# Communication Theory 4th AM10 EC2252

B.E./B.Tech.Degree Examinations,Apr/May 2010
Regulations 2008
Fourth Semester
Electronics and Communication Engineering
EC2252 Communication Theory

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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 figure.
7. What are the characteristics of superheterodyne receivers?
8. What are the methods to improve FM threshold reduction?
9. Define entropy function.
10. Define Rate Bandwidth and Bandwidth eficiency.

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. (10)
(ii) How SSB can be generated using Weaver's method? Illustrate with a neat block diagram. (6)
OR
11. (b) (i) Discuss in detail about frequency translation and frequency division mul-tiplexing technique with diagrams. (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. (10)
(ii) How can you generate an FM from PM and PM from FM? (6)
OR
12. (b) (i) A 20 MHz is frequency modulated by a sinusoidal signal such that the maximum 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. (8)
(ii) Derive the time domain expressions of FM and PM signals.(8)

13. (a) (i) Give a random process, X(t) = Acos(!t+µ), where A and ! are constants and µ is a uniform random variable. Show that X(t) i s ergodic in both mean and autocorrelation. (8)
(ii) Write a short note on shot noise and also explain about power spectral density of shot noise. (8)
OR
13. (b) Write the details about narrow band noise and the properties of quadrature components of narrowband noise. (16)

14. (a) Derive an expression for SNR at input (SNRc) and output of (SNRo) of a coherent detector. (16)
OR
14. (b) (i) Explain pre-emphasis and De-emphasis in detail. (10)
(ii) Compare the performances of AM and FM systems. (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. (10)
(ii) Discuss the source coding theorem. (6)
OR
15. (b) (i) Derive the channel capacity of a continuous band limited white Gaussian noise channel. (10)
(ii) Discuss about rate distortion theory. (6)