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

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Questions
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
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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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