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 bandwidthif 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 signal20 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. Define entropy function.

10. Define 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. (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 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. (8)

(ii) Derive the time domain expressions of FM and PM signals.

(8)

13. (a) (i) Give a random process, X(t) = A cos(!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 quadraturecomponents of narrowband noise. (16)

14. (a) Derive an expression for SNR at input (SNRc) and output of (SNRo) of a

coherent detector. (16)

2 E3077

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 Gaussiannoise channel. (10)

(ii) Discuss about rate distortion theory. (6)

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