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

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## Communication Systems Class 12 MCQs Questions with Answers

Solve these Communication Systems Multiple Choice Questions of Class 12 Physics Chapter 15 MCQ, so as not to miss out on any concept from being clear about what they mean!

Question 1.
A carrier frequency of 1 MHz and peak value of 10 V is amplitude modulated with a signal frequency of 10 kHz with peak value of 0.5 V. Then the modulation index and the side band frequencies respectively are
(a) 0.05 and 1 ± 0.010 MHz
(b) 0.5 and 1 ± 0.010 MHz
(c) 0.05 and 1 ± 0.005 MHz
(d) 0.5 and 1 ± 0.005 MHz
(e) 0.05 and 1 ± 0.100 MHz

Answer: (a) 0.05 and 1 ± 0.010 MHz

Question 2.
The maximum line – of – sight distance dM between two antennas having heights hT and hR above the earth is
(a) $$\sqrt{R(h_T+h_R)}$$
(b) $$\sqrt{2R/(h_T+h_R)}$$
(c) $$\sqrt{Rh_T}$$ + $$\sqrt{2Rh_R}$$
(d) $$\sqrt{2Rh_T}$$ + $$\sqrt{2Rh_R}$$
(e) $$\sqrt{2Rh_T}$$ + $$\sqrt{Rh_R}$$

Answer: (d) $$\sqrt{2Rh_T}$$ + $$\sqrt{2Rh_R}$$

Question 3.
The frequency band used in the downlink of satellite communication is
(a) 9.5 to 2.5 GHz
(b) 896 to 901 MHz
(c) 3.7 to 4.2 GHz
(d) 840 to 935 MHz
(e) 3.7 to 4.2 MHz

Answer: (c) 3.7 to 4.2 GHz

Question 4.
In amplitude modulation, the bandwidth is
(a) twice the audio signal frequency
(b) thrice the audio signal frequency
(c) thrice the carrier wave frequency
(d) twice the carrier wave frequency
(e) sum of audio signal frequency and carrier wave frequency

Answer: (a) twice the audio signal frequency

Question 5.
If both the length of an antenna and the wavelength of the signal to be transmitted are doubled, the power radiated by the antenna
(a) is doubled
(b) is halved
(c) remains constant
(e) increases 16 times

Question 6.
If the maximum amplitude of an amplitude modulated wave is 25 V and the minimum amplitude is 5 V, the modulation index is
(a) $$\frac { 1 }{5}$$
(b) $$\frac { 1 }{3}$$
(c) $$\frac { 3 }{2}$$
(d) $$\frac { 2 }{5}$$
(e) $$\frac { 2 }{3}$$

Answer: (e) $$\frac { 2 }{3}$$

Question 7.
A modem is a
(a) modulating device only
(b) demodulating device only
(c) modulating and demodulating device
(d) transmitting device
(e) receiving device

Answer: (c) modulating and demodulating device

Question 8.
The distance of coverage of a transmitting antenna is 12.8 km. Then, the height of the antenna is (Given that radius of earth = 6400 km)
(a) 6.4 m
(b) 12.8 m
(c) 3.2 m
(d) 16 m
(e) 25.6 m

Question 9.
If Ec = 20 sin 105 πt and Em = 10 sin 400 πt are carrier and modulating signals, the modulation index is
(a) 56%
(b) 30%
(c) 50%
(d) 48%
(e) 60%

Question 10.
Which one of the following is incorrect statement in the transmission of electromagnetic waves?
(a) Ground wave propagation is for high frequency transmission
(b) Sky wave propagation is facilitated by ionospheric layers
(c) Space wave is of high frequency and is suitable for line of sight communication
(d) Space wave is used for satellite communication
(e) Very high frequency waves cannot be reflected by the ionospheric layers

Answer: (a) Ground wave propagation is for high frequency transmission

Question 11.
1000 kHz carrier wave is amplitude modulated by the signal frequency 200 – 4000 Hz. The channel width of this case is
(a) 8 kHz
(b) 4 kHz
(c) 7.6 kHz
(d) 3.8 kHz
(e) 400 kHz

Question 12.
If the TV telecast is to cover a radius of 120 km (given the radius of the earth = 6400 km), the height of the transmitting antenna is
(a) 1280 m
(b) 1125 m
(c) 1560 m
(d) 79 m
(d) 1050 m

Question 13.
Identify the incorrect statement from the following
(a) AM detection is carried out using a rectifier and an envelop detector
(b) Pulse position denotes the time of rise or fall of the pulse amplitude.
(c) Modulation index p is kept ≥ 1, to avoid distortion.
(d) Facsimile (FAX) scans the contents of the document to create electronic signals.
(e) Detection is the process of recovering the modulating signal from the modulated carrier wave.

Answer: (c) Modulation index p is kept ≥ 1, to avoid distortion.

Question 14.
The mobile telephones operate typically in the range of
(a) 1 – 100 MHz
(b) 100 – 200 MHz
(c) 1000 – 2000 MHz
(d) 800 – 950 MHz
(e) 10 – 1000 MHz

Answer: (d) 800 – 950 MHz

Question 15.
A television tower of height 140 m can broadcast its signal upto a maximum area of (Radius of earth = 6.4 × 106m)
(a) 1.56 × 106 km²
(b) 5.6 × 10³ km²
(c) 5.6 × 1010 km²
(d) 1.56 × 109 km²
(e) 1.56 × 104 km²

Answer: (b) 5.6 × 10³ km²

Question 16.
The waves that are bent down by the ionosphere are
(a) ground waves
(b) surface waves
(c) direct waves
(d) space waves
(e) sky waves

Question 17.
A ground receiver in line-of-sight communication cannot receive direct waves due to
(a) its low frequency
(b) curvature of earth
(c) its high intensity
(d) smaller antenna
(e) both its low frequency and high intensity

Question 18.
In an amplitude modulation with modulation index 0.5, the ratio of the amplitude of the carrier wave to that of the side band in the modulated wave is
(a) 4 : 1
(b) 1 : 1
(c) 1 : 2
(d) 2 : 1
(e) 1 : 4

Question 19.
An example of point to point mode of communication is
(c) television
(d) internet
(e) telephony

Question 20.
A signal of peak voltage 5 V at 10 kHz is modulated using a carrier wave of peak voltage 10 V at 2 MHz. The side band frequencies are
(a) 2010 kHz, 1990 kHz
(b) 2000 kHz, 1990 kHz
(c) 2010 kHz, 2000 kHz
(d) 2100 kHz, 1900 kHz
(e) 2200 kHz, 1800 kHz

Answer: (a) 2010 kHz, 1990 kHz

Question 21.
The range of frequency bands used for television VHF service is
(a) 540- 1600 kHz
(b) 88 – 108 MHz
(c) 3.7-4.2 GHz
(d) 54 – 72 MHz
(e) 174 – 216 MHz

Answer: (d) 54 – 72 MHz

Question 22.
The radiating power of a linear antenna of length l for a wave of wavelength λ is proportional to
(a) $$\frac { l }{λ}$$
(b) $$\frac { l^2 }{λ^2}$$
(c) $$\frac { l }{λ^2}$$
(d) $$\frac { l^2 }{λ}$$
(e) $$\frac { l }{√λ}$$

Answer: (b) $$\frac { l^2 }{λ^2}$$

Question 23.
A signal of 5 kHz frequency modulates a carrier of frequency 1 MHz and peak voltage 25 V. If the amplitude at the sidebands of the amplitude modulated signal is 5 V, then the modulation index is
(a) 0.8
(b) 0.6
(c) 0.5
(d) 0.2
(e) 0.4

Question 24.
A repeater is a combination of
(e) amplifier and transmitter

Question 25.
If the pulse duration varies in accordance with the modulating voltage, then the modulation is called as
(a) PAM
(b) PWM
(c) PPM
(d) FM
(e) PCM

Question 26.
The increasing order of the frequency bands used for various communication services is
(a) space waves, sky waves, ground waves
(b) space waves, ground waves, sky waves
(c) sky waves, ground waves, space waves
(d) ground waves, sky waves, space waves
(e) sky waves, space waves, ground waves

Answer: (d) ground waves, sky waves, space waves

Question 27.
A point-to-point communication mode is seen in
(a) Satellite cable communication
(b) Television transmission
(e) Fax transmission

Question 28.
If the heights of transmitting and the receiving antennas are each equal to h, the maximum line-of- sight distance between them is (R is the radius of earth)
(a) $$\sqrt {2Rh}$$
(b) $$\sqrt {4Rh}$$
(c) $$\sqrt {6Rh}$$
(d) $$\sqrt {8Rh}$$
(e) $$\sqrt {Rh}$$

Answer: (d) $$\sqrt {8Rh}$$

Question 29.
The ionospheric layer acts as a reflector for the frequency range
(a) 1 kHz to 10 kHz
(b) 3 to 30 MHz
(c) 3 to 30 kHz
(d) 100 kHz to 1 MHz
(e) 3 GHz to 30 GHz

Answer: (b) 3 to 30 MHz

Question 30.
An AM radio station operating at 630 kHz is permitted to broadcast audio frequencies up to 6 kHz. The band pass filter in its modulation circuit can retain the frequencies
(a) 636 kHz, 630 kHz
(b) 12 kHz, 6 kHz
(c) 1260 kHz, 6 kHz
(d) 1260 kHz, 630 kHz
(e) 6 kHz, 630 kHz

Answer: (a) 636 kHz, 630 kHz

Question 31.
A transducer, in communication system is a device that
(a) is a part of the antenna
(b) is a combination of a receiver and a transmitter
(c) converts audio signals into video signals
(d) detects the incoming signal
(e) converts physical variable into corresponding variations in the electrical signal

Answer: (e) converts physical variable into corresponding variations in the electrical signal

Question 32.
Identify the mismatched pair
(a) Noise – Unwanted signals
(b) Repeater – Communication satellite
(c) Transducer – Energy converter
(d) Demodulation – Retrieval of information
(e) Attenuation – Strengthening of signal

Answer: (e) Attenuation – Strengthening of signal

Question 33.
Pick out the wrong statement
(a) Analog signals provide a continuous set of values
(b) Digital signals represent values as discrete steps
(c) Analog signals cannot utilize the binary system
(d) Digital signals can be processed by logic gates
(e) Digital signals can utilize decimal as well as binary systems

Answer: (e) Digital signals can utilize decimal as well as binary systems

Question 34.
A ground receiver receives a signal at 5 MHz, transmitted by a ground transmitter at a height of 320 m, which is 110 km away from it. Then it can communicate through (radius of earth R = 6400 km)
(a) space waves
(b) ground waves
(c) sky waves
(d) both sky and ground waves
(e) sky waves, ground waves and space waves

Question 35.
The power radiated by a linear antenna of length l at wavelength λ is
(a) directly proportional to l
(b) inversely proportional to λ
(c) inversely proportional to l
(d) directly proportional to λ²