# Analog and Digital Communication June 2009

( Electronics and Communication Engg. Branch)

Note: Attempt one question from each unit.

Total five questions are to be attempted All question carry equal marks.

1. (a)  Find the Fourier transform of the following signals :  (b)      A signal e-3tu(t) is passed through an ideal low pass filter with cut off frequency of 1 rad per second

test wheatear test  input is the energy signal and find the input energy.

OR

2.(a)     Show that the normalized Gaussian pulse is its own Fourier transform.

(b)     Show that the unit impulse response of an ideal low pass filter is non casual.

3. (a)    Discuss the method of detection of AM-SC signal using Cost a receiver

(b)        Discuss the filter method for generation of VSB-SC signal. Also discuss the filter characteric

required

for VSB signal generation.

4. (a)   A single tone modulating signal em =Em  cos ωmt amplitude modulates a carrier ec =  Ec  cos ωct.

Derive an expression for the AM wave and modulation index. Draw the frequency spectrum of the AM wave from and also draw the phasor diagram of AM wave form.

(b) Discuss the detection of AM wave from using linear diode detector. Also discuss the selection of the time constant RC of the circuit.

5.(a) Explain the wide band FM transmission . What should be the value of modulation index for it to

behave as narrow band FM?

(b) A carrier A cos ωc t is modulated by a signal

F(t) = 2 cos 104.2πt + 5 cos 103.2πt + 3 cos 104.4πt

Find the bandwidth of the FM signal by using carson’s rule. assume kf = 15 ×103 Hz per volt . Also

find modulation index mf.

OR

6. (a)    Explain the principle working of ratio detector for FM detection .

(b)     Discuss about the PPL detector used for FM detection

7. (a)    With the help of block diagram explain the working of high level transmitter.

(b)     Explain the principle working of superhetrodyne receiver.

OR

8. (a)   What is image signal why is the local oscillator frequency always kept higher than the signal