1

CHAPTER 1

INTRODUCTION

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.

CHAPTER 2

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.

3

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.

5

CHAPTER 3

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.

7

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 are shown in Fig.

Fig.3.1 Block Diagram of RF Chain (HMB-140)

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.

9

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.

11

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.

13

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.

15

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).

17

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.

19

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 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

21

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.

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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.

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CHAPTER 5

CONCLUSION