mkCOMMUNICATION SYSTEM (T.N.) PAGE # 1 COMMUNICATION SYSTEMS • Communication: It is theprocess bywhichinformationis transferredfaithfullyfromonepoint (Source) totheother point (destination) inanintelligibleform. Information: It is basicallythe news which one wishes to convey. Message : It is the physical manifestation of the information produced bythe source. It mayappear as a sequence of discrete symbols, time varyingquantity, etc. Signal : It is theelectrical analogof themessageat thesource.Asignal maybedefinedas thesingle-valued functionof time(that conveys the information) andwhich, at everyinstant of time, takes a unique value. • Communication system: The set used to transmit information fromone point to another is called a communicationsystem. The essential parts of a communicationsystemaretransmitter, communication channel and receiver, as shown in the block diagramof belowfigure. (i) Transmitter : It is a set - up that transmits the message to the receiving end through a communication channel. Its main function is to convert the message signal produced bythe information source into a formsuitable for transmissionthroughthe channel andtotransmit it. (ii) Communicationchannel ortransmissionmedium: It is themediumor the physical paththat receiver. It can be a transmission line, an optical fibre or free space. Information Soruce Message Signal Transmitter Channel Receiver User of Information Transmitted Signal Received Signal Message Signal Transmitted Signal Communication system Noisa • Antenna : An antenna is basically a small length of a conductor that is used to radiate or receive electromagnetic waves. It acts as aconversiondevice.At thetransmittingend, it transformshighfrequency current intoelectromagneticwaves.At thereceivingend, it transformselectromagneticwaves intoelectrical signal that is fedtotheinput of thereceiver. Adipoleantennais omnidirectional, whereas thedishantenna is highlydirectional. the lengthof dipole antennais usuallyì/2. • Two basic modes of communication : There are as follows. (i) Point-to-point communication. Inthismode, communicationoccursover alinkbetweenasingletransmitter and receiver. Example Telephony. (ii) Broadcast mode. Inthis mode, a largenumber of receivers arelinkedtoa singletransmitter, for example, radioandtelevision. mk COMMUNICATION SYSTEM (T.N.) PAGE # 2 • Aschematicarrangement for transmittingandreceivingamessagesignal figureshows ablockdiagram for anarrangement for transmittingasignal whentransmitter andreceiver areseveral thousandkilometers apart. Amplifier Antenna Amplifier D Microphone Transmitter Antenna Tunable amplifier Demodulator Audio amplifier Loudspeaker Receiver At thetransmittingend, amicrophoneconverts thesoundsignal intoatimevaryingelectrical signal called the message signal, with the help of a modulator, the message signal is translated into radio frequency range. The power of the modulatedwaveis boostedbya suitable amplifier andthen the wave is radiated into space froman antenna. At the receivedand, another antenna picks up the waves fromdifferent transmittingstations. Atunable amplifier can selectivelytune in and boost up the radio frequencywave froma particular station. The original signal is recoveredbyusingademodulator.After beingamplified, theelectrical signal is converted intosound signal byusinga loudspeaker. • Analogsignal : Asignal inwhichcurrent or voltagevaries continuouslywithtimeis calledanalogsignal. Figure shows represents a simplest sinusoidallyvaryinganalogsignal. v =Asin et Digital signal : Asignal that is discrete is called a digital signal. The presence of signal is denoted by digit 1 and absence is denoted bydigit 0. v = +A v v = –A v 1(5V) 0(0V) I • Bandwidthof signals: Therangeover whichfrequenciesinaninformationsignal varyiscalledbandwidth. It is equal to the difference between the highest andlowest frequencies present inthe signal. The type of the communication channel needed for a given signal depends on the bandwidth of the informationsignal. For example, speech signals contain frequencies between 300 Hz to 3100 Hz. Such signals require a bandwidth of 2800 Hz ( 3100Hz - 300 Hz) for telephonic transmission. Audiosignals havefrequencies between20Hz to20KHz. Sothe transmissionof agood music requires a bandwidth of about 20 kHz. mk COMMUNICATION SYSTEM (T.N.) PAGE # 3 Video signals required for transmission of picture have a bandwidth of about 4.2 MHz. ATVsignal, which contains bothvoice and picture, is allocated a bandwidth of 6 MHz for transmission. • Modulation: Ahighfrequencycarrier waveis usedtocarrytheaudiosignal (message) tolargedistances. Modulation is the process by which some characteristic, usuallyamplitude, frequency or frequency audiosignal, calledthemodulatingsignal. Needof modulationincommunicationsystems Audiosignals have a bandwidthof 20 kHz. Suchlowfrequencysignals cannot betransmitteddirectlyto longdistances because of the followingreasons. (i) Practical antennalength: Totransmit asignal effectively, theheight of theantennashouldbecomparable tothewavelengthof thesignal (atleast ì/4inlength) sothat theantennaproperlysenses thetimevariations of the signal. So to transmit a signal of frequency20 kHz, we need an antenna of height = 3 8 10 20 10 3 ÷ × × (ii) Effective power radiatedby anantenna : For liner antenna of length l, it is seen that Power radiated · 2 1 | . | \ | ì Thus for the same antenna length, the power radiated byshort wavelength or high frequencysignals would be large. If the audio signals (of long wavelength) are directlyradiated into space, theydie-out after coveringsome distance due to their lowpower. (iii) Mixing up signals fromdifferent transmitters: When audio signals (of same frequency range are transmitted bymanytransmitters simultaneously, their signals get mixed upand it is not and allottinga bondof frequencies toeachsignal for its transmission. Thus there is a need for translatingthe original lowof frequencymessage (or information) signal intoa highfrequencywavebeforetransmissionsuchthat thetransmittedsignal continuestopossestheinformation containedintheoriginal signal. • Bandwidths of transmission media: Depending on the nature of communication channel, the communicationis classifiedbroadlyintothree categories. (i) Linecommunication: It invokes point-to-point contact between transmitter and receiver. It occur through guided media such as twisted pair and coaxial cable. Coaxial cable is a widelyused wire medium, which offers a bandwidth of 750 MHz. These cables are normallyoperated below18 GHz (1 GHz = 10 12 Hz) (ii) Optical communication: It makes use of a light beamin carrying an information fromone point to another througha guidedmediumlike optical fibre. Optical communicationusingfibres is carriedinthe frequencyrangeof 1THz to1000THz (microwaves to ultraviolet waves, 1 THz = 10 12 Hz). An optical fibre can have a transmission bandwidth above 100 GHz. (iii) Space electromagnetic waves of different frequencies are used to carry the information through the physical space actingas the transmissionmedium. Radio, televisionandsatellitecommunications areall spacecommunications. Communicationthroughfree space usingradiowaves take place over a wide range of frequencies: from a fewhundreds of KHz to a fewGHz. mk COMMUNICATION SYSTEM (T.N.) PAGE # 4 Service Frequency bands Comments Standard AM Broadcast 540-1600 KHz FM Broadcast 88-108 MHz Television 54-72 MHz VHF (very high frequencies) 76-88 MHz TV 174-216 MHz UHF (ultra high frequncies) 420-890 MHz TV Cellular Mobile Radio 896-901 MHz Mobile to base station 840-935 MHz Base station to mobile Satellite communication 5.925-6.425 GHz Uplink 3.7-4.2 GHz Downlink SOLVEDPROBLEMS Q.1 What is amplitude modulation? DrawanA.M. wave. Give its important features. [Sol. Amplitudemodulation. It is theprocessinwhichtheamplitudeof thehighfrequencycarrier wavechanges in accordance with the instantaneous value of modulating signal. Here a voltage proportional to the amplitude of the modulatingsignal gets added tothe carrier amplitude. Waveformof A.M. voltage, an unmodulated carrier voltage, a sinusoidal modulating voltage (audio signal) andamplitude modulatedvoltage. Clearly, the amplitude of the carrier wave changes inaccordance withthe modulatingwave. Duringthe positive half cycle of the modulatingsignal, the amplitude of carrier wave increases. Thus the top and bottomenvelopes of the modulated wave are just the replica of the modulatingsignal. I mportant featuresof theamplitudemodulatedwave: 1. Amplitude of the carrier wave changes accordingtothe modulatingsignal. 2. Amplitude of the carrier wave changes at the frequency'f m ' of the modulatingsignal. 3. Frequencyof theA.M. wave is equal to the unmodulated carrier frequency'f c '.] Q.2 Definemodulationfactor. Express it interms of maximumandminimumvoltages ofA.M. wave. Givethe importance of modulationfactor. [Sol. Modulation factor or depth of modulation. It represents the extendto which the amplitude of the carrier wave is changedbythe modulatingsignal. It is definedas the ratio of the change in the amplitude of the carrier wave to the amplitude of the original carrier wave. It is also known as degree of modulation or modulationindex. IfA m andA c aretheamplitudes of modulatingandcarrier signals respectively, thenthe modulationfactor is givenby µ = wave carrier original of Amplitude wave amplitude in change = c m A A Modulationfactor interms ofA max andA min . LetA max andA min be themaximumandminimumvoltages of A.M. wave. It is clear that A m = 2 A A min max ÷ and AA c =A max – A m =A max – 2 A A min max ÷ = 2 A A min max + Hence, the modulation factor is µ= c m A A = | . | \ | + | . | \ | ÷ 2 A A 2 A A min max min max = min max min max A A A A + ÷ mk COMMUNICATION SYSTEM (T.N.) PAGE # 5 Modulation factor µgenerallylies between 0 and 1. The percentage modulation is given byµ= min max min max A A A A + ÷ ×100% Importance of modulation factor. The modulation factor determines the strength and equalityof the transmittedsignal. If mis small, theamount of variationinthe carrier amplitude is small. As modulation, thestronger andclearer will not bestrongenough. Thegreater thedegree of modulated(µ>1), distortion will occur duringreception.] Q.3 Derive an expressionfor amplitude modulatedwave. [Sol. Let modulatingsignal, m(t) =A m sine m t and carrier signal, c(t) =A c sin e c t where, m(t) instantaneous voltage of modulationwave A m =amplitude of modulatingwave e m =2tf m =angular frequencyof modulating wave c(t) =instantaneous voltage of carrier wave A c =amplitude of carrier wave e c =2tf c =angular frequencyof carrier wave. The amplitude of the carrier wave varies at the frequencye m of the modulatingwave. So, amplitude of the modulated wave is givenby A=A c + m(t) =A c +A m sin e m t =A c + µA c sin e m t =A c (1 + µsin e m t) Here, m= c m A A is the modulation index. The instantaneous voltage of theA.M. is given by c m (t) =Asin e c t =A c (1 + µsin e m t) × sin e c t =A c sin e c t + 2 A c µ × 2sin e c t · sin e m t =A c sin e c t + 2 A c µ cos (e c + e m ) t + 2 A c µ cos (e c + e m ) t. ] Q.4 What is meant bysidebandfrequencies ina carrier wave? Justifythat bandwidthof anA.M. wave is 2f m , where f m is the modulatingfrequency. [Sol. Sideband frequencyand bandwidth of A.M. wave. The equation (1) shows that theA.M. wave is the sumof three sinusoidal waves: A c 2 A c µ 2 A c µ f – f c m f + f c m f m (i) One the original carrier wave of amplitudeA 3 and frequencyf c = t e 2 c . (ii) The second wave of amplitude 2 A c µ and of frequency (f c + f m ). The sum of carrier frequency and modulating frequency(f c +f m ) is calledupper sideband frequency(USB). mk COMMUNICATION SYSTEM (T.N.) PAGE # 6 (iii) The third wave of amplitude is 2 A c and frequency(f c – f m ). The difference of carrier frequencyand modulatingfrequency(f c – f m ) is calledlower sideband frequency(LSB). The difference between the highest and the lowest frequencies present in the A.M. wave is called bandwidth.] Q.5 What is demodulation? With the help of a suitable circuit diagram, briefly explain the working of an amplitudedemodulator. [Sol. Demodulation or detection. The process of recovering the audio signal fromthe modulated wave is known as demodulation or detection. As the modulated wave contains the three waves of frequencies e c' and e c + e m' we need to detect only the original message signal m(t) or frequency e m' . Thus message demodulationis the reverse process of modulation. Ajunctiondiode can be usedas a detector or demodulator. Its action is same as that of a half-wave rectifier. p n L C C H e a d p h o n e Audio Output across R Modulated signal Rectified current pulses Junction diode as a detector Elementarydemodulator circuit for anA.M. wave. Figure shows acircuit diagramfor ajunctiondiode as detector for anamplitude modulated wave. The input circuit is a parallel combination or inductance Land variable capacitor C. It is called turned circuit. Byadjustingthefrequencyof this circuit, thedesiredmodulatedradiosignal is resonantlyselected fromthe different signals picked upthe antenna. Diode rectifies this signal. So the output of the diode is a series of positive half cycles of radio frequency current pulses. The peaks of these pulses vary in accordance withaudiosignal. For extract recoveryof the original message, the followingconditionmust be satisfied. m 1 C f 1 ' C R f 1 < < . ] Q.6 Discuss theadvantages anddisadvantages of amplitude modulation. [Sol. Advantages of amplitude modulation: (i) It is aneasier method for transmittingandreceivingvoice signals. (ii) It requires simple and cheaper transmitters and receivers. (iii) Its transmissionrequires lowcarrier frequencies of 0.5-20 MHz. (iv) Area in whichAMtransmission canbe received is much larger than that in case of FMtransmission. Disadvantages of amplitude modulation: (i) Amplitudemodulationsuffers fromnoise. (ii) Qualityof audioof signal is poor. (iii) EfficiencyofA.M. transmissionis low. ] mk COMMUNICATION SYSTEM (T.N.) PAGE # 7 Q.7 Give the advantages anddisadvantages of frequencymodulationover amplitude modulation. [Sol. Advantages of frequency modulationover amplitude modulation: (i) F.M. transmissionis highlyefficient because all the transmitted poor is useful but inA.M. transmission, most of the power goes waste in transmittingthe carrier alone. (ii) Since amplitude remains constant in F.M. , so amplitude limiters can be used to reduce noise in F.M. (iii) Depth of amplitude modulation cannot be increasedbeyond 100%without causingdistortion. There is no such limit in F.M. Hence noise in F.M. can be reduced further byincreasing deviation o . (iv) In F.M. standard frequencyallocations provide a guard band between commercial F.M. stations. This reduces adjacent channel interference. (v) InF.M. space wave propagation is used. So its radius of operation is slightlymore than the line of sight. It makes possibletooperate several transmitters inthesame frequencyrangewithnegligible interference. (vi) In the VHFor UHFbands where F.M. operates, their is less noise than in HFor MFbands whereAM operates. (vii) F.M. transmission gives high fidelityreception due to the presence of a large number of sidebands. Disadvantages of frequency modulationover amplitude modulation: (i) The channel required in F.M. is about 10 times wider than that inA.M. (ii) F.M. receivers and transmitters are verycomplex and costly. (iii) As F.M. reception is limited to line of sight, so its area of reception is much smaller than that forA.M.] Q.8 Howcanweclassifytransmissionmediaonthebasis of their nature? Givetheir approximatebandwidths. [Sol. Bandwidths of transmission media: Depending on the nature of communication channel, the communicationis classifiedbroadlyinthree categories. (i) Line communication: It invokes point to point contact between transmitter and receiver. It occurs through guided media such as twisted pair and coaxial cable. Coaxial cable is widely used wire medium, which offers a bandwidth of 750 MHz. These cables are normallyoperated below18 GHz (1 GHz = 10 12 Hz) (ii) Optical communication: It makes usedof a light beamincarryinganinformationfromonepoint to another through a guided mediumlike optical fibre. Optical communicationusingfibres is carriedinthe frequencyrange ( of 1THz to100THz microwaves to ultraviolet waves, 1THz =10 12 Hz). An optical fibre canhave a transmission bandwidth above 100 GHz. ] Q.9 Give type of space wave communication. Sol. Here electromagnetic waves of different frequencies are used to carry the information through the physical space actingas the transmissionmedium, radio, television andsatellitecommunications are all spacecommunications. Communications through free space using radio waves takes place over a wide range of frequencies : Froma fewhundreds of kHz to a fewGHz. Servise Frequency bands Comments Standard AM bradcast 540-1800 kHz FM Broadcast 88-108 MHz Television 54-72 MHz VHF (very high frequencies) 76-88 MHz TV 174-216 MHz UHF(Ultra High frequencies) 420-890 MHz TV Cellular Mobile Radio 896-901 MHz Mobile to base station 840-935 MHz Base station to mobile Satellite communication 5.925-6.425 MHz Uplink 3.704.2 MHz Downlink mk COMMUNICATION SYSTEM (T.N.) PAGE # 8 Q.10 Deduce an expression for the distance upto which the T.V. signals be directlybe received froma T.V. tower of height h. [Sol. Range of a transmitting antenna : Let T.V. signal be transmitted froman antenna of height PQ=h r . Du to the curvature of the earth, no direct signals are possible beyond the points S or T, as shown in figure. Let PS = PT = d Fromright angled AOTQ, we get OQ 2 = OT 2 + QT 2 Here OQ = R + h r QT = PT = d OT=R= Radius of the earth (R + h T ) 2 = R 2 + d 2 or R 2 + h T 2 + 2h g R = R 2 + d 2 or d 2 = h T 2 + 2h T R = 2h T R ( ¸ ( ¸ + R 2 h 1 T But h T << R, therefore d 2 = 2h T R or d = R h 2 T . ] Q.11 Write anexpressionfor the maximumlineof sight (LOS) distance betweentwoantennas for space wave propagations. [Sol. Range of space wave propagation between two antennas on earth's surface : Aspace wave travels ina straight line fromtransmittingantenna tothe receivingantenna i.e. it is a line of sight (LOS) communication. Due to the curvature of the earth, direct space waves get blocked at some points. Thus the earth presents a horizon tospace wave propagation commonlycalled ratio horizon. Figureshows the effect of antennaheight ontheradiohorizon. The line of sight radiohorizonfor asingle antenna at sea level is given by d = Rh 2 Where d = distance to radio horizon h =antenna height above sea level R= Radius of the earth The maximumlineof sight distance d M betweenthetransmittingandreceivingantennas is give by d M = d T + d R = R T Rh 2 Rh 2 + Where d T =radiohorizon for transmitting antenna d R =radiohorizon for receivingantenna d T =height of transmittingantenna h g =height of receivingantenna Clearly, if asignal is tobe receivedbeyondthehorizon, thenthe height of receivingantennamust belarge enough soas to intercept line of straight waves directly. More over, the power transmitted from a transmitter decreases nearly as the inverse square of the distance. Sothesignal becomes weaker withtheincreasingdistance. HencetherangeofT.V. transmission can be increased. (i) byincreasingthe highof the transmittingandreceivingantennas. (ii) byusingrepeater transmittingstations. ] mk COMMUNICATION SYSTEM (T.N.) PAGE # 9 Q.12 What is ionosphere ? Explaintheimportance incommunication. [Sol. Ionosphere : It is the outermost regionof the earth's atmosphere which extends fromabout 60-350km fromthe earth's surface. Du to lowpressure in this part of atmosphere, ionisation is produced byUV radiationandX-rays fromthe sun. Sothis regionis mostlycomposedof ionisedmatter, i.e. electrons and positiveions whereastheremainingatmosphereismostlycomposedof neutral molecules. Theionosphere is further subdivided into layer as C,D, E,F 1 and F 2 as shown in figure shown. F 2 F 1 E D C Earth’s surface VHF , UHF transmitted HF reflected Ionosphere MF absorbed The ionosphere plays an important role in space communication. The radio waves of the HFfrequency band havingfrequencyrange 3 - 30 MHz cannot penetrate through the ionosphere. Theyare reflected back towards the earth. This region of theAMband is called shortwave band. Above frequencyof 40 MHz, the ionosphere bends the electromagnetic waves and does not reflect themtowards the earth.] Q.13 What is skypropagation? [Sol. Sky propagation: Aradiowave directed towards the skyand reflected bythe ionosphere towards the desired location of the earth is called a sky wave. Radio wave of frequencies between 2 MHz to 20 MHz can be reflected bythe ionosphere. This region of theAMband is called short wave band. These waves radiated from a certain point and after being reflected by the ionosphere can be received at another point on the surface. this is known sky wave or ionospheric propagation. In this way, radio waves can travel verylarge distance and can even travel round the earth. Critical frequency : It is that highest frequencyof radio wave which when sent straight (normally) towards the given layer of ionospheregets reflectedfromionosphereandreturns tothe earth. Above this frequency, the radiowave will penetrate the ionosphere and is not reflected byit. It is given by f c = 9(N max ) 1/2 Where N max =the maximumelectron densityof the ionosphere.] Q.14 What doyou mean bythe terms internal, Email, world wide web, E-commerce, Chat, Fax and mobile Telephony? [Sol. Internet: It is a global network of computers linked byhigh speed date lines and wireless systems. It allows communicationandsharingof informationbetween anytwoor more computers. E-mail (Electronic mail) : It is a message sent and received through a computer network. File transfer: AnFTP(File transfer protocol) permits the transfer of files / Softwarefromone computer to another connected to the internet. Worldwideweb(WWW): It is a set of protocols that allows us toaccess anydocument onthe internet. E-commerce : It is the collection of tools and practices involving internet technologies that allowa companyto create, maintain and optimize business relations with consumers and other businesses. It permits on-linebankingandshopping. mk COMMUNICATION SYSTEM (T.N.) PAGE # 10 Chat : It is the real time conversation amongpeoplewith common interests throughthe typedmessages on the net. Fax (Facsimile telegraphy) : The electronic reproduction of a document at a distant place is called facsimiletelegraphyor fax. In order tosent a document through fax, the followingthree functions are performed. (i)At the sending end, the original document is read byan optical scanner. (ii) Theblackandwhitedots of thedocument areencodedandcompressedfor transmissionandreception via atelephone line. (iii)At the receivingend, the receiveddate is decompressed andprocessed bya printer toreproduce the original document. Mobile Telephony : Instead of a single high power transmitter for an entire service area, numerous lower power transmitters (base stations) are set upto, each covering a fraction of that service area, called a cell. As a mobile receiver moves fromone cell to another, the mobile user is handedover to the ne cell's basestationthrougha switchingoffice calledmobile telephone switchingoffice (MTSO).] Unsolved Problems Q.1 Name thethree basic units of anycommunication system. Q.2 Define the terms : Signal, noise, attenuationandbaseband inconnectionwith communicationsystems. Q.3 What is a ground wave? Why short wave communication over long distance is not possibly ground waves? Q.4 Sky waves are not used in transmitting T.V. signals. Why? State two factors by which the range of transmission of T.V. signals can be increased. Q.5 What is meant bythe terms "Modulation"? Explainwiththe helpof a blockdiagram, howthe process of modulation is carriedout in radio broadcasts. Q.6 What is demodulation ? With the help of a suitable circuit diagram, brieflyexplain the working of an amplitudedemodulator. Q.7 Atransmitting antenna of the top of a tower has a height 32mand that of the receivingantenna is 50m. What is themaximumdistance betweenthemfor satisfactorycommunicationis LOSmode? Giveradius of earth 6.4 × 10 6 m. (Hint : dm = T e h R 2 + g e h R 2 ) OR Is it necessaryfor a transmittingantenna to be at the same height as that of the receivingantenna for line of slight communication ?ATVtransmittingantenna is 81mtall. Howmuchservice area it cancover it the receivingantenna is at theground level? Q.8 Explaintheneedof modulationincommunicationsystem. Q.9 Derive anexpression for amplitude modulatedwave. Hence explainthe presence of sidebands insucha wave. Q.10 On a particular day, the maximumfrequency reflected fromthe ionosphere in 9 MHz. On another day it was foundtoincreaseto12MHz. Calculatetheratioofthemaximumelectrondensitiesof theionosphere on the two days. Point out a possible explanation for this. mk COMMUNICATION SYSTEM (T.N.) PAGE # 11 Board Problems Q.1 ATVtower has as height of 500 mat a given place. If radius of earth is 6400 km, what is it coverage range? [CBSE2004] Q.2 ATVtower has a height of 400mat agivenplace. Calculate its converge range if radius of earthis 6400 km. [CBSE2004] Q.3 Agroundreceiver station is receivingsignals at (i) 5 MHz and(ii) 100 MHz, transmittedfroma ground transmitter at a height of 300mlocated at a distance of 100 km. Identifywhether the signals are coming via space wave of skywave propagation or satellite transponder. Radius of earth =6400km, maximum electron density in ionosphere N max - 10 12 m –3 . [CBSE2005] Q.4 What should be the left of dipole antenna for a carrier wave of frequency5 ×10 8 Hz. [CBSE2007] Q.5 ATVtower has a height of 71 km. What is the maximumdistance upto which TVtransmissioncan be received ? Radius of earth = 6.4 × 10 6 m [CBSE2007] Q.6 Atransmittingantenna at the top of a tower has a height of 36 mand the height of the receivingantenna is 49m. What is the maximumdistance between them, for satisfactorycommunication inLOSmode? (Radius of earth =6400 km) [CBSE2008] Q.7 Acarrier wave of peak voltage 20 Vis used to transmit a message signal. What should be the peak voltage of the modulatingsignal, in order to have a modulation index of 80%Y. [CBSE2008] Q.8 Write function of (i) Transducer and(ii) Repeater in the context of communication system. [CBSE2009] Q.9 Write factor (two) justifyingthe needof modulationof transmissionfor a signal. [CBSE2009] Q.10 Distinguish between skywave and space wave propagation give a brief description with the help of suitable diagrams indicatinghowthese waves are propagated. [CBSE2010] Q.11 Nameanytwotypes of transmissionmedia that are commonlyusedfor transmissionof signals. Write the range of frequencies of signals for which these transmission media are used. [CBSE2010] Q.12 State the twomain reasons explainingthe needof modulationfor transmissionof audio signals. [CBSE2010] 0.5 1 1.5 2 2.5 3 1 –1 C(t) 0 0.5 1 1.5 2 2.5 3 1 –1 m(t) 0 The diagram, given above, showcarrier wave c(t) that is to be (amplitude) modulated bymodulating signal m(t). Drawgeneral shape of theresultingAMwave. Define it's 'modulation index'. mk COMMUNICATION SYSTEM (T.N.) PAGE # 12 Q.13 Write two factor just flyingthe need of modulating a signal a carrier wave of peak voltage 12Vis used totransmit a message signal. What shouldbe thepeakvoltage of themodulatingsignals inorder tohave a modulationindex 75%. [CBSE2010] Q.14 Whichmodeof propagationis usedbyshort wave broadcast service havingfrequencyrangefroma few MHz upto30 MHz? Explain diagrammaticallyhowlong distance communication can be achieved by this mode. Whyis there an upper limit of frequencyof waves used in this mode. [CBSE2010] Q.15 What are the directions of electric and magnetic field vector relative to each other and relative to the direction of propagationof electromagnetic waves ? [CBSE2012] Ans. B E ± and both are ± to direction of propagation. Q.16 Abar magnet is moved in the direction indicated bythe arrowbetween towcoils PQand CD. Predict the directions of induced current ineach coil. [CBSE2012] A P Q N S A C D Ans. Clock wise in both coils direction is fromP ÷Qand C÷D Q.17 Statetheunderlyingprincipleof atransformer. Howis thelargescale transmissionof electric energyover longdistances done with the use of transformers. [CBSE2012] Q.18 Acircular coil of Nturns and radius Rcarries a current I. It is unwound and rewound to make another coil of radius R/2, current I remainingthe same. Calculate the ratioof the magnetic moments of the new coil andthe original coil. [CBSE2012]