CBSE Class 12 Physics Communication Systems Assignment Set A

Communication Systems MCQ Questions with Answers Class 12 Physics 

Question- A transducer used at the transmitting end, serves the purpose of converting

(a) electrical signal to sound form

(b) sound signal to electrical form

(c) electrical signal to magnetic form

(d) sound signal to magnetic form

Answer-(b)

Question- The AM wave is equivalent to the summation of

(a) two sinusoidal waves

(b) three sinusoidal waves

(c) four sinusoidal waves

(d) None of these

Answer-(b)

Question- Reception of information involves

(a) decoding of signal

(b) storage of signal

(c) interpretation of signal

(d) All of the above

Answer-(d)

Question- If the output of the information source is a non-electrical signal like a voice signal, a ...A... converts it to ...B.. form before giving it as an input to the ...C... . Here, A, B and C refer to

(a) receiver, electrical, channel

(b) channel, magnetic, transducer

(c) transducer, electrical, channel

(d) transducer, electrical, transmitter

Answer-(d)

Question- Communication on ground is through electromagnetic waves of wavelength

(a) larger than 600 m

(b) between 200 and 600 m

(c) between 1 and 5 m

(d) between 10 ^{– 3} and 0.1

Answer-(d)

Question- The term channel is used to indicate

(a) the amplitude range allocated to a given source

(b) the frequency range allocated to a given source

(c) the voltage-range allocated to a given source

(d) All of the above

Answer-(b)

Question- The purpose of ...A... is to convert the message signal produced by the source of information into a form suitable for transmission through the ...B... Here, A and B refer to

(a) channel, transmitter

(b) transmitter, channel

(c) receiver, transmitter

(d) receiver, channel

Answer-(b)

Question-Broadcasting antennas are generally

(a) omnidirectional type

(b) vertical type

(c) horizontal type

(d) None of these

Answer-(b)

Question- Modem is a short form of

(a) modulator-demodulator

(b) multiplexer-demultiplexer

(c) multivibrator-degenerator

(d) None of these

Answer-(a)

Question- In space communication, the sound waves can be sent from one place to another

(a) through space

(b) through wires

(c) by superimposing it on undamped electromagnetic waves

(d) by superimposing it on damped electromagnetic waves

Answer-(c)

Question- Optical fibre are used for long distance communication because

(a) it amplifies signals to be transmitted

(b) it transfer signals faster than electrical cables

(c) it pre-emphasise weak signals

(d) it provide little attenuation as compared to electrical cable for light propagation

Answer-(d)

Question- For transmission of speeches, talks, music, dramas etc-------- is used

(a) radio broadcast transmitter

(b) radio telegraph transmitter

(c) navigation transmitter

(d) None of these

Answer-(a)

Question- Ionosphere as a whole is

(a) +vely charges

(b) –vely charges

(c) electrically neutral

(d) can't say

Answer-(c)

Question-  E.m.wave of audible frequency cannot be directly propagated over a long distance because

(a) they have vary small energy content

(b) the length of antenna required for transmission of these wave is too large

(c) both ( a) and (b)

(d) neither (a) nor (b)

Answer-(c)

Question- As the height of satellite orbit gets lower, the speed of the satellite

(a) increases

(b) decreases

(c) remain same

(d) both (a) and (b)

Answer-(b)

Question-  In sky-wave propagation, skip-distance depends on

(a) frequency of e.m. waves transmitted

(b) critical frequency of the layer

(c) height of layer above earth’s surface

(d) all of the above

Answer-(d)

ONE MARK QUESTIONS:

Question. What are the three basic units of communication system.
Answer :  (i)Transmitter (ii) Communication channel (iii) Receiver

Question. Name the types of communication systems according to the mode of transmission.
Answer : (i) Analog Communication (ii) Digital Communication

Question. Name an appropriate communication channel, needed to communicate a signal of bandwidth 100 kHz over a distance of 8 km.
Answer : Space wave communication

Question. How does the effective power radiated by an antenna vary with wavelength.
Answer :  P is inversely proportional to λ2

Question. What is meaning of the term attenuation in communication system ?
Answer : Loss of strength of signal while propagating through a medium.

TWO MARKS QUESTIONS:

Question. A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?
Answer : modulation index = Am/Ac
Am = 75 x12/100 =9V

Question. What should be the length of the dipole antenna for carrier wave of frequency 6x108 Hz?
Answer : Length of dipole antenna (Image)

Question. Define modulation and demodulation.
Answer : Modulation : process of superimposing message signal on a high frequency carrier wave.
Demodulation : process of retrieval of information from the carrier wave to the receiver. It is a reverse process of modulation.

Question. Name any two types of transmission media that are commonly used for transmission of signals also write their range of frequencies.
Answer : Coaxial Cable : Bandwidth of 750 MHz
Optical Fibre : Frequency range of 1 THz to 1000THz

Question. Identify the parts X and Y in the block diagram of communication system. (Image)
Answer : X - information source , Y- transmission channel

Question. The maximum amplitude of an amplitude modulated wave is found to be 15 V while its minimum amplitude is found to be 3 V. What is the modulation index?
Answer : modulation index, μ = Amax + Amin / Amax - Amin
= 18/12 = 9/6 = 3/2 = 1.5

THREE MARKS QUESTIONS

Question. Give reason, why high frequency carrier waves are needed for effective transmission of information signals.
Answer :  (i) need for long antenna diminishes
(ii) power is inversely proportional to (wavelength)2
(iii) signals from different transmitters can be distinguished

Question. What is the range of frequencies used for T.V. transmission? State two factors by which the range of TV signals can be increased.
Answer : Range: 76-88 MHz and 420-890 MHz
Factors: by increasing height of transmitting antenna and using repeater stations

Question. What is ground wave communication? On what factors the maximum range of propagation in this mode depend.
Answer : The mode of propagation in which wave glides over the surface of the earth is called ground wave communication .The maximum range of propagation in this mode depends on (i) transmitted power(ii) frequency

Question. What is space wave communication ? Write the range of frequencies suitable for space wave communication. Give two examples of space wave mode of propagation is used.
Answer : When a wave propagates in a straight line , from the transmitting antenna to the receiving antenna ,its mode of propagation is called space wave propagation . Frequency range : above 40 MHz Use: Television broadcast, microwave link , satellite communication

Question. (i)Which mode of propagation is used by shortwave broadcast services having frequency range from a few MHz upto 30 MHz. Explain diagrammatically how long distance communication can be achieved by this mode.
Answer : Sky wave propagation, by ionospheric reflection (Image)
(ii) because frequencies higher than 40 MHz sky waves penetrates the ionosphere and are not reflected back.
(ii) Why is there an upper limit to frequency of waves used in this mode?