the signal current results from the change in the potential difference between the two surfaces of the photo layer. Characteristics of Vidicon Camera Tube (i) Dark current sensitivity: When the camera lens is closed, very small signal current flows in the target circuit of the tube as the target is in total darkness. This small current is known as dark current. The value of dark current in vidicon is about 20 nano amp. See Fig. 23.11. The dark current affects the sensitivity of the tube. For higher value of dark current, sensitivity is more. A typical value of sensitivity of vidicon is 120 mA per lumen illumination on the target.

Signal Current (nA) →

500

50

C

r

Da

)

nA

n

rre

u kC

0 t (2

50 Illumination (in lumens) →

Fig. 23.11

(ii) Resolution: As defined already resolution offers the smallest details of the picture that can be resolved by the camera. The resolution of vidicon is about 55% which can be said as satisfactory. (iii) Signal/Noise Ratio: The S/N ratio of vidicon is about 47 dB. It is a quite high value. Applications Vidicon is compact in size and simple in operation. It is just a target and gun assembly. It is used in outdoor and indoor shootings with black and white TV.

23.5 Various B&W TV Cameras

395

(3) Plumbicon Camera Tube Plumbicon is similar to vidicon. The electron gun is also similar to that of vidicon but with a different target plate which is basically a PIN (P, I and N type) semi conductor diode. On one side of the plate, PbO (P type semi conductor material) is deposited and on the other side a layer of SnO2 (N type material) is deposited—both separated again by a PbO layer which is an intrinsic (I type) material. Thus a PIN diode is formed. The SnO2 coated side works as a signal plate. [See Fig. 23.12 (a)]. Glass + + + + + + + + +

Light

SnO2 (N-type material)

Electron beam from electron gun PbO2 (p type material)

Pure PbO (I-type material)

(a) Focussing coil Grid

Cathode

Target

Deflection coil

(b) Fig. 23.12

Construction of Plumbicon [See Fig. 23.12 (b)] The construction of plumbicon is just similar to the vidicon. It is also a target and gun assembly. It also has a cathode, that emits electron beam which is controlled by deflecting and focussing coils.

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Working (Fig. 23.13) The SnO2 (N side) side is connected to a supply of 40 V through a load RL, across which output signal voltage is developed. When the electron beam scans the target, the signal current varies in accordance with the light of each part of the picture. The working of plumbicon is also similar to the vidicon, with the difference that as in vidicon each element acts as a leaky capacitor, in plumbicon, it acts like a capacitor in series with light controlled diode, without light the diode is inverse biased and there is no output. PIN diode

SnO2(Signal plate) C Camera Signal Output

PbO(P)

PbO(I)

RL

Scanning Beam

+ 40 V

Electron Gun

Fig. 23.13

When there is no light, the PIN diode is reverse biased, and a negligible current flows. When light falls on the PIN diode (target), it becomes forward biased in accordance to the intensity of light. The bias is the result of photo excitation of P layer and intrinsic layer of the target, thus the target behaves as a capacitor in series with the PIN diode, and output is obtained. Characteristics of plumbicon (i) Dark current and sensitivity: The dark current of plumbicon is nearly 1 nano amp. and sensitivity is better than vidicon. The value being 400 mA per lumen illumination of the target. (ii) Resolution: The resolution of plumbicon is poor than vidicon. The typical value is 45%. (iii) S/N Ratio: The SN ratio of plumbicon is higher than vidicon. The typical value is 52 dB.

23.6 Picture Tube

397

Application It is also simple in construction and working, therefore, used in studio as well as outdoor shootings specially with colour TV. Table 23.1: Comparison of human eye and Camera S.No.

Human Eye

Camera

1.

The focal length of the eye lens can be changed by the action of ciliary muscles

The focal length of a camera lens is fixed. It cannot be changed.

2.

The focussing in the eye is done by changing the focal length of the eye-lens

The focussing in a camera is done by changing the distance between the lens and the film.

3.

The retina of the eye retains the image only upto 1/20th a second after the object is removed. Thus, the image formed on the retina of an eye is not permanent.

The photographic film of a camera retains the image of the object permanently.

4.

Retina can be used again and again for forming the image.

A photographic film can be used only once for forming the image.

23.6 PICTURE TUBE The picture tube that provides a screen for a TV receiver is basically a cathode ray tube (CRT). It consists of an evacuated glass envelope. At its neck, there is an electron gun which supplies the electron beam. The inner surface of its face plate has a phosphor coating which produces light when the electron beam strikes. A monochrome (B & W) picture tube has one electron gun and a continuous phosphor coating that produces a picture in black and white. For colour picture tubes, the screen is formed of three different phosphors (red, green and blue). The neck of the colour picture tube may have one gun emitting three beams for the three phosphors. These 3 phosphors by combination can produce any colour; or the PT (picture tube) may have three guns each emitting different colour. Table 23.2: Various screen phosphors for Picture Tubes S.No.

Phosphor colour

Applications

1.

White

Monochrome picture tubes

2.

Red, Green, Blue

Tricolour picture tubes

3.

White, Yellow

Two layer screen

4.

Green, ultra violet

Flying spot scanner

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Chapter 23 Television Basics and Monochrome Television

• Monochrome (B & W) Picture Tube The monochrome picture tube is used in B & W-TV. These tubes employ electrostatic focussing and electromagnetic deflection. The composite video signal (picture + sound) obtained is fed to cathode of the picture tube. The picture is constructed bit by bit but the same is perceived by the tube as complete and continuous due to persistence of vision. Phosphor coating +

50 V

–

Control grid

Focussing grid (300 V)

H

Aluminium coating

Electron beam

H

Cathode

Accelerating grid (400 V)

Deflection coil Aquadag coating (18 kV)

Glass face plate

Fig. 23.14

The important parts of a PT are described below (See Fig. 23.14) (i) Electron gun: The electron gun that produces high velocity electron beam consists of an indirectly heated cathode (of tungsten) and 3 grids. These grids are control grid, accelerating grid and focussing grid and constitute the electrostatic focussing system of the tube. (ii) Grids: The grids control the movement of the beam through a smaller area and hence they ‘focus’ the beam at the screen. The control grid is maintained at a negative potential w.r.t. cathode. The accelerating and the focussing grids are maintained at positive potentials between (+200 to 600 V) w.r.t. cathode. (iii) Aquadag coating: Starting from the half way into the neck to about 3 cm of the screen there is a special material coating called aquadag. This is a conducting coating generally of graphite. It is connected with a very high potential. Generally a 48 cm monochrome TV has an aquadag coating at 18 kV. Due to such a high positive potential, it increases the velocity of the electron beam to a very high value. (iv) Electromagnetic deflecting coils: (See Fig. 23.15 (a) & (b)) Two pairs of coils are mounted outside and close to the neck of tube. These coils provide desired deflection (horizontal as well as vertical) to the beam.

23.6 Picture Tube

399 Vertical deflecting coils

am

Be

Horizontal deflecting coil

(a)

a

Gun

Deflection Angle

Deflection coil

(b) Fig. 23.15

These coils maintain a deflection angle between a = 55° to 110°. The deflection angle decreases with length of the tube. 4x (v) Face Plate/Screen: Generally

picture tubes have rectangular

face plate with a length to Aluminium coating breadth ratio 4 : 3 (called 3x 20” aspect ratio). A 20″ screen means the—distance between two diagonal points of the Fig. 23.16 screen (See Fig. 23.16). The thickness of about 1 to 1.5 cm is sufficient to provide it the mechanical strength to withstand the air pressure on the evacuated glass envelope. (vi) Aluminium coating: An aluminium coating is provided at the back surface of face plate of the tube. The coating is very thin and high velocity beam can easily penetrate it to reach the phosphor screen.

About 50% of the light emitted is returned back into the tube after striking the screen. Another 20% is lost in internal reflections within the tube thus, only

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Chapter 23 Television Basics and Monochrome Television

30% light is utilised. The aluminium coating reflects back to the screen much of the light lost in the tube, thus it improves brightness on the screen.

23.7 SCANNING In order to convert a picture into a video signal in the camera, the picture is

divided into number of horizontal lines. The picture tube in turn re-assembles

these lines into the original picture. These horizontal lines are produced by

making the electron beam to scan the picture line by line. There may be 525 (or

625) lines per picture frame. The electron beam scans these lines horizontally

as well as vertically. The process is known as Scanning.

The scanning in camera and scanning in the picture tube should be synchronised.

This is essential to re-assemble the picture on the correct lines. This is known

Synchronisation.

For both the purposes scanning and synchronising circuits are provided in TV

transmitter as well as in the receiver.

The scanning is of two types:

(a) Horizontal scanning: For scanning, saw tooth currents (Fig. 23.17) are used. This current flows through horizontal deflection coils of the picture tube for horizontal scanning and through its vertical deflection coils for vertical scanning. l

ce

tra

Re

ce

a Tr

t Trace

Trace

period

period Retrace period

1st line

2nd line

Fig. 23.17

23.7 Scanning

401

The linear rise of the saw tooth wave deflects the beam across the screen with a continuous uniform motion for the trace from left to right. At peak the wave reverses and decreases rapidly to its initial value. This fast reversal produces retrace or flyback. (See Fig. 23.17 and 23.18) Top

Raster Right

Left

Bottom

ce

Left

Right

Top

(a) horizontal scanning

ce

e

c Tra

tra

Re

tra

Re

e

c Tra

Bottom

(b) vertical scanning Fig. 23.18

The start of the horizontal trace is at the left edge of the raster and the finish is at its right edge. The flyback produces retrace back to the left edge. (See Fig. 23.19) Left

Top

Right

Bottom

Fig. 23.19

(b) Vertical scanning: When made to flow through vertical deflecting coils of the P.T.; The saw tooth current moves the electron beam from top to bottom of the raster. While the electron beam is being deflected horizontally, the vertical deflection coils move the beam downward with uniform speed. Thus the beam produces complete horizontal lines one under the other. The trace part of the wave for vertical scanning deflects the beam to the bottom of the raster, then the rapid vertical retrace returns the beam to top. [See Fig. 23.20 (a), (b) and (c)].

Chapter 23 Television Basics and Monochrome Television

Vertical Trace

Vertical retrace

402

Fig. 23.20 (a) Top

Trace period

Raster

Bottom Retrace period

Fig. 23.20 (b)

l

ce

ce

a Tr

ace

tra

Tr

tr Re

Re

e ac

t

Top Bottom

Trace period

1st line

2nd line

Fig. 23.20 )c)

Retrace Period

23.7 Scanning

403

(1) Scanning Frequencies Both trace and retrace are included in one cycle of the saw tooth wave. Since number of complete horizontal lines scanned in 1 second are 15,750 for horizontal deflection, the frequency of the saw tooth waves is 15750 cycles per second. For vertical scanning, the frequency of saw tooth wave is 60 cycles per second. As vertical scanning frequency is much lower than of the horizontal scanning frequency, many horizontal lines are scanned during one cycle of vertical scanning.

(2) Retrace Time in Scanning During retrace, the beam comes to its original point of start for the next horizontal or vertical line, therefore, this is a waste time, hence retrace time is kept as short as possible. For horizontal scanning, the retrace time is 10 per cent of the total time period of 63.5 mS for a complete line, its 10 per cent i.e., 6.35 mS is the time for retrace or flyback. The lower frequency vertical saw tooth waves usually have a retrace (flyback)

time about 5% of one complete cycle, which comes to as 500 mS. It is note worthy that 500 mS is much more than 63.5 mS. Actually 500 mS includes approximately 8 lines.

(3) Number of Scanning Lines How many scanning lines should be there in a picture to produce it effectively depends upon many factors. Greater the number of lines into which the picture is divided, better will be the resolution. It also very much depends upon the resolving capacity of human eye. There are other factors also which decide the total number of lines. For an ideal case the picture should be divided into 800 lines but above 500, the improvement is not significant. Morever with more number of scanning lines, the bandwidth also increases and this adds to the cost of the system. As a compromise between cost of the system and the quality of the picture, 625 lines have been fixed in India whereas in America this figure is 525 lines.

(4) Scanning Period The horizontal as well as vertical scanning currents are shown in the Fig. 23.21 (a) and (b) respectively. The nominal time for horizontal line = 106/l 5625 = 64 mS out of which trace period is 52 mS and retrace period is 12 mS which is also called as blanking period. Similarly nominal duration for vertical trace is 20 ms (1/50 = 20 ms). Out of this, 18.72 ms is spent to bring the electron beam from top to bottom and remaining 1.28 ms is spent by the beam to return back.

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As horizontal and vertical sweep oscillators operate continuously, 1280 mS/64 mS = 20 lines are traced during each vertical trace interval. In this way, a total = 20 × 2 = 40 lines are lost per frame; as blank lines during the retrace of the two fields. In this way, the lines actually scanned per frame left = 625 – 40 = 585 lines.

f = 15625 Cy/Sec

52 mS

ce tra Re

ce

a Tr

12 mS

52 mS

12 mS

Trace Period

t

Retrace period

(a) Horizontal scanning currents l

ce

tra

Re

ce Tra

18.72 ms

1.28 ms

Trace period

Retrace period

t

(b) Vertical scanning currents Fig. 23.21

(5) Flicker in Scanning Though scanning at the rate of 25 frames (pictures) per second produces illusion of continuity in TV system but when the screen is made alternately bright and dark in between two successive frames, it causes a flicker of light, which is very annoying to the viewer. To reduce the flicker, an improved method of scanning known as interlaced scanning is used.

23.7 Scanning

405

(6) Interlaced Scanning In this scanning alternate lines are scanned, e.g., all the odd lines from top to bottom of the frame are scanned first, leaving out the even lines. After this, a rapid vertical retrace moves the electron beam back to top of the frame and all the even lines on the frame which were omitted in the previous scanning are scanned from top to bottom. This reduces flicker since the area of the screen is covered at a double rate. This is similar to reading alternate lines of the page of a book from top to bottom once and then going back to the top to read the remaining lines down the bottom. Beginning of 1st field

Beginning of 2nd field

1

313

2

314

3

315

311 623

312

624

313

625 End of 1st field

Fig. 23.22

This is illustrated in the Fig. 23.22: In this Fig., (i) Vertical retrace period has been assumed as zero. (ii) Retrace lines not shown. In a 625 lines monochrome TV system, the lines of each picture (or frame) are divided into two sets of 625/2 = 312.5 lines, each set is called a Field. The each field is scanned alternately to cover the entire picture area. For this purpose, a horizontal sweep oscillator is used at a frequency of 15,625 cycles per second (312.5 × 50 = 15625) to scan the same number of lines per frame (15625/25 = 625) but a vertical sweep oscillator is made to work at a frequency of 50 Hz. Since the electron beam is now deflected from top to bottom in half the time and the horizontal oscillator is working at 15625 Hz, only half of the total lines i.e., 312.5 are scanned during each vertical sweep. The first field ends in half line, the second field starts from middle of the line on the top of the screen, the electron scanning beam is able to scan remaining 312.5 lines during its downward journey. In all, the electron beam scans total 625 lines at a rate of 15625 lines per second. Thus we are able to reduce the flicker effect without increasing the speed of scanning and thus no need to increase band width or cost of the system.

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23.8 SYNCHRONIZING PULSES At the transmission end, camera scans the picture and the same process is carried out at the receiving end by picture tube. At the receiver the picture tube should reassemble the picture elements on each horizontal line with the same left-right position as the image at the camera. As the beam scans the successive lines in the picture tube vertically, the screen should show the same picture elements in the corresponding lines as at the camera. Therefore, a horizontal synchronizing pulse is transmitted for each horizontal line to keep the horizontal scanning synchronized and a vertical synchronizing pulse is transmitted for each field to synchronize the vertical scanning. Accordingly the horizontal synchronizing pulses should have a frequency of 15750 cy/s and vertical synchronizing pulses a frequency of 60 cy/s. The term “sync.” is generally used as abbreviation for “synchronizing”. The sync. pulses are transmitted as a part of the picture signal but are sent only during the blanking period when signal is to be transmitted. The Fig. 23.23 shows the waveform of the sync. pulses for the purpose of synchronization. All pulses have the same amplitude but differ in pulse width or waveform. The sync. pulses include—3 horizontal pulses, 6 equalizing pulses, 6 additional equalising pulses and then 3 horizontal pulses. There are many additional horizontal pulses till equalising pulses occur again for beginning of the next field. V

3

6

6

3

t

Fig. 23.23

The sync. pulses do not produce scanning. For scanning, saw tooth pulses must be produced as explained earlier. However, the sync. pulses hold the picture on the screen in still position. If horizontal sync. is not provided, the picture drifts to the left or right on the screen and the picture is torn into diagonal segments. If vertical sync. is not provided, the picture appears rolling up and down on the screen.

23.9 Blanking Pulses

407

23.9 BLANKING PULSES In TV, “blanking” means “going to black”, as part of the video signal, the blanking voltage is at the black level. Video voltage at the black level cuts off the beam currents in the picture tube to black out the light from screen. The purpose of providing the “blanking pulses” is to make invisible the retraces of the scanning process. The horizontal blanking pulse at a frequency of 15750 Hz, blanks out the retrace from right to left for each line. The vertical blanking pulses at 60 Hz blank out the retrace from bottom to top for each field. The time period of blanking pulses is 16% of the each horizontal line i.e., = 16 per cent of 63.5 mS = 10.2 mS. In other words, retrace from right to left must be completed in 10.2 mS. The time period of vertical blank pulses is 8 per cent of each vertical field. It comes equal to 8 per cent of 1/16 S = 0.0013 S. In other words, the vertical retrace must be completed within 0.0013 S. A blanking pulse comes first to put the video signal at black level, then a sync. pulse comes to start the retrace. This sequence applies to blanking, horizontal and vertical retraces. Table 23.3 : Frequencies of Scanning, Synchronising and Blanking Pulses Particulars 1. Horizontal scanning pulses 2. Vertical scanning pulses 3. Horizontal sync. pulses 4. Vertical sync. pulses 5. Horizontal blanking pulses 6. Vertical blanking pulses

Frequency (Hz) 15,750 60 15,750 60 15,750 60

23.10 THE TV STANDARDS We will discuss here about American and Indian standards: (a) American standard: [Fig. 23.24 (a)] The band of frequencies assigned to a station for transmission of the signal is called a channel. Each TV station has a 6 MHz channel within specific bands for commercial broadcasting: (i) Video modulation: The 6 MHz bandwidth is needed for picture carrier signal. The carrier is amplitude modulated by the video signal.

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(ii) Chrominance modulation: For colour broadcasting 3.58 MHz chrominance signal has the colour information. The colour signal (C signal) is combined with luminance signal (Y signal) to form one video signal, that modulates picture carrier wave for the transmission. (iii) Sound signal: In 6 MHz channel, sound carrier signal for the picture is also included. The sound carrier is a “frequency modulated” signal by audio frequencies between 50 Hz to 15 kHz. (iv) Carrier frequencies: The Fig. 23.24 (a) shows how different carrier signals fit into the standard 6 MHz channel. The picture carrier frequency is always 1.25 MHz above lower end of the channel. At opposite end, the sound carrier frequency is 0.25 MHz below the higher end. Picture (P)

Colour Sound (C) (S) 4.5 MHz

0.25 MHz

3.58 MHz

1.25 MHz

6 MHz channel

(a) Broadcasting channel (American standards) P

C

S

5.5 MHz 4.43 MHz

1.25 MHz

0.25 MHz

7 MHz Channel

(b) Broadcasting channel (Indian standards) Fig. 23.24

(b) Indian standards: The total channel width used in India is 7 MHz. The spacing between picture and sound is 5.5 MHz and between picture and colour signal is 4.43 MHz See Fig. 23.24 (b). Table 23.4: Popular TV standards in the World S. No.

Particulars

India, Europe and Asian countries

America, Canada, Mexico and Japan

England

USSR

France

1.

Lines per frame

625

525

625

625

625

2.

Frame per second

25

30

25

25

25

23.11 Composite Video Signal

409

India, Europe and Asian countries

America, Canada, Mexico and Japan

S. No.

Particulars

.5.

Field frequency (Hz)

50 Hz

60

50

50

50

4.

Line frequency (Hz)

15,625

15,750

15,625

15,625

15,625

5.

Channel BW (MHz)

7

6

8

8

8

6.

Picture signal modulation

AM

AM

AM

AM

AM

7.

Sound signal modulation

FM

FM

FM

FM

FM

England

USSR

France

23.11 COMPOSITE VIDEO SIGNAL The video signal used in video system is not simple but it is a complex signal. It is a composite of the following signals: (i) Camera signal—corresponding to the picture information. (ii) Blanking signal—during retrace period to make it invisible. (iii) Synchronizing signal—to synchronise the two scannings occurring in transmitter and receiver. How this signal is formed, has been shown in Fig. 23.25 Blanking signal Camera signal

(a) Camera signal

(b) Camera + blanking signal Sync. signal

Blanking signal

Camera signal

(c) Camera + blanking + sync. signal = Composite video signal Fig. 23.25

Video signal dimensions: The Fig. 23.26 shows detailed dimensions of the composite video signal. The level of the video signal corresponding to the

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maximum whiteness to be handled is known as peak white level. This is about 10 per cent of the maximum value of the signal, while the blank level is about 75 per cent. The sync. pulses are added at about 75 per cent level known as blank level. In actual practice, these two levels are very close and are almost merged with each other. v 100

64 mS Blanking pulses

52 mS

75

Sync. Pulses Blanking level

60

Camera signal (Picture)

D.C. level

Peak white level

10

t

0

Fig. 23.26

In addition to continuous variation in amplitude for the picture elements, the video signal has a d.c. component corresponding to the average brightness of the scene. In its absence, grey picture on a black background will be same as a white picture on a grey background. The video signal also contains blanking pulses which raises the signal amplitude slightly above the black level (75 per cent) so that the retrace is not visible. This signal contains horizontal as well as vertical blanking pulses. The frequency of horizontal blanking pulses is same as that of the horizontal scanning and frequency of the vertical blanking pulses is same as that of vertical scanning that is, 15625 Hz and 50 Hz respectively. The picture to sync. signal ratio (P/S) is kept as 13:5 i.e., 65 per cent of the maximum carrier amplitude is occupied by video signal and about 25 per cent by sync. pulses. This gives best results in reducing the noise level.

23.12 TV Signal Transmission

411

23.12 TV SIGNAL TRANSMISSION In most of the TV transmission systems, the picture signal is amplitude modulated and the sound signal is frequency modulated Need for modulation has already been described. Just to remind, to transmit a signal directly (without modulation) a very very long antenna is required which is impractical. Further picture and sound signals from different stations are concentrated within the same range of frequencies, therefore, both will be mixed up (if they are unmodulated) and it will be difficult to separate them in the receiver. Thus in order to make separation of “intelligence” (signal) from the different stations, it is necessary to keep them at different portions of the spectrum depending upon the carrier frequency assigned to each station. (a) Amplitude Modulation of Video Signal (Fig. 23.27): Modulation of video signals is possible only by amplitude modulation because of their large bandwidth. The modulation can be of low level or high level. The high level amplitude modulation is done at final (power) stage. The intermediate frequency modulation is a low level modulation in which modulation of video signal is carried out at 38.9 MHz. The modulated IF is then changed to channel frequency by heterodyning. Video input (5 MHz)

Modulated video signal

LSB filter

Balanced modulator

Oscillator 40 MHz

USB = 40 MHz LSB = 35 MHz Mixer

Oscillator 102 MHz

Fig. 23.27

To reduce the BW, the upper side band is transmitted fully and lower side band is transmitted partially. This is called vestigial side band (VSB) transmission. The LSB filter is used to allow only the desired frequency range. (b) Frequency Modulation of Sound Signal: The output of all microphones terminate on the sound panel in the control room. Each microphone output is amplified before being fed to mixers. Pre-emphasis and de-emphasis are done to improve the quality of the sound. The modulated audio signal is transferred to the assigned channel sound carrier frequency by the use of multipliers.

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Preference of Modulation for Transmission (a) Preference of AM over FM for picture transmission: The distortion which is produced due to interference between signals is more objectionable in FM then it is in AM, because frequency of the FM signal changes continuously. If FM is used for picture, it will produce the shimmering effect and the picture will not be steady. Further, the BW requirement in AM is lesser as compared to FM and also in FM, complexity of the circuitry increases. This is the reason, that for picture, AM is preferred. (b) Preference of FM over AM for Sound Transmission: Because of crowding in medium and short wave bands in radio transmission, only 5 kHz frequency is used for signal as maximum. This is done to limit the BW to 10 kHz as a maximum. This enables to accommodate large number of radio stations. Further FM provides almost noise free and high fidelity output. The maximum apace it needs is 200 kHz on each side of the picture carrier. As 7 MHz is allotted to the BW, it has no problem to accommodate the sound signal. Hence FM is preferred for sound transmission as, (i) It reduces noise. (ii) It increases, efficiency of transmission. (iii) There is no inter-channel or inter-signal interference.

23.13 TV TRANSMISSION TECHNIQUES Here we discuss three techniques used for transmitting TV signals: DSB transmission, SSB transmission and VSB transmission (a) Double Side Band (DSB) Transmission: In a 625 line TV transmission in which frequency components ranging from 0 to 5 MHz are present, a double side band AM transmission will occupy a total BW of about 10 MHz. In addition, a slope of 0.5 MHz is to be added on both sides. Thus the total BW becomes 11 MHz. Further, each TV channel has an FM sound signal which is situated just outside the upper limit of the picture signal. A 0.25 MHz guard band is also added to this signal, which makes the BW equal to 11.25 MHz. (See Fig. 23.28) Thus in a double side band (DSB) transmission, the BW is very large and would limit the number of channels for a particular station. In order to reduce this BW, single side band transmission is preferred.

23.13 TV Transmission Techniques

413

Total BW = 11.25 MHz

Amplitude

P

Sound carrier

Guard band 0.25 MHz

USB 5.5 MHz

LSB 5.5 MHz

5.5 5

Picture carrier

5 5.5 5.75 Frequency

0

Fig. 23.28

(b) Single Side Band (SSB) Transmission: As discussed earlier, the carrier does not contain signal. The signal is contained in side bands only. But the transmission of carrier along with the side bands is necessary to make transmission and reception simple and inexpensive. However, the two side bands are of the same amplitude, therefore, transmission of only one side band (along with the carrier) is sufficient to convey the total information and it saves a BW of 5 MHz per channel. No doubt magnitude of the detected signal in the receiver will be just half as only one side band is transmitted but it can be amplified by increasing the number of stages of the amplifiers in the receiver to the required value to compensate for the loss. But it will give great advantage as it is going to save a BW of 5 MHz, which in turn will increase the number of channels in a station. Generally lower side band is filtered out and only upper side band is transmitted. This technique has been explained earlier. But this is also not used and VSB transmission is rather preferred. (c) Vestigial Side Band (VSB) Transmission: Practically it is not possible to filter out LSB (Lower Side Band) completely as it has been seen that the lower frequencies contain the most important information of the picture and filtering out the LSB as a whole gives rise to distortions. Therefore, as a compromise full USB (Upper Side Band) and some part (Vestigial part) of the LSB (frequencies up 0.75 MHz) are transmitted. This is known as VSB transmission which is .practically done in 625 lines TV system. Now the composition of the SSB bandwidth comes to be as 7 MHz as shown (Fig. 23.29).

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Chapter 23 Television Basics and Monochrome Television Sound carrier

Amplitude →

Picture carrier

Guard Total BW = 7 MHz

Full USB = 5.0 MHz

5.5 5.75

Slope = 0.5 MHz

5

MHz

Guard 0.25 MHz

0

Slope = 0.5 MHz LSB = 0.75 MHz (Vestige)

1.25 0.75

Fig. 23.29

23.14 TYPES OF TV RECEIVERS Monochrome as well as colour TV receivers may be of the following 3 types: (i) All tube receivers: The type mainly applies to monochrome and few old colour TV receivers. All the functions are carried out by electron tubes. A colour receiver may employ more number of tubes as compared to monochrome receiver. (ii) Solid state receivers: In this type, all the stages use solid state devices, except the picture tube (P.T.). The devices include semi conductor diodes, transistors and ICs etc. (iii) Hybrid receivers: This is a combination of the above two types. The deflection circuits use power tubes, while the signal circuits use solid state devices. (iv) LCD/LED receivers: Recently, these have captured the market, the picture tube has been replaced by LCD/LED respectively. All solid state devices are used. In fact, a TV receiver is a combination of AM receiver (for the picture signal) and FM receiver for sound signal. In addition, it also contains other circuits for scanning, synchronising etc. Here we shall discuss monochrome TV circuitary as all the monochrome circuitary is needed in a colour receiver. The colour TV receiver is just a monochrome receiver plus colours.

23.15 Monochrome TV Receiver

415

23.15 MONOCHROME TV RECEIVER The important sections/stages of a monochrome TV receivers are discussed as under: (See Fig. 23.30) (i) Antenna: The picture and sound signals are intercepted by antenna of the receiver. A wire connects the antenna to the input of the receiver. Twin lead is generally used. There are various antennas used but the most popular is “yagi-uda” which gives good output in fringe areas. If the signal is weak a booster can be used and, if signal is strong or if the transmitting station is near, either no antenna is required or a telescopic indoor antenna may be sufficient. The twin lead from antenna to the receiver has an approximate impedance of 300 ohm. A folded dipole has also the same impedance at its resonant frequency, however, both provide a good impedance matching. (ii) VHF Tuner: The antenna input provides (composite video + sound) signals for the r.f. amplifier stage. The amplified output of amplifier is given to the mixer. Also output of a local oscillator is given to mixer to heterodyne with signals. When the oscillator frequency is set for the particular channel to be tuned, the signals are converted into intermediate frequency. Sound signal Antenna

Sound I.F. amp.

Composite Video signal

RF amp.

Mixer FM sound signal

I.F.

Sound detector

AF amp.

L.S.

Sound signal

Picture signal

Video detector

Picture I.F. amp. AGC

Local osc.

60 Hz Sync. separator

VHF tuner UHF tuner Diode mixer

Vertical deflection osc.

Y Sync. UHF osc.

P.T.

Hor. AFC Horz. sync.

Vert. deflection amp.

Hor. deflection osc.

15.750 Hz Hor. deflection amp. HV rect.

Fig. 23.30

There are two I.F. outputs from the mixer—one for the picture and other for the sound. The standard value for I.F. in a 625 line TV system is 38.9 MHz for picture and 33.4 MHz for sound.

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Chapter 23 Television Basics and Monochrome Television

The tuner selects the channel to be received by converting its picture and sound R.F. carrier frequencies into intermediate frequencies which can be amplified in I.F. amplifiers. The station selector is a gang switch that changes value of capacitor of the tuned circuits of the R.F. amplifier, mixer and local oscillator simultaneously. (See Fig. 23.31) Receiving antenna Picture + Sound (signal)

For picture 38.9 MHz RF amp.

Mixer

I.F. For sound 33.4 MHz

Local osc. Gang switch (channel selector)

Fig. 23.31

(iii) UHF Tuner: When channel selector is set to UHF tuner position, the antenna input is obtained from a separate UHF antenna. This heterodynes the UHF input to the intermediate frequencies. These frequencies are amplified by required number of amplifier stages. (iv) I.F. Amplifiers: These amplify output of the mixer in number of stages to get sufficient voltage for the video detector. The gain of the I.F. amplifier is controlled by an AGC. (v) Video detector: The modulated I.F. picture signal is rectified and filtered in the detector to detect (recover) the amplitude modulated picture signal which is required for driving the picture tube. (vi) Sync. separator: A sync. separator is basically a clipper circuit that separates the sync. pulses from the camera signal contained in the composite video signal. Since there are sync. pulses for horizontal as well as vertical scannings, output of the sync. separator is divided into two parts. (vii) Deflection circuits: These include the vertical and horizontal oscillators for vertical and horizontal scannings. The deflection circuits produce the required scanning currents. The deflection oscillators basically are A-stable multivibrators, which do not need any external triggering pulse for operation. (viii) L.T. (Low Tension) supply: This is needed for tubes and transistors. The dc output upto 280 V is needed to vaccum tube amplifers. A dc supply upto 90 V is needed for rectifier and other circuits. The heater of the vaccum tubes may need dc supply of 6.3 V.

23.15 Monochrome TV Receiver

417

(ix) EHT (Extra High Tension) supply: The high voltage supply needed to rectifiers is given from horizontal amplifiers. The H.V. supply is also needed to picture tube for its suitable operation. Approximate value of EHT supply for anode of the picture tube (PT) is 15–18 kV. This is needed for sufficient brightness. A voltage of 6 to 9 kV is developed across primary winding of horizontal output transformer. This is stepped up by an auto transformer to 15–18 kV (See Fig. 23.32). H.V. rectifier

H.V. winding

From horiz. osc.

EHT 15–18 KV

Horizontal amp.

Auto transformer

Fig. 23.32

(x) AGC: It stands for “Automatic Gain Control”: It helps in getting constant amplitude of the video signal for different carriers. For the picture signal, AGC provides an automatic control of the gain in the reproduced picture. An AGC circuit has been shown in Fig. 23.33.

I.F. amp.

Composite video signal

AGC R.F. amp.

Delay AGC

Fig. 23.33

(xi) Video amplifier: Before giving composite video signal to picture tube, it is amplified in video amplifier sufficiently. It may have number of stages as per requirement. The amount of video signal required for the picture tube is about 100 V for strong contrast. More strong video signal means more contrast.

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The blanking pulses in composite video signal drive the grid voltage of the picture tube to cut off, blanking out the retraces. The function of sync. pulses is to drive the grid more negative than cut off. The Fig. 23.34 shows last stage of the TV receiver. Amplified video signal

Amplified I.F. signal Video amp.

PT

Video det.

EHT to anode Brightness control

Fig. 23.34

SUMMARY 1. The “Television” means to see at a distance. 2. The few TV applications are: cable TV, close circuit TV, Picture phone, TV games etc. 3. The TV camera converts picture into electrical signal. The popular black and white TV camera tubes are: Orthicon, Vidicon and Plumbicon. 4. The B and W cameras are used in different configurations. 5. The picture tube converts the electrical signal into picture signal. It may be a monochrome picture tube or a colour picture lube. 6. The picture is “scanned” by a camera before converting it into electrical signal. Usually “Interlaced Scanning” is used; in which the picture is divided into 625 lines and is then scanned line by line. 7. The Vestigial side band (VSB) transmission is generally used for TV signals. In this technique, full upper side band and some part of lower side band (called Vestigial) are transmitted. 8. Few blocks in monochrome and colour TV are common. 9. In a TV, sound is given to microphone which converts it into electric waves, these waves are processed. The picture is given to a camera, which is also converted into electrical waves and processed. The (sound + picture) signal is transmitted into space through the same antenna.

Summary

419

At the receiver, sound and picture both are detected separately. Sound after processing, is given to loudspeaker, which converts it into the original sound and the picture signal after processing is given to the picture tube, which converts it into original picture. 10. The important TV processes are: scanning, synchronization and blanking. 11. The picture signal alongwith blanking and synchronizing signals is called “composite video signal”. 22. In TV, the sound signal is frequency modulated and the picture signal is amplitude modulated. 13. The TV receiver may be monochrome type (Black and white) or colour TV, solid state TV, LCD/LED TV, hybrid TV, etc. qqq

24 Colour Televisions

The colour television produces transmitted information in the original coloured form. All stages described in the monochrome TV are present in a colour TV. In addition few stages, which are required for treatment of colours are also included.

24.1 COLOUR TELEVISION A colour picture is nothing but “a monochrome picture on a white raster with colours added”. The required information is in chrominance (or colour or C signal) broadcasted along with monochrome (B&W) signal. If we turn down the colour control on the receiver to eliminate the colour signal, what we get will be a black & white picture. With C signal; the picture is reproduced in natural colours. All colours can be produced as a combination of red, green and blue colours. These three are known as Primary Colours.

24.2 PRIMARY, SECONDARY AND COMPLEMENTARY COLOURS (a) The colours which cannot be produced by mixing other colours are called primary colours., Red, Green and Blue are called primary colours. Sometimes Yellow is also taken as primary colour. The primary colours are of two types: (i) Additive Primary Colours: The red, green and blue are called Additive Primaries and are used when coloured light sources are blended to produce the required colour.

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(ii) Subtractive Primary Colours: The red, blue and yellow are called Subtractive primaries and are used when a picture on print is viewed by reflected light from a white source. (b) The colours, which can be produced or obtained by mixing primary colours are called secondary colours. (c) The two colours, which on mixing give white colour are called complementary colours (to each other), e.g. yellow is a complementary to blue, magenta is complementary to green and cyan is complementary to red.

24.3 ADDITIVE AND SUBTRACTIVE MIXING OF COLOURS The mixing of different colours to get new colour is known as additive mixing

of colours.

The Fig 24.1 shows additive mixing of colours:

11% Blue

70% Cyan

41% Magenta 100% White

59% Green

89% Yellow

30% Red

Fig. 24.1

e.g., Red + Green = Yellow

R+G=Y

Red + Blue = Magenta R + B = M Red + Green + Blue = White

R+G+B=W

When we mix pigments, this is known as subtractive mixing of colours. e.g. the additive mixing of red, green and blue gives white colour, while mixing them subtractively gives black colour. The Fig. 24.2 shows subtractive mixing of colours e.g., White – Green – Blue = Red (W – G – B = R) and White – Green = Magenta (W – E = M) etc.

24.4 Types of Colour Video Signals

423

White

– Green

= Magenta

White – Green – Blue = Red White – Blue

White – Blue = Yellow

White – Green – Red

= Blue

– Red –

Green

= Black

White – Red

= Cyan

White – Red – Blue = Green

Fig. 24.2

In colour TV system red, green and blue are used as Primary colours, By mixing these colours in required proportion any colour can be obtained.

24.4 TYPES OF COLOUR VIDEO SIGNALS The important types of colour video signals are three: Red, Green and Blue (R, G, and B), as a TV system starts with R, G, B at the camera and finishes at R, G, B in picture tube. However, colour mixtures are used for coding and decoding, because mixing of two colours can have all the colour (Chrominance) information of the three colours, allowing the third signal to be Y (luminance) signal. The important colour video signals and their combination is given below. 1. I Signal = 0.60 R – 0.28 G – 0.32 B. It is a combination of 60 percent red, 28 percent green and 32 percent blue video signals. The minus sign indicates the addition of video voltages of negative polarity e.g., – 0.32 B means 32 per cent of the total blue video signal, but with inverted polarity, which reproduces blue. Similarly other colour signals are, 2. Q signal = 0.21 R – 0.52 G + 0.31 B. 3. B – Y signal = –0.30 R – 0.59 G + 0.89 B.

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Colour Television

4. R – Y signal = 0.70 R – 0.59 G – 0.11 B. 5. G – Y signal = –0.30 R – 41 G – 0.11 B.

24.5 CHROMINANCE AND LUMINANCE SIGNALS (i) Chrominance or Colour Signal: It is denoted by C. The colour TV system starts and ends with primary colours i.e., red, green and blue colour signals. A colour camera had different camera tubes for red, green and blue colour. The screen of the colour picture tube has red, green and blue colour phosphorus. All the three camera tubes are used to produce the colour signal. Now the colour signals are not directly transmitted or received. At transmitter, light of different colours is converted to different video signal voltages. (ii) Luminance (White) Signal: It is denoted by ‘Y’. Luminance (Y) is the amount of light intensity perceived by the eye as brightness. Different colours have shades of luminance. Luminance signal contains the brightness variations for all the colour information in the picture. In colour televisions, it is obtained by mixing three colours red, green and blue in proportions of 0.3%, 0.59% and 0.11%. Thus, Y = 0.30 R + 0.59 G + 0.11 B The above proportions of colours are selected depending upon the sensitivity of eyes to these colours.

24.6 IMPORTANT TERMS Any colour has 3 characteristics: First is hue or tint, which we generally call the “colour”, the second is saturation. The saturation describes the concentration of the colour. The third is luminance, which indicates brightness or the “shade”. The important terms are described below: (i) Hue (or Tint): The colour itself is known as its hue e.g., green leaves has a green hue and a red flower has a red hue. (ii) Saturation: Saturated colours are deep and strong. Weak colours have no saturation. By saturation we mean how a colour can be diluted by white. When a saturated (strong) red colour is mixed with white, we get pink colour, which can be called as “desaturated” red. (iii) Luminance: By luminance, we mean amount of light intensity or brightness sensed by our eyes. In B & W picture, the lighter parts are

24.7 Visibility Curve

425

more luminous than its darker parts. Some colours appear brighter than others. The luminance indicates how a particular colour will look in black and white reproduction. For example, a monochrome picture will show yellow colour as white, light blue colour as grey and dark red colour as black. Luminance signal is written as Y-signal. (iv) White: Actually white light can be considered as a mixture of red, green and blue (primary colours) in right proportions. A white colour for colour TV is a mixture of 30 per cent red, 59 per cent green and 10 per cent blue. The percentage of the luminance signal is based on sensitivity of the eyes for different colours. (v) Brightness: Each colour produces a certain amount of brightness. It is determined by the amount of light contained in it. “Brightness” is different from “Saturation”, one can be changed keeping the other constant. The colour TV has two different controls: Saturation and Brightness. When saturation control is operated, amount of white light contained in the colour is changed, when the Brightness control is operated white as well as coloured lights are changed. So brightness is the measure of white and colour lights, whereas saturation is the measure of white light only. (vi) Chrominance: The term is used to indicate hue as well as saturation of a colour. The chrominance signal includes all the colour information except the brightness. Chrominance and luminance together give complete information about the picture. Chrominance is abbreviated as chroma and is also written as “C-signal” . As colour TV is concerned, chrominance modulates 3.58 MHz sub carrier before modulation, and after demodulation it is the colour information obtained in red, green and blue signals.

24.7 VISIBILITY CURVE All types of radiations do not produce sensation of light on human eye. The radiations between wave lengths of 4000 Å and 7000 Å (Angstrom, 1 Å = 10–10 m) only produce sensation. The human eye is the most sensitive to radiations of 5500 Å, though it differs from man to man and from age to age. The curve between sensitivity of human eye and the wave length is shown in Fig. 24.3 (a).

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The colour corresponding to 5500 Å is yellowish green which is the most suitable colour tor all purposes.

Sensitivity of human eye (% age)

100% 60% 40% 20% Wavelength (Å) 4000 Å

5500 Å

7000 Å

Fig. 24.3 (a)

The retina of the human eye consists of large number of minute cells called rods and cones, each of which is connected to the brain by optic nerve. These cells are sensitive to the quantity and colour of light falling on them and transmit the colour information to the brain. The ‘cones’ and ‘rods’ are light sensing cells. ‘Rods’ help us to know about ‘light’ and ‘darkness’ while the ‘cones’ give an idea of colours. The ‘cones’ have special attraction for red and yellow wave lengths. This is why we like these colours.

Sensitivity of human eye

The retina has 3 types of cones. The action of light on one kind of cones produces sensation of red colour (R), on a second kind, the sensation of green colour (G) and on third sensation of Blue colour (B). This has been shown in Fig. 24.3 (b), which shows sensitivity of human eye as a function of wave length of light for the three kinds of cones. Note that the wavelength is in mm. 80%

B

60% 40%

G

R

20%

0

0.42

0.46 0.5

0.54 0.58 0.62 0.66

Fig. 24.3 (b)

l(mm)

24.8 Sub Carrier and Multiplexing

427

24.8 SUB CARRIER AND MULTIPLEXING 1. Sub Carrier: This is a carrier which modulates other high frequency carriers. In colour receiver the colour information modulates 3.58 MHz sub carrier. 2. Multiplexing: In this, one carrier is used to modulate two or more signals. In colour TV, 3.58 MHz C-signal is multiplexed with Y signal and then both modulate the main picture carrier.

24.9 POPULAR TV SYSTEMS The popular TV systems are: (a) NTSC system

(b) SECAM system

(c) PAL system

These are discussed below one by one.

(a) NTSC System It stands for “National Television System Committee system”. In this system, the two colour difference signals created by subtraction of red and blue from the total signal (R – Y, B – Y)are transmitted in quadrature (with one quarter cycle behind). The signals are then added together to get the chrominance signal.

R–Y

l

33° 57°

–(B – Y)

Q 57° 33°

B–Y

–Q –(R – Y)

–I

Fig. 24.4

The Fig. 24.4 shows phase relationship in NTSC transmition system.

• Transmitter The Fig. 24.5 shows NTSC transmitter.

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R

M

Colour Television

Y

A G

T

R–Y

0.05 MHz

Balanced modulator

B–Y

1.5 MHz

Balanced modulator

R B

I X

Mixer

Composite video signal

Filters 90° phase shifter

57° phase shift Subcarrier generator Sync. generator

Burst Signal

Sync. and Blanking

Sync.

Fig. 24.5

The three primary signals generated by three electron guns (or pick up tubes) (R, G, B) are amplified and applied to a Matrix where these are combined algebraically to produce one luminance signal (Y) and two chrominance signals (R–Y, B–Y). The chrominance spectras are clipped by Filters. The chrominance signals are modulated on the subcarrier. It accepts all the signals that make up a composite video signal. The sub carrier generator and the sync. generator must be linked in a suitable manner; so that the sub carrier may be an odd harmonic of half the frame (line) scanning frequency. A colour subcarrier burst signal is inserted in the composite video signal during the line blanking interval. The frequency and phase of the burst should be equal to the chrominance subcarrier at the transmitter.

Receiver The Fig. 24.6 shows NTSC receiver. After amplification and detection of R.F. and I.F. signals, the composite signal is passed through filters, demodulators, amplifiers and matrix and then, it is given to the picture tube.

24.9 Popular TV Systems

429 Y Amplifier

B–Y demodulator

M A T

5 MHz filter Composite video signal

Amplifier

Filter

90° Shift

R

R–Y demodulator

Filter

Amplifier

To picture tube

Filter

I X

Burst Subcarrier oscillator Sync. separator

Fig. 24.6

Limitations of NTSC System: (i) The difference of phase between the sub carrier and local oscillator produces incorrect hues. (ii) The crosstalk between demodulator outputs at the receiver causes colour distortions.

(b) The SECAM (Sequential) System It stands for “Sequential Colour a memory’ system The system has a different mode of transmission. The colour difference signals are not arranged a quarter of cycle apart but are kept separate by transmitting them on alternate lines of the picture. Delay lines inside the receiver hold up one set of signals so that they can be recombined to form a picture from alternate lines of the signal. The system avoids the problem of phase error. In SECAM, the two chrominance signals (R-Y and B-Y) are transmitted on alternate lines in sequence (See Fig. 24.7). Though half the colour information is lost but, the human eye cannot distinguish this. Video line input Signal

R–Y

B–Y

R–Y

B–Y

R–Y

B–Y

B–Y

R–Y

B–Y

R–Y

B–Y

R–Y

Delay line Signal Video line output

Fig. 24.7

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The delay time is exactly at one line scan interval, which is about 64 ms. For first time, the signal is taken from input (Video line) to the delay line and second time from output of the delay line.

• Transmitter The Fig. 24.8 shows block diagram of SECAM Transmitter. The colour difference signals (R-Y and B-Y) are band limited to 1.5 MHz by filters. The two signals are applied to FM modulator alternately by using an electronic switch. The switch operates at line frequencies. The colour sub carrier is, therefore, modulated alternately. The channel B.W. is 8 MHz.

Cameras

R

G

B

R amplifier

M

G amplifier

T

B amplifier

Y

A

Electronic switch

R–Y

R I

F.M. Subcarrier modulator

B–Y

X

Composite video signal

Mixer

Fig. 24.8

To obtain the composite signal, the modulated subcarrier is mixed with luminance, Sync. and blanking signals in a mixer.

• Receiver The Fig. 24.9 shows the SECAM receiver. In the receiver, the colour difference signals existing at input and output from the delay line are routed by an electronic switch to the two inputs of a MATRIX which drives the third colour difference signal (G-Y). With the chrominance signal transmitted sequentially (on alternate line scans), the delay line enables the three chrominance signals to be generated in receiver at the same time. This is the reason, that the SECAM system is also called Sequential Simultaneous.

24.9 Popular TV Systems Y signal

Amplifier

(R – Y) and (B – Y)

Delay line 64 ms

To colour picture tube

Advantages of SECAM

R–Y detector

Electronic switch

Composite signal

431

B–Y detector

G–Y matrix

Fig. 24.9

(i) As FM is used, the SECAM receiver is free from phase distortion. (ii) There is no cross talk, as the signals are not present simultaneously. (iii) The SECAM receiver is simpler and cheaper than the NTSC and PAL receivers. Disadvantages of SECAM (i) The vertical resolution of SECAM system is inferior. (ii) The colour is more saturated during fade to black. During fade, the pink colour changes to red.

(c) PAL System PAL stands for “Phase Alternate Line’’. This system is adopted in India. It is an improved NTSC system and stands in between NTSC and SECAM systems. In this system, the signal is transmitted in the same way but difference lies in reception. In receiver, the information is delayed at every line e.g., if a certain line of the picture signal has a strong green signal, the receiver reverses the polarity of alternate lines, it assures that the next line contains very low green signal. The input sent to the picture tube is the average of the delayed line and the corrected line; thus eliminating the error. • Transmitter The Fig. 24.10 shows PAL transmitter. Camera tube converts light into video signals having three primary colours. These colour signals are converted into luminance signal Y and colour difference signals B-Y and R-Y. In this system, B-Y modulates the sub-carrier in phase and R-Y modulates the subcarrier with phase = +90° on one line and –90° on the next line and so on. So, the phase of the subcarrier changes are automatically corrected.

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The phase shifter stage uses an electronic switch to change the phase from + 90 degree on one line to – 90 degree on the next line. Blanking and sync. pulses

Video amp.

Power amp.

Ant.

Light

Video carrier Camera

Matrix

Mod

Sub carrier

Phase shifter

Gate

Adder

Mod VSB signal

Diplexer

Sound

MIC Amp.

FM Mod

Mod FM signal

Audio carrier

Fig. 24.10

The encoded signal is called chroma signal. The Y signal alongwith its control signal is added to the chroma signal by an adder. This colour video signal (CVS) modulates the main video carrier. Audio signal is converted into electrical signal by the microphone. After amplification is made, the audio carrier is frequency modulated, which is located within the 7 MHz channel width. Both modulated signals (video modulated signal and audio modulated signal) are sent to the transmitting antenna with the help of a diplexer circuit. The transmitting antenna transmits the signal into space.

• Receiver The Fig. 24.11 shows PAL receiver. Here, the receiving antenna picks up the signal and feeds it to the tuner stage. The tuner contains a low noise RF amplifier for amplification. The signal is converted into intermediate frequency signal by superheterodyning (superhet) process. Conversion of RF and IF results in better selectivity, higher gain and better stability.

24.9 Popular TV Systems Sound

433 Sound section

Receiving ant.

L.S. Video detector

Scanning ckts

Picture tube

Video amplifier

Luminance amplifier

Tuner (superhet)

I.F. amplifier

Colour Burst

Band pass amplifier

Sub-carrier generator

Chroma

Colour decoder

R.G.B signal Colour amplifier

Matrix

Colour difference signals

Fig. 24.11

The IF signal is amplified by three or four stage of amplification and then goes to video detector.

In the output of video detector, we get:

I. Intercarrier frequency signal (5.5 MHz) II. Chroma signal III. Colour burst (CB) signal IV. Luminance (Y) signal V. Sync. pulses. Now sound IF is amplified and is fed to the FM detector to recover audio signal. After amplification it is fed to the loudspeaker which converts it into sound signal. Chroma signal is decoded to retrieve original colour difference signals. The decoding is done by synchronous demodulation process. Luminance (Y) signal is added to the colour difference signals to get R, G and B signals in the same proportions of amplitude as in the output of the camera tubes. R, G and B signals are fed to a colour picture tube which produces three beams of electrons. These beams are focussed on three vertical stripes of red, green and blue phosphors, respectively, through vertical slits. The closely spaced phosphors produce red, green and blue light intensity in the same proportions as contained in the original light and our eyes integrate these colours to give the resultant colour of the original scene. Deflection of the beams in horizontal and vertical directions is achieved by horizontal and vertical deflection coils.

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24.10 COLOUR TV CAMERA Basically, colour TV camera consists of (Fig. 24.12): • Diachromatic Mirrors • Trimming Filters • Camera Filters • Video pre-amplifiers • Resistive networks Colour Filters R Red camera Red light

Green light

VR R1 R2

G Green camera

Light from

the scene

Zoom lens package

Video pre-amplilfiers

VG R3

Blue light B Blue camera

Y signal voltage

R 30 K

VB

Diachromatic mirrors

Fig. 24.12

Light from the scene is processed by the lens system. Diachromatic mirrors are specially designed which allow a particular colour to pass through and reject all other colours. So these are also a type of colour filters. Diachromatic mirrors are designed to pass one wavelength and reject other wavelengths. So the light is filtered into three colours red, green and blue. These colours also pass through colour filters (R, G, B) which finely tune each colour and provide highly precise colours. Now these colour signals are converted into corresponding electrical signals by camera tubes. So in this way three colour video signals are generated. As the magnitude of video signals is less, so these are amplified by video pre-amplifiers. Now these amplified video signals are passed through resistive network. All the resistors chosen are of different values. These are chosen to produce 30% Red, 59% Green and 11% Blue colour signal.

24.11 Colour Picture Tubes

435

The output of colour TV camera is 0.30 VR + 0.59 VG + 0.11 VB. A colour TV camera is basically an arrangements of three black and white camera tubes. In case of three tube configuration, light falling on the target is split into three primary colours: R, G and B. (Fig. 24.13). Each tube now processes a particular colour.

Camera

R G B Camera Colour lens

The two tubes or single tube arrangement can also work.

Fig. 24.13

24.11 COLOUR PICTURE TUBES The coloured picture tube is provided in colour television. The screen of the colour TV has three i.e., red, green and blue phosphors. These are three primary colours and their combination can produce any desired colour. There are 3 electron beams to excite these colours. A metal plate called shadow mask is also provided which has holes to allow the beam to reach the screen and the electrons that do not have the required angle are blocked. It is important to mention that each beam excites its respective phosphors on the screen. The screen phosphors produce colours, and the electron beams have no colour of their own. If only red gun is operated the whole screen will be red similarly by operating green gun, the screen can be made all green and so on. In normal operation when the three guns are operated, they excite their respective phosphors. We get a picture on screen by superimposition of the three primary colours. For example, yellow colour is obtained by combination of red and green. White colour is obtained by proper combination of red, green and blue and black in the picture is obtained when all the three beams are off.

(1) Types of Colour Picture Tubes According to configuration of guns and the phosphors, three types of colour picture tubes are popular: 1. Delta gun tube (Delta tube) [Fig. 24.14 (a)] 2. Guns in Line tube (GIL tube) [(b)] 3. Single gun tube (S.G. tube) [(c)]

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Neck of the tube Guns

R B

G 120° (a) Delta

R R

G

G

B

B

(c ) S.G.

(b) G.I.L.

Fig. 24.14

The basic construction of all the above tubes is same, the difference lies only in their guns configuration. In delta tube, the guns are arranged in a triangular (delta) form, i.e. 120° apart, in GIL tube the 3 guns are arranged in a horizontal line and in single gun tube, as the name suggests, there is only one gun having 3 cathodes (R, G, B) in a line. The delta gun tube is widely used and is been discussed below:

(2) Delta Gun Colour Picture Lube The Fig. 24.15 shows delta P.T. with 3 electron guns. There are 3 separate cathodes and 3 separate control grids. Each gun has a separate screen grid, focus grid and accelerating grid. Phosphor screen Aperture or shadow mask H.V. Anode Deflection yoke

Internal coating

Convergence yoke Accelerating grid Focus grid

Purity magnet

Screen grids

Control grids Cathodes Heaters

Fig. 24.15

24.12 Colour TV Receivers

437

• Neck of the Picture Tube The Fig. 24.16 shows the neck of the tube separately. This has following components. Phosphor dot screen

Anode

Convergence yoke

Deflection yoke

Purity magnet

Magnetic shield

Fig. 24.16

(i) Deflection yoke: Its horizontal and vertical deflecting coils deflect the three beams. (ii) Convergence yoke: This yoke possesses individual adjustment for R, G and B beams to make them converge through the openings of the shadow mask. For each beam there is a permanent magnet and coil. (iii) Purity magnet: This magnet adjusts the three beams to produce pure red, green and blue colours. A colour P.T. is different from monochrome P.T. in the following ways: 1. It has 3 guns, which provide three electron beams, one each of the three primary colours. 2. The screen of the picture tube is coated with three types of phosphor colours. When light from the three guns is incident on the phosphors, they separately emit red, blue and green lights. 3. The phosphors are embedded on the screen in triangular dot pattern (Triads). Each triad has a group of three phosphor dots.

24.12 COLOUR TV RECEIVERS All stages described in monochrome TV receivers also exist in colour TV receivers (Fig. 24.17). In addition some stages are required for treatment of colour. These are described. (i) Chrominance Band Pass Amplifier: This separates the chrominance signal from the composite video signal, amplifies it and passes to the demodulator. The colour burst is prevented from appearing at its input by horizontal blanking pulses.

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(ii) Colour Demodulators: The colour demodulators detect the original signal. They are a combination of phase and amplitude detectors, because the output is dependent on phase as well as on amplitude of the chroma signal. Each demodulator has two input signals—the chroma which is to the demodulated and a constant output from the local oscillator. (iii) Burst Separator: This circuit has to separate colour burst which is transmitted on the back porch of every horizontal sync. pulse. The circuit is tuned to the subcarrier frequency. The burst output is fed to AFC circuit. L.S.

Sound signal Tuner video I.F. amp.

Video det.

Video preamp.

Video amp.

Y Signal

P.T.

AGC Sync. sep. Burst, blanking and chroma band pass amp.

Subcarrier burst separator

Colour demod.

Filter

M A T R

Colour demod.

Filter

I X

R–Y amp.

R–Y

G–Y amp.

G–Y

B–Y amp.

B–Y

Colour killer

AFC

Fig. 24.17

(iv) Colour Killer Circuit: As the name indicates, this circuit becomes ON and disables the band pass amplifier during monochrome reception. In other words, it prevents any spurious signal from getting through the demodulators which may cause any colour interference on the screen.

24.12 Colour TV Receivers

439

When the colour killer circuit is off, the chroma band pass amplifier is ON for colour information. (v) Matrix: This circuit combines signal in specific proportions. At the receiver, the picture tube acts as matrix for inputs of Y signals and R–Y, G–Y, B–Y signals to produce R, G and B lights on the screen. Problem 24.1. Find time for scanning one horizontal line for frames repeated at 60 Hz and 525 lines per frame. If scanning is progressive and without interlacing. Solution. Time for scanning one line 1 = 31.75 ms 60 × 525

Ans.

Problem 24.2. In a TV picture tube, the cathode is at +230 V and control grid is at 190 V, find grid bias. Solution.

Grid bias = 190 – 230 = – 40 V Ans.

Problem 24.3. The period of an equalizing pulse is 31.77 ms. What is the frequency repetitive rate. Solution. Frequency repetitive rate (FRR) 1 = 31476.23 Hz 31.77 × 10–6

Ans.

Problem 24.4. The time required for horizontal blanking is 16% of each horizontal time. If horizontal time is 63.5 ms, fmd horizontal blanking time for each line. Solution. Horizontal blanking time for each line 63.5 × 16 = 10.2 ms Ans. 100 Problem 24.5. If 400 picture elements are scanned in 50 ms. Find time needed to scan 4 picture elements. Solution. Time to scan 4 elements = 50 × 4 = 0.5 ms 400

Ans.

Problem 24.6. A picture has 625 vertical and 50 horizontal elements. Find total number of elements. Solution. Total number of picture elements = 625 × 500 = 31250 Ans.

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Problem 24.7. Find % modulation for the following frequency deviation in FM sound signal. (a) 5 kHz Solution. (a) (b) (c)

(b) 10 kHz (c) 25 kHz 5 × 100 = 20% Ans. 25 10 × 100 = 40% Ans. 25 25 × 100 = 100% Ans. 25

Problem 24.8. The high tension supply to a colour TV is 24.5 kV and the total beam current from the three guns is 2000 mA. Find power of the picture tube. Solution.

Power = Current × Voltage = (2000 × 10–6) (24.5 × 103) = 49 W Ans.

Problem 24.9. A colour TV has a 53 cm screen. The aspect ratio is 4/3. Find height and width of TV screen (Fig. 24.18). Solution.

(3x)2 + (4x)2 = (53)2 x = 10.6

53

cm

3x

3x = 3 × 10.6 = 31.8. cm Ans. 4x

4x = 4 × 10.6

Fig. 24.18

= 42.4 cm Ans.

Problem 24.10. Find the number of possible colours with a 4 line digital input monitor. Solution. Number of possible colours = 24 = 16 Ans. Problem 24.11. The audio carrier for a TV channel is 529.75 MHz and IF is 33.4 MHz. Find frequency of the local oscillator. (L.O.) (Fig. 24.19). Solution. Frequency of L.O.

Modulator

fL.O. = fc + IF

Mixer fL.O.

= 529.75 + 33.4 = 563.15 MHz

fc

L.O.

Ans.

Fig. 24.19

I.F.

24.13 Special TVs

441

Problem 24.12. Find vertical and horizontal resolution for a TV. Assume K = 0.7, aspect raio 4 : 3, and total no. of lines = 625, while 20 lines are lost per field. Solution.

Total lines = 625 Lines lost, 2 × 20 = 40 Lines = 625 – 40 = 585.

Vertical resolution = 585 × 0.7 = 409.5 Ans. 4 Horizontal resolution = 409.5 × = 546 Ans. 3

24.13 SPECIAL TVs

Few special TVs are listed below (a) Projection TV

(b) Closed circuit TV

(c) Flat Panel TV

(d) Digital TV

(e) Three-dimension TV

(f) HDTV

(g) LCD/LED TV

(h) Plasma TV.

(i) Satellite TV Important TVs are described below:

(a) Projection TV The projection TV is the outcome of the quest of generating a picture whose size considerably exceeds the dimensions of the picture tube available. Such a large sized picture (1 m × l m or even larger) may not be required for home but it certainly has a no. of professional applications. Today, a projection TV is used in a university where the students will follow the lectures on otherwise dull subjects with greater interest. It is used in disco, where the young ones will flock to see their favourites. It is used in hospitals and medical colleges where the medical researchers will be able to follow more closely the achievements of their colleagues in other advanced countries where the video filming of new techniques is very common. Similarly, there are many more similar applications.

A projection TV set up, comprises of a projector and screen [Fig. 24.20]. The

projector houses the three projection tubes for R, G and B (red, green and blue)

signals (in case of a colour TV projector) and the associated optics. But now, floor and ceiling mounted projectors are commercially available. These projectors in addition to showing the normal TV programme can also be hooked up to a video cassette recorder (VCR) or a personal computer (P.C.) for a variety of applications.

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Screen

Projector

Fig. 24.20

(b) Three Dimensional Television (3 DTV) In comparison to a two dimensional picture which appears flat and appears to lie in the plane of the screen, in three dimensional television viewing, all the three dimensions-length, breadth and depth are depicted on the screen. The picture appears to have all the qualities of a live scene as seen with natural vision. We, the human beings are able to see in three dimensions as the left eye and right eye see a slightly different image of the same object while viewing it. These images are transmitted to the brain where they are processed and interpreted as a single composite image in 3-D instead of two overlapping flat images. This in fact is the starting point for achieving a 3-D television broadcast and reception. The stereo video signals can be obtained from a pair of video cameras suitably located to take a binocular view of the scene to be televised. These video signals are kept separate throughout the transmission and reception processes. Various techniques have been demonstrated for the stereo video display. One such technique is to use a television receiver with a shadow mask picture tube. All the phosphor dots of one colour (say Green) are excited by one of the electron beams intensity modulated by the video signal corresponding to one of the cameras. Similarly, all phosphor dots of another colour (say Red) are excited by another electron beam intensity modulated by the video signal corresponding to the second video camera. The third beam is kept off and the third colour phosphors (Blue in this case) are not excited. A viewer wearing green filter glasses in one eye and red filter glasses in the other eye will view the scene in stereo in yellow. In the improved version of the same principle, the picture tube has only two types of phosphor dots-Blue and Yellow. In such a case, a viewer wearing blue and yellow filters in respective eyes will see a 3-D view of the scene in monochrome, as yellow and blue are complementary colours. The 3D television is not yet commercialised.

443

24.13 Special TVs

(c)  HDTV (High Definition Television) It is a new generation TV. Displaying images with much greater resolution on screens having an aspect ratio of 16 : 9 (instead of 4 : 3 at present) will enable the viewers to experience the visual quality comparable to 350 mm film and a sound quality of a CD system. The HDTV system demands complete transformation from the NTSE system. Today there are 3 HDTV systems: The Japanese MUSE system, European EUREKA system and American DIGITAL System. The Fig. 24.21 shows block diagram of HDTV (Japanese muse system). High definition display

Satellite Earth station High definition display

Trans­ mitter (Broad­ casting)

Camera

Decoder

VCR Trans­ mitter

Sound Video disc player Mic.

(General)

Digital video recorder

Video copier

Fig. 24.21

The Fig. 24.22 shows a comparison of HDTV and NTSC TV. 16

H

TV screen

H

9

4 TV screen

3

Where H is the height of the TV screen 30°

1

Observer eye

10°

H

H

7.

3.3

Observer eye

(a) HD TV

(b) NTSC TV Fig. 24.22

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(d) LCD/LED TV We today witness slow death of CRT (Cathode Ray Tube) TVs. Now we have LCD/LED TVs, which replace CRT completely. The LCD TV uses a LCD (Liquid Crystal Display), which gets activated, when an electric current is applied to it. The LCD TVs use LCD as back light. The LED TV works on the same principle with the difference that the CRT has been replaced by an LED (Light Emitting Diode). So difference between LCD and LED TVs is only the back light they use. Moreover, the LED TV has less power consumption but costly than the LCD TV. Here we shall describe LCD TV.

• LCD TV A liquid crystal diode normally is in liquid state but it has a crystalline structure such that when voltage is applied, it changes the arrangement of its molecules. This characteristic is used to make a shutter which alternately shifts off and passes the light. An LCD TV produces picture by the above principle, but picture elements, equivalent to scanning lines of ordinary TV (using CRT) are minute liquid crystal plates. The key to a clear image is, how fast the liquid crystal molecules can be changed in response to the slight fluctuation of voltage.

• LCD-TV Circuit The Fig. 24.23 shows LCD-TV circuit, it uses advanced I.Cs. Antenna

Tuner

Panel I.C.

Video I.C.

Intermediate Frequency I.C.

RGB matrix

I.F. detection

Sync control

Chroma demodulator

Back light driver

Chroma I.C. + –

Fig. 24.23

Battery

X driver

Y driver

TFT colour display LCD or CFL

24.13 Special TVs

445

It is necessary to change the video signals to red, green and blue for each

period of horizontal scanning. The video IC contain RGB matrix and sync.

control circuits.

The panel IC contains X and Y driver circuits. The panel has two ICs for X

driver and two ICs for Y driver.

The outputs of X and Y drivers goes to TFT (Thin Film Transistor) colour

display.

Aa a backlight, a LCD or CFL (Compact Fluorescent Lamp) is used which gets

supply from a battery.

Note: The circuitary of LED TV is almost same except that the LC diode is

replaced by an LE diode.

(e) Plasma TV In a conventional CRT television, a gun fires a beam of electrons (negativelycharged particles) inside a large glass tube. The electrons excite phosphors along the screen of P.T. which causes the phosphors to light up. The television image is produced by lighting up different areas of the phosphor coating with different colours at different intensities. Cathode ray tube produce crisp, vibrant images, but they do have a serious drawback that they are bulky. In order to increase the screen width in a CRT set, you also have to increase the length of the tube (to give the scanning electron gun room to reach all parts of the screen). Consequently, any bigscreen CRT television is going to be heavy and take up a sizable part of a room. Recently, a new alternative has popped up: the plasma flat panel display. These televisions have wide screens, comparable to the largest CRT sets, but they are only about 6 inches (15 cm) thick. Based on the information in a video signal, the television lights up thousands of tiny dots (called pixels) with a high-energy beam of electrons. In most of the systems, there are three pixel colours– red, green and blue – which are evenly distributed on the screen. By combining these colours in different proportions, the television can produce the entire colour spectrum. The basic idea of a plasma display is to illuminate tiny coloured pixels to form an image. Each pixel is made up of three fluorescent lights — a red light, a green light and a blue light. Just like a CRT television, the plasma display varies the intensities of the different lights to produce a full range of colours. Plasma is the fourth state of a matter (other three are solid liquid and gas). The central element in a fluorescent light is a plasma, a gas made up of free-flowing ions (electrically charged atoms) and electrons (negatively charged particles). Under normal conditions, a gas is mainly made up of uncharged particles:

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That is, the individual gas atoms include equal number of protons (positively charged particles in the atom’s nucleus) and electrons. The negatively charged electrons perfectly balance the positively charged protons, so the atom has a net charge of zero. If many free electrons are introduced into the gas by establishing an electrical voltage across it, the situation changes very quickly. The free electrons collide with the atoms, knocking loose other electrons. With a missing electron, an atom loses its balance. It has a net positive charge, making it an ion. In a plasma with an electrical current running through it, negatively charged particles are rushing toward the positively charged area of the plasma, and positively charged particles are rushing toward the negatively charged area. In this mad rush, particles are constantly bumping into each other. These collisions excite the gas atoms in the plasma, causing them to release photons of energy. The Fig. 24.24 explains the plasma technology i.e., how gas atoms emit light. Particles

Nucleus

1 Electron

Electron 2 Electron

1. A collision with a moving particle excites the atom.

Atom

3

Photon of light

2. This causes an electron to jump to

another level.

3. The electron falls back to its original

level, releasing extra energy in the

form of a photon of light.

Fig. 24.24

The Fig. 24.25 shows “plasma display panel”. it is a type of flat panel display that uses small cells containing plasma ionised Front plate gas that responds to electric fields. glass The Xenon and neon atoms, used in plasma screens, release light photons when they are excited. Mostly, these atoms release ultraviolet light photons, which are invisible to the human eye. But ultraviolet photons can be used to excite visible light photons. The xenon and neon gas in a plasma

Dielectric layer with display electrodes Rear plate glass

Pixel

Phosphor coated plasma cells

Fig. 24.25

Summary

447

television is contained in hundreds of thousands of tiny cells positioned

between two plates of glass.

The main advantage of plasma display technology is that you can produce a

very wide screen using extremely thin materials and because each pixel is lit

individually, the image is very bright and looks good from almost every angle.

The image quality isn’t quite up to the standards of the best cathode ray tube

sets, but it is fairly good.

The biggest drawback of this technology is its very high price. But as prices

fall and technology advances, they may start to replace out the old CRT sets.

Note: The close circuit TV (CCTV) and satellite TV are :

SUMMARY 1. The red, green and blue are called primary colours. Any colour can be prepared by mixing primary colours in right proportions. 2. Colour triangle gives the percentage of primary colours to be mixed to obtain a particular secondary colour e.g., 30% red + 59% green and 11% yellow gives white colour. 3. Various TV systems used are: PAL., NTSC and SECAM. In India, PAL (Phase Alternate Line) system is popularly used. 4. A colour camera basically comprises of three Black & White Cameras. 5. A colour picture tube basically comprises of three monochrome picture tubes (guns) of red, green and blue colours in a prescribed configuration. 6. The colour picture tubes are of three types: Delta tube, gun in line and single gun tube. 7. The TV signal is a composite video signal, which contains audio, video and other signals. 8. There are many types of colour TVs such as LCD, LED, Plasma TV etc. 9. Home Theatre TV system gives theatre effect in home. 10. The TV studio equipment are : Microphones, Loudspeakers, TV, Camera etc. qqq

25 Cable Television and DTH Feeding many TV receivers from a single antenna is known as cable TV (CATV). This is a very popular system today. A parabolic dish is used as an antenna. The dish antenna receives signals from a satellite. The TV receivers are joined through co-axial cables. In DTH (Direct to Home) service, the consumers receive signals directly from satellite through their own dish antenna. The cable operator has no role to play.

25.1 CABLE TV (CATV) The feeding of many TV receivers from a single antenna is called cable TV. A single antenna can feed a complete locality. The antenna is installed at the top of a building and the signal is fed to the houses through a cable. The antenna is generally dish antenna which receives national as well as international signals through satellite and this signal is sent to houses through cable. The signal should be sufficiently strong so that the picture is clear at the TV sets at each home. Also there should be no interference between the signal received and the signal sent to the viewers. The weak signal is amplified before sending to the subscribers. The signals from VCR etc., may also be sent to the subscribers through cable. Generally modulation is not required and the cable used is a co-axial cable. The CATV system needs miles of cables. They suffer interference in the way. Moreover, with distance, the signal goes on weaker and when it reaches a TV receiver, it is distorted. To solve this problem, amplifiers are installed at regular distances to keep the signal strong. This also poses problem, if there is a breakdown in any of the amplifiers. The Fig. 25.1 shows block diagram of a cable TV network.

Chapter 25

Consumers

Dish

Cable Television and DTH Consumers

450

Local program

Signal

Amplifier

UHF/VHF processor

Amplifier

Amplifier Consumers

Fig. 25.1

The signals suffer an attenuation @ 15 dB per 500 m cable; so use of repeaters/ amplifiers is necessary. A UHF/VHF signal processor is used, which is positioned at a central place. It is necessary to convert UHF channels to VHF channels, because signal loss at VHF is lesser. Moreover, the processor can accept signals from dish as well as the locally made programs. The local made programs include announcements, local advertisements etc. The connection to consumer may be direct or through a set top box (a converter). The set top box selects channels and controls the operations. The set up box is usually operated through a remote. Note: (i) The signals can also be sent through cable directly from VCR (Video Cassette Recorder) without any modulation. (ii) If there is a breakdown in any of the amplifiers, there is a total breakdown of the signal ahead so the system needs regular checkup.

• CATV Through Internet The CATV through Internet is possible using telephone lines as a medium of transfer and using a dial up modem (modulator-demodulator). See Fig. 25.2. For cable internet access on a PC (personal computer), a cable MODEM is required at the user’s end. A cable modem is an external device that is connected to the computer to provide high speed data access via CATV network. A cable modem sends and receives data from the internet, using the cable network. The modem translates signals in the same way, a telephone modem does from the telephone line. It translates radio frequency signals to (and from) the cable into internet. The modem is connected to the computer through N.I.C. (network interface card). This provides connectivity between cable and computer and translates the signals from the modem so that the computer software displays these signals.

25.1 Cable TV (CATV)

451

The CATV network has high bandwidth about 500 MHz to 720 MHz. It handles more than 100 channels. The CATV through internet is a two way network. Up Stream Link

Conversion

Splitter

Combiner Duplex

Subscribers

Down

F.O. Tx

F.O. Tx

Processor Demodulator Video

Up Conversion

Cable Modem Network Controller

Down F.O. Tx

F.O. Tx

Stream TV Channels

Down Stream Link

Fig. 25.2

This is done by upgrading the amplifiers. The internet signals are digital signals and they are to be interfaced with analog CATV network. This interface is known as “Cable modem termination service” (CMTC), which consists of input interface, cable modem and microprocessor. About 5 to 10 PCs (personal computers) may be connected with the internet through DSL line. The feature films and other programs can be down loaded over the internet and can be put on the cable network; so that the consumer can enjoy them on their PC at their home. The cable connection can also be made through optical fibre cable (OFC) replacing the coaxial cable. This will improve the reception further. The data rate may be as high as 100 megabits per second, so the 3 hour movie can be downloaded in few minutes.

The internet service providers offer telephony, internet access and television

services together, resulting in the economics to the consumers.

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25.2 DTH (DIRECT TO HOME) SERVICE The DTH service is a satellite transmission technology. It has numerous merits for the sender as well as for the receiver. Due to digital technology, it can handle many channels over a single delivery platform as compared to the analog technology. The DTH service is that in which large number of channels are digitally compressed and are sent through high power satellite. The programs can be directly received at home (Fig. 25.3 a). What the consumer is to do is the installation of a small dish antenna at the roof of his house. The transmission eliminates cable operators completely as the consumer receives signals directly from the satellite. The consumer also needs a digital receiver. To receive the multiplexed signals as compared to CATV, this has easy control and clear picture. The set up box is installed near TV, which selects the desired channel and also controls all operations through remote. Transmissions in DTH is done in Ku (or C) band which is most appropriate. As the signals are digital, this band provides high resolution picture and also better sound. All merits of digital transmission are also applicable.

Satellite

Dish

Set top box TV

(a)

Dish

LNB

(b) Fig. 25.3

The Fig. 25.3(b) shows the DTH dish set with LNB [low noise (high gain) block] i.e. amplifier.

• Merits of DTH Communication As DTH is a digital technology, all merits of a digital communications are applicable. It has numerous merits to the subscribers few are listed below: 1. Quality digital picture 2. Stereophonic sound 3. Uninterrupted viewing 4. Capacity upto 500 channels 5. Interactive TV

Summary

453

SUMMARY 1. Through cable TV, information can be sent to many consumers, simultaneously through cable. 2. Usually, coaxial cable is used which runs to all consumers. 3. For this, a dish is used at the roof top. The signal is received from the satellite. This signal is amplified and sent to the consumers. 4. The cable TV system is most popular for sending TV signals and is a means of entertainment. 5. The DTH stands for “Direct to Home”. In this a consumer installs a small dish as an antenna at the roof of his house and receives the signals directly from the satellite (without any cable). qqq

26 Facsimile (FAX) Facsimile is a technique to send printed messages (such as typed or hand written) from one station to another. As compared with the ordinary telegraphic system, the following points are to be noted: (i) By this system, the messages written in the language not known can also be sent, whereas it is not possible in the ordinary system. (ii) A bandwidth of 120 Hz is needed in the facsimile system, where as the ordinary telegraph system needs a very high BW of the order of 2200 Hz. (iii) For facsimile, trained operator is required, whereas in ordinary system this is not essential. (iv) The equipment needed in FAX is costly as compared to the conventional telegraphy. In wider sense, the picture telegraphy and facsimile telegraphy systems are same, Now-a-days these systems are of two types, i.e., over lines as well as over radio (through space). In line facsimile, AM signals are used where as in radio facsimile, FM signals are invariably used. Our modern FAX transmitters and receivers are capable to handle AM as well as FM signals. The word “Facsimile” means “exact reproduction” . In facsimile transmission, an exact reproduction of the picture or document is provided at the receiving end.

26.1 FACSIMILE (FAX) Fax, short for facsimile transmission is a way of sending text of pictures over telephone lines. A device called a facsimile (FAX) machine is used for sending and receiving the messages or pictures. The machine is connected to a telephone. To send a

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document, the sender inserts it into the machine and dials the telephone number of the receiving fax machine. When the connection is made, an electronic scanner in the transmitting machine converts the image of the document into a set of electric signals. The signals travel over the telephone lines to the receiving machine where the signals are reconverted into a copy of the original document. The first fax machine began operation between Paris and Lyons (France) in 1866. The signals were sent through telegraph wires. It could fax documents and drawings.

(1) Fax vs T.V. Transmission A fax is a machine that can transmit photographs, written documents, etc. The same is done in television transmission. The following is the difference between the two: 1. In TV, the scene includes “movement” whereas, such is not the case in fax transmission. The TV can send moving pictures. The FAX can transmit only the still pictures. 2. The transmission in TV is much faster than in FAX. 3. Due to the above, TV transmission needs much larger bandwidth than the FAX. This is the reason that FAX messages can be sent over ordinary telephone lines.

(2) Applications of FAX Important applications of FAX are given below: 1. Business: The business documents like purchase order, invoice, agreement etc., available at one place can be made available at other place in no time through fax without relying on postal department. The most common example is news paper printing. News, photographs, reports available at one place can be sent to other place to be published in the next morning edition. The photographic plates of a complete news paper can be sent in no time to any corner of the world through fax. It has become very easy to print a news paper from many places simultaneously. 2. Weather: The weather reports, maps from one place can be sent to TV stations for broadcasting. Similarly, they can be sent to airports, seaports and to any place through fax for their use. 3. Legal applications: The finger prints, photographs, signatures, and documents of a criminal can be sent from one place to another through FAX for quick verification.

26.2 Basic Fax System

457

4. Personal applications: Personal documents such as certificates, photographs and other documents urgently needed by a person at a distance from his residence can be obtained through FAX in no time. No postal service/courier can be too fast.

26.2 BASIC FAX SYSTEM FAX machines provide an easy way to send documents to any phone number equipped with another fax machine. It is a method by which a page (printed, written or photographic) is converted into electrical signals, sent quickly through the public switching telephone network (PSTN) and recorded as a copy in the remote fax machine. In most cases, faxes are a cost-effectivealternative to overnight delivery and a faster option than regular mail. The Facsimile process involves three basic steps: 1. Reading or converting the documents into electrical signals. 2. Sending or transmitting the signals through a telephone system to another fax unit. 3. Converting a received signals into a “facsimile” of the transmitted documents.” A fax machine works by scanning each outgoing page, converting the image into a series of light and dark dots. This pattern is then translated into audio tones, and sent over regular phone lines. The receiving fax hears the tones and prints the total compilation of dots. The resulting document is black and white like the original page.

26.3 PRINCIPLE OF OPERATION OF FAX Fax involves the following steps of operation which can be very well understood from the block diagram shown in Fig. 26.1. Transmitting Message (Analog) Station

Scanning

Data Compression

Digital

Modulation Analog

PSTN

Receiving Station

Reception (Analog)

Printing

Data Recovery

Fig. 26.1

Digital

Demodulation

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1. Scanning: (Reading and converting the document into electrical signals). The transmitted document originally consists of twodimensional structure. We have to make it one-dimensional structure in order to consider document as the concept of line and then, we convert the line into the dot structure, we regard it as “SCAN”. The scanner is the part of a fax unit that converts the marks on the page being sent into electrical signals. A photosensor looks at a very small “picture element (Pixel), determines whether it is black or white, and generates a strong or weak electrical pulse. To read a page electronically, all of the spots in a thin strip 1/ 100th inch high across the top of the page are read, one at a time, starting with the upper left-hand corner. A stepper motor moves the page down and the photosensor reads the next strip below. Successive strips are read until the whole page is converted into electrical pulses. This is almost same as TV scanning. 2. Modem (Modulation and Demodulation): A modem is a modulationdemodulation device that accepts the digital information and modulates it into an analog signal to be sent over a voice telephone line. We have to modulate the scanned picture element to convert digital signal into analog signal at transmitting station in order to transfer through the PSTN (Public Switching Telephone Network). 3. Printing i.e., converting the received signals into a facsimile of the transmitted documents. Normally we use The Print Head Operation. The Print head has a row of very small resistor-element spots across the recording paper with very-very small width. The thermal recording paper touches this row of resistors. A pulse of current through a resistor causes it to become hot enough to mark the paper in a spot about 0.005 inch in diameter. The spot temperature must .be changed from non marking temperature to marking temperature and return to non marking before the paper steps to the next recording line. Marking temperature is about 200° F. The thermal paper is a special paper that turns black on heating. Either thin-film or thick-film technology is used for making the print heads.

26.4 TRANSMISSION AND RECEPTION The signal obtained from scanning cannot be directly transmitted, as it is difficult to amplify low frequencies. Hence process of modulation is carried out. The

26.5 Types of Fax Machines

459

Fig. 26.2 shows the transmission and reception process. The signal is “amplitude modulated” with a carrier. After this, we use AM/FM converter, as frequency

Original

Signal

Amplitude Modulator

AM/FM Converter

Transmitter

(a) Transmission

Original

Signal

Detector

FM/AM Converter

Receiver

(b) Reception

Fig. 26.2

modulation is superior to that of the amplitude modulation and the signal is transmitted. At reception side we again use an FM/AM converter and the detecter, which detects the original signal and the carrier is separated. The carrier frequencies used for both the modulations are between 1200 Hz to 1800 Hz.

26.5 TYPES OF FAX MACHINES Mainly two types of faxes (fax machines) are used (Fig. 26.3): 1. Thermal Paper Fax 2. Plain Paper Fax.

Fig. 26.3

1. Thermal Paper Fax: Thermal printing works by firing minute dots of heat at a special heat sensitive paper. This triggers a chemical reaction so that where the heat hits the paper, it turns black forming the required image.

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2. Plain Paper Fax. This can be classified as: (a) Thermal Transfer: Thermal transfer adopts a similar process where minute dots of heat are fired at a heat sensitive ribbon, which in turn transfers carbon like material onto plain paper in the form of an image. (b) Laser Fax: Laser faxes use fundamentally the same technology as laser printer which is similar to that used in photocopier. The only difference is that instead of receiving digital pulses from the computer the laser fax receives them over a telephone line. (c) LED Fax: Light Emitting Diode (LED) printing is very similar to laser printing. Instead of firing a laser beam at a drum, LED uses a row of infrared lights to produce a charge which then goes on to produce the image. (d) Inkjet Faxes: Inkjet printing creates an image by firing jet of ink from a matrix of nossels onto a page.

26.6 CONVERSION OF OPTICAL SIGNAL INTO ELECTRICAL SIGNAL This is the basic process which occurs in FAX. Following devices are used for this purpose: 1. Photodiode (Fig. 26.4a): This is the simplest device which can be used to convert light into electrical signal. When light from picture or printed page falls on a photodiode, an electric current flows through the diode. This current is made to flow through a resistive circuit (R) to produce a voltage which is later on amplified. The output of the amplifier is proportional to the light intensity falling on the diode. 2. Photo Multiplier tube (Fig. 26.4b): The photo multiplier tube has so many “photo electrodes” . The light is made to fall on the first electrode which emits electrons. These emitted electrons strike the next electrode and more no. of electrons are emitted from the later. The process is continued and thus multiplied output of electrons i.e., a large photo current reaches at the collecting electrode (anode). The amplification in this way takes place within the device and very little amplification is further needed. The device rather needs much higher voltage to operate and thus it is costly.

26.7 (a) FAX Transmitter

461 Electrode

Photo Multiplier Tube

Light

R Amp.

Amp.

Output Output

Photo Diode

R

Fig. 26.4 (a)

Fig. 26.4 (b)

• Spot Lighting Whatever the type of photodiode used to convert light into the electrical signal, the light should be restricted on the particular part of the picture or document to be transmitted. There may be following methods. (a) In the first approach, the light is focussed through a lens on the part (spot) which is to be transmitted. The reflected light from the spot reaches the photo diode, which converts the lightness (or darkness) of the part into voltages. A pure white part generates a high voltage and a dark part generates a low voltage. (See Fig. 26.5a) (b) In another approach, though the whole surface of the document is illuminated, but reflected light from a restricted area (spot) is allowed to reach the photo diode. The light, out of this (spot) is not detected by the photodiode. (See Fig. 26.5b)

s

Photo diode

Le n

Lig

ht

The first method is superior as it can be achieved by using a LASER beam which gives a better control on the light intensity. Light Photo diode

Spot

Fig. 26.5 (a)

Lens

Spot

Fig. 26.5 (b)

26.7 (a) FAX TRANSMITTER The message is “scanned”, usually by optical scanning in the transmitter. Two methods are adopted for optical scanning:

462

Chapter 26

Facsimile (FAX)

1. Cylindrical Scanning. 2. Tape Scanning. 1. Cylindrical Scanning (Fig. 26.6): The message (signal) is wrapped around a cylindrical drum to allow complete scanning. The cylinder (drum) is rotated and the scanning spot is kept fixed. The light reflected from the drum is fed to a photo diode, which converts the picture signal (light) into electrical signal. The electrical output from the photo diode is amplified and transmitted. The drum is rotated at about 60 RPM and scanning rate is kept at 5 lines/minute. Incident Light

Output Signal

Amp Photo Diode Reflected Light

Spot

Rotating Drum Wrapped with Signal

Fig. 26.6

2. Tape Scanning: This is also an optical scanning method. In this method message is taken directly from the printed tape. Due to width limitations, the method has lesser in use. The Fig. 26.7 shows the method. Lens

Prism

Light

Output

Lens Spot

Photo diode

ng

ti ota

R

Fig. 26.7

Pr

ed int

e

p Ta

26.7 (b) Fax Receiver

463

• Scanning Spot The shape of scanning spot in optical scanning may be rectangular [Fig. 26.8 (a) or a trapezoidal (b)]. The shape of scanning spot decides the size of the wave shape of the output signal.

Rectangular Spot

Trapezoidal Spot

Fig. 26.8

Usually a rectangular spot is used. The size of the spot is very small and covers an area of constant illumination on the message. The change in the average illumination gives the signal output.

26.7 (b) FAX RECEIVER In the receiver, “reverse” is done, of what happened in the transmitter. While transmission, the picture message was converted into an electrical signal, in the receiver, the electrical signal is converted back into picture signal. The equipment (drum etc.) in the receiver is however, almost same as used in the transmitter. We may get an output free of distortion, for this it is necessary that the signal is properly “ synchronised”.

26.8 SYNCHRONISATION OF THE SIGNAL (a) When the signal is a written message (e.g., a page of a book), synchronous motors which rotate at constant speeds are used both at transmission and receiving ends. These motors are operated under controlled frequency supply. If f is the frequency of the supply fed to a synchronous motor with no. of poles P, the synchronous speed of the motor will be 120 × f Ns = P The synchronous speed Ns will remain constant so long frequency of the supply f is kept constant by some means.

464

Chapter 26

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(b) If it is a picture (photograph) signal, a synchronising signal of 1 kHz is sent for synchronization.

When the signal modulates a carrier e.g., in the case of F.D.M.

(Frequency Division Multiplexing), the carrier is sent along with

upper side band (U.S.B.). The carrier helps in the total recovery of the

synchronising signal later on.

SUMMARY 1. The fax telegraphy is a technique to send printed messages from one station to another. 2. In comparison to TV transmission, the fax needs a much shorter bandwidth, this is the reason that fax messages can be sent over ordinary telephone lines. 3. The fax finds application in business, weather calculations and personal use. 4. The fax process involves three basic steps: converting the document into electrical signal, transmitting the electrical signal through telephone wires to another FAX machine and there converting the received signal into the original document. 5. The picture is converted into electrical pulses by a photodiode or photo multiplier. 6. In transmission, AM-FM converter is used. In receiver, FM-AM converter is used. qqq

27 Modern Communication Techniques In this chapter, the following communication techniques/devices will be discussed (i) Wireless telephony

(ii) Mobile phone and W.L.L.

(iii) TV remote

(iv) E-mail

(v) Internet and www

27.1 RADIO TELEPHONY The ordinary telephone system is an example of wire telephony, whereas a pager, mobile, etc. are the examples of wireless or radio telephony, in which signal does not flow through wires but flows through space in the form of radiowaves. Transmitting antenna

Receiving antenna

Elect pulses Mike

Sound

Loudspeaker Modulator

AMP

Demodulator

AMP

Receiving end Mixer Transmitting end

Fig. 27.1

When a person speaks before the microphone of his telephone (see Fig. 27.1), his sound is converted into electrical pulses and modulated, amplified and

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Chapter 27

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relayed in space. At the receiving end, the same signal is demodulated and amplified. The loudspeaker converts the electrical pulses again into original sound.

27.2 (a) MOBILE/CELLULAR PHONE/CELL PHONE The mobile phone has replaced cordless telephone, pager etc.

This is an advanced telephone system in which we can call/can be called by

a person while moving. Today we have world wide roaming mobiles. The

mobile may be a handset or fitted in a vehicle, It is also called cellular phone/ cell phone, as the total area is divided into number of small cells.

The working principle of this system is same as of a cordless telephone with the

difference that it uses high power devices, covers a bigger range and provides

service to many subscribers at a time.

Cellular mobile services offer telephone and data communication services

even when the calling party or the called party or both are mobile (in motion).

(1) Terms Related to Cellular Phone Service Below are given terms which are used regarding the cellular service: AMPS

— Advanced mobile phone service

CTS

— Cellular telephone service

MMC

— Master mobile centre

MTSO

— Mobile telecommunication switching office

MSC

— Mobile switching centre

NCS

— Network control system

PSTN

— Public switching telephone network

RSG

— Remote switch group

SMC

— Satellite mobile centre

SIM

— Subscriber identity module (smart card)

SAT

— Supervisory audio tone.

TDMA

— Time division multiple access.

FDMA

— Frequency division multiple access.

GSM

— Global system for mobiles.

POSCAG — Post office standard code advisery group

27.2 (a) Mobile/Cellular Phone/Cell Phone

467

The Fig. 27.2 shows basic cellular mobile system. PSTN

MTSO Land lines Base station Two way radio

Mobile unit

Fig. 27.2

(2) GSM for Cellular Phones In terms of technology, GSM (Global System for Mobiles) is the most

demanding system with full range of digital techniques.

The GSM air interface provides physical link between mobile phone and

the network. The GSM is a digital system employing TDMA (Time division

multiple access) technique and operates at 900 MHz.

(3) Distribution of Area into Cells (See Fig. 27.3) The total area is splitted into small parts known as “cells”. Each cell covers an area of 15 km and has its own transmitter, receiver and other control equipment and can send/receive calls from the system. Moreover each cell has a particular channel to work and the same channel can be used for more than one conversations. Further, each cell can handle frequencies for two way operation. However, the adjacent cells use different frequencies to avoid interference. Within cells, all communication is performed using a given bandwidth and centre frequency. We arrange such that in Cell A, centre frequency fA is used in Cell B, centre frequency fB is used, etc. If two cells are widely separated so that a receiving antenna in one cell cannot detect the signal transmitted from the other cell, both cells can be given the same centre frequency allocation.

A1 D1 C2

B1 E1 G2

C1 F1 H1

A2 B2

Fig. 27.3

D2

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In Fig. 27.3 each pair of cells A1 and A2, B1 and B2, C1 and C2 and D1 and D2 use the same centre frequency. This technique which is designed to conserve the spectrum is called frequency reuse. The bandwidth allotted to each cell is divided into N channels thus if the bandwidth of a cell is BC. The bandwidth alloted to each user is BC Bu= N In the AMPS (Advanced mobile phone service), the communication is by FM and each user channel is alloted a bandwidth Bu = 30 kHz. The total spectrum allocation per cell is BC = 40 MHz so that N can be calculated. The mobile unit first communicates with a nearby cell site. The cell site is linked to the telecommunication switching office by conventional voice and data circuits (Cell sites and telecommunication switching offices are fixed installations). The telecommunication switching office switches the traffic between the cellular system and rest of the telephone network.

(4) Base Station The base station has a high power transmitter, receiver, control unit and other equipment. It has also an elevated antenna. The base system may be owned by government itself or by private companies. The transmitter of the base station contains modulators, amplifiers and the receiver contains demodulators, amplifiers, etc., for processing the sound signal. The control unit has logic circuits to maintain a total control on all operations.

(5) The Handset Technically it is called a “cell phone” which can be used from any place. But when a cell phone is used in a moving vehicle or it is fitted in a vehicle itself, it is called a mobile phone/unit. Note that “mobile” means “ in motion” . A mobile unit has its own transmitter, receiver and controlling equipment. It works on the same way as an ordinary/conventional telephone except that it is through the base station. A cell phone (handset) consists of—the mobile terminal (phone itself) and the subscriber identity module (SIM), which fits into the handset. Sometime if we do not have our own mobile, with this card, we can use any other person’s mobile, without affecting his billing. Note that the mobile handset should be rugged enough as it may even drop on the ground. The availability

27.2 (a) Mobile/Cellular Phone/Cell Phone

469

of channel is indicated by a visual indication. The mobile unit has 10 digit

Keyboard like other telephones.

The Fig. 27.4 shows the block diagram of the hand set. Note that it is essentially

a microcomputer. The blocks marked as I/O are input/output devices.

I/O Center Processing Unit (C.P.U.)

I/O I/O

Fig. 27.4

(6) Working of Mobile Phone When we dial a number on the mobile phone, it transmits the digital signal through the “inbuilt” radio transceiver to nearby base station, which on turn is connected to MSC (Mobile Switching Center). Once a cell is forwarded to MSC, it determines how to route the call and set up link to the caller. The MSC is to find out where the caller is and then forward the call to the base station which is nearest to it. If the caller is moving (e.g. in a car), the cellular system keeps the track while the call is in progress, so that it can automatically transfer the call to the another cell as the caller moves from one to other area. The receipt of the call is just opposite. The central office acknowledges the caller and connects the concerned MTSO (Mobile telecommunication switching office). A channel is made available and a link is established between “the caller” and “the called”.

(7) Cell Site Transmitter The Fig. 27.5 shows block diagram of the cell site transmitter. F.G.

Exciter

AMP

P.A.

To antenna

Fig. 27.5

According to the instructions obtained from CPU of the cell phone, the frequency generator (FG) generates the desired channel frequency. This signal then reaches to the exciter, where the process of modulation occurs. The output

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Modern Communication Techniques

is then amplified in stages—finally it goes to the power amplifier (P.A.) and to the antenna as shown.

(8) Receiver Block Diagram The Fig. 27.6 shows the block diagram for the receiver. The signal received from antenna goes to the high frequency unit (HFU) where it is preamplified and shifted to the frequency generator (F.G.). It is then lowered by intermediate frequency unit (I.F.U.) to I.F. range and lowered further by Audio Frequency Unit (A.F.U.) to the A.F. range for its further transmission to the MTSO (Mobile Telecom Switching Office). MTSO

AF

A.F.U.

I.F.

I.F.U.

H.F.U.

From antenna

F.G.

Fig. 27.6

The Fig. 27.7 shows outlook of mobile phones.

Service light Earphone Jack

Microphone

Fig. 27.7

Antenna Earpiece Display

27.2 (b) Wire Less Loop (WLL)

471

27.2 (b) WIRE LESS LOOP (WLL) This is a mobile phone with limited mobility. Generally, this does not have “roaming” and S.M.S. (short massaging service) facilities. The W.L.L. works under C.D.M.A. (Code division multiple access), while a mobile phone works under G.S.M. (Global system for mobiles) technology.

27.3 TV REMOTE CONTROL The TV remote control was invented by Mr. Eugene polley. The important functions to be controlled in TV receivers are: 1. ON/OFF control 2. Channel selection 3. Volume control Note that, the contrast and brightness are not included in remote control, because the AGC (Automatic gain control) circuit automatically changes the gain of the TV receiver to maintain desired control of contrast and brightness. The Fig. 27.8 shows working of remote control. It has two parts: (a) Remote Control Handset (b) TV receiver which is to be controlled (a) Remote Control Handset: The Fig. 27.8 (a) shows the basic principle of remote control. The remote control handset is used from a distance from the TV receiver. Ultrasonic elect energy at 40 kHz

Ultrasonic sound signal Mic Amplifier

Oscillator L.S.

Control signal

Elect energy

(a) Remote control handset

(b) TV receiver Fig. 27.8

The remote control handset generates ultrasonic waves of frequencies around 40 kHz. The use of ultrasonic frequencies has the advantages that they are out of reach of the human ears and do not interfere the TV receivers in neighbouring rooms.

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An oscillator can be used to generate the required ultrasonic frequency. Each controlling function needs a different frequency. The typical (approximate) value of the frequencies for few controls are : Channel up ↑

42 kHz

Channel down ↓

41 kHz

Volume up ↑

37 kHz

Volume down ↓

43 kHz

The remote handset may use a Hartley oscillator. When any push button at the remote is pressed, different capacitors get connected across the secondary of output transformer of the oscillator, which changes the resonant frequency of oscillator’s “tank circuit” and the required frequency output is obtained from the oscillator for the particular function to be controlled. The output of the oscillator is fed to the (“piezo electric”) loudspeaker, which converts the output into ultrasonic sound energy and radiates it towards the TV receiver to be controlled. (b) TV Receiver to be controlled [Fig. 27.7 (b)]. In the TV receiver, the ultrasonic energy is picked up by a (Piezo electric crystal) microphone, which converts the sound energy into electrical energy. The microphone uses “Barium Titanate” crystal which when “strained” by the striking sound along one “axis”, a proportional voltage of the same frequency is generated along the other “axis” . The output of the microphone is given to the amplifier. The output of the amplifier is used to provide various controls (Volume, Channel selection, etc.) The control signal received is intercepted by a “sensor” on the TV receiver for the particular control required. The d.c. voltages are varied for controlling the function. For example, for volume or channel control, operating bias of the amplifier is varied, which forms part of the circuit.

Now-a-days for remote control, digital I.C.s are used. The recent

development in high speed I.C. counters have made the remote controls

very sophisticated.

27.4 E-MAIL It is an electronic mail often referred to as E-mail. It is the transmission of textual data from one place to another using electronic means for capture, transmission and delivery of information. In simple words, an electronic mail system is a method of sending messages, mail or documents. It is also

27.4 E-mail

473

known as electronic delivery system or electronic documents distribution/ communication system. Electronic Mail refers to transmission of messages at high speed over communication channels. The simplest form of computer based mail system allows one user of the service to send a message to another. The second user at his or her convenience retrieves the message on his or her display monitor or by taking a hard copy of it. The mail can be duplicated, revised, incorporated into other documents, passed on to new recipients or filed like any other document in the system. E-mail is rather a service with various facilities like: (а) Storing and Forwarding messages. Messages are given to the recipient on request and person to person contact is not required. (b) Advice delivery. The sender can be confirmed of the reception of message to the recipient and an immediate reply can also be demanded. (c) Off time working. During one’s absence the incoming messages can be stored. (d) Gateway. E-mail services include access to other facilities like Telex system, on-line information services, etc. (e) Closed user group. The use of E-mail service can be restricted to smaller or larger areas.

Apart from the above services, E-Mail also offers Radiopaging i.e.,

you will hear a beep (audible tone) while receiving an urgent message,

Telemessages i.e., replacement of old telegram system. Message

translation i.e., message can be translated to the recipient’s own tongue

at the recipient’s end.

• Block Diagram of E-mail We can conclude from the above that E-mail system is actually a message handling system. The Fig. 27.9 shows block diagram of an E-mail.

User Mail Boxes

User Agent

Message Transfer Agent Sub mission

Delivery

Directory

Originator

User Agent

Directory

Network Message Flow

Fig. 27.9

Recipient

User Mail Boxes

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(1) Advantages of E-mail 1. Message can be sent at whatever time or day suits to the user. These can also be prepared in advance for subsequent despatch at convenient times. 2. Messages will be in the recipient’s mailbox within minutes. 3. No need to speak to the recipient in person. 4. Incoming messages can be saved. 5. E-mail reduces the volume of paper that is to be processed. 6. Telex services can be provided as part of the E-mail facility. 7. On-line information services may also be available.

(2) Drawbacks of E-mail 1. Recipients must also be E-mail users. 2. Only text can be sent. 3. Text formatting is restricted to the basic punctuation and alphanumeric characters. 4. Until a mailbox is checked, there is no way of knowing that a message has arrived.

27.5 (a) INTERNET The Internet was conceived in 1969. By 1987, it was called as internet because by this time it was made available to all who want to access it. Thus, the Internet has grown into an immense network of computers and wires interconnecting them. There are millions of computers connected to this “network of networks”, spanning thousands of computer designs, operating various kinds of connections including coaxial cable, optical fibre, etc. The Internet is a computer network made up to thousands of networks worldwide. No one knows how many computers are connected to the internet, or how many people use it, though it is certain that both are expanding at a rapid rate. No one is incharge of the Internet. There are organizations which develop technical aspect of this network, but there is no governing body in control. An Internet is a group of computers connected mutually for exchanging information and sharing equipment. In case something goes wrong with any

27.5 (a) Internet

475

part of the internet, information finds an alternative route around the crippled computers in order to reach its goal. That is why the internet is also called the Information Superhighway or Cyberspace.

(i) Equipment Needed for Internet (Fig. 27.10) We need the following equipment for internet. 1. Computer. A compatible PC (Personal Computer) can be used for an Internet connection. 2. Programs. We require special programs to use the Internet. These programs are given free of charge by most service-providers. 3. Modem. A modem serves as a medium to exchange information between a computer and the Internet.

Modem is a device by which computers exchange information

over telephone lines. The word “Modern” is short for modulator/

demodulator.

Fig. 27.10

4. Telephone line. All Internet-information travels over telephone lines. 5. ISDN line. Integrated Services Digital Network (ISDN) is a line by which data is transmitted over digital telephone lines. ISDN is two to four times quicker than the most speedy modems. Use of ISDN lines on the Internet’s world-web helps to transmit or receive text, graphics, sound and video. Many telephone companies offer ISDN lines.

(ii) Utilities of Internet 1. E-Mail (Electronic-Mail). The internet enables us to exchange messages throughout the world with people, friends, colleagues, relatives and even strangers, we happen to meet on the Internet.

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2. Information. Any information on any subject can be obtained easily on the Internet. We can have a good information from newspapers, magazines, academic papers, government documents, famous speeches, recipies, works of literary figures and what not. 3. Entertainment. The Internet offers hundreds of simple games to various people free of cost. Children as well as grown-ups can enjoy chess, football, snooker, carrom-board, cards, lawn tennis and such other games. We can have a good look at current movies or listen to television theme-songs. We can have several conversations even with famous personalities of the world. 4. Programs. The Internet offers thousands of free programs. Some of these programs are World Processors, Spreadsheets and Games. 5. On-line shopping. We can order desired goods and services on the Internet just sitting comfortably in our home. Items like flowers, books, cars, computer programs, music, CD’s, pizza and many others can be brought. 6. Finding people. If you have lost track of some body, you can find him or her anywhere in the country. 7. Finding businesses products and services. The yellow Page directory services enable you to search the type of company you’re looking for. You can indicate a code to specify the location. People are shopping for that hard-to find special gift item. 8. Education. School teachers coordinate projects with classrooms all over the globe. College students and their families exchange e-mail to facilitate letter writing and keep the cost of phone calls down. Students do research from their home computer. The latest encyclopedias are online. 9. Healing. Patients and doctors keep up-to-date with the latest medical findings, share treatment, experience, and give one another support around medical problems. 10. Investing. People buy stock and invest money. Some companies are online and trade their own shares. The significance of the internet lies not so much in where it is today, but in where it will be in the next five or ten years of time. It is still a technology in its high future prospects of extensive usage in its remarkable early success.

27.5 (b) World Wide Web (www)

477

27.5 (b) WORLD WIDE WEB (WWW) The Internet is the maze of phone, cable lines, satellites and network cables that interconnect computers around the world. The www is all that can be accessed on the Internet using a Uniform Resource Locator, (URL).

SUMMARY 1. In radio telephony, the signal propagates through space in the form of radio waves. 2. The mobile is an advanced telephone by which we may call a person while moving around. The roaming mobile may have theoretically an infinite range. 3. Remote control is a device, which can control an euipment from a distance. 4. The E-mail is a transmission of data/message from one place to another using electronic means. 5. The Internet is a computer network made up of thousands of networks worldwide. Infinite no. of computers are connected to the internet. It is the largest computer system in the world. 6. All modern mobile phones are equipped with email and Internet. qqq

Appendices APPENDIX A Values of General Physical Constants Symbol

Value

Charge of an electron

Constant

e

1.602 × 10–12 C

Mass of an electron

m

9.109 × 10–31 kg

e/m

1.759 × 1011 C/kg

Planck’s constant

h

6.625 × 10–34 J.s

Boltzmann’s constant

k

1.380 × 10–23 J/K

Velocity of light in vacuum

vc

2.998 × 103 m/s

Permeability of space

m

1.257 × 10–4 H/m

Characteristic impedance of free space

Z0

120 p = 377 Ω

Electric permittivity of space

e

8.854 × 10–12 F/m

Standard temperature

T0

17°C = 290 K

Ratio of charge and mass of electron

APPENDIX B SI System of Units Prefix

1012

Preferred Values

Multiplication Factor

tera

T

109

giga

G

106

mega

M

103

kilo

K

102

hecta

H

101

deca

D

1

—

—

–1

deci

d

10–2

centi

c

10–3

milli

m

10–6

micro

m

10–9

nano

n

10–12

pico

p

femto

f

atto

a

10

Preferred Values

Symbol

10

–15

10–18 For example 20 mega ohm = 20 × 5 microfarad = 5 ×

10–6

farads

106

ohms

480

Appendices

APPENDIX C A Page from the Data Book of Diodes Type No.

VF (V)

IF (mA)

VR (V)

IR mA

Ambient Temperature (°C)

Use

BA 100

0.5

0.1

10

9

60

OA 79

1.4

10

10

16

75

OA 81

1.3

10

10

20

60

General purpose diodes

IN 914

1.0

10

75

5

25

OA 200

0.30

0.1

50

1

125

BA 114

1.05

20

–

AA 119

206

30

–

High speed diodes

APPENDIX D Glossary Avalanche breakdown

It takes place in a reverse biased P-N junction resulting from avalanche multiplication. It destroys the Zener diode.

Aspect ratio

the ratio of width to height of picture frame.

Audio

Sound

Audio Frequency (A.F.)

Frequencies in the range 20 Hz – 20 kHz

Admittance

Reciprocal of impedance Y – 1/Z.

Attenuator

A device which reduces the signal level.

Angstrom (Å)

Unit of length (1 Å = 1010 m)

Aquadag

Graphite coating inside CRT

Afterglow

Retention of illumination by a surface after the beam is removed.

BIT

Binary digit 0, 1

Byte

A string of 8 bits

Bandwidth

The range of frequency for which output of a device is uniform and does not fall below 70.7% of the maximum output.

Appendices

Band

481

Range of frequencies: – Longwave band : 0150 – 285 kHz – Medium waveband : 550 – 1650 kHz – Shortwave band : 6 – 25 MHz

Buffer amplifier

The intermediate stage of amplification which isolates the input and output stages.

Booster

Pre-amplifier

Balanced modulator

A modulator which produces only sidebands and no carrier.

Complementary transistors P-N-P and N-P-N are complementary transistors Comparator

This is a circuit, which marks the instant at which a given waveform attains some reference level. It compares two levels.

Cascading

The technique of connecting several amplifiers such that output of one forms the input for the other.

Computer virus

It is a piece of the software, which infects programs, data or disc (computer defect).

Crowbar

It is an over voltage protection for power supplies.

Clock

An electronic device that generates periodic signals.

C Signal/Chroma/Chrominance The colour part of the video signal. CCTV

Closed Circuit TV

Composite signal

The complete TV signal.

Converter

U.H.F./V.H.F. converter device

Conductance

Reciprocal of resistance (G = 1/R)

Doping

The process of adding impurity in a pure semiconductor

dB (decibel)

This is the logarithmic unit of measurement of ratio between two powers dB = 10 log10 P2/P1.

Decoder

Demodulator

482

Appendices

D.C. amplifier

Direct coupled amplifier

Differentiating circuit

An electronic circuit whose output is the derivative of the input

De-emphasis

Reduction in amplitude of HF components of a signal

Darlington pair

Two emitter followers in cascade

Electron volt (eV)

Energy possessed by an electron (1 eV = 1.6 × 10–19 J)

Emitter follower

This is another name for common collector transistor amplifier used for impedance matching.

Emphasis

Boosting H.F. components

Etching

Removing

Extrinsic

Impure

Facsimile (Fax)

A method of transmitting pictures, printed pages or films to remote places.

Forbidden gap

This is the energy gap between conduction and valence bands of an atom.

Fermi level

It is the energy level with 50% probability of being filled if no forbidden band exists in an atom. In intrinsic semiconductor, it lies in the centre of the forbidden band.

Floppy

A storage device

Fidelity

This is the property of an amplifier with output wave exactly same as the input.

Flip flop

A bistable multivibrator

Faithful amplifier

It raises the amplitude without changing the waveshape of the input signal.

Gate

It is an electronic circuit, which has one output and more than one input.

Harmonics

The multiples of fundamental frequency f (2f, 3f, 4f.....)

Holding current (for thyristors)

The minimum current required to keep the device in ON state.

Appendices

483

h(hybrid) parameters

A group of four parameters (for analyzing transistor) having mixed units and dimensions (hybrid-means mixed).

Heat sink

Devices used for heat dissipation of power devices.

Hetrodyning

Beating/mixing

I.C.

Integrated Circuit. It is a complicated circuit on a semiconductor substrate.

Impatt diode

Impact avalanche transit time diode. It is a twoterminal P-N junction operating under reverse bias.

Instrinsic

Pure

Integrator

Its output is the integral of the input.

Inverter

D.C. to A.C. converter.

Latch

Switch

Laching current

This is the minimum anode current, which keeps a thyristor in ON state even if the gate trigger is removed. It is a little more than the holding current.

Lissajous figures

Complex figures obtained on CRO screen to calculate frequency of the input. These patterns are obtained, when the CRO is not calibrated.

Luminance

The black and white part of the video signal

Lumen

Unit of illumination, 1 lm = 1.496 × 10–10 W.

Monolithic IC

This is the most common IC used. Entire circuit is formed on a single silicon chip.

Multivibrator

It is an oscillator that generates non-sinusoidal waveforms.

Max. power transfer (impedance matching) theorem

Max power is transferred from source to the load, If impedance of both are same. It is also called “Impedance Matching Theorem”.

Monitor

It is a video display device. It is a TV receiver with a picture tube and all other components missing.

484

Appendices

Monochrome

Black and white, i.e. luminous or brightness without colour.

Matrix

A circuit which mixes different colours in right proportions.

Multiplexing

Combining two signals on one carrier.

Photon

It signifies a quantum of radiant energy.

Potential barrier

This is the rise in potential across a P-N Junction from P to N region.

Photoconductive effect

This is the absorption of light by a semiconductor resulting in the increase of the conductivity.

Piezoelectirc effect

When pressure is applied on a crystal (like quartz) it produces alternating voltages.

Phosphor

A material which produces glow when light is incident upon it.

Photoemissivity

The emission of electrons with light.

Pre emphasis

To increase amplitude before modulation.

Primary colours

Red, green and blue.

P.A. System

The public address system using microphone, amplifier and loudspeaker.

Quiescent point

This is the operating point of a transistor. It is the coordinates of IC and VCE.

Q. factor

The ratio of XL/R in a circuit.

Regenerative feedback

Positive feedback.

Ramp voltage

A sawtooth voltage.

ROSTAR

An illumination on a TV screen without any video signal.

Resonance (Electrical)

When XL = XC in a circuit, electrical resonance occurs.

Relaxation oscillator

It generates square pulse output.

Retentivity

Property to retain

Reluctance

Resistance of a magnetic material in the flow of magnetic flux.

Appendices

485

Sensistor

It is a highly doped semiconductor device having a positive temperature coefficient of resistance.

Secondary emission

When fast moving electrons strike a surface, electrons are pulled out from inside the surface, the phenomenon is called the secondary emission.

Stability factor

This is the ratio of change in IC and ICBO of a transistor.

Schmitt trigger

It is an inverting comparator with positive feedback.

Seeback effect

When two metals are joined together and their junctions kept at different temperatures, an e.m.f. is induced. The principle is used in thermocouples to measure high temperatures.

Selectivity

This is a property of a receiver to distinguish between desired and undesired signals.

Stereo

Three-dimensional

Servo (Mechanism)

An electro-mechanical device.

Steriophonic

To process low and high frequencies separately.

Thermistor

It is a bulk semiconductor device having negative temperature coefficient of resistance.

Trapatt diode

Trapped plasma avalanche triggered transit diode;

Thyristor

It is a family of solid state devices having more than two P.N junctions and which may be switched ON/OFF between two conductive states.

Threshold

Minimum

Transducer

A device which convets an energy from one form to another.

Telex

A communication system for transmission of plan messages.

Thermal runaway

This is the distraction of a transistor due to rise in temperature.

Thyratron

Hot cathode gas triode

486

Appendices

Tweeter

H.F. (High frequency) loudspeaker

Treble

H.F. control

Undamped (Oscillations)

Oscillations whose amplitude does not decrease with time.

Voltage regulation

The percent change in the output with respect to input from no load to full load.

Work function

For a metal it is the minimum amount of energy required to enable its electrons to escape out.

Wideband

Video band.

White signal

The TV signal containing red, green and blue colours in 30, 59 and 11 per cnet respectively.

Woofer

A low frequency speaker

Y Signal

Lumninance signal

Zener breakdown

In this, direct rupture of covalent bands takes place at the junction of a reverse biased diode. It does not destroy the diode.

Zoom lens

A lens with variable focal length.

APPENDIX E Abbreviations AF

Audio Frequency

AM

Amplitude modulation.

Amp

Ampere or Amplifier

A3

Double-sideband, full carrier AM.

A3A

Single-sideband, reduced-carrier AM.

A3B

Independent-sideband AM.

A3H

Single-sideband, full carrier AM.

A3J

Single-sideband, suppressed-carrier AM.

A5C

Vestigial-sideband AM.

AIR

All India Radio.

AGC

Automatic Gain Control.

Appendices

487

AVC

Automatic Volume Control.

ADM

Adaptive Delta Modulation.

BBC

British Broadcasting Corporation

BD

Breakdown

BE

Base emitter

BW

Bandwidth

Balun

Balance-to-unbalance transformer.

Bit

Binary digit.

B&W

Black & White (monochrome)

CB

Common Base or Collector Base

CE

Common Emitter or Collector Emitter

CC

Common Collector

CG

Current Gain

CT

Centre Tap or Current Transformer

Ckt.

Circuit

Ch.

Characteristic

CRT

Cathode-Ray Tube.

CRO

Cathode-Ray Oscilloscope.

COMSAT

Communications Satellite (Corporation).

CW

Continuous Wave.

Compander

Compressor-expander.

CW

Continuous Wave

CATV

Cable TV.

CCTV

Close Circuit TV.

DC

Direct Current or Direct Coupled

DPCM

Differential pulse Code Modulation.

deci

decimal digit Delta Modulation.

EB

Emitter Base

Eq.

Equivalent or Equation.

EHT

Extra High Tension

E. mail

Electronic mail.

488

Appendices

FB

Feedback or Forward Bias

FOM

Figure of Merit

FDM

Frequency-Division Multiplexing.

FDMA

Frequency-Division Multiple Access.

FHP FM Fax

Fractional Horse Power. Frequency Modulation. Fascimile

Ge

Germanium

HW

Half Wave

HF

High frequency

IC

Integrated Circuit

IF

Intermediate Frequency

I/P

Input

IR

Infra Red

ISB

Independent Sideband (modulation)

ISDN

Integrated Services Digital Network (Computer Communication).

IMU

International Measurement Unit.

LE

Line Equipment.

LASER

Light amplification by stimulated emission of radiation.

MV

Multivibrator

MODEM

Modulator-demodulator.

NASA

National Aeronautics and Space Administration.

NTSC

National Television Standards Committee.

OFC

Optical Fibre Cable.

OC

Open Circuit

OSC

Oscillator

OP

Operational or Operating

O/P

Output

PLCC

Power Line Carrier Communication.

PAM

Pulse-Amplitude Modulation.

PCM

Pulse-Code Modulation.

PG

Power Gain

Appendices

489

PA PT PM PPM

Power Amplifier or Public Address System Potential Transformer / Picture Tube Phase Modulation. Pulse-Position Modulation./Parts Per Million

PWM

Pulse-width Modulation.

PLL

Phase-Locked Loop.

PIN

P-Intrinsic-N (diode).

PSTN

Public Switching Telephone Network.

RB

Reverse Bias

RF

Radio Frequency or Ripple Factor

Radar

Radio Detection and Ranging.

SC

Short Circuit

Si

Silicon

SLV

Satellite Launch Vehicle.

SWR

Standing Wave Ratio.

SSB

Single Sideband

SIM

Subscriber Identity Module (mobile card).

Tr

Transistor

T/F

Transformer

TR

Transmit-Receiver.

TRF

Tuned Radio Frequency.

TDM

Time-Division Multiplexing.

TDMA

Time-Division Multiple Access.

USB

Upper Side Band.

UHF

Ultra High Frequency.

UV

Ultra Violet

VG

Voltage Gain

VCO

Voltage-Controlled Oscillator.

VFO

Variable-Frequency Oscillator.

VHF

Very High Frequency.

VSB

Vestigial Side Band (transmission).

WLL

Wireless Local Loop

490

Appendices

APPENDIX F: DEFINITIONS ACCURACY The accuracy of a D/A converter is the difference between the actual analog output and ideal expected output when a given digital input is applied. AGC AGC stands for Automatic Gain Control. In this technique, a control input to an amplifier is used to control its gain to keep the amplifier output constant irrespective of the changes in the input signal amplitude. AMPLITUDE MODULATION (AM) In amplitude modulation, amplitude of carrier is varied in accordance with the amplitude variation of the intelligence to be transmitted. The carrier frequency remains constant. AM DEMODULATOR It is an electronic circuit that demodulates an amplitude modulated signal fed to it. It recovers the modulating signal from the modulated signal. AND-GATE The output of an AND-GATE is logic ‘1’ only when all of its input are in logic ‘1’ state. For all other possible input combinations, the output is a logic ‘O’. ANTENNA GAIN The antenna gain is defined as the ratio of voltages produced at a given point by the actual and the hypothetical antennas. The hypothetical antenna is nothing but an omni-directional antenna. ANTENNA RECIPROCITY The phenomenon of using the same antenna for transmission as well as for reception is known as antenna reciprocity. ASPECT RATIO The ratio of width to height of the picture frame is called aspect ratio. ASIC ASIC (an abbreviation for Application Specific Integrated Circuit) is an IC belonging to LSI or VLSI category that has been designed for specific applications.

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ASYNCHRONOUS TRANSMISSION It is a transmission mode in which each information character is individually synchronised by the use of start and stop elements. ATM ATM stands for Automatic Teller Machine. An ATM is a computer terminal that allows individual customer to carry out transactions without any human intervention. AUDIO TRANSFORMER It is a transformer used to transform electrical signals spread out in the audio frequency range from one circuit to another. AVC AVC stands for Automatic Volume Control. It is used to keep the strength of the received signal at the output of the detector as constant irrespective of the strength of signal received. BAUD It is a unit of measurement of signalling speed and is equal to the number of signal events per second, or bits per second. BIT BIT stands for Binary Digit and is the smallest unit of information. It is either 0 or 1. BOOLEAN ALGEBRA It is an algebra which has been named after its inventor, George Boole. This algebra is quite similar to ordinary algebra but handles only two valued variables, known as Boolean variables, which can take on one of the two values TRUE or FALSE. BPI Stands for Bits Per Inch. On a disc, data are recorded serially on tracks. On a tape, data are recorded in parallel on several tracks. BPI tells us about the storage density of any mass storage device. BYTE A string of 8 bits is called a byte. Byte is the basic unit of data operated on as a single unit in computers.

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CAD/CAM SYSTEMS CAD/CAM stands for Computer-Aided-design and Computer-AidedManufacturing systems. These systems usually have a CRT display, a keyboard, plotter and some graphics I/O devices.

CARD READER Card reader is an input device which is used to read the punched cards and the data read is then transferred to the computer for processing.

CAT (or CT) CAT stands for Computerised Axial Tomography. It is a type of non-invasive testing that combines X-Ray techniques and computerised processing to carry out medical diagnosis.

CCD The CCD (Charge Coupled Device) is a storage element structured to offer a serial access. It can be regarded as a stretched enhancement MOSFET with a string of gates between source and drain.

CHROMATICITY The chromaticity diagram is a graphical representation in space co-ordinates X, Y, Z, of all spectral colours. The colours which are produced as a result of additive mixing of these colours based on values of the component colours are also represented in this diagram.

CHROMINANCE SIGNAL Chrominance signal is the electrical signal that represents colour information of the scene to be televised.

CITIZEN BAND The range of frequencies from 26.96 MHz to 27.41 MHz allocated to private citizens for short-range radio communication.

CO-AXIAL CABLE A co-axial cable is a two conductor transmission line with a center conductor surrounded by a braided shield. The inner conductor is supported by some form of dielectric insulation.

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CO-AXIAL RELAY A co-axial relay is terminated into some kind of RF connector. These are extensively direct RF switching operations of equipment interconnected by cables.

CLOCK It is an electronic device that generates periodic signals and is used to control the timing of all the CPU operations.

COMPANDING D/A CONVERTER Companding type D/A converters are so constructed that the more significant bits of the digital input have a larger binary relationship than to the less significant bits.

COMPANDING (PCM) Companding mean compressing the signal at the transmitter and expanding it at the receiver. The signal is modified at the transmitter by artificially making the smaller signals immune to quantizing noise by increasing the amplitude of smaller signals. The original amplitudes are restored at the receiver.

CO-PROCESSOR It is a microprocessor that can be plugged into a microcomputer to replace or work with microcomputer’s original (or main) microprocessor.

CROWBARING Crowbaring is a type of over voltage protection for the power supplies. Whenever the output voltage reaches a certain preset limit, the power supply output is short circuited by an SCR which takes the short circuit current through it.

CRT Stands for Cathode Ray Tube. It is an electronic device, upon which the information can be displayed.

DECIBEL (DB) A logarithmic measure of ratio between two powers.

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DIFFERENTIAL PCM In differential PCM, as compared to the conventional PCM, only the relative amplitudes of various samples and not the absolute amplitudes are indicated.

DELTA PCM It is similar to differential PCM. In this technique, only one bit is transmitted per sample just to indicate whether the sample in question is larger or smaller in amplitude than the immediately preceeding sample.

DEMULTIPLEXER It is exactly the reverse of a multiplexer. It receives data from one high speed line and distributes it to one of the low speed output line

DIGITIZER ‘A digitizer (or scanner) is a direct entry input device that can be moved over text, graphics, maps, drawings, pictures etc, to convert them into digital data. This is used to find coded price of books etc.

EDTV EDTV stands for Enhanced Definition Television. It employs newer encoding techniques and a compatible transmission to achieve a resolution better than the conventional system.

ENCODER Encoder is any device which modifies information into the desired pattern or form for a specific method of transmission.

EQUALIZATION Equalization is the process of reducing the effect of amplitude, frequency and/ or phase distortion of a circuit.

FACSIMILE (Or Fax) Facsimile is a method of transmitting pictures, printed pages or film to a remote location, where the transmitted information may be reproduced in the hard copy form.

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FADING Fading phenomenon is the result of electromagnetic waves reaching the receiving antenna via two different paths.

FIDELITY The fidelity of a receiver represents the variation of output with modulation frequency when the output load impedance is a pure resistance.

FLAT PANEL DISPLAY It is an alphanumeric or graphic terminal that uses an LCD or an electroluminescent type display.

FLOPPY DISKETE It is a thin, flexible circular plastic plate, coated with magnetic material and enclosed in a card board jacket. It is used as a secondary storage device.

FREQUENCY MODULATION It is the modulation technique in which frequency of the carrier varies in accordance with the amplitude of the modulating signal. The rate of change of frequency is proportional to the frequency of the modulating signal.

FM DISCRIMINATOR It is a circuit that converts signal frequency variation to corresponding amplitude variation fed at its input. It demodulates a frequency modulated signal.

FREQUENCY DIVISION MULTIPLEXING Frequency Division Multiplexing is the process of utilising the frequency scale for simultaneous transmission of more than one signal on the same carrier.

GEO-STATIONARY It is a satellite orbit where the speed of rotation of satellite is equal to the speed of earth’s rotation with the result that any spot on earth within the coverage of the satellite remains in the same relative position with respect to the satellite.

GPS The GPS (Global Positioning System) is a navigational aid implemented with a series of satellites. System can be used to fix user’s position on earth with an accuracy of few meters.

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HDTV HDTV stands for High Definition Television. It is the new generation TV technology, displaying images with much higher resolution on screen having an aspect ratio of 16 : 9 instead of 4 : 3 at present.

INTERLACED SCANNING In interlaced scanning, all the odd lines are scanned first followed by scanning of all the even lines dividing one complete frame into two parts.

ISDN ISDN stands for “Integrated Services Digital Network.” it integrates different data and voice services so as to transmit them over a single communication channel.

ISO-9000 The term ISO-9000 broadly refers to a series of international standards laid down by the International Standards Organisation.

MAXIMUM FREQUENCY DEVIATION It is the range of frequencies between the lowest signal frequency and the unmodulated carrier frequency.

MODULATION INDEX (for AM) Modulation index in amplitude modulation is defined as the ratio of peak amplitude of the modulating signal to the peak amplitude of unmodulated carrier signal. MODULATION INDEX (for FM) Modulation index in FM is defined as the ratio of maximum frequency deviation and the highest modulating signal frequency. MODEM MODEM stands for Modulator-Demodulator. It is a type of data communication equipment that converts digital data into an analog signal (and vice-versa) for transmission on telephone circuits.

MONOTONOCITY The D/A converter is said to exhibit monotonocity, if its analog output either increases or remains same but does not decrease.

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MULTIPLEXER It is an electronic device that receives input from many low speed lines and then multiplexes or concentrates and transmits a compressed data stream on a high speed and a much more efficient data transmission channel.

NOISE FIGURE Noise Figure (NF) of a receiver is a measure of the extent to which the noise appearing in the receiver output in the absence of signal is greater than the noise that could be present if the receiver were a perfect one.

OMNIDIRECTIONAL ANTENNA An antenna that radiates in or receives equally from all directions is called on omnidirectional antenna.

OVER MODULATION Modulation percentage greater than 100 percent is called over-modulation.

PERCENTAGE OF MODULATION (for FM) The percentage of modulation in an FM signal is defined as the ratio of maximum frequency deviation to the standard deviation (75 kHz) fixed for the service. POINT CONTACT DIODE Point contact diode is primarily intended for RF applications due to its extremely small internal capacitance.

POLARISATION Polarisation is the characteristic of the electromagnetic wave which gives direction of electrical component of the wave with respect to ground.

PRIMARY COLOURS Red, Green and Blue have been identified as the primary colours.

PULSE AMPLITUDE MODULATION (PAM) In a pulse amplitude modulated signal, the amplitude of the unmodulated pulse train varies in accordance with the instantaneous amplitude of the modulating signal

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PULSE CODE MODULATION In Pulse Code Modulation (PCM), the signal to be transmitted is sampled at various instants. These samples are then transmitted.

PUNCH CARDS These are cards made up of thick paper (or card-board) and have 80 columns and 12 rows on which holes can be punched. Each column refers to a character and characters are stored by punching holes into the card.

QUANTIZATION ERROR The error resulting from the resolution limit is known as the Quantization Error.

RADIX Radix of a number system is defined as the number of different symbols used in the number system.

The radix of binary number system, for instance is 2.

RASTAR Rastar is illumination on screen in the absence of any video signal.

RECIPROCITY THEOREM In a linear, active and bilateral network with a single source, the ratio of excitation to response remains unchanged when the excitation and response interchange their positions.

RESOLUTION The resolution of an A/D converter is the number of states that the full scale range is divided or resolved into. RF AND IF TRANSFORMERS These are narrow band transformers that are used to couple signals in a narrow band around a center frequency from one stage to another. SCANNING (Horizontal and Vertical) Picture elements in that line (called horizontal scanning) and then various lines may be scanned sequentially in order to cover the complete picture frame (called vertical scanning).

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SECONDARY COLOURS Yellow (= Red + Green), Cyan (= Blue + Green) and Magneta (= Red + Blue) are known as secondary colours.

SENSITIVITY The sensitivity of a communication receiver is the ability of the receiver to get a standard output.

SMART TERMINAL A smart terminal has a microprocessor and primary storage to enable the user to do some processing, before data is actually sent to the computer.

TELECONFERENCE Teleconference refers to a meeting that occurs via telephone/electronic facilities thereby eliminating need for travel.

TELECOMMUTING Telecommuting refers to working at home and communicate with the office or send data to the office via electronic machines.

TELEX Telex is a communication service used for transmission of plain messages. It makes use if teleprinters as terminals and telephone lines as communication medium.

TELETEXT Teletext service is the use of television sets for display of data, usually alphanumeric, in addition to receiving television broadcast.

TRUTH TABLE A truth table lists all possible combinations of input and the corresponding outputs of a logic system.

TURN-AROUND TIME It is the actual time required to reverse the direction of transmission from sender to receiver or vice versa when using a half duplex circuit.

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TRACKING Tracking in a communication receiver is the ability of different tuned circuits to change their resonant frequencies in the same proportion when a common shaft is rotated.

UPS-(Uninterrupted Power Supply) UPS is a power supply that is not interrupted by mains failure. When the mains power failure occurs, the UPS acts as an auxiliary power source for the equipment. The UPS system draws its power from a storage source such as batteries.

VIDEO TRANSFORMER It is a wideband transformer with a frequency response spread over frequency range from a few Hertz to a few Megahertz.

WAVEGUIDE It is a specially designed and constructed metallic pipe for transmission of microwave electromagnetic radiations. qqq