**B.E. (fourth semester) EXAMINATION, June, 2009**

**(New scheme)**

**(Electronics and Communication Engg. Branch)**

**ANALOG AND DIGITAL COMMUNICATION**

EC-405 (N)

Note: Attempt one question from each unit. All question carry equal marks.

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

(b) Prove that a Dirac comb is its own Fourier transform

OR

2. (a) Show that the practical filter circuit shown in figure two has a transfer function close to idea to

Pass filter and also that its unit impulse response in casual.

(b) A signal e^{-3t}u(t) is passed through an ideal low pass filter with cut-off frequency of one rad per

second (i) test wheatear the input is an energy signal (ii) Fin the input output- energy.

3. (a) With the help of Fourier spectrum explain the chopper type DSB-SC signal generator

(b) A modulating signal f(t) in an AM-SC system is a multitone signal given by:

f (t) = E1 cos ω_{1}t + E_{2} cos ω_{2}t + E_{3} cos ω_{3}t

The signal f (t) modulates a carrier E_{c} cos ω_{3}t. Plot the signal sided trigonometric spectrum and

find the bandwidth of the modulated signal. Assume that:

ω_{3} > ω_{2} > ω_{1} > and E_{3}>E_{2}>E_{1}

_{ } OR

4. (a) Discuss the phase and frequency error in synchronous detection in SSB-SC .Also draw the phase

Diagram of SSB signal.

(b) Explain the filter method for generation of VSB-SC signal.

5. Derive the expression of wideband FM analysis there result in the light power content transmission

Efficiency and spectrum of FM.

OR

6. (a) Explain the parameter variation method for FM generation .

(b) Explain the Foster-seely discriminator method for FM detection.

7. (a) With the help of a block diagram explain the AM transmitter using low level modulation .

(b) With the help of block diagram explain the FM transmitter.

OR

8. (a) Explain the working of superhetrodyne receiver and also discuss its advantages .

(b) Discuss the criteria for selection of intermediate frequency.

9. (a) Derive the expression for the figure od merits why when the modulating f(t) is the signal tone

sinusoid given by f(t) = m_{a} A cos ω_{m}t , where m_{a} is the modulation index and A is the carrier

amplitude .Find the value of why when the depth of modulation is 30%.

(b) Show that an AM system using syschrouns detection does not suffer from the threshold effect.

OR

10. (a) Compare the noise performance in phase modulation and frequency modulation.

(b) Explain how threshold improvement is achieved in discriminators.