AMIETE – ET (NEW SCHEME)   –   Code: AE65

 

Subject: ANALOG COMMUNICATIONS

 

Flowchart: Alternate Process: DECEMBER 2009Time: 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.  In a communication system, noise is most likely to affect the signal

 

                  (A) at the transmitter                             (B) in the channel

                  (C) in the information source                 (D) at the destination

 

             b. If the carrier of a 100% modulated AM wave is suppressed, the % power saving will be

 

                  (A) 50                                                  (B) 100

                  (C) 150                                                (D) 66.66

 

             c.  An FM signal with a modulation index  is passed through a frequency tripler.  The wave at the output of tripler will have a modulation index

                 

                                                                              (A)                                                (B)

                  (C)                                                 (D)

 

             d.  Sensitivity curve is used to check the performance of

 

                  (A) RF amplifier                                   (B) Both RF and IF amplifier

                  (C) IF amplifier                                     (D) Audio amplifier

 

             e.  Identify the false statement SWR is infinity when the load is

 

                  (A) purely reactive                                (B) short circuited

                  (C) open circuited                                 (D) complex impedance

 

             f.                                                               The wave in a waveguide has a full wave of electric intensity change between the two walls and no component in the direction of propagation.  The mode is  

 

                  (A)                                              (B)

                                                                              (C)                                             (D)

       

             g. Indicate which of the following system is digital

 

                  (A) PPM                                              (B) PWM

                  (C) PCM                                             (D) Pulse frequency modulation

 

             h.  Satellites used for intercontinental communications are known as 

 

(A)        Comsat                                       (B) Marisat

                  (C)    Domsat                                        (D) Intelsat

       

             i.   To separate the channels in FDM receiver, it is necessary to use

       

                  (A) AND gates                                     (B) Differentiation

                  (C) BPF                                               (D) Integration

 

             j.   In order to reduce quantization noise, one must

 

                  (A) increase the number of standard amplitudes.                                                                

                  (B) send pulses whose sides are nearly vertical.

                  (C) use an RF amplifier in the receiver. 

                  (D) Increase the number of samples/sec.

 

 

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

 

 

  Q.2     a.   Explain with a schematic block diagram, the basic communication system including transmitter and receiver.                                                                                                                       (10)

 

             b.   Explain the need of modulation in communication system.                                    (6)

        

  Q.3    a.                                                              Obtain the frequency spectrum of a single tone AM and plot it.                                                           (7)

            

b. The antenna current of AM transmitter is 10 amperes for only the carrier component and 11 amperes when carrier is modulated.  Find the depth of modulation.                                             (3)

 

             c.   The waveform  where m is a constant  is applied to the diode demodulator.  Show that, if the demodulator output is to follow the envelop of e(t), it is required that at any time  .                                                                              (6)

 

  Q.4     a.    Show that FM signal contains infinite number of side frequency components along with carrier component.                                                            (6)

       

             b.   What is pre-emphasis?  Why is it used?  Sketch a typical pre-emphasis circuit and also, explain why de-emphasis must be used at receiving end.                                                             (6)

       

             c.  A 400 Hz modulating signal of voltage 2.4 volts is modulated under FM to have a modulation index of 60.  Calculate the maximum deviation and required bandwidth.                                                  (4)

            

Q.5     a.      Explain a two-stage IF amplifier.  List and discuss the factors influencing the choice of IF for a radio receiver.                                                                                                                        (8)

 

        b.      Briefly, explain the function of each of block in the superhetrodyne receiver.  Discuss the advantages and disadvantages of superhetrodyne receiver over TRF receiver.                                           (8)

 

  Q.6     a.   Discuss the standing waves and impedances in a quarter wave and half wave length transmission line.                                                                     (8)

 

             b.   Explain how a smith chart can be used for the calculation of following:

 

                   (i)   Admittance

                   (ii)  Impedance

                   (iii) VSWR                                                                                                        (8)          

  Q.7     a.   With salient figures, show how the field distributes in a circular waveguide.          (8)

 

             b.   Explain the construction and operation of isolators and circulators.                      (8)

 

  Q.8     a.   Define channel capacity and redundancy of a noisy channel.                                (5)

 

             b.   Explain the generation of pulse position modulation using relevant waveforms.                     (5)

       

             c.   Explain a Radio Telemetry Transmitter using FDM.                                             (6)

            

  Q.9     a.   Explain TDM and obtain the required channel bandwidth.                                   (8)

 

             b.   Where and why are troposcatter links used in preference to other two medium distance broadband transmission media?                             (8)