**B.E. (fourth semester) EXAMINATION, Nov. Dec., 2007**

**(Common for CS, EC, EE &EI, IT BM Engg.)**

**ANALOG AND DIGITAL COMMUNICATION**

**(404)**

Note: Attempt any five questions. All question carry equal marks.

1. (a) Define convolution State the prove time convolution theorem .

(b) Find Fourier transform of the following.

(i) e^{bt} u(t)

(ii) Sin ω_{0}t u (t)

2. (a) A single tone modulating signal e_{m} =E_{m} cos ω_{m} t amplitude modulates a carrier e_{e} = E_{e} cos ω_{c} t.

Derive an expression for the AM wave and also derive an expression for modulating isdex.

(b) Explain the working of envelope detector and determine the relation for the obtaining optimum

value of time constant RC in terms of modulation index and modulating frequency .

3. (a) Explain the parameter variation method for generation of FM signal.

(b) A modulating signal 5cos 2π 15 ×10^{3} t, angle modulates a carrier A cos ω_{c }t. Find the modulation

index and the bandwidth for FM and PM system.

4. (a) Explain the working of superhetrodyne receiver with the help of block diagram.

(b) (i) State sampling theorem

(ii) 48 telephone channel each band limited to 3.4 kHz are to be time division multiplexed by using

PCM calculate the bandwidth of the PCM system for 128. Quantization levels and 8kHz sampling

frequency.

5. (a) What is commanding ? Explain. why companding is needed ? Discuss laws of companding .

(b) Explain the delta modulation with block diagram and draw its output wave. Define slop overload

error and explain how it can be reduced.

6. (a) Draw the schematic diagram of QPSK generating system and explain its working .

(b) Discuss the principle of matched filters.

7.(a) How different entropies are related with a communication system ? Define mutual information

and find the channel capacity of binary symmetric channel.

(b) Apply the Shannon-fanno coding producer to find the coding efficiency for the message assembles

the ahead:

X P

X_{1 }1/4

X_{2 }1/8

X_{3 }1/16

X_{4 }1/16

X_{5 }1/16

X_{6 }1/4

X_{7 }1/16

X_{8 }1/8

8. Write short notes on any three of the following:

(i) Parseval’s theorem for energy signal

(ii) Synchronous detection

(iii) Quantization

(iv) Comparison of AM and FM system

(v) SNR in DSB systems

(vi) Pre-emphasis