Chapter Analysis w.r.t, Lost 3 Yeor's Boord Exoms

Transcription

1 - CHAPTER 15 Communication System Chapter Analysis w.r.t, Lost 3 Yeor's Boord Exoms The analysis given here givesyou an analytical picture of this chapter and will helpyou to identify-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 ndia Delhi All ndia All ndia... _.._._._- _ - _ _ , y~ry~~?!~~s..~~e (~...~~.~:~L......J.g. lq :.- -..~... : -1 Short Type Answer (2 marks) 1 Q 1 Q 2Q Short Type Answer (3 marks) Long Answer (5 marks) Value Based Questions (4 marks) n 2015, only one question of 1mark based on Mode. of Communication for Telephonic Communication was asked in Delhi set.; n 2016, in Delhi set, only one question of 2 marks based on Attenuation and Demodulation was asked in Delhi set. n All ndia set, one question of 1mark based on Component of Communication System and one numerical question of 2 mark were asked. n 2017, in Delhi set, one question of two marks each based on Mode of Communication in Satellite Communication System, Sight Communication and TV signals were asked. n All ndia set only one question of 3 marks based on Block Diagram of Communication System was asked. On the basis of above analysis, it can be said that from exam point of view Mode of Communication for Telephonic Communication, Attenuation and Demodulation, Component of Communication System, Mode of Communication in Satellite Communication System, Sight Communication and TV Signals are most important concepts of the chapter. lq ) ~ A.

2 [TOPC 1] Communication Communication is the act of transmission and reception of information. The informations are first converted into electrical signals and then sent electronically. 1.1Communication System A system comprises of transmitter, communication channel and receiver. A block diagram of a generalised communication system is shown as below: Signal t conveys information as single-valued function of time and which, at every instant of time, takes a unique value. Transmitter t consists of transducer/signal generators, modulators and transmitting antenna. For e.g., f information source gives voice signal, transducer converts it into an electrical signal. Receiver ts main function is to decode the original signals. The main function involves picking up the signals, demodulating and displace the original message signal. The process of recovering the original signal is called demodulation. ~~. Communication Channel The physical path between the transmitter and receiver is known as communication 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. Total bandwidth of channel Num ber 0f ch anne s = Bandwidth needed per channel Table shows the various things used in communication system, Source of nformation Speech, pictures, words, codes, symbols, commands and data Transmitter Channel Receiver Modes of Communications Oscillators, amplifiers, filters and antenna Wire links, wireless and optic fibres Radio, TV,computer, telephone, teleprinter, telegraph, fax and internet There are two basic modes of communication given as below: (i) Point-to-point n this type of communication mode, communication takes place over a link between a signal transmitter and a receiver, e.g. telephony. (ii) Broadcast n 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 nformation converted into electrical form and suitable for transmission is called a signal.

3 CHAPTER 15 : Communication System 483 (ii) Transducer Any device/arrangement that converts one form of energy into another is called a transducer, e.g. microphone. i.e. converts physical quantity (message) into electrical signal. (iii) Noise t refers to the unwanted signals that tends to disturb the transmission and processing of message signals in communication system with the interference of information signal. (iv) Attenuation t refers to the loss of strength of a signal during its propagation through the communication channel. (transmission medium) (v) Amplification t is the process by which amplitude of a signal is increased using an electronic circuit called the amplifier. (Consisting of at least one transistor) (vi) Range t is the largest distance between a source and a destination upto which the signal is received with sufficient strength. (vii) Baseband Band of frequencies representing the original signal as is produced by the source of information is called baseband. (viii) Repeater Repeaters are erected at suitable distances between the transmitter and receiver. Repeaters are used to extend the range of a communication system. t retransmits and amplifies signals from the transmitter, to the receiver with a change of carrier frequency. 1.2 Message Signals A time varying electrical signal generated by a transducer out of original signal is termed as message signal. The electrical signals are of two types such as below: Analog Signal A continuous signal value which at any instant lies within the range of a maximum and a minimum value. Graphical representation of analog signal can be represented as given below: As, sinusoidally varying alternating voltage is the simplest analog signal E = Asinrot Eorl Timeperiod : (T)-: Digital Signal (Pulse Signol) Digital signals are those which can take only discrete stepwise values e.g. output of a computer, fax, ete. 1 Eorl Pulse rise Pulse 1 duration Pulse fall T Pulse amplitude + O~--~--~~--o~--~--~o~--~---- Time--+ Coding schemes used for digital communication given as below: Binary Coded Decimal (BCD) L are n this, a digit is represented by two binary numbers o or 1. " American Standard Code for nformation nterchange (ASC) t is a universally popular digital code to represent numbers, letters and certain characters. 1.3 Bandwidth of Signals Bandwidth of signal is defined as the difference between the upper and lower frequencies of signal. n a communication system, the message signal can be voice, music, picture or computer data. Bandwidth = Highest frequency - Lowest frequency This has been shown in the table given as below: Typesof signal Frequencyrange Bandwidth w,."_ Speech signal Hz 2800 Hz ".M Music signal Hz 20 khz Video signal TV signal 4.2 MHz 6 MHz f J

4 M ~MM_MMMHH H.HH HH_.HH HHHH HH H_.HHH_._ HH " Chopterwise CBSE Solved Papers PHYSCS 1.4 Bandwidth of Transmission Medium The commonly used transmission media are wire, free space, fibre optic cable (750 MHz) and optical fibre (100 GHz). This range is sub-divided further and allocated for various services as indicated in the table given as below: Service Frequency bands Remarks Standard AM khz Radio broadcast broadcast FM broadcast MHz Music channel Television MHz MHz MHz MHz VHF (Very High Frequencies ) TV UHF (Ultra High Frequencies) TV Cellular MHz Mobile to base Mobile radio MHz station Base station mobile Satellite GHz Uplink Communication GHz Downlink Antenna Antenna is a device which acts as an emitter of electromagnetic waves and it also acts as a first receiver of energy. t is generally a metallic object often a wire or collection of wires. (i) Hertz Antenna t is a straight conductor of length equal to half the wavelength of radio signals to be transmitted or received. i.e,. l=~ 2 (ii) Marconi Antenna t is a straight conductor of length equal to a quarter of the wavelength of radio signals to be transmitted or received, i.e. l=~ 4 (iii) Dipole Antenna t is used in transmission of radio waves. t is omni directional. to (iv) Dish-Type Antenna t is a directional antenna. Such antenna has a parabolic reflector with an active 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 of Electromagnetic Waves n communication using radio waves, an antenna at the transmitter radiates the EM waves, which travel through the space and reach the receiver at the other end. Depending upon frequency and ways of propagation, electromagnetic waves categorised a", follows: Ground Wave Propagation [f < 2 MHz) n ground wave propagation, the radio waves (AM) travel along the surface of the earth. These waves are guided along the earth's surface and they follow the curvature of the earth. Ground wave propagation is possible only when the transmitting and receiving antenna are close to the surface of the earth. Sky Wave Propagation (2 MHz <t= 30 MHz) Long distance communication can be achieved by a radiowave directed towards the sky and reflected by the ionosphere towards the desired location of the earth is called a sky-wave. This sky wave propagation is used by short wave broadcast services. The ionosphere is so called because of the presence of a large number of ions. t extends from height of 65 km to about 400 km above the earth's surface.

5 ._._H" _.HHH._ "'H _ _ _ _ ~ H~~~ HH H HH _ _ 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) Troposphere ~ ,,-_..- D (part of stratosphere) E (part of stratosphere) Approximate height.. over earth's surface EXsts durmg Frequencies most affected 10 km Day and night VHF (up to several GHz) _ _ H _ _ _. _ H H _ _ _ N _ _ _ km Day only Reflects LF, absorbs MF and HF to some degree 100km F (Part of mesosphere) km F2 (Thermosphere) km Day only Daytime, merges with F2 at night Day and night Helps surface waves, reflects HF Partially absorbs HF waves yet allowing them to reach F2 Efficiently reflects HF waves, particularly at night The degree of ionisation varies with the height. The density of atmosphere decreases with height. The ionospheric layer acts as a reflector for a certain range of frequencies. These phenomena are shown as below: Maximum Usable Frequency (MUF) t is a limiting frequency of radiowave but for some specific angle of incidence other than the normal and gets reflected and returns to the earth. MUF =. sece=~ e cose where, e is the angle between normal and the direction of incidence of waves. Skip Distance t is the shortest distance from a transmitter measured along the surface of earth to the sky wave of fixed frequency but less than critical frequency from the ionosphere. e will return to earth. Dskip= 2h (/j:w r-1 where, h = height of the reflecting layer of atmosphere. MUF =MUF Critical Frequency For a given layer, it is the highest frequency that will return down to earth by that layer. Critical frequency (/el, e = 9 (Nmax)/2 where, N max = maximum ionosphere. electron density of The critical frequency ranges approximately from 5-10 MHz. -, rf"''l Fading t is defined as the variation in the strength of a signal 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 from transmitting 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 waves are get blocked at some point by curvature of earth as shown below: dm dr ---;,1 -- dr T (i) Range of TV transmission, d =.J2hR where, h = height of antenna, R = radius of the earth, d = maximum distance/range of transmission. R

6 486 o chopterwlse esse Solved Papers PHYSCS (ii) Rangeof line of sight distance between two antennas. d M =dt + dr' d M = ~2hTR + ~2hRR where, ht and hr are heights of transmitting and receivingantenna. Satellite Communication n this communication, frequency band 5.9 GHz to 6.4 GHz is used for uplinking and 3.7 GHz to 2 GHz is used for downlinking. PREVOUS YEARS' EXAMNATON QUESTONS TOPC 1 o 1 Mark Questions 1. Name the essential components of a communication system. All ndia Which basic mode of communication is used for telephonic communication? Delhl2D15 3. The figure given below shows the block diagram of a generalised communication system. dentify the element labelled X and write its function. Deihl 2014 C ~ nformation :Transmitter Receiver! source ~ Message Message signal signal 4. What is the meaning of the term attenuation used in communication system? All ndia 2014C 5. Give the one example of point-to-point communication mode. All ndia 2014C 6. Draw a block diagram of a generalized communication system. Delhl2014C 7. What is the function of a transducer used in a communication system? Deihl What does the term attenuation used in communication system mean? Deihl 2D12,200BC 9. What is the function of a repeater in a communication system? Foreign 2011;Deihl What is the function of a transmitter in a communication system? Foreign What is the sky wave propagation? Delhi What is ground wave propagation? Delhi What is space wave propagation? Delhi What does the term transducer mean in an electronic communication system? Delhl2D09C 15. Distinguish between sinusoidal and pulse shaped signals. All ndia 2009C o 2 Marks Questions 16. Which basic mode of communication is used in satellite communication? What type of wave propagation is used in this mode? Write giving reason, the frequency range used in this mode of propagation. Deihl (i) What is the line of sight communication? (ii) Why is it not possible to use sky waves for transmission of TV signals? Upto what distance can a signal be transmitted using an antenna of height h? Delhi Explain the terms (i) Attenuation and (ii) Demodulation used in communication system. Deihl A devices X used in communication system can convert one form'of energy into another. Name the device X. Explain the function of a repeater in a communication system. Foreign (i) Which mode of propagation is used by shortwave broadcast services having frequency range from a few MHz upto 30 MHz? Explain diagrammatically how long distance communication can be achieved by this mode. (ii) Why is there an upper limit to frequency of waves used in this mode? All ndia 2016, 2010

7 CHAPTER 15 : Communication System Write the function of the following in communication systems. All ndio 2014 (i) Transducer (ii) Repeater 22. Write the function of the following in communication systems. All ndio 2014 (i) Transmitter (ii) Modulator 23. Write the function of the followingin communication system. All ndio 2014 (i) Receiver (ii) Demodulator 24. Which mode of wave propagation is suitable for television broadcast and satellite communication and why? Draw a suitable diagram depicting this mode of propagation of wave. Foreign Distinguish between analog and digital signals. Deihl Mention the function of any two of the following used in communication system. (i) Transducer (ii) Repeater (iii) Transmitter (iv) Bandpass filter Delhi What is sky wave communication? Why is this mode of propagation restricted to the frequencies only up to few MHz? All ndio What is space wave communication? Write the range of frequencies suitable for space wave communication. All ndio Draw a block diagram showing the important component in a communication system. What is the function of a transducer? Foreign What is the range of frequencies used for TV transmission? What is common between these waves and light waves? Deihl What is the range of frequencies used in satellite communication? What is common between these waves and light waves? Deihl n standard AM broadcast, what mode of propagation is used for transmitting a signal? Why is this mode of propagation limited to frequencies up to a few MHz? Foreign Name any two types of transmission media that are commonly used for transmission of signals. Write the range of frequencies of signals for which these transmission media are used. All ndio 2010C 34. (i) What is line of sight communication? (ii) Why is it not possible to use sky wave propagation for transmission of TV signals? Foreign A communication satellite is essentially a repeater station in space. Justify this statement by analysing the function of a repeater. All ndio 2009C 36. Write the function of (i) transducer and (ii) repeater in the context of communication system. All ndio 2014, 2009 o 3 Marks Questions 37. Draw a block diagram of a generalised communication system. Write the functions of each of the following: (i) Transmitter (ii) Channel (iii) Receiver All ndio Distinguish between point to point and broadcast modes of communication. Give an example of each. Foreign 2016; Deihl 201SC 39. Describe briefly by suitable diagram of (i) sky wave propagation and (ii) space wave propagation. Mention the frequency range of the waves in these modes of propagation. Foreign Write the function of each of the following used in communication system. (i) Transducer (ii) Repeater (iii) Transmitter Delhl2013C

8 488 o Chapterwise CBSE Solved Papers PHYSCS 41. Name the types of waves which are used for Line of Sight (LOS) communication. What is the range of their frequencies? A transmitting antenna at the top of a tower has a height of 20 m and the height of the receiving antenna is 45 m. Calculate the maximum distance between them for satisfactory communication in line of sight mode. (Radius of the earth = 6.4 x 10 6 km) A,ndia Draw a schematic diagram showing the (i) ground wave (ii) sky wave and (iii) space wave propagation modes for electromagnetic waves. Write the frequency range for each of the following: (i) Standard AM broadcast (ii) Television (iii) Satellite communication HOTS; Delhi Explain briefly the following terms used in communication system (i) Transducer (ii) Repeater (iii) Amplification All ndia Name the three different modes of propagation of electromagnetic waves. Explain using a proper diagram, the mode of propagation used in the frequency range from a few MHz to 40 MHz. Delhi What is the ground wave communication? On what factors does the maximum range of propagation in this mode depend? All ndia Draw a schematic diagram showing the (i) ground wave (ii) sky wave (iii) space wave propagation modes for electromagnetic waves. All ndia 2011C 47. Which mode of propagation is used by short wave broadcast services having frequency range from a few MHz up to 30 MHz? Explain diagrammatically, how long distance communication can be achieved by this mode? Why is there an upper limit to frequency of waves used in this mode? All ndia What is space wave propagation? Give two examples of communication system which uses the space wave mode. A TV tower is 80 m tall. Calculate the maximum distance up to which the signals transmitted from the tower can be received. Delhi (i) Why is communication using line of sight mode limited to frequencies above 40 MHz? (ii) A transmitting antenna at the top of a tower has a height 32 m and the height of the receiving antenna is 50 m. Calculate the maximum distance of them for satisfactory communication in line of sight mode? Delhi The diagram given below represents the block diagram of a generalised communication system. dentify the elements labelled as X, Y, Z in this diagram. Explain the function of each of these elements. 51. Distinguish between sky wave and space wave propagation. Give a brief description with the help of suitable diagram indicating how these waves are propagated? All ndia Distinguish between sky wave and space wave propagation. Explain in brief with the help of suitable diagram, how space waves are used for line of sight communication? HOTS; Delhi 2009C

9 o Explanations 1. Essential components of a communication system are transmitter, transmission medium or communication channel and receiver. [1) 2. Point to point is a basic mode of communication, which is used for telephonic conversation. n this mode, communication, takes place over a link between a single transmitter and a receiver. [1) 3. Labelled element X represents the channel. ts function is to transmit information from one place to another. [1) 4. t refers to the loss of strength of a signal during its propagation through the communication channel output or through the transmission medium. [1) 5. Telephone is the example of point to point communication mode. [1) 6. Refer to theory on page 482. [1) 7. Transducer used as a sensor or detector in communication system. t converts the physical signal into electrical signal. e.g. A photo detector converts light signal into electrical signal. [1) 8. Refer to Ans. 4 [1) 9. Repeater t picks up the signals from the transmitter, amplifies it and transmit it to the receiver. Thus, repeater comprises up of receiver, transmitter and amplifier. ts function is to extend the range of communication. [1) Multiple usage of amplifier repeater extends the range of communication. 10. Transmitter t comprises of message signal source, modulator and transmitting antenna. Transmitter make signals compatible for communication channel via modulator and antenna. [1) 11. Sky wave propagation When radio wave propagates from one place of earth to other after reflection by ionosphere, the range of frequencies from few MHz to 30 MHz gets reflected back by ionosphere. This range also reflected as short wave band. This mode of propagation is used by short wave broadcast service. [1) 12. Ground wave propagation The radio waves whose frequencies ranged up to 1500 khz, propagates from one place of earth to other following its transmission along the surface of earth. These waves get attenuated and hence cannot travel over long distances. This range of frequencies also referred as amplitude modulated band (AM band). [1) 13. Space wave propagation t is also known as Line of Sight propagation (LOS). The radio wave transmitted by antenna directly reaches the receiving antenna travelling along a straight line. TVwaves (80 MHz-200 MHz) propagate through space wave propagation. [1) 14. Transducer Any device which converts one form of energy into other e.g. electric transducer converts pressure, temperature, etc. into varying electrical signals i.e. transducer converts physical signals into electrical signals. 15. A signal in which current or voltage change continuously with time sinusoidally is known as sinusoidal signal. A signal in which current or voltage can take only tw9 discrete values for it is called pulse shaped signals. es. J'!f:' '..,.[(1. 5V OV t- 16. Space wave mode is used in satellite communication. n this type of wave propagation, radio waves are emitted from the transmitter antenna and reach the receiving antenna through space. Frequency range for this communication is very high, so that, it may penetrate the ionosphere and reach satellite. ts range is from 54 MHz to 4.2 GHz. 17. (i) When the radio waves from the transmitting antenna reach the receiving antenna directly, the wave propagation is called line of sight communication. (ii) Sky waves cannot be used for transmission of TV signals because the reflecting layer that is ionosphere becomes transparent for high

10 AgO o Chopterwise CBSE Solved Papers PHYSCS frequency range of TV signal. They go to the sky but do not come back to surface. Radio waves for radio based communication are used as sky waves. An antenna of height h can transmit signal upto a maximum distance of d, where d =.f2hr r where, R is the radius of the earth.,18. (i) Attenuation The loss of strength of signal during its propagation through the transmission medium is called attenuation. Repeater is used to compensate the attenuation. (ii) Demodulation The process ofrecovering the original information signal from the modulated wave at the receiver end is called demodulation. t is the reverse process of modulation. 19. X is transducer. Repeater A repeater accepts the signal from the transmitter amplifies and retransmits it to the receiver. Refer answer 9. (2) ~O. (i) Sky wave propagation is used by shortwave broadcast services having frequency range from few MHz to 30 MHz. F2 F1 onospheric E layer Long distance communication can be achieved by reflection of radio waves by the ionosphere, back towards the Earth. This ionosphere layer acts as a reflector only for a certain range of frequencies (few MHz to 30 MHz). (ii) Electromagnetic waves of frequencies higher than 30 MHz, penetrate the ionosphere and escape, whereas the waves less than 30 MHz are reflected back to the earth by the ionosphere. 21. (i) Transducer Refer to Ans. 14. (ii) Repeater Refer to Ans (i) Transmitter Refer to Ans. 10. (ii) Modulator A modulator is a device. Which does the modulation of low frequency message signal. 23. (i) Receiver A receiver extracts the desired message signal from the received signals at the channel output. (ii) Demodulator A demodulator is a device that performs demodulation, i.e. inverse of modulation. The process of retrieval of information from the carrier wave at the receiver is termed as demodulation. As, signals of these frequency ranges are reflected back by ionosphere up to receiver end after travelling a large distance. 24. Sky wave Television ~ 1710kHz to 40 MHz Sky wave 25. Continuously with time At any time, the value of signal is represented by its amplitude. Analog Signal-output voltage varies continuously with input voltage. Rarely used in modern communication system. Digital signal n digital signal, the amplitudes are not continuous with time. Amplitude of a signal has 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. The radio wave of frequencies up to 30 MHz cannot penetrate the ionosphere and they get reflected back to earth whereas higher frequencies (> 40 MHz) bends slightly but not reflected back to the earth. Because, frequencies up to few MHz «30 MHz) gets reflected back to earth. Hence, this frequency range is used for sky wave communication. (2) 28. Space wave propagation Refer to Ans. 13. (2) (2)

11 CHAPTER 15 Communication System Range of frequency suitable for space wave propagation is 100 MHz to 220 MHz. r ,! Noise 1 - E s ~ c o ~ o 'c :::J E E o () Transducer Refer to Ans. 14. (2j 30. The range of frequencies used for TV transmission is 100 MHz to 220 MHz. Characteristic Light wave Speed Reflection Travel with speed c = 3x 10 8 rn/s Occurs and get affected by ground terrain, atmosphere and other objects. Radio wave ( \ 1:) TV waves Also travel with speed c = 3x10 8 m/s t occurs and also get affected by ground terrain, atmosphere and other objects. 31. The range of frequencies used in satellite communication is 3.7GHz to 6.4GHz. Common between these waves and light waves refer to frequency range for light wave which is of GHz order. 32. n standard AM broadcast, surface wave propagation is used for transmitting the signals. Attenuation of surface wave increases very rapidly with increase in frequency that is wl. it is limited to frequencies up to a few MHz. h. <\M broadcast, range of frequencies are limited to 30 MHz. 1 Ground wave (or medium wave propagation) (f<1500 khz) AM band l (f < 30 MHz) 1 Sky wave (or short) wave propagation (30 khz<f<150 MHz) 33. For the transmission of signals, following two types 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> > 300 MHz (ii) Space wave communication 30 MHz < f < 300 MHz or more. ('12 x 2 = 1) 34. (i) For line of sight communication Refer to Ans. 13. (ii) The frequency of waves used for transmission of TV signals are of range 100 MHz-220 MHz. But ionosphere may be able to reflect waves back on earth of frequency up to 30 MHz. Therefore, ionosphere is unable to reflect TV waves (space waves) back on the earth. 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 communication system is shown below ~ Transmitter. ~. ntelligent Channel nformation Receiver ~ Output (i) Transmitter Amplifies, converts intelligent information into suitable form (electrical) and modulates it, so that it is suitable to transmit through channel of propagation. (ii) Channel Let the signal to pass through it with least possible attenuation. [11 (iii) Receiver Amplifies, demodulates and converts information into a form which is suitable for receiver (user). [11

12 Chopterwise CBSE Solved Papers PHYSCS 38. Point to point communication The communication takes place over a link between a single transmitter and receiver is called point to point communication. Telephony is an example of such a system. Broadcast mode n such a mode, large number of 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 communication involves the transmission from transmitter, travelling along a straight line in space, reaches to receiving antenna. The range of their frequencies is 40 MHz and above. The range of space wave propagation is limited by line of sight distance between transmission to receiver/repeater antenna. Maximum line of sight distance ~=~+~ rn where, d T = Maximum horizon for transmitting antenna, d R = Maximum horizon for receiving antenna and «; =.J2Rfr +.J2Rh R where, R = 6400 X 10 3 m = 6.4 x 10 6 m fir = 20m,h R = 45 m... Maximum line of sight distance d m =.J 2x 6.4 X 106 x 20 +.Jr-2-X-6-.4-X--0-"6-X-4-5 = 2 x 8 X x 8 X 10 3 = 40 km Thus, the maximum distance between the antennas is 40 km. 42. To solve these type of questions first, we should be sure about the transmitter and receiver placing and their height. Then according to the frequency the propagation will be decided. Standard AM broadcast ~ 530 khz to 1710 khz. (ii) Refer to Ans 24. (iii) Space wave Satellite communication ~ upto 1710 khz. Space wave propagation direction propagation. mode is used for 43. (i) Transducer Refer to Ans. 14. (ii) Repeater Refer to Ans. 9. (ill) Amplification t is the process of increasing the strength of the transmitted signal using some suitable electronic circuit. Amplification compensates for the attenuation of the signal. 44. Following are the three different modes of propagation 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. Figure refer to Ans Ground wave communication Refer to sol. 12. The maximum range of propagation of ground wave 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 space communication Earth Three modes of space communication (3)

13 CHAPTER 15 Communication System (i) Sky wave propagation is used by short wave broadcast services having frequency range from a few MHz up to 30 MHz. (ii) Sky wave can travel very long distances andcan even travel round the earth. _.~~~7:~C~~~:~~~.'~7:'~:>~», 48. R1 R2 Earth surface....,.;-..: Refractive index of ionosphere (x) is less than that of free space (x o ), i.e. x< Xo As we go high, the electron density decreases and therefore x' decreases. The refraction occurs till it reaches the critical angle and afterwards it reflects the electromagnetic waves back on the earth's surface. The critical frequency is given by fe = 9 (N max )1/2 n general, 30 MHz> c > 5 MHz. when frequency is greater than fe' then it crosses the ionosphere and never reflects back on the surface of the earth. The ranges of critical frequency up to few MHz sets an upper limit to frequency of waves used in this mode. (1/2) Space wave propagation Refer to Ans. 13. Examples of communication system which uses space wave mode are television channel, UHF, VHF, ete. Maximum distance up to which the signals can be transmitted, d =? d =.,fijlr where, R = radius of earth = 6400 km = 6.4 x 10 6 m.. d = ~2 x 80 x 6.4 X 10 6 d = 32 X 10 3 m = 32 km (112) 49. (i) Communication is using line of sight mode limited to frequency above 40 MHz because the frequency below 40 MHz fall in amplitude modulated band (AM band) which may use ground wave or sky wave mode of propagation. (ii) Refer to Ans. 42 (2) 50. n solving these types of questions, we have to remind the block diagram for generalised communication system. X: Message signal generator Y: Modulator 51. Z: Power amplifier (Yo + Yo + Yo) Function of elements X : t converts one form of message signal into electrical energy. The variation of current or voltage changes in accordance to variation in pressure, etc. also known as transducer. Y: Used for superimposing the low frequency modulating signals over high frequency carrier wave. Z : Power amplifier strengthens the modulated signals for transmission. (Yo + % + %) Differences between sky and space waves are given in the table Characteristic Sky wave Space wave Frequency range 1500 khz to > 40 MHz 30 MHz Channel Distance of transmission After reflection by ionosphere Long distance transmission Figure Refer to Ans. 40. Line of sight communication using repeaters Limited range of transmission can be increased by raising height of antenna. 52. n solving these types of questions, we should have an idea about the frequency and range accordingly we will decide the nature of propagation of the wave. Refer to Ans. 52 for distinguish between sky wave and space wave. (1%) Propagation of space wave Space wave communication involves the wave transmission from transmitter and travelling along a straight line in space, reaches to receiving antenna. The range of space wave propagation is limited by line of sight distance between transmission and receiver/repeater antenna. Range of transmission antenna Let, d be range of transmission antenna of height h. (112) d Q h P S T,,, ire,,,, /Re,,,,,~,., 0 d

14 494 o Chnpterwise ebse Solved Papers PHYSCS n!)'oqt, oo' = QT2 + OT 2 ~ (R. + h)2 = d 2 + R; ~ d 2 = h2 + 2hR, [": h«r] ~ h may be neglected, d 2 = 2hR, ~ d =../2hR, (112) f height of transmitting and receiving antennas are hr and hr respectively, then the maximum line of light distance, d M = d T + d R.. d M =../2hrR, +../2h R R, (1/2) Thus, space wave travels under line of sight mode in space. [TOPC 2] Modulation Modulation is the process of variation of some characteristics of a carrier wave usually amplitude, frequency or phase' angle in accordance with the instantaneous 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 Modulation t 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 different stations. (iii) get attenuated significantly. As power radiated by antenna is given byp oc (ir. 2.1 Amplitude Modulation (AM) n amplitude modulation, the amplitude of the carrier is varied in accordance with the information signal. Amplitude modulation can be represented by expression em(l) = A, sin roc t +!lac cos (ro,-ro m ) t 2 -!lae cos (ro, + rom) t where, 2 Ae = amplitude of carrier wave Am = amplitude of modulating wave (Olc-Olm) Olc Lower side band Graphical Representation Cm(t)forAM_~~ o Modulation ndex Change in amplitude of carrier wave ~ = Amplitude of carrier wave = Am = Amax - Amin Ae A max + Amin where, A max = maximum amplitude of AM wave A min = minimum amplitude of AM wave Upper side band frequency = e + 1m Lower side band frequency = e - 1m where, e and 1m are frequencies and modulating Bandwidth wave. = USB-LSB = (e + 1m) - (e - 1m) = 21m of carrier wave 2.2 Production of Amplitude Modulation Wave One of the simplest method block diagram is shown below by

15 CHAPTER15 : Communication System 495 Modulating signal m(t) Carrier wave c(t) Block diagram of a transmitter shown as below: Message signal m(t) Carrier wave c (t) () Block diagram of a transmitter Receiving antenna and receiver is Condition for satisfactory identification diode, 1 1 -«Rc«- e 1m where, 't = Rcis time constant of the detector 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) Transmitting antenna PREVOUS YEARS' EXAMNATON QUESTONS TOPC 2 o 1 Mark Questions 1. How are side bands produced? All ndio Distinguish between amplitude modulation and frequency modulation. All ndio 2015C Output by a 3. How does the effective power radiated from a linear antenna depend on the wavelength of the signal to be transmitted? Deihl 2D14C 4. The carrier wave of a signal is given by c(t) = 3sin (81t t) volt The modulating signal is a square wave as shown in the figure. Find the modulation index. Deihl 2014 m(t) in volt 5. Why are broadcast frequencies (carrier waves) sufficiently spaced in amplitude modulated wave? Foreign Why do we need a higher bandwidth for transmission of music compared to that for commercial telephonic communication? Deihl 2009 o 2 Marks Questions 7. Distinguish between a transducer and a repeater. Deihl Write two factors which justify the need of modulating a low frequency signal into high frequencies before transmission? Deihl Define the term modulation. Draw a block diagram of a simple modulator for obtaining AM signal. Foreign A message signal offrequency 10 khz and peak voltage 10 V is used to modulate a carrier of frequency 1 MHz and peak voltage 20 V. Determine (i) the modulation index (ii) the side bands produced. Delhl2013C 11. n the block diagram of a simple modulator for obtaining an AM signal shown in the figure, identify the boxes A and B. Write their function. x(t) -:-:-~-:-:-:-. -l + A Modulating signal Carrier wave y(t) AM wave All ndia 2013

16 496 o ehapterwise ebse Solved Papers PHYSCS 12. (i) Describe briefly the three factors which justify the need for translating a low frequency signal into high frequency before transmission. (ii) Figure shows a block diagram of a detector for AM signal. AM wave., r---~ TRectlfler Output ABC Draw the waveforms for the (a) input AM wave at A, (b) output B at the rectifier and (c) output signal at C. Allndio 2013C 13. Block diagram of a receiver is shown in the figure below: Receivingantenna (i) dentify X and Y. (ii) Write their functions. Oelhi 2013; Allndio Figure shows a block diagram of a transmitter identify the boxes X and Y and write their functions. Foreign 2012 m(t) ~Y signal Jransmitling antenna 15. A carrier wave of peak voltage 18 V is used to transmit a message signal. Calculate the peak voltage of the modulating signal in order to have a modulation index of 50%. Oelhi For an amplitude modulated wave, the maximum amplitude is found to be 10V while the minimum amplitude is 2 V. Calculate the modulation index. Why is modulation index generally kept less than one? Foreign For an amplitude modulated wave, the maximum amplitude is found to be 12 V. Calculate while minimum amplitude is 2 V the modulation index. Why is modulation index generally kept less than one? Foreign (i) Define the modulation index. (ii) Why is the amplitude of modulating signal kept less than the amplitude of carrier wave? Oelhi Write two factors justifying the need of modulating a signal. A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%? Allndio A carrier wave, c (t) = Ae sin wet is amplitude modulated by a modulating signal m(t)= A", sinwmt. The maximum and minimum amplitudes of the resulting AM wave are found to be 16V and 4V, respectively. Calculate the modulation index. Oelhi 2010C 21. Why are high frequency carrier waves used for transmission? Oelhi Write two factors justifying the need of modulation for transmission of a signal. Allndio A message signal of frequency 10 khz and peak voltage of 10 V is used to modulated frequency of 1 MHz and peak voltage of 20 V. Determine the (i) modulation index (ii) the sidebands produced Allndio 2009C o 3 Marks Questions 24. (i) How is amplitude modulation achieved? (ii) The frequencies of two sidebands in an AM wave are 640 khz and 660 khz respectively. Find the frequencies of carrier and modulating signal. What is the bandwidth required for amplitude modulation? Allndio (i) Explain any two factors which justify the need of modulating a low frequency signal. (ii) Write two advantages of frequency modulation over amplitude modulation. Oelhi 2016

17 CHAPTER 15 : Communication System Define the modulation index. Why is its value kept, in practice, less than one? A carrier wave of frequency 1.5 MHz and amplitude 50 V is modulated by a sinusoidal wave of frequency 10 khz producing 50% amplitude modulation. Calculate the amplitude of the AM wave and frequencies of sidebands produced. Delhi Draw a block diagram of a detector for AM signal and show, using necessary processes and the waveforms, how the original message signal is detected from the input AM wave. All ndia Write two basic modes of communication. Explain the process of amplitude modulation. Draw a schematic sketch showing how amplitude modulated signal is obtained by superposing a modulating signal over a sinusoidal carrier wave. All ndia What is meant by detection of a modulated signal? Draw block diagram of a detector for AM waves and state briefly showing the waveforms, how the original message signal is obtained. Deihl 2013C 30. Write three important factors which justify the need of modulating a message signal. Show diagrammatically how an amplitude modulated wave is obtained when a modulating signal is superimposed on a carrier wave. Oelhi Define the modulation index. Give its physical significance. For an amplitude modulated wave, the maximum amplitude is found to be 10 V while the minimum amplitude is 2 V. Determine the modulation index u. Fariegn (i) Distinguish between sinusoidal and pulse shaped.signals. (ii) Explain, showing graphically, how a sinusoidal carrier wave is superimposed on a modulating signal to obtain the resultant amplitude modulated (AM) wave. All ndia 2012C 33. Write briefly any two factors which demonstrate the need for modulating signal. Draw a suitable diagram to show amplitude modulation using a sinusoidal signal as the modulating signal. HOTS; Delhi 2012, All ndia Give reasons for the following. (i) For ground wave transmission, size of antenna (l) should be comparable to wavelength (A.) of signal, i.e. l = A./4. (ii) Audio signals converted into an electromagnetic wave are not directly transmitted. (iii) The amplitude of a modulating signal is kept less than the amplitude of carrier wave. Delhi 2012, 2011C 35. State the two main reasons explaining the need of modulation for transmission of audio signals. C(t~~NVVVVVWVVVVW\MM o m(t)~~ The diagrams given above shows a carrier wave c (t), that is to be (amplitude) modulated by a modulating signal m(t). Draw the general shape of resulting AM wave. Define its modulation index. All ndia 2010C o Explanations 1. Sidebands are produced during the process of modulation. During modulation, the audio frequency modulating signal wave is superimposed on a high frequency wave is called carrier wave. Any form of modulation produces frequencies that are the sum and the differences of the carrier and modulating frequencies. These frequencies are called sidebands. Lower sideband frequency = fc - 1m Upper sideband frequency = ; + 1m

18 498 o Chopterwise ebse Solved Papers PHYSCS ~ ~ Amplitude modulation The amplitude of carrier wave changes in accordance with the information (signal) Frequency modulation The frequency of carrier wave changes in accordance with the information (signal) wave. wave n AM most of power goes waste in transmitting carrier. Highly efficient all the transmitted power is useful. 3. The power radiated by a linear antenna of length 1. is proportional to (L / A.)2, where A. is the signal wavelength. Since for efficient transmission, the signal should be transmitted with high power, the signal should be of small wavelength or high frequency. 4. According to the diagram, Amplitude of modulating signal Am = 1 V Amplitude of carrier wave Ae = 2V Modulation index, Jl = Am = ~ = 0.5 Ae 2 5. To avoid mixing up of signals from different transmitters the broadcast frequencies are sufficiently spaced in amplitude modulated wave. This can be done by modulating the signals on high frequency carrier waves, e.g., frequency band for satellite communication is G& 6. The range of frequencies of music is higher than commercial telephone conversation and therefore, greater bandwidth is needed for music. Also, it is free from noise. 7. Transducer is a device which converts one form of energy into another form. Microphone is a transducer because it converts sound energy into electrical energy. Repeater is a device which picks up electromagnetic signal, amplifies it and then again transmits it to the receiver. t is used when transmitted signal is attenuated beyond a certain limit or distance between a transmitter and a receiver is very large. 8. (i) Size ofthe antenna Since, an antenna is needed both for transmission and reception. Each antenna should have a size comparable to the wavelength of the signal. (1/2) m For an EM wave of frequency 20 khz, wavelength is 15 krn. 15 :. Length of antenna = - km = 3.75km. 4 t is an obvious that such a long antenna is not possible. Hence, antenna length can be made reasonable, if the frequency is high. So, there is a need to convert low frequency signal into high frequency signal before transmission. (112) (ii) Effective power radiated by an antenna Power P radiated from nr linear antenna of length is proportional to Hence, antenna length can be made reasonable, if the frequency is high. So, there is a need to convert low frequency signal into high frequency signal before transmission. 9. Modulation is the process in which low frequency message signal is superimposed on high frequency carrier wave so that they can be trasmitted over long distances. The block diagram for a simple modulator for obtaining AM signal is shown as below: (2).,...,...,...,...,..,......(+ x(t) Square law Modulating device signal m(t) Carrier wave c(t) 10. (i) Modulation index, Em = ~ = 0.5 e, 20 (ii) Side band frequencies = fc ± fm fm = 10kHz ~ fc = 1 MHz = 1000 khz :. Side band frequencies = 1000 ± 10 = 1010 khz, 990 khz 11. n the block diagram of modulator, A is square law device and B is bandpass filter. Bandpass filter rejects low and high frequencies and allows a band of frequencies to pass through. Square law device is a non-linear device for obtaining AM wave. t produces a non-linear output of message and carrier signals. The output from square law device is y(t) = Bx(t) + Cx 2 (t). where, B and C are constants and x(t) = message signal (Am sin wmt) + carrier signal (A e sin we t). Non-linear element used may be suitably biased diode or transistor.

19 CHAPTER 15 Communication System 499" 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 can not be distinguished. (1] AM input wave Rectified wave Output (without RF component) Block diagram of detector for AM signal (1] 13. From the given block diagram of demodulation of a typical receiver, we can conclude the following. (i) X represents ntermediate Frequency (F) stage while Y represents an amplifier. (1] (ii) At F stage, the carrier frequency is changed to a lower frequency and in this process, the modulated signal is detected while the function of amplifier is to amplify the detected signal which may not be strong enough to be made use of and hence is required. 11] 14. Block diagram of a transmitter m(t) Message signal '----,.----' Transmitting antenna Carrier wave Modulator Since, the frequency range of signal is quite low and it is associated with very small amount of energy. t dies out very soon if transmitted as such. So, it is tnodulated by mixing with very high frequency waves called carrier waves. This is done by modulator power. (1] Amplifier Since, the signal gets weaken after travelling through long distances it cannot be transmitted as such. Thus, we use a power amplifier to provide it necessary power before feeding the signal to the transmitting antenna. (1] 15. Here, A, = 18 V, A", =? Modulation ndex u, = 50% = 0.50 Since, ~a = ---.!!!.., A 0.50 = ---.!!!.. A A 18.. Am = 9V (2] 16. For AM wave, 11. Maximum amplitude, A max = 10 V Minimum amplitude, Amin = 2 V.. Modulation index, Amax-Amin ~ = ~ ~ = = _ = _ (112] Amax + Amin ~=- 3 (1/2] f the modulation index (JJ.) is greater than 1, the carrier wave is said to be over modulate and distortion will occur during reception as negative peak of modulating signal will be missing. Therefore, ~ is kept less than one. (1] Refer to Ans ~= (i) Modulation index The modulation index is defined as the ratio of change in the amplitude of carrier wave to the amplitude of carrier 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 and minimum 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 frequency signal into high frequency signal before transmission. (ii) To transmit a low frequency signal to a distant place so that it may not die out in the way itself. (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 modulating signal and carrier wave voltage.. (1] (1]

20 500 o Chopterwise CBSE Solved Papers PHYSCS 20. Here, Am = 7, A, = 12 V, ~ = 75% 75= Am X ~Am=12X ~Am=9V Here, maximum amplitude, Amax= 16 V Minimum amplitude, A",;" = 4 V Modulation index, ~ = Am A = Amax- A",;" Amax+ A",;" ~=--=-= High frequency carrier waves are used to increase operating range, to reduce antenna length and convert the wide band signal into narrow band signal. Then the signal can be easily recovered and distinguished from other signals at the receiving station. (2) 22. The needs of modulation for transmission of a signal are given as below: (i) The transmission of low frequency signal needs antenna of height 4-5 krn which is impossible to construct. So, there is a need to modulate wave in order to reduce the height of antenna to a reasonable height. (ii) Effective power radiated hy antenna for low wavelength or high frequency wave as 1 p oc "),}" So, for effective radiation by antenlla,-there is a need to modulate the wave. (1 + 1) 23. (i) Peak voltage of modulating signal, A m =lov Peak voltage of carrier signal, A, = 20 V.. Modulation index, ~=Am=~=~ A, 20 2 (ii) USB (Upper Side Band) = fc + 1m = 1 MHz + 10 khz 10 4 = 1 MHz+-MHz 10 6 ='1 MHz = 1.01 MHz LSB (Lower Side Band) = fc - m= 1 MHz- 10 khz (1/2) = 0.99 MHz 24. (i) For amplitude modulation, a message signal is used to modulate amplitude of a high frequency wave in input transistor of CE amplifier. m(t) c(t) c[m(t)] n this way, a modulated wave is obtained The output voltage is carrier signal varying in amplitude in accordance with biasing modulation voltage. (ii) Given, USB frequency = 660 khz and LSB frequency = 640 khz As USB = fc + 1m = 660 khz and LSB = fc - 1m = 640 khz.. 2fc = = 1300 So, carrier frequency t; = 650 khz and 21m = 20 khz Message frequency is, ~ 1m =10 khz Bandwidth of frequencies required = USB - LSB = = 20 khz (2) 25. (i) Refer Ans. 19. (1'12) (ii) Advantages of frequency modulation over amplitude modulation. (a) Frequency of a wave does not change while travelling through different media. An amplitude' modulated wave carrier information in terms of variation of amplitude, which can get disturbed. This is why FM signal is less susceptible to noise than AM signal. (b) n FM transmission, all the transmitted power is useful, whereas in AM transmission most of the power wastes in transmitting carrier wave, with no useful information. (1'12)

21 CHAPTER 15 : Communication System Amplitude modulation index is the ratio of the modulating signal to the maximum amplitude of carrier wave. t.. b Am S given y 11 = - A 27. Since, the amplitude modulation index (11) determines the quality of the transmitted signal. When modulation index is small, then variation in carrier amplitude will be small. Therefore, audio signal being transmitted will be weak. As, the modulation index increases, the audio signal on reception becomes clearer. Given, frequency of carrier wave, v, = 1. 5MHz = 1500 khz Frequency of sinusoidal (modulated) wave, Amplitude vm = 10kHz of carrier wave, A = 50 V Modulation index (l) = 50% ~ 2Q. = ~ A.: Modulation index, 11 = --"'- ~ A, ~ = Am ~ Am = 25V 2 50 So, the amplitude of AM wave, Am = 25V. As, we know, the side bands are USB = v, + Vm /= = 1510 khz LSB = v, - Vm = = 1490 khz These are the required frequencies of the side bands produced. Receiving antenna Detector Block diagram for detection A 'f' Output mp ler of AM signal When a message is received, it gets attenuated through the channel. Therefore, the receiving antenna is to be followed by an amplifier and a detector. The carrier frequency is usually changed to a lower frequency (F) stage. The detected signal may not be strong enough to be use and hence, is required to be amplified. n order to obtain the original message signal m(t) of angular frequency, a simple method is used which is shown below in the form of a block diagram. AM wave AM input wave When the received modulated signal is passed through a rectifier, an envelope signal is produced. This envelope signal is the message signal. n order to retrieve the message, the signal is passed through an envelope detector. (2)

22 502 o Chapterwise CBSE Solved Papers PHYSCS 28. The two basic modes of communication are given as below: (i) Point-to-point (ii) Broadcast (3) The process of amplitude modulation is the process of varying the amplitude of the sinusoidal carrier wave by the amplitude of the modulating signal. + 1'\-Sinusoidal r. modulating signal Time - V + / / '\ -... /- Unmodulated sinusoidal carrier wave 1\ ti"r\ J J'~\,if \ l \ Envelope of modulated carrier wave / Time 29. The transmitted messages get attenuated in propagating through the channel. The receiving antenna is to be followed by an amplifiers and a detector. Detection is the process ofrecovering the modulating signal from the modulated carrier wave.,---;:-----;---, met) The block diagram of a detector for AM waves are shown as below: AM wave Rectifier - Output L...:::=:P----' (c) ~ ~(a)~ As high powers are needed for good transmission, therefore, for given antenna length, wavelength X should be small or frequency v should be high. Thus, this factor also points out to need of using high frequency transmission. (iii) The interference of signals from different transmitters. To avoid the interference of the signals, there is a need of high frequency which can be achieved by the modulation. 1 C(t)~~ m(t~0rs:zszs:j Carrier wave 1 1 MOdulating signal 2 Cm(t)~~ 31. Refer to Ans. 18 (i). Amplitude modulated Wave m(~l~.", ~~Ol~ M~M "' ~me AM input wave time Rectified wave time Output (without RF component) The modulated signal of the form given in Fig. (a) is passed through a rectifier to produce the output shown in Fig. (b). This envelope of signal Fig. (c) is the message signal. n order to retrieve m (f), the signal is passed through an envelope detector. 30. Three important factors which justify the need of modulating a message signal (i) Size of effective power radiated by antenna. (ii) Theoretical studies reveal that power P radiated from a linear antenna of length is proportional to (1/')..)2 Le.P oc (1/')..)2 Refer to Ans. 14.

23 CHAPTER 15 : Communication System 503 Physical significance of modulation index t is used to determine the strength and quality of transmitted signal. f the modulation index is small, the amount of variation in the carrier amplitude is small. Thus, the audio signal transmitted will not be strong. The greater the degree of modulation, the stronger and clearer will be the audio signal during reception. Maximum amplitude, Am.x = 10 V Minimum amplitude, Amin = 2V Thus, modulating index, ~ = Am.x - Amin 10-2 = ~ = 0.66 (2) Am.x + Amin A continuous time varying current or voltage signal is called sinusoidal signals. The signal which two levels of current or voltage represented by 0 and are called pulse shaped signal. Three important factor to justify the need of modulating a message signal (i) Practical antenna length. (ii) Effective power radiated by antenna. (Hi) Mixing up of signals from different transmitters. Amplitude Modulation When the modulating wave is superimposed on a high frequency carrier wave in a manner that the frequency of a modulated wave is same as that of carrier wave but amplitude of carrier wave is modified in accordance with the modulating wave, the process is called amplitude modulation. Amplitude-modulated wave 33. The circuital arrangement for a modulator should be such that carrier wave should be mixed with the modulating signal. The output will be obtained through the LC filter. Need for modulation Refer to Ans. 22. (2) Figure shows the amplitude modulation using a sinusoidal signal as the modulating signal. C Rs Am cos romt nput (modulating... - signal) V\ t JMOutput : : Vcc modulated wave 34. (i) To radiate the signals with high efficiency. (ii) Because they are of large wavelength and power radiated by antenna is very small as p ee l()..4. (ill) t is so to avoid making over modulated carrier wave. n that situation, the negative half cycle of the modulating signal is dipped and distortion occurs in reception. (1 x 3 = 3) 35. When we are drawing waveform for AM wave the amplitude variation of carrier wave should be considered and also that of modulating signal. Need for modulation Refer to Ans. 22. (2) 2 Carrier wave /\ /\ ~\J Modulating sine-wave signal O~~~~F+~~~+4~

24 Value Based Questions (From Complete Chapter) 1. Anuj's mother was having constant headaches. After a medical checkup, she was diagnosed with tumour. Anuj realised there was a telecommunication tower very close to their house. He enquired from the doctor if the radiation from the tower could have caused the tumour. As the doctor supported his anxiety, he lodged a complaint with the police and ultimately succeeded in getting the tower removed to a distant place away from the residential colony. (i) What values were displayed by Anuj? (ii) Anuj made a rough estimate about the height of the antenna to be about 20 m from the ground. Calculate the maximum distance up to which radiations from the tower are likely to reach. Use the value of radius of the earth = 6400 x 10 3 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 10 3 m We know that, d = -J2JlR = ~r2-x-2-0-x-6-.4-x-o-6 ~ d=16km (2) 2. When Sunita, a class X student, came to know that her parents are planning to rent out the top floor of their house to a mobile company she protested. She tried hard to convince her parents that this move would be a health hazard. Ultimately her parents agreed. (i) n what way can the setting up of transmission tower by a mobile company in a residential colony prove to be injurious to health? (ii) By objecting to this move of her parents, what value did Sunita display? (iii) Estimate the range of electromagnetic waves which can be transmitted by an antenna of height 20m. (Given, radius of the earth = 6400 km) Delhi 2014 Ans. (i) A transmission tower transmits electromagnetic waves such as microwaves, exposure to these waves can cause severe health hazards like cancer and tumour. Also transmission tower (antenna) works on a very high power, so the risk of someone severely gets burnt increased in residential area. (2) (ii) Sunita has displayed awareness towards the health and environment of society by objecting to this move of her parents. (ill) Here, R = 6400 km = 64 X10 5 m; h= 20m, d=? d=..}2 hr = ~2 x (20) x (64 x 10 5 )= ~4 x 64 x 0 6 ~ d = m 3. Arnab was talking on his mobile to his friend for a long time. After his conversation was over, his sister Anita advised him that if his conversation was of such a long duration, it would be better to talk through a land line. (i) Why is it considered harmful to use a mobile phone for a long duration? (ii) Which values are reflected in the advice of his sister Anita? (iii) A message signal of frequency 10 khz is superimposed to modulate a carrier wave of frequency 1 MHz. Determine the sidebands produced. All ndia 2014C Ans. (i) The ultra high frequency of EM radiations continuously emitted by a mobile phone, may harm the system of the human body. (ii) Anita shows the following values. (a) Concern about her brotherlcommand (b) Awareness about the likely effects of electromagnetic radiations on human body. (ill) Given, fc = 1000 khz, 1m = OkHz The side bands are (fc + 1m) and (fc - 1m) or (l )and (l000-10) khz = 10O and 990 khz (2) 4. Mohan went to the market to purchase a TV set. He got confused with so many features and functions of electronic appliances. He took the help of his friend Sohan, a science student. Sohan explained him about the communication system,

25 CHAPTER 15 : Communication System 505 digital and analog signals. This knowledge proved of great help to Mohan in purchasing a colour TV. (i) What do you mean by the term comm 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 for global communication coverage? Ans. (i) A set up that transfer information implicitly from one point to another is called communication system. Analog signal is better. A signal in which current or voltage continuously vary with time. [1+1) (ii) Sohan is an intelligent and knowledgable person has good command over subject. [1) (iii) There are three communication satellites required for global communication coverage.[1) 5. Group discussion was arranged in class X on the topic of atmosphere. Three groups were made. Teacher asked the question "why can Moon be not used as a communication satellite?" Answers were given by all the three groups. Each group can give only one reason. Teacher told them that reason given by each group is ' correct. The groups collected all the three reasons and came to correct conclusion. (i) Give the correct reason for the above question. (ii) What values were showed by all the three groups? Ans. (i) Moon is a natural satellite of the earth but it is not an active satellite as it carries no electronic equipments for receiving, amplifying and transmitting the signal back to the earth. Moon cannot be used as a communication satellite because (a) the distance between the earth and the moon is not proper. (b) the period of revolution of the moon is not 24 hr. (c) the moon is not revolving in the equitorial plane of the earth. [2) (ii) The three groups showed the values of eagerness to engage in a debate, promptness in taking decision and ability to work in a group in a constructive manner. [2) 6. n the famous conversation, Rakesh Sharma, the first ndian Astronaut in space, was asked by the Prime Minister ndira Gandhi as to how ndia looked from space. To which he replied, 'Sare Jahan Se Achcha' (better than the whole world). Answer the following questions based on above passage: (i) Which scientific mode of communication enabled the Prime Minister to speak to the Astronaut? (ii) Name the scientific values displayed in this anecdote. (iii) Which values are being reflected in the reply given by the astronaut? (iv) Give one more example of this scientific mode of communication in everyday life situations. Ans. (i) Radio wave communication system. (ii) Use of scientific and technological advancement in service to mankind. Human passion for continued progress and advancement. (iii) Patriotism of mind. and love for the country, presence (iv) Television communication system. [1-4) 7. Raj was cycling from his village to a neighbouring village. On the way, he met an old man who was walking in the direction of the same neighbouring village. Raj invited the old man to sit on,qis cycle and took him to the neighbouring village. The old man got down from the cycle, thanked Raj and gave his blessings. (i) What according to you, are the values displayed by Raj? (ii) Can you relate the given paragraph with the process of modulation? Ans.(i) Raj showed concern for the stranger and respect for the old age of the person. [1) (ii) Just as the old man sat on the cycle, the message signal is superposed on the carrier wave. The modulated carrier wave travels some distance. The message signal is then recovered from the modulated wave in the same way as the old man got down from the cycle. [1+1+1)