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 best alternative in the following:                                         (2x10)

a.   Air for wireless communication and wire for landline communication are called

(A)   channels.                                      (B)  transmitters.

(C)  information sources                        (D)  receivers.

2. The type of noise associated with all amplifying or active devices is the

(A) solar noise                                      (B) man-made noise    

(C) shot noise                                      (D) cosmic noise

3. If the SNRs at the input and output of a receiver are 36 dB and 20 dB respectively, then the receiver noise figure will be   

(A) 1.8 dB                                           (B) 16 dB

(C) –16 dB                                          (D) 56 dB

4. For a modulating sinusoidal voltage with  the AM wave has . The modulation index is            

(A) 0.5                                                 (B) 0.8            

(C) 0.6                                                 (D) 0.7

5. By Carson’s rule, the bandwidth of an FM signal (of modulating frequency  and maximum frequency deviation  proportional to ) is

(A) (B)

(C) (D)

6. The average information per message interval for M different and independent messages  with probabilities of occurrence  is given by

(A) (B)

(C) (D)

7. Distortion of modulation envelope and sideband overlap with adjacent channels are caused when modulation index m of AM is  

(A) m > 1                                             (B)  m < 1       

(C) m = 1                                             (D) m = 0

h.   Indicate the digital system of modulation

(A) PAM                                             (B)  PCM       

(C) PFM                                              (D) PWM

       i.     A sinusoid of amplitude A, frequency f has a maximum slope =  while passing through zero.  To avoid slope-overload in linear delta modulation with step-size S, it must satisfy the condition    

(A) (B)

             (C)  (D) 

      j.     The triangular noise distribution for FM, called the noise triangle, and the corresponding AM distribution are represented as

(A) (B)

             (C) (D) 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q 2.     a. Sketch an AM wave for m < 1 and draw its frequency spectrum.  Show that total power , where  is carrier power and m is modulation index.                   (8)

b.  Consider an amplifier operating at  over the frequency range of 455 to 460 KHz, having a 200 K input resistor.  Find the rms noise voltage at the input of the amplifier.                             (4)

             c.  A receiver connected to an antenna of 50 has an equivalent noise resistance of 30.  Calculate the noise figure (dB) and the equivalent noise temperature of the receiver operating at .       (4)

Q 3.     a.  List the advantages of FM over AM and of AM over FM for communication.                (8)

             b.  For the FM wave:  V, what are the values of amplitude, carrier and modulating signal frequencies in Hz?  Find the modulation index, maximum frequency deviation, and the power dissipated in a 10 resistor.                                                                              (8)

Q 4.     a.  What are pre-emphasis and de-emphasis operations?  Why are they used?  Explain the function of a typical pre-emphasis circuit.  Why de-emphasis must also be used?                                          (8)

             b.  Suppose a carrier of angular frequency  is amplitude modulated by a sinusoid of angular frequency  to give a resultant waveform: .  If one of the sidebands is removed to give waveform , show that the amplitude of  is given by: . (8)

  Q5.     a.  Explain the difference between Natural Sampling and Flat-top Sampling, with illustrative waveforms.                                                                                                                                     (8)

             b.  Explain the principle of Adaptive Delta Modulation.  How does it differ from Linear Delta Modulation?                                                                                                                                    (8)

  Q6.     Write short notes on:

(i) PCM                                                                                                                (6)

(ii) PWM                                                                                                               (5)

(iii) Moving target indicator                                                                                      (5)

  Q7.          Explain the following terms with respect to information theory

(i) Block-codes                            

(ii) Systematic codes

(iii) Hamming distance                    

(iv) Entropy                                                                                                       (4 x 4)

  Q8.     a.  With the help of a block-diagram, explain briefly the function of a pulsed radar.                    (8)

b.  Consider a radar operating at 1.25 GHz using peak pulse power of 3MW to detect targets of  radar cross-section at a range of 185.2 km.  Determine the smallest diameter D of the reflector antenna, if the minimum acceptable power by receiver is  and effective antenna               area = 0.5 .                                                                                                                                 (8)

  Q9.     a.  Explain the terms referred to in television:                                                                

(i) Compatibility in colour TV

(ii) Blanking pulse

(iii) Scanning

(iv) Synchronisation pulse                                                                                        (8)

             b.  An analog signal is band limited to B Hz, sampled at the Nyquist rate and the samples are quantised into 4 levels  to  (messages) assumed independent and occurring with probabilities  and .  Find the information rate R of the source.  If the four levels have equal probability of occurrence, will there be an improvement on R?                                                            (8)