Code: D - 12                                                     Subject: COMMUNICATION ENGINEERING

Time: 3 Hours                               June 2006                                                                       Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1       Choose the correct or best alternative in the following:                                         (2x10)

a. In commercial FM broadcast systems, the modulating signal frequency is limited to about

(A)  3,400 Hz.                                     (B)  5,000 Hz.

(C)   15,000 Hz.                                  (D)  20,000 Hz.

b. In a certain system, the signal power is 13 dBm and the noise power is         – 1 dBm.  The SNR will be  

(A)  -13 dB.                                       (B)  14 dB.

(C)  12 dB.                                         (D)  12 dBm.

             c.   A DSB signal can be demodulated by using

(A) a synchronous detector.                (B)  a low pass filter.

(C)  a phase shift discriminator.            (D)  an envelope detector.

             d.   The time constant of a pre-emphasis circuit in FM systems is usually

(A)  20 ms.                                        (B)  1.25 ms.

(C)  75 .                                        (D)  1 .

             e.   The number of bits in a PCM system is increased from 7 to 8.  As a result, the signal-to-quantization noise ratio increases by

(A) 3 dB.                                           (B)  6 dB.

(C)  8 dB.                                           (D)  12 dB.

             f.    A PLL can be used to demodulate

(A) an AM signal.                               (B)  a VSB-SC signal.

(C)  a PCM signal.                              (D)  an FM signal.

             g.   Reflection coefficient on a transmission line

(A) is always real and lies between 0 and 1.         

(B) is always real and lies between –1 and 0.

(C) is always real and lies between –1 and +1.

(D) can be complex-valued.               

             h.   A rhombic antenna

(A) is a resonant antenna.                    (B)  is a multiband antenna.

(C) consists of   dipoles.               (D) is used in the UHF band.

             i.    The propagation time from one earth station to another via a geo-stationary satellite is about

(A) 300 ms.                                        (B) 1.25 ms.

(C) 125 .                                        (D) 25 .

             j.    A cavity resonator finds application as

(A)  an wideband attenuator.                (B)  an isolator.

(C)  a directional coupler.                    (D)  a frequency meter.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   Draw the block diagram of a superheterodyne radio receiver having one RF amplifier stage and also having AGC loop.  List the stages that mainly control

                   (i)   adjacent channel selectivity            (ii)  image channel selectivity

                   (iii) AGC performance  and                  (iv) AF fidelity.                                        (8)

             b.   Describe shot noise and find out its rms value in a diode when the average current is 1 mA and the bandwidth is 10 kHz.                           (4+4)

Q.3     a.   Describe an envelope detector for detection of AM signals and clearly explain how diagonal clipping may arise.                                                   (8)

             b.   Describe the ‘THIRD METHOD’ of generation of SSB signals.                          (8)

Q.4     a.   Explain the need for pre-emphasis (PE) and de-emphasis (DE) as used in FM systems.  Give simple PE and DE circuits and their response.             (8)

b.   Describe an FM signal.  Sketch its spectrum marking important values and from this spectrum give an estimate of the bandwidth of the FM signal.                                                        (8)

Q.5     a.   State sampling theorem and explain natural and flat topped sampling.  How do you reconstruct the original signal from its samples?                        (2+2+2+2)

     b.   Describe a PCM system with the help of a suitable block diagram.  What is companding, give its advantages.                                                                                                                         (8)

  Q.6     a. Describe a Phase-Locked Loop (PLL) with the help of a block diagram.  Describe the function of each block.  Give two applications of a PLL.                                                              (8)

             b.   Find the input impedance of a short-circuited transmission line having           parameters L = 4  and C = 100 pF/m.  The length of the line .                                                          (8)

Q.7     a.   What do you understand by the dominant mode in a rectangular waveguide?  How will you couple a rectangular waveguide to a source for exciting the dominant mode?                                     (8)

b.   Describe three types of optical fibers and compare their dispersion characteristics.              (8)

Q.8           Explain the following terms in relation to sky wave propagation

(i) Skip distance.

(ii) Fading.

(iii) MUF.

(iv) Critical frequency.                                                                (4 x 4)

Q.9           Write short notes on any TWO of the following:

(i) Dipole arrays.

(ii) Extension of monochrome television system to colour system.         

(iii) Pulse Position Modulation (PPM).                                        (8 x 2)