NEET Physics Communication Systems Questions Solved

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Give reason, why high frequency carrier waves are needed for effective transmission of information signals.

need for long antenna diminishes, with explanation           (1) mark

power is inversely proportional to (wavelength)2               ( ½ ), mark

signals from different transmitters can be distinguished     ( ½ ) mark

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What is the range of frequencies used for T.V. transmission? State two factors by which the range of TV signals can be increased.

Range: 76-88 MHz and 420-890 MHz                                                                      (1) mark

Factors: by increasing height of transmitting antenna and using repeater stations.     (1) mark

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Distinguish between broadcast mode and point-to-point mode of communication and give one example for each.                                                          2 marks

In point-to-point communication mode, communication takes place over a link between a single transmitter and a single receiver.                           12 marks

In the broadcast mode, there are a large number of receivers corresponding to a single transmitter.                                                                          12 marks

Example :

                Point-to-point :       telephone (any other)                       12 marks  

                Broadcast :             T.V., Radio (any other)                      12 marks

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(a) Define the term 'modulation index,' used in communication system. Why is

     its value kept less than or equal to one ?

(b) A message signal of frequency 10 kHz and peak voltage of 10V is used to

      modelate a carrier frequency 1 MHz and peak voltage 10 V. Determine the

      (i) modulation index, and 

      (ii) side bands produced.

                                                                                                      3 marks

a)     μ=AmAc                                                                                           1 marks         μ1 to avoid distortion of signal.                                              12 marks        

 

b)   μ=10V10V=1                                                                               12 marks       Vc-Vm = (1000-10)kHz=990kHz                                     12 marks                Vc+Vm=(1000+10)kHz=1010kHz                            12 marks

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A message signal of frequency 20 KHz and peak voltage of 20 volts is used to modulate a carrier signal of frequency 2 MHz and peak voltage of 40 volts. Determine (i) modulation index, (ii) the side bands produced. Draw the corredponding frequency spectrum of amplitude modulated signal.

(i) modulation index =  Am/ Ac=20/40=0.5

    The side bands are (2000+20) KHz

                          =2020 KHz and (2000-20)KHz

                          = 12980 KHz                                           (1/2 +1/2) mark

 Amplitude versus w for amplitude modulated signal: page number 525 NCERT part (ii) Figure 15.9, Ac=40 volts, μAc/2=10 volts   (1) marks

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Draw a block diagram of a generalized communication system. Write the functions of each of the following :

(a) Transmitter 

(b) Channel

(c) Receiver                                     (3)

[Alsp accept the following diagram

                12

(a) Transmitter:A transmitter process the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception.    12

(b)Channel:It carries the message signal from a transmitter to a receiver.   12

(c)Receiver:A receiver signals at the channel output.           12

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(a) How is amplitutde modulation achieved?

(b) The frequencies of two side bands in an AM wave are 640 kHz and

660kHz respectively. Find the frequencies of carrier and modulating 

signal. What is the bandwidth required for amplitude modulation?            (3 marks)

(a) Amplitude modulation can be achieved by applying the 

message signal, and the carrier wave, to a non linear

(square law device) followed by a band pass filter.

(Alternatively, The student may just draw the block diagram.)

(Alternatively, Amplitude modulation is achieved by superposing a message

signal on a carrier wave in a way that causes the amplitude of the carrier

wave to change in accordance with the message signal.)                        (1 mark)

(b) Frequencies of side bands are:                                                     

               (vc+vm) and (vc-vm)                                                           (1/2 mark)

so,       vc+vm=660kHz

and        vc-vm=640kHz

so,        vc=650 kHz                                                                         (1/2 mark)

so,       vm=10 kHz                                                                            (1/2 mark)

Bandwidth = (660-640)kHz = 20kHz                                                        (1/2 mark)

 

 

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Explain the terms (i) Attenuation and (ii) Demodulation used in communications system.                          (1)

(i) The loss of strength of a signal while propagating through a medium.        (1)

(ii) The process of retrival of information, from the carrier wave, at the receiver. (1)

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(a) Explain any two factors which justify the need of modulating a low frequency

signal.                                                                                                         (3 marks)

(b) Write two advantages of frequency modulation over amplitude modulation.

 

(a) A low frequency signal is modulated for the following purposes:

(i) It reduces the wavelength of transmitted signal, and the minimum

height of antenna becomes practically achievable.                               (1 mark)

(ii) Power radiated into the space by an antenna is inversely

proportional to λ2. Therefore, the power radiated into the 

spaces increases and signal can travel larger distance.

(Give full credit of this part for any other correct answer)                       (1 mark)

(b) (i) High efficiency

(ii) Less noise

(iii) Maximum use of transmitted power (any two)                            (1/2+1/2 marks)

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A signal of 5 kHz frequency is amplitude modulated on a carrier wave of frequency 2 MHz. What are the frequencies of the side bands produced?

νside bands= vc±vm                                           (1/2) marks

=2005 kHz; 1995 kHz                                       (1/2) marks

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