Code: D - 12                                                     Subject: COMMUNICATION ENGINEERING

Time: 3 Hours                                                                                                     Max. Marks: 100

NOTE: There are 11 Questions in all.

· Question 1 is compulsory and carries 16 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Answer any THREE Questions each from Part I and Part II. Each of these questions carries 14 marks.

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

Q.1       Choose the correct or best alternative in the following:                                           (2x8)

a. The voice frequency band accommodated in telephone systems is

(A)  300-3400 Hz.                              (B)  50-5000 Hz.

(C)  100 Hz -15 kHz.                          (D) 20 Hz – 20 kHz.

             b.   A system has a 3 dB noise figure.  Its noise temperature is about

                  (A)  1160 K.                                       (B)  870 K.

(C)  580 K.                                         (D)  290 K.

             c.   An SSB signal can be demodulated by using

(A) an envelope detector.                   (B)  an average detector.

(C)  a discriminator.                             (D)  a synchronous detector.

             d.   Frequency range used in a geostationary satellite lies in the

(A)  EHF band.                                   (B)  SHF band.

(C)  UHF band.                                   (D)  HF band.

             e.   The number of fields/sec. in TV system in India is

(A) 15625.                                         (B)  625.

(C)  50.                                               (D)  25.

             f.    The magnitude of reflection coefficient on a transmission line lies in the range

(A) 0 to 1.                                          (B)  0 to .

(C)  0 to 100.                                      (D)  1 to .

             g.   The dominant mode in a rectangular waveguide is

(A)  .                                        (B)  .

(C)  .                                          (D)  TEM.

             h.   A 100 MHz carrier is frequency modulated by a 1 KHz tone to a modulation index of 10.  The bandwidth of the resulting FM signal is 

(A)  2 kHz.                                          (B)  22 kHz.

(C)  75 kHz.                                        (D)  200 kHz.

PART I

Answer any THREE Questions. Each question carries 14 marks.

Q.2     a.   Describe shot noise.                                                                                           (7)

             b.   A receiver connected to an antenna whose resistance is , has an equivalent noise resistance of 50.  Find the receiver noise figure in decibels.                                                                             (7)

Q.3     a.   Describe the operation of a diode envelope detector for demodulation of AM signals.                     (7)

             b.   Consider an AM signal  volt applied across a  resistor.  Find

                   (i)   the carrier frequency

                   (ii)  the depth of modulation and

                   (iii) signal power.                                                                                                (7)

Q.4     a.   Describe a balanced modulator for generation of DSB-SC signals.                      (7)

             b.   Give a labelled block diagram of a medium wave AM superhet receiver.  Which stages generally determine the image channel selectivity?               (7)

Q.5     a.   Explain stereophonic FM broadcasting.                                                              (7)

             b.   Explain the reasons for employing preemphasis - deemphasis in FM systems.  Show simple PE-DE circuits and give their performance.                  (7)

  Q.6           Write short notes on

(i) Phase Locked Loop.

(ii) Pulse width modulation.                                                            (14)

PART II

Answer any THREE Questions. Each question carries 14 marks.

Q.7     a.   A transmission line with  is terminated into an unknown resistor R.  The maximum and minimum voltages on the line are found to be 10 mV and 4 mV.  Final all possible values of R.        (7)

             b.   Find the input impedance of a short circuited lossless line of  length and .  At 10 MHz, what component value does it represent?                                                                  (7)

  Q.8     a.   Explain the following terms as related to antennas:

(i) Radiation resistance

(ii) Polarisation and

(iii) Beamwidth.                                                                                           (7)

b. Find the gain of a 2.8m paraboloid reflector at a frequency of 6 GHz.                  (7)

Q.9           Describe space wave propagation including super-refraction or ducting.            (14)

Q.10           a.                                                        Discuss principles of scanning of pictures in television transmission.                                                           (7)

             b.   Describe cavity resonators and their applications.                                                (7)

Q.11                                                                      Write short notes on

(i) Geostationary communication satellites.

(ii) PPM.                                                                                                  (14)

                                                                                                                                                                                                                                                                                                          (14)