# Analog and Digital Communication June 2002

(Electrical Engineering Branch)

ANALOG AND DIGITAL COMMUNICATION

Note: Attempt any five questions.

Answer to all parts of the question should be attempt toghater .

Assume suitable value for missing data.

1. (a)  Evaluate the Fourier transform of a saw tooth pulse define by :

X (t)  α t,  -T<= t>= T

=0, elsewhere

Where α is a constant

(b)       State the parseval’s power theorem for periodic and non- periodic signals.

2. (a)   What is the concept of convolution ? Derive an expression for convolution of two time function

X1 (f) and X(f) .

(b)       Given the Fourier transform of f(t) is f(Ω) . Find the Fourier transform of the function f(t).cos Ωt.

3. (a)    Explain the difference in theory in practical implementation between frequency modulation and

face modulation .

(b)        Explain with block diagram the principle of operation of any modulation method to generate

Double side band suppressed carrior (DSBSC) signal. Describribe the advantage and

Disadvantage of (DSBSC) when compared with DSB with carrior.

4. (a)    Consider the modulation wave :

V(t) = Ae cos [2πfc t] + e(t) cos (2πfc t)  + e(t) sin [2πfc t]

Which represents carrior puls on SSB- SC (LSB) wave? Determine the condition for which an

ideal envelope detector with v(t) as the input would provide a good approximation to be message

signal e(f).

(b)           Draw the discuss the balance modulator.

5. (a)       Why fm reception is supposed to be better than AM  reception ? Draw the block diagram of an

(b)    Discuss various source of the noice. What do you understand by the term noise figure ?

(a)     Xand X2 are zero mean Gaussian random variable with

σ21= σ22 σ2 and E [X1, X2] = 0.5

Find the probability of x1 and x2 lying the shaded area as shown in the ahead fig. :

(b)     Describe the basic idea behind QPSK 4-phase PSK) system. Write down all possibilities input to

QPSK transmitters and their corresponding output waveforms.

(a)     When is source referred to as ‘memory less’? Define the entropy of a discrete memory less source.

Give an example of such a source.

(b)   Consider a system of band with 10 kHz and a signal to noice ratio as 19Db. Calculate the channel

capacity of the system.

Write short notes on any three of the following:

(a)         Sampling theorem

(b)         Shannon’s theorem for channel capacity

(c)          Companding

(d)          Convolution codes