Unit: COMMUNICATION SYSTEM
1mark
Q1.What is the function of a transmitter in a communication system?(2011)
Q2.Which part of the electromagnetic spectrum is used in satellite communication?(2010).
Q3.What is sky wave propagation(2009)
Q4. What is space wave propagation.(2009)
2marks
Q1.In the block diagram of a simple modulator for obtaining an AM signal , shown in the fig. identify the
boxes A and B and write their functions .(2013)
X(t)
y(t)
modulating signal
+
AM waves
A
B
carrier wave
Q1.
(a) Identify 'X' and 'Y'
(b) Write their functions
A1.(a) X represents Intermediate Frequency (IF) stage while Y represents an Amplifier.
(b) At IF stage, the carrier frequency is changed to a lower frequency and in this process, the modulated
signal is detected. While the function of amplifier is to amplify the detected signal which not be strong
enough to be made use of and hence is required.
Q2.Mention the function of any 2 of the following:
(i).transducer
(ii).repeater
(iii).transmitter
(iv).bandpassfilter (2012)
Ans.A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver
sometimes with a change in carrier frequency. Repeaters are used to extend the range of a
communication system.
Q3.(i).Define modulation index.
(ii)Why is the amplitude of modulating signal kept less than the amplitude of carrier wave?(2011)
Ans.Modulation index is the ratio of amplitude of modulating signal and amplitude of carier wave.
Modulation index is kept less than 1, that is Am<Ac, in order to avoid distortion.
Q4.What is the range of frequencies used in satellite communication? What is common between these
waves and light waves?(2010)
Q5.By what percentage will the transmission range of a T.V tower be affected when the height of the
tower is increased by 21%?(2009)
3marks
Write three important factors which justify the need of modulating a message signal. Show
diagrammatically how an amplitude modulated wave is obtained when a modulating signal is
superimposed on carrier wave.
iii)
to avoid intermixing of signals
Q1. Name the type of waves which are used for LOS communication? What is the range of their
frequencies ?
A transmitting antenna at the top of the tower has a height of 20 m. and the height of the receiving
antenna is 45m. calculate the max. distance between them for satisfactory communication in LOS mode.
(radius of earth =6.4x106m)(2013)
Q2.Name three different modes of propagation of electromagnetic waves .explain , using a proper
diagram the mode of propagation used in the frequency range above 40MHz.(2012).
Ans: Same asans below
Q3.Draw a schematic diagram showing the (i)ground wave (ii)sky wave (iii)space wave propagation
modes for em waves.
Write the frequency range for each of the following :
(i)standard AM broadcast
(ii)television
(iii)satellite communication(2011)
AnsDiagram showing the (i) ground wave (ii) sky wave and (iii) space wave propagation modes for em
waves:
Frequency range:
(i) Standard AM broadcast: 540-1600 kHz
(ii) Television: 54-72 MHz
76-88 MHz: VHF (very high frequencies)
174-216 MHz
420-890 MHz: UHF (ultra high frequencies)
(iii) Satellite communication: 5.925-6.425 GHz (uplink)
3.7 - 4.2 GHz (downlink)
Q4.(i)Why is communication using LOS mode limited to frequencies above 40MHz.
(ii) A transmitting antenna at the top of the a tower has a height 32m. and the height of the receiving
antenna is 50m. what is the max. distance between them for communication in LOS mode?(2010)
Q4 .What is space wave propagation? Give two examples of communication system which use space
wave mode.
A TV tower is 80 m tall. Calculate the maximum distance upto which the signal transmitted from the
tower can be received. [Marks:3]
2010
Ans.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. At frequencies
>40 MHz, communication is essentially limited to line-of-sight paths. At these frequencies, the antennas
are relatively smaller and can be placed at heights of many wavelengths above the ground. Because of
line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the
earth. 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.
Two examples of communications systems which use space wave mode are - Television broadcast,
microwave links and satellite communication.
Given hT = 80 m
Taking hT = hR = 80 m. , we get
dM = 64000 m. = 64 km.
Therefore the maximum distance upto which the signal transmitted from the tower can be received is
64 km.
Q5. What is meant by the detection of a signal in a communication system? With the help of block
diagram explain the detection of AM signal.(2009).