# WBUT Question Papers

## Analog Communication B Tech Sem Forth June 2008

[ Full Marks : 70

GROUP-A (Multiple Choice Type Questions )

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

i) If X (to) is the Fourier transform of x(t), then the Fourier transform of x(t)e jw0t a) X (to – to0)        b) X (a)0– to)                                                     .

c) X (to + to0)                                             d) X (to0).

11)         For envelop detection In AM the value of RC should be

a)        17W<<RC<<1/fc         b) l/W»RC»l/fc

c)               l/W«l/RC«l/fc

Ill)      The capacity C of AWGN channel Is given by

a)         Blog10(l+S/N)b/s      b) Blog2(l+S/N)b/s

c)        Blog,0(l+N/S)b/s      d) Blog2(l+N/S)b/s.

iv)            According to Carson’s rule the bandwidth of FM signal is expressed as

a)              B=2A f+fm                                                  b) B=Af+f„

c)               B=Af+2 fm                                                d) B=Af+fm/2.

v)               Varactor diode is used for

 a) FM generation c) PM generation
 b) AM generation d) All of these.

rV-245055 (?A)

vl) IF frequency for a superheterodyne commercially available AM receiver is

a)    460 kHz                                             b) 500 kHz

c) 455 kHz                                                d) 355 kHz.

vli) The bandwidth required for transmission of SSB SC signal Is

a)     more than AM signal          b)

c) more than VSB signal            d)

vlii) A slgnum function is

a)              zero for t greater than zero b) c) unity for t greater than zero d)

ix)            A pre-emphasis circuit provides extra noise immunity to

a)              boost the bass frequencies

b)              amplify the higher audio frequencies

c)              pre-ampllfy the whole audio band

d)              convert the PM to FM.

x)               FWM may be generated

a)              by differentiating PPM

b)              with a monostable multivibrator

c)               by integrating the signal

d)              with a free running multivibrator.

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

a)              needs less power

b)              needs less bandwidth

c)               needs simple circuit

d)              gives better signal to noise ratio.

a)

an aperture effect      b) aliasing effect

c)       loss of signal  d) none of these.

GROUP – B (Short Answer Type goeitlona)

Answer any three of the following.                                     3×5= 15

1. A modulating signal given by vm=2 sin (2n x 5001) amplitude modulates a carrier given by ve=10 sin (2n x 106t). Determine                                                            1

i)              modulation index

11)    frequencies present in the modulated signal

til)     total transmitter power.                                                                                        2+1+2

1. a)       State and prove Parseval’s theorem.                                                                             1+ 2
2. b)   Verify Parseval’s theorem for the signal g(t)= e”*tu(t);a>0.                                                          2
1. Explain the working principle of PLL for FM demodulation.                                             5
2. a)       What do you mean by distortionless transmission ?                                              2

b)              Obtain the conditions fifr distortionless transmission of signals through a system.      3

1. a) Distinguish between ‘auto-correlation’ and ‘cross-correlation’ functions.

b)              What is a ‘balanced modulator’ ? Explain the main advantages and disadvantages of such circuits.                                                                                                                                           5

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6

GROUP -C (Long Answer Type Questions)

Answer any three of the following questions.                            3 x 15 = 45

1. a) Explain with suitable block diagram the generation of FM signal using Armstrong
2. method.                                                                                                                              5

• b) What is Narrowband FM and Wideband FM ? ‘                                                                            2

c)               Explain with proper expression

I   modulation index for FM                                                                           ,

II)    bandwidth required for transmission of FM.                                                           4

d)              The maximum deviation allowed in an FM broadcast system is 75 kHz. If the modulating signal is a single tone sinusoid of 10 kHz, find the bandwidth of the FM signal. What will be the change in the bandwidth, if the modulating frequency is doubled ? Determine the bandwidth when modulating signal amplitude Is also doubled.                                                                                     4

1. a) Define DSB-SC and SSB-SC.                                                                                                 4

b)              With neat block diagram explain the principle of SSB-SC generation by phase shift method. What is VSB-SC modulation ?                                                                                       4 + 3

c)               Briefly explain QAM.                                                                                               4

1. a) What do you mean by ‘switching modulator’ ? How can ring modulation be acted
2. . as switching modulator ?                               ,                                                                5

b)              Show that an AM system using synchronous detection does not suffer from threshold effect.           5

c)               Compare FDM with a quadrature carrier multiplexing.                                        5

IV-245033 (3-A)

7                                                              ‘

1. a) Discuss the methods for modulation and demodulation of PAM signal.                               6

b)              Compare PAM with FWM system of signal/data transmission.                            6

c)               Explain the terms ‘ sensitivity1, ‘selectivity’ and ‘fidelity’ of a receiver.                   3

1. a) In a multipath transmission system, the input and output are related by :

y (t) = x (t) + ax (t – t), where a and t are constants. Determine the transfer function of the equalizer to compensate the channel induced distortion.                                                                   6

b)              Determine the performance of an FM system in the presence of additive Gaussian noise. Discuss briefly the improvement offered in noise performance by pre­emphasis and de-emphasis concepts.  *                                                                                                                 9

1. Write short notes on any three of the following :                                                                     3×5