Optics and Modern Physics for JEE (Advanced) [3 ed.] 9353503752, 9789353503758

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Communication Systems

9 NTRODUCTION e

term

communication

refers to the

transmitting, receiving, ormation by electronic

comm

ocessing of

means.

nd

SASIC COMMUNICATION SYSTEM communication

system consists of an information source,

nsmitter, a link, and a receiver. Source of

Transmitter-

information

nformation:

The

-

Receiver

(Channel)

Output

3. Facsimile

signal

exact

Transmitter: In radio transmission, the transmitter consists of a nsducer, modulator, amplifier, and transmitting antenna. Transducer: Converts sound signals into electric signal. Modulator: Mixing of audio electrie signal with high frequency radio wave. Amplifier: Boosting the power of modulated signal. Signal is radiated in the space with the aid

Antenna:

an

Antenna Amplifier

mmunication channel: The function of communication Ver. The communication channel is also called transmission

edium or link. channel refers to the frequency range allocated to a

ATicular service or transmission. eceiver: A consists

receiver the following components: Ckup antenna: To pick theof signal

odulator: To separate out the audio signal from the

modulated signal

plifier: To boost up the weak audio signal

Transducer To convert back audio signal in the form of clectri pulses into sound waves. Antenna

Output Je-

modulator

Amplifier

Receiver

reproduction

of

a

(FAX):

document

or

This

picture

involves which

are

Classification according to the mode of transmission: 1. Analog communication: The communication system which makes use of analog signals is called analog communication system.

2.

Digital communication: In this system, digital signals are used.

Classification according

to the 1. Line communication

transmission channel:

Classification 1.

according to the type of modulation: Amplitude modulation (AM)

2.

Frequency modulation (FM) (PAM)

5. Pulse time modulation (PTM) 6. Pulse code modulation (PCM)

nel is to carry the modulated signal from transmitter to

cal

static.

transmission

3. Phase modulation (PM) 4. Pulse amplitude modulation

Transmitter

m

source:

2. Space communication of

antenna.

a

information

1. Speech transmission

idea/message

Modulator

COMMUNICATION SYSTEM

Communication systems can be classified according to the nature of information or mode of transmission or types of transmission channel or types of modulation. Classification according to the nature of 2. Picture transmission

Link

that is to be conveyed is nomation. The message may be individual one or a set off ages. It may be a symbol, code, group of words, or any zdecided unit.

Source of nformation

TYPES OF

Transducer

ANALOG AND DIGITAL SIGNALS In communication system, a signal means a time, varying electrical signal containing informations. Analog signals: It is a continuous wave form which changes smoothly over time. Such signals can be easily generated from the source of information by using an appropriate transducer, e.g.. pressure variations in the sound waves can be converted into corresponding eurrent or voltage pulses with the help

of a microphone. A simple analog signal

is

represented by

a

sine

wave.

9.2

Optics

and Modern Physics MODULATION

The frequency of analog signals associated with speed or music varies over a range between 20 Hz to 20 kHz.

Pulse wave

Continuous wave

modulation

modulation

Amplitude|Frequency

Phase

modulationmodulationmodulation

(PM)

(FM)

(AM)

Frequency

Pulse

BW= f-f The

range

over

which the frequencies

of a signal

signal supplied by

the

source

(PTM)

Pulse width modulation

of

modulation (PCM)

odulation

(PAM)

vary is

called bandwidth. The term base band designates the band of frequencies representing the

Pulse timePulse code

amplitude modulation

(PWM)

Pulse

position

modulation

(PPM)

informat1ion.

consists of two or

more

waves

of different

A signal frequencies known as a complex analog signal.

NEED FOR MODULATION A sound wave (20 Hz to 20 kHz) cannot be transmitted directlv from

A

one

place to another for the following reasons.

Height of Antenna For efficient radiation and reception, the height of transmitting and receiving antennas should be comparable to a quarter of

AA

15 kHz it is 5000 m ftoo wavelength of the frequency used. For The m. 75 is energy radiated from an and for 1 MHz it

AAA A

Digital signals:

large)

antenna is

practically

zero, when the

frequency of the

signal to

be transmitted is below 15 Hz.

digital signal

is

a

discontinuous function of

time. It has only two voltage levels, i.e., either low (0) or high is called (1). Either of 0 andl is known as bit. A group of bit code combinations, four A byte comprising 2 bits can give

byte.

i.e., 00, 01, 10, and 11. Pulse duration

Detecting Signals All audible signals are in the range of 20 Hz to 20 kHz So the in air. It will signals from all sources remain heavily mixed up be very difficult to differentiate or detect the broadcast signal at a lowthe receiving station. Thus, modulation is necessary for distant a to place. frequency signal especially when it is to be sent itself. the in out not die information way so that the may

Pulse

amplitude

Source of

O

Originalnrormaton Baseband SIgnal

dator Modulator

signal

number of bits The number of code combination increases with number of bits in a byte. in a byte is given by N=2", where x describe a The number of binary digits (bits) per second, which rate is expressed in bits per digital signal is called its bit rate. Bit

T Modulated signal

Carrier wave

=

Oscillator

second (bps). AMPLITUDE MODULATION

MODULATION

to be transmitted are usually of low Digital and analog signals be transmitted as such. These frequency and hence cannot carriers are carrier to be transported. These signals require some The process

known

as

carrier

waves

or

high-frequency signals.

over the highlow-frequency (LF) signal Various types of is known as modulation. signal (HF) frequency modulations are shown in figure.

of placement of

a

carrier wave n The process of changing the amplitude of a (AF) SIgnai accordance with the amplitude of the audio frequency or AM frequency In tn Is known as amplitude modulation (AM). of modulated carrier wave remains unchanged. The amplitude modulanng the amplitude of wave is varied in accordance with wave.

Communication Systems

9.3

side band frequencies).

G+S,): Upper side band (USB) frequency .-): Lower side band (LSB) frequency frequencies

Side band

(b)

HF

are

frequency.

carier wave

Bandwidth in AM side of the carrier

bandwidth

generally

close to the

on

either

equal frequency interval

So,

The two side bands lie

wave:

frequency

at

carrier

+)-0-S)= 21

,

E

(b) LF modulating

E+mE.Sin o, -m) -2m

min

(+m) Frequency

Power in AM waves: Power dissipated in any circuit, (c) AM wave

adulation

mer wave

P

index: The ratio of change of amplitude of the wave is called to the amplitude of original carrier or modulation modulation factor or degree of

modulation

Change in amplitude

Amplitude

of carrier

wave

of original carrier wave

JR. Hence, ms

Carrier power, Pe

=

(E,2E2 2R

R

Total power of side bands,

P. Psb

kEm

(m,E /2N2)

(m,E./22)

R

E in the

change zrek is a factor which determines thethe of modulating signal. If litude for a given amplitude E maximum

Total power of AM wave, P, =P. *Pb

**

=1, then

and P

min

mE 4R

R

1 2R

2

m/2 m

max Tmin ta carier wave is modulated

sine waves, the total

by several

zxdnlated index m, is given by

will occur when Maximum power in AM (without distortion) m= 1, i.e., P = 1.5P, = 3P

IfT= unmodulated current and I,=

m,= ms+mz +mg + tage equation for AM

wave:

CaTier wave and modulating

Suppose voltage equations and e, =E, COS !

=

the instantaneousS

instantaneous voltage

ular velocity at carrier frequency f, e UOdulating, E amplitude ofmodulating wave, and gplar velocity of modulating frequency J

27)

olage equation for AM wave is

eE sin E+em)sin o (E+E

or modulated current.

wave are

mE, sin@1, where e, is ge of carrier wave, E, amplitude of carrier wave, o=27t)

*ESin 01

total

Limitations of Amplitude Modulation

.

Noisy reception

2. Low eficiency 3. Small operating range 4. Poor audio quality FREQUENCY MODULATION of a carrier wave in The process of changing the trequency accordance with the audio frequency signal is known as trequency

sin o,/)sin»

modulation.

ccos(o, E, sinot+m,E cos(o, - , - 2

+0

bOve AM wave indicated that the AM wave is equivalent tumation of three sinusoidal waves, one having amplitude d the other two having

amplitude m .

Me and frequencies: An AM wave contains three frequencies:

calledarTier c frequency), (. +f), and (

-S,) (latter two called

There is always a Audio quality of AM transmission is poor. need to eliminate amplitude sensitive noise. This is possible a need to keep the if we eliminate amplitude variation. (i.e., of the carrier constant). This is precisely what we

amplitude

do in FM. In FM the overall amplitude of FM

all times. In FM, the total

wave

remains constant at

transmitted power remains constant.

9.4

Optics

and Modern Physics

Deviation ratio: The ratio of maximum permitted freat audio deviation to the maximum permitted frequency is known as deviation ratio. Thus,

equency

wwwwwwv

Deviation ratio =

(a) HF carrier wave

(AJ )max

(mmax

PERCENT MoDULATION

The ratio of actual frequency deviation to the maximu allowed frequency deviation is defined as percent modulation (b) LF modulating wave

Thus, Percentmodulation, m

(A )actual (A )max

PULSE MODULATION In pulse modulation, the carrier wave is in the form of pulses

() FM wave

Mentioned below are the types of pulse modulation. Frequency deviation: The maximum change in frequency from mean value (f) is known as frequency deviation. This is also the change or shift either above or below the frequency f and is called as frequency deviation.

lses.

Pulse amplitude modulation (PAM): The amplitude of the pulse varies in accordance with the modulating signal. Pulse width modulation (PWM): The pulse duration varies in accordance with the modulating signal.

d

=Umaxf)=f -Jmin =kr

Pulse position modulation (PPM): In PPM, the position of the pulses of the carrier wave train is varied in accordance with the

27

where k, is the constant of proportionality. It determines the maximum variation in frequency of the modulated wave for a

instantaneous value of the modulating signal.

given modulating signal. Carrier swing (CS): The total variation in frequency from the lowest to the highest is called the carrier swing, CS 2xAf i..

(a) Unmodulated pulse

Frequency modulation index (m): The ratio of maximum frequency deviation to the modulating frequency is called modulation index.

mm,-

(b) Modtlating signal

maxJ_ -Iminb m

hinnD

Jm

Frequency spectrum: FM side band modulated signal consist of infinite number of side bands whose frequencies are

(c) Pulse amplitude modulation (PAM)

hnnn The number of side bands

D

(d) Pulse width modulation (PWM)

depends on the modulation index m

Original pulse positions

(e) Pulse position modulation (PPM) e-2fm)fe - f m ) f (

+ fm) t + 21m) Frequency

PULSE CODE MODULATION In FM signal, the information (audio signal) is contained in the side bands. Since the side bands are separated from each other by the frequency of modulating signalf, so

Bandwidth= 2n xf, where n = number of significant side band pairs

Sition

pulse amplitude, pulse width, and pulse pos digital modulations are not completely digital. A completey modulation is obtained by pulse code modulation The

An analog signal is pulse code modulated by following

operations:

ree

ommuncaulon

ystems

9.5

6.91

4.2 v

AM

Wave inpu

R

|14v al Analog

(b) Sampling at three

voltage signal

Output

Q

discrete time

Um On.

100 001 111

7V

In the actual circuit the value of RC is chosen such that

1f RC, where f is the frequency of carrier signal.

(d) Coding

(c) Q u a n t i z a t i o n

DATATRANSMISSION AND RETRIEVAL es.

inC: It is the process of generat1ng pulses of zero width mpnt

amplitude equal to the instantaneous amplitude of the signal.

The number

of samples taken per second is called

alog

ampling r a t e .

The term data is applied to a representation of facts, concepts, or instructions suitable for communication, interpretation, or processing by human beings or by automatic means. Data in

most cases consist of pulse type of signals.

antization: The process of dividing the maximum amplitude

analog voltage signal into fixed number of levels is he lbd quantization. For example, amplitude 5 V of the analog a

age signal divides into six quantization levels, viz., 0, 1, 2,

(b) Modulating source code

(a) Carrier wave

45.

Pulses having amplitude between -0.5 V

and

V

0.5

approximated (quantized) to a value of 0 V, amplitude

ween 0.5 V and 1.5 V are approximated to a value of 1 V,

(c)

ad so on.

ASK modulated

wave

(d)

FSK modulated

wave

Coding: The process of digitizing the quantized pulses according some code is called coding. For example, consider that voltage

mplitude of an analog signal varies

between 0 and 7 V.

(e)

PSK modulated

wave

The PCM signal is a series of ls and Os. Following three

DEMODULATION e process of extracting the audio signal from the modulated

modulation techniques are used to transmit a PCM signal: Two different amplitudes of the carrier represent the two binary values of the PCM signal. This

RVe is known as demodulation or detection. The wireless

Amplitude shift keying (ASK):

sgnals consist of radio frequency (high frequency) carrier wave odulated by audio frequency (low frequency). The diaphragm

method is also known as on-off keying (OOK):

ua elephone receiver or a loud

speaker

cannot vibrate with

high

quency. So it is necessary to separate the audio frequencies 0n the radio frequency carrier wave.

Simple demodulator cireuit: A diode can be used to detect

amplitude modulated (AM) wave. A diode caly acts as a rectifier, i.e., it reduces the modulated carrier ave into positive envelope only. An AM wave input is shown in figure. It appears at the output

demodulate

1: Presence of carrier of same constant amplitude.

0: Carrier of zero amplitude. Frequency shift keying (FSK): The binary values of the PCM signal are represented by two frequencies. 1: Increase in frequency. 0: Frequency unaffected.

an

diode across PQ as a

rectified

wave

(since

a

diode conducts

uy in the positive half cycle). This rectified wave after passing

ough the RC network does not contain the hroug

er component. nodulated wave.

radio frequency

Instead, it has only the envelope

of the

Phase shift keying (PSK): The phase of the carrier changed in accordance with modulating data signal.

wave

is

1: Phase changed by T.

0: Phase remains unchanged. The analog signal is sampled by the sampler. The sampled

quantized. The encoder codes the quantized the PCM pulses according to the binary codes. After modulating modulated the signal issignal (by ASK, FSK, or PSK method),

pulses

are

then

of bits. then transmitted into free space in the form

9.6

Optics and Modern Physics Carrier oscillator

Information

Source

Channel

=

10x 10 Hz

IFNis the maximum electron density, then/,= 9,/N max

N encoder

Output

10 MHz

Sol. Given =

(10x10 =1.2x 10 m 81 81

=

max

Modulator

encoder

Satellite communication

Communication satellite

Transmitter

h 35800 km

Transmitting

Receiver

Receiving antenna

antenna

Infomation Source Output

Demodulator

Channel decoder

decoder

or

detector

ILLUSTRATION 9.1 of range of S0 km. What is the height x 10m. the TV transmission tower? Radius ofearth R, 6.4 ATV transmitter has

a

=

Sol. Here d= 50 km Height of TV

50

=

x

10

m

the entire globe of earth.

transmission tower

(50x 10

REMOTE SENSING Remote sensing is the process of finding information about an object at a distance without going near it. It is achieved by launching satellites in sun synchronous orbits.

= 195.3 mn

2x6.4x10 ILLUSTRATION 9.2

A TV tower has a height of 100 m. How much population is covered by the TV broadcast if the average population density around the tower is 1000 km

Sol

Here h

=

The height of communication satellite from earth's surface is 35,800 km. These communication satellites are required to cover

100 m,

R,

=

(radius ofearth 6.4 x 10° m). 6.4

x

10

m

Average population density = 1000 km= 1000 x (1000 m) The TV transmission range, d=

10 m2

2hR

LINE COMMUNICATION The line communication is conventional and it is used to transmit electricity and telephone signals through a pair of wires. The most used transmission lines are (i) a pair of conducting wires and (ii) a coaxial cable. It consists of two concentric conductors separated by an isolated material. The equivalent circuit of a transmission line consists of a sequence of inductance L and capacitance C.

Therefore, area of TV transmission range A = nd2= T 2h R, = 21 hR,

=2 x3.14 x 100 x 6.4x 10 m2=4.0x 10' m2 Therefore, population covered by TV broadcast =

4.0x

10

x

103

=

4.0 x

10

(b) Usual lines

(a) Twisted pair of wires L

L

L

L

ILLUSTRATION 9.3 On a particular day, the maximum frequency reflected from the ionosphere is 10 MHz. On another day, it was found to increase to 11 MHz. Calculate the ratio of maximum electron (c) Equivalent circuit

densityofthe ionosphere on the two days. Point out a plausible explanation for this.

OPTICAL COMMUNICATION In optical communication, communication

SoL f 9N max

is

through

cari

optical signals. Optical signals cannot be transmitted througn

.

81

(Mma10

12-1.21 100

The variation may be due to atmospheric disturbances.

ILLUSTRATION 9.4 Frequencies higher than 10 MHz are found not to be reflected by the ionosphere on a particular day at a place. Calculate the maximum electron density of the ionosphere.

cables. The essentials of optical communication are (i) Light source and nmodulator: Light sources are LE diode lasers. (1) Communication channel: The optical fibres of sma have i diameters 10% m based on total internal refiection =

been discovered. (1) Optical signal detectors: These are photodiode, Avalancue

photodiode.

Communcdon ystems communica cation are s

band width has

large

channel

carrying

T00% communication optical

is shown in the

figure.

Optical fiber

Output

Receiver

Light source

of light amplification of stimulated . It is based on stimulated emission. A laser 1ofradiation. resonant

nd

one

totally

and

one

atoms are

in excited metastable state

is achieved. When a photon of is incident on the active material, it stimulates inversion

population

hot

reguency

Finally 1ofphotons. Fin: tional coherent

CONCEPT LA sky

an intense, monochromatic, highly

laser beam

is available.

EXERCISE

APPLICATIDN

refraction cm) 60

frequency 55 MHz is incident on of earth's atmosphere at 45°. The angle of electron/ is (electron density forD-region is 400

(c) 150 w

(d) 200 W

and

a

modulating signal 10 kHz (b)

is

10

will be

(a) 4.5 kHz

(b) 54 kHz

(c) 27 kHz

(d) 15 kHz

rate of 10. The bit rate for a signal, which has a sampling used is been have levels 8 kHz and where 16 quantisation

(a) 32000 bits/sec

(b)

(c) 64000 bits/sec

(d) 72000 bits/sec

16000 bits/sec

(a) 70%

(b) 65.4%

(c) 94.4%

(d) 25.5%

wave wave and medium 12. In a radio receiver, the short of coils but stations are tuned by using the same capacitor

(b) 45

(d) 15

c)30°

100 w

50%. What 11. An amplitude modulated wave is modulated to of the side is the saving in power if carrier as well as one bands are suppressed?

9.1

wave with a

D-region

(b)

voltage kept kHz, then the new deviation

The active substance has metastable

reflecting face.

most of the

cavity having

each side

(d) 25 (c) 20 9. A 500-Hz modulating voltage fed into an FM generator produces a frequency deviation of 2.25 kHz. If amplitude to 6 constant but frequency is raised of the is

hhreviation of a.

(a) 50 W

Swing of 200 kHz (a) 5

abbr

consists

by

8. The modulation index of an FM carrier having a carrier

display

modulator

the power transmitted

modulation,

band is

Signal security

SER

(d) 1000 W

.The total power content of an AM wave is 900 W. For

capacity Lowtransmission losse sses

matic

(b) 700 W

(a) 500 W (c) 750 W

of optical

jide c h a n n e

different inductance

circuit consist of R in a diode AM-detector, the output 100 kHz is to be k and C= 10 pF. A carrier signal of detected. Is it good? =

L, and L

(a)L, (c) L,=

respectively, then

b)L,