A Quick Recapitulation of the Chapter

1. Communication is the act of transmission and reception of information.

2. Every communication system has three essential elements which are as
transmitter, medium or communication channel and receiver.
3. Basic terminology used in electronic communication system
(i) Signal Information converted into electrical form and suitable for transmission is
called a signal.
(ii) Transducer Any device/arrangement that converts one form of energy into
another is called a transducer, e.g. microphone.
(iii) Noise It refers to the unwanted signals that tend to disturb the transmission and
processing of message signals in communication system.
(iv) Attenuation It refers to the loss of strength of a signal during its propagation
through the communication channel.
(v) Amplification It is the process by which amplitude of a signal is increased using
an electronic circuit called the amplifier.
(vi) Range It is the largest distance between a source and a destination up to which
the signal is received with sufficient strength.
(vii) Repeater Repeaters are erected at suitable distances between the transmitter
and receiver. Repeaters are used to extend the range of a communication system.
4. Message signals A time varying electrical signal generated by a transducer out of
original signal is termed as message signal.
The message signals are of two types as given below.
(i) Analog signal A continuous electrical signal which at any instant lies within the
range of a maximum and a minimum value.
(ii) Digital signal (Pulse signal) Digital signals are those which can take only discrete
stepwise values, e.g., output of a computer, fax etc.
5. Bandwidth of communication channel is the range of frequencies used to pass
through channel.
6. Ground wave propagation (f <2 MHz ) In ground wave propagation, the radio waves
(AM) travel along the surface of the earth. These waves are guided along the earth's
surface and they follow the curvature of the earth.
7. Sky wave propagation ( 2 MHz< f <30 MHz ¿ Long distance communication can be
achieved by ionospheric reflection of radio waves back towards earth. This mode of
 propagation is called sky wave propagation and is used by short wave broadcast
services.
8. Space wave propagation (f >30 MHz) A space wave travels in a straight line from
transmitting antenna to the receiving antenna. Space waves are used for Line-of-
Sight (LOS) communication as well as satellite communication.
(i) Range of TV transmission, d= √ 2 hR
where, h=¿ height of antenna, R=¿ radius of the earth,
d =maximum distance/range of transmission
(ii) Range of line-of-sight distance between two antennas,
d M =d T + d R , d M =√ 2 hT R+ √ 2 hR R
where, hT and h R are heights of transmitting and receiving antenna and d T is the
radio horizon of the transmitting antenna.
9. Satellite communication Signals which are reflected by ionosphere is reflected back
by satellite. In this communication, frequency band 5.9 GHz to 6.4 GHz is used for
uplinking and 3.7 GHz to 4.2 GHz is used for downlinking.
10. For transmitting a signal, we need an antenna. This antenna should have a size
comparable to the wavelength of the signal (atleast λ /4 is dimension).
11. The effective power radiated by an antenna is proportional to ¿, where l is the
length of linear antenna. For a good transmission, we need high power.
12. Baseband Band of frequencies representing the original signal is called baseband.
13. Modulation Modulation is the process of variation of some characteristics of a carrier
wave in accordance with the instantaneous value of a modulating signal.
14. Types of pulse modulation
(i) PAM (Pulse Amplitude Modulation)
(ii) PDM (Pulse Duration Modulation)
(iii) PPM (Pulse Position Modulation)
(iv) PCM (Pulse Code Modulation)
15. Amplitude modulation In amplitude modulation, the amplitude of the carrier is
varied in accordance with the information signal. (i) AM can be represented by
expression
μ Ac μ Ac
C m (t)= A c sin ⁡ωc t + cos ⁡( ωc −ω m ) t−
2 2 $$
cos ⁡( ωc +ω m ) t

Am = amplitude of modulating wa ve

f c = frequency of carrier wave an f m= modulating wave frequency.

16. Modulation index
Change in amplitude of carrier wave
μ=
Amplitude of carrier wave
A m A max − A min
¿ = , A max = Ac + A m , A min = A c −A m
A c A max + A min
where, Amax =¿ maximum amplitude of AM wave
A min = minimum amplitude of AM wave
17. Upper side band frequency ¿ f c + f m Lower side band frequency ¿ f c −f m where, f c and f m
are frequencies of carrier wave and modulating wave.
Bandwidth ¿ USB −LSB= ( f c + f m )− ( f c−f m ) =2 f m
18. Demodulation It is the process of extracting the audio frequency message signal
from the modulated wave.

[Objective Questions Based on NCERT Text

Topic 1
Basic Terminology used in Electronic Communication System
1. A modern communication system acts as a
(a) transmitter
(b) receiver
(c) messenger
(d) None of these
2. Languages and methods used in communication have kept evolving from
prehistoric to modern times, to meet the growing demands in terms of ..... and .....
of information.
(a) receive, sender
(b) receiver, sender
(c) speed, complexity
(d) language, quality
3. A communication system consists of
(a) transmitter and receiver
(b) receiver and communication channel
(c) transmitter and communication channel
(d) transmitter, communication channel and receiver
4. Identify the parts … X … and … Y … in the block diagram of a generalised
communication system.
Communication system

(a) channel, transmitter

(b) transmitter, receiver
(c) receiver, channel
(d) receiver, transmitter
5. If the output of the information source is a nonelectrical signal like a voice signal,
a ..... converts it to .... form before giving it as an input to the ..... .
(a) receiver, electrical, channel
(b) channel, magnetic, transducer
(c) transducer, electrical, channel
(d) transducer, electrical, transmitter
6. When noise adds to the transmitted signal, which part of the generalised
communication system receives a corrupted version of the transmitted signal?
(a) Channel
(b) Receiver
 (c) Transducer
(d) Transmitter
7. ..... reconstructs a recognisable form of the original message signal for delivering it
to the user information.
(a) channel
(b) transducer
(c) receiver
(d) transmitter
8. In a point to point communication mode, communication takes place over a link
between
(a) transmitter and channel
(b) channel and transmitter
(c) transmitter and receiver
(d) channel and receiver
9. Which of the following is an example of point to point communication mode?
(a) Radio
(b) Television
(c) Telephone
(d) Microwave
10. The transmission media can be
(a) only guided
(b) only unguided
(c) Both (a) and (b)
(d) None of these
11. Point to point communication requires the use of
(a) only a guided medium
(b) only unguided medium
(c) any medium
(d) None of these
12. Quality of transmission depends upon
(a) nature of the medium only
(b) nature of signal only
(c) Both (a) and (b)
(d) Neither (a) nor (b)
13. Which of the following project was undertaken by US defence department?
(a) ETHERNET
(b) ARPANET
(c) INTERNET
(d) INTRANET
14. ARPANET allowed
(a) file transfer from one computer to another connected to the network
(b) a message signal representing the original signal delivered by the source of
information
(c) communicating without using wires
(d) message signals and make it suitable for transmission
15. Which of the following optical systems are superior and more economical as
compared to traditional communication system?
(a) Ray optical
 (b) Fibre optical
(c) Wave optical
(d) Light optical
16. With the advancement of technology, a communication system is based on
(a) electrical
(b) electronic
(c) optical
(d) All of these
17. Which type of communication system is used in FAX machine?
(a) Binary
(b) Analog
(c) Hybrid
(d) All of these
18. Tremendous growth of message transmission through post offices is due to
(a) E-mail
(b) FAX
(c) television
(d) telegraph
19. Which of the following is not a transducer?
(a) Loudspeaker
(b) Amplifier
(c) Microphone
(d) All of these
20. A device that converts some physical variable (pressure, displacement, force,
temperature, etc.) into the corresponding variation in the electrical signal at its
output is that
(a) transducer
(c) noise
(b) receiver
(d) transmitter
21. Audio signal cannot be transmitted directly upto large distance
(a) the signal has more noise
(b) the signal cannot be amplified for distance communication
(c) the transmitting antenna length is very small to design
(d) the transmitting antenna length is very large and cannot be achieved practically
22. In electronic communication systems, we usually come across devices that have
either their inputs and outputs in the form of
(a) magnetic
(c) Both (a) and (b)
(b) electrical
(d) None of these
23. Signals are essentially single-valued function of
(a) distance
(c) speed
(b) displacement
(d) time
24. Which of the following is the fundamental analog signal?
(a) cos wave
 (b) cosec wave
(c) sine wave
(d) tan wave
25. Which of the following statement is incorrect?
(a) All analog signals can be fully understood in terms of their sine wave
components
(b) Sound and picture signals in TV are analog in nature
(c) Digital signals are those which cannot take only discrete stepwise values
(d) Binary system that is extensively used in electronics employs just two level of a
signal
26. Fading in the variation in the strength of a signal at a receiver is due to
(a) interference of waves
(c) polarisation of waves
(b) diffraction of waves
(d) None of these 27. The loss of strength of a signal while propagating through a
medium is known as
(a) amplification
(b) receiver
(c) attenuation
(d) noise
27. Why is the amplification necessary in the communication system?
(a) To compensate for the attenuation of the signal
(b) To compensate for the range of the signal
(c) To compensate for the receiver of the signal
(d) To compensate for the transmitter of the signal
28. The energy needed for additional signal strength is obtained from
(a) AC power source
(c) generator
(b) DC power source
(d) sunlight
29. Which is the largest distance between a source and a destination upto which the
signal is received with sufficient strength?
(a) Bandwidth
(b) Demodulation
(c) Repeater
(d) Range
30. Consider the transmission of singal as shown in figure.

Why are repeaters used?

(a) To discrete the range of a communication system
(b) To extend the range of a communication system
(c) Both (a) and (b)
(d) None of the above
32. Which one of the following is essentially repeater station in space?
(a) Artificial satellite
(b) Natural satellite
(c) Communication satellite
(d) All of the above

Topic 2
Bandwidth of Signals & Transmission Medium
33. In a communication system, message signal can be
(a) voice
(b) music
(c) picture or computer data
(d) All of these
34. The type of communication system needed for a given signal depends on the which
is considered essential for the communication process.
(a) range of wavelength
(b) band of frequencies
(c) Either (a) or (b)
(d) None of the above
35. Rectangular wave can be decomposed into a superposition of sinusoidal waves of
frequencies are
(a) v 0 , 3 v 0 , 6 v 0 , 9 v 0 , ⋯ , n v 0
(b) 4 v 0 , 8 v 0 , 12 v 0 ,16 v 0 , ⋯ , n v 0
(c) v 0 , 2 v 0 , 3 v 0 , 4 v 0 , ⋯ , n v 0
(d) 2 v 0 , 4 v 0 ,6 v 0 , 8 v 0 ,⋯ , n v 0
36. What happen if the bandwidth is large enough to accommodate a few harmonics?
(a) The information is not lost
(b) The rectangular signal is more or less recovered
(c) Both (a) and (b)
(d) Only (b)
37. Large bandwidth for higher data rate is achieved by using
(a) high frequency carrier wave
(b) low frequency carrier wave
(c) high frequency audio wave
(d) low frequency audio wave
38. A laser is a coherent source because it contains
(a) many wavelengths
(b) in ordinate wave of particular wavelength
(c) coordinate waves of many wavelength
(d) coordinate waves of particular wavelength
39. Message signals are also called
(a) band signals
(b) electronic signals
(c) electromagnetic signals
(d) baseband signals
40. How many signals, is a single frequency sinusoid?
(a) Two bandwidth
(b) No signal
 (c) One signal
(d) Three signals
41. A singal spreads over a range of frequencies called
(a) signal bandwidth
(b) signal baseband
(c) signal audio frequency
(d) signal video frequency 42. The effective power radiated by a long wavelength
baseband signal would be
(a) large
(b) small
(c) either small or large
(d) neither small nor large
42. The radio waves frequency 80 MHz of 300 MHz belong to
(a) high frequency band
(b) very high frequency band
(c) ultra frequency band
(d) super high frequency band
43. Coaxial cables are normally operated below
(a) 18 GHz
(b) 18 kHz
(c) 18 MHz
(d) 18 THz
44. Communication through free space using ..... waves takes place over a very wide
range of frequencies : from a few hundreds of kHz to a GHz .
(a) gamma waves
(b) microwaves
(c) radio waves
(d) None of these
45. For mobile communication to base station, the required frequency band is
(a) 896−901 MHz
(b) 896−901 kHz
(c) 840−935 MHz
(d) 840−935 kHz
46. Optical communication is performed in the frequency range of
(a) 100 GHz
(b) 1 THz to 1000 THz
(c) 1 THz to 100 Tz
(d) Only 1000 THz
47. How much bandwidth, is required by an optical fibre for data transmission?
(a) 100 GHz
(b) 1000 GHz
(c) 100 kHz
(d) 1000 kHz
48. The range of frequency allotted for UHF TV broadband is
(a) 470−960 kHz
(b) 47−960 MHz
(c) 470−960 MHz
(d) 174−216 MHz
49. The Cellular Mobile Radio wave of frequency 840 - 935 MHz belongs to
(a) base station to mobile
(b) mobile to base station
(c) ultra high frequency
(d) very high frequency
50. Which type of communication uses carrier signals having frequencies in the range
of 1012 Hz to 1016 Hz ?
(a) Optical communication
(b) Analog communication
(c) Digital communication
(d) None of these
51. 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 MHz
(b) 30 to 300 MHz
(c) 30 to 470 MHz
(d) 30 to 600 MHz
52. 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 radio waves
(d) All of the above

Topic 3

Propagation of Electromagnetic Waves

54. As the electromagnetic waves travel away from the transmitter, the strength of the
wave keeps on
(a) increasing
(b) decreasing
(c) Either increasing or decreasing
(d) Both (a) and (b)
55. Which size of the antenna should have comparable to the wavelength λ of the
signal, to radiate signals with high frequency?
(a) Atleast ∼ λ /2
(b) Atleast ∼ λ /4
(c) At maximum ∼ λ /2
(d) At maximum ∼ λ /4
56. A ground receiver in line-of-sight communication cannot receive direct waves due to
(a) its low frequency
(b) curvature of earth
(c) its high intensity
(d) smaller antenna
57. Sky wave is used by
(a) long wave broadcast services
(b) short wave broadcast services
 (c) Both (a) and (b)
(d) middle wave broadcast services
58. The ionosphere is acting as reflector due to
(a) Presence of a short number of ions
(b) Presence of a large number of ions or charged particles
(c) Presence of neither short nor large number of ions
(d) Both (a) and (b)
59. From which height does the ionosphere extends above the earth's surface?
(a) 65 to 75 km
(b) 170 to 190 km
(c) ∼ 65 km to 400 km
(d) ∼ 65 to ∼ 75 km 60. Ionisation occurs due to
(a) absorption of the gamma rays
(b) absorption of the infrared rays
(c) absorption of the X-rays
(d) absorption of the ultraviolet rays
60. The degree of ionisation depends on
(a) height
(c) thickness
(b) intensity
(d) All of these
61. Which of the following is the correct statement?
(a) Electromagnetic wave of frequencies higher than 30 MHz penetrates the
ionosphere and escape
(b) Electromagnetic wave of frequencies higher than 30 kHz , penetrates the
ionosphere and escape
(c) Electromagnetic wave of frequencies lower than 30 MHz penetrates the
ionosphere and escape
(d) Electromagnetic wave of frequencies lower than 30 kHz penetrates the
ionosphere and escape
62. Radio waves are propagated through
(a) ground wave
(b) sky wave
(c) space wave
(d) All of these
63. Which of the following statements is incorrect?
(a) Ground wave propagation can be sustained at frequencies 500 kHz to 1500 kHz
(b) Satellite communication is useful for the frequencies above 30 MHz
(c) Sky wave propagation is useful in the 50 to 80 MHz
(d) Space wave propagation takes place through tropospheric space
(e) The phenomenon involved in sky wave propagation is total internal reflection
64. The part of the ionosphere which exists during day and night time is
(a) D -layer
(b) E-layer
(c) F 1-layer
(d) F 2-layer
65. On which path, a space wave travels from transmitting antenna to the receiving
antenna?
 (a) Parabola
(b) Straight line
(c) Circular path
(d) Rectangular path
66. Space waves are used for
(a) Line-of-Sight (LOS) communication
(b) satellite communication
(c) Both (a) and (b)
(d) None of the above
67. Line-of-Sight (LOS) communication is only possible when the frequency is
(a) above 40 MHz
(b) above 40 kHz
(c) equal to 40 MHz
(d) below 40 MHz
68. Where the receiving antenna must be stand so that the signal is to be received
beyond the horizon?
(a) At lowest
(b) At highest possible
(c) In the middle
(d) Neither low or high
69. Which of the following are the examples of communication system using space
wave mode of propagation?
(a) Telephonic communication, microwave links and television broadcast
(b) Telephonic communication, satellite communication, and microwave links
(c) Radio broadcast, television broadcast and telephonic communication
(d) Microwave link, satellite communication and television broadcast
70. What does the given figure depict?

(a) various propagation modes for electromagnetic waves

(b) various propagation modes for magnetic waves
(c) sight communication by space waves
(d) sky wave propagation
72. A transmitting antenna at the top of tower has a height 32 m and the height of the
receiving antenna is 50 m . What is the maximum distance between them for
satisfactory communication is LOS mode?
(given, radius of earth ¿ 6.4 × 106 m )
(a) 45.5 km
(b) 4.55 m
(c) 45.5 m
(d) 45.5 cm
73. Frequencies in the UHF range, normally propagate by means of
(a) ground waves
 (c) surface waves
(b) sky waves
(d) space waves
74. A radar has a power of 1 kW and is operating at a frequency of 10 GHz . It is located
on a mountain top of height 500 m . The maximum distance upto which it can detect
object located on the surface of the earth (Radius of earth ¿ 6.4 × 106 m ) is
(a) 80 km
(c) 40 km
(b) 16 km
(d) 64 km
[AIEEE 2012]
75. The highest frequency of radio waves which when sent at some angle towards the
ionosphere, gets reflected from that and returns to the Earth is called
(a) critical frequency
(c) polarisation of waves
(b) maximum usable frequency
(d) None of the above
76. Through which mode of propagation, the radio waves can be sent from one place to
another
(a) ground wave propagation
(b) sky wave propagation
(c) space wave propagation
(d) All of the above
77. A transmitting antenna of height 20 m and the receiving antenna of height h are
separated by a distance of 40 km for satisfactory communication in line of sight
mode. Then, the value of h is (Given radius of the earth is 6400 km )
(a) 40 m
(b) 45 m
(c) 30 m
(d) 25 m
78. What should be the height of a transmitting antenna, if the TV telecast is to cover a
radius of 128 km ?
(a) 1280 m
(b) 1280 km
(c) 1540 m
(d) 1140 km
79. The maximum distance upto which TV transmission from a TV tower of height h can
be received is proportional to
(a) h1 /2
(b) h
(c) h3 /2
(d) h2
80. A TV transmission tower antenna is at a height of 20 m . The percentage increase in
area covered in case if the receiving antenna is at ground level to that at a height of
25 m is (Radius of earth ¿ 6.4 × 106 m )
(a) 248 %
(b) 348.9 %
 (c) 150 %
(d) 360.2 %
81. A transmitting antenna at the top of a tower has a height of 36 m and the height of
the receiving antenna is 49 m. What is the maximum distance between them for
satisfactory communication in LOS mode?
(a) 46.5 km
(b) 45.5 km
(c) 42.75 km
(d) 35.77 km
82. The area of the region covered by the TV broadcast by a TV tower of 100 m height is
(radius of the earth ¿ 6.4 ×10 6 m )
(a) 12.8 ×10 8 km2
(b) 1.28 π × 103 km2
(c) 0.64 π ×103 km2
(d) 1.28 π × 104 km2
83. For transmitting a signal, we need an
(a) signal
(b) modulation
(c) receiver
(d) antenna or aerial
84. Antenna should have a size comparable to the
(a) bandwidth of the signal
(b) baseband of the signal
(c) wavelength of baseband
(d) wavelength of the signal
85. For an electromagnetic wave of frequency 20 kHz , the wavelength λ is that
(a) 20 km
(b) 20 m
(c) 15 km
(d) 15 m
86. To transmit a signal of 3 kHz frequency, the minimum length of antenna is ........ km .
(a) 20
(b) 25
(c) 50
(d) 75
87. What should be the length of the dipole antenna for a carrier wave of frequency
3 ×10 Hz ?
8

(a) 1 m
(b) 1 cm
(c) 0.5 m
(d) 5 cm
88. A theoretical study of radiation from a linear antenna (length l ) shows that the
power radiated is
(a) proportional to ¿
(b) inversely proportional to ¿
(c) proportional to ¿
(d) inversely proportional to ¿
89. For a good transmission, we need
(a) high power
(b) low power
(c) never low power
(d) neither low power nor high power
90. The fundamental radio antenna is a metal rod which has a length equal to
(a) λ in free space at the frequency of operation
(b) λ /2 in free space at the frequency of operation
(c) λ /4 in free space at the frequency of operation
(d) 3 λ /4 in free space at the frequency of operation
Topic 4
Modulation
91. A figure shows the nature of wave is

(a) pulse
(b) sinusoidal
(c) Both (a) and (b)
(d) None of these
92. A figure shows the nature of wave is

(a) sinusoidal
(b) pulses
(c) Both (a) and (b)
(d) None of these
93. A process which attaches information to the carrier wave, known as
(a) modulation
(b) demodulation
(c) amplification
(d) transmission
94. During the process of modulation, any of the three parameters, viz., Ac , ωc and φ , of
the carrier wave can be controlled by
(a) baseband
(b) bandwidth
(c) message or information signal
(d) Both (a) and (b)
95. Choose the correct statements.
[JEE Main 2016]
(a) In amplitude modulation, the amplitude of the high frequency carrier wave is
made to vary in proportion to the amplitude of the audio signal.
 (b) In amplitude modulation, the frequency of the high frequency carrier wave is
made to vary in proportion to the amplitude of the audio signal.
(c) In frequency modulation, the amplitude of the high frequency carrier wave is
made to vary in proportion to the frequency of the audio signal.
(d) In frequency modulation, the amplitude of the high frequency carrier wave is
made to vary in proportion to the frequency of the audio signal.
96. The significant characteristics of a pulse are
(a) pulse amplitude, pulse duration, pulse receiver
(b) pulse amplitude, pulse repeater, pulse width
(c) pulse amplitude, pulse duration or pulse width, pulse repeater
(d) pulse amplitude, pulse duration or pulse width, pulse position
97. Various types of pulse modulation are
(a) pulse amplitude modulation, pulse position modulation, pulse frequency
modulation
(b) pulse frequency modulation, pulse phase modulation, pulse position modulation
(c) pulse amplitude modulation, pulse phase modulation, pulse position modulation
(d) pulse amplitude modulation, pulse duration or pulse width modulation, pulse
position modulation
98. In an amplitude modulation with modulation index 0.5 , the ratio of the amplitude of
the carrier wave to that of the sideband in the modulated wave is
(a) 4 :1
(b) 1 :4
(c) 1 :2
(d) 2 :1
99. The modulation techniques used for transforming digital data into analog signals
are
(a) only Amplitude Shift Keying (ASK)
(b) only Frequency Shift Keying (FSK)
(c) only Phase Shift Keying (PSR)
(d) All of the above
100. In amplitude modulation, the amplitude of the carrier is varied in accordance with
the
(a) message signal
(c) bandwidth
(b) baseband
(d) information signal
101. To avoid distortion modulation index μ is kept.
(a) ¿ 1
(b) ¿ 1
(c) ¿ 1
(d) ≤ 1
102. Using trigonometric relation sin ⁡A sin ⁡B ¿ 1/2[cos ⁡( A−B)]−cos ⁡( A + B) , we can write
c m (t )= Ac sin ⁡ω c t+ μ A c sin ⁡ω m t sin ⁡ω c t as follows
μ Ac −μ A c
(a) c m (t )= Ac sin ⁡ω m t + cos ⁡( ω c −ω m ) t cos ⁡( ωc +ω m ) t
2 2
μ Am −μ A m
(b) c m (t )= Ac sin ⁡ω c t+ cos ⁡( ω c −ω m ) t cos ⁡( ω c +ω m ) t
2 2
 μ Ac −μ A c
(c) c m (t )= Ac sin ⁡ω c t+ cos ⁡( ω c −ωm ) t cos ⁡( ωc +ω m ) t
2 2
μ Ac −μ A c
(d) c m (t )= Ac sin ⁡ω c t+ cos ⁡( ω c + ωm ) t ( ω c −ωm ) t
2 2
103. A signal of 5 kHz frequency is amplitude modulated on a carrier wave of frequency
2 MHz . The frequencies of the resultant signal is/are
(a) 2 MHz only
(b) 2005 kHz and 1995 kHz
(c) 2005 kHz ,2000 kHz and 1995 kHz
(d) 2000 kHz and 1995 kHz
[JEE Main 2015]
104. A signal wave of frequency 12 kHz is modulated with a carrier wave of frequency
2.51 MHz. The upper and lower sideband of frequencies are respectively,
(a) 2512 kHz and 2508 kHz
(b) 2522 kHz and 2488 kHz
(c) 2502 kHz and 2498 kHz
(d) 2522 kHz and 2498 kHz
105. As carrier wave,
μ Ac
c m (t )= Ac sin ⁡ω c t+ cos ⁡( ω c −ωm ) t
2
−μ A c /2 cos ⁡( ω c +ω m ) t
In the given equation, ( ω c −ω m ) and ( ω c + ωm ) are
(a) upper side frequency, lower side frequency
(b) lower side frequency, lower side frequency
(c) upper side frequency, lower side frequency
(d) lower side frequency, upper side frequency
106. The modulated signal consists of the carrier wave of frequency ω c plus two
sinusoidal waves each with a frequency slightly different, known as
(a) sidebands
(b) baseband
(c) width band
(d) All of these 107.

Which one of the following graphs is shown in figure?

(a) Modulation of carrier wave
(b) Modulation of sky wave
(c) Frequency spectrum of the amplitude modulated signal
(d) Pulse modulation of carrier wave
108. A message signal of frequency 10 kHz and peak voltage of 10 V is used to modulate a
carrier of frequency 1 MHz and peak voltage of 20 V . Determine the modulation
index.
(a) 0.5
(b) 0.2
 (c) 2
(d) 5
109. In the above question, sidebands produced are
(a) 1010 kHz and 990 kHz
(b) 1010 MHz and 990 MHz
(c) 1010 Hz and 990 Hz
(d) 1010 THz and 990 THz
110. A sinusoidal carrier voltage of amplitude 100 V is amplitude modulated by a
sinusoidal voltage to give as amplitude modulation wave to minimum voltage
amplitude of 70 V . Find the modulation index.
(a) 0.03
(b) 30
(c) 0.3
(d) 0.003
111. A carrier wave of peak voltage 10 V is used to transmit a message signal. What
would be the peak voltage of the modulating signal in order to have a modulation
index of 50 % ?
(a) 5 V
(b) 3 V
(c) 9 V
(d) 10 V
112. If the minimum voltage in an AM wave was found to be 2 V and maximum voltage
10 V . Find per cent modulation index.
(a) 80 %
(b) 66.67 %
(c) 64.25 %
(d) 76.25 %
113. A carrier is simultaneously modulated by two sine waves having modulation index
of 0.3 and 0.4 . The total modulation index will be
(a) 0.1
(b) 0.5
(c) 0.7
(d) 0.35
114. The modulator is to be followed by a
(a) baseband
(c) signal
(b) bandwidth
(d) power amplifier
115. Identify the part A in the block diagram of transmitter.

(a) square law device

(c) power amplifier
 (b) band pass filter centred
(d) modulating signal
116. In the communication system, AM is used for broadcasting because
(a) it avoids receiver complexity
(b) it is more noise immune than other modulation system
(c) it requires less transmitting power
(d) no other modulation system can give the necessary bandwidth for faithful
transmission
117. Identify the parts A and B of a diagram of a receiver.

(a) detector, amplifier

(b) signal, carrier wave
(c) amplifier, detector
(d) carrier wave, signal
118. Which of the following is the process of recovering the modulating signal from the
modulated carrier wave?
(a) Amplification
(b) Detection
(c) Rectifier
(d) Demodulation
119. The method shown in the block diagram is used to obtain.

(a) the original message signal m(t) of angular frequency ω m

(b) the original information signal c (t ) of a simpler modular
(c) an AM signal
(d) modulation of carrier wave
120. In a modulated signal, an envelope of rectified wave is
(a) message signal
(b) informal signal
(c) Both (a) and (b)
(d) None of these
121. In order to retrieve m(t) , the signal is passed through
(a) rectifier
(b) amplifier
(c) envelope detector
(d) Both (a) and (c)
Special Format Questions

Assertion and Reason

Directions (Q. Nos. 122-125) In the following questions, a statement of
assertion is followed by a corresponding statement of reason. Of the
following statements, choose the correct one.
(a) Both Assertion and Reason are correct and Reason is the correct explanation of
Assertion.
(b) Both Assertion and Reason are correct but Reason is not the correct explanation
of Assertion.
(c) Assertion is correct but Reason is incorrect.
(d) Assertion is incorrect but Reason is correct.
122. Assertion Digital signals are preferred over analog signals for transmission of
information.
Reason Analog signals require amplification and correction at suitable intervals.
123. Assertion In real life, an approximate bandwidth of 20 kHz is required to transmit the
music.
Reason High frequencies are produced by the musical instruments.
124. Assertion A TV signal requires the bandwidth of 76-88 MHz for transmission.
Reason A TV signal contains both voice and picture.
125. Assertion Antenna should have a size comparable to the wavelength of the signal
(atleast λ /4 in dimension). Reason The antenna properly senses the time variation
of the signal.

Statement Based Questions

126. Digital signals
I. do not provide a continuous set of a values.
II. represent values as discrete values.
III. can utilise binary system and.
IV. can utilise decimal as well as binary systems.
Which of the above statements are correct?
(a) I and II
(b) II and III
(c) I, II and III
(d) All of these
127. Read the following statements and select the correct option from the options given
below.
I. To reproduce the rectangular wave shape exactly we need to superimpose all the
harmonics v 0 , 2 v 0 , 3 v 0 , 4 v 0 ⋯ , which implies an infinite bandwidth.
II. For practical purposes, the contribution from higher harmonics cannot be
neglected, thus limiting the bandwidth.
III. Received waves are distorted version of the transmitted one.
(a) I and III
(b) I and II
(c) Only III
(d) All of these
128. Which of the following statement(s) is/are correct?
 I. Similar to message signals, different types of transmission media offer different
bandwidths.
II. The commonly used transmission media are wire, free space and optical fibre.
III. Coaxial cable is widely used wire media, which offers a bandwidth of
approximately 750 MHz .
(a) I and II
(b) II and III
(c) I, II and III
(d) Only I
129. Which of the following statement(s) is/are correct?
I. The frequency used in satellite communication lies between 5 MHz and 1 MHz.
II. The uplink and downlink frequencies are different.
III. The orbit of geostationary satellite lies in the equatorial plane at inclination of 0∘ .
(a) I and II
(b) I and III
(c) II and III
(d) I, II and III
130. Which of the following statement(s) is/are correct regarding ground wave
propagation?
I. A wave induces current in the ground over which it passes and surrounding
particle.
II. It is attenuated as a result of absorption of energy by the Earth and surrounding
particle.
III. The attenuation of surface waves decreases very rapidly with increase in
frequency.
IV. The maximum range of coverage depends on the transmitted power and
frequency (less than a few MHz ¿ .
(a) I and II
(b) I and III
(c) I, II and IV
(d) All of these
131. Consider the following figure

Line of sight communication by space waver

Which statement of the following is/are correct?
I. Because of line of sight nature of propagation, direct waves get blocked at some
point by the curvature of the Earth.
II. Because of sky wave nature of propagation, direct waves get blocked at some
point by the curvature of the Earth.
III. Because of surface wave nature of propagation, direct waves not blocked at
some point by the curvature of the Earth.
(a) Only I
(b) II and III
(c) I and III
(d) I and II
132. High frequency carrier waves are needed for effective transmission of signals.
I. To reduce the length (l) of the transmitting antenna as l=λ /4 .
II. To increase the power (P) radiated by the antenna (as P∝ 1/ λ2 ) so as to increase
the range of transmission.
III. To transmit a large number of signals simultaneously.
(a) I and II
(b) Only I
(c) I and III
(d) All of these
133. A schematic arrangement for transmitting a message signal ¿ to 20 kHz ) is given in
figure

Which of the following statement(s) is/are correct?

I. The message signal is in the AM range and cannot be transmitted beyond some
distances as it possess small energy.
II. An Amplitude Modulation (AM) signal does not possess enough energy to travel
an appreciable distance from the antenna.
III. The heights of the antennas required are so large that is impossible to build
them.
(a) Only I
(b) I and II
(c) I and III
(d) All of the above

Matching Type
134. Match the following Column I with names given in Column II and choose the correct
option from the codes given below.

135. Match the following Column I with names given in Column II and choose the correct
option from the codes given below.
[HOTS]
Column I Column II

1
A. Speech signals Bandwidth of 20 kHZ
 Speech signals for
2 Frequencies extends
B. commercial telephonic
from 20 Hz to 20 kHz
communication

3 Frequency range 300 Hz

C. Music transmission
to 3100 Hz .

Audible range of 4
D. 6 MHz of bandwidth
frequencies

Video signals for 5 Bandwidth of 2800 Hz

E.
transmission of picture (3100 Hz−300 Hz )

6
F. TV signal 4.2 MHz of bandwidth

ABCDEF
(a) 6 4 3 2 1 5
(b) 5 3 2 1 4 6
(c) 3 5 1 2 6 4
(d) 4 2 1 6 5 3
136. Consider the following figure.

Match the following columns.

Column I Column II

A. ( p) 1. Fundamental wave ( ν 0 )

Fundamental ( ν 0 ) +second ⁡ harmonics

B. (q) 2.
( 2 v 0 ) + third harmonics ( 3 v 0 )
Rectangular
C. (r ) 3.
wave

Fundamental wave ( v 0 ) + second

D. (s) 4.
harmonics ( 2 v 0 )

A B C D
1 ¿ (b) ¿ 1 ¿ 3 ¿ 4 ¿ 2 ¿ (c) ¿ 3 ¿ 2¿ 1 ¿ 4 ¿ (d) ¿ 4 ¿ 1 ¿ 2¿ 3 ¿
(a) 2 4 ¿
137. Match the following Column I with names given in Column II and choose the correct
option from the codes given below.
Column I Column
II
 Standard AM
A. 1. 54−72 MHz
broadcast

B. FM broadcast 2. 5.925−6.425GHz

Satellite communication
C. 3. 540−1600 kHz
(uplink)

D. Television (VHF) 4. 88−108 MHz

ABCD ABCD
(a) 4 2 3 1 (b) 2 1 4 3
©1342 (d) 3 4 2 1
138. Match the following Column I with names given in Column II and choose the correct
option from the codes given below.

139. Match the following Column I with names given in Column II and choose the correct
option from the codes given below.

A B C ¿ A B C
3 ¿ 1 ¿ (c) ¿ 2¿ 1 ¿ 3 ¿ (d) ¿1 ¿ 3 ¿ 2 ¿
(a) 3 2 1 (b) ¿

Passage Based Questions

Directions (Q. Nos. 140-142) These questions are based on the following
situation. Choose the correct options from those given below.
Space waves are used for line-of-sight (LOS) communication as well as satellite
communication. Because of line-of-sight nature of propagation, direct waves gets
blocked at some point by the curvature of the earth as illustrated in figure. If the
 signal is to be received beyond the horizon then the receiving antenna must be high
enough to intercept the line-of-sight waves.

140. If the transmitting antenna is at a height hT , the distance to the horizon d T is given
as
(a) d T =√ 2 h R T
(b) d T =√ 2 R h T
(c) d T =2 R hT
hr
(d) d T =2 R hT 1+
2R
141. In the above question, d T is also called the radio
(a) horizon of the transmitting antenna
(b) horizon of the transmitter
(c) horizon of the modulation
(d) horizon of the signal
142. The maximum line of sight distance d M between the two antennas having heights hT
and h R above the Earth is
(a) d M =1+ √ 2 R h T
(b) d M =1+ √ 2 R h R
(c) d M =√ 2 R h T + √ 2 R h R
(d) d M =2 R hT ( 1+ √ 2 R hR )
Directions (Q. Nos. 143-145) These questions are based on the following
situation. Choose the correct options from those given below.
The modulated signal c m (t ) can be written as
c m (t ) ¿ ( A c + A m sin ⁡ωm t ) sin ⁡ωc t
¿ ¿
Am
Here, μ= is the modulation index; in practice μ is kept ≤ 1 to avoid distortion.
Ac
μ Ac
Now, c m (t )= Ac sin ⁡ω c t+ cos ⁡( ω c −ωm ) t
2
−μ A c
cos ⁡( ωc +ω m ) t
2
Here, ω c −ω m and ω c + ωm are respectively called the lower side and upper side
frequencies.
143. An amplitude modulation wave is represented as
c m (t )=10(1+0.4 sin ⁡3140 t)sin ⁡( 2.2× 106 t ) V . The minimum and maximum voltage applied
amplitude of the wave are
(a) 10 V , 2 V
(b) 8 V , 10 V
(c) 14 V ,6 V
(d) 8 V , 14 V
144. The frequency components contained in the wave are
(a) 350 kHz ,349.5 kHz ,350.5 kHz
(b) 350 MHz , 349.5 MHz , 350.5 MHz
 (c) 250 kHz ,249.5 kHz ,250.5 kHz
(d) 250 MHz , 249.5 MHz , 250.5 MHz
145. Amplitudes of frequency components are
(a) 10 V , 6 V , 8 V
(b) 10 V , 2 V and 2 V
(c) 8 V , 4 V and 2 V
(d) 8 V , 6 V , 2 V

More than One Option Correct

146. 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 earth
is to be 6.4 × 106 m
(a) 100 km
(b) 24 km
(c) 55 km
(d) 50 km
147. An audio signal of 15 kHz frequency cannot be transmitted over long distances
without modulation, because
(a) the size of the required antenna would be atleast 5 km which is not convenient
(b) the audio signal cannot be transmitted through sky waves.
(c) the size of the required antenna would be 5 m
(d) effective power transmitted would be very low, if the size of the antenna is less
than 5 km
148. 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 6 kHz .
(c) The bandwidth required for amplitude modulation is 3 MHz .
(d) The sideband frequencies are 1503 kHz and 1497 kHz .
149. In amplitude modulation, the modulation index m , is kept less than or equal to 1
because
(a) m>1, will result in interference between carrier frequency and message
frequency, resulting into distortion
(b) m>1, will result in overlapping of both sidebands resulting into loss of information
(c) m>1, will result in change in phase between carrier signal and message signal
(d) m>1, indicate amplitude of message signal greater than amplitude of carrier
signal resulting into distortion
NCERT & NCERT Exemplar Questions
NCERT
150. Frequencies in the UHF range normally propagate by means of
(a) ground waves
(b) sky waves
(c) surface waves
(d) space waves
151. 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 m tall. How much service area can it cover, it the receiving antenna is at the
ground level?
(a) 3258.5 km2
 (b) 2200.5 km2
(c) 4512 km2
(d) 1212 km2
152. For an amplitude modulated wave, the maximum amplitude is found to be 10 V
while the minimum amplitude is found to be 2 .V. Determine the modulation index μ
.
What would be the value of μ, if the minimum amplitude is 0 V ?
(a) 3/2 , 1.5
(b) 2/3 , 1
(c) 2/3 , 0.5
(d) 3/2 , 1.33

NCERT Exemplar
153. Three waves such as 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 is sky wave and C is 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
154. A 100 m long antenna is mounted on a 500 m tall building. The building can become
a transmission tower for waves with λ is
(a) ∼ 400 m
(b) ∼ 25 m
(c) ∼ 150 m
(d) ∼ 2400 m
155. A 1 kW signal is transmitted using a communication channel which provides
attenuation at the rate of −2 dB per km . If the communication channel has a total
length of 5 km, the power of signal received is
(a) 900 W
(b) 100 W
(c) 990 W
(d) 1010 W
156. 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
157. A message signal of frequency ω m is superposed on a carrier wave of frequency ω c
to get an Amplitude Modulated (AM) wave. The frequency of the AM will be
(a) ω m
(b) ω c
ω +ω
(c) c m
2
ωc −ωm
(d)
2
158. I −V characteristics of four devices are shown in figure (i)

Identify devices that can be used for modulation.

(a) (i) and (iii)
(b) Only (iii)
(c) (ii) and some regions of (iv)
(d) All devices can be used
159. 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 basic communication
system.
(a) ABCDE
(b) BADEC
(c) BDACE
(d) BEADC
160. Which of the following would produce analog signals?
(a) A vibrating tuning fork
(b) Light pulse
(c) Output of NAND gate
(d) All of the above
161. Which of the following would produce digit signals?
(a) Musical sound
(b) A vibrating tuning fork
(c) Sound and picture signals in TV
(d) Light pulse
162. The frequency response curve for the filter circuit used for production of AM wave
should be
 (a) (i) followed by (ii)
(b) (ii) followed by (i)
(c) (iii)
(d) (iv)
163. Would sky waves be suitable for transmission of TV signals of 60 MHz frequency?
(a) Yes
(b) No
(c) May be
(d) Never yes

Answers
1. (c) 2. (c) 3. (d) 4. (b) 5. (d) 6. (b) 7. (c) 8. (c) 9. (c) 10. (c)
11. (a) 12. (c) 13. (b) 14. (a) 15. (b) 16. (d) 17. (a) 18. (d) 19. (b)
20. (a) 21. (d) 22. (b) 23. (d) 24 (c) 25. (c) 26. (a) 27. (c) 28. (a) 29. (b) 30. (d)
31. (b) 32. (c) 33. (d) 34. (b) 35. (c) 36. (c) 37. (a) 38. (d) 39. (d) 40. (b)
41. (a) 42. (b) 43. (a) 44. (a) 45. (c) 46. (a) 47. (b) 48. (a) 49. (d) 50. (a)
51. (a) 52. (c) 53. (d) 54. (b) 55. (b) 56. (b) 57. (b) 58. (b) 59. (c) 60. (d)
61. (a) 62. (a) 63. (d) 64. (c) 65. (d) 66. (b) 67. (c) 68. (a) 69. (b) 70. (d)
71. (a) 72. (a) 73. (d) 74. (a) 75. (b) 76. (d) 77. (b) 78. (a) 79. (a) 80. (b)
81. (a) 82. (b) 83. (d) 84. (d) 85. (c) 86. (b) 87. (c) 88. (c) 89. (a) 90. (c)
91. (b) 92. (b) 93. (a) 94. (c) 95. (b) 96. (d) 97. (d) 98. (a) 99. (d) 100. (d)
101. (d) 102. (c) 103. (c) 104. (d) 105. (d) 106. (a) 107. (c) 108. (a) 109. (a) 110. (c)
111. (a) 112. (b) 113. (b) 114. (d) 115. (c) 116. (a) 117. (c) 118. (b) 119. (a) 120.
(a)
121. (c) 122. (b) 123. (a) 124. (a) 125. (a) 126. (c) 127. (a) 128. (c) 129. (c) 130. (c)
131. (a) 132. (d) 133. (c) 134. (b) 135. (c) 136. (c) 137. (d) 138. (a) 139. (b) 140.
(b)
141. (a) 142. (c) 143. (c) 144. (a) 145. (b) 146. (b,c,d)147. (a,b,d)148. (b,d)149.
(b,d)150. (d)
Hints and Explanations
1. (c) A modern communication system acts as a messenger. It sends as a input signal
in the form of message through communication channel to receive an output signal
at the end of receiver.

2. (c) Languages and methods used in communication have kept evolving from
prehistoric to modern times, to meet the growing demands in terms of speed and
complexity of information such as videophones, voicemail and satellite conferences.
3. (c) A receiver reconstructs a recognisable form of the original message signal for
delivering it to the user's information.
4. (c) In a point to point communication mode, communication takes place over a link
between a transmitter and a receiver.It is also known as peer-to-peer
communication.
5. (c) The transmission media can be both, guided as well as unguided. In guided
medium, signal is guided along a solid medium such as transmission line, while in
unguided medium, signal is transmitted and received by wireless medium.
6. (a) A guided medium can provide point to point communication.
7. (b) ARPANET was a project undertaken by the US department of defence.
8. (a) ARPANET allowed file transfer from one computer to another connected to the
network by using File Transfer Protocol (FTP).
9. (b) Fibre optical systems are superior and more economical as compared to
traditional communication systems because it is used to transmit more information
and is immune to electromagnetic interference.
10. (d) The modern communication system is based on electrical, electronic or optical
signals. For example, a microphone converts speech signal into electrical signals.
11. (a) The fax or facsimile telegraphy is the electronic transmission and reproduction
of a document at a distinct place. It is the most popular example of a digital
communication system.
12. (b) A transducer is a device that converts a signal in one form of energy to another
form of energy. So, amplifier is not a transducer.
13. (a) An electrical transducer may be defined as a device that converts some physical
variable/pressure, displacement, force, temperature, etc) into corresponding
variations in the electrical signal at its output.
14. (d) For efficient transmission, size of antenna ¿ λ /4
15. (c) Digital signals are those which cannot take only discrete stepwise values.
16. (a) Fading in the signal received is due to interference of waves or attenuation
which decreases in strength of a signal.
17. (b) The energy needed for additional signal strength is obtained from a DC power
source in order to amplify the amplitude of a signal.
18. (c) Rectangular wave can be decomposed into a superposition of sinusoidal waves
of frequencies v 0 , 2 v 0 , 3 v 0 , 4 v 0 , ⋯ , n v 0, where n is an integer extends to infinity.
19. (c) The information is not lost and the rectangular signal is more or less recovered.
This is so because the higher the harmonic less is its contribution to the waveform.
20. (a) High frequency carrier wave provides a larger bandwidth for higher date rate
transmission from sender to receiver or vice-versa.
21. (b) No signal, in general, is a single frequency sinosoid, but it spreads over a range
of frequencies called the signal bandwidth.
22. (a) A range of frequencies between two bands, i.e., upper and lower limit is called
the signal bandwidth.
23. (b) Optical communication using fibres is performed in the frequency range of 1 THz
to 1000 THz (microwaves to ultraviolet).It lies in extremely-high-frequency band
(EHF).
24. (c) A communication link between a fixed base station and mobile units on a ship or
aircraft works on 30 to 470 MHz .
25. (b) To radiate signals with high efficiency, the antennas should have a size
comparable to the wavelength λ of the signal (atleast ∼ λ /4 ). So, lesser the
wavelength, the higher will be frequency of a signal.
26. (b) A ground receiver in line-of-sight communication cannot receive direct waves
due to curvature of earth.
27. (d) Ionisation occurs due to the absorption of ultraviolet and very high energy
radiation coming from the sun by air molecules.
28. (a) The degree of ionisation varies with the height.So,the density of atmosphere
decreases with height.
29. (a) Electromagnetic waves of frequencies higher than 30 MHz penetrate the
ionosphere and escape. These phenomena are shown in the figure.

63. (d) Radio waves are propagated through ground, sky and space waves. It has
frequency from 300 Hz to as low as 3 kHz and corresponding wavelength ranging
from 1 mm to 100 km.
64. (c) Sky wave propagation is possible for the radio waves of frequency range from
3 MHz to 30 MHz and not in the range 50-80 MHz.
65. (b) A space wave travels in a straight line from transmitting antenna to the
receiving antenna because its frequency is below 2 MHz. So, it cannot travel over the
horizon or behind obstacles.
66. (b) If the signal is to be received beyond the horizon, then the receiving antenna
must be high enough to intercept the line of sight waves.
67. (a) The maximum Line of Sight (LoS) distance d M between the two antennas having
heights hT and h R is
 dM ¿ √ 2 R hT + √ 2 R h R
d ¿ √ 2× 64 ×10 × 32+ √ 2 ×64 ×10 ×50 m
5 5

¿ ¿ 144 ×10 2 × √ 10 m=45.5 km

74. (a) Range of radar on earth's surface (optical distance, for space wave, i.e., line of
view).
Range ¿
75. (b) The highest frequency of radio waves that can be reflected by the ionosphere is
Critical frequency
called maximum usable frequency (MUF) , so MUF ¿ .
cos ⁡θ
76. (b) d M =√ 2 R h R + √ 2 R hT
where, h R and hT are the heights of receiving and transmitting antenna and R is the
radius of the earth.
40× 103=√ 2 ×6400 ×103 × h+ √ 2 × 6400× 103 × 20
40 × 10 =√ 2 ×6400 ×10 ×h+ 16 ×10
3 3 3

2
[(40−16)×103 ]
h= 6
=45 m
2× 6.4 ×10
78. (a) Here, height of a transmitting antenna, i.e., d T =128 km ¿ r radius of Earth, i.e.,
R=6400 km
∴ Height of a transmitting antenna, d T =√ 2 R h T
2
dT
hT = =¿ ¿
2R
79. (a) As maximum distance on Earth from transmitter upto which a signal can be
received is d T =√ 2 Rh , d T ∝h 1/ 2 where, R is radius of Earth and is constant.
80. (b) Here, hT =20 m, R=6.4 ×106 m=64 ×105 m If the receiving antenna is at ground level,
d ¿ √ 2 hT R
Range, 3
¿ ¿ 16 ×10 m=16 km
2 22
Area covered, A=π d = ׿
7
If the receiving antenna at a height of 25 m , then
Range, d 1= √ 2 hT R+ √ 2 h R R
¿ √ 2× 20 × ( 64 × 10 ) + √ 2 ×25 ×64 × 10
5 5

¿ 16 ×103 +17.9 ×103

3
¿ 33.9× 10 m=33.9 km
2 22
Area covered, A1=π d 1= × ¿
7
A −A
Percentage increase in area ¿ 1 ×100
A
3611.8−804.6
¿ × 100=348.9 %
804.6
81. (a) Given, hT =36 m
hR ¿ 49 m
R ¿ 6400 km
Maximum distance between transmitting and receiving antenna,
i.e., d M =d T + d R =√ 2 R hT + √ 2 R h R
¿ √ 2(6400) ( 36 × 10−3 ) + √ 2(6400) ( 49 ×10−3 km )
¿ 21.5+25=46.5 km
2
82. (b) Area of broadcast, A=π d =π (2 hR)
 Given, h=100 m and R=6.4 ×106 m
⇒ A ¿ π ( 2× 100× 6.4 ×10 6 )
¿ ¿
85. (c) Frequency, i.e., v=20 kHz=20 ×10 Hz Speed of light, i.e., c=3 × 108 ms−1
3

c
So, v=
λ
8
c 3× 10 4
Wavelength, i.e., λ= = 4
=1.5 ×10 m=15 km
v 2× 10
(∵ 1 km=1000 m)
For an electromagnetic wave of frequency 20 kHz , the wavelength λ is 15 km
.Obviously, such a long antenna is not possible to construct and operate. 87. (c)
8 −1
c 3× 10 ms
Wavelength of an antenna, λ= = =1 m Length of the dipole antenna
v 3 ×10 8 Hz
1
¿ λ /2= =0.5 m
2
89. (a) For good transmission, we need high powers and hence this also points out to
the need of using high frequency transmission.
90. (b) The fundamental radio antenna is a metal rod which has a length equal to λ /2 in
free space at the frequency of operation.
91. (b) As, we know, an amplitude modulated wave, the bandwidth is twice the
frequency of modulating signal. Therefore, amplitude modulation (AM), the
frequency of the high frequency carrier wave is made to vary in proportion to the
amplitude of the audio signal.
92. (a) Here, modulation index, μ=0.5
Amplitude of the carrier wave ¿ Ac
A
Amplitude of the sideband ¿ μ c
2
2 2 4
Ratio ¿ = =
μ 0.5 1
102. (c) Using the trigonometric relation sin ⁡A sin ⁡B=1/2
[cos ⁡(A−B)−cos ⁡( A+ B)], we can write c m (t ) of equation c m (t )= Ac sin ⁡ω c t+ μ A c sin ⁡ω m t sin ⁡ω c t
μ Ac
as c m (t )= Ac sin ⁡ω c t+ cos ⁡( ω c −ωm ) t
2
−μ A c
cos ⁡( ωc +ω m ) t
2
Here, ω c −ω m and ω e + ωm are respectively called the lower side and upper sideband
frequencies.
103. (c) Frequency associated with AM are f c −f m , f c , f c + f m Thus, frequency of the resultant
signal is/are carrier frequency f c =2000 kHz , LSB frequency f c −f m=2000 kHz−5 kHz
¿ 1995 kHz and USB frequency f c + f m =2005 kHz
104. (d) Here, v s=12 kHz
v c =2.51 MHz=2510 kHz
Upper side band frequency ¿ 2510+12=2522 kHz
Lower side band frequency ¿ 2510−12=2498 kHz
106. (a) The modulated signal consists of the carrier wave of frequency ω c plus two
sinusoidal waves each with a frequency slightly different known as sidebands.
107. (c) The frequency spectrum of the amplitude modulated signal is shown in figure.
108. (a) Here, Am =10 V , A c =20 V
As we know modulation index,
(μ)= A m / A c =10/20=0.5
109. (a) The sidebands of a modulated message signal is ω c −ω m and ω c + ωm are
(1000+10)=1010 kHz andd (1000−10 kHz)=990 kHz
110. (c) Here, Ac =100 V , M 2 (minimum voltage amplitude of AM wave) ¿ 70 V
Since, M 2= A c (1−μ)
So, modulation index
M2 70 V
i.e., μ=1− =1− =1−0.7=0.3
Ac 100 V
50 1
111. (a) Given, Ac =10 V , μ=50 %= =
100 2
As modulating index
Am 1
μ= , A m=μ A c= (10 V )=5 V
Ac 2
112. (b) Modulation index,
E max−E min 10−2 2
μ= = = × 100=66.67 %
E max + Emin 10+2 3
113. (b) Here, m1=0.3 and m2=0.4
Total modulation index i.e.,
m ¿ √ m 1 +m 2=√ 0.3 +0.4
2 2 2 2

¿ ¿
114. (d) The modulator is to be followed by a power amplifier which provides the
necessary power and the modulated signal is fed to an antenna of appropriate size
for radiation.
115. (a) In the communication system, AM is used for broadcasting because it avoids
receiver complexity level to maintain high strength of a modulating signal.
116. (b) Digital signals can be transmitted over long distances without error because of
their ability to overcome with noise. Analog signals determine the amplitude and
frequency, respectively of the waveform of the voltage current. It pick up unwanted
singnals.
117. (a) To transmit music, an approximate bandwidth of 20 kHz is required because the
high frequencies are produced by the musical instruments.
118. (a) A TV signal contains both voice and picture and is usually allocated 6 MHz of
bandwidth for transmission.
119. (a) For transmitting a signal, aerial or antenna should have a size comparable to the
wavelength of the signal atleast λ /4 in dimension. So, it properly senses the time
variation of the signal.
120. (c) Digital signals are the values in the form of 0 or 1 . It represents discrete values
in the binary bits which are noncontinuous set of values. 127. (a)To reproduce the
rectangular wave shape exactly we need to superimpose all the harmonics
v 0 , 2 v 0 , 3 v 0 , 4 v 0 , ⋯ , which implies an infinite bandwidth. However, for practical
 purposes, the contribution from higher harmonics can be neglected, thus limiting
the bandwidth. As a result, received waves are a distorted version of the
transmitted one.
121. (c) In satellite communication, the frequency used is more than 40 MHz . The uplink
and downlink frequencies are different to avoid distortion of signal and the orbit of
geostationary satellite lies in the equatorial plane at an inclination of 0∘.
122. (a) In sky wave propagation, radio wave reaches the receiver after the reflection
from ionosphere.
123. (c) (i) The message signal is in the AM range and cannot be transmitted beyond
some distance as it possesses small energy (due to low frequency).
(ii) The height of the antennas required are so large that it is impossible to build
them.
134. (b) ' 0 ' corresponds to a low level and ' 1 ' corresponds to a high level of
voltage/current. There are several coding schemes useful for digital communication.
They employ suitable combinations of number system such as the Binary Coded
Decimal (BCD). American Standard Code for Information Interchange (ASCII) is a
universally popular digital code to represent number, letters and certain characters.
135. (c) For speech signal, frequency range from 300 Hz to 3100 Hz is considered
adequate. Therefore, speech signal requires a bandwidth of
2800 Hz (3100 Hz−300 Hz) for commercial telephonic communication.
To transmit music, an approximate bandwidth of 20 kHz is required because of the
high frequencies produced by the musical instruments.
The audible range of frequencies extends from 20 Hz to 20 kHz .
Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV
signal contains both voice and picture and is usually allocated 6 MHz of bandwidth
for transmission.
142. (c) The maximum Line of Sight (LOS) distance d M between the two antennas having
heights hT and h R is d M =√ 2 R h T + √ 2 R h R , where h R and hT is the height of receiving and
transmitting antenna.
143. (c) Comparing the given equation with
Amplitude of carrier wave
c m (t )= Ac ( 1+ μ sin ⁡ωm t ) sin ⁡ωc t
i.e.,
A c =10 V , μ=0.4
ω 3140
ν m= m = =0.5 kHz
2 π 2× 3.14
Frequency of carrier wave,
ω c 2.2 ×10 6
vc= = =350 kHz
2π 2π
Minimum amplitude,
Maximum amplitude,
M 1= Ac (1+ μ)=10 (1+ 0.4)=14 V
M 2= A c (1−μ)=10(1−0.4)=6 V
144. (a) Frequency components of AM wave are v c , ( v c −v m ) and ( v c + v m ) i.e.,
350 kHz ,(350−0.5)=349.5 kHz and (350+0.5)=350.5 kHz.
145. (b) Amplitude of the frequency components are
μ Ac μ A c
Ac , , , i.e., 10 V , 2 V and 2 V
2 2
146. (b , c , d) Range, d T =√ 2 R h T
Given, height of tower h=240 m
For LOS (Line of Sight) communication.
d ¿ √ 2 Rh=√ 2× 6.4 × 106 × 240
¿ ¿
Thus, the range of 55.4 km covers the distance 24 km, 55 km and 50 km.
147. (a ,b ,d ) Transmission of a signal depends on three factors. These are size of
antenna, medium of transmission and power of transmitted wave.
Given, frequency of the wave to be transmitted is
3
v m=15 kHz=15 ×10 Hz
8
c 3× 10 1 5
Wavelength, λ m= = 3
= ×10 m
v m 15 ×10 5
λ 1 1 5
Size of the antenna required, l= = × ×10
4 4 5
3
¿ 5 ×10 m=5 km
The audio signals are of low frequency waves. Thus, they cannot be transmitted
through sky waves as they are absorbed by atmosphere.
If the size of the antenna is less than 5 km, the effective power transmission would
2
l
be very low because l → high, λ → low, P ∝
λ
ω
148. (b , d ) Given, m =3 kHz
ω c =1.5 MHz=1500 kHz
Now, sideband frequencies
ω c ± ω m ¿(1500 ± 3)
¿ ¿
Also, bandwidth ¿ 2 ωm =2× 3=6 kHz
149. (b , d ) The modulation index (m) of amplitude modulated wave is
amplitude of message signal ( A m )
m=
amplitude of carrier signal ( A c )
If m>1, then Am > A c.
In this situation, there will be distortion of the resulting signal of amplitude
modulated wave. Maximum modulation frequency ( mf ) of Am wave is
Δ v max
mf ¿
v m (max)
¿ ¿
If mf > 1, then Δ v max > v m. It means, there will be overlapping of both sidebands of
modulated wave resulting into loss of information.
150. (d) Space wave range ¿ 40 MHz
UHF range, 170−200 MHz
151. (a) Range, d= √ 2 hR
22 6
∴ Service area ¿ π ×2 hR= ×2 ×81 ×6.4 ×10
7
¿ ¿
152. (b) Given, maximum amplitude max A =10 V
Minimum amplitude, A min =2 V
Let Ac and Am be the amplitudes of carrier wave and signal wave.
∴ A max =A c + A m=10
and A min =A c − A m=2
 Adding the Eqs. (i) and (ii), we get
2 A c =12 or A c =6 V
and
Am =10−6=4 V
Am 4 2
Modulation index, μ= = =
Ac 6 3
When the minimum amplitude is zero, then i.e., Amin =0
Ac + A m=10
A c − A m=0
By solving Eqs. (iii) and (iv), we get
2 A c =10 or A c=5 and
A 5
Modulation index, μ= m = =1
Ac 5
154. (a) For given building complex, l=100 m, λ=¿ ?
As length of the antenna, l=λ /4
λ ∼ 4 l⇒ λ=4 × 100=400 m
155. (b) Here, Pi=1 kW =1000 W , Length of path ¿ 5 km , Po=¿ ? Loss of power at a distance
Po
of 5 km=2× 5=10 dB As loss in dB=10 log ⁡
Pi
Po
∴ ¿−10=10 log ⁡
Pi
Pi
⇒ ¿ log ⁡ =1=log ⁡10
Po
P
⇒ ¿ i =10
Po
P 1000
⇒ ¿ Po = i = =100 W
10 10
156. (a) Here, v=1 MHz , Δ v=3 kHz=0.003 MHz
Using amplitude modulation, the frequencies of the sideband
¿(v + Δ v ) and (v−Δ v) = v + v =1 MHz+ 3 kHz ¿=1 MHz +0.03 MHz=1.003 MHz ¿ Lower sideband ¿= ν −v =1 M
¿
( c m) ( c m)
157. (b) The amplitude modulation (AM) carrier signal is given by
c m (t )=( Ac + A m sin ⁡ωm t ) sin ⁡ωc t frequency is ω c.
158. (c) The device which follows square law is used for modulation purpose.
Characteristic shown by (i) and (iii) corresponds to linear devices. Characteristic
shown by (ii) and (iv) corresponds to square law.
159. (a) A vibrating tuning fork produces analog signals which is a continuous set of
values.
160. (a ,b ,c ) As bandwidth of AM wave should be 2 ωm =( ω c + ωm ) −( ω c −ωm ). Therefore, it is
applicable for cases (i), (ii) and (iii). The case (iv) will reject the required signal for
amplitude modulated wave.
161. (b) No. sky waves of Very High Frequency (VHF) and above, i.e., above 30 MHz pass
through the ionosphere into outer space.