GATE Communications Book

8/10/2019 GATE Communications Book

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COMMUNICATIONS

for

ECE / IN

By

www .the gate academy. com
http://www.thegateacademy.com/http://www.thegateacademy.com/http://www.thegateacademy.com/http://www.thegateacademy.com/http://www.thegateacademy.com/http://www.thegateacademy.com/http://www.thegateacademy.com/


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Syllabus Communication

THE GATE ACADEMY PVT.LTD. H.O.: #74, Keshava Krupa (third Floor), 30 th Cross, 10 th Main, Jayanagar 4 th Block, Bangalore-11080 65700750 i f @ h C i h d W b h d

Syllabus for Communication

Random signals and noise: probability, random variables, probability density function,autocorrelation, power spectral density. Analog communication systems: amplitude and anglemodulation and demodulation systems, spectral analysis of these operations, super-heterodynereceivers; elements of hardware, realizations of analog communication systems; signal-to-noiseratio (SNR) calculations for amplitude modulation (AM) and frequency modulation (FM) for lownoise conditions. Fundamentals of information theory and channel capacity theorem. Digitalcommunication systems: pulse code modulation (PCM), differential pulse code modulation(DPCM), digital modulation schemes: amplitude, phase and frequency shift keying schemes

(ASK, PSK, FSK), matched filter receivers, bandwidth consideration and probability of errorcalculations for these schemes. Basics of TDMA, FDMA and CDMA and GSM.

Analysis of GATE Papers(Communication)

Year ECE IN

2013 9.00 4.00

2012 8.00 0.00

2011 10.00 0.00

2010 9.00 2.00

Over AllPercentage

9.00 1.5


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Contents Communication

THE GATE ACADEMY PVT.LTD. H.O.: #74, Keshava Krupa (third Floor), 30 th Cross, 10 th Main, Jayanagar 4 th Block, Bangalore-11080 65700750 i f @ h d C i h d W b h d

C O N T E N T S

Chapter Page No.

#1. Amplitude Modulation AM) 1-25 Communication Systems 1 Modulation 2-3 Amplitude Modulation 3-9 Generation on AM Signals 9-14 Assignment 1 15-18 Assignment 2 18-20

Answer Keys 21 Explanations 21-25

#2. DSBSC, SSB and VSB Modulation 26-55 Double-Sideband Suppressed Carrier (DSB-SC) 26-30 Demodulation of DSB Signals 30-32 Single Sideband Modulation 32-33 Generation of SSB Signal s 33-35 Vestigial Sideband (VSB) Modulation 35-43 Frequency Division Multiplexing 43-47

Assignment 1 48-50 Assignment 2 50-51 Answer Keys 52 Explanations 52-55

#3. Angle Modulation 56-87 Phase Modulation 56 Frequency Modulation 56-59 Generation of NBFM signal 59-67 Balanced Slope Detector 67

Foster-Seeley Discriminator 67-69 Ratio Detector 69-76 Assignment 1 77-80 Assignment 2 80-81 Answer Keys 82 Explanations 82-87

#4. Receivers 88-107 AM Receivers 88 TRF Receiver 88-89

Super Heterodyne Receiver 89-97 RF Amplifier 98


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Contents Communication

THE GATE ACADEMY PVT.LTD. H.O.: #74, Keshava Krupa (third Floor), 30 th Cross, 10 th Main, Jayanagar 4 th Block, Bangalore-11080 65700750 i f @ h d C i h d W b h d

IF Amplifier 98-100 Assignment 1 101-103

Assignment 2 104 Answer Keys 105 Explanations 105-107

#5. Noise in Analog Modulation 108-144 Noise 108 Noise Analysis of AM 109-132 Assignment 1 133-136 Assignment 2 136-139 Answer Keys 140

Explanations 140-144

#6. Digital Communications 145-195 Introduction 145-152 Baseband Data Transmission 152-155 Pulse Digital Communication 155-157 Differential Pulse Code Modulations 157-159 Electrical Representation of Binary Data 159-162 Bandpass Data Transmission 162-166 Differential Phase Shift Keying 167-179

Assignment 1 180-183 Assignment 2 183-187 Answer Keys 188 Explanations 188-195

Module Test 196-216 Test Questions 196-206 Answer Keys 207 Explanations 207-216

Reference Books 217


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Chapter 1 Communication

THE GATE ACADEMY PVT.LTD. H.O.: #74, Keshava Krupa (third Floor), 30 th Cross, 10 th Main, Jayanagar 4 th Block, Bangalore-11080 65700750 i f @ h d C i h d W b h d P 1

CHAPTER 1

Amplitude Modulation AM)

Communication Systems

Irrespective of the form of communication process being considered, there are three basicelements to every communication system, namely, transmitter, channel and receiver. Thetransmitter is located at one point in space, the receiver is located at some other pointseparate from the transmitter and the channel is the physical medium that connects them. Thepurpose of the transmitter is to convert the message signal produced by the source ofinformation into a form suitable for transmission over the channel.

Generalized Block Diagram

However, as the transmitted signal propagates along the channel, it is distorted due to channelimperfections. Moreover, noise and interfering signals are added to the channel output, withthe result that the received signal is a corrupted version of the transmitted signal. The receiverhas the task of operating on the received signal so as to reconstruct it to a recognizable form ofthe original message signal.

Normally used communication channels are twisted pair, coaxial cable, fiber optic cable andfree space.

Primary Communication Resources

In a communication system, two primary resources are employed: transmitted power andchannel bandwidth. The transmitted power is the average power of the transmitted signal.The channel bandwidth is defined as the band of frequencies allocated for the transmission ofthe message signal. A general system design objective is to use these two resources asefficiently as possible. In most communication channels, one source may be considered moreimportant than the other. Therefore, communication channels are classified as power limitedor band limited.

When the spectrum of a message signal extends down to zero or low frequencies, we definethe bandwidth of the signal as that upper frequency above which the spectrum content of thesignal is negligible and therefore unnecessary for transmitting information. The importantpoint is unavoidable presence of noise in a communication system. Noise refers to unwantedwaves that tend to disturb the transmission and processing of message signals in acommunication system. The source of noise may be internal or external to the system.

A quantitative way to account for the effect of noise is to introduce signal to noise ratio(SNR) as a system parameter. We may define the SNR at the receiver input as the ratio of theaverage signal power to the average noise power, both being measured at the same point.

Transducer Modulator orTransmitter

Demodulatoror Receiver Transducer

Channelm(t)

O/P


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Modulation

Modulat ion is defined as the process in which some characteristic parameter of a highfrequency carrier is varied linearly with which contains information message signal.

Generally, the carrier is represented by c (t) = A c cos (2f ct + ).

The three characteristic parameters of the carrier are A c (peak amplitude), f c (frequency) and (phase). Accordingly, the three type of modulation are

(1) Amplitude modulation (A.M)(2) Frequency modulation (F.M)(3) Phase modulation (P.M)

In frequency domain, modulation is defined as the process of translating the spectrum of asignal from low frequency region to high frequency region

Modulator converts

(1) Low frequency signal to a high frequency signal,(2) A wide band signal into narrow band signal,(3) A baseband signal into band pass signal.

Need for Modulation1) To reduce the antenna height

The antenna height required to transmit a signal depends on operating wavelength. Forefficient radiation, the minimum height should be /10. To transmit a low freque ncy signalantenna height required is very high. To reduce the antenna height, the low frequencysignal is converted into a high frequency signal by modulation.

2) For multiplexing of signalsMultiplexing allows transmission of more than one signal through the samecommunication channel. By modulation it is possible to allot different frequencies tovarious signals so that there is no interference.

3) To reduce noise and interferenceSometimes the effect of noise will be more at some frequencies and the effect will be lessat some other frequencies. lf the effect of noise is more at some particular frequency, bymodulation the spectrum is shifted to higher frequencies where the effect of noise is less.

4) For narrow banding of signalsNot only the antenna height, the antenna dimensions also depends on operatingwavelength to transmit a wideband signal. Single antenna will not be sufficient becausethe ratio between the highest frequency to lowest frequency is very much greater thanone.Modulation converts a wideband signal into a narrow band signal whose ratio betweenhighest frequency to lowest frequency is approximately one and single antenna will besufficient to transmit the signal.

>> 1 Wideband signal

1 Narrow band signal


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Example

Fig. 1.1 Spectrum is shifted by MHz using modulator

1

5) To overcome equipment limitation

The design of a communication system may be constrained by the cost and availability ofhardware, whose performance often depends upon the frequencies involved. Modulationpermits the designer to place a signal in some frequency range that avoids hardwarelimitations. A particular concern along this line is the question of fractional bandwidth,defined as the, absolute bandwidth divided by the center frequency. Hardware costs andcomplication are minimized if the fractional bandwidth is kept 1 to 10 percent. Fractionalbandwidth considerations account for the fact that modulation units are found in receivers

as well as in transmitters.

Amplitude Modulation AM)

In A.M. the amplitude of carrier wave c(t) = cos 2f t is varied linearly with the amplitude ofmessage signal.

0

Ac[1+ ]

Ac

Ac[1- ]

0

-Ac[1- ]

-Ac

-Ac[1+ ]

m t) 0< 1

S t)

1 MHz + 300 Hz 1 MHz+3.5 KHz00 Hz 3.5 KHz

V

f

V

f


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Time domain equation of AM

The standard form of AM wave is defined by,

S(t)= cos 2f t+ k m(t) cos 2f t = [1+ k m(t)]cos2f t .(1.1)

The amplitude of the carrier before modulation is and the amplitude of the carrier aftermodulation is [1+ m(t)] (After modulation the carrier amplitude depends on the messagesignal), k =amplitude sensitivity of the modulator.

Envelope of AM wave S(t) is given by,

a(t) = 1 + m(t)| ..(1.2)

The maximum absolute value of m(t) multiplied by 100 is referred as percentagemodulation.

When | m(t)| 1 for all t, the term [1 + m(t)] is always non-negative, on the other handwhen m(t)| >1 for all t, the term [1 + m(t)] will not be always non- negative and the AMwave is said to be over modulated and it is said that wave suffer from envelope distortion.

0t

0

0

0

t

t

tMessage

AM signal

e -t

Fig. 1.2 Amplitude Modulation


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By taking Fourier transform of both side of equation (1.1) the AM wave in frequency domain is,

S(f) = [(f -f )+(f+f ) ] + [M(f- f )+M(f+ f )]

0

C)

t t

Ac

0

-Ac

>1

-2Ac

Overmodulated

signal

-2Ac

Fig. 1.3

0

(B)

t

Ac

0

-Ac

=1

t

0

A)

t

Ac[1+ ]

Acc[1- ]

0t


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Fig. 1.4 a) Spectrum of message signal b) spectrum of AM wave

The condition > W ensures that the side bands do not overlap otherwise, the modulated waveexhibits spectral overlap and therefore, frequency distortion.

Bandwidth of the A.M signal = 2W= 2 message bandwidth,Bandwidth of USB=WBandwidth of LSB=W

Single Tone Modulation of AM

When the message contains single frequency or single tone, the modulation is called single tonemodulation. Message signal, M(t)= cos (2f t)

Spectrum of message is given by, M(f) = [(f -f ) + (f + f )]

The time domain equation of AM for single tone modulation is,

S(t) = cos (2f t) + cos (2f t).c os (2f t)

-fm fm f

0Fig. 1.5 Spectrum of message signal

fc fc+wc-wfc+w-fc-w -fc

Ac/2 ( )

S f)

USBLSB

2 wb)

f

M f)

M 0)

0 ww fa)


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GATE Communications Book

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