Principles of Communication
Unit X: Communication Systems
Elements of a communication system (block diagram only); bandwidth of signals (speech, TV and digital
data); bandwidth of transmission medium. Propagation of electromagnetic waves in the atmosphere, sky and
space wave propagation. Need for modulation. Production and detection of an amplitude-modulated wave.
[2001]
Q.1
State two factors by which the rage of T-V signal can be increased?
[2002]
Q.1
Derive an expression for covering range of T-V transmission tower.
[2003]
Q.1
Q.2
Q.3
What is amplitude modulation? Represent the process graphically. Write its two limitations and two
advantages.
Schematic arrangement for transmitting a message signal (20 Hz to 20 kHz) is given below : Give two
drawbacks from which this arrangement suffers. Describes briefly with the help of a block diagram
the alternative arrangement for the transmission and reception of message signal.
What are analog and digital signals? How can an analog signal be converted into a digital signal?
[2004]
Q.1
Q.2
Q.3
Q.4
Q.5
Q.7
Q.8
Q.9
Q.10
Q.11
Q.12
Q.13
Why is short-wave band? Used for long distance radio broadcast?
Name the type radiowave propagation involved. When TV signals, broadcast by a tall antenna, are
intercepted directly by the receiver antenna.
Why is the transmission of signals through a co-axial cable not possible for frequencies greater than
20 MHz?
Name the type of modulation scheme preferred for digital communication.
Name two main parts of an optical fibre. Which of the two main parts of an optical fibre has higher
value of refractive index?
Write the full form of the terms (i) M.S.I. and (ii) V.L.S.I. used form different types of integrated
circuits.
Name the types of the communications that uses carrier signal having frequencies in the range 1012 Hz
to 1016 Hz.
Explain briefly the principle of transmitting signals using a satellite. State two main advantages of
using a satellite for transmitting signals.
What is remote sensing? Explain how is it carried out. Mention its two applications.
With the help of block diagram, explain the principle of an optical communication system. Give its
two advantages over cable communications system.
Name the device used for data transmission from one computer to the other. Justify the name. Using
the device, draw the block diagram for data communication and explain it briefly.
A T-V tower has a height of 500 m at a given place. If radius of earth is 6400 km, what it is coverage
range.
[2005]
Q.1
Q.2
Explain the following terms:
(i)
ground waves
(ii)
space waves and
(iii)
sky waves.
Aground receiver station is receiving signals at (i) 5MHz and (ii) 100 MHz, transmitted from a ground
transmitter at a height of 300 m located at a distance of 100km. Identify whether the signals are
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coming via space wave or sky wave propagation or satellite transponder. Radius if earth = 6400 km;
maximum electron density in ionosphere, Nmax = 1012m-3.
[2006]
Q.1
Q.2
Define the term modulation. Explain the need of modulation. Name three different types of
modulation used for a message signal using a sinusoidal continuous carrier wave. Explain the meaning
of any one of them.
What is the type of communication in which signal is discrete and binary coded version of the
message (or information).
[2007]
Q.1
Q.2
Q.3
Q.4
Q.5
Q.6
Q.7
What should be the length of dipole antenna for a carrier wave of frequency 6  108 Hz?
What is modulation? Explain the need of modulating a low frequency information signal. With the
help of diagrams, differentiate between PAM and PDM.
Write the acronym LASER in expanded form. State any four reasons for preferring diode lasers as
light sources for optical communication links.
Why is the frequency modulation is required for television broadcast?
What is the purpose of modulating a signal in transmission?
A message signal has a bandwidth of 5 MHz. Suggest a possible communication channel its
transmission.
A TV tower has height of 71 km. What is the maximum distance upto which TV transmission can be
received? Radius of earth = 6  106 m.
[2008]
Q.1
Q.2
Q.3
Q.4
Q.5
Q.6
Q.7
Draw a block diagram of a simple a amplitude modulation. Explain briefly how amplitude modulation
is achieved.
Explain, why high frequency carrier waves are needed for effective transmission of signals.
A message signal of 12 kHz and peak voltage 20 V is used to modulate a carrier wave of frequency
12 MHz and peak voltage 30 V. Calculate the (i) modulation index (ii) side-band frequencies
Why do we need carrier waves of very high frequency in the modulation of signals?
Draw a plot of variation of amplitude versus  for an amplitude-modulated wave. Define modulation
index. State its importance for effective amplitude modulation.
What does the term LOS communication mean? Name the types of waves that are used for this
communication. Give typical examples, with the help of suitable figure of communication system that
use space wave mode propagation.
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? (Radius of earth = 6400 km).
A carrier wave of peak voltage 20 V is used to transmit a massage signal. What should be the peak
voltage of the modulating signal, in order to have a modulation index of 80% Y.
[2009]
Q.1
Write the function of (i) Transducer and (ii) Repeater in the context of communication system.
Or
Write two factors justifying the need of modulation for transmission of a signal.
[2]
[2]
[2010]
Q.1
Write two factors justifying the need of modulating a signal. A carrier wave of peak voltage 12V 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%?
[2]
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Q.2
Which mode of propagation is used by short wave broadcast services having frequency range from a
few MHz upto 30 MHz? Explain diagrammatically how long distance communication can be achieved
by this mode. Why is there an upper limit to frequency of waves used in this mode?
[3]
[2011]
Q.1
Q.2
What is sky wave communication? Why is this mode of propagation restricted to the frequencies only
upto few MHz?
Write briefly any two factors which demonstrate the need for modulating a signal.
Draw a suitable diagram to show amplitude modulation using a sinusoidal signal as the modulating
signal.
[3]
[2012]
Q.1
Mention three different modes of propagation used in communication system. Explain with the help of
a diagram how distance communication can be achieved by ionospheric reflection of radio waves. [3]
[2013]
Q.1
In the block diagram of a simple modulator for obtaining an AM signal, shown in the figure, identify
the boxes A and B. Write their functions.
Q.2
Name the type of waves which are used for line of sight (LOS) communication. What is the range of
their frequencies? A transmitting antenna at the top of a tower has a height of 45 m and the height of
the receiving antenna is 80 m. Calculate the maximum distance between them for satisfactory
communication in LOS mode. (Radius of the Earth = 6·4 × 106 m)
[2014]
Q.1
Write the functions of the following in communication systems :
(i)
Q.2
Transducer
(ii)
Repeater
Write two basic modes of communication. Explain the process of amplitude modulation. Draw a
schematic sketch showing how amplitude modulated signal is obtained by superposing a modulating
signal over a sinusoidal carrier wave.
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SAMPLE PAPER
Q.1
Q.2
Q.3
Q.4
Q.5
Q.6
Q.7
Q.8
Q.9
Q.10
Q.11
Q.12
Q.13
Q.14
Q.15
Q.16
Q.17
Q.18
Q.19
Define the term modulation? Name three different types of modulation used for a message signal
using a sinusoidal continuous carrier wave? Explain the meaning of any one of them?
Explain how the optical communication system offers the possibility of millions of channels with
increased bandwidth? Given an additional advantage of optical communication system over a system
employing a co–axial cable?
State the two functions performed by a modem?
A ground receiver station is receiving a signal at (a) 5MHz and (b) 100 MHz, transmitted form a
ground transmitter at a height of 300m, and located at a distance of 100km. from the receiver station.
Identify whether the signal is coming via space wave or sky wave propagation or satellite
transponder? Radius of earth = 6.4  106 m. , Nmax of Inospehre = 1012m3
What is the function of a fax machine?
Why do we require a satellite for long distance TV transmission?
A fax message is to be sent from Delhi to Washington via a geo-stationary satellite. Calculate the
minimum time delay between the dispatch and its getting received? (Take height of the geostationary
satellite  36,000km).
What is pulse modulation? With the help of diagrams, differentiate between pulse amplitude
modulation and pulse duration modulation?
State the main function of a modem?
The truth table of a logic gate is given below. Name the gate and draw its logic symbol?
INPUTS
OUTPUT
A
B
Y
0
0
1
1
0
1
0
1
1
1
1
0
Mention any two special characteristics of light sources used in optical communication. Explain the
meaning of the terms (i) sensitivity (ii) responsivity of the detector used in an optical communication
system. Name two such optical detectors?
Why are infra red waves preferred for optical fiber communication?
Explain the working of a typical fiber optic communication link, using a block diagram?
What will be the required height of a T.V tower, which can cover the population of 60.3 lakhs if
average population density around the tower is 1000km-2? (Radius of earth = 6.4  106m)
How do we make the choice of a communication channel? A massage signal has a bandwidth of
5MHz. Suggest a possible communication channel for its transmission?
Name the type of communication corresponding to the case where the signal is
(i) A continuous signal essentially similar to the message or information.
(ii) A discrete and binary coded version of the message or information.
What does the term LOS communication mean? Name the types of waves that are used for this
communication. Which of the two-height of transmitting antenna and height of receiving antenna can affect the range over which this mode of communication remains effective?
Define the term ‘modulation index’ for an AM wave. What would be the modulation index for an AM
wave for which the maximum amplitude is ‘a’ while the minimum amplitude is ‘b’?
Distinguish between ‘point to point’ and ‘broadcast’ communication modes. Give one example of
each.
What is space wave propagation? Which two communication methods make use of this mode of
propagation? If the sum of the heights of transmitting and receiving antennae in line of sight of
communication is fixed at h, show that the range is maximum when the two antennae have a height
each.
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Q.20 Give one example each of a ‘system’ that uses the
(i)
Sky wave
(ii)
Space wave
(iii)
mode of propagation
[1]
Q.21 The modulation index of an amplitude modulated wave is 0.5. What does it mean?
Calculate the modulation index for an AM wave for which the maximum amplitude is ‘a’ while the
minimum amplitude is ‘b’.
[3]
Q.22
A distorted signal is received by a receiver set. Suggest a method to rectify this while doing the
modulation process?
[1]
Q.23 A company would like to transmit message signal as amplitude modulated wave. Suggest a simple
method in the form of block diagram to produce A M wave. What are the functions of the blocks
(devices) used? With a block diagram show how this AM wave will be transmitted?
[3]
Q.24 What do you mean by ‘modulation’?
Q.25 Label the unlabeled box in the given figure.
Q.26 With the help of Block Diagram explain how an amplitude modulated wave can be demodulated.
OR
Explain the following terms :
(1)
Ground waves
(2)
Space waves
(3)
Sky waves
Q.27 Block diagram of receiver is as shown :-
Identify X and Y.
State their function.
Q.28 Give reasons for the following:(i)
For ground wave transmission, size of antenna () should be comparable to wavelength of signal i.e.   /4
(ii)
Audio Signals, converted into an emwave, are not directly transmitted as such.
(iii)
The amplitude of modulating signal is kept less than the amplitude of carrier wave.
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Q.29 Draw a plot of the variation of ‘amplitude’ versus ‘’ for an amplitude modulated wave. Hence
explain the need for keeping the broadcast frequencies sufficiently spaced out?
Q.30 A device X can convert one form of energy into another. Another device Y can be regarded as a
combination of a transmitter and a receiver. Name the devices X and Y.
Q.31 An audio signal frequency va is to be transmitted as an electromagnetic wave
(i)
directly as such,
(ii)
through its use as the modulating signal on a carrier wave of frequency vc.
State the ratio of the size of the transmitting antenna, that can properly sense the time variation of the
signal, in terms of va and vc.
Q.32 Complete the following block diagram depicting the essential elements of a basic communication
system.
Name the two basic modes of communication. Which of these modes is used for telephonic
communication?
OR
Is it necessary for the transmitting antenna and the receiving antenna to be of the same height for line
of sight communication? Find an expression for maximum line of sight distance dm between these two
antennas of height hT and hR.
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