Chapter 15: Communication Systems

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 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

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 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

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

I-V characteristics of four devices are shown in the figure.

Identify devices that can be used for modulation:

(a) ‘i’ and ‘iii’

(b) only ‘iii’

(c) ‘ii’ and some regions of ‘iv’

(d) All the devices can be used.

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

A basic communication system consists of

(A) 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

Identify the mathematical expression for amplitude modulated wave:

(a) Ac sin [{ωc + k1vm(t)}t + φ ]

(b) Ac sin {ωc t + φ + k2 vm(t)}

(c) {Ac + k2 vm(t)} sin (ωc t + φ )

(d) Ac vm(t) sin (ωc t + φ )

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