communication system · for a certain range of frequencies. these phenomena are shown as below:...
TRANSCRIPT
- CHAPTER 15
CommunicationSystem
Chapter Analysis w.r.t, Lost 3 Yeor's Boord ExomsThe analysis given here givesyou an analytical picture of this chapter and will helpyou toidentify-the concepts of the chapter that are to befocussed morefrom exam point of view.
Number of Questions asked in lost 3 years--- ..-.-.--.--.--.-.- -- - - _ - ..........•.....- .•._ ..•......•................····r-· ·····························-···········-_·_·.--v-r ..-- -- -.-...•.....•...•..•........-..,
Delhi All India Delhi All India
Long Answer (5 marks)
) ~ A.
2017 I I2015 2016
..............y~ry~~?!~~s..~~E...(~...~~.~:~L......J.g.Short Type I Answer (2 marks)Short Type II Answer (3 marks)
.............................................._. ._._._- · ···_····-·_·······_·-····1 ···········---,---··-····.-....... ·-4························· + · ·1lQ. -.. ..- - ..- ..-.+.-.--.-.-.:.- -..~............. :···································· -1
1 Q 1 Q 2Q
Value Based Questions (4 marks)
All India
I lQ
• In 2015, only one question of 1mark based on Mode. of Communication forTelephonic Communication was asked in Delhi set.;
• In 2016, in Delhi set, only one question of 2 marks based on Attenuation andDemodulation was asked in Delhi set.In All India set, one question of 1mark based on Component of CommunicationSystem and one numerical question of 2 mark were asked.
• In 2017, in Delhi set, one question of two marks each based on Mode ofCommunication in Satellite Communication System, Sight Communication andTV signals were asked. In All India set only one question of 3 marks based on BlockDiagram of Communication System was asked.
On the basis of above analysis, it can be said that from exam point of view Mode ofCommunication for Telephonic Communication, Attenuation and Demodulation,Component of Communication System, Mode of Communication in SatelliteCommunication System, Sight Communication and TV Signals are most important conceptsof the chapter.
[TOPIC1] CommunicationCommunication is the act of transmission and reception of information. The informations are firstconverted into electrical signals and then sent electronically.
1.1Communication SystemA system comprises of transmitter, communication channel and receiver.A block diagram of a generalised communication system is shown as below:
• Signal It conveys information as single-valued function of time and which, at every instant of time,takes a unique value.
• Transmitter It consists of transducer/signal generators, modulators and transmitting antenna. Fore.g., If information source gives voice signal, transducer converts it into an electrical signal.
• Receiver Its main function is to decode the original signals. The main function involves picking upthe signals, demodulating and displace the original message signal. The process of recovering theoriginal signal is called demodulation.~~.
• Communication Channel The physical path between the transmitter and receiver is known ascommunication channel. e.g. transmission channel, optical fibre. They are of two types namely(itGuided (point-to-point) e.g.- telephony (ii) Unguided e.g.- free space
• Bandwidth The range of frequencies used to pass through channel is known as bandwidth.
N b f h I Total bandwidth of channelum er 0 c anne s = ------------Bandwidth needed per channel
Table shows the various things used in communication system,Source of Information Speech, pictures, words, codes, symbols, commands and dataTransmitter Oscillators, amplifiers, filters and antennaChannel Wire links, wireless and optic fibresReceiver Radio, TV,computer, telephone, teleprinter, telegraph, fax and
internet
Modes of CommunicationsThere are two basic modes of communication given as below:(i) Point-to-point In this type of communication mode, communication takes place over a link between a
signal transmitter and a receiver, e.g. telephony.(ii) Broadcast In the broadcast mode, there are a large number of receivers corresponding to a signal
transmitter, e.g. radio and Tv.
Basic Terminology used in Electronic Communication Systems(i) Signal Information converted into electrical form and suitable for transmission is called a signal.
CHAPTER 15 : Communication System
(ii) Transducer Any device/arrangement thatconverts one form of energy into another iscalled a transducer, e.g. microphone. i.e.converts physical quantity (message) intoelectrical signal.
(iii) Noise It refers to the unwanted signals thattends to disturb the transmission andprocessing of message signals incommunication system with the interferenceof information signal.
(iv) Attenuation It refers to the loss of strengthof a signal during its propagation through thecommunication channel. (transmissionmedium)
(v) Amplification It is the process by whichamplitude of a signal is increased using anelectronic circuit called the amplifier.(Consisting of at least one transistor)
(vi) Range It is the largest distance between asource and a destination upto which the signalis received with sufficient strength.
(vii) Baseband Band of frequencies representingthe original signal as is produced by the sourceof information is called baseband.
(viii) Repeater Repeaters are erected at suitabledistances between the transmitter andreceiver. Repeaters are used to extend therange of a communication system. Itretransmits and amplifies signals from thetransmitter, to the receiver with a change ofcarrier frequency.
1.2 Message SignalsA time varying electrical signal generated by atransducer out of original signal is termed asmessage signal. The electrical signals are of twotypes such as below:
Analog SignalA continuous signal value which at any instantlies within the range of a maximum and aminimum value. Graphical representation ofanalog signal can be represented as given below:As, sinusoidally varying alternating voltage isthe simplest analog signal E = Asinrot
483
Eorl
Timeperiod :(T)-:
IIIII
Digital Signal (Pulse Signol)Digital signals are those which can take onlydiscrete stepwise values e.g. output of a computer,fax, ete.
1Eorl
Pulserise
Pulsefall
Pulse1 duration
TPulseamplitude+O~--~--~~--o~--~--~o~--~-----
Time--+Coding schemes used for digital communication aregiven as below:
Binary Coded Decimal (BCD) L I IIf
In this, a digit is represented by two binary numberso or 1. "
J
American Standard Code for InformationInterchange (ASCII)It is a universally popular digital code to representnumbers, letters and certain characters.
1.3 Bandwidth of SignalsBandwidth of signal is defined as the differencebetween the upper and lower frequencies of signal.In a communication system, the message signal canbe voice, music, picture or computer data.Bandwidth = Highest frequency - Lowest frequencyThis has been shown in the table given as below:
Typesof signal Frequencyrange BandwidthSpeech signal 300-3100 Hz 2800 Hz
__ w ••• , __ ."_
Music signal 20-20000 Hz 20 kHz" .M •• • • __ • •
Video signal 4.2 MHz
TV signal 6 MHz
484
1.4 Bandwidth ofTransmission Medium
The commonly used transmission media arewire, free space, fibre optic cable(750 MHz) and optical fibre (100 GHz). Thisrange is sub-divided further and allocated forvarious services as indicated in the table givenas below:
Service Frequency Remarksbands
Standard AM 540-1600 kHz Radio broadcastbroadcast
FM broadcast 88-108 MHz Music channel
Television 54-72 MHz76-88 MHz
174-216 MHz420-890 MHz
VHF (Very HighFrequencies )
TV
UHF (Ultra HighFrequencies) TV
••••••••••• M •• ~MM_MMMHH ••• H.HH •• HH_.HH •••• HHHH •• HH •• H_.HHH_._ •• HH • ". •
Cellular 896-901 MHz Mobile to baseMobile radio 840-935 MHz station
Base station tomobile
Satellite 5.925-6.425 GHz Uplink
Communication 3.7-4.2 GHz Downlink
AntennaAntenna is a device which acts as an emitter ofelectromagnetic waves and it also acts as a firstreceiver of energy. It is generally a metallicobject often a wire or collection of wires.(i) Hertz Antenna It is a straight conductor
of length equal to half the wavelength ofradio signals to be transmitted or received.. l=~i.e,
2(ii) Marconi Antenna It is a straight conductor
of length equal to a quarter of the wavelengthof radio signals to be transmitted or received,
l=~i.e.4
(iii) Dipole Antenna It is used in transmissionof radio waves. It is omni directional.
.0 Chopterwise CBSE Solved Papers PHYSICS
(iv) Dish-Type Antenna It is a directional antenna.Such antenna has a parabolic reflector with anactive element. The gain of a dish antenna is
P = 6(i-r,where, D = diameter of reflector,
A. = wavelength of electromagnetic radia tion
1.5 Propagation ofElectromagnetic Waves
In communication using radio waves, an antenna atthe transmitter radiates the EM waves, which travelthrough the space and reach the receiver at the otherend. Depending upon frequency and ways ofpropagation, electromagnetic waves categorised a",follows:
Ground Wave Propagation [f < 2 MHz)In ground wave propagation, the radio waves (AM)travel along the surface of the earth. These waves areguided along the earth's surface and they follow thecurvature of the earth. Ground wave propagation ispossible only when the transmitting and receivingantenna are close to the surface of the earth.
Sky Wave Propagation(2 MHz <t= 30 MHz)Long distance communication can be achieved by aradiowave directed towards the sky and reflected bythe ionosphere towards the desired location of theearth is called a sky-wave. This sky wave propagationis used by short wave broadcast services. Theionosphere is so called because of the presence of alarge number of ions. It extends from height of65 km to about 400 km above the earth's surface.
CHAPTER 15 : Communicotion System 485
The details are in the table as below:Different layers of atmosphere and their interaction with the propagating electromagnetic waves
Name of the stratum(layer) Frequencies most affectedApproximate height . .
over earth's surface EXIsts durmg
Troposphere.-- ..- -.--.~.•.--- -- --- ,,-_ ..-
D (part of stratosphere)10km Day and night VHF (up to several GHz)
••• - •• --.-- •• -.- •••••• -- •••• --- •••• - •• ---- •••••• __ ··_·· •• __ ··_·· •• H· ·····_·· ··_···_··.·_------- ••••••••••• --- ••• - •••••••••• --- •• ··H··H •••••• _ •••• _ •••••••• _ •• •• __ •••••••••••• __ •••••••• N ••••••••••••••••••••••• _ ••• _ ••••••• •• •••••••• _
65-75 km Day only Reflects LF, absorbs MF and HFto some degree
••••• __ •••••• __ ._ •••••••••••••••• • •• • ._H" __ ••••• ••••• _ •••• __ ••• __ .HHH •••• __ ._ •••••• • • __ "'H __ •••••• •• _ •• _ •• _ •••••• _ •••••••• ~ •••••••••••••• H~~~ •••• HH •• H •••• HH •••••••••••• _ •••••••••••••••••••••••••• _ ••••••••••••••••••••••
Helps surface waves, reflects HFE (part of stratosphere) 100kmPartially absorbs HF waves yetallowing them to reach F2
FI (Part of mesosphere) 170-190km
Efficiently reflects HF waves,particularly at night
F2 (Thermosphere) 250-400 km
The degree of ionisation varies with the height.The density of atmosphere decreases withheight. The ionospheric layer acts as a reflectorfor a certain range of frequencies.These phenomena are shown as below:
Maximum Usable Frequency (MUF)It is a limiting frequency of radiowave but forsome specific angle of incidence other than thenormal and gets reflected and returns to theearth.
MUF = I. sece=~e cose
where, e is the angle between normal and thedirection of incidence of waves.
Skip DistanceIt is the shortest distance from a transmittermeasured along the surface of earth to the skywave of fixed frequency but less than criticalfrequency from the ionosphere. Ie will return toearth.
Dskip= 2h (/j:W r -1
where, h = height of the reflecting layer ofatmosphere.
IMUF =MUF
Day onlyDaytime, merges withF2 at nightDay and night
Critical FrequencyFor a given layer, it is the highest frequency thatwill return down to earth by that layer.Critical frequency (/el, Ie = 9 (Nmax)I/2
where, N max = maximum electron density ofionosphere.The critical frequency ranges approximately from5-10 MHz. -, rf"''l
FadingI I;I "'
It is defined as the variation in the strength of asignal at a receiver due to interference of waves. ".
Space Wave Propagation (LOS)(Tropospheric wave propagation)(f >30MHz)A space wave travels in a straight line fromtransmitting antenna to the receiving antenna.Space waves are used for Line-of-Sight (LOS)communication as well as satellite communication.Because of LOS nature of propagation, these wavesare get blocked at some point by curvature of earthas shown below:
+----- dM -------+- dr ---;,1 -- dR -----+
T R(i) Range of TV transmission, d = .J2hR
where, h = height of antenna,R = radius of the earth,
d = maximum distance/range of transmission.
486
(ii) Rangeof line of sight distancebetweentwoantennas.
dM =dT + dR' dM = ~2hTR + ~2hRRwhere, hT and hR are heights of transmittingand receivingantenna.
Satellite CommunicationIn this communication, frequencyband 5.9 GHzto 6.4 GHz is used for uplinking and 3.7 GHz to2 GHz is used for downlinking.
PREVIOUS YEARS'EXAMINATION QUESTIONSTOPIC 1o 1Mark Questions
1. Name the essential components of acommunication system. All India 2016
2. Which basic mode of communication isused for telephonic communication?Delhl2D15
3. The figure given below shows the blockdiagram of a generalised communicationsystem. Identify the element labelled Xand write its function. Deihl 2014 C
~------------------------I
II
Information :Transmitter Receiver!source ~------------------------
Message Messagesignal signal
4. What is the meaning of the termattenuation used in communicationsystem? All India 2014C
5. Give the one example of point-to-pointcommunication mode. All India 2014C
6. Draw a block diagram of a generalizedcommunication system. Delhl2014C
7. What is the function of a transducer usedin a communication system? Deihl 2012
8. What does the term attenuation used incommunication system mean?Deihl 2D12,200BC
o chopterwlse eSSE Solved Papers PHYSICS
9. What is the function of a repeater in acommunication system?Foreign 2011;Deihl 2010
10. What is the function of a transmitter in acommunication system? Foreign 2011
11. What is the sky wave propagation?Delhi 2009
12. What is ground wave propagation? Delhi 2009
13. What is space wave propagation? Delhi 2009
14. What does the term transducer mean inan electronic communication system?Delhl2D09C
15. Distinguish between sinusoidal and pulseshaped signals. All India 2009C
o 2 Marks Questions16. Which basic mode of communication is used
in satellite communication? What type ofwave propagation is used in this mode?Write giving reason, the frequency rangeused in this mode of propagation. Deihl 2017
17. (i) What is the line of sightcommunication?
(ii) Why is it not possible to use skywaves for transmission of TV signals?Upto what distance can a signal betransmitted using an antenna ofheight h? Delhi 2017
18. Explain the terms (i) Attenuation and(ii) Demodulation used in communicationsystem. Deihl 2016
19. A devices X used in communication systemcan convert one form'of energy intoanother. Name the device X. Explain thefunction of a repeater in a communicationsystem. Foreign 2016
20. (i) Which mode of propagation is used byshortwave broadcast services havingfrequency range from a few MHz upto30 MHz? Explain diagrammaticallyhow long distance communication canbe achieved by this mode.
(ii) Why is there an upper limit tofrequency of waves used in this mode?
All India 2016, 2010
CHAPTER 15 : Communication System
21. Write the function of the following incommunication systems. All Indio 2014
(i) Transducer (ii) Repeater22. Write the function of the following in
communication systems. All Indio 2014
(i) Transmitter (ii) Modulator23. Write the function of the followingin
communication system. All Indio 2014
(i) Receiver (ii) Demodulator24. Which mode of wave propagation is
suitable for television broadcast andsatellite communication and why? Drawa suitable diagram depicting this mode ofpropagation of wave. Foreign 2012
25. Distinguish between analog and digitalsignals. Deihl 2012
26. Mention the function of any two of thefollowing used in communication system.
(i) Transducer (ii) Repeater(iii) Transmitter (iv) Bandpass filter
Delhi 2012
27. What is sky wave communication? Why isthis mode of propagation restricted to thefrequencies only up to few MHz?All Indio 2011
28. What is space wave communication?Write the range of frequencies suitablefor space wave communication.All Indio 2011
29. Draw a block diagram showing theimportant component in a communicationsystem. What is the function of atransducer? Foreign 2011
30. What is the range of frequencies used forTV transmission? What is commonbetween these waves and light waves?Deihl 2010
31. What is the range of frequencies used insatellite communication? What iscommon between these waves and lightwaves? Deihl 2010
487
32. In standard AM broadcast, what mode ofpropagation is used for transmitting asignal? Why is this mode of propagationlimited to frequencies up to a few MHz?Foreign 2010
33. Name any two types of transmission mediathat are commonly used for transmissionof signals. Write the range of frequenciesof signals for which these transmissionmedia are used. All Indio 2010C
34. (i) What is line of sight communication?(ii) Why is it not possible to use sky wave
propagation for transmission of TVsignals? Foreign 2009
35. A communication satellite is essentially arepeater station in space. Justify thisstatement by analysing the function of arepeater. All Indio 2009C
36. Write the function of(i) transducer and
(ii) repeater in the context ofcommunication system.All Indio 2014, 2009
o 3 Marks Questions37. Draw a block diagram of a generalised
communication system. Write thefunctions of each of the following:
(i) Transmitter (ii) Channel(iii) Receiver All Indio 2017
38. Distinguish between point to point andbroadcast modes of communication. Givean example of each. Foreign 2016; Deihl 201SC
39. Describe briefly by suitable diagram of(i) sky wave propagation and (ii) spacewave propagation. Mention the frequencyrange of the waves in these modes ofpropagation. Foreign 2014
40. Write the function of each of the followingused in communication system.
(i) Transducer (ii) Repeater(iii) Transmitter Delhl2013C
488
41. Name the types of waves which are usedfor Line of Sight (LOS) communication.What is the range of their frequencies? Atransmitting antenna at the top of atower has a height of 20 m and the heightof the receiving antenna is 45 m.Calculate the maximum distance betweenthem for satisfactory communication inline of sight mode. (Radius of the earth= 6.4 x 106km) AII,India 2013
42. Draw a schematic diagram showing the(i) ground wave (ii) sky wave and(iii) space wave propagation modes forelectromagnetic waves.Write the frequency range for each of thefollowing:
(i) Standard AM broadcast(ii) Television
(iii) Satellite communicationHOTS;Delhi 2012
43. Explain briefly the following terms usedin communication system
(i) Transducer (ii) Repeater(iii) Amplification All India 2012
44. Name the three different modes ofpropagation of electromagnetic waves.Explain using a proper diagram, themode of propagation used in thefrequency range from a few MHz to40 MHz. Delhi 2012
45. What is the ground wave communication?On what factors does the maximum rangeof propagation in this mode depend?All India 2011
46. Draw a schematic diagram showing the(i) ground wave (ii) sky wave
(iii) space wave propagation modes forelectromagnetic waves. All India 2011C
47. Which mode of propagation is used byshort wave broadcast services havingfrequency range from a few MHz up to
o Chapterwise CBSE Solved Papers PHYSICS
30 MHz? Explain diagrammatically, howlong distance communication can beachieved by this mode? Why is there anupper limit to frequency of waves used inthis mode? All India 2010
48. What is space wave propagation? Give twoexamples of communication system whichuses the space wave mode.A TV tower is 80 m tall. Calculate themaximum distance up to which the signalstransmitted from the tower can bereceived. Delhi 2010
49. (i) Why is communication using line ofsight mode limited to frequenciesabove 40 MHz?
(ii) A transmitting antenna at the top of atower has a height 32 m and theheight of the receiving antenna is50 m. Calculate the maximum distanceof them for satisfactory communicationin line of sight mode? Delhi 2010
50. The diagram given below represents theblock diagram of a generalisedcommunication system. Identify theelements labelled as X, Y, Z in thisdiagram.Explain the function of each of theseelements.
51. Distinguish between sky wave and spacewave propagation. Give a brief descriptionwith the help of suitable diagramindicating how these waves arepropagated? All India 2009
52. Distinguish between sky wave and spacewave propagation. Explain in brief withthe help of suitable diagram, how spacewaves are used for line of sightcommunication? HOTS;Delhi 2009C
o Explanations1. Essential components of a communication system
are transmitter, transmission medium orcommunication channel and receiver. [1)
2. Point to point is a basic mode of communication,which is used for telephonic conversation. In thismode, communication, takes place over a linkbetween a single transmitter and a receiver. [1)
3. Labelled element X represents the channel. Itsfunction is to transmit information from oneplace to another. [1)
4. It refers to the loss of strength of a signal duringits propagation through the communicationchannel output or through the transmissionmedium. [1)
5. Telephone is the example of point to pointcommunication mode. [1)
6. Refer to theory on page 482. [1)
7. Transducer used as a sensor or detector incommunication system. It converts the physicalsignal into electrical signal. e.g. A photo detectorconverts light signal into electrical signal. [1)
8. Refer to Ans. 4 [1)
9. Repeater It picks up the signals from thetransmitter, amplifies it and transmit it to thereceiver. Thus, repeater comprises up of receiver,transmitter and amplifier. Its function is toextend the range of communication. [1)
Multiple usage of amplifier repeaterextends the range of communication.
10. Transmitter It comprises of message signalsource, modulator and transmitting antenna.Transmitter make signals compatible forcommunication channel via modulator andantenna. [1)
11. Sky wave propagation When radio wavepropagates from one place of earth to other afterreflection by ionosphere, the range of frequenciesfrom few MHz to 30 MHz gets reflected back byionosphere. This range also reflected as shortwave band. This mode of propagation is used byshort wave broadcast service. [1)
12. Ground wave propagation The radio waveswhose frequencies ranged up to 1500 kHz,propagates from one place of earth to otherfollowing its transmission along the surface ofearth. These waves get attenuated and hencecannot travel over long distances. This range offrequencies also referred as amplitude modulatedband (AM band). [1)
13. Space wave propagation It is also known asLine of Sight propagation (LOS). The radio wavetransmitted by antenna directly reaches thereceiving antenna travelling along a straight line.TVwaves (80 MHz-200 MHz) propagate throughspace wave propagation. [1)
14. Transducer Any device which converts oneform of energy into other e.g. electric transducerconverts pressure, temperature, etc. into varyingelectrical signals i.e. transducer converts physicalsignals into electrical signals. (1)
15. A signal in which current or voltage changecontinuously with time sinusoidally is known assinusoidal signal.
A signal in which current or voltage can takeonly tW9 discrete values for it is called pulseshaped signals.
5V
(1)
es. J'!f:'
'..,.[(1. OVt-
16. Space wave mode is used in satellitecommunication. In this type of wavepropagation, radio waves are emitted from thetransmitter antenna and reach the receivingantenna through space. Frequency range forthis communication is very high, so that, it maypenetrate the ionosphere and reach satellite. Itsrange is from 54 MHz to 4.2 GHz.
17. (i) When the radio waves from the transmittingantenna reach the receiving antenna directly,the wave propagation is called line of sightcommunication.
(ii) Sky waves cannot be used for transmission ofTV signals because the reflecting layer that isionosphere becomes transparent for high
AgO
frequency range of TV signal. They go to thesky but do not come back to surface. Radiowaves for radio based communication areused as sky waves.An antenna of height h can transmit signalupto a maximum distance of d, whered = .f2hR r where, R is the radius of the earth.
,18. (i) Attenuation The loss of strength of signalduring its propagation through thetransmission medium is called attenuation.Repeater is used to compensate theattenuation. (1)
(ii) Demodulation The process ofrecoveringthe original information signal from themodulated wave at the receiver end is calleddemodulation. It is the reverse process ofmodulation. (1)
19. X is transducer.Repeater A repeater accepts the signal fromthe transmitter amplifies and retransmits it tothe receiver. Refer answer 9. (2)
~O. (i) Sky wave propagation is used by shortwavebroadcast services having frequency rangefrom few MHz to 30 MHz. (1)
F2F1
Ionospheric Elayer
Long distance communication can beachieved by reflection of radio waves by theionosphere, back towards the Earth. Thisionosphere layer acts as a reflector only for acertain range of frequencies (few MHz to30 MHz).
(ii) Electromagnetic waves of frequencies higherthan 30 MHz, penetrate the ionosphere andescape, whereas the waves less than 30 MHzare reflected back to the earth by theionosphere. (1)
21. (i) Transducer Refer to Ans. 14. (1)(ii) Repeater Refer to Ans. 9. (1)
o Chopterwise CBSE Solved Papers PHYSICS
22. (i) Transmitter Refer to Ans. 10. (1)(ii) Modulator A modulator is a device. Which
does the modulation of low frequency messagesignal. (1)
23. (i) Receiver A receiver extracts the desiredmessage signal from the received signals at thechannel output. (1)
(ii) Demodulator A demodulator is a device thatperforms demodulation, i.e. inverse ofmodulation. The process of retrieval ofinformation from the carrier wave at thereceiver is termed as demodulation.As, signals of these frequency ranges arereflected back by ionosphere up to receiver endafter travelling a large distance. (1)
24. Sky wave Television ~ 1710kHz to 40 MHz
Skywave
(2)
25. Continuously with time At any time, the valueof signal is represented by its amplitude. AnalogSignal-output voltage varies continuously withinput voltage. Rarely used in moderncommunication system.Digital signal In digital signal, the amplitudes arenot continuous with time. Amplitude of a signalhas only its two levels (i.e. low or high). (2)
26. (i) Transducer Refer to Ans. 14.(ii) Repeater Refer to Ans. 9.
(ill) Transmitter Refer to Ans. 10.(iv) Bandpass filter A device which passes the
signals with certain frequency range only. (2)
27. Sky wave propagation Refer to Ans. 11.Reason behind restriction up to few MHz. Theradio wave of frequencies up to 30 MHz cannotpenetrate the ionosphere and they get reflectedback to earth whereas higher frequencies(> 40 MHz) bends slightly but not reflected back tothe earth. Because, frequencies up to few MHz« 30 MHz) gets reflected back to earth. Hence,this frequency range is used for sky wavecommunication. (2)
28. Space wave propagation Refer to Ans. 13. (2)
CHAPTER 15 Communication System
29. Range of frequency suitable for space wavepropagation is 100 MHz to 220 MHz.
IEs~co~o'c:::JEEo()
r----------.,! NoiseI1 -
Transducer Refer to Ans. 14.30. The range of frequencies used for TV transmission
is 100 MHz to 220 MHz. (1)
Characteristic Light waveRadio wave(100 -2001\ 1:)TV waves
Speed Travel withspeedc = 3x 108 rn/s
Also travel withspeedc = 3x108 m/s
Reflection Occurs and getaffected byground terrain,atmosphere andother objects.
It occurs and alsoget affected byground terrain,atmosphere andother objects.
31. The range of frequencies used in satellitecommunication is 3.7GHz to 6.4GHz. (1)
Common between these waves and light wavesrefer to frequency range for light wave which is ofGHz order. (1)
32. In standard AM broadcast, surface wavepropagation is used for transmitting the signals.Attenuation of surface wave increases veryrapidly with increase in frequency that is wl . itis limited to frequencies up to a few MHz. h. <\M
491
broadcast, range of frequencies are limited to30 MHz. (1)
AM band (f < 30 MHz)l
1Ground wave (or medium
wave propagation)(f<1500 kHz)
1Sky wave (or short)wave propagation
(30 kHz<f<150 MHz)(1)
(2j
33. For the transmission of signals, following twotypes of transmission media are used.(i) Sky wave propagation or short wave
propagation.(ii) Space wave communication or line of sight
communication. ('12 x 2 = 1)
Range of frequencies(i) Sky wave propagation 3 MHz> I> 300 MHz
(ii) Space wave communication 30 MHz < f <300 MHz or more. ('12 x 2 = 1)
34. (i) For line of sight communicationRefer to Ans. 13. (1)
(ii) The frequency of waves used for transmissionof TV signals are of range 100 MHz-220 MHz.But ionosphere may be able to reflect wavesback on earth of frequency up to 30 MHz.Therefore, ionosphere is unable to reflect TVwaves (space waves) back on the earth. (1)
35. A communication satellite(i) pick up the signal transmitted by transmitter
(ii) amplifies it(iii) retransmit it towards information users. These
all are also a function of repeater to receive,amplify and retransmission of signal. (2)
36. Refer to Ans. 14 and 9. (2)
37. Block diagram for a generalised communicationsystem is shown below
(1)
~ . I ~Transmitter ~. Intelligent ChannelInformation
I Receiver ~ Output
(i) Transmitter Amplifies, converts intelligentinformation into suitable form (electrical) andmodulates it, so that it is suitable to transmitthrough channel of propagation. (1)
(ii) Channel Let the signal to pass through it withleast possible attenuation. [11
(iii) Receiver Amplifies, demodulates andconverts information into a form which issuitable for receiver (user). [11
492
38. Point to point communication Thecommunication takes place over a link between asingle transmitter and receiver is called point topoint communication. Telephony is an example ofsuch a system.Broadcast mode In such a mode, large numberof receivers is linked to a single transmitter.Radio is an example of such a system.
39. (i) Sky wave propagation. Refer to Ans. 11.
(ii) Space wave propagation. Refer to Ans. 13. (3)
40. (i) Transducer Refer to Ans. 14.
(ii) Repeater Refer to Ans. 9.
(iii) Transmitter Refer to Ans. 10. (3)
41. Space waves are used for the Line of Sight (LOS)communication. Space wave communicationinvolves the transmission from transmitter,travelling along a straight line in space, reaches toreceiving antenna. The range of their frequenciesis 40 MHz and above.The range of space wave propagation is limited byline of sight distance between transmission toreceiver/repeater antenna.Maximum line of sight distance
~=~+~ rnwhere, dT = Maximum horizon for transmittingantenna, dR = Maximum horizon for receivingantennaand «; = .J2RfIr + .J2RhR
where, R = 6400 X 103 m = 6.4 x 106 m
fir = 20m,hR = 45 m. (1)
.. Maximum line of sight distancedm
= .J 2x 6.4 X 106 x 20 + .Jr-2-X-6-.4-X-I-0-"6-X-4-5
= 2 x 8 X 103 + 3 x 8 X 103 = 40 km
Thus, the maximum distance between theantennas is 40 km. (1)
42. To solve these type of questions first, we should besure about the transmitter and receiver placing andtheir height. Then according to the frequency thepropagation will be decided.
Earth
121 Chopterwise CBSE Solved Papers PHYSICS
Standard AM broadcast ~ 530 kHz to 1710kHz. (1)
(ii) Refer to Ans 24. (1)
(iii) Space wave Satellite communication ~ upto1710 kHz.
(1)Space wave propagation mode is used fordirection propagation.
43. (i) Transducer Refer to Ans. 14. ·(1)
(ii) Repeater Refer to Ans. 9. (1)
(ill) Amplification It is the process of increasing thestrength of the transmitted signal using somesuitable electronic circuit. Amplificationcompensates for the attenuation of the signal. (1)
44. Following are the three different modes ofpropagation of electromagnetic waves.(i) Ground or surface wave propagation
(ii) Space wave or tropospheric wave propagation(iii) Sky or ionospheric wave propagation
(i) Refer to Ans. 12. (1/2)
(ii) Refer to Ans. 13. (1/2)(ill) Referto Ans. 11. (1)
Figure refer to Ans. 47. (1)
45. Ground wave communication Refer to sol. 12.(1)
The maximum range of propagation of groundwave propagation depends on the(i) attenuation due to absorption by the earth and
the diffraction.(ii) frequency of transmitted wave.
(ill) the power of transmitter. (2)
46. Figure shows the three modes of spacecommunication
Three modes of space communication (3)
CHAPTER 15 Communication System
47. (i) Sky wave propagation is used by short wavebroadcast services having frequency rangefrom a few MHz up to 30 MHz. (1)
(ii) Sky wave can travel very long distances and-can even travel round the earth.
_.~~~7:~C~~~:~~~.'~7:'~:>~»,............ .,.;-.
R1 R2Earth surface
.: Refractive index of ionosphere (x) is less thanthat of free space (xo), i.e. (1)
48.
x< XoAs we go high, the electron density decreasesand therefore x' decreases. The refractionoccurs till it reaches the critical angle andafterwards it reflects the electromagneticwaves back on the earth's surface.The critical frequency is given by
fe = 9 (Nmax)1/2
In general, 30 MHz> Ic > 5 MHz. whenfrequency is greater than fe' then it crossesthe ionosphere and never reflects back on thesurface of the earth. The ranges of criticalfrequency up to few MHz sets an upper limitto frequency of waves used in this mode. (1/2)
Space wave propagation Refer to Ans. 13. (1)
Examples of communication system which usesspace wave mode are television channel, UHF,VHF, ete. Maximum distance up to which thesignals can be transmitted, d = ?
d = .,fiJlR (1)
where, R = radius of earth = 6400 km = 6.4 x 106 m
.. d = ~2 x 80 x 6.4 X 106
d = 32 X 103 m = 32 km
(112)
49.(1)
(i) Communication is using line of sight modelimited to frequency above 40 MHz becausethe frequency below 40 MHz fall inamplitude modulated band (AM band)which may use ground wave or sky wavemode of propagation.
(ii) Refer to Ans. 42
50. In solving these types of questions, we have toremind the block diagram for generalisedcommunication system.
X: Message signal generatorY: Modulator
(1)
(2)
493
51.
Z: Power amplifier (Yo + Yo + Yo)Function of elements
X : It converts one form of message signal intoelectrical energy. The variation of current orvoltage changes in accordance to variation inpressure, etc. also known as transducer.
Y: Used for superimposing the low frequencymodulating signals over high frequency carrierwave.
Z : Power amplifier strengthens the modulatedsignals for transmission. (Yo + % + %)
Differences between sky and space waves are givenin the table
Characteristic Sky wave Space wave
Frequency range 1500 kHz to > 40 MHz30 MHz
Channel Afterreflection byionosphere
Line of sightcommunicationusing repeaters
Distance oftransmission
Longdistancetransmission
Limited range oftransmission can beincreased by raisingheight of antenna.
Figure Refer to Ans. 40.
52. In solving these types of questions, we should have anidea about the frequency and range accordingly we willdecide the nature of propagation of the wave.
Refer to Ans. 52 for distinguish between sky waveand space wave. (1%)
Propagation of space wave Space wavecommunication involves the wave transmissionfrom transmitter and travelling along a straightline in space, reaches to receiving antenna. Therange of space wave propagation is limited by lineof sight distance between transmission andreceiver/repeater antenna.Range of transmission antenna Let, d berange of transmission antenna of height h. (112)
Q
d h dPIII
S I TI, I ,,iRe
,,, ,,, ,,, I ,, I /Re, I,, I, I,, I ,,, I ,,,~,.,0
494
In !)'OQT, oo' = QT2 + OT2 ~ (R. + h)2 = d2 + R;
~ d2 = h2 + 2hR, [": h« R]
~ h may be neglected, d2 = 2hR,
~ d = ../2hR, (112)
[TOPIC 2] ModulationModulation is the process of variation of somecharacteristics of a carrier wave usually amplitude,frequency or phase' angle in accordance with theinstantaneous value of a modulating signal.A sinusoidal carrier wave can be represented as,
c(t) = Acos (roct + <Po)
Types of Modulations(i) Amplitude modulation
(ii) Frequency modulation(iii) Phase modulation(iv) Pulse modulation
Need for ModulationIt is due to the fact that low frequency signal(i) needs antenna of very large length ('" 5 km).
(ii) mixes up of signal transmitted from differentstations.
(iii) get attenuated significantly. As power radiated
by antenna is given byp oc (ir.2.1 Amplitude
Modulation (AM)In amplitude modulation, the amplitude of thecarrier is varied in accordance with theinformation signal.Amplitude modulation can be represented byexpression
em(l) = A, sin roc t + !lAc cos (ro,-rom) t2
- !lAe cos (ro, + rom) t2
where, Ae = amplitude of carrier waveAm = amplitude of modulating wave
o Chnpterwise eBSE Solved Papers PHYSICS
If height of transmitting and receiving antennasare hr and hR respectively, then the maximum lineof light distance, d M = dT + dR
.. dM = ../2hrR, + ../2hRR,(1/2)
Thus, space wave travels under line of sight modein space.
(Olc-Olm) Olc
Lower sideband
Graphical Representation
Cm(t)forAM_~~
o 0.5 1 1.5 2 2.5 3
Modulation IndexChange in amplitude of carrier wave
~ = Amplitude of carrier wave
= Am = Amax - Amin
Ae Amax + Amin
where, Amax = maximum amplitude of AM waveAmin = minimum amplitude of AM wave
• Upper side band frequency = Ie + 1mLower side band frequency = Ie - 1mwhere, Ie and 1m are frequencies of carrier waveand modulating wave.
• Bandwidth = USB-LSB= (Ie + 1m) - (Ie - 1m) = 21m
2.2 Production of AmplitudeModulation Wave
One of the simplest method is shown below byblock diagram
CHAPTER15 : Communication System
Modulatingsignalm(t) Carrier wave
c(t)
• Block diagram of a transmitter and receiver isshown as below:
Transmittingantenna
Message signalm(t)
Carrier wavec (t)
(I) Block diagram of a transmitterReceiving antenna
Output• Condition for satisfactory identification by a
diode,1 1-«Rc«-Ie 1m
where, 't = Rcis time constant of thedetector circuit of AM.
Types of Pulse Modulation(i) PAM (Pulse Amplitude Modulation)(ii) PDM (Pulse Duration Modulation)(iii) PPM (Pulse Position Modulation)(iv) PCM (Pulse Code Modulation)
PREVIOUS YEARS'EXAMINATION QUESTIONSTOPIC 2o 1Mark Questions
1. How are side bands produced?All Indio 2015
2. Distinguish between amplitudemodulation and frequency modulation.All Indio 2015C
495
3. How does the effective power radiatedfrom a linear antenna depend on thewavelength of the signal to betransmitted? Deihl 2D14C
4. The carrier wave of a signal is given byc(t) = 3sin (81t t) volt
The modulating signal is a square waveas shown in the figure. Find themodulation index. Deihl 2014
m(t)in volt
5. Why are broadcast frequencies (carrierwaves) sufficiently spaced in amplitudemodulated wave? Foreign 2013
6. Why do we need a higher bandwidth fortransmission of music compared to thatfor commercial telephoniccommunication? Deihl 2009
o 2 Marks Questions7. Distinguish between a transducer and a
repeater. Deihl 2017
8. Write two factors which justify the need ofmodulating a low frequency signal into highfrequencies before transmission? Deihl 2015
9. Define the term modulation. Draw ablock diagram of a simple modulator forobtaining AM signal. Foreign 2014
10. A message signal offrequency 10 kHzand peak voltage 10 V is used tomodulate a carrier of frequency 1 MHzand peak voltage 20 V.Determine
(i) the modulation index(ii) the side bands produced. Delhl2013C
11. In the block diagram of a simplemodulator for obtaining an AM signalshown in the figure, identify the boxes Aand B. Write their function.
x(t) y(t)-:-:-~-:-:-:-.-l + A
Modulatingsignal
Carrier wave
AMwave
All India 2013
496
12. (i) Describe briefly the three factorswhich justify the need for translatinga low frequency signal into highfrequency before transmission.
(ii) Figure shows a block diagram of adetector for AM signal.
AM wave . , r---~TRectlfler
ABC
Draw the waveforms for the (a) inputAM wave at A, (b) output B at therectifier and (c) output signal at C.
AllIndio 2013C
13. Block diagram of a receiver is shown inthe figure below:
Receivingantenna
Output
(i) Identify X and Y.(ii) Write their functions.
Oelhi 2013; AllIndio 2012
14. Figure shows a block diagram of atransmitter identify the boxes X and Yand write their functions. Foreign 2012
Jransmitlingm(t) antenna
~Ysignal
15. A carrier wave of peak voltage 18 V is used totransmit a message signal. Calculate thepeak voltage of the modulating signal inorder to have a modulation index of 50%.Oelhi 2012
16. For an amplitude modulated wave, themaximum amplitude is found to be 10Vwhile the minimum amplitude is 2 V.Calculate the modulation index. Why ismodulation index generally kept less thanone? Foreign 2011
17. For an amplitude modulated wave, themaximum amplitude is found to be 12V.Calculate while minimum amplitude is 2 Vthe modulation index. Why is modulationindex generally kept less than one? Foreign 2011
o ehapterwise eBSE Solved Papers PHYSICS
18. (i) Define the modulation index.(ii) Why is the amplitude of modulating
signal kept less than the amplitudeof carrier wave? Oelhi 2011
19. Write two factors justifying the need ofmodulating a signal. A carrier wave ofpeak voltage 12 V is used to transmit amessage signal. What should be the peakvoltage of the modulating signal in orderto have a modulation index of 75%?AllIndio 2010
20. A carrier wave, c (t) = Ae sin wet isamplitude modulated by a modulatingsignal m(t)= A", sinwmt. The maximumand minimum amplitudes of the resultingAM wave are found to be 16V and 4V,respectively. Calculate the modulationindex. Oelhi 2010C
21. Why are high frequency carrier wavesused for transmission? Oelhi 2009
22. Write two factors justifying the need ofmodulation for transmission of a signal.AllIndio 2009
23. A message signal of frequency 10 kHzand peak voltage of 10 V is used tomodulated frequency of 1 MHz and peakvoltage of 20 V. Determine the
(i) modulation index(ii) the sidebands produced
AllIndio 2009C
o 3 Marks Questions24. (i) How is amplitude modulation
achieved?(ii) The frequencies of two sidebands in an
AM wave are 640 kHz and 660 kHzrespectively. Find the frequencies ofcarrier and modulating signal. What isthe bandwidth required for amplitudemodulation? AllIndio 2017
25. (i) Explain any two factors which justifythe need of modulating a lowfrequency signal.
(ii) Write two advantages of frequencymodulation over amplitudemodulation. Oelhi 2016
CHAPTER 15 : Communication System
26. Define the modulation index. Why is itsvalue kept, in practice, less than one? Acarrier wave of frequency 1.5 MHz andamplitude 50 V is modulated by asinusoidal wave of frequency 10 kHzproducing 50% amplitude modulation.Calculate the amplitude of the AM waveand frequencies of sidebands produced.Delhi 2015
27. Draw a block diagram of a detector forAM signal and show, using necessaryprocesses and the waveforms, how theoriginal message signal is detected fromthe input AM wave. All India 2015
28. Write two basic modes of communication.Explain the process of amplitudemodulation. Draw a schematic sketchshowing how amplitude modulated signalis obtained by superposing a modulatingsignal over a sinusoidal carrier wave.All India 2014
29. What is meant by detection of a modulatedsignal? Draw block diagram of a detector forAM waves and state briefly showing thewaveforms, how the original message signalis obtained. Deihl 2013C
30. Write three important factors whichjustify the need of modulating a messagesignal.Show diagrammatically how an amplitudemodulated wave is obtained when amodulating signal is superimposed on acarrier wave. Oelhi 2013
31. Define the modulation index. Give itsphysical significance. For an amplitudemodulated wave, the maximum amplitudeis found to be 10 V while the minimumamplitude is 2 V. Determine themodulation index u. Fariegn 2012
32. (i) Distinguish between sinusoidal andpulse shaped.signals.
(ii) Explain, showing graphically, how asinusoidal carrier wave issuperimposed on a modulating signalto obtain the resultant amplitudemodulated (AM) wave. All India 2012C
497
33. Write briefly any two factors whichdemonstrate the need for modulatingsignal. Draw a suitable diagram to showamplitude modulation using a sinusoidalsignal as the modulating signal.HOTS;Delhi 2012, All India 2011
34. Give reasons for the following.(i) For ground wave transmission, size
of antenna (l) should be comparableto wavelength (A.) of signal, i.e.l = A./4.
(ii) Audio signals converted into anelectromagnetic wave are not directlytransmitted.
(iii) The amplitude of a modulating signalis kept less than the amplitude ofcarrier wave. Delhi 2012, 2011C
35. State the two main reasons explaining theneed of modulation for transmission ofaudio signals.
C(t~~NVVVVVWVVVVW\MMo 0.5 1.0 1.5 2.0 2.5 3
m(t)~~
-10 0.5 1.0 1.5 2.0 2.5 3
The diagrams given above shows a carrierwave c (t), that is to be (amplitude)modulated by a modulating signal m(t).Draw the general shape of resultingAM wave. Define its modulation index.All India 2010C
o Explanations1. Sidebands are produced during the process of
modulation. During modulation, the audiofrequency modulating signal wave issuperimposed on a high frequency wave is calledcarrier wave. Any form of modulation producesfrequencies that are the sum and the differencesof the carrier and modulating frequencies. Thesefrequencies are called sidebands.Lower sideband frequency = fc - 1mUpper sideband frequency = I; + 1m (1)
498
~------------------~------------Amplitudemodulation
Frequencymodulation
The amplitude of carrierwave changes inaccordance with theinformation (signal)wave------------------------------
The frequency of carrierwave changes inaccordance with theinformation (signal)wave.
In AM most of powergoes waste intransmitting carrier.
Highly efficient all thetransmitted power isuseful.
(1)
3. The power radiated by a linear antenna of length1. is proportional to (L / A.)2, where A. is the signalwavelength. Since for efficient transmission, thesignal should be transmitted with high power,the signal should be of small wavelength or highfrequency. (1)
4. According to the diagram,Amplitude of modulating signal Am = 1VAmplitude of carrier wave Ae = 2V
Modulation index, Jl = Am = ~ = 0.5Ae 2 (1)
5. To avoid mixing up of signals from differenttransmitters the broadcast frequencies aresufficiently spaced in amplitude modulatedwave.This can be done by modulating the signals onhigh frequency carrier waves, e.g., frequencyband for satellite communication is 5.925-6.425G& m
6. The range of frequencies of music is higher thancommercial telephone conversation andtherefore, greater bandwidth is needed formusic. Also, it is free from noise. (1)
7. Transducer is a device which converts one formof energy into another form. Microphone is atransducer because it converts sound energy intoelectrical energy. Repeater is a device whichpicks up electromagnetic signal, amplifies it andthen again transmits it to the receiver. It is usedwhen transmitted signal is attenuated beyond acertain limit or distance between a transmitterand a receiver is very large.
8. (i) Size ofthe antennaSince, an antenna is needed both fortransmission and reception. Each antennashould have a size comparable to thewavelength of the signal. (1/2)
o Chopterwise eBSE Solved Papers PHYSICS
For an EM wave of frequency 20 kHz,wavelength is 15 krn.
15:. Length of antenna = - km = 3.75km.4
It is an obvious that such a long antenna is notpossible. Hence, antenna length can be madereasonable, if the frequency is high. So, there isa need to convert low frequency signal intohigh frequency signal before transmission. (112)
(ii) Effective power radiated by an antennaPower P radiated from linear antenna of length
I is proportional to nr·Hence, antenna length can be made reasonable,if the frequency is high.So, there is a need to convert low frequencysignal into high frequency signal beforetransmission. (1)
9. Modulation is the process in which low frequencymessage signal is superimposed on high frequencycarrier wave so that they can be trasmitted overlong distances. The block diagram for a simplemodulator for obtaining AM signal is shown asbelow: (2).,....,.......,....,...,..,..... ..(+ x(t) Square lawModulating device
signalm(t) Carrier wave
c(t)
10. (i) Modulation index, Em = ~ = 0.5e, 20(ii) Side band frequencies = fc ± fm
fm = 10kHz ~ fc = 1 MHz = 1000 kHz:. Side band frequencies = 1000 ± 10
= 1010 kHz, 990 kHz
(1)
(1)
11. In the block diagram of modulator, A is square lawdevice and B is bandpass filter.Bandpass filter rejects low and high frequenciesand allows a band of frequencies to pass through.
(1)
Square law device is a non-linear device forobtaining AM wave. It produces a non-linearoutput of message and carrier signals. The outputfrom square law device is y(t) = Bx(t) + Cx2(t).
where, B and C are constants andx(t) = message signal (Am sin wmt) + carriersignal (Ae sin we t). Non-linear element used may besuitably biased diode or transistor. (1)
CHAPTER 15 Communication System
12. (i) The three factors of modulation are(a) Transmission of audio frequency electrical
signals need long impracticable antenna.(b) The power radiated at audio frequency is
quite small, hence transmission is quite lossy.(c) They various information signals transmitted
at low frequency get mixed and hence cannot be distinguished. (1]
AM input wave Rectified wave Output (withoutRF component)
Block diagram of detector for AM signal (1]
13. From the given block diagram of demodulation ofa typical receiver, we can conclude the following.(i) X represents Intermediate Frequency (IF)
stage while Y represents an amplifier. (1]
(ii) At IF stage, the carrier frequency is changed toa lower frequency and in this process, themodulated signal is detected while thefunction of amplifier is to amplify the detectedsignal which may not be strong enough to bemade use of and hence is required. 11]
14. Block diagram of a transmitterTransmitting
antenna
m(t)
Messagesignal '----,.----'
Carrier waveModulator Since, the frequency range of signalis quite low and it is associated with very smallamount of energy. It dies out very soon iftransmitted as such. So, it is tnodulated bymixing with very high frequency waves calledcarrier waves. This is done by modulator power. (1]
Amplifier Since, the signal gets weaken aftertravelling through long distances it cannot betransmitted as such. Thus, we use a poweramplifier to provide it necessary power beforefeeding the signal to the transmitting antenna. (1]
499"
15. Here, A, = 18 V, A", =?
Modulation Index u , = 50% = 0.50A ASince, ~a = ---.!!!.., 0.50 = ---.!!!..A 18
.. Am = 9V (2]
16. For AM wave,Maximum amplitude, Amax = 10 VMinimum amplitude, Amin = 2 V.. Modulation index,
Amax-Amin 10-2 8 2~ = ~ ~ = __ = _ = _ (112]Amax + Amin 10 + 2 12 3
11.
2~=-3 (1/2]
If the modulation index (JJ.) is greater than 1, thecarrier wave is said to be over modulate anddistortion will occur during reception as negativepeak of modulating signal will be missing.Therefore, ~ is kept less than one.Refer to Ans. 16.
(1]
(1]
(1]5~=-7
18. (i) Modulation index The modulation index isdefined as the ratio of change in theamplitude of carrier wave to the amplitude ofcarrier wave.
_ Change in amplitude of carrier wave.. fJ.,.,-, Amplitude of carrier wave (1/2]
But change u;. 'amplitude of carrier wave=Amplitude of modulating wave (Am).Jt A Am-Am•• L ~ = ---.!!!.. = ax in (112]
l' r··.. A Amax + Amm
r, where, Amax and Amm aretrnaximum andminimum voltage of AM ~ave, respectively.
(ii) Refer to Ans. 16. (1]
19. Need for modulation(i) To reduce the length of antenna from 15 km to
reasonable height by converting low frequencysignal into high frequency signal beforetransmission.
(ii) To transmit a low frequency signal to a distantplace so that it may not die out in the wayitself. (V. x2 = 1]
(iii) For protecting the waveform of the signal%Modulation index, ~ = Am X 100 (1/2]
A,where, Am and A, are peak voltage of modulatingsignal and carrier wave voltage ..
500
Here, Am = 7, A, = 12 V, ~ = 75%
75= Am X 10012
~Am=12X75100
20.~Am=9V
Here, maximum amplitude, Amax= 16 VMinimum amplitude, A",;" = 4 V
Modulation index,~ = Am
A= Amax- A",;"
Amax+ A",;"16-4 12 3
~=--=-=-16+ 4 20 5
21. High frequency carrier waves are used toincrease operating range, to reduce antennalength and convert the wide band signal intonarrow band signal. Then the signal can beeasily recovered and distinguished from othersignals at the receiving station. (2)
22. The needs of modulation for transmission of asignal are given as below:(i) The transmission of low frequency signal
needs antenna of height 4-5 krn which isimpossible to construct. So, there is a needto modulate wave in order to reduce theheight of antenna to a reasonable height.
(ii) Effective power radiated hy antenna for lowwavelength or high frequency wave as
1p oc "),}"
So, for effective radiation by antenlla,-thereis a need to modulate the wave. (1 + 1)
23. (i) Peak voltage of modulating signal,Am=lOV
Peak voltage of carrier signal, A, = 20 V.. Modulation index,
~=Am=~=~A, 20 2 (1)
(ii) USB (Upper Side Band)= fc + 1m = 1 MHz + 10 kHz
104= 1 MHz+-MHz
106
='1 MHz+ 0.01 = 1.01 MHzLSB (Lower Side Band)
= fc - Im= 1 MHz- 10 kHz= 0.99 MHz (1)
o Chopterwise CBSE Solved Papers PHYSICS
24. (i) For amplitude modulation, a message signal isused to modulate amplitude of a high frequencywave in input transistor of CE amplifier. (1)
m(t)
(1/2)
c(t)
(1)c[m(t)]
(1)
In this way, a modulated wave is obtained
The output voltage is carrier signal varying inamplitude in accordance with biasingmodulation voltage.
(ii) Given, USB frequency = 660 kHz
and LSB frequency = 640 kHzAs USB = fc + 1m = 660 kHzand LSB = fc - 1m = 640 kHz.. 2fc = 660 + 640 = 1300So, carrier frequency
t; = 650 kHzand 21m = 20 kHzMessage frequency is,
~ 1m =10 kHzBandwidth of frequencies required
= USB - LSB = 660 - 640 = 20 kHz (2)25. (i) Refer Ans. 19. (1'12)
(ii) Advantages of frequency modulation overamplitude modulation.
(a) Frequency of a wave does not change whiletravelling through different media. Anamplitude' modulated wave carrierinformation in terms of variation ofamplitude, which can get disturbed. This iswhy FM signal is less susceptible to noise thanAM signal.
(b) In FM transmission, all the transmitted poweris useful, whereas in AM transmission most ofthe power wastes in transmitting carrier wave,with no useful information. (1'12)
CHAPTER 15 : Communication System 501
26. Amplitude modulation index is the ratio of the modulating signal to the maximum amplitude of carrier. . b Amwave. It IS given y 11 = -
ASince, the amplitude modulation index (11) determines the quality of the transmitted signal. When modulationindex is small, then variation in carrier amplitude will be small. Therefore, audio signal being transmitted willbe weak. As, the modulation index increases, the audio signal on reception becomes clearer.Given, frequency of carrier wave, v, = 1.5MHz = 1500 kHzFrequency of sinusoidal (modulated) wave,
vm = 10kHzAmplitude of carrier wave, A = 50 V
Modulation index (Il) = 50% ~ 2Q. = ~100 2
(1)
A.: Modulation index, 11 = --"'- ~
A,~ = Am ~ Am = 25V2 50 (1)
So, the amplitude of AM wave, Am = 25V.As, we know, the side bands are USB = v, + Vm /= 1500 + 10 = 1510 kHz
LSB = v, - Vm = 1500 -10 = 1490 kHzThese are the required frequencies of the side bands produced.
Receiving antenna
(1)
27.
A I'f' OutputDetector mp I ler
Block diagram for detection of AM signal
When a message is received, it gets attenuated through the channel. Therefore, the receiving antenna is to befollowed by an amplifier and a detector. The carrier frequency is usually changed to a lower frequency (IF)stage. The detected signal may not be strong enough to be use and hence, is required to be amplified. (1)In order to obtain the original message signal m(t) of angular frequency, a simple method is used which isshown below in the form of a block diagram.
AM wave
AM input waveWhen the received modulated signal is passed through a rectifier, an envelope signal is produced. Thisenvelope signal is the message signal. In order to retrieve the message, the signal is passed through anenvelope detector. (2)
502
28. The two basic modes of communication are givenas below:(i) Point-to-point (ii) Broadcast (3)
The process of amplitude modulation is the processof varying the amplitude of the sinusoidal carrierwave by the amplitude of the modulating signal.
+I1'\-Sinusoidal modulating sign
I r.V Time
- I/ '\ // 1\
ti"r\ ,ifJI'~\ Time
IJ
\ l \- /-.....
+
Unmodulated sinusoidalcarrier wave
Envelope of modulatedcarrier wave
29. The transmitted messages get attenuated inpropagating through the channel. The receivingantenna is to be followed by an amplifiers and adetector. Detection is the process ofrecoveringthe modulating signal from the modulated carrierwave. (1)
The block diagram of a detector for AM waves areshown as below:
AM waveRectifier
~(a)~
~me time
AM input wave Rectified wave
,---;:-----;---,met)- Output
L....:::=:P----' (c) ~
timeOutput (without RF
component) (1)
The modulated signal of the form given in Fig. (a)is passed through a rectifier to produce the outputshown in Fig. (b). This envelope of signal Fig. (c)is the message signal. In order to retrieve m (f), thesignal is passed through an envelope detector. (1)
30. Three important factors which justify the need ofmodulating a message signal (1)
(i) Size of effective power radiated by antenna.(ii) Theoretical studies reveal that power P
radiated from a linear antenna of length I isproportional to (1/')..)2 Le.P oc (1/')..)2
o Chapterwise CBSE Solved Papers PHYSICS
al
As high powers are needed for goodtransmission, therefore, for given antennalength, wavelength X should be small orfrequency v should be high.Thus, this factor also points out to need ofusing high frequency transmission. (1)
(iii) The interference of signals from differenttransmitters. To avoid the interference of thesignals, there is a need of high frequencywhich can be achieved by the modulation. (1)
1
C(t)~~
Carrier wave1m(t~0rs:zszs:J1 MOdulating signal2
Cm(t)~~
Amplitude modulated Wave
31. Refer to Ans. 18 (i).
m(~l~ .",
~~Ol~¥M~M"'
(1)
Refer to Ans. 14.
CHAPTER 15 : Communication System
Physical significance of modulation indexIt is used to determine the strength and qualityof transmitted signal. If the modulation index issmall, the amount of variation in the carrieramplitude is small. Thus, the audio signaltransmitted will not be strong.The greater the degree of modulation, thestronger and clearer will be the audio signalduring reception.Maximum amplitude, Am.x = 10 VMinimum amplitude, Amin = 2VThus, modulating index,
~ = Am.x - Amin 10 - 2 = ~ = 0.66 (2)Am.x + Amin 10 + 2 12
32. A continuous time varying current or voltagesignal is called sinusoidal signals. The signalwhich two levels of current or voltagerepresented by 0 and I are called pulse shapedsignal. (1)
Three important factor to justify the need ofmodulating a message signal(i) Practical antenna length.
(ii) Effective power radiated by antenna.(Hi) Mixing up of signals from different
transmitters. (1)
Amplitude Modulation When themodulating wave is superimposed on a highfrequency carrier wave in a manner that thefrequency of a modulated wave is same asthat of carrier wave but amplitude of carrierwave is modified in accordance with themodulating wave, the process is calledamplitude modulation.
Carrier wave
/\ /\~\J
Modulating sine-wave signal
503
Amplitude-modulated wave (1)
33. The circuital arrangement for a modulator should besuch that carrier wave should be mixed with themodulating signal. The output will be obtained throughthe LC filter.
Need for modulation Refer to Ans. 22. (2)Figure shows the amplitude modulation using asinusoidal signal as the modulating signal.
C JIMOutput
:§: Vcc modulatedwave
RsInput
Am cos romt(modulating
signal)IV\
•... -IIII It __ I
(1)
34. (i) To radiate the signals with high efficiency.(ii) Because they are of large wavelength and
power radiated by antenna is very small asp ee l()..4.
(ill) It is so to avoid making over modulated carrierwave. In that situation, the negative half cycleof the modulating signal is dipped anddistortion occurs in reception. (1 x 3 = 3)
35. When we are drawing waveform for AM wave theamplitude variation of carrier wave should beconsidered and also that of modulating signal. (1)
Need for modulation Refer to Ans. 22. (2)
2
O~~~~F+~~~+4~-----
-1
-2
Value Based Questions (From Complete Chapter)1. Anuj's mother was having constant
headaches. After a medical checkup, shewas diagnosed with tumour. Anuj realisedthere was a telecommunication tower veryclose to their house. He enquired from thedoctor if the radiation from the towercould have caused the tumour.As the doctor supported his anxiety, helodged a complaint with the police andultimately succeeded in getting the towerremoved to a distant place away from theresidential colony.
(i) What values were displayed by Anuj?(ii) Anuj made a rough estimate about
the height of the antenna to be about20 m from the ground.Calculate the maximum distance upto which radiations from the towerare likely to reach. Use the value ofradius of the earth = 6400 x 103 m.Delhi 2014C
Ans. (i) Anuj was displayed following values:(a) Concern (b) Scientific temperament(c) Keen observer (2)(d) Alertness
(ii) Given, h = 20 m. R = 6400 X 103 mWe know that, d = -J2JlR = ~r2-X-2-0-x-6-.4-X-IO-6
~ d=16km (2)
2. When Sunita, a class XII student, came toknow that her parents are planning torent out the top floor of their house to amobile company she protested. She triedhard to convince her parents that thismove would be a health hazard.Ultimately her parents agreed.
(i) In what way can the setting up oftransmission tower by a mobilecompany in a residential colony proveto be injurious to health?
(ii) By objecting to this move of her parents,what value did Sunita display?
(iii) Estimate the range ofelectromagnetic waves which can betransmitted by an antenna of height20m.
(Given, radius of the earth = 6400 km)Delhi 2014
Ans. (i) A transmission tower transmitselectromagnetic waves such as microwaves,exposure to these waves can cause severehealth hazards like cancer and tumour. Alsotransmission tower (antenna) works on a veryhigh power, so the risk of someone severelygets burnt increased in residential area. (2)
(ii) Sunita has displayed awareness towards thehealth and environment of society byobjecting to this move of her parents. (1)
(ill) Here, R = 6400 km = 64 X105 m;h= 20m, d=?
d=..}2 hR = ~2 x (20) x (64 x 105)= ~4 x 64 x I06
~ d = 16000 m (1)
3. Arnab was talking on his mobile to hisfriend for a long time. After hisconversation was over, his sister Anitaadvised him that if his conversation wasof such a long duration, it would be betterto talk through a land line.
(i) Why is it considered harmful to use amobile phone for a long duration?
(ii) Which values are reflected in theadvice of his sister Anita?
(iii) A message signal of frequency10 kHz is superimposed to modulatea carrier wave of frequency 1 MHz.Determine the sidebands produced.All India 2014C
Ans. (i) The ultra high frequency of EM radiationscontinuously emitted by a mobile phone, mayharm the system of the human body. (1)
(ii) Anita shows the following values.(a) Concern about her brotherlcommand(b) Awareness about the likely effects of
electromagnetic radiations on human body. (1)
(ill) Given, fc = 1000 kHz, 1m = IOkHzThe side bands are (fc + 1m) and (fc - 1m)or (l000 + 10)and (l000 -10) kHz
= 10IO and 990 kHz (2)
4. Mohan went to the market to purchase aTV set. He got confused with so manyfeatures and functions of electronicappliances. He took the help of his friendSohan, a science student. Sohan explainedhim about the communication system,
CHAPTER 15 : Communication System
digital and analog signals. Thisknowledge proved of great help to Mohanin purchasing a colour TV.
(i) What do you mean by the termcomm unica tion?Which type of signals are better?
(ii) What type of nature Sohan has?(iii) What is the minimum number of
communication satellites required forglobal communication coverage?
Ans. (i) A set up that transfer information implicitlyfrom one point to another is calledcommunication system. Analog signal is better.A signal in which current or voltagecontinuously vary with time. [1+1)
(ii) Sohan is an intelligent and knowledgableperson has good command over subject. [1)
(iii) There are three communication satellitesrequired for global communication coverage.[1)
5. Group discussion was arranged in classXII on the topic of atmosphere. Threegroups were made. Teacher asked thequestion "why can Moon be not used as acommunication satellite?" Answers weregiven by all the three groups. Each groupcan give only one reason. Teacher toldthem that reason given by each group is 'correct. The groups collected all the threereasons and came to correct conclusion.
(i) Give the correct reason for the abovequestion.
(ii) What values were showed by all thethree groups?
Ans. (i) Moon is a natural satellite of the earth but it isnot an active satellite as it carries no electronicequipments for receiving, amplifying andtransmitting the signal back to the earth.Moon cannot be used as a communicationsatellite because
(a) the distance between the earth and themoon is not proper.
(b) the period of revolution of the moon is not24 hr.
(c) the moon is not revolving in the equitorialplane of the earth. [2)
(ii) The three groups showed the values ofeagerness to engage in a debate, promptness intaking decision and ability to work in a groupin a constructive manner. [2)
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6. In the famous conversation, RakeshSharma, the first Indian Astronaut inspace, was asked by the Prime MinisterIndira Gandhi as to how India lookedfrom space. To which he replied, 'SareJahan Se Achcha' (better than the wholeworld). •Answer the following questions based onabove passage:
(i) Which scientific mode ofcommunication enabled the PrimeMinister to speak to the Astronaut?
(ii) Name the scientific values displayedin this anecdote.
(iii) Which values are being reflected inthe reply given by the astronaut?
(iv) Give one more example of thisscientific mode of communication ineveryday life situations.
Ans. (i) Radio wave communication system.(ii) Use of scientific and technological
advancement in service to mankind. Humanpassion for continued progress andadvancement.
(iii) Patriotism and love for the country, presenceof mind.
(iv) Television communication system. [1-4)
7. Raj was cycling from his village to aneighbouring village. On the way, he metan old man who was walking in thedirection of the same neighbouringvillage. Raj invited the old man to sit on
,qis cycle and took him to theneighbouring village. The old man gotdown from the cycle, thanked Raj andgave his blessings.
(i) What according to you, are the valuesdisplayed by Raj?
(ii) Can you relate the given paragraphwith the process of modulation?
Ans.(i) Raj showed concern for the stranger andrespect for the old age of the person. [1)
(ii) Just as the old man sat on the cycle, themessage signal is superposed on the carrierwave. The modulated carrier wave travelssome distance. The message signal is thenrecovered from the modulated wave in thesame way as the old man got down from thecycle. [1+1+1)