11 Communication Systems

8/3/2019 11 Communication Systems

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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,

stored in a retrieval system, transmitted in any form or by any means-

Electronic, Mechanical, Photocopying, Recording or otherwise, without

prior permission of the Authors and Publisher

SAVANT INSTITUTE

TM

CLASS XII

PHYSICS


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Physics Communication Systems

SAVANT EDUCATION GROUP E-17, East of Kailash, New Delhi 110065. Ph.: +91-11-26224417 www.savantgroup.org

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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256 Communication Systems Phy


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


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


10. Carrier swing in frequency modulated wave is given b

(a)f

CS2=

(b) CS = 2f(c) CS = f(d) CS = 0


14/15



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


15/15



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


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

Documents

11 Communication Systems