# Analog and Digital Communication June 2010

(Electronics and Communication Engg. Branch)

Note: Attempt one question from each unit.

All question carry equal marks.

1. (a)  Determine the Fourier transform of the following  –

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

2. (a)  For a low pass filter , show that the rise time is inversely proportional to the system bandwidth.

(b)      Determine the expression of power density spectrum of A periodic function f(t) With period T.

3. (a)  Discuss the phase discrimination method for generation  of SSB-SC signals.

(b)     The carrier A cos ωmt, is modulated by a signal tone modulating signal. f(t) = Em  cos  ωmt, Find the

Modulated power r.m.s. value of the modulated signal and transmission efficiency for a 10%

modulation.

OR

4.   Explain the demodulation of AM wave using a linear diode detector. Justify the choice optimum value

of time constant RC.

5. (a)  A carrier wave A cos  ωct is frequency modulated by a signal tone modulated signal tone

modulated signal f(t) = Em  cos  ωmt:

(i)      Find the expression of FM wave.

(ii)    Also find the expression for a narrow band FM.

(b)    A modulating signal 5cos 2π 15× 103t, angle modulates a carrier A cos  ωct:

(i)     Find the modulation index and the bandwidth for F.M. and P.M. system.

(ii)    Determine the change in the bandwidth and the modulation index for both FM and PM, if fm is

Reduced to 5 kHz.

6. (a) Discuss the Armstrong method of FM signal generation .

(b)    Explain ratio detector used for Demodulating FM signal.

7. (a) With the help of A block diagram Explain the AM transmitter using high level modulation .

(b)     How and why in frequency multiplication applied to FM signal.

8. (a) Explain how tracking and alignment is achieved in AM radio receiver . Also discuss why local

Oscillator frequency is always kept higher than the signal frequency.