Communications : Instrumentation Engineering, THE GATE ACADEMY

COMMUNICATIONS

for

ECE / IN

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Syllabus Communication

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Syllabus for Communication

Random signals and noise: probability, random variables, probability density function,

autocorrelation, power spectral density. Analog communication systems: amplitude and angle

modulation and demodulation systems, spectral analysis of these operations, super-heterodyne

receivers; elements of hardware, realizations of analog communication systems; signal-to-noise

ratio (SNR) calculations for amplitude modulation (AM) and frequency modulation (FM) for low

noise conditions. Fundamentals of information theory and channel capacity theorem. Digital

communication 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 error

calculations 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 All

Percentage

9.00% 1.5%

Contents Communication

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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

Contents Communication

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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

Chapter 1 Communication

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CHAPTER 1

Amplitude Modulation (AM)

Communication Systems

Irrespective of the form of communication process being considered, there are three basic elements to every communication system, namely, transmitter, channel and receiver. The transmitter is located at one point in space, the receiver is located at some other point separate from the transmitter and the channel is the physical medium that connects them. The purpose of the transmitter is to convert the message signal produced by the source of information 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 channel imperfections. Moreover, noise and interfering signals are added to the channel output, with the result that the received signal is a corrupted version of the transmitted signal. The receiver has the task of operating on the received signal so as to reconstruct it to a recognizable form of the original message signal.

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

Primary Communication Resources

In a communication system, two primary resources are employed: transmitted power and channel 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 of the message signal. A general system design objective is to use these two resources as efficiently as possible. In most communication channels, one source may be considered more important than the other. Therefore, communication channels are classified as power limited or band limited.

When the spectrum of a message signal extends down to zero or low frequencies, we define the bandwidth of the signal as that upper frequency above which the spectrum content of the signal is negligible and therefore unnecessary for transmitting information. The important point is unavoidable presence of noise in a communication system. Noise refers to unwanted waves that tend to disturb the transmission and processing of message signals in a communication 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 the average signal power to the average noise power, both being measured at the same point.

Transducer Modulator or

Transmitter

Demodulator

or Receiver Transducer

Channel m(t)

O/P



Modulation

Modulation is defined as “the process in which some characteristic parameter of a high frequency carrier is varied linearly with which contains information message signal”.

Generally, the carrier is represented by c (t) = Ac cos (2πfct + ϕ).

The three characteristic parameters of the carrier are Ac (peak amplitude), fc (frequency) and ϕ (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 a signal 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 Modulation (1) To reduce the antenna height

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

(2) For multiplexing of signals Multiplexing allows transmission of more than one signal through the same communication channel. By modulation it is possible to allot different frequencies to various signals so that there is no interference.

(3) To reduce noise and interference Sometimes the effect of noise will be more at some frequencies and the effect will be less at some other frequencies. lf the effect of noise is more at some particular frequency, by modulation the spectrum is shifted to higher frequencies where the effect of noise is less.

(4) For narrow banding of signals Not only the antenna height, the antenna dimensions also depends on operating wavelength to transmit a wideband signal. Single antenna will not be sufficient because the ratio between the highest frequency to lowest frequency is very much greater than one. Modulation converts a wideband signal into a narrow band signal whose ratio between highest frequency to lowest frequency is approximately one and single antenna will be sufficient to transmit the signal.

>> 1→ Wideband signal

1→ Narrow band signal



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 of hardware, whose performance often depends upon the frequencies involved. Modulation permits the designer to place a signal in some frequency range that avoids hardware limitations. A particular concern along this line is the question of fractional bandwidth, defined as the, absolute bandwidth divided by the center frequency. Hardware costs and complication are minimized if the fractional bandwidth is kept 1 to 10 percent. Fractional bandwidth 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 2πf t is varied linearly with the amplitude of message signal.

0

Ac[1+μ]

Ac

Ac[1-μ]

0

-Ac[1-μ]

-Ac

-Ac[1+μ]

m(t) 0< μ 1

S(t)

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

V

f

V

f



Time domain equation of AM

The standard form of AM wave is defined by,

S(t)= cos 2πf t+ k m(t) cos 2πf t = [1+k m(t)]cos2πf t …….(1.1)

The amplitude of the carrier before modulation is and the amplitude of the carrier after modulation is [1+ m(t)] (After modulation the carrier amplitude depends on the message signal),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 percentage modulation.

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

0

t

0

0

0

t

t

t

Message AM signal

e-t

Fig. 1.2 Amplitude Modulation



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

S(f) =

[δ(f-f )+δ(f+f ) ] +

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

0

(C

)

t t

Ac

0

-Ac

μ>1

-2Ac

Over

modulated

signal

-2Ac

Fig. 1.3

0

(B)

t

Ac

0

-Ac

μ=1

t

0

(A)

t

Ac[1+μ]

A

c Ac[1-μ]

0

t

μ<1

Ac[1-μ]

-Ac[1+μ]



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 wave exhibits spectral overlap and therefore, frequency distortion. Bandwidth of the A.M signal = 2W = 2× message bandwidth, Bandwidth of USB=W Bandwidth of LSB=W

Single Tone Modulation of AM

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

Spectrum of message is given by, M(f) =

[δ(f-f ) + δ(f +f )]

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

S(t) = cos (2πf t) + cos (2πf t).cos (2πf t)

-fm fm f

0

Fig. 1.5 Spectrum of message signal

fc fc+w fc-w -fc+w -fc-w -fc

Ac/2

( )

S(f)

USB

LSB

2 w (b)

f

M(f)

M(0

)

0 w -w f

(a)

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Communications : Instrumentation Engineering, THE GATE ACADEMY