AMIETE – ET (OLD SCHEME)

Code: AE15                                                      Subject: COMMUNICATION ENGINEERING

Time: 3 Hours                                                                                                     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 the best alternative in the following:                                  (210)

a.   Unwanted energy, usually of random nature, present in a transmission system, due to any cause is called

(A)   noise.                                           (B)  random signal.

(C)  information.                                   (D)  carrier.

2. For a receiver of noise figure F operating at , K, the equivalent noise temperature is given by 

(A) .                                      (B) .     

(C) .                                     (D) .

3. Modulation index of an AM wave with  is   

(A) 40%.                                              (B) 80%.

(C) 125%.                                            (D) 360V.

4. If there are  (N an integer) equally likely and independent messages, then information bits in each message is         

(A) .                                               (B) .               

(C) .                                     (D) N.

5. The Hamming distance of an orthogonal code of k information bits is equal to

(A) .                                               (B) .       

(C) .                                            (D) .

6. If sampling of 20 Hz bandwidth signals is at Nyquist rate using 3 bits per sample, then the bit rate (bits/s) is 

(A) 17. (B) .

(C) 60. (D) 120.

7. The nonlinear device with  type characteristics used in a square-law AM demodulator is a  

(A) diode.                                            (B)  capacitor. 

(C) resistor.                                          (D)  comparator.

h.   An amplitude-modulated signal, carrier plus double sideband is passed through a filter before transmission to the receiving end in the 

(A) PCM system.                                 (B)  VSB system.        

(C) SSB system.                                   (D)  DSB system.

       i.     The amplitude-to-step-size ratio is 128 in a linear delta modulation for a sinusoid of frequency 800 Hz.  To avoid slope-overload, the sampling rate  must be      

(A) .                               (B) .

             (C)  =102.4 KHz.                               (D)  .

      j.     In a PCM system, the error between a digit sent and the actual signal at that instant is random and is called

(A)   distortion.                                    (B)  random noise.

             (C)   nonlinear noise.                           (D) quantisation noise.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q 2.     a.  Determine the noise figure of a microwave amplifier operating with a bandwidth of 27 MHz and internal noise power of .                                                                                                  (8)

             b.  Obtain a relationship between the noise figure and noise temperature of a two-port network (like an amplifier).                                                                                                                     (8)

Q 3.     a. Derive an expression for the instantaneous value of FM signal voltage  in terms of its amplitude A, modulation index , carrier frequency  and modulating signal frequency .  Sketch a typical FM wave.                                                                                                                           (8)

b.  A certain AM transmitter radiates 3 kW at 6 MHz with the carrier unmodulated and 3.735 kW when the carrier is sinusoidally modulated with 15 KHz signal.  Calculate the modulation index, sideband frequencies, powers and the bandwidth. (8)

Q 4.     a.  Explain briefly

(i) How voltage variable capacitor is used to frequency modulate an LC oscillator?

(ii) Armstrong system as an indirect method of frequency modulation.                (8)

             b.  A 25 MHz carrier is modulated by a 400 Hz audio sine-wave.  If the carrier voltage is 4V and the maximum deviation is 10 KHz, find the expressions for instantaneous voltage for frequency modulated and phase-modulated waves.  Write the new expressions if the modulating frequency is now changed to 2 KHz, with no other change.                                                                                                                (8)

Q 5.     a.  Compare wideband FM and narrowband FM.                                                              (8)

             b.  Explain the meaning of the following terms:

(i) Discriminator.                                                                                                       

(ii) Frequency division multiplexing.

(iii) Suppressed carrier.

(iv) Power and bandwidth saving.                                                                            (8)

Q 6.     a.  State and explain the sampling theorem for low-pass signals.  How can the original signal be recovered?                                                                                                                                     (8)

             b.  With the help of a block-diagram, explain the working of a linear delta modulator.          (8)

Q 7.     a.  Describe the basic error control coding that detects and corrects errors.                         (5)

     b.  State Shannon theorem for the capacity of a Gaussian channel.  What is its implication if a non-Gaussian channel is used?                                                                                                            (6)

             c.  Show that for n = 3 type repeated codes the matrix is given by .            (5)

Q 8.          Explain how codewords are formed and represented in

(i) Hadamard code                                                                                                (6)

(ii) Hamming code                                                                                                  (5)

(iii) Cyclic code                                                                                                      (5)

Q 9.     a.  Write briefly about the following:          

(i) factors influencing colour transmission

(ii) phased-array radar                                                                                           (8)

             b.  Calculate the Doppler frequency seen by a stationary CW Doppler radar operating at 5GHz transmit frequency when the target radial velocity is 100Km/h.  Will this radar be able to provide audio-range signals for booking speeding motorists by highway police patrol?                                              (8)