S U B M I T T E D B YD E E P A K K U M A R

ACKNOWLEDGEMENT

Training is an essential part of our four-year B.TECH. Degree program. I feel

proud to have the opportunity to join ALL INDIA RADIO, Chandigarh, a

government undertaking company with unique dynamic vision and extremely

professional staff. I wholeheartedly thank the company as well as the Technical

Department, for giving me the opportunity to step into the professional era.

The training in ALL INDIA RADIO is quite rewarding. It is a fruitful experience,

working in the largest sound broadcasting companies of INDIA.

I would like to express my heart-felt gratitude to Mr.D.R Sharma. I will be failing

in my duties if I do not extend my sincere thanks to highly experienced technical

staff comprising Mr.S.K Yadav, Mr.Amarjeet Dhiman, Mrs.Alka vatsa,

Mr.Ravinder SinghThakur and Mr.Sandeep Verma for helping me and providing

proper guidance.

A national service planned, developed and operated by the Prasar Bharati Broadcasting Corporation of India 

Sound broadcasting started in India in 1927 with the proliferation of private radio clubs. The operations of All India Radio began formally in 1936, as a government organisation, with clear objectives to inform, educate and entertain the masses.  

When India attained Independence in 1947, AIR had a network of six stations and a complement of 18 transmitters. The coverage was 2.5% of the area and just 11% of the population. Rapid expansion of the network took place post Independence.  

AIR today has a network of 229 broadcasting centers with 148 medium frequency (MW), 54 high frequency (SW) and 168 FM transmitters. The coverage is 91.79% of the area, serving 99.14% of the people in the largest democracy of the world. AIR covers 24 Languages and 146 dialects in home services. In External services, it covers 27 languages; 17 national and 10 foreign languages. 

Growth & Development of AIR

The first radio programme in India was broadcast by the Radio Club of Mumbai in June 1923. It was followed by the setting up of a Broadcasting Service that began broadcasting in India in July 1927 on an experimental basis at Mumbai and Kolkata simultaneously under an agreement between Government of India and a private company called the Indian Broadcasting Company Ltd.

When India became independent, the AIR network had only six Stations located at Delhi, Mumbai, Kolkata, Chennai, Lucknow and Tiruchirapalli with a total complement of 18 transmitters - six on the medium wave and the remaining on short wave. Radio listening on medium wave was confined to urban limits of these cities. As against a mere 2,75,000 receiving sets at the time of Independence, now there are about 132 million estimated radio sets in the country.

Now the broadcast scenario has drastically changed with 215 broadcasting centers, including 77 local Radio Stations, covering nearly cent-per-cent country's population.

IMPORTANT MILESTONES SINCE

INDEPENDENCE

August 15,1947 There were Six Radio stations at Delhi, Bombay, Calcutta, Madras, Tiruchirapalli and Lucknow.

July 20, 1952 First National Programme of Music broadcast from AIRJuly 29. 1953 National Programme of Talks (English) commenced from AIR.1954 First Radio Sangeet Sammelan held.October 3, 1957

Vividh Bharati Services started.

November 1, 1959

First TV station in Delhi started (at that time, it was part of AIR).

July 21, 1969 Yuvavani services started at Delhi.August 15, 1969

1000 KW Superpower Medium Wave Transmitter commisioned at Calcutta(Mogra).

January 8, 1971

1000 KW Superpower Medium Wave Transmitter commissioned at Rajkot

1974 Akashvani Annual Awards instituted.July 23, 1977 First ever FM service was started from Madras.September 14, 1984

Two High Power250 KW shortwave transmitters inaugurated at Aligarh.

October 30, 1984

First Local Radio Station at Nagarcoil started.

1985 All AIR stations were provided with 5 channel satellite receiver terminals.

May 18, 1988 Introduction of National Channel.April 8, 1989 Commissioning of Integrated North East Service.March 2, 1990 The 100th station of AIR commissioned at Warangal (Andhra

Pradesh) March 10, 1990 Two 500 KW Superpower shortwave transmitters

commissioned at Bangalore. October 2, 1992

Commissioning of FM Chanel at Jalandhar.

April 1, 1993 The 150 th station of AIR commissioned at Berhampur (Orissa).

August 15, Introduction of Times slots on FM Channel to private Parties at

1993 Delhi-Bombay. September 1, 1993

Time slots on FM Chanel to private parties at Chennai.

January 24, 1994

FM Channel at Panaji.

July 25, 1994 Time slots on FM channel to private parties at Calcutta.September 10, 1994

Multi-track recording studios commissioned at Mumbai.

September 28, 1994

Four 500 KW Superpower Shortwave transmitters at Bangalore inaugurated. This has made Bangalore one of the biggest transmitting centres in the world.

October 31, 1994

The 175th station of AIR commissioned at Nasik.

November 13, 1994

Time slots on FM channel to private parties at Panaji.

August 5, 1995 Multi-track recording studios commissioned at Chennai.February 1, 1996

Foundation stone laid for New Broadcasting House at New Delhi.

May 2, 1996 Launching of AIR on-line Information Services on Internet. January 13, 1997

Started Audio on demand on Internet Service.

April 1, 1997 Digital Audio Broadcasting (DAB) introduced at Delhi on experimental basis.

January 26, 1998

'Radio on Demand' service on 2nd FM Channel Transmission.

February 25, 1998

AIR 'News on Telephone' and AIR 'live on Internet'.

August 15, 1999

Radio station commissioned at Kokrajhar in Bodo Land Autonomous Council Area.

August 15, 1999

Second FM Channels commissioned at Delhi and Calcutta with Yuvavani service.

July 17, 2000 Regional Staff Training Institute (Tech.) started functioning at Bhubaneshwar (Orissa)

Sept 1, 2001 AIR launched Infotainment channel known as FM-II at four metros, Mumbai, Kolkata, Chennai, Delhi, in addition to the Metro Channel FM-I.

Nov 12, 2001 Museum of Radio and Doordarshan was inaugurated. Declared as The Public Service Broadcasting day to commemorate Gandhiji's visit to AIR

Feb 27, 2002 AIR launched its first ever digital statellite home service which will cater to Indian sub-continent and South-East Asia.

July, 2002 Celebrated 75 years of Broadcasting.April, 2003 Marketing Division of Prasar Bharati Inaugurated.Jan 26, 2004 Bhasha Bharati Channel of AIR launched at Delhi and

Classical Music Channel launched at Bangalore.Apr 01, 2004 Launch of Kisan Vani Programme from 12 Stations of AIR.Dec 16, 2004 DTH Service of Prasar Bharati, with 12 AIR Channels,

launched.

INTRODUCTION

COMMUNICATION: It is the process where by information is transferred from one point called source to the other point called destination (receiver).

RADIO: Radio comes after telephone in the communication history. It means wireless communication by electromagnetic waves or precisely saying radio waves.

BROADCASTING: Broadcasting comes under the wide definition of communication, it means one way communication, where one speaks and other listens only without interfacing. The basic requirement is that the signal to be broadcast must be distortion free.

RADIO STATION: For the purpose of entertainment and NEWS broadcasting, radio stations are established. First radio station started in 1935 in the state of Hyderabad. In Rajasthan first radio station was established on 9th of April 1955 in Jaipur, which was medium wave station of 1KV power capability.

AIR CHANDIGARH STATION: Medium wave Transmitter of NEC Company having 10KV power capabilities was inaugurated in April 1963 as an auxiliary centre of AIR, Jaipur. Till year 1980-81 the station was originated three hours of program daily, there activity of station was increased and station functioning as full-filled station from 1981-82. In August 1992, 2*10KV transmitter has been installed which increased power of transmission to 20KV.

STUDIO CATEGORY: The radio station categorized according to the number of studios in the station. Broadly it is divided into five categories

(A) Studio 1(B) Studio 2(C) Studio 3(D) Studio 4(E) Studio 5: This type of studio is present metro cities having two music

studios, two drama studios as per requirement.

CHANDIGARH STUDIO: Chandigarh radio station is II type of radio station having four studios:- (A) Music studio (B) Drama studio (C) Talk studio (D) Play back studio

1.1 INTRODUCTION :

The broadcast of a programme from source to listener involves use of

studios, microphones, announcer console, switching console, telephone lines / STL

and Transmitter. Normally the programmes originate from a studio centre located

inside the city/town for the convenience of artists. The programme could be either

“live” or recorded”. In some cases, the programme can be from OB spot, such as

commentary of cricket match etc. Programmes 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 programmes are then selected and routed from studio to transmitting centre

through broadcast quality telephone lines or studio transmitter microwave/VHF

links

A broadcast studio is an acoustically treated room. It is necessary that the

place where a programme 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 programme. 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 studio is ‘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.

STUDIO OPERATIONAL REQUIREMENTS

Many technical requirements of studios like minimum noise level, optimum

reverberation time etc. are normally met studio at the time of installation of studio.

However for operational purposes, certain basic minimum technical facilities are

required for smooth transmission of programmes and for proper control. These are

as follows:

Programme in a studio may originate from a microphone or a tape deck, or a

turntable or a compact disc or a R-DAT. So a facility for selection of output of

any of these equipments at any moment is necessary. Announcer console does

this function.

Facility to fade in/fade out the programme smoothly and control the programme

level within prescribed limits.

Facility for aural monitoring to check the quality of sound production and

sound meters to indicate the intensity (VU meters).

For routing of programmes from various studios/OB spots to a central control

room, we require a facility to further mix/select the programmes. The Control

Console in the control room performs this function. It is also called switching

console.

Before feeding the programmes to the transmitter, the response of the

programme should be made flat by compensating HF and LF losses using

equalised line amplifiers.(This is applicable in case of telephone lines only)

Visual signalling facility between studio announcer booth and control room

should also be provided.

If the programmes from various studios are to be fed to more than one

transmitter, a master switching facility is also required.

2.1 ANNOUNCER CONSOLE:

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 console called Announcer Console. The Announcer Console

is used for mixing and controlling the programmes 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 programmes either live or recorded.

The technical facilities provided in a typical announcer booth, besides an

Announcer Console are one or two microphones for making announcements, two

turn tables for playing the gramophone records and two playback decks or tape

recorders for recorded programmes on tapes. Recently CD and Rotary Head

Digital Audio Tape Recorder (R-DAT) are also included in the Transmission

Studio.

2.2 CONTROL ROOM: For two or more studios set up, there would be a

provision for further mixing which is provided by a control console manned by

engineers. Such control console is known as switching console. 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 :

i) Console tape recorders

ii) Console tape decks

iii) Recording/dubbing panel having switches, jacks and keys etc.

Switching of different sources for transmission like News, O.Bs. other

satellite based relays, live broadcast from recording studio.

Level equalisation and level control.

Quality monitoring.

Signalling to the source location.

Communication link between control room and different studios.

RADIO NETWORKING TERMINAL

SATELLITE Since the advent of freedom, India has embarked upon a program of national development & has attempted to use very consciously, since technology as an instrument for rapidly accelerating national growth.

Radio & television can serve as efficient tools for learning & distortion free information transfer for instruction. The need for developmental information is

Music studioDrama studioTalk studioPlayb

ack studioDubbi

ng studio

Control room

maximum in rural & economically backward villages, which are remote & isolate from urban centers. The absence of an extensive was ameliorated by the successful launchings of INSAT-1D & INSAT-2D.

The first generation India satellite system (INSAT-1D) built by the ford aerospace & communication corporation FACC of USA, to Indian specification & requirements under a contact from the development of space (DOS) is located at 83 degree east longitude INSAT- 1D became operational on June 1990 & INSAT-2A during July 1992.

Each of the multipurpose INSAT-1 satellites is designed to provide the following capabilities over there individual seven year in orbit life.

Fixed satellites service (FSS): Twelve transponders operating in 5935-6415 MHz (up-link), 3710-4200 (downlink). Utilization for thick route, thin route and remote area communication and TV program distribution.

Broadcast satellite service (BSS): Two transponders in operating in 5855-5935MHz (uplink) and 2555-2635MHz (downlink). Utilization for direct TV broadcasting to augmented low cost community TV sets in rural areas radio program distribution, national TV networking disaster warning.

R.N. TERMINAL : The various stations of AIR spread throughout the nation are required to relay certain programs and news services, centrally originated at New Delhi. There are also events of popular interests, taking place any where in the country, which need national or regional coverage. The programs for external service also originate at New Delhi and broadcast round the clock from transmitting stations located at Aligarh, Calcutta, Jullunder, Rajkot, Bombay, & Madras. In order to meet these varied requirements, AIR needs an elaborated networking systems, confirming to a set of quality and reliability objectives.

Any one of the six carrier signals is selected in one module and is down converted to 5.5MHz in the synthesizer. Now in the demodulator unit original audio signals are extracted.R.N. Terminal has been developed at Space application centre (SAC) Ahemdabad of ISRO as a joint ISRO program. The uplinks are provided by P & T. Radio networking refers to National networking of AIR stations through the series of satellite for radio programming distribution. The Radio Network terminals (RNT) located at AIR stations receives the S-BAND.

R.N. transmission & audio program thus received after processing is fed to transmitter to be broadcasted. The RNT acts as the ground terminal for satellite signal reception. The transponder is INSAT-2 satellite that can accommodate 28 channels. The RNT is thus designed to receive any of these channels and six of them simultaneously. In, addition, the equipment is confined incorporate redundancy.

The block diagram of R.N. terminal is shown in figure.

The system is considered by the following components:-1. 12ft. chickens mesh reflector antenna + helical feed.2. Low noise amplifier unit (LNA).3. Front-end converter unit (FEC).4. Passive frequency translator unit (FTP).5. Active frequency translator unit (FTA).6. Synthesized translator unit.7. Audio demodulator unit (DEM).8. Power supply (PS).

The 12ft. parabolic antenna collects the R.N. carriers transmitted by the satellite and feed them to the feed mounted LNA unit. The LNA unit contains two channels of LNA PCB’s in the redundant mode any of which can be selected by means of an RF switch. The outputs of the LNA’s are combined in powers to give the output of two LNA’s unit.

The S-BAND output of two LNA is given to FEC through low loss cable. The FEC contains two channels of down converter & IF pre amplifier. An RF switch does channel selection. The FEC is located close to the antenna & is converted to the indoor unit by means of a coaxial cable for carrying the IF signal & a separate three core cable for the +24 volt DC supply. Power of LNA module is taken from the power connector points of FEC.

Power to the normal/redundant sites of the LNA/FEC is switched by a toggle switch provided on the front panel of the FAT unit. In the indoor part of the equipment, the signal from the FBC is fed to the power divider in the passive frequency translator. The two wide band (92MHz) filters in the passive frequency translator, separate the 52MHz components of the IF. Again the 52MHz/92MHz signals are divided.

The power dividers are used to get normal and redundant channels, which can be selected by changing the cable connection.

The four outputs are then fed to the audio frequency translator. The signal of 92 MHz band undergoes frequency down conversion, amplification and power division in the active translator. Also 52MHz band signals undergo power division in the translator unit.

The output signal of the active translator, which is now in the 52MHz band and fed to the synthesized translator via coaxial cables. Each such translator has six numbers of synthesizer + translator plug in modules. Any of the channels can be selected/tuned by varying the entry of front panel thumb wheel switch of the synthesized plug-in.

The selected input signals are down converted to 5.5 MHz in the synthesized translator output signal is undergone demodulation, de-emphasis filtering (L.P.) in the demodulator plug-in module in the demodulator unit. The outputs of these are in a demodulator unit. The output of this audio is across 600 ohm (balanced). In the active frequency translator facility is provided to monitor the carrier levels on the front panel level meter.

The power supply unit for the R.N. terminal is meant to cater the power supply requirement of R.N. terminal equipment.

This 19th rack is mountable unit, provides both +24 volt and -24 volt tracking regulated DC output, the positive supply has a current rating of 5 A., while the negative supply output is rated for 3 A. The power supply unit features over voltage and short circuit protection. The unit operates at 230 volt, 50 Hz single-phase mains.

AMPLIFIERS USED IN AIR STUDIOS

Amplifier is one of the basic building blocks of modern electronics. The

present day electronics would not exist without this. Amplification is necessary

because the desired signal is usually too weak to be directly useful. Present day

amplifiers used in studios are mostly employing ICs and transistors.

3.1 TERMS USED WITH REFERENCE TO AMPLIFIERS:

If you look at the technical specifications of any amplifier used in a studio,

you will come across number of terms such as

Input Impedance

Input Level

Output Impedance

Output Level

Gain

Noise and Equivalent Input Noise

Frequency response

Distortion.

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

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 pre-amplifier will get from a

microphone. The input impedance also decides the noise performance of the

amplifier. For best noise performance, the input impedance of a pre amplifier

should exceed ten times the source impedance. It is because of this reason that the

input impedance of a pre amplifier is always 2000 ohm or more. In some

amplifiers 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.

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 a 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 impedance of equalised line amplifier

type lab 568 is less than 50 ohm while the specified load impedance is 600 ohm.

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.

NON LINEAR DISTORTION:

This type of distortion results from the production of new

frequencies in the output which are 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.

FREQUENCY DISTORTION:

This type of distortion exists when the signal components of

different frequencies are amplified 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.

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.

NOISE AND EQUIVALENT INPUT NOISE:

The term noise used broadly to describe any spurious electrical

disturbances 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 voltages at the input that would cause the actual noise output.

This noise is amplified along with the signal and tends to mask up the signal at the

output. If in an amplifier, the noise at output is 50dbelow 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.

3.2 MEDIUM WAVE TRANSMITTER:

RF circuits consists of a crystal oscillator, transistor power amplifier, RF.

Driver and Power Amplifier of 100 kW HMB 140 MW transmitter.

i)CRYSTAL OSCILLATOR: To oscillate at a consistent

frequency, the crystal is kept in a oven. The temperature of the oven is

maintained between 68 to 72o C and the corresponding indication is

available in the meter panel. Crystal oven is heated by + 12 V. One crystal

oscillator with a stand by has been provided. It gives an output of 5 V

square wave which is required to drive the Transistor Power Amplifier. The

crystal oscillator works between 3 MHz and 6 MHz for different carrier

frequencies. Different capacitors are used to select different frequency

ranges. In addition, variable capacitor is used for varying the frequency of

the crystal within a few cycles. The oscillator frequency is divided by 2, 4,

or 8 which is selected by jumpering the appropriate terminals. The oscillator

Unit gives 3 outputs, one each for RF output, RF Monitoring and RF output

indication.

ii)TRANSISTOR POWER AMPLIFIER:

Oscillator output is fed to the transistor Power amplifier (TRPA). It

gives an output of 12 Watt across 75 ohms. It works on + 20 V DC, derived from a

separate rectifier and regulator. For different operating frequencies, different

output filters are selected. (Low Pass Filter).

iii)RF DRIVER :

A 4-1000 A tetrode is used as a driver which operates under class AB

condition, without drawing any grid current. About 7 to 10 Watts, of power is fed

to the grid of the driver through a 75 : 800 ohms RF Transformer, which provides

proper impedance matching to the TRPA output and also provides the necessary

grid voltage swing to the driver tube.

Because the cathode is at -600 V, the effective grid to cathode bias voltage (fixed)

is -50V and the effective plate voltage is 2500 V. The driver develops a peak grid

voltage of 800 to 900 V at the grid of PA and PA grid current of about 0.3 A to 0.4

Amps. The required wave form for operating the PA as class -D operation is also

developed at the output of the driver by mixing about 20% third harmonic with the

fundamental which is the operating frequency of the transmitter.

iv)RF POWER AMPLIFIER:

CQK - 50, condensed vapour cooled tetrode valve is used as a PA stage.

High level anode modulation is used, using a class B Modulator stage. The screen

of the PA tube is also modulated by a separate tap on modulation transformer.

Plate load impedance of the PA stage is about 750 ohms and the output impedance

is 120 ohms, and it is matched by L-C components. Using various combination of

the L-C circuits plate impedance of third harmonic is created, the Harmonics also

are filtered imaginatively at the output side. 11 kV DC, the HT voltage is

connected to the plate of the PA valves through the secondary of the modulation

transformer and RF chokes : hence the AF signal is super imposed on the DC for

the PA plate.

3.3 AF STAGE:

FIG.3.2 AF Stage

The AF stage supply the audio power required to amplitude modulate the

final RF stage. The output of the AF stage is superimposed upon the DC voltage to

the RF PA tube via 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.

i)HIGH PASS FILTER:

The audio input from the speech rack is fed to active High Pass Filter. It

cuts off all 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.

ii)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 feed back network from the

secondary of the modulation transformer and the signals from the compensator also

are fed to this unit.

iii)AF PRE-CORRECTOR:

Pre- amplifier output are 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.

iv)AF DRIVER :

The two 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 transistor in

series as it works with a high voltage of about -400 V. 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 exceeded.

v)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 vapour cooled tetrodes,

deionised water is used for cooling. The valve required about 11.5 litres/min. of

water. Two water flow switches WF1 and WF2 in the water lines of each of the

valves protect against low or no water flow. Thermostats WT1 and WT2 in each

water line provide protection against excessive water temp. by tripping the

transmitter up to stand-by if the temperature of the water exceeds 70o C.

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 :

1. HT -11 kV PA & Modulator : thyristor controlled for smooth variation of HT

2. 800 V Power Supply : Screen voltage to PA valve.

3. 1070 V : Screen voltage to modulate valve.

4. 1900 V : Plate voltage to RF Driver

5. - 650 V : (i) Grid Bias to PA Modulator & RF Driver

(ii) A tap on -650 V provides -600 V supply to

the

cathode of RF Driver

(iii) -100 V for the screen of RF Driver.

6. Main supply to transmitter : 415 V. 3 Phase 50 Hertz.

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 C 210 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.

3.4 CONTROL AND INTERLOCK SYSTEMS IN TRANSMITTER:

Switching Sequence of Transmitter:

Ventilation.

Filament

Grid Bias/Medium Tension

High Tension.

3.4.1VENTILATION :

All the transmitters handle large amount of power. Basically the

transmitters convert power from AC main's to Radio Frequency and Audio

Frequency energy. The conversion process always result 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 cubics.

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.

The water flow and the air flow provided by the cooling equipments to the

various 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.

3.4.2 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 switched on and requisite air and water flow

established, the filament of the tubes can be switched on. While switching on

filament of the tube, the control and 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 filament. Hence, it becomes necessary to apply the filament

voltage in steps. Various methods adopted are :

i. Use of step starter resistance : Here the filament voltage of the tubes are given

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.

ii. Use of special filament transformer which allows slow build up of the filament

voltage.

iii. Application of filament voltage in 3 or 4 steps.

The emission from the tubes depend 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.

3.4.3 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 bias generally anode and screen voltages of intermediate stages and

driver stages are also switched on. Application of bias and medium tension makes

available very high voltages for the various transmitter equipment. Hence, in order

to ensure the safety of the personnel access to these 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 be put on only after all the

transmitter and other HV equipment doors are closed to prevent access.

3.5 CONNECTION OF LOAD (ANTENNA/DUMMY LOAD):

After the application of ventilation, 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 process of the various RF

stages are complete and none of the tuning motors are moving.

APPLICATION OF SCREEN VOLTAGE :

In the case of tetrode tubes, the screen voltage to the tube 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 reach 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 carrier

power would result in the operation of modulation transformer with no load

connected. This is not desirable. Therefore, the AF signal should be applied to the

Audio frequency stages only when the RF power amplifier is delivering the

nominal power. Normally AF frequency signal to the AF stage is released only

when the carrier power is approximately 80% of the normal power.

3.6 MEDIUM WAVE ANTENNA:

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 metres, 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 MW radiation is horizontal, they will be attenuated very

fast with distance due to the proximity of the earth. MW antenna have to be placed

vertically, so that they radiate vertically polarised signals. It is for this reason, all

the MW antenna 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.

3.6.1 SELF RADIATING MW 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

Figure 3.1 :MW Antenna isolated from ground

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).

CHAPTER 4

FM TRANSMITTER

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.

4.1 Salient Features of FM Transmitters :

1. Completely solid state.

2. Forced air cooled with the help of rack-integrated blowers.

3. Parallel operation of two transmitters in passive exciter standby mode.

4. Mono or stereo broadcasting

5. Additional information such as SCA signals and radio traffic signals (RDS) can

also be transmitted.

6. Local/Remote operation

7. Each transmitter has been provided with a separate power supply.

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

using

a synthesizer.

4.2 Modern FM Transmitter:

Simplified block diagram of a Modern FM Transmitter is given in Fig.1.

The left and right channel 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 10 kHz generated by a crystal

oscillator of 10 MHz and divided by a divider (1/1000). The output of phase

detector is an error voltage, which is fed to VCO for correction of its frequency

through rectifier and low pass filter.

Figure 4.1: Block Diagram of Modern FM Transmitter

4.2 2 X 3 KW FM TRANSMITTER:

Simplified block diagram of a 2 x 5 kW FM transmitter is shown in

Fig.2. 2 x 5 kW Transmitter setup, which is more common, consists of two 3 kW

transmitters, 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) which, with the help of the Adapter

plug-in-unit (KA 033A1), also ensures the parallel operation of transmitter B.

Combining unit is housed in a separate rack.

Figure 4.2: 2 x 3 kW FM Transmitter

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 5 kW power. The transmitter is forced

air-cooled with the help of a blower. A standby blower has also been provided

Studios

Splitter

Control room

Audio processer

Exciter

PAPAPAPA Comb

iner5kW

which is automatically selected when the pre-selected blower fails. Both the

blowers can be run if the ambient temperature exceeds 40oC.

Power stages are protected against mismatch (VSWR > 1.5) or

excessive heat sink temperature by automatic reduction of power with the help of

control circuit. Electronic voltage regulator has not been provided for the DC

supplies of power amplifiers but a more 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.

4.2.1 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

Authorisation) and RDS (Radio Data System) signals. It can give three output

powers of 30 mW, 1 W or 10 W by means of internal links and switches. The

output power is stabilized and is not affected by mismatch (VSWR > 1.5),

temperature and AC supply fluctuations. Power of the transmitter is automatically

reduced in the event of mismatch. The 10 W output stage is a separate module that

can be inserted 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.

This unit processes the incoming audio signals both for mono and

stereo transmissions. In case of stereo transmission, the incoming L and R channel

signals are processed in the stereo coder circuit to yield a stereo base band signal

with 19 kHz pilot tone for modulating the carrier signal. It also has a multiplexer

wherein the coded RDS and SCA signals are multiplexed with the normal stereo

signal on the modulating base band. The encoders for RDS and SCA applications

are external to the transmitter and have to be provided separately as and when

needed.

4.2.2 FREQUENCY GENERATION, CONTROL AND MODULATION:

The transmitter frequency is generated and carrier is modulated in the

Synthesiser module within the Exciter. The carrier frequency is stabilized with

reference to the 10 MHz frequency from a crystal oscillator using PLL and

programmable dividers. The operating frequency of the transmitter can be selected

internally by means of BCD switches or externally by remote control. The output

of these switches generates the desired number by which the programmable divider

should divide the VCO frequency (which lies between 87.5 to 108 MHz) to get a

10 kHz signal to be compared with the reference frequency. The stablised carrier

frequency is modulated with the modulating base band consisting of the audio

(mono and stereo), RDS and SCA signals. The Varactor diodes are used in the

synthesizer to generate as well as modulate the carrier frequency.

4.2.3 SWITCH-ON CONTROL UNIT:

The switch-on-control unit can be termed as the “brain” and controls the working

of the transmitter ‘A’. It performs the following main functions:

1. It controls the switching ON and OFF sequence of RF power amplifiers, rack

blower and RF carrier enable in the exciter.

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

3. Provides automatic switch over operation of the exciter in the passive exciter

standby mode in which either of the two exciters can be selected to operate as the

main unit.

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

amplifiers.

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

6. It evaluates the fault signals provided by individual units and generates an

overall sum fault signal which is indicated by an LED on the front panel. The fault

is also stored in the defective unit and displayed on its front panel.

4.3 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.

4.4 FM ANTENNA AND FEEDER CABLE SYSTEM:

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

a) Supporting tower

b) Main antenna

c) Feeder Cable

4.4.1 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 on the top of the tower. At

some places existing towers of Doordarshan have also been utilized for mounting

the FM antenna. Provision has also been made on the AIR towers for top

mounting of TV antenna below FM antenna (Aperture for Band III).

4.4.2 ANTENNA:

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

Wide-band usage from 88 to 108 MHz range.

Omni-directional horizontal pattern of field strength.

Circular polarization for better reception.

High gain for both vertical and horizontal signals.

Two degrees beam tilt below horizontal

Sturdy design for maintenance-free service.

Further, depending on the type of tower available for mounting the

requirement is for two types of antenna. The first type is to be mounted on a small

cross-section AIR Tower. For which a pole type FM antenna has been selected.

For mounting on the existing TV towers, a panel type antenna has been used. The

cross section of the TV tower at the AIR aperture is 2.4 x 2.4 m. the pole type

antenna is quite economical as compared to panel type antenna, but it can not be

used on large area towers. For our requirement, the antennae supplied by M/s.

SIRA have been found suitable.

i)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 above

the other on the same face.

Lengths of feed cables of dipoles will be different and has been calculated to give

a beam tilt of 2o below horizontal.

The feed point of the antenna is looking towards ground so as to avoid

deterioration of the insulating flange. This flange consists of high density PVC.

The life of this is expected to be about 7 to 10 years.

The distance of the feeding strip is 240 mm from edge and this should not be

disturbed. All the six dipoles are mounted on a 100 mm dia Pole. This pole is

supported by the main tower.

The antenna is fed through a power divider which divides total power into 6

outlets for feeding the 6 dipoles. The power divider is mounted on a different face

of the tower.

The main feeder cables, power divider branch feeder cables, and dipoles are of

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 in

case of failure of any dipole.

ii)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 10 kW power. The reflector panels are constructed of GI bars whereas the

dipoles are made out of steel tubes. Since each panel consists of 4 dipoles, there

are a total of 64 dipoles for all the 16 panels. Therefore the power divider has 64

outlets to feed each of the dipoles. The power divider will be mounted inside the

tower. This antenna gives an omni-directional pattern when the panels are

mounted on all the four faces.

4.4.3 FEEDER CABLE:

For connecting the output power of the transmitter to the dipoles through the power

divider, a 3” dia feeder cable has been used.

This cable is of hollow type construction and has to be handled very

carefully. From the building to the base of the tower, the cable is laid on

horizontal cable tray. Along with the tower this is fixed on the cable rack provided

for this purpose. The cable is clamped at every 1.5 m and the minimum radius of

bending of this cable is about 1 m. The cable has been 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

Teflon insulator inside the connector. A dummy hole (drilled only half way) is

already provided by the manufacturer for this purpose.

CONCLUSION:

In India Sound Broadcasting started in 1927 with the

proliferation of private Radio clubs. All India Radio began formally in1936,

as a government organization, with clear objectives of providing

information, to educate and entertain the masses. After Independence in

1947, AIR had a network of six stations and a complement of eighteen

transmitters with coverage was about 2.5% of the area and just 11% of the

population. Later Rapid expansion of the network took place.

AIR today has a network of 229 broadcasting centre with

148 medium frequency (MW), 54 high frequency (SW) and 168 FM

transmitters. The coverage is 91.79 % of the area, serving 99.14 % of the

people in the largest democracy of the world. AIR covers 24 languages and

146 dialects in home services. In External services, it covers 27 languages,

17 national and 10 foreign languages. Hoping its expansion covers entire

India reaching every corner of country and connects entire country.