WBUT Question Papers EC Analog Communication B Tech Sem Forth 2010

WBUT Question Papers EC

Analog Communication B Tech Sem Forth 2010


Time Allotted : 3 Hours

Full Marks i 70

The figures in the margin indicate Jull marks.

Candidates are required to give their answers in their own words

as far as practicable.

GROUP – A (Multiple ChoiceTyp® Questions)

  1. Choose the correct alternatives for any ten of the following :10 x 1 = 10

ill) The Fourier transform of real valued time signal has

a)            odd symmetry

b)            even symmetry

c)             conjugate symmetry

d)            no symmetry.

iv)           The most suitable method for detecting a modulated singal ( 2-5 + 5 cos wmt) cos wc t is

a)            Envelope detector

b)            Synchronous detector

c)             Ratio detector                                            .

d)            both (a) and (b).

v)             In a commercial radio receiver, a PLL can be used to demodulate

a) an AM Signal                         b) aPCMsign&l

c)    an FM signal            d) a PM signal,

vl) The main advantage of TDM over FDM is that it

a)            needs less power

b)   needs less bandwidth (                                                        .

c)             needs simple circuitry

d)            gives better SNR.

vii)         Flat-top sampling leads to

a)            an aperture effect

b)            aliasing

c)             loss of signal                                                                 t

d)            none of these.

viii)       Quantization noise occurs In

a) PAM                                       b) PWM

c) DM                                         d) none of these.

ix)           Companding Is used In PCM to

a)            reduce bandwidth

b)            reduce power

c)             Increase SNR

d)            get almost uniform SNR.

x)             The aperture effect in flat top pulses is reduced using

a)            Predictor                                    b) Integrator

c)             Equalizer                                    d) Compander.

xl) SNR in dB for PCM linear quantization with n as no. of bits is   ,

a)                                                                                               n2 / 12            b) 6 ( 1 + n)

c)                                                                                                (6-8 + 4n)        d) ( 4-8 + 6n).

 xii) IF frequency for FM receiver is

a)    10-7 MHz           b) 12-7 MHz

c)    13-71 MHz    d) 10-3 MHz. ,

xttU . Zero crossing detectors are used to detect

a)    SSB-SC        b) DSB-SC

c)  FM    d) none of these.

xiv)       An ideal ramp signal is a/an

a)            energy signal

b)            powersignal

c)            both of these

d)            .       none of these. .

xv)          Bandwidth required for PM is

a)            same as FM signal

b)            greater than FM signal /

c)             less than FM signal

d)            less than SSB-SC signal.

Answer any three of the following. 3 x 5 « 15

2; A single tone FM signal is given by ■

e FM * 10 sin ( 16« x 10 6t + 20 sin 2n x 10 3t ) volts. Determine the modulation index modulating frequency, frequency deviation and carrier frequency^

  1. Explain the elements of a communication system with suitable block diagram.

I          ….    ‘

  1. What is a slope detector ? What are the problems of slope detectors and how is it overcome using a balanced detector ?                                                                    2 + 3
  2. Explain pre-emphasis and de-emphasis in FM.
  3. Determine the Fourier transform erf x (t):

x«GROUP-C (Long Answer Type Questions)

Answer any three of the following. 3 x 15 * 45

  1. a) What is meant by autocorrelation ? Explain with power

expressions.                                                                      ‘• 3

b)      State and prove time convolution theorem.                                                                                                            3

cj Find the transfer function of a system for distortionless ■ transmission,   3

d)            Given transfer function for a channel with ideal amplitude response and non-ideal phase response :

I H( co) |=1

® <*>)■- <o t0 – fcsln co T k « 1 i

Then show that output for an input g (t)

y (t)*S(t-t0) + (fc/2) [git-t0-T)-g[t-t0 + T)\

  1. a) What are sensitivity and selectivity of radio receiver ? 3

b)            Explain with proper circuit diagram how DSB-SC signal is obtained using ring modulator.    5

c)            What is meant by synchronous detection of DSB-SC signal?     2

d)            Discuss the effect of phase and frequency error in synchronous detection.       5

  1. a) Considering a message signal e m = Em cos Wmt and a

carrier signal by ec = Ec sin ( W ct + 0 ), find the expression of the resultant FM wave.        7

b)              Explain FM stereo Tx / Rx system with block schematic diagrams. g

  1. a) Justify how FM can be obtained from PM and vice

versa.                                                                                         g

b)            Describe a method of Indirect way of FM generation. 7

  1. Write short notes on any three of the following : 3×5 a} VSB modulation

b)            QAM system       •

c)             Pre-emphasis and de-emphasis

d)            S/N of DSB-SC system

e)             Envelope detector.


In FM sound broadcasting system, the maximum frequency deviation is usually

b)            75 kHz d) 5-2 kHz.

ii) A superheterodyne receiver with an I.F. of 450 kHz, is tuned to a signal at 1200 kHz. The image frequency is

a)     750 kHz            b) 900 kHz

c)     1650 kHz            d) 2100 kHz.

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