Chapter 15 Communication Systems fotreexecnione © Introduction Introduction + Elements of ‘The act of transmission and reception of information is known as Communication System ‘communication. Every living creature in the world experiences the need to impart or receive information almost continuously with others Baste Terminology Used in ation Cone eaten sn the surrounding world. For successful communication. itis essential faieme that the sender and the receiver understand a common language. Languages and methods used in communication have changed from © Band Width of Signals prehistoric to modern times, to meet the growing demands in terms of © Band Width of Transmission speed and complexity of information. It would be useful to look at the Mectuan. major milestones In events that promoted developments in + Propagation of lectromagnetc Waves “The aim of this chapter Is to introduce the concepts of communication, namely the mode of communication, the need for modulation, production and deduction of amplitude modulation, communications. © Modulation and its Necessity © Amplitude Modulation Ee of Arpitade ELEMENTS OF A COMMUNICATION SYSTEM Modulated Wave Irrespective ofthe nature, every communication system has three essential , °e epamcsl clements, transmiter, medium (also known as channel) and receiver Modulated Wave Communication System BS cern en ; : A rca oe = — Sl fo Fig. : Block diagram of a gor ralised communication system. ‘Aakash Educational Services Pvt. Lid: - Rood) Ofico Aakash Tow, 6, Puse Rood, Now Dafi-110008 Ph 017-47623456)B12 Communication Systems. Board & Competitive Exams. In a communication system, the transmitter is located at one place, the receiver Is located at some other place (far or near) separated from the transmitter and the channel is the physical medium that connects them. Depending upon the type of communication system, channel may be in the form of wires or cables: connecting the transmitter and the receiver or it may be wireless. The purpose of the transmitter is to convert the message signal, produced by the source of information, into a form, suitable for transmission through the channel. If the output of the information source is a non-electrical signal like a voice signal, transducer conver it to electrical form before giving it as an input to the transmitter. When a transmitted signal propagates along the channel it may get distorted due to channel imperfection. Moreover, noise adds to the transmitted signal and the receiver receives a corrupted version of the transmitted signal. The receiver hhas the task of operating on the received signal. It reconstructs a recognisable form of the original message signal for delivering it to the user of information “There are two basic mades of communication : point-to-point and broadoast In point-to-point communication mode, communication takes place over a link between a single transmitter ‘and a receiver, Telephony is. an example of such a mode of communication, In contrast, in the broadcast ‘mode, there are a large number of receivers corresponding to a single transmitter. Radio and television are ‘examples of broadcast mode of communication. ‘Modern communication has its roots in the 19" and 20" century in the work of scientists like JC ‘bose, FB Morse, G.Marconi and Alexander Graham Bel. Table 1 : Some major milestones in the history of communication Year | Event Remarks ‘Around | The reporting of the delivery of a | Itis believed that minister Birbal experimented| 4565.0. | child by queen using drum beats | with the arrangement to decide the number of| fromadistantplace to KingAkbar.” | drummers posted between the place where| the queen stayed and the place where the king stayed, 1035, Invention of telegraph by Samual | It resulted in tremendous growth of messages| FB. Morse and Sir Charles | through post offices and. reduced Wheatstone travel messengors considerably 1876 ‘Telephone invented by Alexander | Perhaps the most widely used means of GrahamBellandAntonia Meuce!” | communication in the history of mankind, 1895 Jagadis Chandra Bose and | It meant a giant leap — from an era off ‘Gigjicimo Marcon! demonstrated | communication using wires to communicating ‘wirelesstelegraphy. ‘wathoutusing wires. (wireless) 1936 Television broadcast (John Logi | First television broadcastby BBC. Baird) 1955 First racio FAX transmitted across | The idea of FAX transmission was patented by continent. (Alexander Bain) ‘Atexander Bainin 1843. 1968, ARPANET: the fist intomet came | ARPANET was a project undertaken by the| intoexistenceJ.C R.Licklider) | U.S. defence department. It allowed. fle ‘transfer from one. computer to another ‘connectedto the network 1975 Fiber optics developed at Bell | Fiber optical systems are superior and more| Laboratories economical compared. to traditional ‘communication systems 1989.91 | Tim Bemers-Loe invented the | WWW may be regarded as the mammoth| World Wide Web. ‘encyclopedia of ‘knowledge accossibie to ‘everyone round the clock throughout the year. ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delh-110005 Ph. 011-47623456Board & Competitive Exams. Communication Systoms 413 BASIC TERMINOLOGY USED IN ELECTRONIC COMMUNICATION SYSTEMS ‘To understand the principles of communication, we must know some basic terms which will be used frequent. o w co) ow “ wi ‘Transducer : ‘a device that converts one form of energy into another. In electronic communication systems, we usually come across devices that have either their inputs or outputs in the electrical form. ‘An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, etc.) into corresponding variations in the electrical signal at its output. ‘Signal : It is the Information converted in electrical form and suitable for transmission. Signals can be either anaiog or digital. Analog signals are continuous variations of voltage or current. They are essentially Single-valued functions of tine. Sine wave is a fundamental analog signal. All other analog signals can be fully understood in terms of their sine wave components. Sound and picture signals in TV are analog in nature. Digital signals are those which can take only discrete stepwise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. ‘0’ corresponds to a low level {and "1' corresponds to a high level of voltagelcurrent. There are several coding schemes useful for digital communication. They employ suitable combinations of number systems such as the binary coded decimal (BCD). American Standard Code for Information Interchange (ASCII) is a universally popular digital code to represent numbers, letters and certain characters. Noise : The unwanted signals that tend to disturb the transmission and processing of message signals in a communication system is known as noise. The source generating the noise may be located inside oF outside the system ‘Transmitter : It processes the incoming message signal so as to make it suitable for transmission through a channel and subsequent reception. Receiver : It extracts the desired message signals from the received signals at the channel output. Attenuation : Its the loss of strength of a signal while propagating through a medium, (vil) Amplification : Amplification refers to the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier. Amplification is necessary to compensate for the attenuation of the signal in communication systems. The energy needed for ‘additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength. (ulti Range : Its the largest distance between a source and a destination up to which the signal is received wo w ‘with sufficient strength Bandwidth : tis the frequency range over which an equipment operates or the portion of the spectrum ‘occupied by the signal Modulation : The original low frequency message/information signal cannot be transmitted to tong distances because of obvious reasons. Therefore, at the transmitter, information contained in the low frequency message signal is superimposed on a high frequency wave, which acts as a carrier of the Information. This process is known as modulation. There are several types of modulation, abbreviated as AM, FM and PM, ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi 110005 Ph. 011-47623458a4 Communication Systems Board & Competitive Exams. (Xi) Demodulation : Its the process of extraction of information from the carrier wave at the receiver. It is the reverse process of modulation. (xil) Repeater : It is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in cartier frequency. Repeaters are used to extend the range of a communication system as shown in figure. A. communication satelite is essentially a repeater station in space. “The undesirable effects in the course of signal Wanamission are () Attenuation : decrease in signal strength due to energy loss. (Distortion : waveform perturbation, (i) Interference = contamination by extraneous signals. (0) Nolse : due to random electrical signal. BAND WIDTH OF SIGNALS ‘The message signal can be voice, music, picture or computer data. In a communication system, each of these signal has different ranges of frequencies. The types of communication system needed for a given signal depends on the range of frequencies which is considered essential for the communication process. For speech signals frequency range has a band width of 2800 Hz (3100 Hz ~ 300 Hz) for commercial telephonic communication. To transmit music an approximate band width of 20 kHz is required because of high frequencies produced by the musical instruments. The audible range of frequencies extends from 20 Hz to 20 kez Video signals for transmission of pictures require about 4.2 MHz of band width. A LV. signals contains both voice and picture and is usually allocated 6 MHz of band width for transmission. ‘The transmission medium or communication channel is link through which information/message signal may propagate from the source to the destination, without any noise or distortion, Its a sort of electronic roadways along which signals travel. ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph. 011-47623456,Board & Competitive Exams. Communication Systeme 415) Jagadis Chandra Bose (1858-1937) He developed an apparatus for generating lltrashon electromagnetic waves and studied their optical properties, Ho was said to be the frst to employ a semiconductor like galena as a self recovering detector of electromagnetic waves. Bose published three papers in the British magazine, “The Electrician’ of 27 Dec. 1895. His invention was published in the ‘Proceedings of The Royal Society’ Con 27 April 1899 over two years before Marcon's first wireless communication on 13 Decomber 1901. Bose aleo invented highly sensitive instruments for the detection of minute responses by living organisms, established and similarity between animal and plant tissues. Broadly, transmission media have been divided into two types: 1, Guided transmission medium 2. Unguided transmission medium. Guided transmission medium : Its that communication medium or channel which is used in point to point communication between a single transmitter and a receiver. For example, parallel wre line, twisted pair and co-axial cable are guided transmission media. Optical fibres are other examples of guided transmission medium. Thus, guided transmission medium is used in line communication. Unguided transmission medium : Its that communication medium which is used, where there is no. point to point contact between the transmitter and receiver. There are a large number of receivers. Corresponding to a single transmitter. Free space is an example of unguided transmission medium. It is Used in space communication and satellite communication, such as in radio and television. The characteristics and quality of transmission medium depends upon (© Nature of transmission medium (W) Nature of signat We have already studied that there are different band widths for different message signals. In fact different transmission media for communication of signals offer different band widths. The commonly used transmission media are wire, free space and optical fibre cable. Co-axial cable is a widely used wire medium. It offers a band width of 750 MHz. These cables are generally operated below 18 GHz ‘Communication through free space using radiowaves occurs at frequencies ranging from 10° Hz to 10? Hz. This range is subdivided further and allocated for various services. ‘The optical fibre communication is used in the frequency range of 1 THz to 1000 THz (microwaves to Ultraviolet). An optical fibre can offer a transmission band width in excess of 100 GHz. Knowledge ~~ Did You Know? {In a digital signal, information is carried by the patter of pulses and not by the shape of pulses. ‘Aakash Educational Services Pvt. Lid. - Regd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph. 011-4762345616 Communication Systems. Board & Competitive Exams. BAND WIDTH OF TRANSMISSION MEDIUM Different types of transmission media offer different bandwidths similar to massage signals, The commonly Used transmission media are wire, ffee space and fiber optic cable. Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz. Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. This range of frequencies is further subdivided and allocated for various services as indicated in Table 2. An optical fiber can offer a transmission bandwidth in excess of 100 GHz. Table-2 : Some important wireless communication frequency bands 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 wv 174-216 MHz LUE (ultra high frequencies) 420-890 MHz Ww Cellular Mobile Radio 896-901 MHz Mobile to base station 840-035 MHz Base station to mobile Satellite Communication | 5.925-6.425GHz | Uplink 37-42 GHz Downlink We may define message signal as a single valued function of time that conveys the information. Such a function has a unique value at any instant of time. The message signals have to be converted into electrical signals to make them suitable for transmission through the channel. As explained already, we use transducers: for the conversion, ‘lectrical signals are of two types 1. Analog signals 2. Digital signals, 1. Analog sign: with time, In the simplest form of an analog signals, amplitude of the signal varies sinusoidally with time, as shown in figure. Itis represented by the equation +: An analog signal is that in which current or voltage value varies continuouslyBoard & Competitive Exams. Communication Systems 417 were 6 maximum valu of vote, caled the ample, tne para and = 2 is anger teqncy of he agra ard egrecert the pha ania, Buch spain cxt havea sr of vara Sirens tanta thew voce thal aman suse tang’ as msm wae (0 6 and a ina wl €-69 Examples of analog signals are speech, music, sound produced by a vibrating tuning fork, variations. in light intensity from a picture, These are converted into currentvoltage variations using suitable transducers as mentioned. For speech signals, frequency range is from 300 Hz to 3100 Hz. Thus band ‘width of speech signals = 3100 Hz ~ 300 Hz = 2800 Hz, ‘As is known, the audible range of frequencies is from 20 He to 20 kH2. As the frequencies produced by musical instruments are high, therefore, approximate band width for music is 20 kHz For transmission of pictures, video signals require a band width of 4.2 MHz. T-V. signals contain both, voice/music and picture signals. Therefore, band width allocated for transmission of TLV. signals is 6 MHz. 2. Digital signals : A digital signal is a discontinuous function of time, in contrast to an analog signal, ‘wherein current or voltage value varies continuously with time. ‘A typical digital signal is shown in figure. Such a signal is usually in the form of pulses. Each pulse has two levels of current or voltage, represented by 0 and 1. Zero (0) of a digital signal refers to open ireult and (1) of a digital signal refers to closed circult. Zero (0) is also referred to as "No" or space tnd (1) is referred to as "Yes' or mark. Both 0 and 1 are called bits. Pulse Beil ee 1 1 1 t Ipuse] | Purse Puse Rise a ‘Arpitude NV + Crane o o_o Time ‘The significant characteristics of a digital signal are: pulse amplitude; Pulse duration or pulse width and pulse position, representing the time of rise and time of fall ofthe pulse amplitude, as shown in figure. Examples of digital signals are ()loters printed in a book, (i listing of any data, (i) output of a digital computer, (iv) electronic transmission of a document at a distant place via telephone line 16, Fax etc. Note that for computer data, the bandwidth allocated is about 600 MHz ‘An analog signal can be converted suitably into a digital signal and vice-versa, ‘There are several coding schemes useful for digital communication. For example (Binary coded decimal (&CD) in which a digit is represented by two binary bits (0 or 1). (i) American standard code for information interchange (ASCII) is a universally popular digital code to represent numbers, letters and certain charactors.1B Communication Systems. Board & Competitive Exams. PROPAGATION OF ELECTROMAGNETIC WAVES ‘An antenna at the transmitter in communication using radio waves, radiates the electromagnetic waves which travel through space and reach the receiving antenna at the other end. Several factors affect the propagation of em waves and the path, they follow. i <7 Radio waves, visible light rays, ultraviolet rays, infra-red rays, X-ray, rays etc. are ALL em waves. ‘The energy of a photon of em wave is calculated by formula E= "©, where h = 6.6 x 10 Js, © = 3 x 10 ms". Taking 2 for visible ight as 5000 A (on average), 5 A for X-rays (on average) and 0.005 A for y-rays (on average), the value of X-rays photon energy is.~ keV, visible light photon energy ~ eV and ‘prays photon energy ~ MoV Ground Wave The antennas should have a size comparable to the wavelength ?. of the signal (at least ~ 2/4) to radiate signals with high efficiency. At longer wavelengths (Le., at lower frequencies), the antennas have large physical size and they are located on or very near to the ground. In standard AM broadcast, ground based vertical towers are generally used as transmitting antennas. For such antennas, ground has a strong influence on the propagation of the signal. The mode of propagation is called surface wave propagation and the wave glides: over the surface of the earth. A wave induces current in the ground over which it passes and itis attenuated 8 a result of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with Increase in frequency. The maximum range of coverage depends on the transmitted power and frequency (less than a few MHz). Table-3 : Different layers of atmosphere and their interaction with the propagating electromagnetic waves aaa ny [Eee ‘stratum (layer) over earth's surface affected renee = Cay encom | Vir upto several rs) Oper rr Goreng eee | ecaeaslun tees i ce cea courer os a |e Sse Sher) rates eae Se Se Mesosphere) | win Fat right | yet allowing them to reach recreronctee|# | soownamse | peysranot | emconty retacie, HE ae Seen eres Se ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph. 011-47623456,Board & Competitive Exam: Communication Systems (419) ‘Sky waves Long distance communication can be achioved by ionospheric reflection of radio waves back towards the earth. ‘Sky wave propagation and is used by short wave broadcast services. The ionosphere is so called because Of the presence of a large number of ions or charged particles. It extends from a height of ~ 65 km to about 400 km above the earth’s surface. lonisation occurs due to the absorption of the ultraviolet and other high- ‘energy radiation coming from the sun by air molecules. The ionosphere is further subdivided into several layers, the details of which are given in Table 3. The degree of ionisation varies with the height. The density of ‘atmosphere decreases with height. At great heights, the solar radiation is intense but there are few molecules 10 be ionised, Close to the earth, even though the molecular concentration is very high, the radiation intensity {s low 80 that the lonisation is again low. However, at some intermediate heights, there occurs a peak of ionisation density. The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 20 MHz penetrate the ionosphere and escape. These ‘phenomena are shown in the figure. The phenomenon of bending of em waves so that they are diverted towards the earth is similar to total internal reflection in optics F lonospheric F, Fig. : Sky wave propagation. The layer nomenclature is given in Table 3 7 —— ee ‘Total internal reflection occurs when 1. Light rays travel from denser medium towards rarer medium. 2. Angle of incidence is greater than the critical angle ‘3. _Crtical angle depends upon the media combination Phenomenon of mirage is based upon total internal reflection of light waves. It occurs due to variation in refractive index of different atmospheric air layers. ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi 110005 Ph, 011-47623458420 Communication Systems. Board & Competitive Exams. ‘Space wave ‘A space wave travels in a straight line from transmiting antenna to the receiving antenna. Space waves are Used for line-of-sight (LOS) communication as well as satelite communication. At frequencies above 40 MHz, ‘communication is essentially limited to line-of-sight paths. At these frequencies, the antennas are relatively ‘smallor and can be placed at heights of many wavelengths above the ground. Because of line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth as ilustrated in Figure. If the signat is to be received beyond the horizon then the receiving antenna must be high enough to intercept the line-of-sight waves. «~<—$ $$ 4, —_________.. i. 4, ——_. Fig. : Line of sight communication by space waves: 11 the transmitting antenna is at a height f, then it can be shown that the distance to the horizon d, is given as dy = /2Rhr . where Ris the radius of the earth (approximately 6400 km). d, is also called the radio horizon of the transmitting antenna, With reference to figure, the maximum line-of-sight distance dj, between the two antennas having heights fy and Ng above the earth is given by yy = JOR + JPRhig where hp isthe height of receiving antenna Television broadcast, microwave links and satelite communication are some examples of communication systems that use space wave mode of propagation. Figure summarises the various modes of wave propagation discussed 80 far. ‘Communication satelite Space wave LosBoard & Competitive Exams. ‘Communication Systems 421 Determination of range “The range is dotermined by the height of transmitting antenna, The range AP or PB can be easily calculated by geometrical consideration. Suppose height of the tower is and the radius of earth is r (hat Is OA = OB = OP = 7). In the right-angled tangle OQA, we have OG? = Qa? + Oo”? QA=ap=a Therefor, (Ronee Ree Ree b+ 2Rn= Ree Ree f= 2Rh a= V2Rh For a height of 500 m, d= 80 km Hence maximum line-of-sight distance between two antennas will be calculated as follows, ¢, ———+ jy : Maximum line-of-sight distance between the two antennas d,,: Radio horizon of transmitting antenna th Height of transmitting antenna in : Height of receiving antenna Example 1: A transmitiing antonna at tho top of a tower has a helght 32 m and the height of the receiving ‘antenna is 50 m. What is the maximum distance betwaen them for satisfactory communication in the line of sight mode. Given radius of earth 6.4 » 10° m. Solutions: = /2Rh, + /2Rhg = \2x64 x10 232 + \2x64 x10" x50 = 64x10? x V10 + 8x10? x 10 m = 144 x10? x J10 m= 48.5 km 4. Atranemiting antenna a the top of tower 32 m and height of receiving antenna is 200 m. What ‘is the maximum distance between them for satisfactory communication in LOS mode? Radius. of earth 6.4 = 10° m. ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph, 011-47623458422 Communication Systems. Board & Competitive Exams. MODULATION AND ITS NECESSITY ‘As already mentioned, the purpose of a communication system is to transmit information or message signals. Message ‘signals are also called baseband signals, which essentially designate the band of frequencies representing the original signal, as delivered by the source of information. No signal, in general, Is a single frequency sinusoid, but it spreads over a range of frequencies called the signal bandwidth, Suppose we wish to transmit an electronic signal in the audio frequency (AF) range (baseband signal frequency less than 20 kHz) over a long distance direcly. Let us find what factors prevent us from doing #0 and how we overcome those factors, ‘An antenna plays a vital role in a communication system. Its used in both, the transmission and reception Of radio frequency signals. Infact, an antenna is a structure that is capable of radiating electromagnetic waves or receiving them, as the ‘case may be. Basically, an antenna is generally a metalic object, often a wire or collection of wires, used {0 convert high frequency current into electromagnetic waves and vice-versa, Thus, a transmitting antenna ‘converts electrical energy (high freq. current) ino electromagnetic waves, whereas a receiving antenna converts ‘electromagnetic waves into electrical energy (high frequency current). Apart from their diferent functions, ‘ransmiting and receiving antennas behave identically. When a transmitting antenna is held vertically, the electromagnetic waves produced are polarised vertically. ‘When the same antenna is held horizontally, the em waves produced are polarized horizontally. ‘A Hertz antenna is a straight conductor of length equal to half the wavelength of radio signals to bbe transmitted or received Le., != 7/2. This antonna is not grounded. ‘A Marconi antenna is a straight conductor of length equal to a quarter of the wavelength of radio signals to bbe transmitted or received /., = 1/4. itis held vertically with its lower end touching the groud. The ground ‘provides a reflection of the voltage and current distibutions set up in the antenna. The electromagnetic waves. ‘emitted from (Marconi) antenna ground system are the same as those emitted from Hertz antenna, which is not grounded. “The design of an antenna depends on frequency of carrier wave and directivity of the beam etc. Two common types of antenna are : 1. Dipole antenna, shown in fig(a) is used in transmission of radio waves. it is omni directional. 2. Dish type antenna, shown in fig.(b) is a directional antenna. ‘Such an antenna has a parabolic reflector with an active element, called the dipole or horn feed at {focus of the reflector. The dish type antenna can transmit waves in a particular direction. Also, it can rocoive only those waves which are directed towards it. For transmission, the signal is fed to the active ‘element, which directs it on to the reflector. The signals is then transmitted in the form of a parallel beam ‘as shown in fig. (b). For reception, the waves directed towards the dish are reflected on to the active ‘element, which converts them into electical signals. “The dish type antenna are used commonly in radar and satelite communication, ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delh-110005 Ph. 011-47623456Board & Competitive Exams. Communication Systoms 423 1 Size of the antenna or aerial : An antenna or aerial is needed, both for transmission and reception. Each antenna should have a size comparable to the wavelength of the signals, (at least 2/4 in size), ‘0 that time variation of the signal is properly sensed by the antenna, For an audio frequency signal of frequency v The lena of the antenna = * = 2000 5000 mote, To set up an antenna of vrcal height 5000 metre is practically impossible to construct and operate. iwanomision frequency were raised 161 MHZ, then 2 = © ‘The length of antenna would be 300/4 = 75 m, which is reasonable. Therefore, there is an urgent need Cf converting the information contained in our original low frequency base-band signal into high or radio frequencies before transmission. Soe ae eee setenna tung propertonat tte, (2) ‘As high powers are needed for good transmission, therefore, for given antenna length, wavelength i should bbe small or frequency v should be high ‘Thus, this factor also points out to the need of using high frequency transmission. Mixing up of signals from different transmitters. When many people are talking at the same time, we just cannot make out who is talking what. Similarly, when many transmitters are transmitting baseband information signals simultaneously, they get mixed up and there is: no way to distinguish between them. The possible solution is, communication at high frequencies and allotting a band of frequencies to each transmitter so that there is no mixing. This Is what is being done for different radio and T.V. broadeast stations. {Al the three reasons explained above suggest that there is a need for transmission at high frequencies. This is achieved by a process, called modulation. Modulation is the phenomenon of superimposing the low audio frequency baseband message or information signals (called the modulating signals) on a high frequency wave (called, the carrier wave). The resultant wave Is called the modulated wave, which Is transmitted. Example 2: Show that the minimum length of antenna required to transmit a radio signal of requency 10 MHz Solution : ie 75m, 2210" 50m $739 =10MH2=107H2, Misr gt of antenna = 2: = 7.5 Try Yourself 2.__Find the minimum length of antenna used to transmit a radio signal of frequency of 30 MHz. ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph, 011-47623458(Frequency modulation (FM) and Ww) PrBoard & Competitive Exams. Communication Systoms 425 AMPLITUDE MODULATION “The amplitude of the carrier wave is varied in accordance with the amplitude of the audio frequency ‘modulating signal. However, the frequency of amplitude modulated wave remains the same as that of the ‘camer wave, ‘To explain amplitude modulation, let us take a sinusoidal modulating signal, represented by m0) = Axsin gf a where A,, is amplitude of modulating signal, and ©, ‘This is ropresented in fg. (a) Lot the sinusoidal wave be represented as ett) = Asien @ where A, is amplitude of carrier wave, and e, = 2 vis angular frequency of carior wave. This is represontedtin fig.(b) Ko) T ° (@) Modulating Signal ao t e = (©) Carior Wave (A.+A, sino0, top envelope) 7 | A A Aw ', | (©) Amplitude Modulated Wave ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi 110005 Ph. 011-47623458426 Communication Systems. Board & Competitive Exams. In amplitude modulation, the amplitude of modulated wave ¢,() Is (A, + Ay sini,f). AS its frequency fe unchanged (= 0,), therefore, we represent amplitude modulated wave as: Galt) = (A, + Ay sine sin @ = A(1+ 422 sinogt} sino, = (1+ 42 sno gt) ino,t ut [An put [Je =i where jis called amplitude modulation index enll) = Asinod + WAsinagh. ina “) In practice, the value of jis kept < 1 to avoid distortion. Often, we represent iin percent. The amplitude modulated wave is shown in fig. (c) Using the trigonometric relations sinAsine = Nc08(A-)~c09(A +B). We get rom (4 calls Arnagt+ A sfeostng og C0805 +m) (8) E-qustion (5 show that the ample moda stn const of te carer wave of Fequency piso sinusoidal waves, one of frequency (0, —«,) and other of frequency (0, + ©,). These two additional waves are called side bands. Their frequencies are called side band frequencies. (eee =, = Om © Frequency of lower side band, [ise = @.— ” land frequency of upper side band, ® Pn ane enn Ee =e ae (044 Op) ~ (04~ O) ee Le. Band with = ce the equenoy of modulating sina The frequency spectrum of amplitude modulated wave is shown in fig. The two side band frequencies have equal amplitude (= 1 A/2), which never exceeds half the carrior Co) % ©.) amplitude. (° 4 1) —+ o(raass) ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph. 011-47623456,Board & Competitive Exams. Communication Systems 427 Power and current relation in AM wave ‘Average powerlcycie in the unmodulated carrier wave is a [Pe = 351 (10) where R is resistance (of antenna) in which power is dissipated. It can be shown that total powerfcycte in the modulated wave is 78) = Pel te ay42B_ Communication Systems. Board & Competitive Exams. PRODUCTION OF AMPLITUDE MODULATED WAVE ‘One of the simplest methods for producing amplitude modulated wave is shown as a block diagram in fig. Modulating signal m(), cy (0, Comier wave 0, Lot the modulating signal be represented by KE) = Aging and the carior wave be represented by 0) = A,sinegt ‘When the modulating signal is added to the cartier wave, let the signal produced be X10 = Aysinogt + Asinog 13) Let this signal be passed through a square law device, which is a non linear device. Let it produce an output AO = Bat) + COOP «ay where B and C are arbitrary constants, Using equation (13), we get 10 = BIA, sinogf + Aino! + CA,siningt + Aino ff = BIA,sinogf + Asino,| + CIAZSiN ont + AP sin? gt + 2AyAe Sine int] Using the trigonometrical relations aaa 328 2 od sinAsinB= 3 foouA-8)-couA +B Wo got 10 = Bay sigt + BA sina.t SUAS © A2)-L A co829t EAB cos2ot + CAyA. COS ~ In this equation, there is a dic. term S(A% + AZ) and sinusoidal waves of frequency ing: 2i%q Wy 20. ( ‘This signal is passed trough Band pass filer centered at o,, Such a fiter rejects low and high frequencies, and alow a particular band of frequencies to pasa. In this case, the filer rejects dic. and the sinusoids of frequencis 1, 2 and 20,. The frequencies passed are m, (0) and (0, +0). The output ofthe band pass fiter, therefore, contains waves of frequencies, «, and (0%,— jy) and (2, + @,). This Is an amplitude modulated wave. and (01, 6) ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph. 011-47623456,Board & Competitive Exams. Communication Systems 429) Note that the amplitude modulated signal so obtained cannot be transmitted as such. It is to be amplified fist ‘and then fed to an antenna of appropriate size for radiation. The block diagram of a transmitter is shown in 0: (@) Transmiting “Antenna Fimpitade] _Medvleted (Power y Meee eer a wave L_Ambiior Carrer wave et) ‘The basic circuit of an amplitude modulator is shown in fig. : (b). It is essentially a common emitter amplifier for the carrier wave signal. The base biasing voltage, in this case, is the sum of d.c. and the ‘modulating signal. As the base biasing voltage changes, amplification changes. Therefore, output voltage will be a carrier wave varying in amplitude in accordance with the biasing modulating valtage. This 1s the ‘amplitude modulated wave. | “Amplitude = edad DETECTION OF AMPLITUDE MODULATED WAVE Demodulation is the reverse process of modulation, which Is performed in a receiver to recover the original ‘modulating signals, [At the receiving end, the signal is generally quite weak due to attenuation in the channel. Therefore, the receiver must amply the received signal fst. As the signal is usually accompanied by lots of other unwanted signals (noise), only the desired signal is selected and others are rejected by the receiver. Finally, ‘demodulation is performed in the recenver to recover the original modulating signals, ‘Thus a demodulator or a recever performs the folowing functions. (Selecting the desired signal and rejecting the unwanted signals. WAmpltying and demodulating the desired signal, (i) Displaying the original modulating signal in a desired manner. Figure (c) show the basic circuit of a demodulator, wherein a junction diode acts as the detector. ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi 110005 Ph. 011-47623458430 Communication Systems. Board & Competitive Exams. The input circuit consists of a tuned circuit comprising an inductor L and a variable capacitor C. Thus circuit Selects the desired amplitude modulated wave signal from the different signals picked up by the receiver antenna. When this wave is passed through junction diode, we obtain rectified modulated wave containing only positive haif cycles. This occurs due to rectifying action of the junction diode. Receiver Do on 'pn junction diode GO OHS Rectified Audio output modutatod'wave modulstod wave across ®) ua— [Rae] | peataes | [Emenee os sae | Bae | Comer” Fig. = (c) ‘The rectified output is then fed to the parallel combination of a capacitor C’ and a resistor R. The value of 1 1 2 chowon tat te reectnce (xg = 1.) tthe hgh eqtency canier wave alow The rectnce wi obviously be high for the low frequency modulating signal. Therefore, the capacitor C’ acts as a bypass for the cartier waves and audio frequency modulating signal voltage appears across R. This sends current through headphone, and the original speech or music is reproduced. A block diagram of a typical receiver is shown infg. (c). Receiving Antenna Output Received signal Fig. : (d) Note that IF stage represents intermediate frequency stage preceeding the detection. The carrier frequency is usually changed to a lower frequency by the IF stage. ‘The essential condition for demodulation is that time period of high frequency carrier wave must be much smatr than the tine constant of AC cout Je, T, << + or << where vg te the raquancy of r wave. Maximum modulated frequency could be detected by diode detector is ¢, - —t - ~ SaROR (= modulation index of AM wave) ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph. 011-47623456,Board & Competitive Exams, Communication Systems “431 Knowledge Cloud Fax (Facsimile) “The fax is the electronic transmission and reproduction of a document at a distant place. Its the most popular example of a digital communication system. Fax transmission usually involves tranamission of document or photograph. The different regions of the document to be transmitted are frst ‘scanned by a light source. ‘The scanner gives optical signals carrying the information regarding the writings, patterns, signatures etc. in different parts of the document. These optical signals are converted into electrical signals by a photodetector. ‘The electrical signals are coded and transmitted by some suitable communication method. Figure represents block diagram of transmission and reproduction of documents by Fax machines. Original Analog Document = Digital Signal ‘signal a Digital J signal eal ==] poe =o (b) Fax Machine at Receiver's end Did You Know? A modem acts as a modulator in transmission mode and as a demodulator in receiving mode. Additional Information = ‘The Internet : It is a system with bilions of users worldwide. It permits communication and sharing Of all types of information between any two or more computers connected through a large and network. It was started in 1960's and opened for public use in 1990's. With the passage Of time it has witnessed tremendous growth and it stil expanding its reach. Its applications include © Email ~ it permits exchange of textigraphic material using email software. We can write a letter land send it to the recipient through ISP's (Internet Service Providers) who work like the ispatching and receiving post offices ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi-110005 Ph, 011-47623458432 Communication Systems. Board & Competitive Exams. (@ Filo transfer — A FTP (Filo Transfer Programmes) allows transfer of files/software from one ‘computer to another connected to the Internet. (wi) World Wide Web (WWW) — Computers that store specific information for sharing with others provide websites either directly or through web service providers. Government departments, ‘companies, NGO's (Non-Government Organisations) and individuals can post information about ‘theie activities for restricted or free use on their websites. This information becomes accessible to the users. Several search engines like Google, Yahoo! etc. help us in finding information by listing the related websites. Hypertext is a powerful feature of the web that automatically links relevant information from one. page on the web to another using HTML (hypertext markup language). (@) E-commerce ~ Use of the Internet to promote business using electronic means such as using credit cards is called E-commerce. Customers view images and receive all the information about various products or services of companies through their websites. They can do on-line shopping from home/office. Goods are dispatched or services are provided by the company through mail/ courier. (Chat — Real time conversation among people with common interests through typed messages: is called chat. Everyone belonging to the chat group gets the message instantaneously and ‘can respond rapidly Mobile telephony : The concept of mobile telephony was developed first in 1970's and it was fully implemented in the following decade. The central concept ofthis system is to divide the service area Into a suitable number of colls centred on an office called MTSO (Mobile Telephone Switching Office). Each cell contains a low-power transmitter called a base station and caters to a large number Cf mobile receivers (popularly called cell phones). Each cell could have a service area of a few square kilometers or even less depending upon the number of customers. When a mobile receiver crosses the coverage area of one base station, It is necessary for the mobile user to be transferred to another base station. This procedure is called handover or handoff. This process is carried out ‘very rapidly, to the extent that the consumer does not even notice it. Mobile telephones operate ‘typically in the UHF range of frequencies (about 800-950 MHz). Example 4: Calculate the longth of half wave dipole antenna at 30 MHz. Solution : 10 Mizz = 30 = 108 He © _3xto'ms Vv" 20x10%s om Hence length of antenna 1 = Try Yourself 3. Calculate the length of half wave dipole antenna at 300 mez (3000 mz. 4. What inference do you draw from above result? ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi 110005 Ph. 011-47623456Board & Competitive Exams. Communication Systems 433 Example 5: An audio signal of 15000 Hz is modulating @ carrier generated by a tank circuit containing 1 pF ‘capacitor and 10 micro henry inductor. Calculate the frequencies of Tet pair of side bands. 15000 Hz, C= 1 pF = 10" F,L= 10% 10H Solution: Here, Vy 1 1 S = 1592 KHz. v, 'e == € Bell” 9.72 frOx10* 10" 1607 kbtz oF 1877 khtz Try Yourself 5. Find the amplitude of side bands in the above problem in terms of modulation index 4 and carrier wave amplitude A. Example 6 : A message signal of frequency 10 kHz and peak voltage 10 volt Is used to modulate a carrier of frequency 1 Mbtz and peak voltage 20 volt. Find the modulation index. Solution: Here, vq =10 kHz, A,, = 10 V, vo = 1 MHz = 1000 kHz A, = 20 6. Calculate the upper and lower side band in the above problem. Example 7: A Broadcast AM transmitter radiates 50 kW of carrier power. What will be the radiated power at {85% modulation? Solution : Here, P, = 50 KW, = 85% F,= P 080x009 f nf te] sof 2885088) _ oot kw Try Yourself 7. A400 W carer wave is modulated to a depth of 75%. Calculate the total power in the Peete ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi 110005 Ph. 011-47623458434 Communication Systems. Board & Competitive Exams. Example 8 : A TV broadcasting tower has a height of 100 m. How much population is covered by the TV Broadcast. i average population density around the tower is 1500 knr#? Radius of earth 6.37 = 10° m, 100 m, R= 6.37 x 108m, P= 1800 x 10 mr? Population covered = P x nd? = Px2Rh Solution = = 0010 22.2 100% 0.972108 = 0104 4, The maximum value of modulation index in case of amplitude modulation system is oa @ 4 @) 100 (4) tity 2. The process of changing some characteristic of a carrier wave in accordance with the intensity of signal to be transmitted is called (1) Amplification 2) Rectitication (8) Modulation (@) None of these 3, Modulation is done in (1) Transmittor (2) Radio receiver @) Communication channel (4) None of these 4. Am information signal of 150 Miz is to be sent across a distance of 40 km. Which mode of communication fs suitable? (1), Ground wave propagation (2) Space wave propagation @) Sky wave propagation (4) Satettte communication 5. For VHF television broadcasting, the frequency employed is generally (1) 54-72 Mite (@) 420 - 890 MHz 8) 540 - 1600 Kez @) 88 - 108 kHz 6. Line of sight communication is also known as (1), Ground wave communication (2) Space wave communication @) Sky wave communication (@) lonospheric communication 7. Digital signals, (1) Do not provide a continuous set of values 2) Utiize binary systems @) Ensures error and noise free communication (4) All of these 8. Which of the following is not used in analog communication? (1) PAM 2) AM @ pM (@) ASK ‘Aakash Educational Services Pvt. Lid. - Rogd. Office : Aakash Tower, 8, Puse Road, New Delhi 110005 Ph. 011-47623456