Physics NCERT Exemplar Solutions Class 12th Chapter Fifteen

A basic communication system consists of transmitter.
B. Information source.
C. User of information.
D. Channel.
E. Receiver.
Choose the correct sequence in which these are arranged in a basic communication system.
(a) ABCDE   (b) BADEC   (c) BDACE    (d) BEADC

A male voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to
(a) Poor selection of modulation index (selected 0 < m <1)
(b) Poor bandwidth selection of amplifiers
(c) Poor selection of carrier frequency
(d) Loss of energy in transmission.

I-V Characteristics of 4 devices are shown in figure.

A message signal of frequency ω_mis superposed on a carrier wave of frequency ω_c to get an Amplitude Modulated Wave (AM). The frequency of the AM wave will be

A speech signal of 3 kHz is used to modulate a carrier signal of frequency 1 MHz, using amplitude modulation. The frequencies of the side bands 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

A1 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  d_B=10 log₁₀(p₀/p_i)]
(a) 900 W    (b) 100 W    (c) 990 W    (d) 1010 W

A loom long antenna is mounted on a 500 m tall building. The complex can become a transmission tower for waves with λ.
(a) ~400m    (b) -25 m    (c) -150 m     (d) -2400 m

Three waves A, B and C of frequencies 1600 kHz, 5 MHz and 60 MHz respectively are to be transmitted from one place to another. Which of the following is the most appropriate mode of communication?
(a) A is transmitted via space wave while B and C are transmitted via sky wave
(b) A is transmitted via ground wave, B via sky wave and C via space wave
(c) B and C are transmitted via ground wave while A is transmitted via sky wave
(d) B is transmitted via ground wave while A and C are transmitted via space wave

On radiating (sending out) and AM modulated signal, the total radiated power is due to energy carried by ω_c, (ω_c– ω_m) and (ω_c + ω_m). Suggest ways to minimise cost of radiation without compromising on information.

The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is found to be f_max= 9(N_max)^1/2, where N_maxis the maximum electron density at that layer of the ionosphere.
On a certain day it is observed that signals of frequencies higher than 5 MHz are not received by reflection from the F₁ layer of the ionosphere while signals of frequencies higher than 8 MHz are not received by reflection from the F₂ layer of the ionosphere. Estimate the maximum electron densities of the F₁ and F₂ layers on that day.