# MCQ Questions for Class 12 Physics Chapter 15 Communication Systems with Answers

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## Communication Systems Class 12 MCQs Questions with Answers

Question 1.
Modulation index of an AM signal:
(a) depends upon the peak amplitude of the modulating signal and the carrier.
(b) depends upon the peak amplitude of modulating signal.
(c) is a function of carrier frequency.
(d) None of these.

Answer: (a) depends upon the peak amplitude of the modulating signal and the carrier.

Question 2.
In amplitude modulation
(a) the amplitude of the carrier remains constant.
(b) the amplitude of the carrier varies in accordance with the amplitude of the modulating signal.
(c) the amplitude of carrier varies in accordance with the frequency of the modulating signal.
(d) None of these.

Answer: (b) the amplitude of the carrier varies in accordance with the amplitude of the modulating signal.

Question 3.
In modulation process, radio frequency wave is termed as
(a) modulating wave
(b) modulated wave
(c) carrier wave
(d) none

Question 4.
Digital signals:
(a) represent values as discrete steps.
(b) utilise binary code system.
(c) do not provide a continuous set of values.
(d) All of these.

Question 5.
Modulation index in an FM signal:
(a) Varies directly as the modulating frequency.
(b) Varies directly as frequency deviation and inversely as the modulating frequency.
(c) varies inversely as the frequency deviation. ‘
(d) None of these.

Answer: (b) Varies directly as frequency deviation and inversely as the modulating frequency.

Question 6.
The most commonly employed analog modulation technique in satellite communication is the
(a) amplitude modulation
(b) frequency modulation
(c) phase modulation
(d) All of the above.

Question 7.
Satellite transponders
(a) use a high frequency for reception and a lower frequency for retransmission of information to and from earth
(b) use a lower frequency for reception and a higher frequency for retransmission of information to and from earth.
(c) Use a single frequency for reception and retransmission of information to and from earth.
(d) none of the above.

Answer: (a) use a high frequency for reception and a lower frequency for retransmission of information to and from earth

Question 8.
For television transmission, the frequency employed is normally in the range
(a) 30-300 Hz
(b) 30-300 kHz
(c) 30-300 MHz
(d) 30-300 GHz.

Question 9.
Which one of the following can happen to the electromagnetic waves travelling in free space:
(a) Reflection
(b) Refraction
(c) absorption
(d) attenuation

Question 10.
High frequency waves are
(a) reflected by E layer.
(b) absorbed by F layer.
(c) affected by the solar cycle.
(d) capable of use for long-distance transmission.

Question 11.
Which of the following is invariably found on all communication satellites as a payload
(a) transponder
(b) photographic camera
(c) optical telescope
(d) None of these

Question 12.
An antenna is
(a) capacitive
(b) reactive
(c) inductive above its resonance frequency.
(d) None of these

Answer: (c) inductive above its resonance frequency.

Question 13.
Microwave link repeaters are typically 50 km apart,
(a) because of atmospheric attenuation.
(b) because of the earth’s curvature.
(c) to ensure that signal voltage may not harm the repeater.
(d) None of these.

Answer: (b) because of the earth’s curvature.

Question 14.
Optical communication uses the frequency range of
(a) 109 to 1010 Hz
(b) 105 to 106 Hz
(c) 1014 to 1015 Hz
(d) 1016 to 1017 Hz.

Answer: (c) 1014 to 1015 Hz

Question 15.
Audio signals have frequency range.
(a) 20 Hz to 20 kHz
(b) 20 kHz to 20 MHz
(c) 20 MHz to 20 GHz
(d) All of these.

Answer: (a) 20 Hz to 20 kHz

Question 16.
The numbers of geostationary satellites needed for uninterrupted global coverage is
(a) 3
(b) 4
(c) 2
(d) 1

Question 17.
The maximum distance upto which T.V. signals can be received from an antenna of height h is
(a) $$\sqrt{2hr}$$
(b) h$$\sqrt{2R}$$
(c) R$$\sqrt{2h}$$
(d) None of these.

Answer: (a) $$\sqrt{2hr}$$

Question 18.
The information bearing signal is called
(a) Base band signal
(b) digital signal
(c) Analog signal
(d) None of these

Question 19.
In a single reflection from the ionosphere, the sky waves cover a distance on ground not more than
(a) 400 m
(b) 4,000 m
(c) 400 km
(d) 4,000 km

Question 20.
AM is used for broadcasting because
(a) it is more noise immune than other modulation systems.
(b) it requires less transmitting power compared with other systems.
(c) its use avoids receiver complexity.
(d) no other modulation system can provide the necessary band width faithful transmission.

Fill in the Blanks

Question 1.
………………. is a device which converts energy from one form to another.

Question 2.
………………. separates out the audio signal from the modulated signal.

Question 3.
Marconi antenna has length ………………. while Hertz antenna has a length equal to ……………….

Answer: $$\frac {λ}{4}$$, $$\frac {λ}{2}$$

Question 4.
The length of antenna required to transmit a radio signal of frequency 1 MHz is about ……………….

Question 5.
The bandwidth of a complex audio signal is equal to ………………. of the audio signal.

Question 6.
In-frequency modulated signal the informatibn is contained in the ………………. and the bandwidth is ……………….

Answer: side bands, 2n × frequency of modulating signal where n is the number of significant side band pairs.

Question 7.
To achieve pulse code modulation an analog signal is ………………., ………………. and then coded.

Question 8.
A digital signal is described by the ……………….

Answer: bit rate i.e., number of binary digits (bits) per second.

Question 9.
The electromagnetic waves of radio frequencies are reflected by ………………. and this effect is used for round the curved surface of earth.

Question 10.
The bandwidth for AM radio station is ………………. and for an FM radio station, the bandwidth is ……………….

Question 11.
The modulation index of an AM wave lies between ……………….

Question 12.
The rate at which a modem can transmit and receive data is expressed as ……………….

Question 13
………………. is preferred for high fidelity reception.

Question 14.
Bit rate = ………………. × number of bits per sample.

Question 15
………………. are the relevant waves in telecommunication.

Question 16.
We use short wave bands only for long distance radio broadcast because ……………….

Answer: Ionosphere easily reflects the waves in this band.

Question 17
………………. uses carrier signals having frequencies in the range 1012 Hz to 1016 Hz.

Question 18.
The electromagnetic wave of frequency 120 MHz belongs to ……………….

Question 19.
The essential condition for demodulation is that the time period of high frequency carrier wave must be ……………….

much smaller than the time constant of RC circuit i.e.,
Tc < < τ
$$\frac {1}{f_c}$$ << RC

Question 20.
In amplitude modulation, the modulation index or degree of modulation is the ratio of ……………….

m = $$\frac {E_m}{E_c}$$ = $$\frac {V_m}{V_c}$$