B.E./B.Tech.Degree Examinations,Apr/May 2010
Regulations 2008
Fourth Semester
Electronics and Communication Engineering
EC2252 Communication Theory
Time: Three Hours Maximum: 100 Marks
Answer ALL Questions
Part A - (10 x 2 = 20 Marks)
1. How many AM broadcast stations can be accommodated in a 100 kHz bandwidth
if the highest frequency modulating a carrier i s 5 kHz?
2. What are the causes of linear distortion?
3. Draw the block diagram of a method for generating a narrowband FM signal.
4. A carrier wave of frequency 100 MHz i s frequency modulated by a signal
20 sin(200¼ £ 103t):
What i s bandwidth of FM signal if the frequency sensitivity of the modulation is
25kHz=V .
5. When i s a random process called deterministic?
6. A receiver connected to an antenna of resistance of 50 has an equivalent noise
resistance of 30. Find the receiver noise ¯gure.
7. What are the characteristics of superheterodyne receivers?
8. What are the methods to improve FM threshold reduction?
9. De¯ne entropy function.
10. De¯ne Rate Bandwidth and Bandwidth e±ciency.
Part B - (5 x 16 = 80 Marks)
11. (a) (i) Draw an envelope detector circuit used for demodulation of AM and ex-
plain its operation. (Marks 10)
(ii) How SSB can be generated using Weaver's method? Illustrate with a neat
block diagram. (Marks 6)
OR
11. (b) (i) Discuss in detail about frequency translation and frequency division mul-
tiplexing technique with diagrams. (Marks 10)
(ii) Compare Amplitude Modulation and Frequency Modulation.
(6)
12. (a) (i) Using suitable Mathematical analysis show that FM modulation produces
in¯nite sideband. Also deduce an expression for the frequency modulated
output and its frequency spectrum. (Marks 10)
(ii) How can you generate an FM from PM and PM from FM?
(Marks 6)
OR
12. (b) (i) A 20 MHz is frequency modulated by a sinusoidal signal such that the max-
imum frequency deviation i s 100 kHz. Determine the modulation index
and approximate bandwidth of the FM signal for the following modulating
signal frequencies,
(1) 1 kHz (2) 100 kHz and (3) 500 kHz. (Marks 8)
(ii) Derive the time domain expressions of FM and PM signals.
(8)
13. (a) (i) Give a random process, X(t) = Acos(wt+µ), where A and ! are constants
and µ is a uniform random variable. Show that X(t) i s ergodic in both
mean and auto correlation. (Marks 8)
(ii) Write a short note on shot noise and also explain about power spectral
density of shot noise. (Marks 8)
OR
13. (b) Write the details about narrow band noise and the properties of quadrature
components of narrow band noise. (Marks 16)
14. (a) Derive an expression for SNR at input (SNRc) and output of (SNRo) of a
coherent detector. (Marks 16)
OR
14. (b) (i) Explain pre-emphasis and De-emphasis in detail. (Marks 10)
(ii) Compare the performances of AM and FM systems. (Marks 6)
15. (a) (i) Find the code words for ¯ve symbols of the alphabet of a discrete memory-
less source with probability f0.4, 0.2, 0.2, 0.1, 0.1g, using Hu®man coding
and determine the source entropy and average code word length. (Marks 10)
(ii) Discuss the source coding theorem. (Marks 6)
OR
15. (b) (i) Derive the channel capacity of a continuous band limited white Gaussian
noise channel. (Marks 10)
(ii) Discuss about rate distortion theory. (Marks 6)
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