Code: A-15                                                        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 output and input SNR’s of an amplifier are 16 dB and 20 dB respectively.  Its noise figure is

(A) 0.8.                                               (B)  1.25.

(C)  4 dB.                                           (D)  36 dB.

b. In a superhet AM receiver, most of the adjacent channel rejection comes from

(A)  RF tuned circuits.                         (B)  1F tuned circuits.

(C)  Detector stage.                             (D)  AF amplifiers.

c. A slope detector is used for detecting

(A)  VSB signals.                                 (B)  PPM signals.

(C)  DM signals.                                  (D)  FM signals.

d. Flat top sampling leads to

                   (A)  aliasing.                                        (B)  undersampling.

(C)  distortion in received signal.          (D) aperture effect.

e. The core diameter of a single mode fiber is

(A)  0.2 – 1 mm.          (B)  20 – 100 .

(C)  2 – 10 .                                 (D)  20 – 100 nm.

             f.    The impulse response of a filter matched to rectangular pulse signals is

(A) an impulse.                                   (B)  a rectangular pulse.

(C)  a saw-tooth.                                 (D)  a triangular pulse.

             g.   The blind speed of an MTI radar varies (where PRF stands for pulse repetition frequency)

(A)  directly as PRF.                            (B)  inversely as PRF.

(C) inversely as .                   (D)  directly as .

             h.   The channel capacity of a 4 KHz channel with a 15 dB SNR is about

(A)  20 kbps.                                      (B) 32 kbps.         

(C) 40 kbps.                                       (D) 124 kbps.

PART I

Answer any THREE Questions. Each question carries 14 marks.

Q.2     a.   Obtain a relationship between the noise figure and noise temperature of a two-port network (like an amplifier).                                                                                                         (7)

             b.   Consider a parallel combination of two resistors  and  and a capacitor C.  Find the total mean squared thermal noise voltage across C.                                                             (7)

Q.3     a.   Describe any one method of generation of AM signals.                                       (7)

             b.   Consider an FM signal 10 cos  volt applied across a 600 resistor.  Find

i. the bandwidth of the FM signal.

ii. the frequency range occupied by the FM signal and

iii. the expression for the modulating signal.                                (7)

Q.4     a.   Compare the average transmitter power and channel bandwidth for AM, DSB-SC and SSB systems for 10 KHz modulating signal and required output SNR of 50 dB.  The channel loss is 60dB and the two-sided channel noise power spectral density is .  For AM, the depth of modulation is 70% and the mean squared value of the audio signal is 0.5.                                                     (7)

       b.    A DSB-SC signal= , with  bandlimited to , multiplied with itself.  The product signal is applied to an ideal LPF of cutoff 2B.  The filter output is given to a square-rooter circuit.  Find the output of the square-rooter and comment on the result.                                      (7)

Q.5           An FM system uses a 75 preemphasis and the corresponding deemphasis.  Find the improvement in noise performance for modulating signal frequency of 15 KHz.  Derive the formula used.             (7+7)

  Q.6     a. Give a labelled block diagram of a stereophonic FM broadcast system.               (7)

b.   Describe diagonal clipping in diode envelope detection of AM signals.                 (7)

PART II

Answer any THREE Questions. Each question carries 14 marks.

Q.7     a.   Derive an expression for signal-to-quantisation noise ratio in a Delta Modulation (DM) system with no slope overload.                                             (7)

             b.   In a single-integration DM system with 4 KHz, 1 V max amplitude modulating signal and with 64,000 / sec sampling, find

(i) minimum step size to avoid slope overload.

(ii) granular noise power.

(iii) Signal-to-quantisation noise ratio.                                               (7)

Q.8     a.   Compare the performance of

(i) six bit binary PCM with linear quantisation and

(ii) six bit binary PCM with -law compression with =100.  The message signal is a full load test tone at 4 KHz.                                (7)

             b.   Explain the reasons for normally selecting (R-Y) and (B-Y) signals in colour demodulator in a TV receiver.  How is (G-Y) signal derived?               (7)

Q.9           A receiver is to receive binary pulses s(t) and –s(t) using a suboptimum filter.  The signal s(t) is a rectangular pulse of duration .  The filter is a simple RC-LPF and the noise is AWGN.  Find the value of the RC product in terms of  for optimum performance.                                             (14)

Q.10                                                                      Derive an expression for the capacity of a Gaussian channel.                                                           (14)

Q.11           a.                                                        Find the maximum range of a deep space radar operating at 2.5 GHz and with a peak power of 25 MW.  The antenna is a 6.4 m dish and the target cross-section is .  The amplifier has a 1.1 dB noise figure and the receiver bandwidth is only 5 KHz.                     (7)

b.   Describe the operation of an MTI radar.                                                             (7)