11 communication systems
TRANSCRIPT
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By:-
DR. VIK RAM SINGH
TANUSHREE SINGH
YEAR OF PUBLICATION-2010
All rights reserved. No part of this publication may be reproduced,
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prior permission of the Authors and Publisher
SAVANT INSTITUTE
TM
CLASS XII
PHYSICS
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11
COMMUNICATION SYSTEMSPre-requisites
Elementary ideas of wave phenomena.
Basic ideas of waves.
___________________________________________________
Concept Map
______________________Slide 1
______________________
History
The history of communication dates back to theearliest sign of life
Communication ranges from very subtle process of
exchange to full conversation and mars communicationhuman communication was revolutionized with speechabout 200000 year ago and writing about 30000 year
ago. Symbols were developed about 7000 year there have
been major development in the field of
telecommunication in the past few centuries
______________________Slide 2______________________
The transmission of signal over a distance for the
purpose of communication began thousands of years
ago with the use of smoke signals and drums in Africa,
America and in the parts of Asia.
In the 1790 the first fixed some a phone s
emerged in Europe However it was not until 183
electrical telecommunication system started to ap
______________________Slide 3_________________
Heinrich Hertz
Heinrich Rudolf Hertz (February 22, 1857 Janu
1894) was a German physicist who clarified
expanded the electromagnetic theory of light th
been put forth by Maxwell.
He was the first to satisfactorily demonstrat
existence of electromagnetic waves by buildi
apparatus to produce and detect VHF or UHF
waves.
______________________Slide 4_________________
Marconi
Guglielmo Marconi was born at Bologna, Italy, on
25, 1874, the second son of Giuseppe Marconi, an
country gentleman, and Annie Jameson, daugh
Andrew Jameson of Daphne Castle in the County WIreland.
He was educated privately at Bologna, Florenc
Leghorn.
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Slide 5
In 1899 he established wireless communication between
France and England across the English Channel.
He erected permanent wireless stations at The Needles, Isle
of Wight, at Bournemouth and later at the Haven Hotel,
Poole, and Dorset.
In 1900 he took out his famous patent No. 7777 for "tuned
or syntonic telegraphy" and, on an historic day in December
1901, determined to prove that wireless waves were notaffected by the curvature of the Earth, he used his system
for transmitting the first wireless signals across the Atlantic
between Poldhu, Cornwall, and St. John's, Newfoundland, a
distance of 2100 miles.
______________________Slide 6______________________
Between 1902 and 1912 he patented several new
inventions. In 1902, during a voyage in the American liner
"Philadelphia", he first demonstrated "daylight effect" relative
to wireless communication and in the same year patented
his magnetic detector which then became the standard
wireless receiver for many years.
In December 1902 he transmitted the first complete
messages to Poldhu from stations at Glace Bay, Nova
Scotia, and later Cape Cod, Massachusetts, these early
tests culminating in 1907 in the opening of the first
transatlantic commercial service between Glace Bay and
Clifden, Ireland, after the first shorter-distance public service
of wireless telegraphy had been established between Bari in
Italy and Avidari in Montenegro.
______________________Slide 7______________________
In 1905 he patented his horizontal directional aerial and in
1912 a "timed spark" system for generating continuous
waves.
In 1914 he was commissioned in the Italian Army as a
Lieutenant being later promoted to Captain, and in 1916
transferred to the Navy in the rank of Commander.
______________________Slide 8______________________
He was a member of the Italian Government mission to the
United States in 1917 and in 1919 was appointed Italian
plenipotentiary delegate to the Paris Peace Conference.
He was awarded the Italian Military Medal in 1919 in
recognition of his war service.
In 1931 Marconi began research into the propagation
characteristics of still shorter waves, resulting in the opening
in 1932 of the world's first microwave radiotelephone link
between the Vatican City and the Pope's summer residence
at Castel Gandolfo.
Slide 9
J C Bose
Sir Jagadish Chandra Bose CSI CIE FRS (B
Jogodish Chondro Boshu) (November 30, 1858 Nov
23, 1937) was a Bengali polymath: a physicist, bio
botanist, archaeologist, and writer of science fiction.
_____________________Slide 10_________________
He pioneered the investigation of ratio and micr
optics, made very significant contributions to plant sc
and laid the foundations of experimental science
Indian subcontinent. He is considered one of the fathers of radio science,
also considered the father of Bengali science fiction.
_____________________Slide 11_________________
He was the first from the Indian subcontinent to get
patent, in 1904.
Although Bose filed for patent for one of his inventio
to peer pressure, his reluctance to any form of pat
was well known.
Now, some 70 years after his death, he is being reco
for many of his contributions to modern science.
_____________________Slide 12_________________
Communication
The act of transmission of information from one pl
another place is called communication.
For successful communication, sender and receiver
have common language of information exchange.
_____________________Slide 13_________________
Elements of communication system
Every communication system has three essential ele
as shown in diagram.
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Slide 14
Types of communication
1. Point to point communication
In such a mode communication takes place over a link
between a single transmitter and a receiver such as
telephony.
_____________________Slide 15______________________
Unidirectional antennas
_____________________
Slide 16
______________________
2. Broadcast communication
In such a mode of communication, there are a large
number of receiver corresponding to a single transmitter
such as radio and television.
_____________________Slide 17______________________
Omnidirectional antenna
_____________________Slide 18______________________
Basic Terminology Used In Electronic
Communication Systems
1. Transmitter
It is a device that transmits a message/signal from source
into a form suitable for transmission over the channel
2. Channel
It is a connecting link between transmitter and receiver.
Slide 19
3. Receiver
It is a device which decodes the transmitted sign
fed to other devices which are connected to the u
obtain the information.
4. Transducer
It is a device which converts energy in one fo
another and empowers the transmitter.
5. Repeater
A repeater is a combination of a receiver
transmitter.
_____________________Slide 20_________________
6. Modulator
It translates message signal to the radio freq
range.
7. Demodulator
It is used to translate radio signals back to original
8. Antenna
It is used on both sides to radiate and pick up s
respectively.
_____________________Slide 21_________________
9. Signal
Information converted in electrical form and suita
transmission is called a signal.
Amplitude, frequency and phase are the characte
of a signal.
Signal is of two types: analog and digital.
_____________________Slide 22_________________
(a) Analog signal
Analog signal is a continuous function of tim
continuous amplitude.
Within a given range of values, analog signa
take any value.
Sine wave is a fundamental analog signal.
Human voice, video and music when conve
electrical signals produce analog signal.
_____________________Slide 23_________________
(b) Digital signals
Digital signals are those which can takdiscrete stepwise values.
Binary system that is extensively used in
electronics employs just two levels of a sig
corresponds to a low level and 1 correspon
high level of voltage/current.
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Slide 24
10. Bandwidth
The range over which frequencies in a signal vary is
called bandwidth.
Baseband is the band of frequency which represents
the original signal.
11. Noise
Noise refers to the unwanted signals that tend to disturb
the transmission and processing of message signals.
______________________Slide 25_____________________
12. Attenuation
Attenuation is the loss of strength of a signal while
propagating through a medium.
13. Amplification
Amplification is the process of increasing the amplitude
and hence the strength of signal to overcome from the
attenuation.
14. Range
The largest distance between source and destination upto which signal is received with sufficient strength is
called its range.
______________________Slide 26_____________________
Bandwidth of Signals
Speech signals in telephonic communication required a
bandwidth of 2800 Hz.
To transmit music a bandwidth around 20 kHz is needed.
Video signals for picture transmission require 4.2 MHz
bandwidth while a TV signal usually requires 6 MHz
bandwidth for transmission.
______________________Slide 27_____________________
Rectangular wave
Digital signals are in the form of rectangular wave can be
decomposed into a superposition of sinusoidal waves of
frequencies 0, 20, 30. 40nv0 where n is an integerextending to infinity and0 = 1 /T0.
Slide 28
The fundamental (0), fundamental (0)+ first Harmonic (20), and fundamental (0) + second har
(20) + third harmonic (30). To reproduces the rectangular wave shape exactly we
to superimpose all the harmonics 0, 20. 30, 40implies and infinite bandwidth.
_____________________Slide 29_________________
Bandwidth of Transmission Medium
The commonly used transmission media are wire
space and fiber optic cable.
Coaxial cable is a widely used wire medium, which o
bandwidth of approximately 750 MHz
The international telecommunication union (ITU) adm
the present system of frequency allocations as shown
table.
_____________________Slide 30_________________
Some Important Wireless Communication Frequency Ba
Service Frequencybands
Comments
Standard AM
broadcast540-1600 kHz
FM broadcast 88-108 MHz
Television
54-72 MHz
76-88 MHz
174-216 MHz
420-890 MHz
VHF (very high frequen
TV
UHF (ultra high frequ
TV
Cellular Mobile
Radio
896-901 MHz
840-935 MHz
Mobile to base stat
Base station to mo
Satellite
Communication
5.925-6.425 GHz
3.7-4.2 GHz
Uplink
Down link
_____________________Slide 31_________________
Wireless transmission frequency
Band Range Propagation Application
VLF 330 KHz Ground Long-range radio navi
LF 30300 KHz GroundRadio beacons an
navigational locato
MF 300 KHz3 MHz Sky AM radio
HF 330 MHz SkyCitizens band (CB
ship/aircraft communi
VHF 30300 MHz
Sky and
line-of-sight VHF TV, FM radi
UHF 300 MHz3 GHz Line-of-sightUHF TV, cellular pho
paging, satellite
SHF 330 GHz Line-of -sight Satellite communica
EHF 30300 GHz Line-of -sight Long-range radio navi
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Slide 32
Terrestrial Radio (All forms)
Ground-wave propagation follows the curvature of the earth Frequencies below 2MHz
AM radio (550-1600KHz)
Sky-wave propagation relies on the ionosphere and thesurface of the earth to refract waves back-and-forth
Frequencies 2MHz-30MHz
Short-wave Radio, HAM radio
_____________________Slide 33______________________
Line of site is point-to-point in a nearly straight line
Frequencies 30MHz and up FM radio, TV, Mobile phones
_____________________Slide 34______________________
Ground Wave
Ground waves are the one which propagate from one point
to another following the surface of earth. To radiate signals with high efficiency, the antennas should
have a size comparable to the wavelength of the signal atleast /4.
In standard amplitude modulated broadcast, ground based
vertical towers are generally used as transmitting antennas.
The mode of propagation and the wave glides over thesurface of earth and the propagation is called surface wave
propogation.
_____________________Slide 35______________________
A wave induces current in the ground over which it passes
and it is attenuated as a result of absorption of energy by
the earth. The attenuation of surface waves increases very rapidly with
increase in frequency.
The maximum range of converge depends on thetransmitted power and frequency.
_____________________Slide 36______________________
Sky Waves
Sky waves are the one which are received after being
reflected from the ionosphere.
When long distance communication takes place byionospheric reflection of radio waves back towards the earth
in the frequency range from few MHz up to 30 40 MHz, is
called sky wave propogation and used by short wavebroadcast services.
The ionospheric layer acts as a reflector for the frequ3 MHz to 30 MHz.
_____________________Slide 37_________________
_____________________Slide 38_________________
_____________________Slide 39_________________
Electromagnetic waves of frequencies higher than 30
penetrate the ionosphere and escape.
The phenomenon of bending of electromagnetic waves
they are diverted towards the earth is similar to total refl
In ionosphere, there is a large number of an ion or c
particles and it extends from a height of 65 km to 4above the earths surface.
_____________________Slide 40_________________
Ionisation occurs due to the absorption of ultraviolet an
high energy radiation coming from sun by air molecules
The degree of ionization varies with the height.
_____________________Slide 41_________________
Space Wave
Space waves travel in straight line and used for line o
(Los) communication as well as satellite communicatio
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Slide 42
If the transmitting antenna is at height ht and distance to
horizon dT is equal to T e td 2R h= where Re is radius of
earth and dT is also called the radio horizon of the
transmitting antenna.
The maximum line of sight (Los) distance dm between the
two antennas having heights ht and hr above the earth is
given by m e t e rd 2R h 2R h= +
_____________________Slide 43______________________
FSS - Fixed Satellite system
_____________________Slide 44______________________
Television broadcast, microwave links and satellite
communication use space wave propagation.
A communication satellite acts like a microwave link
repeater which receives signals from earth station, amplifies
and returns it back to earth at a different frequency to avoid
interference.
_____________________Slide 45______________________
Modulation and Its Necessity
Message signals are also called baseband signals which
essentially designate the band of frequencies representing
the original signal, as delivered by the source of information.
No signal, in general, is a single frequency sinusoid, but is
spreads over a range of frequencies called the signal
bandwidth.
There are various factors for which modulation of signals is
needed to transmit information. i.e. size of antenna or aerial,
Effective Power radiated by an antenna, mixing up of signals
from different transmitters.
_____________________Slide 46______________________
Size of Antenna or Aerial
For an electromagnetic wave of frequency 20 Hz. the
wavelength is 15 km and such a long antenna is notpossible.
Translating the information contained in our origin
frequency baseband signal into high or radio frequ
before transmission is thus needed for possible heig
antenna.
_____________________Slide 47_________________
Effective Power Radiated by an Antenna
For transmitting a signal, an antenna or an aerial ha
size comparable to the wavelength of the signal i.e.
/4 in dimension is needed for properly senses thvariation of the signal. The power radiated by linear antenna having lengt
proportional to ( )2L . Where is the wavelength
radiated frequency.
_____________________Slide 48_________________
Thus, for the same length of antenna the power of ra
increases by decreasing i.e. increasing frequency. Hence, the effective power radiated by a lone wave
baseband signal should be small.
For a good transmission, we need high powers by
high frequency transmission.
_____________________Slide 49_________________
Mixing up of Signals from Different Transmitters
If many people are talking at the same time or
transmitters are transmitting baseband information
mixing it is very difficult to distinguish between them.
Thus, there is a need for translating the origin
frequency baseband massage or information signal in
frequency wave before transmission.
The translated signal continues to possess the inform
continues to posses the information in the original sign
_____________________Slide 50_________________
Modulation
It is the process of changing some characteristic
amplitude, frequency or phase) of a carrier waaccordance with the intensity of the signal.
Therefore, modulation permits the transmission to oc
high frequency while it simultaneously allows the carrthe audio signal.
_____________________Slide 51_________________
1. Carrier wave
A wave of high frequency which carries a
frequency wave is called a carrier wave.
2. Signal wave These are the waves which are to be sen
transmitter to receiver through modulation.
3. Modulated wave
It is the superimposed wave of carrier wave and
wave.
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Slide 52
Types of modulation
There are three types of modulation (a) amplitude modulation (b)
frequency modulation (c) Pulse modulation.
(a) Amplitude modulation
When the amplitude of high frequency carrier wave is
changed in accordance with the intensity of the signal, it is
called amplitude modulation.
_____________________Slide 53______________________
_____________________Slide 54______________________
AM Envelope
Slide 55
(b) Frequency Modulation
In frequency modulation, the frequency of the mod
wave varies with the frequency of the modulating
whereas its amplitude is same as that of the carrier wa
_____________________Slide 56_________________
Amplitude, Phase, and Frequency Modulatioa digital baseband signal
_____________________Slide 57_________________
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Slide 58
Pulse Modulation
In such a modulation, the modulation of radio frequency
carrier is done by short pulses.
_____________________Slide 59______________________
Theory of Amplitude modulation
Let c (t) = Ac sin (ct) represent carrier wave and m(t) = Am sin(m t) represent the message or the modulating signal, where
m = 2fm is the angular frequency of the message signal. The modulated signal cm (t) can be written as cm(t) = (Ac +
Am sin m t) sin c t mc m cc
AA 1 sin t sin t
A
= +
_____________________Slide 60______________________
The modulated signal now contains the message signal.
cm (t) = Ac sin c t + Ac sin m t sin ctWhere = Am / Ac is the modulation index .
The modulation index ?is kept 1 to avoid distortion.
_____________________Slide 61______________________
Using the trigonometric relation, sin A sin B = [(cos (A B) cos (A + B)],
( ) ( ) ( )c cm c c c m c mA A
c t A sin t cos t cos t2 2
= + +
Here, (c m) and (c + m) are respectively called thelower side and upper side frequencies.
The modulated signal now consists of the carrier wave of
frequency c plus two sinusoidal waves each with afrequency slightly different from, known as side bands.
_____________________Slide 62______________________
Slide 63
Production of amplitude modulated wave
_____________________Slide 64_________________
The modulating signal Am sin (m t) is added to the signal Ac sin (c t)o produce the signal x (t)
Signal x (t) = Am sin (m t) + AC sin (c t) is passed ta square law device which is a non-linear device
produces an output.
y(t) = Bx (t) + Cx2 (t)Where B and C are constants.
_____________________Slide 65_________________
Thus y(t) = B Am sin t + B Ac sin ct2 2 2
m m c c m c m cC[A sin2 t A sin t 2A A sin t sin t]+ + + =
Am sin m t + B Ac sin c t
( ) (
2 2 22m m m
c m c
m C c m m c c
CA CA CA
A cos2 t cos2 t2 2 2
CA A cos tCA A cos+ +
+ +
Now, there is a dc term2 2m c
c" A A2
+ and sinuso
frequencies m, 2 m, c, 2c, c m and c + m.
_____________________Slide 66_________________
Signal is passed through a band pass filter which reje
sinusoids of frequencies m, 2m and 2c and retafrequencies c, cm and c + m.
The output of the band pass filter produce a
modulated wave.
_____________________
Slide 67
_________________
Detection of amplitude modulated wave
Demodulation
Detection is the process of recovering the modulating
from the modulated carrier wave.
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Demodulation is popularly known as detection
The modulated carrier wave contains the frequencies c andcm.
The modulated signal is passed though a rectifier to produce
the output.
_____________________Slide 68______________________
In order to retrieve m(t), the signal is passed thought an
envelope detector (which may consist of a simple RC circuit.
_____________________Slide 69______________________
_____________________Slide 70______________________
Disadvantages of Amplitude Modulation
No doubt amplitude modulation is very important but still
there are some disadvantages of amplitude modulation.
(i) Low effacing
(ii) Noisy reception
(iii) Small operating range_____________________Slide 71______________________
The modulator portion of modem converts the incoming
digital signal into a standard form suitable for transmission
over telephone channel.
The demodulator portion of the modem receives the channel
output and reconverts it into the original digital signal
formed.
_____________________Slide 72______________________
Internet
Internet permits communications and sharing of all types of
information between any two or more computers connected
through a large and complex network.
Email permits exchange of text/graphic material using email
software through internet service providers.
Slide 73
World Wide Web (WWW) computers that store s
information for sharing with others provides websites
directly or thorough web service providers.
Hypertext is a powerful feature of the web that automa
links relevant information from one page on the w
another using HTML.
E- commerce is the use of the internet to promote bu
using electronic means such as using credit cards.
Chat is real time conversation among people with co
interests through typed messages.
_____________________Slide 74_________________
Fax
Electronic reproduction of the document at a distant p
known as facsimile (fax) and it involves three steps.
(a) Optical scanning in which conversion of o
document takes place into digital data.
(b) Transmission of digital data through a
communication channel.
(c) Reproduction of original document at the receivin
_____________________Slide 75_________________
Mobile Telephony
In mobile telephony, all the service areas are divided
suitable number of cells centred on an office called
i.e. mobile telephone switching office.
Each cell contains a low-power transmitter called a
station and caters to a large number of mobile rec
popularly called cell phones.
Each cell has a service area of few kilometers depe
upon number of customers.
_____________________Slide 76_________________
The transfer of phone services from one coverage a
another coverage area is called handover or handoff.
Mobile phones operate typically in the VHF ran
frequencies about 800 950 MHz.
Modem is combination of modulator and demodulator
_____________________Slide 77_________________
Modem converts digital data into analog form.
The analog signals are modulated and transmitt
communication network.
At the receiving end modern converts analog signa
digital signals.
_____________________Slide 78_________________
Solved Example 1
If a broadcasting station is transmitting but not modulating
is heard in a receiver tuned to its frequency?
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Slide 79
Solution
If a broadcasting station is transmitting but not modulating, if
means that the station is sending the carrier wave only. The
loudspeaker in the radio receiver will produce no sound from this
carrier. It is because the carrier frequency in generally very high
and the loudspeaker cannot respond to such rapid variations due
to mechanical inertia.
_____________________Slide 80______________________
Solved Example 2
What is carrier communication?
_____________________Slide 81______________________
Solution
Carrier communication involves the use of a high-frequency
alternating current (called carrier) as the basic communication
agent and the modification of this high frequency current rent by
the audio signal (i.e., comparatively low frequency current) which
contains the message to be transmitted.
_____________________Slide 82______________________
Solved Example 3
The maximum peak-to-peak voltage of an AM wave is 16 mV
and the minimum peak-to-peak voltage is 4 mV. Calculate the
modulation factor.
_____________________Slide 83______________________
Solution
Fig shows the conditions of the problem.
Maximum voltage of AM wave, max16
V 8mV
2
= =
Minimum voltage of AM wave, min4
V 2mV2
= =
_____________________Slide 84______________________
max mina
min min
V V 8 2 6m 0.6
V V 8 2 10= = = =
+ +
Slide 85
Class Assignment 1
A sinusoidal carrier voltage of frequency 1 MHz and am
100 volts is amplitude modulated by sinusoidal volta
frequency 5 kHz producing 50% modulation. Calcula
frequency and amplitude of lower an upper sideband term
_____________________Slide 86_________________
Class Assignment 2
Why does FM give noiseless reception?
_____________________Slide 87_________________
Class Assignment 3
Why does amplitude modulation give noisy reception?
_____________________Slide 88_________________
Class Assignment 4
What is the importance of modulation factor (m a) in am
modulation?
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CURRICULUM BASED WORKSHEET
Topics for Worksheet
Elements of communication and types of
communication system.
Basic terminology used in Electrons.
Analog and Digital system.
Band with of Transmission Medium.
Modulation and Demodulation. Types of modulation.
Worksheet
1. Which part of the electromagnetic spectrum has the
largest penetrating power?
2. Earth stations play a vital role in satellite
communication. Mention their two functions.
3. Give the radio frequency spectrum.
4. A transmitting antenna at the tip of a 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 in LOS mode? Given radius
of earth 6.4 x 106 km.
5. A T.V. tower has height of 400 m at a given place.
Calculate its coverage range, if the radius of the earth is
6400 km.
6. What is a communication channel? Discuss Briefly the
various communication channels employed in
communication.
7. Explain the term FAX? Distinguish between FAX and e-
mail.
8. What is frequency modulation?
9. What is a carrier wave? Why high frequency carrier
waves are employed for transmission
10. The height of T.V. tower at a place is 400 cm. Calculate).(i) The maximum range upto which signals can be
received from the tower and
(ii) Area covered by the transmission. (Radius of earth
= 6400 km).
11. What is communication system? Describe briefly the
major constituents of a communication system.
12. Distinguish between frequency modulation and
amplitude modulation. Why is an FM signal less
susceptible to noise than an AM signal?
13. What is an electromagnetic wave? Explain briefly.
CURRICULUM BASED CHAPTER ASSIGNMENT
1 Mark Questions
1. Why long distance radio broadcasts use short-wave
bands?
2. A radio can tune to any station in the 7.5 MHz
MHz band. What is the corresponding wave
band?
3. Long distance radio broadcasts use short-wave
Why?
4. Why is it necessary to use satellites for long di
TV transmission?
5. Greater the height of a TV transmitting antenna, g
is its coverage. Comment.
6. Name one factor by which the range of transmis
signals by a TV tower can be increased.
7. The charging current for a capacitor is 0.25 A. w
the displacement current across its plates?
8. Which of the following has the lowest frequency?
Microwaves, UV rays and X-rays.
9. What is a sky wave?
10. What is amplitude modulation?
2 Marks Questions
11. A radar using wavelength 5 cm and having an a
disc of diameter 10 m has an angular resolution sthan 0.01 radian. Is this statement correct?
12. What is the wavelength of a TV station, which tra
on 500 MHz?
13. Compare and contrast radio waves and gamma r
14. How do we make the choice of a commun
channel? A message signal has a bandwidth of
suggest a possible communication channel
transmission?
15. It is necessary to use satellites for long distan
transmission. Why?
3 Marks Questions
16. It is necessary to use satellites for long distan
transmission. Why?
17. Why ground wave propagation of Radio is not s
for high frequencies?
18. Why is an FM signal less susceptible to noise t
AM signal?
19. Define the term modulation. Name three differe
of modulation used for a message signal u
sinusoidal continuous carrier wave. Explai
meaning of any one of these.
20. A radio broadcast is transmitted using am
modulation at a carrier frequency of 680 kHz. E
the meaning of each of the words.
21. What mode of communication is employed f
transmission of TV signals? Explain wh
transmission towers are usually made very high.
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22. What is sky wave propagation of waves? Explain why
sky wave transmission of electromagnetic waves
cannot be used for TV trans-mission.
23. A ground receiver station is receiving a signal at
(a) 5 MHz, and (b) 100 MHz, transmitted from a ground
transmitter at a height of 300 of 300 m located at
distance of 100 km. Identify whether it is coming via
space wave or sky wave propagation or satellite
transponder. [Radius of earth 6.4 106 m; N MAX ofionosphere = 1012m3].
5 Marks Questions
24. A TV tower has a height of 75 m. What is the maximum
distance and area upto which this TV transmission can
be received? Take radius of the earth as 6.4 106
m.
Solve with proper diagram.
25. Discuss main types of radio wave propagation.
26. A schematic arrangement for transmitting a message
signal (20 Hz to kHz)is given below:
Give two drawback from which this arrangement suffers.
Describe briefly with the help of a back diagram the
alternative arrangement for the transmission and
reception of the message signal.
27. What is the behavior of atmosphere towards
(i) Visible light
(ii) Infra-red radiation and
(iii) Ultraviolet radiation?
QUESTION BANK FOR COMPETITIONS
1. A digital signal possesses:
(a) continuously varying values
(b) only two discrete values
(c) only four discrete values
(d) none of the above
2. The audio signal:
(a) can be sent directly over the air for large distance
(b) cannot be sent directly over the air for large distance
(c) possess very high frequency
(d) none of the above
3. If the audio signal is transmitted directly into spac
length of transmitting antenna required will be:
(a) extremely small
(b) extremely large
(c) infinitely large
(d) none of these
4. If a carrier wave of 1000 kHz is used to carry the sign
length of transmitting antenna will be equal to:
(a) 3m
(b) 30 m
(c) 300 m
(d) 3000 m
5. Degree of modulation:
(a) can take any value
(b) should be less than 100%
(c) should exceed 100%
(d) none of these
6. The AM wave is equivalent to the summation of:
(a) two sinusoidal waves
(b) three sinusoidal waves
(c) four sinusoidal waves
(d) none of these
7. The AM wave contains three frequencies, viz,:
(a) e c s c sf f f f f
, ,2 2 2
+
(b) 2fc, 2(fc + fs), 2(fc fs)
(c) fc, (fc + fs), (fc fs)
(d) fc, fc , fc
8. Fraction of total power carried by side bands is given b
(a)2s
T
Pm
P=
(b) s2
T
P 1
P m=
(c)2
s2
T
P 2 m
P m
+=
(d)2
s2
T
P m
P 2 m=
+
9. In frequency modulation the amount of frequency de
depends on the:
(a) frequency of audio signal
(b) amplitude of audio signal
(c) both the frequency and amplitude of audio signal
(d) none of the above
10. Carrier swing in frequency modulated wave is given b
(a)f
CS2=
(b) CS = 2f(c) CS = f(d) CS = 0
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11. For a carrier frequency of 100 kHz and a modulating
frequency of 5 kHz, what is the band of AM transmission?
(a) 5 kHz
(b) 10 kHz
(c) 20 kHz
(d) 200 kHz
12. Optical fibre communication is generally preferred over
general communication system because:
(a) it is more efficient
(b) of signal security(c) both (a) and (b)
(d) none of these
13. Which one of the following subsystems is used for satellites
orbit position and altitude?
(a) Thrust subsystem
(b) Power subsystem
(c) Antenna subsystem
(d) Stabilization subsystem
14. Which of the following satellites is used in ship-to-shore and
shore-to-ship communication?
(a) Intelsat
(b) Marisat
(c) Comsat(d) Telestar
15. When microwave signals follow the curvature of earth, this is
known as:
(a) window
(b) the Faraday effect
(c) ionospheric reflection
(d) ducting
16. The absorption of radio waves by the atmosphere depends
on:
(a) their distance from the transmitter
(b) the polarization of the wave
(c) their frequency
(d) the polarization of the atmosphere
17. Modem is a device used for:
(a) modulating signals
(b) converting analog signals to digital signals
(c) either of the above
(d) none of above
18. What should be the maximum acceptance angle at the air-
core interface of an optical fibre, it n1 and n2 are the
refractive indicates of the core and the cladding
respectively?
(a) sin1
(n2/n1)
(b)1 2 2
1 2sin n n
(c)1 2
1
ntan
n
(d)1 1
2
ntan
n
19. In frequency modulation:
(a) the amplitude of modulated wave varies as frequ
carrier wave
(b) the frequency of modulated wave varies as ampli
modulating wave
(c) the amplitude of modulated wave varies as ampli
carrier wave
(d) the frequency modulated varies as frequen
modulating wave
20. Audio signal cannot be transmitted because:(a) the signal has more noise
(b) the signal cannot be amplified for d
communication
(c) of the transmitting antenna length
(d) None of these
21. In which of the following remote sensing technique
used?
(a) forest density
(b) pollution
(c) wetland mapping
(d) ground water survey
(e) medical treatment22. Given below is the circuit diagram of an AM demod
For good demodulation of AM signal of carrier frequ
the value of RC should be:
(a)1
RCf
=
(b)1
RCf
<
(c) 1RCf
(d)1
RCf
>>
23. Refractive index of ionosphere is:
(a) zero
(b) more than one
(c) less than one
(d) one
24. To cover a population of 20 lakh, a transmission
should have a height: (radius of the earth = 640
population per square km = 1000)
(a) 25 m(b) 50 m
(c) 75 m
(d) 100 m
(e) 39 m
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25. The range of frequencies allotted for FM radio is:
(a) 88 to 108 kHz
(b) 88 to 108 MHz
(c) 47 to 230 kHz
(d) 47 to 230 MHz
(e) 470 to 960 MHz
26. The skip zone in radio wave transmission is that range
where:
(a) there is no reception of either ground wave or sky wave
(b) the reception of ground wave is maximum but that ofsky wave is minimum
(c) the reception of both ground and sky wave is maximum
(d) the reception of both ground and sky wave is minimum
27. Phase shift keying is used:
(a) in digital data transmission
(b) in analog data transmission
(c) in both digital and analog data transmission
(d) none of the above
28. The frequency of an FM transmitter without signal input is
called:
(a) the centre frequency
(b) modulation factor(c) the frequency deviation
(d) the carrier swing
29. The fundamental radio antenna is a metal rod which has a
length equal to:
(a) in free space at the frequency of operation(b) /2 in free space at the frequency or operation(c) /4 in free space at the frequency of operation 3/4 in
free space at the frequency of operation
(d) None of these
30. In frequency modulated wave:
(a) frequency varies with time
(b) amplitude varies with time
(c) both frequency and amplitude vary with time both
frequency and amplitude are constant
(d) None of these