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1. Introduction

Radio broadcasting is an audio broadcasting service, broadcast through the air as radio waves

from a transmitter to a radio antenna and, thus, to a receiver. Stations can be linked in radio

networks to broadcast common radio programs, either in broadcast syndication, simulcast or

sub channels. Broadcasting by radio takes several forms. These include AM and FM stations.

There are several subtypes, namely commercial broadcasting, non-commercial educational

public broadcasting and non-profit varieties as well as community radio, student-run campus

radio stations and hospital radio stations can be found throughout the world. Many stations

broadcast on shortwave bands using AM technology that can be received over thousands of

miles.

AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the

first method of impressing sound on a radio signal and is still widely used today. Commercial

and public AM broadcasting is carried out in the medium wave band worldwide. Once AM was

the only commercially important method for broadcast signal modulation. Am stations were

the earliest broadcasting stations to be developed. AM refers to amplitude modulation, a mode

of broadcasting radio waves by varying the amplitude of the carrier signal in response to the

amplitude of the signal to be transmitted. The AM range is 535 - 1605 KHz stations are assigned

between 540 and 1600 KHz every 10 KHz.AM transmissions cannot be

ionospherically propagated during the day due to strong absorption in the D-layer of the

ionosphere. During the night, this absorption largely disappears and permits signals to travel to

much more distant locations via ionosphere reflections. AM radio transmitters can transmit

audio frequencies up to 10 kHz, and only capable of reproducing frequencies up to 5 kHz. AM

stations are never assigned adjacent channels in the same service area. This prevents the

sideband power generated by two stations from interfering with each other.

FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which

uses frequency modulation to provide high-fidelity sound over broadcast radio. In FM the

instantaneous frequency of a carrier is varied, the instantaneous changes of the amplitude of

broadband signal. The FM range is 88 - 108 MHz (with broadcast frequencies, or stations,

assigned between 88.1 and 107.9MHz every 0.2 MHz).

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The broadcast of a program from source to listener involves use of studios, microphones, announcer console, switching console, telephone lines / STL and Transmitter. Normally the programmers originate from a studio centre located inside the city/town for the convenience of artists. The program could be either “live” or recorded”. In some cases, the program can be from OB spot, such as commentary of cricket match etc. Programs that are to be relayed from other Radio Stations are received in a receiving centre and then sent to the studio centre or directly received at the studio centre through RN terminal/telephone line. All these programs are then selected and routed from studio to transmitting centre through broadcast quality telephone lines or studio transmitter microwave/VHF links. The broadcasting of program from source to destination the use of involves the use of

Studios

Microphones

Announcer console

Switching console

Telephone lines / STL and

Transmitter

1.1 STUDIO CHAIN IN AN AIR STATION The studios have separate announcers booths attached to them where first level fading, mixing and cueing facilities are provided. A simplified block schematic showing the different stages is as shown in the figure.

Fig. 1.1 Simplified block schematic of broadcasting chain

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1.2 STUDIO CENTER A broadcast studio is an acoustically treated room. It is necessary that the place where a program for broadcast purposes is being produced should be free of extraneous noise. This is possible only if the area of room is insulated from outside sound. Further, the microphone which is the first equipment that picks up the sound is not able to distinguish between wanted and unwanted signals and will pick up the sound not only from the artists and the instruments but also reflections from the walls marring the quality and clarity of the program. So the studios are to be specially treated to give an optimum reverberation time and minimum noise level. The entry to the studios is generally through sound isolating lobby called sound lock. Outside of every studio entrance, there is a warning lamp, which glows ‘Red’ when the studious ‘ON-AIR’. The studios have separate announcers booths attached to them where first level fading, mixing and cueing facilities are provided. In addition to control room and studios, dubbing/recording rooms are also provided in a studio complex. Following equipments are generally provided in a recording/dubbing room:

Console tape recorders

Console tape decks

Recording/dubbing panel having switches, jacks, and keys etc. The above equipments can be used for the following purpose

For recording of programs originating from any studio.

For recording of programs available in the switching consoles in control room.

For dubbing of programs available on cassette tape.

For editing of programs

For mixing and recording of program

1.3 STUDIO OPERATIONAL REQUIREMENTS Many technical requirements of studios like minimum noise level, optimum reverberation time etc. are normally met at the time of installation of studio. However for operational purposes, certain basic minimum technical facilities are required for smooth transmission of programs and for proper control. These are as follows:

Program in a studio may originate from a microphone or a tape deck, or a turntable or a comp act d i s c or a R- D AT. So a f ac i l i t y f or se lect ion o f ou t p u t o f an y o f t h ese equipments at any moment is necessary. Announcer console does this function.

Fac i l i t y t o f ad e in / f ad e ou t t h e p rogram smoot h ly an d con t ro l t h e p rogram leve l within prescribed limits.

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Facility for aural monitoring to check the quality of sound production and sound meters to indicate the intensity (VU meters).

For routing of programs from various studios/OB spots to a central control room, we require a facility to further mix/select the programs. The Control Console in t h e control room performs this function. It is also called switching consol.

Before feeding the programs to the transmitter, the response of the program should be made flat by compensating HF and LF losses using equalized line amplifiers.(This is applicable in case of telephone lines only)

Visual signaling facility between studio announcer booth and control room should also be provided.

If the programs from various studios are to be fed to more than one transmitter, a master switching facility is also required.

1.4 MIXING As already mentioned, various equipments are available in a studio to generate program as given below:

Microphone, which normally provides a level of –70 dBm.

Turntable which provides an output of 0 dBm.

Tape decks which may provide a level of 0 dBm.

CD and R-DAT will also provide a level of 0 dBm.

The first and foremost requirement is that we should be able to select the output of any of these equipments at any moment and at the same time should be able to mix output of two or more equipments. However, as we see, the level from microphone is quite low and need to be amplified, so as to bring it to the levels of tape recorder/ tape decks. Audio mixing is done in following two ways:

Required equipments are selected and then outputs are mixed before feeding to an amplifier. This is called low level mixing (Fig. 2). This is not commonly used now days.

Low-level output of each equipment is pre-amplified and then mixed. This is called high level mixing.

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Figure1.2: Low Level Mixing

Low Level Mixing Low leve l mix in g s yst em ma y look econ omi ca l s in ce i t req u i res on e s in g le p re - amplifier for all low level inputs, but quality of sound suffers in this system as far as S/N ratio is concerned. Noise level at the input of best designed pre-amplifier is of the order of – 120 dBm and the output levels from low level equipment –70 dBm. In low level mixing, there is signal loss of about 10 to 15 dB in mixing circuits. Therefore, the S/N ratio achieved in low level mixing is 35 to 40 dB only.

Figure1.3: high level mixing

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High level mixing system requires one pre-amplifier in each of the low level channels but ensures a S/N of better than 50 dB. All India Radio employs high level mixing.

2. STUDIO EQUIPMENT 2.1 ANNOUNCER CONSOLE In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining (also called "mixing"), routing, and ch an g in g t h e leve l , t imb re an d /or d yn amics o f au d io s ign a ls . A mixe r can mi x an a lo g or digital signals, depending on the type of mixer. The modified signals (voltages or digital samples) are summed to produce the combined output signals. Mixing consoles are used in many applications, including recording studios, publicad d ress sys t em s, sou n d re in f orce men t syst ems, b ro a d cast in g , t e le v i s -io n , an d f i lm p ost - production. An example of a simple application would be to enable the signals that originated from two separate microphones (each being used by vocalists singing a duet, perhaps) to be heard through one set of speakers simultaneously. When used for live performances, the s ign a l p rod uced b y t h e mixer w i l l u su a l l y b e sen t d i rect ly t o an amp l i f ie r , u n less t h at particular mixer is "powered" or it is being connected to powered speakers. Most of the studios have an attached booth, which is called Transmission booth or Announcer booth or Play back studio. This is also acoustically treated and contains a mixing c o n s o l c a l l e d A n n o u n c e r C o n s o l e t h e A n n o u n c e r C o n s o l e i s u s e d f o r m i x i n g a n d controlling the programs that are being produced in the studio using artist microphones, tape playback decks and turn tables/CD players. This is also used for transmission of programs either live or recorded. The technical facilities provided in a typical Announcer Booth, besides an Announcer C the gramophone records and two playback decks or tape recorders for recorded programs on console are one or two microphones for making announcements, two turn tables for playing t ap es . Recen t ly CD an d Rot a ry Head D i g i t a l Au d io Tap e Record er (R- D AT) a re a l so included in the Transmission Studio. 2.2 AUDIO CONSOLE Th i s i s wh ere a l l t h e sou n d sou rces are mixed b ef ore b e in g sen t t o t h e t ran smit t er . Each s l id er , somet imes k n own ’s as a "p ot " on o ld er b oard s , con t ro ls t h e vo lu me of on e sou n d source: microphone, CD player, digital recorder, network feed, etc. Each slider channel has an on/off switch at the bottom and various switches at the top which can divert to more than one destination.

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A VU meter, such as the square box-like area toward the top of the console with the two green horizontal lines (center top), shows the operator the level of sound output. The top horizontal line is the left channel and the bottom line is the right channel.

The audio console converts analog audio (voice via microphone) and phone calls to a digital output. It also allows for the mixing of digital audio from CDs, computers, and other digital sources with the analog audio.

Figure2.1: Audio consol

In the case of Internet radio, the audio output would be uploaded to a server which then distributes the audio - or streams it - to listeners. 2.3 MICROPHONE Most radio stations have an assortment of microphones. Some microphones are especially designed for voice and on-air work. Often, these microphones will also have wind-screens over them, as this one does. The wind-screen keeps extraneous noise to a minimum such as the sound of breath blowing into the microphone or the sound of a "popping" "P". (Popping Ps occurs when a person pronounces a word with a hard "P" in it and in the process, expels a pocket of air that hits the microphone creating undesired noise.

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Figure2.2: Microphone This is another example of a high-end professional microphone. Most mikes of this caliber easily cost hundreds of dollars.

Figure2.3: Microphone

This microphone does not have an external windscreen. It is also on an adjustable mike stand and in this case is usually used for studio guests. M ost rad io s t at ion s h ave en t ered t h e d ig i t a l age wh ere n ot on ly i s a l l t h e mu s i c , commercials, and other sound elements stored digitally on hard drives, but sophisticated software is also used to either automatically run the station when a human can't be there or to help in assisting a live DJ or personality in running the station.

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2.3.1 WORKING OF A MICROPHONE Microphones are a type of transducer - a device which converts energy from one form to another. Microphones convert acoustical energy (sound waves) into electrical energy (the audio signal).Different types of microphone have different ways of converting energy but they all share one thing in common: The diaphragm. This is a thin piece of material (such as paper, plastic or aluminum) which vibrates when it is struck by sound waves. In a typical hand-held mic like the one below, the diaphragm is located in the head of the microphone.

Figure2.4: Location of Microphone Diaphragm

Wh e n t h e d ia p h ragm v ib rat es , i t cau se s ot h er comp on en t s in t h e microp h on e t ovibrate. These vibrations are converted into an electrical current which becomes the audio sign.

2.3.2 Types of Microphone There are a number of different types of microphone in common use. The differences can be divided into two areas: The type of conversion technology they use This refers to the technical method the mic uses to convert sound into electricity. The most common technologies are dynamic, condenser, ribbon and crystal. Each has advantages and disadvantages, and each is generally more suited to certain types of application. The following pages will provide details. The type of application they are designed for

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Some mics are d es ign ed f or gen era l u se an d can b e u sed e f f ect ive ly in man y d i f f eren t situations. Others are very specialized and are only really useful for their intended purpose. Characteristics to look for include directional properties, frequency response and impedance (more on these later).

Mic Level & Line Level The electrical current generated by a microphone is very small. Referred to as mic level, this signal is typically measured in mill volts. Before it can be used for anything serious the signal needs to be amplified, usually to line level (typically 0.5 -2V). Being a stronger and more robust signal, line level is the standard signal strength use d by audio processing equipment and common domestic equipment such as CD players, tape machines, VCRs, etc This amplification is achieved in one or more of the following ways:

Some microphones have tiny built-in amplifiers which boost the signal to a high mic level or line level.

The mic can be fed through a small boosting amplifier, often called a line amp

Sound mixers have small amplifiers in each channel. Attenuators can accommodate mics of varying levels and adjust them all to an even line level.

The audio signal is fed to a power amplifier - a specialized amp which boosts the signal enough to be fed to loudspeakers.

Figure2.5: Radio Station software

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There are various types of software designed to do this and it usually displays directly in front of the audio console where it clearly seen by the person on-air. This screen is displaying each element that has played and will play over the next 20minutes or so. It is a digital version of the station's log. 2.4 OTHER EQUIPMENT OF STUDIO 2.4.1 Head phones Radio personalities and deejays wear headphones to avoid feedback. When a microphone is turned on in a radio studio, the monitors (speakers) automatically mute. Th is way , t h e sou n d f rom t h e mo n i t ors won ' t re - en t er t h e microp h on e, cau s in g a feedback loop. If you've ever heard someone talking on a P.A. system at an event when it feedback, you know how annoying that noise can be.

Figure2.6: Head phone

So, when the monitors are muted because somebody turns on the microphone, the only way to monitor the broadcast is by using headphones to hear what's going on. As you can see, these are pretty weathered. But, then again professional headphones cost more and last longer. These are 10 years old!

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2.4.2 Radio Station Studio Sound proofing In order to keep the sounds of radio personality's voice sounding as good as possible, it’s important to soundproof a radio studio. Sound proofing takes the "hollow sound" out of a room. You know what it sounds like in your shower when you speak or sing? That effect is the sound waves bouncing off of smooth surfaces, like porcelain or tile.

Figure2.7: sound proofing S o u n d p r o o f i n g i s d e s i g n e d t o t a k e t h e b o u n c e o f t h e v o i c e ' s s o u n d w a v e w h e n i t h i t s the walls. Soundproofing flattens the sound wave. It does this by creating a special texture on the radio studios walls. Cloth and other designs on the wall are usually employed to flatten out the sound. 2.4.3 Radio Station Studio Phone Interface A key piece of equipment in most radio studios is the phone interface. It's a vital tool which allows the on-air personality to easily answer incoming phone calls, whether for on-air or off-air reasons. Phone interfaces come in various sizes and capacities but share the abilities. They allow for answering calls quietly and almost always have the capability of joining one or more calls together in conference mode which is especially useful for some talk shows.

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Figure2.8: phone Interface The phone interface's output goes directly to the audio console where the caller’s audio can be manipulated by the on-air personality.

2.4.4 Radio Station Studio Monitor When the microphones are turned off, the on-air personality can listen to the radio station through monitors which are hung in the radio studio. Monitors are basically speakers and for a stereo radio station, there would be one for each channel.

Figure2.9: studio monitor

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2.4.5 Radio Station Studio CD player Although most radio stations have converted to digital operations, some still do use CDs to play at least some of the audio used in their daily operations. CD players used in radio stations are professional grade and have LED information displayed about the CD to help the DJ or show host quickly "cue up" the track he needs.

Figure2.10: Radio station studio CD player 2.5 CONTROL ROOM For t wo or more s t u d ios set u p , t h ere wou ld b e a p rov is ion f or f u r t h er mix in g wh i ch i s p r ov id ed b y a con t ro l con so le man n ed b y en g in eers . Su ch con t ro l con so le i s k n own as Switching Console. Broad functions of switching console in control room are as follows:

Switching of different sources for transmission like News, O.Bs, and other satellite based relays, and live broadcast from recording studio.

Level equalization and level control

Quality monitoring.

Signaling to the source location

Communication link between control room and different studios.

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3. RADIO SIGNALS AND MODULATION Radio signals or radio waves are a form of electromagnetic wave. Although this may sound complicated, it is possibly sufficient to say that these waves have both electric and magnetic components. They are the same as light rays, ultra-violet and infra-red. The only difference is in the wavelength of the waves. To gain more of an idea of how the wave travels it can be likened to the surface of a pond when a stone is dropped into the water. Ripples spread out all around, decreasing in amplitude as they travel outwards. So it is with an electromagnetic wave although its action is somewhat more complicated.

Figure3.1: Radio signal analogy with ripples on a pond

3.1 Speed of a radio signal Another feature which can be noted about an electromagnetic wave is its speed. As it is the same as a l i gh t wa ve i t h as t h e sam e sp eed . Nor mal ly t h i s i s t a ken t o b e 3 x 10 ^8 meters a second but a more exact figure is 299 792 500 meters a second in a vacuum.

3.2 Frequency and wavelength There are a number of properties of the radio wave that can be measured. One of the first to be measured was the wavelength. Radio stations originally used to have their position on a radio dial determined by the wavelength. For example the BBC used to have one of their transmitters broadcasting on a wavelength of 1500 meters. The wavelength of a radio wave is the distance between a point on one wave to the identical

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point on the next. One of the most obvious points to choose as a reference is the peak as this can be easily identified although any point is equally valid provided the same point on each wave is taken.

The frequency can be explained using the pond analogy. It is the number of times the wave goes up and down in a given time and at a particular point in the pond.

Figure3.2: The wavelength of a radio signal

The unit used for f r e q u e n c y i s t h e H e r t z a n d t h i s c o r r e s p o n d s t o o n e c y c l e o r w a v e p e r s e c o n d . A s frequencies which are encountered can be very high the standard prefixes of kilo (kilohertz, k H z ) f o r a t h o u s a n d H e r t z , M e g a ( M e g a h e r t z , M H z ) f o r a m i l l i o n H e r t z , a n d G i g a (Gigahertz, GHz) for a thousand million Hertz are commonly used.

Wavelength and frequency conversion The speed, frequency and wavelength of a radio wave are all related to one another. As the speed is virtually the same whether the signal is travelling in free space, or in the atmosphere, it is very easy to work out the wavelength of a signal if its frequency is known. Conversely the frequency can be calculated if the wavelength is known. The formula is very simply:

v = λ x f

Where v =the velocity of the radio wave in meters per second (normally taken as 3 x 10^8m/s λ =the wavelength in meters f =the frequency in Hertz For example a signal with a frequency of 1 MHz will have a wavelength of 300 meters.

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R a d i o w a v e s a r e o n e f o r m o f e l e c t r o m a g n e t i c r a d i a t i o n . T h e y h a v e t h e l o w e s t frequency, and hence the longest wavelengths. Above the radio spectrum, other forms of radiation can be found. These include infra red radiation, light, ultraviolet and a number of other forms of radiation. Although they have different names, and they are often thought of as d i f f e r e n t e n t i t i e s , t h e y a r e a l l f o r m s o f e l e c t r o m a g n e t i c w a v e . T h e o n l y f u n d a m e n t a l difference is the wavelength / frequency. As a result of this difference they act in slightly different ways, and they may be used for different purposes. For example infra-red radiation may be used for heating, while light is used for illuminating areas and visibly seeing things. Nevertheless they are all fundamentally the same.

3.3 RADIO SPECTRUM Th e d i f f eren t t yp es o f e lect romagn et ic wave an d t h e i r re la t ive f req u en c ies an d wavelengths may be displayed on what is often termed the electromagnetic spectrum. This covers radio waves at the lower end with the lowest frequencies and longest wavelengths to infra-red, light and ultraviolet radiation and extending further up in frequency to radiation such as gamma and x-rays.

Figure3.3: Electro-magnetic wave spectrum Wh i le t h e wh ole o f t h e e lect r omagn e t i c wa v e sp e ct ru m cover s a h u ge ran ge o f frequencies, radio waves themselves extend over a very large range as well. Again it is useful t o b e ab le t o eas i l y re f er t o d i f f eren t sect ion s o f t h e sp ect ru m. T o ach i eve t h i s d i f f eren t d es ign at ion s are g iven t o d i f f eren t areas . Th e f req u en c ies t h at are cover ed are sp l i t in t o sections that vary by a factor of ten, e.g. from 3 MHz to 30 MHz Each section is allocated a name such as high

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frequency and these areas are abbreviated to give terms like HF, VHF and so forth that are often used. Often talk is heard of VHF FM, or UHF television. The VHF and UHF refer to the areas of the radio spectrum where these transmissions take place.

Figure3.4: various frequency ranges It can be seen from the diagram that transmissions in the long wave broadcast band which extends from 140.5 to 283.5 kHz available in some parts of the world falls into the lowf r e q u e n c y o r L F p o r t i o n o f t h e s p e c t r u m . T h e r e a r e a l s o a n u m b er o f o t h e r t y p e s o f transmission which are made here. For example there are a number of navigational beacons which transmit on frequencies around 100 kHz or less. Moving up in frequency, the medium wave broadcast band falls into the medium f requ en cy o r M F p ort ion o f t h e sp ect ru m. Ab o ve t h i s b road cast b an d i s o f t en wh ere t h e lowest frequency short wave bands start. Here there is an amateur radio band together with allocations for maritime communications. Between 3 and 30 MHz is the high frequency or HF portion. Within this frequencyr a n g e l i e t h e r e a l s h o r t w a v e b a n d s . S i g n a l s f r o m a l l o v e r t h e w o r l d c a n b e h e a r d . Broadcasters, radio amateurs and a host of others use them. M o v i n g u p f u r t h e r t h e v e r y h i g h f r e q u e n c y o r V H F p a r t o f t h e s p e c t r u m i s encountered. This contains a large number of mobile users. "Radio Taxis" and the like have allocations here, as do the familiar VHF FM broadcasts.

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In t h e u l t ra h igh f req u en cy o r U HF p art o f t h e sp e ct ru m most o f t h e t er rest r ia l television stations are located. In addition to these there are more mobile users including the increasingly popular cellular telephones. Above this in the super high frequency or SHF and extremely high frequency or EHF portions of the spectrum there are many uses for the radio spectrum. They are being used increasingly for commercial satellite and point to point communications.

Figure3.4: radio spectrum Radio signals are used for a huge variety of tasks. Radio signals are used to carry radio broadcasts, they are used to send signals to astronauts, establish Wi-Fi connections, for cellular communications and many, many more applications. Radio signals are essential to enable today's technology to function.

3.4 TYPES OF RADIO PROPAGATION Th ere are a nu mber o f cat egor ies in t o wh ich d i f f erent t yp es o f rad io p rop agat ion can b e placed. These relate to the effects of the media through which the signals propagate.

Free space propagation : Here t h e rad io wave s t rave l in f re e sp ace , o r awa y f rom ot h er ob ject s wh ich in f l u en ce t h e wa y in wh ich t h e y t rave l . I t i s on l y t h e distance from the source which affects the way in which the signal strength reduces. This type of radio propagation is encountered with radio communications systems including satellites where the signals travel up

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to the satellite from the ground and b ack d o wn aga in . Typ i ca l l y t h ere i s l i t t le in f lu en ce f rom e lemen t s su ch as t h e atmosphere, etc.

Ground wave propagation: When signals travel via the ground wave they are modified by the ground or terrain over which they travel. They also tend to follow the Earth’s curvature. Signals heard on the medium wave band during the day use this form of radio propagation.

Ionospheric propagation: Here the radio signals are modified and influenced by a region high in the earth's atmosphere known as the ionosphere. This form of radio propagation is used by radio communications systems that transmit on the HF or short wave bands. Using this form of propagation, stations may be heard from the other side of the globe dependent upon many factors including the radio frequencies used, the time of day, and a variety of other factors.

Tropospheric propagation: Here the signals are influenced by the variations of refractive index in the troposphere just above the earth's surface. Tropospheric radio propagation is often the means by which signals at VHF and above are heard over extended distances.

In addition to these categories, many short range wireless or radio communications systems have radio propagation scenarios that do not fit neatly into these categories. Wi-Fi systems, for example, may be considered to have a form of free space radio propagation, but there will be will be very heavily modified because of multiple reflections, refractions and diffractions. Despite these complications it is still possible to generate rough guidelines and models for these radio propagation scenarios. There are many radio propagation scenarios in real life. Often signals may travel by several means, radio waves travelling using one type of propagation interacting with another. However to build up an understanding of how a radio signal reaches a receiver, it is necessary t o h a v e a g o o d u n d e r s t a n d i n g o f a l l t h e p o s s i b l e m e t h o d s o f r a d i o p r o p a g a t i o n . B y understanding these, the interactions can be better understood along with the performance of any radio communications systems that are used.

3.5 SIGNAL MODULATION Amplitude modulation or AM as it is often called is a form of modulation used for rad io t ran smiss i on s f or b road cast in g an d t wo wa y rad i o commu n icat ion ap p l i cat ion s . Although one of the earliest used forms of modulation it is still in widespread use today. The first amplitude modulated signal was transmitted in 1901 by a Canadian

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engineer named Reginald Fessenden. He took a continuous spark transmission and placed a carbon microphone in the antenna lead. The sound waves impacting on the microphone varied its resistance and in turn this varied the intensity of the transmission. Although very crude, signals were audible over a distance of a few hundred meters, although there was a rasping sound caused by the spark. Wit h t h e in t rod u ct ion o f con t in u ou s s in e wave s ign a ls , t ran sm iss ion s imp roved s ign i f i c an t ly , an d A M soon b ecame t h e s t an d ard f or vo ice t ran smiss ion s . Nowad ay s , amplitude modulation, AM is used for audio broadcasting on the long medium and shortwave bands, and for two way radio communicati on at VHF for aircraft. However as there now are more efficient and convenient methods of modulating a signal, its use is declining, although it will still be very many years before it is no longer used.

3.5.1 Amplitude Modulation In order that a radio signal can carry audio or other information for broadcasting or for two way radio communication, it must be modulated or changed in some way. Although there are a number of ways in which a radio signal may be modulated, one of the easier, and one of the first methods to be used was to change its amplitude in line with variations of the sound. T h e b a s i c c o n c e p t s u r r o u n d i n g w h a t i s a m p l i t u d e m o d u l a t i o n , A M , i s q u i t e st ra igh t f orward . Th e amp l i t u d e o f t h e s ign a l i s ch an ged in l ine wi t h t h e in st an t an eou s intensity of the sound. In this way the radio frequency signal has a representation of the sound wave superimposed in it. In view of the way the basic signal "carries" the sound or modulation, the radio frequency signal is often termed the "carrier".

Wh en a carr i er i s mod u lat ed in an y wa y, f u r t h er s i gn a ls are creat ed t h at carry t h e actual modulation information. It is found that when a carrier is amplitude modulated, further signals are generated above and below the main carrier. To see how this happens, take the example of a carrier on a frequency of 1 MHz which is modulated by a steady tone of 1 kHz.

T h e p r o c e s s o f m o d u l a t i n g a c a r r i e r i s e x a c t l y t h e s a m e a s

M i x i n g t w o s i g n a l s together and as a result both sum and difference frequencies are produced. Therefore when atone of 1 kHz is mixed with a carrier of 1 MHz, a "sum" frequency is produced at 1 MHz + 1kHz, and a difference frequency is produced at 1 MHz - 1 kHz, i.e. 1 kHz above and below the carrier.

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If the steady state tones are replaced with audio like that encountered with speech of music, these comprise many different frequencies and an audio spectrum with frequencies over a band of frequencies is seen. When modulated onto the carrier, these spectra are seen above and below the carrier.

Figure3.5: Amplitude Modulation, AM

It can be seen that if the top frequency that is modulated onto the carrier is 6 kHz, then the top spectra will extend to 6 kHz above and below the signal. In other words the bandwidth occupied by the AM signal is twice the maximum frequency of the signal that is used to modulated the carrier, i.e. it is twice the bandwidth of the audio signal to be carried.

Amplitude demodulation Amplitude modulation, AM, is one of the most straight forward ways of modulating a radio signal or carrier. The process of demodulation, where the audio signal is removed from the radio carrier in the receiver is also quite simple as well. The easiest method of achieving amplitude demodulation is to use a simple diode detector. This consists of just a handful of components:- a diode, resistor and a capacitor.

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Figure3.6: AM Diode Detector

In t h i s c i rcu i t , t h e d iod e rect i f ies t h e s ign a l , a l lo win g on ly h a l f o f t h e a l t ern at in g waveform through. The capacitor is used to store the charge and provide a smoothed output f rom t h e d et ect o r , and a l so t o remove a n y u n wan t ed rad io f req u e n cy comp on en t s . Th e resistor is used to enable the capacitor to discharge. If it was not there and no other load was present, then the charge on the capacitor would not leak away, and the circuit would reach a peak and remain there.

Advantages of Amplitude Modulation, AM There are several advantages of amplitude modulation, and some of these reasons have meant that it is still in widespread use today:

It is simple to implement

it can be demodulated using a circuit consisting of very few component

AM receivers are very cheap as no specialized components are needed.

Disadvantages of amplitude modulation Amplitude modulation is a very basic form of modulation, and although its simplicity is one of its major advantages, other more sophisticated systems provide a number of advantages. Accordingly it is worth looking at some of the disadvantages of amplitude modulation.

It is not efficient in terms of its power usage

It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency

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AM has advantages of simplicity, but it is not the most efficient mode to use, both in terms of the amount of space or spectrum it takes up, and the way in which it uses the power t h a t i s t r a n s m i t t e d . T h i s i s t h e r e a s o n w h y i t i s n o t w i d e l y u s ed t h e s e d a y s b o t h f o r broadcasting and for two way radio communication. Even the long, medium and short wave broadcasts will ultimately change because of the fact that amplitude modulation, AM, is subject to much higher levels of noise than are other modes. For the moment, its simplicity, and its wide usage, means that it will be difficult to change quickly, and it will be in use for many years to come.

While changing the amplitude of a radio signal is the most obvious method to modulate

it, it is by no means the only way. It is also possible to change the frequency of a signal to give frequency modulation or FM. Frequency modulation is widely used on frequencies above 30 MHz, and it is particularly well known for its use for VHF FM broadcasting.

Although it may not be quite as straightforward as amplitude modulation,

neverthelessf requ en cy mod u lat io n , FM , o f f ers some d is t in c t ad van t age s . I t i s ab le t o p ro v id e n ea r interference free reception, and it was for this reason that it was adopted for the VHF sound broadcasts. These transmissions could offer high fidelity audio, and for this reason, frequency modulation is far more popular than the older transmissions on the long, medium and shortwave bands.

In addition to its widespread use for high quality audio broadcasts, FM is also sued

for a var ie t y o f t wo way r ad io commu n i cat ion syst em s. Wh et h e r f or f i xed o r mob i le rad io communication systems, or for use in portable applications, FM is widely used at VHF and above.

3.5.2 Frequency Modulation To generate a frequency modulated signal, the frequency of the radio carrier is changed in line with the amplitude of the incoming audio signal.

When the audio signal is modulated onto the radio frequency carrier, the new radiofrequency signal moves up and down in frequency. The amount by which the signal moves up and down is important. It is known as the deviation and is normally quoted as the number of kilohertz deviation. As an example the signal may have a deviation of ±3 kHz. In this case the carrier is made to move up and down by 3 kHz. Broadcast stations in the VHF portion of the frequency spe ctrum between 88.5 and 108 MHz use large values of deviation, typically ±75 kHz. This is known as wide-band FM (WBFM). These signals are capable of supporting high quality transmissions, but occupy a l a r g e a m o u n t o f b a n d w i d t h . U s u a l l y 2 0 0 k H z i s a l l o w e d

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f o r e a c h w i d e - b a n d F M t ran smiss ion . Fo r commu n icat ion s p u rp ose s less b an d wid t h i s u sed . Narrow b an d F M (NBFM) often uses deviation figures of around ±3 kHz. It is narrow band FM that is typically used for two-way radio communication applications. Having a narrower band it is not able to provide the high quality of the wideband transmissions, but this is not needed for applications such as mobile radio communication.

Figure3.7: Frequency Modulation, FM Advantages of frequency modulation, FM F M i s u s e d f o r a n u m b e r o f r e a s o n s a n d t h e r e a r e s e v e r a l a d v a n t a g es o f f r e q u e n c y modulation. In view of this it is widely used in a number of areas to which it is ideally suited. Some of the advantages of frequency modulation are noted below:

Resilience to noise: One particular advantage of frequency modulation is its resilience to signal level variations. The modulation is carried only as variations infrequency. This means that any signal 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 radio communication applications including more general two -way radio communication or portable applications where signal levels are likely to vary considerably. The other advantage of FM is its resilience to noise and interference. It is for this reason that FM is used for high quality broadcast transmissions.

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Easy to apply modulation at a low power stage of the transmitter : Another advantage of frequency modulation is associated with the transmitters. It is possible to apply the modulation to a low power stage of the transmitter, and it is not n ecessar y t o u se a l in ear f orm of amp l i f i cat ion t o in creas e t h e p ower leve l o f t h e signal to its final value.

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 transmitter and these are more efficient than the linear ones required for signals with an y amp l i t u d e var ia t ion s (e .g . AM an d SS B ) . Th i s mean s t h at f or a g iven p ower output, less battery power is required and this makes the use of FM more viable for portable two-way radio applications.

F req u en cy mod u lat i on i s wid e l y u sed in man y areas o f rad i o t ech n o logy in c lu d in g broadcasting and areas of two way radio communication. In these applications its particular advantages can be used to good effect. For the future, other forms of digital modulation are becoming more widely used - DAB for radio broadcasting and a number of other formats such as TETRA for two-way radio communication systems. Despite these changes, FM will remain in use for many years to come as there are many advantages of frequency modulation for the areas in which it has gained a significant foothold in recent years.

As the name implies, wideband FM (WFM) requires a wider signal bandwidth

than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio"( a l t h o u g h f o r m a n y y e a r s t h e B B C c a l l e d i t " V H F r a d i o " , b e c a u s e c o m m e r c i a l F M broadcasting uses a well-known part of the VHF band—the FM broadcast band).

FM receivers employ a special detector for FM signals and exhibit a

phenomenon called capture effect, where the tuner is able to clearly receive the stronger of two stations being broadcast on the same frequency. Problematically however, frequency drift or lack of se lect iv i t y ma y cau se on e s t at ion or s ign a l t o b e su d d en ly overt a ke n b y an ot h er on an adjacent channel. Frequency drift typically constituted a problem on very old or inexpensive receivers, while inadequate selectivity may plague any tuner. An FM signal can also be used to carry a stereo signal: see FM stereo. However, this is done by using multiplexing and de-multiplexing before and after the FM process. The rest of this article ignores the stereo multiplexing and de-multiplexing process used in "stereo FM",

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and concentrates on the FM modulation and demodulation process , which is identical in stereo and mono processes. A high-efficiency radio-frequency switching amplifier can be used to transmit FM signals (and other constant-amplitude signals). For a given signal strength (measured at the receiver antenna), switching amplifiers use less battery power and typically cost less than a l in ear amp l i f ie r . Th i s g ives FM an ot h er ad van t age ove r ot h er mod u lat io n sch eme s t h at require linear amplifiers, such as AM and QAM.

FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech. Normal (analog) TV sound is also broadcast using FM. A narrow band form is u se d f or vo ice commu n ica t ion s in com merc ia l an d amat eu r rad io set t in gs . In b road c a st services, where audio fidelity is important, wideband FM is generally used. In two-way radio, n arrowb a n d FM (NBFM ) i s u sed t o con ser ve b an d wid t h f or lan d mob i le rad io s t at ion s , marine mobile, and many other radio services.

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4 .AMPLIFIERS USED IN AIR STUDIOS 4.1 INTRODUCTION Amp l i f ie r i s on e o f t h e b as ic b u i ld in g b lock s o f mod er n e lect ron ics . Th e p resen t d ay electronics would not exist without this. Amplification is necessary because the desired signals usually too weak to be directly useful. Present day amplifiers used in studios are mostly employing ICs and transistors.

4.2 TERMS USED WITH REFERENCE TO AMPLIFIERS

Input Impedance

Output Impedance

Distortion

Gain

Noise and Equivalent Input Noise

Frequency response Some of these terms have been explained briefly in the following paragraphs

4.2.1 Input impedance It is defined as the impedance which we get while looking into the input terminals of an amplifier. The input impedance of a pre-amplifier determines the amount of A.C. voltage the p re - amp l i f ie r wi l l get f rom a mi crop h on e. Th e in p u t imp ed an ce a l so d ec id es t h e n o i se performance of the amplifier. For best noise performance, the input impedance of a preamplifier should exceed ten times the source impedance. It is because of this reason that the in p ut imp ed an ce o f a p re amp l i f ie r i s a l way s 2000 oh m or more . In so me amp l i f ie rs a bridging input is provided. This implies that the input impedance is 10,000 ohm or greater and this impedance is achieved by using a special input transformer. Bridging input permits several amplifiers to be connected across a line without upsetting the impedance match of other equipment.

4.2.2 Output impedance The actual impedance seen when looking into the output terminals of an amplifier is called its output impedance. This term should not be confused with load impedance. Load impedance is defined as the specified impedance into which a device is designed to work. Many times the load impedance is higher than the output impedance. For example the output

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impedanceo f e q u a l i s e d l i n e a m p l i f i e r t y p e l a b 5 6 8 i s l e s s t h a n 5 0 o h m w h i l e t h e s p e c i f i e d l o a d impedance is 600 ohm.

4.2.3 Distortion in amplifiers The amplification of a sinusoidal signal to the input of an ideal class - A amplifier will result in a sinusoidal output wave. Generally the output waveform is not an exact replica of the input signal waveform because of various types of distortions that may arise either from the inherent non-linearity in the characteristics of the active device or from the influence of the associated circuit. The types of distortions that may exist either separately or simultaneously are called

Non-linear distortion

Frequency distortion and

Delay or phase shift distortion

4.2.3.1 Non linear distortion This type of distortion results from the production of new frequencies in the output, which is not present in the input signal. These new frequencies or harmonics result from the existence of non-linear dynamic curve for the active devices. The distortion is sometimes referred to as amplitude distortion or harmonic distortion. This type of distortion is more prominent when the signal levels are quite large so the dynamic operation spreads over a wide range of the characteristics.

4.2.3.2 Frequency distortion Th i s t yp e o f d i s t or t ion ex i s t s wh en t h e s ign a l comp on en t s o f d i f f eren t f requ en c ies areamplified differently. In a transistor amplifier, this type of distortion may be caused either by the internal device capacitances or it may arise because of the associated circuit such as, the coupling components. If the frequency response characteristic is not a straight line over the range of frequencies under consideration, the circuit is said to exploit frequency distortion over this range.

4.2.3.3 Phase shift or delay distortion Phase shift distortion results from unequal phase shifts of signals of different frequencies. This type of distortion is not important in audio frequency amplifiers since the human ear is incapable of distinguishing relative phases of different frequency components. But it is very objectionable in the system that depends on the wave shape of the signal for their operation e.g. in television.

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4.2.4 Gain: The gain of an amplifier of unequal input and output impedance is given as

Gain (in db) = 20 log Where E1 is the voltage at the input E2 is the voltage across the output load terminations

Z1 is the input impedance Z 2 is the output load impedance

.

4.2.5 Noise and Equivalent Input Noise The term noise used broadly to describe any spurious electrical disturbance that causes an output when the signal is zero. Noise may be produced by causes which may be external to the system or internal to the system regardless of where it originates in the amplifier, the noise is conveniently expressed as an equivalent noise voltage at the input that would cause the actual noise output. This noise is amplif ied along with the signal and tends to mask up the signal at the output. If in an amplifier, the noise at output is 50dB below the output signal level, then the equivalent noise at the input of the amplifier, which has a gain of 70 dB, will be -120 dbm.

4.3 AUDIO AMPLIFIERS USED IN AIR STUDIO Th e f o l lowin g are some of t h e au d io amp l i f ie rs u sed in A IR . A l l t h ese amp l i f ie rs are designed to have a frequency response within ±1 d B f rom ab ou t 30 Hz t o 10 KHz wi t h respect to 1 KHz and a total harmonic distortion (THD) of less than 1% at maximum rated output power .

Pre-Amplifier Pre-amplifier is the first amplifier in the broadcast chain. The output from a microphone or a p icku p wh ich i s a t very lo w leve l ( - 70 d Bm ) i s f ed t o i t s in p u t . Th e amp l i f ied s ign a l s obtained from this amplifier are given to the programme amplifier through a fader box or through a mixing console. The normal gain of this amplifier is about 50 dB.

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Programme Amplifier Programme amplifier provides second stage of amplification. The output obtained from the fader box or mixing console is fed to the input of this amplifier. The normal input level to this amplifier varies from -45to 20 dBm. This amplifier gives a maximum output of +27d Bm. I t h as a ga in o f 70 d B wh ich i s var ia b le f rom 0 t o 70 d B. Th e in p u t an d ou t p u t impedance are usually 600 ohm.

Monitoring Amplifier: T h e o u t p u t a v a i l a b l e f r o m t h e p r o g r a m m e a m p l i f i e r i s h o w e v e r , n o t e n o u g h t o d r i v e loudspeaker. Therefore, monitoring amplifiers are provided to boost these signals further. A part of the output signal from the programme amplifier is given to the monitoring amplifier. The output of the monitoring amplifier is usually fed to a monitoring bus for further feeding to the loudspeakers. A separate monitoring amplifier is used for a group of loudspeakers which are located in studios, control room, duty room and other selected places. Monitoring amplifiers of different wattage ratings are used in AIR. But 8 watt monitoring amplifier is very common.

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5. MEDIUM WAVE TRANSMITTER M ed iu m wave (M W ) i s t h e p art o f t h e med iu m f req u en cy (M F) rad io b an d Used main ly for AM broadcasting. Medium wave signals have the property of following the curvature of the earth (the ground wave) at all times, and also refracting off the ionosphere at night (sky wave ) . Th is m ake s t h i s f req u en c y b a n d id ea l f or b ot h loca l an d con t in en t - wid e serv ice , depending on the time of day.

When the electromagnetic waves in the medium wave (MW) range are directed towards the Ionosphere, they are absorbed by the D-region during the day time and are reflected from the E layer during the night time, which may travel longer distances to cause interferences. Th e wave len gt h o f M W s ign a ls a re ver y l a rg e , o f t h e ord er o f f ew h u n d red met ers , an d therefore the antenna cannot be mounted a few wavelengths above the earth to radiate as space waves. MW antenna, therefore, have to exist close to the surface of the earth and the Radio waves from them have to travel close to the earth as ground waves. If the electric vector of such MW radiation is horizontal, they will be attenuated very fast with distance due to the proximity of the earth. MW antennas have to be placed vertically, so that they ra diate vertically polarized signals. It is for this reason; all the MW antennas are installed vertically close to the ground. However vertical wire antenna, inverted 'L' type antenna, top loaded antenna and umbrella antenna are at a few All India Radio stat ions.

Directional antenna systems also exist in many All India Radio stations. A.M. Transmitter of any power in general will have a separate HF and AF stages. In the conventional transmitters, vacuum tubes are used right from the first stage to the final stage and the preliminary stages are solid state devices. A brief description of RF and AF stages and Power Supply of 10 kW transmitters...

Figure5.1 : block diagram of AM Transmitter

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5.1 RF SECTION

RF section consists of crystal oscillator, buffer, intermediate power Amplifier,

and Exciter and power amplifier.

Figure5.2 Block Diagram of RF chain

5.1.1 CRYSTAL OSCILLATOR AND BUFFER STAGE

The crystal oscillator with buffer stage is generally kept together and is shielded by a metal cover to isolate from other circuits. This crystal oscillator employs a pentode tube 6 AU 6 or its equivalent, connected as a triode. The frequency of oscillation is controlled by a quartz crystal and by a variable trimmer capacitor.

The frequency of the medium wave transmitter should be highly stable. For medium wave transmitter operating in the range of 540 kHz to 1602 kHz, the variation of a transmitter frequency should be within a tolerance of + 10 Hertz. To maintain a high stability of the t ran smit t er f requ en cy i t i s n eces sar y t h at t h e osc i l la t or sh ou ld osc i l la t e a t a p ar t i cu lar frequency against variations in voltage and ambient temperature. Hence the crystal is kept in constant temperature ovens whose temperature is controlled by a thermostat and maintained at 75 degrees +/- 1.5 degrees centigrade.

The oscillator frequency changes considerably under initial transient condition that is when power is switched ON. However, it is essential to keep it always ready at a stable condition. To facilitate a separate power supply is provided to feed the oven which can b e swi t ch ed ON an d OFF wi t h t h e h e lp o f a sn ap swi t ch locat ed on t h e AE p an e l o f t h e transmitter. Two crystal units X1 and X2 housed separately in different ovens Z1 and Z2viz. a normal and a stand by unit are provided. Either one of them can be selected by means of change over switch S2. However, both the ovens Z1and Z2 are kept ON all the time. The oscillator output comes to the buffer stage or its equivalent. It acts as a buffer between the oscillator and the intermediate power amplifier (IPA). Its output can be tuned byan adjustable dust iron core of coil L.

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5.1.2 IPA STAGE Th i s s t age emp loy s an in d i rect ly h eat e d b eam p ower t u b e an d i t op erat es as a c la ss amplifier.

5.1.3 EXCITER This stage is operated as a class - C amplifier, employing air cooled tetrode type and drives P.A. stage. Screen supply is taken from plate supply. The output is a tuned circuit consists of a fixed capacitor C and coil. L3 is having a flipper, through it, fine tuning can be made. This stage is modulated about 10 to 20%. A small secondary tap from the modulation transformer supplies the necessary audio and super-imposes on the DC Plate supply. When the triodes are anode modulated, the grid must be overdriven in the carrier condition in order that the drive level will be adequate to sustain the peak anode current at 100% modulation. Alternatively the drive must be modulated. Hence the 10 to 20% modulation. With tetrode the same effect is achieved by modulating the screen enabling the anode current peaks to be attained with the same drive level as that required for the carrier only condition. To some extent this ceases the grid dissipation limit. .

5.1.4 POWER AMPLIFIER STAGE This is a class - C power amplifier obtaining the required output by means of three parallel connected forced air cooled, directly heated triode tubes type BEL 3000. As a triode tube issued in this stage, neutralization technique is adopted to neutralize, the grid-plate capacitance. T h e o u t p u t c i r c u i t i s f o r m e d b y P I ( π) s e c t i o n a n d ' L ' s e c t i o n m a d e u p o f c o i l s a n d condensers. There is a variable coil to tune the output. A second harmonic filter is connected at the output which attenuates the harmonics. This filter is a simple L C circuit tuned to the second harmonic frequency. The output circuit also matches the plate impedance of about1 1 0 0 o h m s t o t h e f e e d e r i m p e d a n c e o f 2 3 0 o h m s , w h i c h i s c a r r i e d o u t a t t h e t i m e o f in st a l la t ion o f th e t ran smit t er u s in g Imp ed an ce B r id ge .At t h e t ime o f main t en an ce , care should be taken that the coil settings are not disturbed.

5.2 AF CIRCUITS The audio frequency amplifier consists of two voltage amplifiers, a cathode follower which serves as a driver to the modulator and the modulator is a class B push pull power Amplifier .

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5.2.1 First and Second AF Amplifier Stages: This stage is operated as a class A push pull connected amplifier employing two indirectly heated pentode type which provides about 30 dB gain. The output from the first AF stage is coupled to the second stage through the coupling condensers. Plate supply is obtained from the neutral of the Transformer.

5.2.2 Sub Modulator Stage: This stage employs two in push-pull mode to excite the modulator. The sub-modulator is a cathode follower. As the grid current flows in the modulator tube, the input impedance varies widely with different input levels and hence a cathode follower which possesses low output imp ed ance , very s mal l n on - l in ear d i s t or t ion f or load imp ed an c e var ia t ion s an d good frequency and phase shift characteristics is used. The DC Potential of the cathodes of sub-modulator and the grid of the modulator stages are kept nearly at the same negative voltage of about 200 volt.

Modulator amplifier Th is i s t h e f in a l s t age au d io f re q u en cy p ower amp l i f ie r wh ich su p p l ies t h e RF p ower amplifier, the required modulating power. The HT and the superimposed audio signals are connected to the plate of the PA valves. It may be noted that the negative feedback Network is connected in the primary of the modulation transformer. Low tension 3 p h a s e 2 2 0 V A C i s s t e p p e d u p t o 3 p h a s e 5 2 0 V A C u s i n g a D e l t a / S t a r c o n n e c t e d transformer. It is rectified using silicon diodes and filtered using L C components. It gives DC voltage to the following.

Bias 3 phase 400 V AC is stepped up to 3 phase 470 V using Delta/Star connected transformer and rectified using silicon diodes in two sets SE 2 and SE3 and filtered using L -C components. SE 2 output supply is connected to the cathode Bias of sub modulator. The output of SE 3 is connected to control grid of Exciter and Grid of P.A.

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High tension 3 p h ase 400 V AC i s s t ep p ed u p t o 2300 V 3 p h ase an d rect i f ied u s in g s i l i con d iod e s assembly SE4 and filtered using L-C components. Full HT is supplied to plate of modulator and PA valves. The filtered DC from the star point of the HT transformer is connected to the plate of 2nd AF and plate and screen grid of Exciter.

Figure5.3: high tension

The AF stage supply the audio power required to amplitude modulates the final RF stage.

Theo u t p u t o f t h e A F s t a g e i s s u p e r i m p o s e d u p o n t h e D C v o l t a g e t o

t h e R F P A t u b e v i a modulation transformer. An Auxiliary winding in the modulation

transformer, provides the AF voltage necessary to modulate the screen of the final stage. The

modulator stage consists of two CQK-25 ceramic tetrode valves working in push pull class

B configuration. The drive stages up to the grid of the modulator are fully transistorized.

5.2.2.1 High pass filter Th e au d io in p u t f rom t h e sp eech ra c k i s f ed t o a ct ive High Pa ss F i l t er . I t cu t s o f f a l l frequencies below 60 Hz. Its main function is to suppress the switching transistors from the audio input. This also has the audio attenuator and audio muting relay which will not allow AF to further stage till RF is about 70 kW of power.

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5.2.2.2 AF Pre-amplifier The output of the High Pass Filter is fed to the AF Pre-amplifier, one for each balanced audio line. Signal from the negative feedback network from the secondary of the modulation transformer and the signals from the compensator also are fed to this unit .

5.2.2.3 AF Pre-corrector Pre- amplifier output is fed to the AF Pre-correctors. As the final modulator valve in the AF is operating as Class B, its gain will not be uniform for various levels of AF signal. That is the gain of the modulator will be low for low level, input, and high for high level AF input because of the operating characteristics of the Vacuum tubes. Hence to compensate for the non linear gain of the modulator. The Pre-corrector amplifies the low level signal highly and high level signal with low gain. Hum compensator is used to have a better signal to noise ratio. 5.2.2.4 AF Driver 2 AF drivers are used to drive the two modulator valves. The driver provides the necessary DC Bias voltage and also AF signal sufficient to modulate 100%. The output of AF driver stage is formed by four transistors in series as it works with a high voltage of about -400V.The transistors are protected with diodes and Zener diodes against high voltages that may result due to internal tube flashovers. There is a potentiometer by which any clipping can be avoided such that the maximum modulation factor will not exceed. 5.2.3 AF FINAL STAGE AF final stage is equipped with ceramic tetrodes CQK -25. Filament current of this

tube is about 210 Amps. at 10V. The filament transformers are of special leakage

reactance type and their short circuit current is limited to about 2 to 3 times the normal load

current. Hence the filament surge current at the time of switching on will not exceed the

maximum limit.

A varistor at the screen or spark gaps across the grid are to prevent over voltages. As the

modulator valve is condensed vapor cooled tetrodes, de ionized water is used for cooling. The

valve required about 11.5 liters/min. of water. Two water flows switches WF1 and WF2in the

water lines of each of the valves protect against low or no water flow. Thermostats

WT1 an d WT2 in each wat er l in e p rov id e p ro t ect ion aga in st exce ss i ve

wat er t emp . b y tripping the transmitter up to stand-by if the temperature of the water

exceeds 70 degrees centigrade.

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Modulation condenser and modulation choke have been dispensed with due

to the special design of the modulation transformer. Special high power varistor is provided

across the secondary winding of the modulation transformer to prevent transformer over

voltages.

Power Supply in 100 kW HMB 140 MW Transmitter

Earthing switch operated by a handle from the front of the rack has been provided in the filter tank. The main HT terminal and also the live ends of the filter condensers C201 to C210 have been brought to the earthing switch. In addition all the MT voltage (- 650, 800,1070, 1900) are also brought to the earthing switch. The 11 kV point is discharged initially through a resistor R - 543 before it is grounded. The earthing switch is interlocked to the main transmitter by micro switches S 302, S 303 and S 304. In addition, a key interlock system is provided to prevent accidental contact with high voltages.

5.3 MEDIUM WAVE ANTENNA5. 5.3.1 Introduction When the electromagnetic waves in the medium wave (MW) range are directed towards the Ionosphere, they are absorbed by the D-region during the day time and are reflected from the E-layer during the night time, which may travel longer distances to cause interferences. The wave length of MW signals are very large, of the order of few hundred meters, and therefore the antenna cannot be mounted a few wavelengths above the earth to radiate as space waves.MW antenna, therefore, have to exist close to the surface of the earth and the Radio waves from them have to travel close to the earth as ground waves. If the electric vector of such M W rad iat ion i s h or i z on t a l , t h ey wi l l b e at t en u at ed very f ast wi t h d i s t an ce d u e t o t h e p rox imit y o f t h e eart h . M W an t en n a h as t o b e p laced vert i c a l ly , so t h at t h ey rad iate vertically polarized signals. It is for this reason; all the

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MW antennas are installed vertically close to the ground. However vertical wire antenna, inverted 'L' type antenna, top loaded.

Antenna and umbrella antenna are at a few All India Radio stations. Directional antenna systems also exist in many All India Radio stations

5.3.2 Self Radiating Mast Antennas They are broadly of two types:

Mast isolated from ground and fed at its base.

Grounded mast fed at a suitable point along its height The first consideration of such mast is its height in terms of the wave length. What is the optimum height ? Obviously the main considerations are economy consistent with maximum coverage and minimum high angle radiation (sky wave).

Figure5.4: MW Antenna isolated from ground

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6. TRANSMITTER CONTROL AND I NTERLOCKING SYSTEM 6.1 SWITCHING SEQUENCE OF TRANSMITTER

Ventilation.

Filament

Grid Bias/Medium Tension

High Tension

6.1.1 Ventilation Al l t h e t ran smit t ers h an d le la rge amou n t o f p ower . Bas ica l ly t h e t ran smit t ers con ver t p ower f rom AC m ain 's t o Rad io Freq u en cy an d Au d io F req u en cy . The conversion process always results in some loss. The loss in energy is dissipated in the form of heat. The dissipated energy has to be carried away by a suitable medium to keep the raise in temperature of the transmitting equipment within limits. Hence, in order to ensure that the heat generated by the equipment is carried away as soon as it is generated the ventilation equipment need to be switched on first. Normally the cooling provided in a transmitter could be classified on the following lines:

Cooling for the tube filaments.

Cooling for the tube Anodes.

General cooling of the cubic’s.

Cooling for coils, condensers, Resistors etc

The cooling equipments comprise of blowers, pumps and heat exchangers. Another important consideration is that during the switching off sequence the cooling equipments should run a little longer to carry away the heat generated in the equipments. This is ensured by providing a time delay for the switch off of the cooling equipment. Normal time delay is of the order of 3 to 6 Minutes.

Th e wat er f lo w an d t h e a i r f lo w p ro v i d ed b y t h e coo l in g eq u ip men t s t o t h e var iou s equipments are monitored by means of air flow and water flow switches. In case of failure of water or air flow, these switches provide necessary commands for tripping the transmitter.

6.1.2 Filaments All the transmitters invariably employ tubes in their drive and final stages of RF amplifiers and sub modulator and modular stages of AF amplifiers. After ventilation equipments are

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switched on and requisite air and water flow established, the filament of the t u b e s c a n b e s w i t c h e d o n . W h i l e s w i t c h i n g o n f i l a m e n t o f t h e t u b e , t h e c o n t r o l a n d interlocking circuits have to take care of the following points.

The cold resistance of the filament is very low and hence application of full filament voltage in one strike would result in enormous filament current and may damage the tube f i lamen t . Hen ce , i t b ecomes n ecessa ry t o ap p ly t h e f i lamen t vo l t age in s t ep s . Var iou s methods adopted are:

Use of s t ep s t ar t er res i s t an ce : Here t h e f i lamen t vo l t ages o f t h e t u b es are g iven through a series resistance (called step starter resistance). The series resistance which limits the initial filament current is shorted and after a time interval by the use of a timer switch.

Use of sp ec ia l f i l am en t t ran sf ormer wh ich a l lo ws s lo w b u i l d u p o f t h e f i lamen t voltage.

Application of filament voltage in 3 or 4 steps

The emission from the tubes depends upon the temperature of the filament. Generally it

takes some time for the filament to reach a steady temperature after it is switched on. Hence, it is not desirable to draw any power from the tube till it attains a stable temperature. This means that the further switching on process has to be suspended till the filament temperature and hence the emission becomes stable. This aspect is taken care of by providing a time delay of 3 to 5 minutes between the filament switching on and the next sequence namely bias switching on.

6.1.3 bias and medium tension For obvious reasons the control grid of the tube has to be given the necessary negative bias voltage before its anode voltage can be applied. Hence, after the application of full filament voltage and after the lapse of necessary delay for the filament temperature to become stable bias voltage can be switched on. Along with biasgen era l ly an od e an d screen vo l t age s o f in t ermed iat e s t ages an d d r iver s t ages are a l so switched on. Application of bias and medium tension makes available very high voltages for the various transmitter equipments. Hence, in order to ensure the safety of the personnel, access to this equipment should be forbidden before the application of bias and medium tension. This is ensured by providing the interlocking so that the bias and medium tension can b e p ut on on ly a f t er a l l t h e t ran smit t er an d ot h er HV eq u ip men t d oors are c losed t o prevent access.

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6.1.4 Connection of load (Antenna/Dummy load): A f t er t h e ap p l i cat ion o f ven t i la t ion , filament and bias the anode voltage can be switched on. But before the anode voltage can be increased the interlocking circuit is to ensure that the load of the transmitter namely antenna or dummy load is connected to the transmitter. The tuning processes of the various RF stages are complete and none of the tuning motors are moving Application of screen voltage In t h e case o f t et r od e t u b es , t h e scr een vo l t age t o t h e t u b e should not be applied before the application of anode voltage to keep the screen current and screen dissipation within limits. This is taken care of by an interlocking provision that the screen voltage is applied only after the anode voltage reaches a certain pre-determined value well above the normal screen voltage. Release of Audio frequency The application of AF signal to the AF stage in the absence of c a r r i e r p o w e r w o u l d r e s u l t i n t h e o p e r a t i o n o f m o d u l a t i o n t r a n s f o r m e r w i t h n o l o a d connected. This is not desirable. Therefore, the AF signal should be applied to the Audio f req u en cy s t ages on ly wh en t h e RF p ower a mp l i f ie r i s d e l iv er in g t h e n omin a l p ower . Normally AF frequency signal to the AF stage is released only when the carrier power is approximately 80% of the normal power.

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7. FM TRANSMITTER 7.1 INTRODUCTION There is too much over-crowding in the AM broadcast bands and shrinkage in the night-time service 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, interference, etc.

7.2 SALIENT FEATURE OF FM TRANSMITTER

7.3 MODERN FM TRANSMITTER

Simplified block diagram of a Modern FM Transmitter is given in the figure below. The left

and right channels 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 transmission. 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 comparator (phase detector). The factor ‘N’ is automatically selected when we set the station carrier frequency. The other input of phase detector is a reference signal of 10kHz generated by a crystal oscillator of 10 MHz and divided by a divider (1/1000). The o u t p u t o f p h a s e d e t e c t o r i s a n e r r o r v o l t a g e , w h i c h i s f e d t o V C O f o r c o r r e c t i o n o f i t s frequency through rectifier and low pass filter.

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Figure7.1: Block Diagram of Modern FM Transmitter 7.4 2 x 3 kW FM TRANSMITTER 2 x 3 k W T r a n s m i t t e r s e t u p , w h i c h i s m o r e c o m m o n , c o n s i s t s o f t w o 3 k W

t r a n s m i t t e r s , designated as transmitters 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) w h i c h , w i t h t h e h e l p o f t h e A d a p t e r p l u g -

i n - u n i t ( K A 0 3 3 A 1 ) , a l s o e n s u r e s t h e p a r a l l e l operation of transmitter

B. Combining unit is housed in a separate rack.

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 basic 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.

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Figure7.2: block diagram of 2x3kW FM Transmitter

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 efficient 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

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7.5 2 x 5 KW FM Transmitter: A simplified block diagram of a 2 x 5 kW FM Transmitter is as shown in the figure……

Figure7.3: RF Schematic block diagram of a 2x5 kW FM Transmitter

7.6 EXCITER The Exciter 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 information such as traffic radio, SCA (Subsidiary Channel Authorization) and RDS (Radio Data System) signals. I t c a n g i v e t h r e e o u t p u t p o w e r s o f 3 0 m W , 1 W o r 1 0 W b y m e a n s o f i n t e r n a l l i n k s a n d 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 inthe event of mismatch. The 10 W output stage is a separate module that can be inserted between 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.

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T h i s u n i t p r o c e s s e s t h e i n c o m i n g a u d i o s i g n a l s b o t h f o r m o n o a n d s t e r e o t r a n s m i s s i o n s . I n c a s e o f s t e r e o t r a n s m i s s i o n , t h e i n c o m i n g L a n d R c h a n n e l s i g n a l s a r e 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 normal stereo signal on the modulating base band. The e n c o d e r s f o r R D S a n d S C A a p p l i c a t i o n s a r e e x t e r n a l t o t h e t r a n s m i t t e r a n d h a v e t o b e provided separately as and when needed.

Frequency generation, control and modulation The transmitter frequency is generated and carrier is modulated in the Synthesizer module w i t h i n t h e E x c i t e r . T h e c a r r i e r f r e q u e n c y i s s t a b i l i z e d w i t h r e f e r e n c e t o t h e 1 0 M H z frequency from a crystal oscillator using PLL and programmable dividers. The operating f r e q u e n c y o f t h e t r a n s m i t t e r c a n b e s e l e c t e d i n t e r n a l l y b y m e a n s o f B C D s w i t c h e s o r 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 87.5t o 1 0 8 M H z ) t o g e t a 1 0 k H z s i g n a l t o b e c o m p a r e d w i t h t h e r e f e r e n c e f r e q u e n c y . T h e stabilized carrier frequency is modulated with the modulating base band consisting of the audio (mono and stereo). The Varactor diodes are used in the synthesizer to generate as well as modulate the carrier frequency.

Power coupler The output from the RF switch is fed to the two transmitters A and B from where the signals are fed to power coupler. There it generates a power of 2.5/3w and the output is fed to the harmonic filter where it generates the harmonics and the increases its output power to1.25kw which is fed to the power coupler again and is finally fed to the antenna.

1.5 KW VHF Amplifier This amplifier is the basic power module in the transmitter. It has a broad banded sign 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 configuration of the transmitter, output of several such amplifiers is combined to get the desired output power of the transmitter. For instance, for a 3 kW set-u p t w o p o w e r a m p l i f i e r s a r e u s e d w h e r e a s f o r a 2 x 3 k W s e t -u p , 4 s u c h a m p l i f i e r s a r e needed.

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The simplified block diagram of 1.5 kW Power Amplifier is as shown in the figure…

Figure7.4:Block Diagram of 1.5 kW Amplifier

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 to1500 W in the final stage is achieved with the help of eight 200 W stages. Each 200 W stage consists of two output 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 contacts to obviate the need for soldering. The output of all these stages is c o m b i n e d v i a c o u p l i n g n e t w o r k s t o g i v e t h e f i n a l o u t p u t o f 1 . 5 k W . A m o n i t o r i n e a c h amplifier controls the power of the driver stage depending on the reference voltage produced b y t h e s w i t c h - o n - c o n t r o l u n i t . S i n c e t h i s r e f e r e n c e v o l t a g e i s t h e s a m e f o r a l l t h e V H F amplifiers being used, all of them will have the

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same output power. Each amplifier has a meter for indicating the forward and reflected voltages and transistor currents. 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 transistor.

7.7 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.

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.

7.8 FM ANTENNA AND FEEDER CABLE SYSTEM The Antenna system for FM Transmitters consists of 3 main sub-systems, namely:

Tower A tower of good height is required for mounting the FM antenna since the coverage of the transmitter 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 onthe top of the tower. At some places existing towers of Doordarshan have also been utilized f o r m o u n t i n g t h e F M a n t e n n a . P r o v i s i o n h a s a l s o b e e n m a d e o n t h e A I R t o w e r s f o r t o p mounting of TV antenna below FM antenna (Aperture for Band III)

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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 is2.4 x 2.4 m. The pole type antenna is quite economical as compared to panel type antenna but it cannot be used on large area towers.

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 required to enhance the signal. 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 abovethe other on the same face.

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

T h e f e e d p o i n t o f t h e a n t e n n a i s l o o k i n g t o w a r d s g r ou n d s o a s t o a v o i d 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.

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T h e d i s t a n c e o f t h e f e e d i n g s t r i p i s 2 4 0 m m f r o m e d g e a n d t h i s s h o u l d n o t b e disturbed. All the six dipoles are mounted on a 100 mm die Pole. This pole is supported by the main tower.

T h e a n t e n n a i s f e d t h r o u g h a p o w e r d i v i d e r w h i c h d i v i d e s t o t a l p o w e r i n t o 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 hollow 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 incases of failure of any dipole.

PANEL TYPE ANTENNA 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 10kW 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 the16 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, a3” 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 every1.5 m and the minimum radius of bending of this cable is about 1 m. The cable has been provided with two numbers of EIA flange connectors of 3 1/8” size on both ends. Both the connectors 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 T e f l o n i n s u l a t o r

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i n s i d e t h e c o n n e c t o r . A d u m m y h o l e ( d r i l l e d o n l y h a l f w a y ) i s a l r e a d y provided by the manufacturer for this purpose.

8. DIGITAL FM EXCITERS Generating the FM signal with “Direct Digital Synthesis” (DDS) technology, the digital FM exciter uses Numerically Controlled Oscillator for program modulation instead of the Voltage Controlled Oscillator (VCO) traditionally used in analog exciters.

8.1 DIRECT DIGITAL SYNTHESIS (DDS) It is a technique for using digital data processing blocks as a means to generate a frequency-and phase-tunable output signal referenced to a fixed-frequency precision clock source. In e s s e n c e , t h e r e f e r e n c e c l o c k f r e q u e n c y i s “ d i v i d e d d o w n ” i n a D D S a r c h i t e c t u r e b y t h e scaling factor set forth in a programmable binary tuning word. The tuning word is a typically24-48 bit long which enables a DDS implementation to provide superior output frequency tuning resolution. T o d a y ’ s c o s t c o m p e t i t i v e , h i g h - p e r f o r m a n c e , f u n c t i o n a l l y - i n t e g r a t e d , a n d s m a l l package-sized DDS products are fast becoming an alternative to traditional frequency-agile a n a l o g s y n t h e s i z e r s o l u t i o n s . T h e i n t e g r a t i o n o f a h i g h -s p e e d , h i g h - p e r f o r m a n c e , D / A converter and DDS architecture onto a single chip (forming what is commonly known as a Complete-DDS solution) enabled this technology to target a wider range of applications and provide, in many cases, an attractive alternative to analog-based PLL synthesizers. For many applications, the DDS solution holds some distinct advantages over the equivalent agile analog frequency synthesizer employing PLL circuitry.

8.1.1 DDS advantages

Micro-Hertz tuning resolution of the output frequency and sub -degree phase tuning capability, all under complete digital control.

E x t r e m e l y f a s t “ h o p p i n g s p e e d ” i n t u n i n g o u t p u t f r e q u e n c y (o r p h a s e ) , p h a s e continuous frequency hops with no over/undershoot or analog-related loop settling time anomalies.

The DDS digital architecture eliminates the need for the manual system tuning and tweaking associated with component aging and temperature drift in analog synthesizer solutions.

The digital control interface of the DDS architecture facilitates an environment where systems can be remotely controlled, and minutely optimized, under processor control.

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When utilized as a quadrature synthesizer, DDS afford unparal leled matching and control of I and Q synthesized outputs.

8.1.2 THEORY OF OPERATION I n i t s s i m p l e s t f o r m , a d i r e c t d i g i t a l s y n t h e s i z e r c a n b e i m p l e m e n t e d f r o m a p r e c i s i o n reference clock, an address counter, a programmable read only memory (PROM), and a D/A converter as shown in the figure below

In this case, the digital amplitude information that corresponds to a complete cycle of a sine wave is stored in the PROM. The PROM is therefore functioning as a sine lookup table. The address counter steps through and accesses each of the PROM’s memory locations and the c o n t e n t s ( t h e e q u i v a l e n t s i n e a m p l i t u d e w o r d s ) a r e p r e s e n t e d t o a h i g h - s p e e d D / A converter. The D/A converter generate an analog sine wave in response to the digital input words from the PROM. The output frequency of this DDS implementation is dependent on

The frequency of the reference clock, and

The sine wave step size that is programmed into the PROM.

While the analog output fidelity, jitter, and AC performance of this simplistic architecture can be quite good, it lacks tuning flexibility. The output frequency can only be changed by changing the frequency of the reference clock or by reprogramming the PROM. Neither of these options supports high-speed output frequency hopping. With the introduction of a phase accumulator function into the digital signal chain, this architecture becomes a numerically-controlled oscillator which is the core of a highly-flexible DDS device.

As the figure below shows, an N-bit variable-modulus counter and phase

register are implemented in the circuit before the sine lookup table, as a

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replacement for the address counter. The carry function allows this function as a “phase wheel” in the DDS architecture.

T o u n d e r s t a n d t h i s b a s i c f u n c t i o n , v i s u a l i z e t h e s i n e w a v e o s c i l l a t i o n a s a v e c t o r rotating around a phase circle (see Figure 1 -3). Each designated point on the phase wheel c o r r e s p o n d s t o t h e e q u i v a l e n t p o i n t o n a c y c l e o f a s i n e w a v e f o r m . A s t h e v e c t o r r o t a t e s around the wheel, visualize that a corresponding output sine wave is being generated. One revolution of the vector around the phase wheel, at a constant speed, results in one complete cycle of the output sine wave.

T h e p h a s e a c c u m u l a t o r i s u t i l i z e d t o p r o v i d e t h e e q u i v a l e n t o f t h e v e c t o r ’ s l i n e a r rotation around the phase wheel. The contents of the phase accumulator correspond to the points on the cycle of the output sine wave.

The number of discrete phase points contained in the “wheel” is determi ned by

the resolution, N, of the phase accumulator. The output of the phase accumulator is linear and c a n n o t d i r e c t l y b e u s e d t o g e n e r a t e a s i n e w a v e o r a n y o t h e r w a v e f o r m e x c e p t a r a m p . Therefore, a phase-to-amplitude lookup table is used

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to convert a truncated version of the phase accumulator’s instantaneous output value into the sine wave amplitude information that is presented to the D/A converter.

M o s t D D S a r c h i t e c t u r e s e x p l o i t t h e s y m m e t r i c a l n a t u r e o f a s i n e w a v e a n d u t i l i z e mapping logic to synthesize a complete sine wave cycle from ¼ cycle of data from the phase accumulator. The phase-to-amplitude lookup table generates all the necessary data by reading forward then back through the lookup table.

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The phase accumulator is actually a modulus M counter that increments its stored number each time it receives a clock pulse. The magnitude of the increment is determined by a digital word M contained in a “delta phase register” that is summed with the overflow of the counter. The word in the delta phase register forms the phase step size between reference clock updates; it effectively sets how many points to skip around the phase wheel. The larger the jump size, the faster the phase accumulator overflows and completes it’s equivalent of a sine wave cycle. For a N=32-bit phase accumulator, an M value of 0000…0001(one) would result in the phase accumulator overflowing after 232 reference clock cycles (increments). If the M value is changed to 0111…1111, the phase accumulator will overflow after only 21clock cycles, or two reference clock cycles. This control of the jump size constitutes the f r e q u e n c y t u n i n g r e s o l u t i o n o f t h e D D S a r c h i t e c t u r e . T h e r e l a t i o n s h i p o f t h ep h a s e a c c u m u l a t o r a n d d e l t a p h a s e a c c u m u l a t o r f o r m s t h e b a s i c t u n i n g e q u a t i o n f o r D D S architecture. FOUT = (M (REFCLK)) /2N Where: FOUT = the output frequency of the DDS

M = the binary tuning word REFCLK = the internal reference clock frequency (system clock) N = the length in bits of the phase accumulator

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Changes to the value of M in the DDS architecture result in immediateandphasec o n t i n u o u s c h a n g e s i n t h e o u t p u t f r e q u e n c y . I n p r a c t i c al a p p l i c a t i o n , t h e M v a l u e , o r f r e q u e n c y t u n i n g w o r d , i s l o a d e d i n t o a n i n t e r n a l s e r i a l o r b y t e - l o a d e d r e g i s t e r w h i c h p r e c e d e s t h e p a r a l l e l o u t p u t d e l t a p h a s e r e g i s t e r . T h i s i s g e n e r a l l y d o n e t o m i n i m i z e t h e package pin count of the DDS device.

Once the buffer register is loaded, the parallel-output delta phase register is clocked and the DDS output frequency changes. Generally, the only speed limitation to changing the output frequency of a DDS is the maximum rate at which the buffer register can be loaded and executed.

A numerically-controlled oscillator (NCO) is a digital signal generator which creates Asynchronous (i.e. clocked), discrete-time, discrete-valued representation of a waveform, usually sinusoidal. NCOs are often used in conjunction with a digital-to-analog converter (DAC) at its output to create a direct digital synthesizer (DDS).

8.3 OPERATION An NCO, generally, consists of two parts:

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Figure 8.5: Numerically controlled oscillator with optional quadrature output

When clocked, the phase accumulator (PA) creates a modulo-2N Saw tooth waveform

Which is then converted by the phase-to-amplitude converter (PAC) to a sampled sinusoid, where N is the number of bits carried in the phase accumulator. N sets the NCO frequency resolution and is normally much larger than the number of bits defining the memory space of the PAC look-up table. If the PAC capacity is 2M, the PA output word must be truncated to M b i t s a s s h o w n i n F i g u r e 1 . T h e t r u n c a t i o n o f t h e p h a s e o u t p u t w o r d d o e s n o t a f f e c t t h e frequency accuracy but produces a time-varying periodic phase error which is a primary source of spurious products.

Another spurious product generation mechanism is finite word length effects of the PAC output (amplitude) word. T h e f r e q u e n c y a c c u r a c y r e l a t i v e t o t h e c l o c k f r e q u e n c y i s l i m i t e d o n l y b y t h e precision of the arithmetic used to compute the phase. NCOs are phase- and frequency-agile, a n d c a n b e t r i v i a l l y m o d i f i e d t o p r o d u c e p h a s e - m o d u l a t e d o r f r e q u e n c y -m o d u l a t e d b y summation a t the appropriate node, or provide quadrature outputs as shown in the figure.

8.4 PHASE ACCUMULATOR A binary phase accumulator consists of an N-bit binary adder and a register configured as shown in Figure 1. Each clock cycle produces a new N-bit output consisting of the previous output obtained from the register summed with the frequency control word (FCW) which is constant for a given output frequency. The resulting output waveform is a staircase with step size ΔF, the integer value of the FCW. In some configurations, the phase output it taken from the output of the register which introduces a one clock cycle latency but allows the adder to operate at a higher clock rate.

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Figure 8.6: Normalized phase accumulator output The adder is designed to overflow when the sum of the absolute value of its operands

exceeds its capacity (2N−1). The overflow bit is discarded so the output word width is always equal to its input word width. The remainder φn, called the residual, is stored in the register and the cycle repeats, starting this time from φn (see figure 2). Since a phase accumulator is a finite state machine, eventually the residual at some sample K must return to the initial value φ0. The interval K is referred to as the grand repetition rate (GRR) given by

Where GCD is the greatest common divisor function. The GRR represents the true

periodicity f o r a g i v e n Δ F w h i c h f o r a h i g h r e s o l u t i o n N C O c a n b e v e r y l o n g . U s u a l l y w e a r e m o r e interested in the operating frequency determined by the average overflow rate, given by

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The frequency resolution, defined as the smallest possible incremental change in frequency, is given by

Equation (1) shows that the phase accumulator can be thought of as a programmable

non-integer frequency divider of divide ratio ΔF / 2 N

8.5 PHASE-TO-AMPLITUDE CONVERTOR The phase-amplitude converter creates the sample-domain waveform from the truncated p h a s e o u t p u t w o r d r e c e i v e d f r o m t h e P A . T h e P A C c a n b e a s i m p l e r e a d o n l y m e m o r y containing 2M c o n t i g u o u s s a m p l e s o f t h e d e s i r e d o u t p u t w a v e f o r m w h i c h t y p i c a l l y i s a sinusoid. Oftentimes though, various tricks are employed to reduce the amount of memory required. This includes various trigonometric expansions; trigonometric approximations and m e t h o d s w h i c h t a k e a d v a n t a g e o f t h e q u a d r a t u r e s y m m e t r y e x h i b i t e d b y s i n u s o i d s . Alternately, the PAC may consist of random access memory which can be filled as desired to create an arbitrary waveform generator.

8.6 PARTS OF DIGITAL FM EXCITER Digital modulator module The heart of the exciter, the modulator includes a Numerically Controlled Oscillator (NCO) w h i c h g e n e r a t e s a m o d u l a t e d F M w a v e f o r m . T h i s o u t p u t , a v e r y p u r e s i g n a l a t a n intermediate frequency of about 5 MHz, is converted to a precise FM signal.

Up convertor A single stage up converter mixes the output of the digital modulator with a synthesized local oscillator, producing the on-channel FM-carrier.

Filters Unwanted mixing products are removed by very stable, well -controlled band pass filters. Several stages and types of filters are used after up conversion to ensure

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mixing productions are at least 80 dB below the un modulated carrier. Special techniques are used to preserve the Bessel sideband components.

PLL and VCO Modules While traditional analog exciters use VCO/PLL circuitry for program modulation, digital FM exciters use the VCO/PLL only to provide an injection frequency for the up converter. Consequently, problems traditionally associated with VCO/PLL circuitry, such as automaticf r e q u e n c y c o n t r o l u n l o c k c a u s e d b y l o w f r e q u e n c y t r a n s i e n t s a n d p o o r l o w f r e q u e n c y response, are eliminated.

Externally controlled frequency synchronization This option gives it the ability to sync to an external reference for use in on-channel booster systems.

8.7 ADVANTAGES

T h e D i r e c t D i g i t a l S y n t h e s i s p r o c e s s e l i m i n a t e s t h e n e e d f o r l i n e a r l y c o r r e c t i o n circuitry and related adjustments

M o d u l a t i o n i s c o n t r o l l e d b y a d i g i t a l w o r d , h e n c e t h e r e i s n o c h a n c e o f o v e r modulation.

The FM exciter directly accepts a digital audio signal thereby making it possible to eliminate analog-to-digital and digital-to-analog conversations that add hum, noise, distortion to a signal

Among benefits are improved stereo signal-to-Noise, AM noise null, and the ability to synchronize to an external reference for use on-channel booster systems.

P r o b l e m s a s s o c i a t e d w i t h V C O / P L L t e c h n o l o g y s u c h a s p o o r l o w f r e q u e n c y separation and PLL unlock from audio transients, are eliminated

With flat low-frequency response to below 10 Hz, the digital FM exciter can pass such low sounds as music synthesizers, pipe organ pedal stops, and large bass drums.

Among other performance advantages, they are immune to subsonic transients that can put an analog exciter in to a fault mode and force a station off air.

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S u s t a i n a b l e p e r f o r m a n c e i s a n o t h e r k e y b e n e f i t o f t h e d i g i t a l F M e x c i t e r . U n l i k e analog exciters whose performance can degenerate overtime, digital FM exciters deliver the same exceptional signal quality years after installation as it did in the factory on the day of final test.

9.RADIO RECEIVERS A radio receiver is an electronic circuit that receives its input from an antenna, uses electronicf i l t e r s t o s e p a r a t e a w a n t e d r a d i o s i g n a l f r o m a l l o t he r s i g n a l s p i c k e d u p b y t h i s a n t e n n a , a m p l i f i e s i t t o a l e v e l s u i ta b l e f o r f u r t h e r p r o c e s s i n g , a n d f i n a l l y c o n v e r t s through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc… 9.1 TYPES OF RADIO RECEIVER • Basic crystal set. • A T.R.F. Receiver. • A Super heterodyne Receiver.

Mainly FM receivers are of the super heterodyne variety. Before we go into any depth a b o u t F M r a d i o r e c e i v e r s l e t ' s c o n s i d e r t h e p r i n c i p a l d i f f e r e n c e s b e t w e e n a . m . a n d F M . s i g n a l s . A t f i r s t g l a n c e i t m i g h t s e e m I a m m e r e l y s t a t i n g t h e b l i n d i n g o b v i o u s b u t t h e differences are indeed quite profound. 9.1.1 Crystal radio (A Crystal Set):

Antenna T h e a n t e n n a c o n v e r t s t h e e n e r g y i n t h e e l e c t r o m a g n e t i c r a di o w a v e s s t r i k i n g i t t o an alternating electric current in the antenna, which is connected to the tuning coil. Since in a crystal radio all the power comes from the antenna, it is

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important that the antenna collect as much power from the radio wave as possible. The larger an antenna, the more power it can intercept.

Figure9.1: crystal radio

Ground

The wire antennas used with crystal receivers are monopole antennas which develop their o u t p u t v o l t a g e w i t h r e s p e c t t o g r o u n d . T h e y r e q u i r e a r e t u r n c i r c u i t c o n n e c t e d to ground (earth) so that the Current from the antenna, after passing through the receiver, can flow into the ground. The ground wire is attached to a radiator, a water pipe, or a metal stake driven into the ground. A good ground is more important for crystal sets than for powered receivers, because crystal sets have low input impedance to transfer power efficiently from the antenna, so significant current flows in the antenna/ground circuit.

Tuned circuit A tuned circuit to select the signal of the radio station to be received, out of all the signals received by the antenna. This consists of a coil of w ire called an inductor or

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tuning coil and a capacitor connected together, one or both of which is adjustable and can be used to tune indifferent stations. In some circuits a capacitor is not used, because the antenna also serves as the capacitor. The tuned circuit has a natural resonant frequency, and allows radio signals at this frequency to pass while rejecting signals at all other frequencies.

Crystal detector A semiconductor crystal detector which extracts the audio signal (modulation) from the radiof r e q u e n c y c a r r i e r w a v e . I t d o e s t h i s b y o n l y a l l o w i n g c u r r e n t t o p a s s t h r o u g h i t i n o n e direction, blocking half of the oscillations of the radio wave. This rectifies the alternating current radio wave to a pulsing direct current, whose strength varies with the audio signal. This current can be converted to sound by the earphone. Finally, An earphone to convert the audio signal to sound waves so they can be heard. The low power produced by crystal radios is insufficient to power a loudspeaker so earphones are used.

9.1.2 TUNED RADIO FREQUENCY RECEIVER A tuned radio frequency receiver (TRF receiver) is a radio receiver that is usually composedo f s e v e r a l t u n e d r a d i o f r e q u e n c y a m p l i f i e r s f o l l o w e d b y c i r c u i t s to d e t e c t a n d a m p l i f y the audio signal.

Figure9.2:tuned radio frequency receiver

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The T.R.F. (tuned radio frequency) receiver was among the first designs available in the early days when means of amplification by valves became available. The basic principle was that all RF stages simultaneously tuned to the received frequency before detection and subsequent amplification of the audio signal. The main drawback for this is it is exceedingly difficult or near impossible to build LC Filters with impressive channel spacing and shape factors at frequencies as high as the broadcast band .

9.1.3 SUPER HETERODYNE RECEIVER A super heterodyne receiver (sometimes shortened to super het) uses frequency mixing or heterodyning to convert a received signal to a fixed intermediate frequency, which can be more conveniently processed than the original radio carrier frequency.

Design and principle of operation:

Figure9.3:super heterodyne receiver T h e p r i n c i p l e o f o p e r a t i o n o f t h e s u p e r h e t e r o d y n e r e c e i v e r d e p e n d s o n th e u s e of heterodyning or frequency mixing. The signal from the antenna is filtered sufficiently at least to reject the image frequency and possibly amplified. A local oscillator in the receiver p r o d u c e s a s i n e w a v e w h i c h m i x e s w i t h t h a t s i g n a l , s h i f t i n g i t t o a s p e c i f i c i n t e r m e d i a t e frequency (IF), usually a lower frequency. The IF signal is itself filtered and amplified and possibly processed in additional ways. The demodulator uses the IF signal rather than the original radio frequency to recreate a copy of the original modulation (such as audio).To r e c e i v e a r a d i o s i g n a l , a s u i t a b l e a n t e n n a i s r e q u i r e d . T h i s i s o f t e n b u i l t i n t o a r e c e i v e r , especially in the case of AM broadcast band radios. The output of the antenna may be very s m a l l , o f t e n o n l y a f e w m i c r o v o l t s . T h e s i g n a l f r o m t h e a n t e n n a i s t u n e d a n d m a y b e amplified in a so-called radio frequency (RF) amplifier, although this stage is often omitted. One or more tuned circuits

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at this stage block frequencies which are far removed from the intended reception frequency.

Intermediate frequency stage The stages of an intermediate frequency amplifier are tuned to a particular frequency not dependent on the receiving frequency; this greatly simplifies optimization of the circuit. The IF amplifier (or IF strip) can be made highly selective around its center frequency f If, whereas achieving such a selectivity at a much higher RF frequency would be much more difficult.

Band pass filter:

T h e I F s t a g e i n c l u d e s a f i l t e r a n d / o r m u l t i p l e t u n e d c i r c u i t s I n o r d e r t o a c h i e v e t h e desired selectivity. This filtering must therefore have a band pass equal to or less than the frequency spacing between adjacent broadcast channels. Ideally a filter would have a high a t t e n u a t i o n t o a d j a c e n t c h a n n e l s , b u t m a i n t a i n a f l a t r e s p o n s e a c r o s s t h e d e s i r e d s i g n a l spectrum in order to retain the quality of the received signal. This may be obtained using one or more dual tuned IF transformers or a multi pole ceramic crystal filter.

Demodulation The received signal is now processed by the demodulator stage where the audio signal (or other baseband signal) is recovered and then further amplified

Figure9.4: transmission and reception or radio signals

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10. APPLICATIONS OF RADIO Radio technology is used in a wide number of applications, and the list is growing all the time. Some of the earliest applications were to enable communications where wired links were not possible. Marconi, one of the early pioneers saw a need for radio communications between ships and the shore, and of course radio is still used for this today. However as radio became more established people started to use the medium for broadcasting. Today a huge number of stations broadcast both sound and vision using radio to deliver the programs to the listener. There are many more applications for radio. Apart from being used for ship to shore communications, radio is used for other forms of communications. Short wave radio was one of the first applications for radio. With ships sailing over vast distances it was seen that radio could provide a means for them to communicate when they were in the middle of an ocean .By "bouncing" the signals off reflecting layers in the upper atmosphere, great distances could be achieved. Once it was sent hat this could be done, many others also started to use the shortwave bands were long distance communications could be made. It was used by everyone from the military to news agencies, weather stations, and even radio hams.R a d i o i s a l s o u s e d f o r t e l e c o m m u n i c a t i o n s l i n k s . S i g n a l s w i t h f r e q u e nc i e s i n t h e microwave region are normally used. These signals have frequencies much higher than those in the short wave band and they are not affected by the ionosphere. However they provide reliable direct line of sight links that are able to carry many telephone conversations or other f o r m s o f t r a f f i c . H o w e v e r a s t h e y a r e o n l y l i n e o f s i g h t , t h e y r e q u i r e t o w e r s o n w h i c h t o mount the antennas to enable them to transmit over sufficiently long distances.

10.1SATELLITES S a t e l l i t e s a r e a l s o u s e d f o r r a d i o c o m m u n i c a t i o n . A s s h o r t w a v e c o m m u n ic a t i o n s a r e unreliable, and cannot carry the level of traffic required, higher frequencies must be used. It is possible to transmit signals up to satellites in outer space. These can receive the signals and broadcast them back down to Earth. Using this concept it is possible to transmit signals over vast distances, such as over the oceans. Additionally it is possible to use the satellites for b r o a d c a s t i n g . T r a n s m i t t i n g a s i g n a l u p t o t h e s a t e l l i t e , i t i s t h e n r e l a y e d o n a d i f f e r e n t frequency, and can give coverage over a whole country using just one satellite. A land based system may require many transmitters to cover the whole country.S a t e l l i t e s m a y a l s o b e u s e d f o r m a n y o t h e r a p p l i c a t i o n s . O n e o f t h e s e i s f o r observation. Weather satellites, for example, take images of the Earth and relay them back to Earth using radio signals. Another application for satellites is for navigation. GPS, the Global Positioning System uses a number of satellites in orbit around the Earth to provide very accurate positioning. Now further systems including Galileo (a European based system) and Glonass (a Russian based system) are being planned and put into operation.

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10.2RADAR Radar is an application of radio technology that has proved to be very useful. It was first used by the British in the Second World War (1939 - 1945) to detect incoming enemy bombers. B y k n o w i n g w h e r e t h e y w e r e , i t w a s p o s s i b l e t o s e n d u p f i g h t e r s t o i n t e r c e p t t h e m a n d thereby gain a significant advantage. The system operates by sending out a short burst of wireless energy. The signal is sent out and reflects back from the objects in the area that is ‘illuminated' by the radio signal. By knowing the angle at which the signal is returned, and the time it takes for the reflection to be received, it is possible to pinpoint the object that reflected the signal.

10.3MOBILE COMMUNICATIONS

In recent years there has been an explosion in personal communications. One of the first major applications was the mobile phone. Since their introduction in the last 20 years of the20th century, their use has mushroomed. Their growth has shown the value of mobile Communications and mobile connectivity. Accordingly other applications such as Bluetooth, Wi-Fi and others been developed and are now part of the wireless scene.

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11. CONCLUSION It is evident that digital FM exciters are incredibly versatile and will far exceed the currentr e q u i r e m e n t s w i t h o u t c o m p r o m i s i n g f u t u r e n e e d s . I t i s F M e x c i t e r s t h a t w i l l c h a n g e e x p e c t a t i o n s b y p r o v i d i n g t h e c l e a r e s t c l e a n e s t F M s o u n d , t h e b e s t q u a l i t y , t h e l e a s t maintenance, and the highest value of FM exciter. Application of frequency modulation technique for superimposing audio signals on t h e V H F c a r r i e r w a s a n o t a b l e d e v e l o p m e n t i n r a d i o b r o a d c a s t i n g i n 1 9 5 0 . 8 8 - 1 0 8 M H z f r e q u e n c y b a n d i s r e s e r v e d f o r F M B r o a d c a s t S e r v i c e . T h e m a j o r a d v a n t a g e o f F M broadcasting is its better noise tolerance and higher fidelity compared to AM broadcasting.T h e m a j o r d i s a d v a n t a g e o f F M i s i t s s h o r t r a n g e , o n l y t e n s o f k i l o m e t e r s . V H F / F M technology has since been extensively used for broadcasting in India. With the growth in the requirement for mobile connectivity, it is certain that wireless technologies with radio at the core will continue to thrive and become more widespread. To meet the demand it is likely that new technologies will be developed to maximize the use of t h e a v a i l a b l e r a d i o s p e c t r u m . I t i s a l s o a n t i c i p a t e d t h a t t h e u s e r w i l l b e l e s s a w a r e o f t h e u n d e r l y i n g t e c h n o l o g y . W i t h t h e i n c r e a s i n g c o m p l e x i t y , i t w i l l b e n e c e s s a r y t h a t a l l t h e technicalities are handled by the software, leaving the user free to use the device, whatever it may be, easily and freely. Radio transmission technology is a one way sound broadcasting service, transmitted over radio

waves (a form of electromagnetic radiation) from a transmitter to a receiving antenna and

intended to reach a wide audience. Stations can be linked in radio networks to broadcast

common programming, either in syndication or simulcast or both. Audio broadcasting are also

can be done via cable FM, local wire networks, satellite and the internet.

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12. BIBLIOGRAPHY

http://en.wikipedia.org/wiki/Numerically-controlled_oscillator

www.electronics-radio.com

http://en.wikipedia.org/wiki/Radio_broadcasting

http://www.transmitter.be/

A technical tutorial on digital signal synthesis – analog devices

Harris DIGIT CD FM exciters

http://www.intechopen.com