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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 i n a 100 kHz bandwidth if t he highest frequency modulating a carrier i s 5 kHz? 2. What are the causes of linear distortion? 3. Draw t he 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(200π ×
103t). What i s bandwidth of FM signal if the frequency sensitivity of t he modulation is 25kH z/V . 5. When i s a random process called deterministic? 6. A receiver connected to a n antenna
of resistance of 50Ω has an equivalent noise resistance
of 30Ω.
Find the receiver noise figure. 7. What are t he characteristics of super heterodyne receivers? 8. What are the methods to improve FM threshold reduction? 9. Define entropy function. 10. Define Rate Bandwidth an d Bandwidth efficiency.
Part B - (5 x 16 = 80 Marks)
11. (a) (i) Draw an envelope detector circuit used for demodulation of AM an d ex- plain its operation. (10) (ii) How SSB can be generated us
ing Weaver’s method? Illustrate with a neat block diagram. (6)
OR 11. (b) (i) Discuss i n detail about frequency translation and frequency division multiplexing technique with diagrams. (10) (ii) Compare Amplitude Modulation and Frequency Modulation. (6) 12. (a) (i) Using suitable Mathematical analysis show that FM modulation produces infinite 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 i s frequency modulated by a sinusoidal signal such that t he maximum frequency deviation is 100 kHz. Determine the modulation index and approximate bandwidth of the FM signal for t he 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(ωt
+
θ), where
A an d
ω a
re constants and
θ i
s a uniform random variable. Show that X (t) is 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 t he details about narrow band noise and t he properties of quadrature components of narrowband noise. (16) 14. (a) Derive a n 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 i n detail. (10) (ii) Compare the performances of AM and FM systems. (6) 15. (a) (i) Find the code words for five symbols of t he alphabet of a discrete memory- less source with probability {0.4, 0.2, 0.2, 0.1, 0.1}, using Huffman coding and determine the source entropy and average code word length. (10) (ii) Discuss the source coding theorem. (6) OR 15. (b) (i) Derive t he channel capacity of a continuous band limited white Gaussian noise channel. (10) (ii) Discuss about rate distortion theory. (6)

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