Class 12 Physics Chapter 15 Communication Systems MCQs

(1) Digital signals:

[A] deal with all sort of values

[B] can use decimal system

[C] can use binary system

[D] (b) and (c) both

Answer: (b) and (c) both

(2) Digital signals:

[A] represents values as discrete steps

[B] Do not represents values as discrete steps

[C] represent values steps

[D] represent random steps

Answer: represents values as discrete steps

(3) Increase in frequency deviation results into:

[A] increase in noise of FM receiver

[B] decrease in noise of AM receiver

[C] increase in noise of Am receiver

[D] decrease in noise of FM receiver

Answer: decrease in noise of AM receiver

(4) Communication channel consists of:

[A] transmission line only

[B] optical fibre only

[C] free space only

[D] All of the above

Answer: All of the above

(5) Which is more advantageous?

[A] analog data communication

[B] digital data communication

[C] both (a) and (b) are equally good

[D] depends on the situation

Answer: digital data communication

(6) A transmitting antenna at the top of a tower has a height 3200 cm and that of the receiving antenna is 5000 cm. The maximum distance between them for satisfactory communication in line of sight mode is:

(Radius of earth = 6.4 X 10⁶ m)

[A] 1.5 × 10³ m

[B] 5.5 × 10³ m

[C] 4.5 × 10³ m

[D] 4.5 × 10⁴ m

Answer: 4.5 × 10⁴ m

(7) Which mode of communication is/are not employed for the transmission of T.V. signals?

[A] Ground wave propagation

[B] Sky wave propagation

[C] Both (a) and (b)

[D] None of these

Answer: Both (a) and (b)

(8) If both the length of an antenna and the wave length of the signal to be transmitted are doubled, the power radiated by the antenna

[A] is doubled

[B] is halved

[C] increases 16 times

[D] remains constant

Answer: remains constant

(9) Modulation is the process of superposing

[A] high frequency audio signal on low frequency audio signal

[B] high frequency message signal on low frequency audio waves

[C] low frequency message signal on high frequency waves

[D] low frequency radio signal on high frequency audio waves

Answer: low frequency message signal on high frequency waves

(10) A TV tower has a height of 100 m. The maximum distance upto which the T.V. Trans- mission can be received (Radius of earth = 8 X 10⁶ m)?

[A] 34.77 km

[B] 32.70 km

[C] 40 km

[D] 40.70 km

Answer: 40 km

(11) Which of the following mode of propagation is used to send radio-waves from one place to another?

[A] Space wave propagation

[B] Sky wave propagation

[C] Ground wave propagation

[D] All of them

Answer: All of them

(12) The radio waves of frequency 300 MHz to 3000 MHz belong to

[A] High frequency band

[B] Ultra high frequency band

[C] Very high frequency band

[D] Super high frequency band

Answer: Ultra high frequency band

(13) 12 A TV transmission tower has a height of 240 m. Signals broadcast from this tower will be received by LOS communication at a distance of (assume the radius of the earth to be 6.4 × 10⁶ m)

[A] 100 km

[B] 24 km

[C] 55 km

[D] 50 km

Answer: B, C, D

(14) Audio sine waves of 3 kHz frequency are used to amplitude modulate a carrier signal of 1.5 MHz. Which of the following statements are true?

[A] The sideband frequencies are 1506 kHz and 1494 kHz

[B] The bandwidth required for amplitude modulation is 6kHz

[C] The bandwidth required for amplitude modulation is 3 MHz

[D] The sideband frequencies are 1503 kHz and 1497 kHz.

Answer: B, D

(15) An audio signal of 15kHz frequency cannot be transmitted over long distances without modulation because

[A] the size of the required antenna would be at least 5 km which is not convenient

[B] the audio signal can not be transmitted through sky waves

[C] the size of the required antenna would be at least 20 km, which is not convenient

[D] effective power transmitted would be very low, if the size of the antenna is less than 5 km

Answer: A, B, D

(16) A message signal of frequency ωm is superposed on a carrier wave of frequency ωc to get an amplitude modulated wave (AM). The frequency of the AM wave will be

[A] ωm

[B] ωc

[C] ωc + ωm/2

[D] ωc – ωm/2

Answer: ωc

(17) A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude modulation. The frequencies of the sidebands will be

[A] 1.003 MHz and 0.997 MHz

[B] 3001 kHz and 2997 kHz

[C] 1003 kHz and 1000 kHz

[D] 1 MHz and 0.997 MHz

Answer: 1.003 MHz and 0.997 MHz

(18) A 1 KW signal is transmitted using a communication channel which provides attenuation at the rate of – 2dB per km. If the communication channel has a total length of 5 km, the power of the signal received is [gain in dB = 10 log P0/P1]

[A] 900 W

[B] 100 W

[C] 990 W

[D] 1010 W

Answer: 100 W

(19) A 100m long antenna is mounted on a 500m tall building. The complex can become a transmission tower for waves with λ

[A] ~ 400 m

[B] ~ 25 m

[C] ~ 150 m

[D] ~ 2400 m

Answer: ~ 400 m

(20) AM is used for broadcasting because:

[A] it is more noise immune than other modulation system

[B] it requires less transmitting power compare with other systems

[C] its use avoids receiver complexity

[D] No other modulation system can provide the necessary bandwidth faithful transmission

Answer: its use avoids receiver complexity

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NCERT Solutions for class 12 Physics | Chapter 15 - Communication Systems

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