# WBUT Previous Years Question Papers

## Principles Of Communication Engineering B Tech 4th Sem June 2008

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

9 A broadcast radio transmitter radiates 20 kW when the modulation percentage la ; 60. The carrier power will be                                                                                         •

a) 1.2 kW                 b) 1.45 kW .

c)  16.94 kW         d) 20 kW.

U) In TV system, picture and sound respectively use                                         .

a)       AM, FM           b) FM, FM

. c) FM, AM               d) AM, AM.

In a narrow band FM the highest modulating frequency is fin. The bandwidth of the system will be

aO 6fm                                                     b) fin

c) 2fm                                             . d) lOfrn.

Recovering information from a carrier is known as

a) demultiplexing                                     b) modulation

c) detection t                                            d) carrier recovery.

In an envel6pe detector for AM signal                                                   •

a)              only diode is used

b)               only capacitor is used

c)               only diode and capacitor are used

d)              only inductor and capacitor are used.

vi) Indicate which of the following modulation is analog

a) PCM                                                     b) Differential PCM

c) PAM                                                      d) Delta Modulation.

vli) The bandwidth required for transmitting a 4 kHz signal using PCM with 128 quantization levels is

a)       8 kHz                                               b) 16 kHz

c) 28 kHz                                                  d) 32 kHz.

viii) The sampling frequency/s, must be (B = bandwidth)

a)     equal to B       b) greater than B

c)   greater than 2B      d) must lie between B and 2B.

PWM signal can be generated by

a)              a monostable multi-vibrator

b)               a astable multi-vibrator

c)               integrating the PPM signal

d)              differentiating the PPM signal.

Quantization noise occurs In

a)      TDM   b) FDM

c)        PCM   d) PWM.

For global communication, the number of satellites needed is

 b) d)
 3 7.
 a) c)
 1 5

xii)           Entropy is basically a measure of

a)   rate of Information b)

c) probability of information d)

xiii)        The IF used for a superhet, receiver is

a)              455 kHz

b)              455 MHz

c)               910 kHz

d)             fc + 455 kHz

where fc = carrier frequency.

average information disorder of information.

GROUP – B (Short Answer Type Questions) ,

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

a)               Explain briefly, why modulation is needed in communication system. .                                                   2

b)               Draw the spectrum of (i) DSB – SC (AM ), (ii) SSB signal (iii) VSB signal.   3 Briefly explain FM demodulation scheme using PLL.

a)               Explain what you understand by the term ‘Aliasing*. 1

b)               To avoid aliasing, find the Nyquist rate of the signal x( t) = 8 cos 200 nt.                                              2

c)                Encode the bit sequence 1011011 in the NRZ-polar and RZ-bipolar format.                                            2

Distinguish between ASK, ESK and PSK in terms of their performances.                                                   5

Draw a diagram of A/D converter and explain its working principle.  5

Draw the block diagram of a satellite transponder and briefly explain the role of each block.                                         5

GROUP -C (Long Answer Type Questions)

a)               ‘FM and PM are basically same’ — comment on the statement and justify. 3

b)               Give a block diagram of WBFM modulation for practical use (Armstrong method). Explain the principle of working.         ‘ 6

c)                Define selectivity’ and ‘sensitivity’ of a receiver. A superheterodyne receiver is . tuned to a signal frequency of 655 kHz. The LO frequency is 1110 kHz. Find the

image frequency.

a)               Discuss the relative advantages and disadvantages of ‘digital communication’ over ‘analog communication’.        3

b)               Explain briefly with block diagrams the generation and detection processes of PCM.                                   5

c)               A telephone signal has a maximum frequency of 4 kHz. It is limited in voltage between ± IV. It is transmitted by using PCM. The required signal-to- quantization noise ratio is 40 dB. What is the minimum bandwidth required for transmission ?                                                                 ‘                              ^

1. a) Explain the principle of detection of FM signed using balanced slope detector
2. circuit with proper sketch.
3. b)  What Is Carson’s rule ?
4. c)  Derive an expression for the signal to noise ratio of DSB-SC systems.    7
1. £0 Discuss the generation of time division multiplexed PAM signal.  4
2. b)   Write the advantages and disadvantages of TDM over FDM. 3

c)               With the help of block diagram, explain the working principles of coherent FSK generation and detection.             5

d)              What is DPSK ? Write down the DPSK format for bit pattern  1011011 considering Initial bit to be 1. / #                    3

1. a) Derive Hartley Shanon Law.
2. b)    Explain how a single bit error differs from burst error. * 3
3. c)    Discuss the purpose of Huffman encoding.               2
4. d)  Represent the block code in Matrix form.               6
1. Write short notes on any three of the following :            15

a)               Reactance Modulator

b)               Foster-Seeley Discriminator   .

c)               Pre-emphasis and De’-emphasis

d)               MODEM

e)               Ring Modulator.