BHARAT SANCHAR NIGAM LIMITED

A Mini project report submitted

In partial fulfilment of the requirements for the award of the degree of

BACHELOR OF TECHNOLOGY

In

ELECTRONICS AND COMMUNICATION

ENGINEERINGBy

K. Shiva prashanth(06331A0483)

N.Rajsekhar Ch. Pavan Kumar (06331A0461) (06331A0451)

K.Krishna Rao(06331A0436)

Under the esteemed guidance of

Sri B.Sada Siva Rao M.E.[PhD]Associate Professor

Department of Electronics and communication EngineeringM. V.G. R College of Engineering

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

MAHARAJ VIJAYARAM GAJAPATHI RAJ COLLEGE OF ENGINEERING

(Affiliated to Jawaharlal Nehru Technological University, Kakinada)

VIZIANAGARAM

2006-2010

ACKNOWLEDGEMENTS

As a note of acknowledgement, with great solemnity and sincerity, we of-

fer our profuse thanks to Sri. B.Sada Siva Rao M.E.[PhD], Associate Pro-

fessor, Dept. of ECE, for guiding us through our project work, giving a right direc-

tion and shape to our learning by extending his expertise and experience in the

field of education. Really we are indebted to him for his excellent and enlightened

guidance.

We consider it our privilege to express our deepest gratitude to

Mr. M. Sunil Prakash, Associate Professor, Head of the Department, for his valu-

able suggestions and constant motivation that greatly helped the project work to

get successfully completed.

We also thank Dr. K.V.L. Raju, Principal, for extending his utmost sup-

port and cooperation in providing all the supervisions for the successful completion

of the project.

We sincerely thank all the members of the staff in the Electronics and Com-

munication Engineering department for their sustained help in our pursuits.

We thank all those who contributed directly or indirectly in the successfully

carrying out this work.

K. Shiva prashanth (06331A0483)

N. Rajsekhar (06331A0461)

Ch. Pavan Kumar (06331A0451)

K. Krishna Rao (06331A0436)

HISTORY OF RADIO COMMUNICATION

The radio as we know today had its beginning when Hein-rich Hertz (in whose honors is named the unit of frequency, namely, Hertz abbreviated Hz) experimentally produced electromagnetic waves in 1888 by inducing a spark between two electrodes. These waves were detected by him a few meters away. These waves were called Hertzian waves and remained a laboratory curiosity. Hertz’s motivation for his experiments was the work of James Clerk Maxwell who had derived his famous equation and predicted that a variable current in a conductor would produce electromagnetic waves in space and that these waves will travel with the velocity of light.

Guglielmo Marconi got interested in Hertzian waves and start experimenting in his father’s villa near Bologna. He transmitted wireless signals over tens of kilometers in 1895, seven years after the first demonstration of electromagnetic waves in space by Hertz. In the meanwhile several other inventions were being made which ultimately led Marconi to develop long distance wireless telegraphy. It was the fore-runner of radio and Marconiis wrongly called the ‘inventor of radio’. Ra-dio as we know it today, that is, audio broadcast by wireless was the cul-mination of many other inventions starting with the work of Thomas Alva Edison. Edison observed that if plates sealed in an incandescent lamp, which he himself had developed, were connected to the positive end of the filament through a galvanometer, current would flow. If the galvanometer lead was connected to the negative end of the filament, current flowed. This phenomenon was known as the Edison effect (1883). J J Thompson showed that this current was due to the travel of electrons from the filament to the plate (1899). Later J A Fleming patented two electrode vacuum tube as a detector of high frequency os-cillations through the rectifying action of the device. These discoveries laid the foundation of telecommunication and broadcasting. Marconi was not only a good scientist and inventor but was also good at exploit-ing the business potential of telecommunication. the death-knell of the ‘spark gap’ and the ‘alternator’ type of transmitters. David Sarnoff – a vi-sionary at the American Marconi Company was instrumental in giving a push to radio broadcasting. Voice was broadcast in the real sense of the word for the first time in 1915 in USA. World’s first radio broadcasting station was set up in USA followed by the second one in UK in 1922.

Radio Wave Propagation

The long distance transmission of electromagnetic waves (popularly known as radio waves) was explained by the discovery of ionized layers in the upper atmosphere which could serve as a reflect-ing surface for radio waves and confine the radiation to Earth. It was also established that the radio waves entering the ionized medium were bent. The amount of bending depended upon the degree of ionization and its gradient as well as upon the frequency of the incident waves. It could also be established that the intensity of electromagnetic waves emanating from a vertical quarter wave radiator and traveling along the surface of the Earth (ground waves) were attenuated which increased with the frequency of the wave and the electrical conductivity of the soil through which these waves travel. Long waves or LF (153-279

KHz) which are not used for the purpose of broadcasting in the tropical region, do not suffer diurnal and seasonal variations. The difference be-tween day and night field strengths becomes quite marked for medium waves or MF (531-1602 KHz). Threturn of the sky wave to Earth at night produces serious fading at nominal distances where these two waves are of comparable intensities. Short waves or HF (3-27 MHz) are eminently suite for long distance broadcasting within the country as well as over-seas. Propagation of HF is dependent on the vagaries of the ionized lay-ers, which sends them back to Earth and they are therefore unsuitable for listening pleasure. One of the reasons why the advantages of HF transmissions remained undiscovered until about 1922 was that exten-sive investigations which had been made on long distance transmission all showed that the attenuation of the signals increased rapidly as the frequency At the turn of the nineteenth century Marconi conceived the idea of wireless telegraphy and formed in 1897, the Wireless Telegraph and Signal Company for manufacturing wireless apparatus. Marconi’s system was adopted for ship to shore communication after he was suc-cessful in receiving a prearranged Morse code signal, which was sent from the far side of Atlantic in December 1901. The year 2001 was cele-brated as the centenary of the first transatlantic wireless transmission. Marconi type of ‘transmitters’ used a spark between two electrodes, which produced a small-scale discharge of energy resulting in crackles in headphones of a distant receiver. The spark between the electrodes was produced by a telegrapher’s key making it possible to communicate by a code. The transmitters were called the ‘spark transmitters’. Around the same time R A Fessenden devised another type of wireless transmit-ter equipment employing ‘alternator’ – an electromechanicaldevice which produced continuous waves (CW) of a single frequency rather than burst of energy as in the case of Marconi type of transmitters. The ‘CW transmitters’ could cover longer distances with lesser power. They could transmit and receive Morse code better. The first ever 1 KW con-tinuous wave transmitters operating at 42 KHz was amplitude modu-lated with the Morse code signal in 1902. J A Fleming’s invention of two element vacuum tube with rectifying properties in 1904 and the devel-

opment of three element vacuum tube triode by Lee De Forest which could perform the function of an amplifier, sounded was raised. It may be pointed out that the choice of frequency for communication over a given distance depends upon a number of factors. Radio waves with higher frequencies penetrate through ionosphere and escape from Earth. VHF/UHF or the space waves (30-300 MHz) travel as the crow flies and behaves like abeam of light. VHF/UHF waves are used extensively for communication purposes over distances up to 60 km. SHF or the mil-limeter waves (1-10 GHz) are considered best for communication from and to geostationary communication satellites. EHF(10-100 GHz), though used for satellite communication, are attenuated due to water va-por in the atmosphere.

The Radio Transmitter

Antennas help to radiate radio frequency energy (elec-tromagnetic waves) into space. They can be designed to radiate energy in all directions or in a particular direction. Radio wave at a constant fre-quency (called base band) has to be suitably modified to carry a message signal. Any one of the three characteristic of the varying sinusoidal wave (amplitude, frequency or phase) may be modified to carry the program-mer (voice or music)signals. The process of changing the characteristics of the radio wave is called ‘modulation’. The radio transmitter generates the radio frequency energy on which the voice or music rides ‘piggy-back’. The main components of the transmitter are the oscillator; the modulator and the power amplifier. The oscillator generates the contin-uous waves, one of the three characteristics(amplitude, frequency and phase) of which is changed by the modulator. The energy level of the modulated continuous wave is raised to the desired level by the power amplifier which pumps its output into the antenna which radiates the ra-dio frequency energy into space. At the receiving end, the signal is ‘de-modulated’ and the original voice or music signal is obtained. The earli-est receivers called crystal receiving sets were based on the discovery that if certain kind of crystals were touched in the right place with a fine wire called ‘cat-whiskers’ they could detect radio waves and transform them into electric current. The radio waves were captured by a receiving antenna. The listening was via a pair of headphones. The crystal receiv-ing sets were replaced in 1923 by the ‘valve wireless’ with improved re-ception and listening via aloud speaker. The radio set of the1940s and 1950s vintage appeared to be a jumble of wires and components such as vacuum tubes, capacitors, resistors and inductors and were indeed a fashionable piece of furniture in the household.

The Technology Push

The entire foundation of electronics, which had been so carefully built upon triode and other vacuum tubes, was shaken in1947 by the techni-cal innovation called the transistor – a thin Semiconductor device. The new transistor radios could make use of dry cells since they consumed much less power, were more reliable and inexpensive, much lighter and smaller than the wireless sets so that people could easily carry them around. Transistor radios were first marketed in USA (1953). By the end of 1970s, 70% of the radio receivers were either portable or mobile. The transistor caused a revolution in the way radio broadcasting could be used. But what followed the invention of transistor was even more amaz-ing. Several transistors, diodes, resistors, and capacitors were combined to form a complete circuit on a single germanium chip, which came to be known as integrated circuit (IC). IC chips brought about a major concep-tual change in the design of electronic systems. Attempts to make elec-tronics systems failsafe and foolproof resulted in the development of printed circuit boards (PCB) and the concept of modular construction of electronic systems. Application of frequency modulation technique for superimposing audio signals on the VHF carrier was a notable develop-ment in radio broadcasting in 1950. 88-108MHz frequency band is re-served for FM Broadcast Service. The major advantage of FM broadcast-ing is its better noise tolerance and higher fidelity compared to AM broadcasting. The major disadvantage of FM is its short range, only tens of kilometers. VHF/FMtechnology has since been extensively used for broadcasting in India. The advent of man-made geostationary communi-cation satellites in 1960s, which in effect are ‘radio repeaters’ in the sky, was indeed an innovation in radio technology. The idea of communica-tion through a satellite was conceived by a British scientist Arthur C Clarke. He had pointed out that a satellite in the circular equatorial orbit with a radius of 42, 242 km would have an angular velocity equal to that of the Earth. Thus, such a satellite would always remain above the same spot on ground and it could receive and relay signals from most of the country. He had also stated that the electrical power for the satellite would be obtained from conversion of the Sun’s radiation by means of solar cells. However, it was AT&T of the USA which established the mod-

ern technique of communication satellite thereby ushering in a new era of broadcasting. An innovation in sound broadcasting called stereophony was developed in the late 1950s and used in the UK in 1966. In this sys-tem sound is split into parts and reproduced by two separate channels in order to create spatial effect. Currently FM broadcasting in India uses stereo sound.

Digital Audio and Radio

Analogue audio signal is continuous in time. In other words, attach in-stant the signal has an instantaneous level, which can assume any value within the permissible dynamic range of the Analogue system. On the contrary, digital audio signal is binary. In other words it can assume any one of the two levels namely 0or 1. Audio signal in digital form is mere numbers. Samuel F Morse, without being aware of it, established the foundation of electrical digital signaling in 1944. In his telegraph the electrical signal had only two options. Either some current flowing through the circuit or no current at all. Most telegraph systems used a special code to represent the characters of the alphabet. He used a short duration electrical pulse known as a ‘dot’ and along duration pulse known as ‘dash’ with gaps of no voltage in between. He combined these dots and dashes into a pattern known as the Morse code to send text messages overlong conducting wires. When automation came along in the field of communication, Morse code gave way to newer binary codes. A method of digitizing audio was first proposed by Alec Reeves of Eng-land and is known as pulse code modulation (PCM). In PCM, the analogue signal is sampled at a rate slightly greater than twice the highest fre-quency contained in it. Each sample is then converted into a code of bi-nary bits. In professional audio systems 16-bit PCM is used. In order to eliminate the bit error in the digital audio code upon reproduction, addi-tional data bits are encoded along with the digital audio data. All these result in incredibly large number of bits, the bandwidth of which is at least 25 times that of the analogue audio signal, and requirement of enormous amount of storage space. It is thus necessary to compress the signal without sacrificing quality. The audio compression system de-pends on the fact that human ear perceives good quality sound despite data compression. The basic principle of the psycho-acoustic model of audio bit reduction is to reduce redundancy and irrelevance in source signal thus significantly lowering the requirement of bandwidth for transmission and storage space. The foremost requirement of any real time compression system is to achieve a low bit rate with minimum de-lay and perceived loss of quality from either small signal distortion or injected noise. The art of compressing the data stream into narrower

and narrower pipes continues. The current standard is MP3 As a result of the cost-effective digitalization of analogue audio signal, additional data could now be broadcast giving added values to traditional voice and music. Audio broadcasting in the digital era has indeed become multime-dia broadcasting. Multimedia broadcasting means use of existing broad-cast infrastructure to a variety of devices such as PC, Smart TV receiver and personal digital assistant. Digital technology is breaking down the barriers that separate the technologies and service characteristics which exist between broadcasting, telecommunication and computers.

First transatlantic message to:His Majesty, Edward VII, London, England.In taking advantage of the wonderful triumphof scientific research and ingenuity which hasbeen achieved in perfecting a system of wirelesstelegraphy, I extend on behalf of the Americanpeople most cordial greetings and goodwishes to you and all the people of the BritishEmpire.Theodore RooseveltSouth Wellfleet, Massachusetts, Jan. 19, 1903The Answer came back:The President, White House, Wash., America.I thank you most sincerely for the kind messagewhich I have just received from youthrough Marconi’s transatlantic wireless telegraph.I sincerely reciprocate in the name ofthe people of the British Empire the cordialgreetings and friendly sentiment expressed by

you on behalf of the American nation, and Iheartily wish you and your country every possibleprosperity.Edward R. and I.

Sandringham, Jan. 19, 1903

About All India Radio:

ALL INDIA RADIO

ALL India Radio officially known as Akashvani, is the radio broad-caster of India and a division of Prasar Bharathi, an autonomous corporation of the Ministry of Information and Broadcasting ,Government of India. Established in

1936 today it is the sister service of Prasara Bharati”s Doordarshan, the national television broadcaster.

The word Akashvani was coined by Professor Dr.M.V.Gopalaswamy for his radio station in Mysore during 1936.

Broadcasting started in India in 1927 with two privately –owned transmitters at Mumbai and Calcutta which were taken over by the Government in 1930.These were operating under the name “Indian Broadcasting Service” until 1936 when it was given the present name “All India Radio(AIR)”.It also came to be known as “Akashvani” from 1957.

Today AIR “s network provides radio coverage to 93.7% of the population and reaches 90% percent of the total area .Social responsibility and Public Service broadcasting continue to be hallmark of AIR .The services provided by AIR on its primary channel including local radio stations is a vital part of life in the country .It educates, entertains and provides information for enrichment of lives of people and it seeks to cater to the interests of the few as well as of the many. It provides Information through news and current affairs programmes. En-tertainment through Music –devotional, classical, Film songs etc. Education through extension programmes for specific audience including farmers,women,children,youth,troops,Formal and nonformal education ,Adult education ,IGNOU,UGC etc.

The AIR network comprises the National Channel, Regional Stations, Local Radio Stations, Vividh Bharati Centres, FM Stereo Service, External Services and North-Eastern Services.

FM SERVICES:

AIR FM Rainbow is a group of FM radio channels across India. It is run by All India Radio, a government owned enterprise. It features Hindi and regional language songs and occasional English songs along with hourly news in English, regional language and/or Hindi. In Bhopal, it operates on 102.1 megahertz in Hindi cum. English language covering more than 12 districts of Madhya Pradesh and 21 Sub Urbs in Bhopal City. In MP, it is also known as "Rainbow FM". In Delhi, it operates on 102.6 megahertz and AIR FM Rainbow Delhi is the only FM Channel to be aired in as many as ten cities. In Mumbai, it operates on 107.1 megahertz. In Lucknow, it uses the 100.7 MHz slot. In vizag, it operates on 102 megahertz. In

hyderabad, it operates on 101.9 megahertz and in Vijayawada it operates on 102.2 megahertz. It operates in Bangalore as well with a frequency of 101.3. It was earlier called FM Metro, the name was changed to FM Rainbow in 2002.

CONTENTS

1. BASIC COMMUNICATION SYSTEM

2. MODULATION AND ITS NECESSITY

3. FREQUENCY DIVISION MULTIPLEXING

4. FREQUENCY MODULATION

5. FM RADIO

6. ITS PERFORMANCE

7. FM TRANSMITTER

Basic System

The basic communications system has:

Transmitter: The sub-system that takes the information signal and processes it prior to transmission. The transmitter modulates the information onto a carrier sig-nal, amplifies the signal and broadcasts it over the channel

Channel: The medium which transports the modulated signal to the receiver. Air acts as the channel for broadcasts like radio. May also be a wiring system like cable TV or the Internet.

Receiver: The sub-system that takes in the transmitted signal from the channel and processes it to retrieve the information signal. The receiver must be able to dis-criminate the signal from other signals which may using the same channel (called tuning), amplify the signal for processing and demodulate (remove the carrier) to retrieve the information. It also then processes the information for reception (for example, broadcast on a loudspeaker).

MODULATION:

Modulation is the process of varying one waveform in relation to another waveform. In telecommunications, modulation is used to convey a message, or a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch.

Often a high-frequency sinusoid waveform is used as carrier signal to convey a lower frequency signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low fre-quency information signal to obtain the modulated signal.

A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (short for "Modulator-Demodulator").

Why is Modulation Required?

1. To achieve easy radiation: If the communication channel consists of free space, antennas are required to radi-ate and receive the signal.Dimension of the antennas is limited by the corresponding wavelength

Example: Voice signal bandwidth f = 3kHz

λ = c/f

λ = (3x10^8) / (3x10^3 ) = 100000m

λ/4 = 25000m

If we modulate a carrier wave of frequency f = 100MHz with the voice signal

λ = c/f

λ = (3x10^8)/(100x10^6) = 3m

λ/4 = 75cm

2. To expand the bandwidth of the transmitted signal for better transmission quality (to reduce noise and interference):

C = B.log2(1+SNR)

Channel capacity Bandwidth Signal to noise ratio

Channel capacity: Maximum achievable information rate that can be transmitted over the channel.

As bandwidth increases, the required SNR (for fixed noise level, corresponding signal power) decreases.

3. To accommodate for simultaneous transmission of several signals :

In many communication systems, a single, large frequency band is assigned to the system and is shared among a group of users. Examples of this type of system include:

1. A microwave transmission line connecting two sites over a long distance. Each site has a number of sources generating independent data streams that are transmitted simultan-eously over the microwave link.

2. AM or FM radio broadcast bands, which are divided among many channels or stations. The stations are selected with the radio dial by tuning a variable-frequency filter.

3. A satellite system providing communication between a large number of ground stations that are separated geographically but that need to communicate at the same time. The total bandwidth assigned to the satellite system must be divided among the ground stations.

4. A cellular radio system that operates in full-duplex mode over a given frequency band. The earlier cellular telephone systems, for example AMPS, used analog communication methods. The bandwidth for these systems was divided into a large number of channels. Each pair of channels was assigned to two communicating end-users for full-duplex com-munications.

Frequency division multiplexing (FDM):

Frequency division multiplexing (FDM) means that the total bandwidth available to the system is divided into a series of non overlapping frequency sub-bands that are then assigned to each communicating source and user pair.

Figures 1-a and 1-b show how this division is accomplished for a case of three sources at one end of a system that are communicating with three separate users at the other end. Note that each transmitter modulates its source's information into a signal that lies in a different frequency sub-band (Transmitter 1 generates a signal in the frequency sub-band between 92.0 MHz and 92.2 MHz, Transmitter 2 generates a signal in the sub-band between 92.2 MHz and 92.4 MHz, and Transmitter 3 generates a signal in the sub-band between 92.4 MHz and 92.6 MHz). The sig-nals are then transmitted across a common channel.

Figure 1-a—A system using frequency division multiplexing

Figure 1-b—Spectral occupancy of signals in an FDM system.

At the receiving end of the system, bandpass filters are used to pass the desired signal (the sig-nal lying in the appropriate frequency sub-band) to the appropriate user and to block all the un-wanted signals. To ensure that the transmitted signals do not stray outside their assigned sub-bands, it is also common to place appropriate passband filters at the output stage of each trans-mitter. It is also appropriate to design an FDM system so that the bandwidth allocated to each sub-band is slightly larger than the bandwidth needed by each source. This extra bandwidth, called a guardband, allows systems to use less expensive filters (i.e., filters with fewer poles and therefore less steep rolloffs).

The main advantage is that unlike TDM, FDM is not sensitive to propagation delays. Channel equalization techniques needed for FDM systems are therefore not as complex as those for TDM systems.

Disadvantages of FDM include the need for bandpass filters, which are relatively expensive and complicated to construct and design (remember that these filters are usually used in the transmitters as well as the receivers). TDM, on the other hand, uses relatively simple and less costly digital logic circuits.

Another disadvantage of FDM is that in many practical communication systems, the power am-plifier in the transmitter has nonlinear characteristics (linear amplifiers are more complex to build), and nonlinear amplification leads to the creation of out-of-band spectral components that may interfere with other FDM channels. Thus, it is necessary to use more complex linear ampli-fiers in FDM systems.

Example —FDM for commercial FM radio

The frequency band from 88 MHz to 108 MHz is reserved over the public airwaves for com-mercial FM broadcasting. The 88–108 MHz frequency band is divided into 200 kHz sub-bands. The 200 kHz bandwidth of each sub-band is sufficient for high-quality FM broadcast of music. The stations are identified by the center frequency within their channel (e.g., 91.5 MHz, 103.7 MHz). This system can provide radio listeners with their choice of up to 100 different radio sta-tions

Frequency modulation (FM):

Frequency modulation (FM) conveys information over a carrier wave by varying its frequency In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying

The amplitude of the carrier remains unchanged at all times. In other words, the amplitude of the modulated wave remains the same as the amplitude of the carrier wave. The frequency of the carrier is made to fluctuate symmetrically above and below its unmodulated frequency. As an ex-ample, a carrier frequency, of 1000kHz may be caused to swing between 925kHz and 1075kHz or any other amount chosen in accordance with the signal voltage.

Advantages of frequency modulation:

1. Resilience to noise:  One particular advantage of frequency modulation is its resilience to signal level varia-tions. The modulation is carried only as variations in frequency. This means that any sig-nal level variations will not affect the audio output, provided that the signal does not fall to a level where the receiver cannot cope. As a result this makes FM ideal for mobile ra-dio communication applications including more general two-way radio communication or portable applications where signal levels are likely to vary considerably. The other ad-vantage of FM is its resilience to noise and interference. It is for this reason that FM is used for high quality broadcast transmissions.

2. Easy to apply modulation at a low power stage of the transmitter: Another advantage of frequency modulation is associated with the transmit-ters. It is possible to apply the modulation to a low power stage of the transmitter, and it is not necessary to use a linear form of amplification to increase the power level of the signal to its final value.

3. It is possible to use efficient RF amplifiers with frequency modulated signals:   It is possible to use non-linear RF amplifiers to amplify FM signals in a transmit-ter and these are more efficient than the linear ones required for signals with any ampli-tude variations (e.g. AM and SSB). This means that for a given power output, less battery power is required and this makes the use of FM more viable for portable two-way radio applications.

4. Also the main advantage of fm is that frequency reuse is possible.

For example we have the same frequency for an fm service provider operat-ing in different cities. That is although it is operating in different areas with different pro-grammes there is no effect of ones’ over the others’..

Disadvantages of frequency modulation:

The biggest disadvantage of FM is the high bandwidth required for transmitting the same signal compared AM.

Also FM receivers and transmitters are more costly, thus limiting the applicability of FM.

FM Radio

FM radio uses frequency modulation, of course. The frequency band for FM radio is about 88 to 108 MHz. The information signal is music and voice which falls in the audio spectrum. The full audio spectrum ranges form 20 to 20,000 Hz, but FM radio limits the upper modulating fre-quency to 15 kHz (cf. AM radio which limits the upper frequency to 5 kHz). Although, some of the signal may be lost above 15 kHz, most people can't hear it anyway, so there is little loss of fi-delity. FM radio maybe appropriately referred to as "high-fidelity."

If FM transmitters use a maximum modulation index of about 5.0, so the resulting bandwidth is 180 kHz (roughly 0.2 MHz). The FCC assigns stations ) 0.2 MHz apart to prevent overlapping signals (coincidence? I think not!). If you were to fill up the FM band with stations, you could get 108 - 88 / .2 = 100 stations, about the same number as AM radio (107). This sounds convinc-ing, but is actually more complicated (agh!).

FM radio is broadcast in stereo, meaning two channels of information. In practice, they generate three signals prior to applying the modulation:

the L + R (left + right) signal in the range of 50 to 15,000 Hz. a 19 kHz pilot carrier.

the L-R signal centered on a 38 kHz pilot carrier (which is suppressed) that ranges from 23 to 53 kHz .

So, the information signal actually has a maximum modulating frequency of 53 kHz, requiring a reduction in the modulation index to about 1.0 to keep the total signal bandwidth about 200 kHz.

FM PerformanceBandwidth

As we have already shown, the bandwidth of a FM signal may be predicted using:

BW = 2 ( + 1 ) fm

where is the modulation index and

fm is the maximum modulating frequency used.

FM radio has a significantly larger bandwidth than AM radio, but the FM radio band is also larger. The combination keeps the number of available channels about the same.

The bandwidth of an FM signal has a more complicated dependency than in the AM case (recall, the bandwidth of AM signals depend only on the maximum modulation frequency). In FM, both the modulation index and the modulating frequency affect the bandwidth. As the information is made stronger, the bandwidth also grows.

Efficiency

The efficiency of a signal is the power in the side-bands as a fraction of the total. In FM signals, because of the considerable side-bands produced, the efficiency is generally high. Recall that conventional AM is limited to about 33 % efficiency to prevent distortion in the receiver when the modulation index was greater than 1. FM has no analogous problem.

The side-band structure is fairly complicated, but it is safe to say that the efficiency is generally improved by making the modulation index larger (as it should be). But if you make the modula-tion index larger, so make the bandwidth larger (unlike AM) which has its disadvantages. As is typical in engineering, a compromise between efficiency and performance is struck. The modula-tion index is normally limited to a value between 1 and 5, depending on the application.

Noise

FM systems are far better at rejecting noise than AM systems. Noise generally is spread uni-formly across the spectrum (the so-called white noise, meaning wide spectrum). The amplitude of the noise varies randomly at these frequencies. The change in amplitude can actually modulate the signal and be picked up in the AM system. As a result, AM systems are very sensitive to ran-dom noise. An example might be ignition system noise in your car. Special filters need to be in-stalled to keep the interference out of your car radio.

FM systems are inherently immune to random noise. In order for the noise to interfere, it would have to modulate the frequency somehow. But the noise is distributed uniformly in frequency and varies mostly in amplitude. As a result, there is virtually no interference picked up in the FM receiver. FM is sometimes called "static free, " referring to its superior immunity to random noise.

Summary In FM signals, the efficiency and bandwidth both depend on both the maximum modulating fre-

quency and the modulation index. Compared to AM, the FM signal has a higher efficiency, a larger bandwidth and better immunity

to noise.

Broadcast bands

Throughout the world, the broadcast band falls within the VHF part of the radio spectrum. Usually 87.5 - 108.0 MHz is used, or some portion thereof, with few exceptions:

In the former Soviet republics, and some Eastern Bloc nations, an older band from 65-74 MHz is also used. Assigned frequencies are at intervals of 30 kHz. This band, sometimes referred to as the OIRT band, is slowly being phased out in many countries.

In Japan, the band 76 - 90 MHz is used.

The frequency of an FM broadcast station (more strictly its assigned nominal centre frequency) is usually an exact multiple of 100 kHz. In most of the Americas and the Caribbean, only odd multiples are used. In some parts of Europe, Greenland and Africa, only even multiples are used. In Italy, multiples of 50 kHz are used. There are other unusual and obsolete standards in some countries, including 0.001, 0.01, 0.03, 0.074, 0.5, and 0.3 MHz.

SMALL SCALE USE OF FM BROADCAST BAND

FM radio microphones

The FM broadcast band can also be used by some inexpensive wireless microphones, but professional-grade wireless microphones generally use bands in the UHF region so they can run on dedicated equipment without broadcast interference. Such inexpensive wireless microphones are generally sold as toys for karaoke or similar purposes, allowing the user to use an FM radio as an output rather than a dedicated amplifier and speaker.

Microbroadcasting

Low-power transmitters such as those mentioned above are also sometimes used for neighborhood or campus radio stations, though campus radio stations are often run over carrier current. This is generally considered a form of microbroadcasting. As a general rule, enforcement towards low-power FM stations is stricter than AM stations due to issues such as the capture effect, and as a result, FM microbroadcasters generally do not reach as far as their AM competitors.

Clandestine use of FM transmitters

FM transmitters have been used to construct miniature wireless microphones for espionage and surveillance purposes (covert listening devices or so-called "bugs"); the advantage to using the FM broadcast band for such operations is that the receiving equipment would not be considered particularly suspect. Common practice is to tune the bug's transmitter off the ends of the broadcast band, into what in the United States would be TV channel 6 (<87.9 MHz) or aviation navigation frequencies (>107.9); most FM radios with analog tuners have sufficient overcoverage to pick up these beyond-outermost frequencies, although many digitally tuned radios do not

Modulation characteristics Pre-emphasis and de-emphasis

Random noise has a 'triangular' spectral distribution in an FM system, with the effect that noise occurs predominantly at the highest frequencies within the baseband. This can be offset, to a limited extent, by boosting the high frequencies before transmission and reducing them by a corresponding amount in the receiver. Reducing the high frequencies in the receiver also reduces the high-frequency noise. These processes of boosting and then reducing certain frequencies are known as pre-emphasis and de-emphasis, respectively.

It is important that stereo broadcasts should be compatible with mono receivers. For this reason, the left (L) and right (R) channels are algebraically encoded into sum (L+R) and difference (L−R) signals. A mono receiver will use just the L+R signal so the listener will hear both channels in the single loudspeaker. A stereo receiver will add the L+R and L−R signals to recover the Left channel, and subtract the L+R and L−R signals to recover the Right channel.

The (L+R) Main channel signal is transmitted as baseband audio in the range of 30 Hz to 15 kHz. The (L−R) Sub-channel signal is modulated onto a 38 kHz double-sideband suppressed carrier (DSBSC) signal occupying the baseband range of 23 to 53 kHz.

A 19 kHz pilot tone, at exactly half the 38 kHz sub-carrier frequency and with a precisely defined phase relationship to it, is also generated. This is transmitted at 8–10% of overall modulation level and used by the receiver to regenerate the 38 kHz sub-carrier with the correct phase.

The final multiplex signal from the stereo generator contains the Main Channel (L+R), the pilot tone, and the sub-channel (L−R). This composite signal, along with any other sub-carriers (SCA), modulates the FM transmitter.

Converting the multiplex signal back into left and right audio signals is performed by a stereo decoder, which is built into stereo receivers.

In order to preserve stereo separation and signal-to-noise parameters, it is normal practice to apply pre-emphasis to the left and right channels before encoding, and to apply de-emphasis at the receiver after decoding.

Stereo FM signals are more susceptible to noise and multipath distortion than are mono FM signals. This is due to imbalance of FM sideband ratios of the additional modulating signals created by the pilot tone and the sub-carrier channel.

In addition, for a given RF level at the receiver, the signal-to-noise ratio for the stereo signal will be worse than for the mono receiver. The point at which the receiver input RF level reaches maximum monaural signal-to-noise ratio will be 23 dB lower than the receiver input RF level for maximum stereo signal-to-noise ratio. For this reason many FM stereo receivers include a

stereo/mono switch to allow listening in mono when reception conditions are less than ideal, and most car radios are arranged to reduce the separation as the signal-to-noise ratio worsens, eventually going to mono while still indicating a stereo signal is being received.

Deviation and bandpass

Normally, each channel is 200 kHz (0.2 MHz) wide, and can pass audio and subcarrier frequencies up to 100 kHz. Deviation is typically limited to 150 kHz total (±75 kHz) in order to prevent interference to adjacent channels on the band. Stations in the U.S. may go up to 10% over this limit if they use non-stereo subcarriers, increasing total modulation by 0.5% for each 1% used by the subcarriers

FM TRANSMITTER

Introduction:

There is too much over-crowding in the AM broadcast bands and shrinkage in the night-time ser-vice area due to fading, interference, etc. FM broadcasting offers several advantages over AM such as uniform day and night coverage, good quality listening and suppression of noise, inter-ference, etc. All India Radio has gone in for FM broadcasting using modern FM transmitters in-corporating state-of-art technology.

The configurations of the transmitters being used in the network are :

2 x 3 kW Transmitter

2 x 5 kW Transmitter

Salient Features of BEL/GCEL FM Transmitters:

1. Completely solid state.2. Forced air cooled with the help of rack-integrated blowers.3. Parallel operation of two transmitters in passive exciter standby mode.4. Mono or stereo broadcasting5. Additional information such as SCA signals and radio traffic signals (RDS) can also

be transmitted.6. Local/Remote operation7. Each transmitter has been provided with a separate power supply.

8. Transmitter frequency is crystal controlled and can be set in steps of 10 kHz using a synthesizer.

Modern FM Transmitter:

Simplified block diagram of a Modern FM Transmitter is given in Fig.1. The left and right chan-nel of audio signal are fed to stereo coder for stereo encoding. This stereo encoded signal or mono signal (either left or right channel audio) is fed to VHF oscillator and modulator. The FM modulated output is amplified by a wide band power amplifier and then fed to Antenna for trans-mission.

Voltage controlled oscillator (VCO) is used as VHF oscillator and modulator. To stabilize its frequency a portion of FM modulated signal is fed to a programmable divider, which divides the frequency by a factor ‘N’ to get 10 kHz frequency at the input of a phase and frequency compar -ator (phase detector). The factor ‘N’ is automatically selected when we set the station carrier fre-quency. The other input of phase detector is a reference signal of 10 kHz generated by a crystal oscillator of 10 MHz and divided by a divider (1/1000). The output of phase detector is an error voltage, which is fed to VCO for correction of its frequency through rectifier and low pass filter.

Fig. 1 Block Diagram of Modern FM Transmitter

2 x 3 kW FM Transmitter:

Simplified block diagram of a 2 x 3 kW FM transmitter is shown in Fig.2. 2 x 3 kW Trans-mitter setup, which is more common, consists of two 3 kW transmitters, designated as transmit-ters A and B, whose output powers are combined with the help of a combining unit. Maximum of two transmitters can be housed in a single rack along with two Exciter units. Transmitter A is provided with a switch-on-control unit (GS 033A1) which, with the help of the Adapter plug-in-unit (KA 033A1), also ensures the parallel operation of transmitter B. Combining unit is housed in a separate rack.

Fig.2 Block Diagram Of 2x3 Kw Fm TransmitterRef.Drg.No.:-STI(T)395,(DC147)

Low-level modulation of VHF oscillator is carried out at the carrier frequency in the Exciter type SU 115. The carrier frequency can be selected in 10 kHz steps with the help of BCD switches in the synthesizer. The exciter drives four 1.5 kW VHF amplifier, which is a ba-sic module in the transmitter. Two such amplifiers are connected in parallel to get 3 kW power. The transmitter is forced air-cooled with the help of a blower. A standby blower has also been provided which is automatically selected when the pre-selected blower fails. Both the blowers can be run if the ambient temperature exceeds 40oC.

Power stages are protected against mismatch (VSWR > 1.5) or excessive heat sink temperature by automatic reduction of power with the help of control circuit. Electronic voltage regulator has not been provided for the DC supplies of power amplifiers but a more effi -cient system of stabilization in the AC side has been provided. This is known as AC-switch over. Transmitter operates in the passive exciter standby mode with help of switch-on-control unit. When the pre-selected exciter fails, standby exciter is automatically selected. Reverse switch over, however, is not possible.

2 x 5 kW FM Transmitter

A simplified block diagram of a 2 x 5 kW FM Transmitter is also given in Fig. 3.

Fig.3 RF Block Schematic of 2x5 kW FM TransmitterRef.Drg.No.-STI (T) 557,(DC309)

Exciter (SU 115):

The Exciter (SU115) is, basically, a self-contained full-fledged low power FM Transmitter. It has the capability of transmitting mono or stereo signals as well as additional in-formation such as traffic radio, SCA (Subsidiary Channel Authorisation) and RDS (Radio Data System) signals. It can give three output powers of 30 mW, 1 W or 10 W by means of internal links and switches. The output power is stabilized and is not affected by mismatch (VSWR > 1.5), temperature and AC supply fluctuations. Power of the transmitter is automatically reduced in the event of mismatch. The 10 W output stage is a separate module that can be inserted be-tween 1 W stage and the low pass harmonics filter. This stage is fed from a switching power supply which also handles part of the RF output power control and the AC supply stabilizations. In AIR set up this 10 W unit is included as an integral part of the Exciter.

This unit processes the incoming audio signals both for mono and stereo transmissions. In case of stereo transmission, the incoming L and R channel signals are processed in the stereo coder circuit to yield a stereo base band signal with 19 kHz pilot tone for modulating the carrier signal. It also has a multiplexer wherein the coded RDS and SCA signals are multiplexed with the nor -mal stereo signal on the modulating base band. The encoders for RDS and SCA applications are external to the transmitter and have to be provided separately as and when needed.

Frequency Generation, Control and Modulation:

The transmitter frequency is generated and carrier is modulated in the Synthesiser module within the Exciter. The carrier frequency is stabilized with reference to the 10 MHz frequency from a crystal oscillator using PLL and programmable dividers. The operating frequency of the trans-mitter can be selected internally by means of BCD switches or externally by remote control. The output of these switches generates the desired number by which the programmable divider should divide the VCO frequency (which lies between 87.5 to 108 MHz) to get a 10 kHz signal to be compared with the reference frequency. The stablised carrier frequency is modulated with the modulating base band consisting of the audio (mono and stereo), RDS and SCA signals. The Varactor diodes are used in the synthesizer to generate as well as modulate the carrier frequency.

Switch-ON Control Unit (Type GS 033 A1):

The switch-on-control unit can be termed as the “brain” and controls the working of the transmit-ter ‘A’. It performs the following main functions:

1. It controls the switching ON and OFF sequence of RF power amplifiers, rack blower and RF carrier enable in the exciter.

2. Indicates the switching and the operating status of the system through LEDs.

3. Provides automatic switch over operation of the exciter in the passive exciter standby mode in which either of the two exciters can be selected to operate as the main unit.

4. It provides a reference voltage source for the output regulators in the RF amplifiers.

5. It is used for adjusting the output power of the transmitter.

6. It evaluates the fault signals provided by individual units and generates an overall sum fault signal which is indicated by an LED on the front panel. The fault is also stored in the defective unit and displayed on its front panel.

Adapter Unit (KA 033A1):

Adapter Unit is a passive unit which controls transmitter B for its parallel operation with trans-mitter A in active standby mode. The control signals from the Switch-on control unit are ex-tended to transmitter B via this Adapter unit. If this unit is not in position the transmitter B can not be energized.

1.5 kW VHF Amplifier (VU 315):

This amplifier is the basic power module in the transmitter. It has a broad band design so that no tuning is required for operation over the entire FM Broadcast band. RF power transistors of its output stages are of plug in type which are easy to replace and no adjustments are required after replacement. Each power amplifier gives an output of 1.5 kW. Depending on the required con-figuration of the transmitter, output of several such amplifiers is combined to get the desired out -put power of the transmitter. For instance, for a 3 kW set-up two power amplifiers are used whereas for a 2 x 3 kW set-up, 4 such amplifiers are needed. The simplified block diagram of 1.5 kW Power Amplifier is given in Fig. 4.

Fig. 4 Block Diagram of 1.5 kW Amplifier VU 315Ref. Drg.No.:-STI(T)444(DC196)

This amplifier requires an input power of 2.5 to 3 W and consists of a driver stage (output 30 W) followed by a pre-amplifier stage (120 W). The amplification from 120 W to 1500 W in the fi -nal stage is achieved with the help of eight 200 W stages. Each 200 W stage consists of two out -put transistors (TP 9383, SD1460 or FM 150) operating in parallel. These RF transistors operate

in wide band Class C mode and are fitted to the PCB by means of large gold plated spring con-tacts to obviate the need for soldering. The output of all these stages is combined via coupling networks to give the final output of 1.5 kW. A monitor in each amplifier controls the power of the driver stage depending on the reference voltage produced by the switch-on-control unit. Since this reference voltage is the same for all the VHF amplifiers being used, all of them will have the same output power.

Each amplifier has a meter for indicating the forward and reflected voltages and transistor cur-rents. Also a fault is signaled if the heat sink temperature or the VSWR exceed the prescribed limits. In both cases, the amplifier power is automatically reduced to protect the transistors.

Power Supply System:

The FM transmitter requires 3-phase power connection though all the circuits, except the power amplifiers, need only single phase supply for their operation. An AVR of 50 kVA capacity has been provided for this purpose.

Power consumption of the transmitters under various configurations is as follows :

Frequency of Power Consumptionoperation 3 kW 5 kW 2 x 3 kW 2 x 5 kW

87.5 to 100 MHz 5100 W 8500 W 10200 W 17000 W100 to 108 MHz 5320 W 8860 W 10640 W 17720 W

These figures do not include the power consumption of blowers which is 200 W for each blower.

For each transmitter, there is a separate power distribution panel (mounted on the lower portion on the front of the rack). Both the distribution panels A&B are identical except for the difference that the LEDs, fuses and relays pertaining to switching circuit of blowers and absorber are mounted on the ‘A’ panel.

Power Reduction in case of Amplifier or Transistor Failure:

When an amplifier module or a push-pull output stage in an amplifier module fails due to failure of any one transistor, the output gets reduced according to the following formula. :

P =Po {(m-n)/m}2

WherePo = nominal powerP = reduced power available at the antennaM = total number of amplifier modules or of push

Pull output stages in circuitN = number of faulty amplifiers or push pull output stages.

The power consumed in the absorber resistors is calculated according to the formula :

Pabsorber = Po – Pn

Where Pn is the faulty partial power, which in case of failure of an entire amplifier module equals 1250 W.

If power reduction occurs due to failure of one or more VHF amplifiers, the transmitter should be switched off immediately and the working transmitter should be selected on the antenna using the U-links on the Combining unit.

FM Antenna and Feeder Cable System:

The Antenna system for FM Transmitters consists of 3 main sub-systems, namely :

a) Supporting towerb) Main antennac) Feeder Cable

Tower

A tower of good height is required for mounting the FM antenna since the coverage of the trans-mitter is proportional to the height of the tower. For a 100 m height, the coverage is about 60 km. Wherever new towers were to be provided, generally they are of 100 m height since beyond this height; there is steep rise in their prices because of excessive wind load on the top of the tower. At some places existing towers of Doordarshan have also been utilized for mounting the FM antenna. Provision has also been made on the AIR towers for top mounting of TV antenna below FM antenna (Aperture for Band III).

Antenna

The main requirements of the antenna to be used for FM transmitters are :

- Wide-band usage from 88 to 108 MHz range.- Omni-directional horizontal pattern of field strength.- Circular polarization for better reception.- High gain for both vertical and horizontal signals.- Two degrees beam tilt below horizontal- Sturdy design for maintenance-free service.

Further, depending on the type of tower available for mounting the requirement is for two types of antenna. The first type is to be mounted on a small cross-section AIR Tower. For which a pole type FM antenna has been selected. For mounting on the existing TV towers, a panel type

antenna has been used. The cross section of the TV tower at the AIR aperture is 2.4 x 2.4 m. the pole type antenna is quite economical as compared to panel type antenna, but it can not be used on large area towers. For our requirement, the antennae supplied by M/s. SIRA have been found suitable.

Pole Type Antenna

The pole type antenna is mounted on one of the four faces of the tower. This system will give a field pattern within a range of 3 dB. The antenna is mounted in such a direction in which it is re-quired to enhance the signal. The important parameters for this antenna are :

Weight 200 Kg. (for 6 dipoles).VSWR 1.4 : 1Gain 5 dBRating of each dipole 5 kW

The other important features are :

Very low power radiation towards Transmitter building.

Spacing between dipoles is 2.6 m and all the dipoles are mounted one above the other on the same face.

Lengths of feed cables of dipoles will be different and has been calculated to give a beam tilt of 2o below horizontal.

The feed point of the antenna is looking towards ground so as to avoid deterioration of the insulating flange. This flange consists of high density PVC. The life of this is expected to be about 7 to 10 years.

The distance of the feeding strip is 240 mm from edge and this should not be dis-turbed. All the six dipoles are mounted on a 100 mm dia Pole. This pole is supported by the main tower.

The antenna is fed through a power divider which divides total power into 6 outlets for feeding the 6 dipoles. The power divider is mounted on a different face of the tower.

The main feeder cables, power divider branch feeder cables, and dipoles are of hol-low construction to enable pressurization of the system.

The antenna can handle two channels with diplexing.

Suitable terminations are supplied for terminating the output of power divider in case of failure of any dipole.

Panel Type Antenna

The panel type antenna is to be used on TV tower. Doordarshan have provided an aperture for FM antenna on their towers. The size of this section is 2.4 x 2.4 mtrs. and its height is different at different places. The antenna system envisaged for FM broadcasting consists of a total of 16 panels. For omni-directional pattern 4 panels are mounted on each side of the tower. Ladders for mounting these panels have already been provided on the four sides of the tower.

Each panel consists of :

Reflector panel Two numbers of bent horizontal dipoles and Two numbers of vertical dipoles

The capacity of each dipole is 2.5 kW. Therefore, each panel is able to transmit 10 kW power. The reflector panels are constructed of GI bars whereas the dipoles are made out of steel tubes. Since each panel consists of 4 dipoles, there are a total of 64 dipoles for all the 16 panels. Therefore the power divider has 64 outlets to feed each of the dipoles. The power divider will be mounted inside the tower. This antenna gives an omni-directional pattern when the panels are mounted on all the four faces.

Feeder Cable

For connecting the output power of the transmitter to the dipoles through the power divider, a 3” dia feeder cable has been used.

This cable is of hollow type construction and has to be handled very carefully. From the building to the base of the tower, the cable is laid on horizontal cable tray. Along with the tower this is fixed on the cable rack provided for this purpose. The cable is clamped at every 1.5 m and the minimum radius of bending of this cable is about 1 m. The cable has been pro-vided with two numbers of EIA flange connectors of 3 1/8” size on both ends. Both the con-nectors are of gas-stop type. The cable connector on the antenna end i.e. on top of the tower is made gas-through before hoisting. This is achieved by drilling a hole through the Teflon insulator inside the connector. A dummy hole (drilled only half way) is already provided by the manufacturer for this purpose.

The weight of the cable is about 2.7 kg per meter and the power handling capacity is about 27 kW. Since enough safety margin has been provided in the power handling capacity, no

standby cable has been provided. This cable can be used later for two transmitters by diplex-ing. The attenuation loss of the cable is about 0.44 dB per 100 meter length. The cable and the antenna system should be fed with dry air by means of a dehydrator provided with the transmitter.

Conclusion

The objective of radio broadcasting is to entertain, inform and educate people. The develop-ments in radio technology are aimed at providing reliable, interference free and high quality voice and music to the listener at home or those moving in as wide area as possible. Rapid technological developments in the areas of digital signal processing and transmission, global cable networks and satellite technology have made their way into radio broadcasting. These factors have resulted in the convergence of computing, telecommunication and broadcasting. Voice and music can now be combined with data and text for broadcasting. The listener can also be provided with interactivity using the return channel. Way has been paved for high quality broadcasts directly to the listeners from space. Internet too is fast emerging as a means of delivering programs. Radio broadcasting has indeed become multimedia broadcast-ing and the world truly a global village.