Anna University

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 t*h*e causes of linear distortion?

3. Draw *t *he block diagram o*f *a method for generating a narrowband FM signal.

4. A carrier wave *o*f frequency 100 MHz *i *s frequency modulated by a signal 20 sin(200*π × *103*t*)*.*

What *i *s bandwidth of FM signal if th*e *frequency sensitivity of *t *he modulation i*s *25*kH z/V *.

5. When *i *s a random process called deterministic?

6. A receiver connected to *a *n antenna *o*f resistance of 50Ω has an equivalent noise resistance *o*f 30Ω. Find th*e *receiver noise figure.

7. What a*r*e *t *he characteristics *o*f super heterodyne receivers?

8. What *a *re the methods to improve FM threshold reduction?

9. Define entropy function.

10. Define Rate Bandwidth a*n *d Bandwidth efficiency.

**Part B - (5 x 16 = 80 Marks)**

11. (a) (i) Draw an envelope detector circuit used for demodulation *o*f AM a*n *d 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 *i *n detail about frequency translation *a*nd frequency division multiplexing technique with diagrams. (10)

(ii) Compare Amplitude Modulation *a*nd Frequency Modulation. (6)

12. (a) (i) Using suitable Mathematical analysis show that FM modulation produces infinite sideband. Also deduce an expression for th*e *frequency modulated output an*d *its frequency spectrum. (10)

(ii) How can you generate a*n *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 th*e *modulation index an*d *approximate bandwidth o*f t *he FM signal for *t *he following modulating signal frequencies,

(1) 1 kHz (2) 100 kHz *a*nd (3) 500 kHz. (8)

(ii) Derive the time domain expressions o*f *FM an*d *PM signals. (8)

13. (a) (i) Give a random process, *X *(*t*) = *A *cos(*ωt *+ *θ*), where *A *a*n *d *ω a *re constants *a*nd *θ i *s a uniform random variable. Show that *X *(*t*) is ergodic in both mean *a*nd autocorrelation. (8)

(ii) Write a short note on shot noise an*d *also explain about power spectral density o*f *shot noise. (8)

OR

13. (b) Write *t *he details about narrow band noise *a*nd *t *he properties of quadrature components of narrowband noise. (16)

14. (a) Derive *a *n expression for SNR at input (SNR*c *) an*d *output o*f *(SNR*o*) *o*f a coherent detector. (16)

OR

14. (b) (i) Explain pre-emphasis *a*nd De-emphasis *i *n detail. (10)

(ii) Compare the performances o*f *AM an*d *FM systems. (6)

15. (a) (i) Find t*h*e code words for five symbols *o*f *t *he alphabet *o*f a discrete memory- less source with probability *{*0.4, 0.2, 0.2, 0.1, 0.1*}*, using Huffman coding and determine t*h*e source entropy an*d *average code word length. (10)

(ii) Discuss t*h*e source coding theorem. (6)

OR

15. (b) (i) Derive *t *he channel capacity *o*f a continuous band limited white Gaussian noise channel. (10)

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