Anna University Chennai 2005 B.E Electrical and Electronics Engineering Fourth Semester EC 256 — COMMUNICATION ENGINEERING - Question Paper

B.E./B.Tech. DEGREE exam
Fourth Semester
Electrical and Electronics Engineering
EC 256 — COMMUNICATION ENGINEERING
(Common to Electronics and Instrumentation Engineering and Instrumentation and
Control Engineering)
Time : 3 hours Maximum : 100 marks
ans ALL ques..
PART A — (10 ? two = 20 marks)
1. describe the term ‘‘conversion conductance’’.
2. What is AGC?
3. provide the expression for path loss ‘‘L’’ of satellite communication system in dB.
4. describe Pulse Amplitude Modulation.
5. describe Companding.
6. describe Time Division Multiplexing.
7. provide the expression for the capacity of a Gaussian Channel.
8. What is the function of a PLL?
9. What is Quadrature Phase Shift Keying (QPSK)?
10. discuss the term ‘‘Interlaced Scanning’’ as applicable to Television.
PART B — (5 ? 16 = 80 marks)
11. Bandwidth of the input to a pulse code modulator is restricted to four kHz. The
signal varies from –3.8 V to 3.8 V and has the avg. power of 30 mW. The
needed signal to quantisation noise power ratio is 20 dB. The modulator
produces binary OUTPUT. obtain
D 253
2 D 253
(i) No. of bits needed per sample.
(ii) OUTPUT of 30 such PCM codes are time multiplexed. What is the
minimum needed transmission bandwidth for the multiplexed signal?
12. (a) Determine the Bandwidth occupied by a sinusoidally frequency
modulated carrier for which the modulation index is 2.4 and the
maximum frequency deviation is 15 kHz.
Or
(b) In a FM system the frequency deviation is one kHz/V. A sinusoidal
modulating voltage of amplitude 15 V and frequency of three Hz applied.
Find
(i) the maximum frequency deviation
(ii) modulation index.
13. (a) A Telephone Transmitter using AM has un–modulated carrier output
power of 20 kW and can be modulated to a maximum depth of 80% by a
sinusoidal modulating voltage without causing overloading. obtain the
value to which unmodulated carrier power may be increased without
resulting in overloading if the maximum permitted modulation index is
restricted to 60%.
Or
(b) Determine the carrier swing, the maximum and minimum frequencies
attained, and the modulation index of the FM signal generated by
frequency modulation at 101.6 MHz carrier with an eight kHz sine wave
causing a frequency deviation of 40 kHz.
14. (a) obtain an expression for the signal ( ) three V t in figure for
( ) 10 cos (2000 ) four sin (200 ) one V t ? ? t ? ? t .
presume that ( ) ( ) 0.1 two ( )
2 one 1 V t ? V t ? V t and the BPF has an ideal unity
gain with pass band from 800 Hz to 1200 Hz.
3 D 253
Or
(b) For binary data transmission find the expressions and waveforms for
ASK, FSK and PSK schemes.
15. (a) Here X is uniformly distributed as shown in fig. obtain E (x) ,
E (x2 ), E (cosx) and ( )2 x E x ? m .
Or
(b) obtain the output signal to noise ratio of a matched filter where the signal
is S (t) ? A, for 0 ? t ? T . The noise is white with power spectral density
/2 0 N .
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Earning:   Approval pending.