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Roll No. ..................................

End-Term Examination

Fourth Semester [B.Tech] - May 2005

Paper Code : ETEC – 208 Subject : Communication Systems and Circuits

Time : 3 Hours

Maximum Marks : 75

Note : Attempt any 5 questions in all.

Q1

( a )

Give the time-domain and frequency domain representation of the following signals:

(i) AM-DSB/FC (ii) AM-DSB/SC
(iii) AM-SSB/FC (iv) AM-SSB/SC

Give graphical representation wherever necessary.

8

( b )

In a certain SSB generator of phase shift type, the phase shift for modulating signal is deviated by 20 from its ideal value of 900. Find the ratio of undesired sideband amplitude to desired sideband amplitude.

7

Q2

( a )

Explain the indirect method for generation of FM signals.

7

( b )

List the difference between NBFM and WBFM signals.

3

( c )

Am FM signal is given by
V(t) = 20 cos(108t + 20cos(3000t))

Find the various parameters associated with this signal.

5

Q3

( a )

State and explain sampling theorem for and limited signals.

6

( b )

Calculate Nyquist rate of sampling for the following signal:
V(t) = 20 cos(2000pt) . cos(1000pt) + sin2(3000pt)

3

( c )

Explain the principle of various pulse modulated signals using necessary waveforms.

6

Q4

( a )

Explain the difference between PCM and DM systems. How the slope overload is minimized using ADM systems.

8

( b )

A modulator transmits 1000 symbols /sec. Each symbol has 32 different possible states. Find:
(i) bit rate     (ii) baud rate

3

( c )

A signal of 1V amplitude and 1 KHz frequencies transmitted by DM system. Find minimum step size so that slope overloading is avoided if the sampling frequency = 40 samples /sec.

4

Q5

( a )

Explain the principles of PSK and DPSK systems. How will you modulate and demodulate such signals?

10

( b )

Why are the various line coding formats required? Encode the bit stream 11010101 using:
(i) RZ (ii) NRZ
(iii) AMI (iv) Manchester

5

Q6

( a )

A matched filter is to be designed for PCM systems. The PCM pulse has amplitude A for 0<t<T. Find

  1.  H(w) and h(t) of the matched filter.
  2. Maximum SNR at the output.
  3. Minimum probability of error.

12

( b )

Explain the principle of companding.

3

Q7

( a )

Explain Shannon Hartley theorem of channel capacity.

3

( b )

Find:
(i) Shannon Fano's codes     (ii) Huffman's codes
for the following digits with specified probabilities:
i A B C D E F
P (i) 0.30 0.24 0.21 0.13 0.07 0.05

Calculate efficiency of transmission before and after coding using Shannon Fano's codes.

5

Q8

( a )

A random variable has probability density function:

 p(x) = a e-bx ; x>0
        = 0         ; x<0

Find the total probability that the random variable lies between 1 and 2.

5

( b )

Find he mean and variance of sum of two random variables given by
  Z = x + y

5

( c )

Define (i) stationary and random processes
          (ii) ergodic random process

5

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