Master Resource Book in JEE Main Physics

D. B. Singh

Chapter 28

Communication Systems - all with Video Answers

Educators

Section 1

Round 1

Problem 1

For an amplitude modulated wave, the maximum amplitude is found to be $10 \mathrm{~V}$ while the minimum amplitude is found to be $2 \mathrm{~V}$. Determine the modulation index $\mu$.
(a) $\frac{2}{3}$,
(b) $4 \frac{2}{3}$
(c) $\frac{4}{5}$,
(d) $6 \frac{4}{5}$

Hunza Gilgit

Hunza Gilgit

Numerade Educator

Problem 2

The modulation techniques used for transforming digital data into analog signals are
(a) amplitude shift keying (ASK) only
(b) frequency shift keying (FSK) only
(c) phase shift keying (PSK) only
(d) All of the above

Narayan Hari

Numerade Educator

Problem 3

The speech signal of $3 \mathrm{kHz}$ is used to modulate a carrier signal of frequency $1 \mathrm{MHz}$, using amplitude modulation. The frequencies of the side bands will be [NCERT Exemplar]
(a) $1.003 \mathrm{MHz}$ and $0.997 \mathrm{MHz}$
(b) $1.954 \mathrm{MHz}$ and $2.0 \mathrm{MHz}$
(c) $40.5 \mathrm{MHz}$ and $27.6 \mathrm{MHz}$
(d) $5 \mathrm{MHz}$ and $0.997 \mathrm{MHz}$

Narayan Hari

Numerade Educator

Problem 4

Three waves $A, B$ and $C$ of frequencies $1600 \mathrm{kHz}$, $5 \mathrm{MHz}$ and $60 \mathrm{MHz}$, respectively are to be transmitted from one place to another. Which of the following is the most appropriate mode of communication?
[NCERT Exemplar]
(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

Prem Bijarniya

Numerade Educator

Problem 5

Range of frequencies allotted for commercial UHF TV broadeast is
(a) $470-960 \mathrm{kHz}$
(b) $47-960 \mathrm{MHz}$
(c) $470-960 \mathrm{MHz}$
(d) $47-960 \mathrm{kHz}$

Narayan Hari

Numerade Educator

Problem 6

Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for line of sight communication? A TV transmitting antenna is $81 \mathrm{~m}$ tall. How much service area can it cover, if the receiving antenna is at the ground level?
(a) $3800 \mathrm{~km}^{2}$
(b) $3260 \mathrm{~km}^{2}$
(c) $7400 \mathrm{~km}^{2}$
(d) $3320 \mathrm{~km}^{2}$

Narayan Hari

Numerade Educator

Problem 7

A transmitter transmits the message in original.
[a) True
(b) False
(c) Sometimes true and sometimes false
(d) Never true

Narayan Hari

Numerade Educator

Problem 8

A receiver reconstructs the original message after propagation through the channel
[a) may be true
(b) may be false
(c) may be true or false
(d) is certainly true

Narayan Hari

Numerade Educator

Problem 9

A $100 \mathrm{~m}$ long antenna is mounted on a $500 \mathrm{~m}$ tall building. The complex can become a transmission tower for waves with $\lambda \quad$ INCERT Exemplar]
(a) $\sim 400 \mathrm{~m}$
(b) $-25 \mathrm{~m}$
(c) $\sim 150 \mathrm{~m}$
$(\mathrm{d})=2400 \mathrm{~m}$

Narayan Hari

Numerade Educator

Problem 10

In amplitude modulation, carrier wave frequencies are
(a) lower compared to those in frequency modulation
(b) higher compared to those in frequency modulation
(c) same as in frequency modulation
(d) lower sometimes and higher sometimes to those in frequency modulation

Narayan Hari

Numerade Educator

Problem 11

Digital signals [NCERT]
(i) do not provide a continuous set of values
(ii) drepresent values as discrete steps
(iii) can utilize binary system and
(iv) can utilize decimal as well as binary systems Which of the above statements are true?
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i), (ii) and (iii)
(d) All of (i), (ii), (iii) and (iv)

Hunza Gilgit

Numerade Educator

Problem 12

In pulse modulation of analog signals, common pulse systems employed are
(a) Pulse Amplitude Modulation (PAM)
(b) Pulse Position and Pulse Duration Modulation [PPM and PDM)
(c) Pulse Code Modulation (PCM)
(d) All of the above

Narayan Hari

Numerade Educator

Problem 13

A $1 \mathrm{~kW}$ signal is transmitted using a communication channel which provides attenuation at the rate of $-2 \mathrm{~dB}$ per $\mathrm{km}$. If the communication channel has a total length of $5 \mathrm{~km}$, the power of the signal received is $\left[\right.$ Gain in $\left.\mathrm{dB}=10 \log \left(\frac{P_{0}}{P_{1}}\right)\right]$ [NCERT Exemplar]
(a) $900 \mathrm{~W}$
(b) $100 \mathrm{~W}$
(c) $990 \mathrm{~W}$
(d) $1010 \mathrm{~W}$

Narayan Hari

Numerade Educator

Problem 14

Sinusoidal carrier voltage of frequency $1.5 \mathrm{MHz}$ and amplitude $50 \mathrm{~V}$ is amplitude modulated by sinusoidal voltage of frequency $10 \mathrm{kHz}$ producing $50 \%$ modulation. The lower and upper side band frequencies in $\mathrm{kHz}$ are
(a) 1510,1490
(b) 1820,1012
(c) $\frac{1}{1510}, \frac{1}{1490}$
(d) 2490,1510

Narayan Hari

Numerade Educator

Problem 15

A signal emitted by an antenna from a certain point can be received at another point of the surface in the form of
(a) sty wave
(b) ground wave
(c) sea wave
(d) Both (a) and (b)

Hunza Gilgit

Numerade Educator

Problem 16

In satellite communication, the communication 2 satellite;
(a) acts as a reflector for a beam of modulated microwave from transmitter sent directly towatds it
(b) acts as a repeater for a signal reaching there, without any change in frequency
(c) receives the coming modulated microwave signal, amplifies it and returns it to earth at a different frequency
(d) None of the above

Narayan Hari

Numerade Educator

Problem 17

Which of the following frequencies will be suitable for beyond the horizon communication using sky waves?
[NCERT Exemplar]
(a) $10 \mathrm{kHz}$
(b) $10 \mathrm{Mlk}$
(c) $1 \mathrm{GHz}$
(d) $1000 \mathrm{GHz}$

Narayan Hari

Numerade Educator

Problem 18

The space wave propagation is utilised in
(a) only television communication
(b) can be reflected by ionosphere
(c) can be reflected by mesosphere
(d) cannot be reflected by any layer of earth's atmosphere

Narayan Hari

Numerade Educator

Problem 19

The radiowaves of frequency $30 \mathrm{MH}_{2}$ to $300 \mathrm{MHz}$ belong to
(a) high frequency band
(b) very high frequency band
(c) ultra high frequency band
(d) super high frequency band

Hunza Gilgit

Numerade Educator

Problem 20

In earth's atmosphere, for F_layer; the virtual height and critical frequency are
(a) $150 \mathrm{~km}$ and $3 \mathrm{MHz}$
(b) $160 \mathrm{~km}$ and $3.5 \mathrm{MH}$.
(c) $170 \mathrm{~km}$ and $4.5 \mathrm{MHz}$
(d) $180 \mathrm{~km}$ and $5 \mathrm{MH}$ z.

Narayan Hari

Numerade Educator

Problem 21

Frequency range for microwaves is
(a) $3 \times 10^{4}$ to $3 \times 10^{9} \mathrm{~Hz}$
(b) $3 \times 10^{9}$ to $3 \times 10^{13} \mathrm{~Hz}$
(c) $3 \times 10^{9}$ to $3 \times 10^{14} \mathrm{~Hz}$
(d) $1 \times 10^{9}$ to $3 \times 10^{11} \mathrm{~Hz}$

Narayan Hari

Numerade Educator

Problem 22

Ozone layer above earth's atmosphere will not
(a) prevent infrared radiations from sun reaching on earth
(b) prevent infrared radiations originated from carth from exaping earth's atmosphere
(c) prevent ultraviolet tays from sun
(d) reflect back radiowaves

Narayan Hari

Numerade Educator

Problem 23

A geo synchronous satellite is
(a) located at a height of $34860 \mathrm{~km}$ to ensure global coverage
(b) appears stationary over a place on earth's magnetic pole
(c) not teally stationary, but orbits the earth within $24 \mathrm{~h}$
(d) always at a fixed location in state and simply spins about its own axis

Hunza Gilgit

Numerade Educator

Problem 24

A message signal of frequency $\omega_{m}$ is superposed on a carrier wave of frequency 0 , to get an amplitude modulated wave (AM). The frequency of the AM wave will be
(a) $\omega_{-1}$
(b) $\omega_{f}$
(c) $\frac{\underline{\omega}_{z}+\omega_{m}}{2}$
(d) $\frac{\omega_{c}-\omega_{1}}{2}$

Narayan Hari

Numerade Educator

Problem 25

Which one of the following is correct?
(a) A single geostationary satellite can cover the whole part of the earth for microwave communication
(b) Atleast three geastationary satellites in the same orbit around earth's can cover the whole part of the earth for microwave communication
(c) The first Indian communication satellite is Apple
(d) The satellite communication is not like the line of sight microwave communication

Hunza Gilgit

Numerade Educator

Problem 26

When electromagnetic waves enter the ionised layer of ionosphere, then the relative permittivity i.e., dielectric constant of the ionised layer
(a) does not change
(b) appears to increase
(c) appears to decrease
(d) sometimes appears to increase and sometimes to decrease

Narayan Hari

Numerade Educator

Problem 27

Fading is the variation in the strength of a signal at a receiver due to
(a) interference of waves
(b) diffraction of waves
(c) polarisation of waves
(d) None of these

Narayan Hari

Numerade Educator

Problem 28

The highest frequency of radiowaves which when sent at some angle towards the ionosphere, gets reflected from that and returns to the earth is called
(a) critical frequency
(b) maximum unusable frequency
(c) polarisation of waves
(d) None of the above

Narayan Hari

Numerade Educator

Problem 29

A carrier wave of peak voltage $12 \mathrm{~V}$ is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75\%?
(a) $7 \underline{\mathrm{V}}$
(b) $6 \mathrm{~V}$
(c) $9 \mathrm{~V}$
(d) $8 \mathrm{~V}$

Narayan Hari

Numerade Educator

Problem 30

A communication between a fixed base station and several mobile units, located on ships or aircraft utilising two way radio communication in the VHF and UHF is of frequency band
(a) 3 to $30 \mathrm{MHz}$
(b) 30 to $300 \mathrm{MHz}$
(c) 30 to $470 \mathrm{MHz}$
(d) 30 to $600 \mathrm{MHz}$

Narayan Hari

Numerade Educator

Problem 31

In earth's atmosphere, for $E$-layer, the virtual height and critical frequency are
(a) $80 \mathrm{~km}$ and $3 \mathrm{MHz}$
(b) $90 \mathrm{~km}$ and $3.5 \mathrm{MHz}$
(c) $120 \mathrm{~km}$ and $4.5 \mathrm{MHz}$
(d) $110 \mathrm{~km}$ and $4 \mathrm{MHz}$

Narayan Hari

Numerade Educator

Problem 32

In space communication, the information can be passed from one place to another at a distance of $100 \mathrm{~km}$ in
(a) $1 \mathrm{~s}$
(b) $0.5 \mathrm{~s}$
(c) $0.003 \mathrm{~s}$
(d) None of these

Narayan Hari

Numerade Educator

Problem 33

The layer of earth's atmosphere responsible for absorbing a large portions of ultraviolet radiations by the sun is
(a) mesosphere
(b) troposphere
(c) ozone layer
(d) ionosphere

Narayan Hari

Numerade Educator

Problem 34

Intel-sat satellite is used for
(a) radio communication
(b) radar communication
(c) intercontinental communication
(d) None of the above

Narayan Hari

Numerade Educator

Problem 35

$1000 \mathrm{~Hz}$ carrier wave is amplitude modulated by the signal frequencies $200-4000 \mathrm{~Hz}$. The channel width of this case is
(a) $3 \mathrm{kHz}$
(b) $8 \mathrm{kHz}$
(c) $10 \mathrm{kHz}$
(d) $14 \mathrm{kHz}$

Narayan Hari

Numerade Educator

Problem 36

The range of characteristic impedance for coaxial wire lines is
(a) $40 \Omega$ to $150 \Omega$
(b) $400 \Omega$ to $1500 \Omega$
(c) $4 \Omega$ to $15 \Omega$
(d) $4 \mathrm{k} \Omega$ to $15 \mathrm{k} \Omega$

Narayan Hari

Numerade Educator

Problem 37

The diameter of an optical fibre is
(a) $10^{-5} \mathrm{~m}$
(b) $10^{-4} \mathrm{~cm}$
(c) $10^{-3} \mathrm{~cm}$
(d) $10^{-2} \mathrm{~cm}$

Narayan Hari

Numerade Educator

Problem 38

Optical fibre communication is generally preferred over general communication system because
(a) it is more efficient
(b) it has signal security
(c) it cannot be jammed as easily as radiowaves
[d) All of the above

Narayan Hari

Numerade Educator

Problem 39

A laser is a coherent source because it contains
(a) mamy wavelengths
(b) uncoordinate waves of particular wavelength
(c) coordinated waves of many wavelength
(d) coordinated waves of particular wavelength

Narayan Hari

Numerade Educator

Problem 40

If $\mu_{1}$ and $\mu_{2}$ are the refractive indices of the materials of core and cladding of an optical fibre, then the loss of light due to its leakage can be minimised by having
$(\mathrm{a}) \boldsymbol{\mu}_{1}>\boldsymbol{\mu}_{2}$
(b) $\mu_{1}<\mu_{2}$
$(\mathbf{c}) \mu_{1}=\mu_{2}$
(d) None of these

Narayan Hari

Numerade Educator

Problem 41

Numerical aperture of optical fibre is a measure of
(a) its resolving power
(b) its light gathering power
(c) the pulse dispersion
(d) the attenuation of light through it

Narayan Hari

Numerade Educator

Problem 42

The population inversion in helium-neon laser is produced by
(a) photon excitation
(b) clectron excitation
(c) inelastic atomic collisions
(d) chemical reaction

Narayan Hari

Numerade Educator

Problem 43

The number of telephonic messages which are carried by a fibre at an instant with much less intensity loss are
(a) 2400
(b) 2200
(c) 2000
(d) 1800

Narayan Hari

Numerade Educator

Problem 44

Point to point communication requires the use of
(a) a guided medium only
(b) unguided medium only
(c) any medium
(d) None of these

Narayan Hari

Numerade Educator

Problem 45

The characteristic impedance $\left(Z_{0}\right)$ of parallel line wire is
(a) $Z_{0}=\frac{276}{\sqrt{k}} \log \frac{2 s}{d}$
(b) $Z_{0}=\frac{376}{\sqrt{k}} \log \frac{2 s}{d}$
(c) $Z_{0}=\frac{276}{k} \log \frac{25}{d}$
(d) $Z_{a}=\frac{276}{\sqrt{k}} \log \frac{d}{2 s}$

Narayan Hari

Numerade Educator

Problem 46

An optical fibre made of glass with a core of refractive index of $1.55$ and include with another glass with a refractive index of 1.51. Launching takes place from air. What is the value of eritical angle for core-clad boundary?
(a) $65^{\prime}$
(b) $72^{=}$
(c) $77^{*}$
(d) $82=$

Narayan Hari

Numerade Educator

Problem 47

I-V characteristics of four devices are shown in figure.
(i)
(ii)
(iii)
(iv)
Identify devices that can be used for modulation |NCERT Exemplar]
(a) (i) and (iii)
(b) only (iii)
(c) (ii) and some regions of (iv)
(d) All the devices can be used

Prem Bijarniya

Numerade Educator

Problem 48

In coaxial cables, the repeater spacing is of the order of
(a) $20 \mathrm{~km}$
(b) $2 \mathrm{~km}$
(c) $200 \mathrm{~km}$
(d) $2000 \mathrm{~km}$

Narayan Hari

Numerade Educator

Problem 49

Quality of transmission depends upon
(a) nature of medium only
(b) nature of signal only
(c) Both (a) and (b)
(d) Neither (a) nor (b)

Narayan Hari

Numerade Educator

Problem 50

The losses in transmission lines are
(a) radiation losses only
(b) canductor heating only
(c) dielectric heating only
(d) All of the above

Narayan Hari

Numerade Educator

Problem 51

The resistance of a copper wire of length $1 \mathrm{~m}$ diameter $=2 \mathrm{~mm}$ is of the order of
(a) $0.1 \Omega m$
(b) $0.01 \Omega \mathrm{km}^{-1}$
(c) $1 \Omega \mathrm{m}$
(d) $1 \mathrm{k} \Omega$

Narayan Hari

Numerade Educator

Problem 52

Primary constants of a transmission line are
(a) resistance and inductance
(b) capacitance and conductance
(c) Both (a) and (b)
[d) None of the above

Narayan Hari

Numerade Educator

Problem 53

Which fibres are less expensive and simple to construct?
(a) Single-mode step index fibre
(b) Multi-mode step index fibre
(c) Multi graded index fibre
(d) All are equally expensive

Narayan Hari

Numerade Educator

Problem 54

A male voice after modulation-transmission sounds like that of a female to the receiver. The problem is due to $\quad$ NCFRT Exemplar]
(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

Hunza Gilgit

Numerade Educator

Problem 55

A basic communication system consists of [NCFRT Exemplar]
(A) transmitter
(B) information source
(C) user of information
(D) channel
(E) receiver
Choose the correct sequence in which these are arranged in a basie communication system
(a) $\mathrm{ABCDE}$
(b) BADEC
(c) BDACE
(d) BEADC

Prem Bijarniya

Numerade Educator

Problem 56

Identify the mathematical expression for amplitude modulated wave $\quad$ [NCERT Exemplar]
(a) $A_{c} \sin \left[\left\{e_{-}+k_{1} v_{m}(t)\right\} t+\phi\right]$
(b) $A_{c} \sin \left\{a_{c} t+\phi+k_{2} v_{m}(t)\right\}$
(c) $\left\{A_{c}+k_{2} v_{m}(t)\right\} \sin \left(\omega_{e} t+\phi\right)$
(d) $A_{c} v_{m}(t) \sin (\omega, t+\phi)$

Prem Bijarniya

Numerade Educator