1

CHAPTER-1

INTRODUCTION

Doordarshan is the public television broadcaster of India and a division of Prasar

Bharati, a public service broadcaster nominated by the Government of India. It is one of the

largest broadcasting organizations in the world in terms of the infrastructure of studios and

transmitters. On September 15, 2009, Doordarshan celebrated its 50th anniversary.

Doordarshan had a modest beginning with the experimental telecast starting in Delhi

on 15 September 1959 with a small transmitter and a makeshift studio. The regular daily

transmission started in 1965 as a part of All India Radio. The television service was extended

to Mumbai and Amritsar in 1972. Up until 1975, only seven Indian cities had a television

service and Doordarshan remained the sole provider of television in India. In 1976 each

office of All India Radio and Doordarshan were placed under the management of two

separate Director Generals in New Delhi. Finally Doordarshan as a National Broadcaster

came into existence.

1.1 Channels

Presently, Doordarshan operates 21 channels

2 All India channels-DD National and DD News

11 Regional languages Satellite Channels (RLSC)

4 State Networks (SN)

1 International channel

1 Sports Channel (DD Sports)

2 channels (Rajya Sabha TV & DD-Lok Sabha)

1.1.1 List of Channels

National

DD National

DD News

Rajya Sabha TV

DD-Lok Sabha

2

DD Sports

DD Bharathi

Regional

DD Bangla

DD Kashmir

DD Urdu

DD Punjabi

DD NorthEast

DD Sahyadri

DD Gujarati

DD Malayalam

DD Podhigai

DD Saptagiri

DD Odia

Fig 1.1: Logos of some Dhoordarshsan channels

3

1.1.2International broadcasting

DD India broadcasts internationally via satellite and it is available in 146 countries

worldwide.

4

CHAPTER-2

TYPES OF BROADCASTING

We basically know 3 types of methods by which the signal is received by the

customers. Now let us see the 3 types of methods

1. Cable television

2. Direct To Home (DTH)

3. Transmitter service

Basically the signals are sent over a band of frequencies. There are several bands are

available with their own frequency ranges. And here for this transmitting and receiving we

see the bands namely Ku and C bands. So let us see about Ku band.

2.1 Ku band

The Ku band is a portion of the electromagnetic spectrum in the microwave range of

frequencies. This symbol refers to "K-under" (originally German: Kurz-unten) in other

words, the band directly below the K-band.

Ku band is primarily used for satellite communications, most notably for fixed and

broadcast services, and for specific applications such as NASA's Tracking Data Relay

Satellite used for both space shuttle and ISS communications. Ku band satellites are also used

for backhauls and particularly for satellite from remote locations back to a television

network's studio for editing and broadcasting. The band is split into multiple segments that

vary by geographical region by the International Telecommunication Union (ITU). NBC was

the first television network to uplink a majority of its affiliate feeds via Ku band in 1983.

2.1.1 Other Microwave Bands

Compared with C-band, Ku band is not similarly restricted in power to avoid

interference with terrestrial microwave systems, and the power of its uplinks and downlinks

can be increased.

2.2 Cable transmission

Cable television is a system providing a distribution of signal via co-axial cables.

5

It originally stood for Community Antenna Television, from cable television's origins in

1948. In areas where Over-the-air reception was limited by distance from transmitters or

mountainous terrain, large "community antennas" were constructed, and cable was run from

them to individual homes.

Tab 2.1 :Various Bands of frequencies

In the 1990s cable providers began to invest heavily in new digital based distribution

systems. Increased competition and programming choices from Direct-broadcast satellite

services such as DirecTV, Dish Network, and Prime Star caused cable providers to seek new

ways to provide more programming. Customers were increasingly interested in more

channels, pay-per-view programming, digital music services, and high speed internet services

Fig 2.2: A cable wire

A-Outer protective layer B- Outer copper wire

C- Inner protective layer D- Inner copper wire

L band 1 to 2 GHz

S band 2 to 4 GHz

C band 4 to 8 GHz

X band 8 to 12 GHz

Ku band 10.95-14.5 GHz

K band 18 to 26.5 GHz

Ka band 26.5 to 40 GHz

Q band 30 to 50 GHz

U band 40 to 60 GHz

V band 50 to 75 GHz

D band 110 to 170 GHz

6

2.3 DTH

D.T.H stands for Direct-To-Home television. It refers to a digital satellite service

that provides television services direct to subscribers anywhere in the country. Since it makes

use of wireless technology, TV channels/programs are sent to the subscriber's television

direct from the satellite, eliminating the need for cables and any cable infrastructure.

In DTH telecast, the signals are transmitted in Ku band (10.7 GHz to 18 GHz) and are

received by the subscribers through a small dish antenna (about 45cm in dia.) and a set-top box (or an

integrated receiver decoder). The DTH system can also provide many value-added services such as

the Internet, e-mail, data casting, e-commerce, and interactive multimedia.

In order to receive DTH service, you need the following two basic components:

1. Dish Antenna.

2. Receiver /Set Top Box (STB).

2.3.1 Dish Antenna

It is a dish-shaped type of parabolic antenna (as small as 45 cm in diameter) designed

to receive microwaves from communications satellites, which transmit data transmissions or

broadcasts, such as satellite television. When the antenna is properly pointed, the dish

"catches" the signal and reflects it to the Low-Noise Block (LNB) converter - included with

the purchase of the antenna.

2.3.2 Set Top Box

It is a digital integrated receiver/decoder (IRD) also called a receiver, which separates

each channel and decompresses and translates the digital signal for viewing over a television;

and a remote control.

7

Fig 2.3: Process of DTH method

2.4 Transmitter service:

Transmitter service is a service which is provided by certain broadcasters such that

the signal is propagated into the free space and then the signal is received by the user or the

customer.

The propagation of the signal strength depends on the power to which it is

transmitted. For different ranges of power different distances will be covered from the centre

of the transmitting section or transmitter.

Let us see some of the transmitting services that we see nearby transmitting stations.

High Power Transmitter (HPT):

Transmitter power 10KW

Distance covered by above transmitter is 60km-100km

E.g.: Located in Rajahmundry

8

Low Power Transmitter (LPT):

Transmitted power 100w-500w

Local area transmitter covers distance around 21kms

E.g.: Located in Kakinada

Very Low Power Transmitter (VLPT):

Transmitted power ± 10w

Distance covered is around 5-10Km

E.g.: Located in Peddapuram

9

CHAPTER-3

LOW POWER TRANSMITTER

A transmitter station is considered as a low power transmitter if the transmitted power and the

area of distance it can cover are as follows:

Transmitted power 100w-500w

Local area transmitter covers distance around 21kms

Fig 3.1: Block diagram of LPT

3.1 Receiver Dish Area:

In receiver dish area parabolic dipole antennas (P.D.A) are used. The shape of the

dish must be parabola because the parabola has specific focal point. When the information

from satellites through space is incident on parabolic dishes and it reflects back from

parabolic surfaces to the principle of foci. So that, the received information by the dish at the

focus point, is the exact replica of information transmitted by the satellite.

3.2 Parabolic Antenna:

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface

with the cross-sectional shape of a parabola, to direct the radio waves. The most common

10

form is shaped like a dish and is popularly called a dish antenna or parabolic dish. The main

advantage of a parabolic antenna is that it is highly directive.

Main advantages to use the parabolic dish antenna are:

It is a Passive receiver

It receives signal from satellite

If the size of the dish increases gain is also increases. So that receiving capability

increases.

A typical parabolic antenna consists of a parabolic reflector with a small feed antenna

at its focus, pointed back toward the reflector. The reflector is a metallic surface formed into

a parabola and usually truncated in a circular rim that forms the diameter of the antenna.

Fig 3.2 Parabolic dish antenna

11

3.2.1 Parking Angle

The angle at which the satellite placed in geostationary satellite is called parking

angle.

3.2.2 Look angle

The angle at which the P.D.A is placed on earth with respect to latitudes and

longitudes is called look angle.

A satellite‟s location can impact the quality of network. Identified by its longitude, a

satellite‟s location is also referred to as its orbital slot. The orbital slot of the satellite

determines the angle at which a ground antenna needs to be positioned to see the satellite, or

“look angle.” Higher look angles provide greater reliability by improving the quality of the

communication link. A higher look angle also allows more earth stations to communicate

with the satellite. On the other hand, low or shallow look angles may face obstructions from

trees, nearby buildings, or other objects and are more subject to interference, particularly in

heavy rain.

3.2.2.1 Azimuthal angle

Azimuthal angle determines the look angle in horizontal direction (longitudinal angle). An

azimuth is an angular measurement in a spherical coordinate system. The vector from an

observer (origin) to a point of interest is projected perpendicularly onto a reference plane, the

angle between the projected vector and a reference vector on the reference plane is called the

azimuth angle.

Fig 3.3: a) Elevation angle b) Azimuth angle

12

3.2.2.2 Elevation angle

Elevation angle determine the look angle in vertical direction (latitudinal angle).

To fix the look angle, azimuth angle and elevation angle should be fixed. Latitudes

and longitudes steels about the situation of P.D.A in geometrical plan.

So by checking and fixing this look angle towards the geo stationary satellite we can

receive the signal transmitted by the satellite using parabolic dish antenna/ receiver.

13

CHAPTER-4

INPUT RACK

For case of understanding we can divide the functioning of input rack in to three blocks

1. Receiving section

2. Transmitting section

3. Antenna section

Fig 4.1: Input rack in detail

4.1 Receiving Section

Fig 4.2: Block diagram of receiving section

The parabolic dish antenna is metal structure with a shape of half circle, and apart

from that at a distance a feed arm is held with support in air to which a low noise amplifier in

addition to the low noise block convertor and the internal relay station there is a digital

broadcast receiver in for monitoring and later on re-transmission of the signal is done in the

transmitting section

For transmission, a signal (sound or light) is first converted to an electrical signal

using a microphone or a video camera. These signals are then superimposed on to a carrier

wave before being sent to the transmitter. Different channels of a radio or television use

14

carrier waves of different frequencies. Radio/TV receivers are so designed that they can tune

in to a particular frequency of electromagnetic waves at a time they ignore other signals.

4.2 Transmitter Section

Fig 4.3: Block diagram of transmitter section

The images and sounds corresponding to one particular signal can then be selectively

processed and reproduced by the receivers. It is important to remember that radio waves are

only part of an extensive spectrum of electromagnetic waves. This spectrum includes such

familiar radiations as visible light; ultraviolet and infrared radiations, as well as X-rays and

gamma rays. As you may be aware they all exhibit phenomenon of reflection, refraction,

diffraction, interference and absorption. Since sound or the image signals both travel as an

electromagnetic wave, their velocity is the speed of light and they reach any destination on

Earth almost instantaneously. The process of mixing electrical signals from a microphone or

a TV/video camera with an electromagnetic carrier wave is known as modulation. At present

two kinds of signal modulation are commonly used

Amplitude Modulation (AM) and

Frequency Modulation (FM).

In amplitude modulation the amplitude of the carrier electromagnetic wave at any instant of

time is changed corresponding to the amplitude of the signal electric current and in frequency

modulation, the frequency of the carrier wave at a particular instant of time is changed

according to the amplitude of the signal wave. Normally amplitude modulation is used for

15

carrier frequencies corresponding to the short, medium and long wavelength bands of radio

frequencies. The television signal is made up of two parts, both related to each other by the

frequency of the carrier signal. The image signal is amplitude modulated and occupies about

three fourth of the total bandwidth (which is usually 6 MHz) the audio component of the TV

signal is frequency modulated and has a frequency in the range of the upper quarter of the

band frequency range. Television and radio receivers are generally designed to process

signals in a certain frequency range which are globally allotted for the respective signals.

Normal radio receivers therefore cannot process sound signals meant for televisions. But such

radio receivers can be designed.

A Radio/TV transmitter performs essentially three functions: generation of the carrier

currents for the sound and/or light signals, modulation and amplification of the resulting

signal (so that it has enough energy to dissipate over a large area). The carrier currents have

frequencies accurate to roughly one part in200, 000. The signals are then sent to the

transmission antenna, which in turn, sends the signal out into air as electromagnetic waves.

The receiver receives the electromagnetic waves through its own antenna, demodulates the

received signal (by mixing with an electromagnetic wave corresponding to the carrier wave

frequency generated within itself) and then recreates the original sounds and/or images. The

broadcasting range of a transmitter depends both on its power (measured in watts) as well as

the frequency of the transmitted signals. For example, a typical strong AM radio station

which broadcasts signals in the range of 1000 KHz (in medium wave band) has a power of

50, 000 watts and its signals can be received far away. For example, programs broadcast on

Delhi-B by a 50, 000 watts transmitter in New Delhi can be heard at night as far as Calcutta

(about 1500 km away). The least powerful AM stations operate at 250 watts and usually

serve only neighbouring areas. AIR Chandigarh programmes can be received in Delhi but not

beyond. The power of FM stations which broadcast signals in the range of 100 MHz, ranges

from 100 watts (which can broadcast up to 30 kilometres) to 100,000 watts, (which can

broadcast up to about 100 kilometres). The dependence of the range of a radio signal on its

frequency is essentially due to absorption of these waves by matter. As the frequency of the

carrier wave increases it is absorbed more readily by the structures in its way. Since the

television signals are generally transmitted on carrier waves popularly known as VHF (Very

High Frequency waves) they can only be transmitted and received more like light i.e. in

straight lines. VHF signals, unlike short-wave radio signals transmitted by radio stations,

transmitting in short-wave frequency band, cannot be reflected by the upper layers of

16

atmosphere. Television or FM signals therefore cannot be transmitted directly to receivers

located at very long distances. But now with the advent of satellites it is possible to beam

signals both in VHF and UHF (Ultra High Frequency) range to geostationary satellites, which

can then transmit these signals back to another far off location on Earth.

4.3 Propagation Unit

For the propagation, the electrical energy is converted into electro-magnetic wave. This is

done by antenna section and the different types of propagation are explained below as

4.3.1. Sky wave or Ionospheric wave propagation [between 2 to 30MHz]:

The sky wanes are of practical importance for every long radio communications at

medium and high frequencies i.e. medium waves and short waves. In this mode the EM

waves transmitted by the transmitting antenna reach the receiving antenna at very long

distance away from transmitting antenna after the reflection from the ionized region in the

upper part of the atmosphere of the earth. This part is called ionosphere and it is located

above earth‟s surface at about 70km to 400km height. The ionosphere acts as the reflecting

surface and reflects the EM wave back to the earth if the frequency is between 2 to 30

MHz‟s. As the sky wave propagation is useful for the frequencies between 2MHz to 30MHz

only this mode of propagation is also called short wave propagation. As the waves propagate

due to the reflection by the ionosphere the mode of propagation is also called Ionospheric

propagation using the sky wave propagation is also called Ionospheric propagation. Using the

sky wave propagation a long distance point to point communication is possible and hence it is

also called point to point propagation or point to point communication.

4.3.2. Space wave propagation [above 30MHz]:

When the frequency of the EM wave is between 30MHz to 300MHz the space wave

propagation mode is of importance. The EM waves in the space wave propagation mode

reach the receiving antenna either directly from the transmitting antenna or after reflection

from the atmosphere above the earth‟s surface up to 16km of height called troposphere.

Space wave consists of two components i.e. direct wave and indirect wave. The space wave

propagation is mainly used in VHF band as both previous modes namely ground wave

propagation and sky wave propagation both fail at very high frequencies.

17

4.3.3. Tropospheric scatter propagation or forward scatter propagation [above 30MHz

i.e. UHF and micro wave range]:

The UHF and microwave signals are propagated beyond line of sight propagation

through the forward scattering in the tropospheric irregulations. This mode of propagation is

of practical significance at UHF and microwave frequency ranges. This mode uses the

properties of the troposphere. Hence it is also known as troposphere scatter propagation. This

type of scatter propagation also needs to the Ionospheric scatter propagation for frequencies

in the lower range. Both Ionospheric scatter and tropospheric scatter produce undesirable

noise and fading which can be taken with diversity reception.

4.3.4. Ground wave propagation- plane wave earth reflection:

When the transmitting and receiving antennas are elevated the useful propagation can

be achieved by means of the space wave propagation. As the two antennas are within the line

of sight of each other the propagation of such space wave is also called line of sight

propagation. Basically for the line of sight propagation the resultant signal obtained is the

combination of the space wave and the surface wave whereas the VHF and UHF

transmissions are different. Here the antennas are of two types where the propagation of the

signal is done.

The word mast means that a supporting structure.

4.3.1.1. Self-supporting mast:

It is a general broadcasting purpose antenna here the antenna is held at height so that

the transmission of the signals would be without any obstacles. It is generally almost used in

all media using sectors.

4.3.1.2. Guided wire mast:

The mast here is suspended from the ground and it is supported by some wires so that it

would with stand to the climatic conditions.

18

Fig 4.4: a) Self-support mast b) Guided wire mast

19

CHAPTER-5

TRANSMITTER RACK

Fig 5.1: Transmitter rack

5.1 Audio- Video switcher:

This unit performs the function of selecting one of the four sets audio and video

inputs. The video input levels to the unit are 0.5 ± 1.5 Vp-p and +10 dB/m respectively. This

unit as an associated power supply to derive +15v, +5v and -15v required for its sub units

from 230V AC. One of the programme sources (video or audio) can be selected using PUSH

button switches available on the front panel.

5.2 Exciter:

The audio and video outputs from audio-video switcher unit are fed to exciter unit.

The audio input is fed directly to the aural modulator while the video signal is passed through

a low pass filter before being fed to its respective modulator. The audio is frequency

modulated using 33.4MHz IF. While video signal is amplitude modulated using 38.9 MHz

IF. The modulated signals are combined and then up converted to the desired transmitted

channel frequency. The video output power level after vestigial sideband filter and mixer is

10MW synchronous peak while audio is 1mW ALC (automatic level control) input is

available on VSBF mixer unit which can be fed from P.A stages to keep the overall

transmitter power output constant. The power supply need +16V and +28V for the unit is

supplied by P.S.U

5.3 Driver Amplifier:

The up- convertor signal from the exciter is fed to an attenuator which is placed at the

front panel and adjusting the input levels suitably. The signal is amplified using class A

20

driver stages. The overall gain of the amplifier can be adjusted by the front panel attenuator

control to be about 33db. The output of the amplifier is fed to the directional coupler where in

samples of transmitted and reflected power is obtained and fed to metering unit which defects

the signal and feds suitable voltage to a DC meter placed at the front panel. The three position

switch on the front panel selects the parameters to be monitored viz. vision, power, aural

power and reflected power. Readings are to be read with black picture aural power indication

is valid for black picture only. A separate exhaust fan operating at 230V AC is provided for

blowing off air in the driver unit to control the temperature raise for operation of driver

amplifier. A portion of output power is taken to the back panel of the driver unit for

monitoring purposes. The front panel output constant called „Ale‟ can be fed to the exciter

ALC in to the driver output constant at the set level. The availability of the input power

“28V” to the unit is indicated through a green LED on the front panel „DC Check‟ facility is

provided to monitor currents of 4 stages of power amplifiers by patching a „chord‟ meter on

combiner /divider unit.

5.4 Divider:

The linearity corrector output is to divide into four equal amplitude and phase outputs

to fed four PA to get the required output power. To achieve this connection, a four way

splitter is used by terminating unused parts. The four ways splitter doesn‟t have any achieved

components for isolation resistor. It is a micro strip circuit design based on Wilkinson‟s

power divided principles.

5.5 Combiner:

The two way power combiner is a sub unit in the 500W transmitter there are such

units. A two way combiner is used to combine the outputs of four amplifiers. For the first

level combining pairs of amplifiers are combined output or pairs of amplifiers is combined in

a second level of combining resulting in 600W peak sync output power. All units are

identical electrically and mechanically and are interchangeable. It is based on the Wilkinson‟s

power combiner principle. The combiner is realized as a micro strip line on a PCB substrate

with an isolation resistor for isolating all the ports.

21

5.6 Power Amplifier Unit:

The power amplifier unit comprises of two similar 60W power amplifier modules.

The R.F power output from the driver unit is divided in to two parts using the divider in the

divider/combiner unit and fed to each 5.0W power amplifiers. Each power amplifier is fed

with power input which is amplified to 50W (Sync peak) by four class A paralleled power

amplifier stages with a gain of approximately 6 & 10dB for channel 9.10 & 11; 12

respectively. - this output is fed to a directional coupler for obtaining samples of forward &

reflected power (30 dB coupling) for monitoring purposes for the control unit. The control

unit also obtains the temperature of heavy sink assembly through a thermistor. Separate

power supply is made available for each power amplifier (28V, 20A). The power supplies are

placed at the bottom portion of chassis assembly. A DC voltage proportional to current drawn

by each of the transistor in power amplifier is available from bias unit on DC check connector

placed over the front panel. This can be monitored on the current meter provided on divider

combiner unit through suitable patch cord provided separately.

There are two types of transmitters:

1. V.H.F transmitter

2. U.H.F transmitter.

22

CHAPTER-6

VHF TRANSMITTER

In this transmitter the frequency ranges from 224 MHz-231MHz. It requires 100W Power.

DD is presented by this transmitter in channel No.11

Fig 6.1: Block diagram of exciter unit

6.1 Exciter:

Exciter provides amplitude modulated visual drive of 10MW. Sync peak and a frequency

modulated all drive of 1mW required for the power amplifier stages of 100W TV transmitter at the

designated channel frequencies. It consists of the following individual units:

1. Video signal

2. Low pass filter

3. Video processor

4. Vision modulator

23

5. IF oscillator

6. Control oscillator

7. Aural modulator

8. Audio signal

9. Power combiner

10. Vestigial side band filter

11. Driver

12. +12V regulated power supply

6.1.1 Video Signal:

The video signal is limited to 5MHz by the low pass filter and the group delay by it is

corrected. Group delay introduced by it is corrected by the active group delay equalizer.

6.1.2 Low Pass Filter:

The LPF is used to limit the Video frequency to 5MHz only and it attenuates the Video signal

more than 20dB above 5.5MHz. The group delay introduced by steep falling characteristic at 5.5MHz

is corrected using 5-6 active group delay equalizer. LPF unit consists of single PCB consisting of a

video amplifier section and clamp pulse generator section.

6.1.3 Video Amplifier:

It amplifies the video signal to level sufficient to modulate the vision carrier in the vision

modulator unit. The video input to this unit is at level of 1Vp-p clamp pulse.

6.1.4 Generator Section:

The Sync component of the input video signal is separated and differentiated to produce a

trigger pulse which operates as multi vibrator.

6.1.5 Vision Modulator:

It consists of an IF amplifier, a modulator and band pass amplifier sections.

24

6.1.6 IF Amplifier:

This section amplifies the input form.The IF oscillator is to provide sufficient excitation to the

modulator. It operates as an amplitude limiter and thus maintain a constant output even with changes

in the IF oscillator output.

6.1.7 Modulator:

The video signal is applied in parallel to the modulator while the IF is fed in push pull

amplifier.

6.1.8 Band Pass Amplifier:

The modulated signal is amplified to a level of 10MW (approx.). A double tuned circuit with

a response flat within ±0.5dB in 7MHz band forms the collector load

6.1.9 IF Oscillator:

This unit generates the visual IF frequency at 38.9 MHz four outputs is available from this

unit. It gives four equal outputs of +14dBm each. The first one goes to the visual modulator unit

second one is used for monitoring, third one is taken to the local oscillator and the fourth one to the

aural modulator unit.

6.1.10 Control Oscillator:

The basic function of this oscillator is to supply three equal outputs of +8dBm each at a

frequency of (FV+FVIF).

6.1.11 Aural Modulator:

The aural modulator unit consists mainly of the following sections in a simple PCB.

6.1.12 Audio Amplifier:

The balanced audio signal at a level of +10dBm from the studio is converted into unbalanced

signal by the audio transformer the output of the audio transformer is taken through a rotary fader to

the input of the audio amplifier. The rotary fader is mounted on the front panel. The audio level can be

varied by adjusting the fader.

6.1.13 Voltage Controlled Oscillator:

The VCO is a Varactor tuned oscillator, the frequency of which can be varied manually by the

coil. Transistor forms the oscillator. The output of the oscillator is taken through a buffer amplifier by

25

two stages. The output of one of the amplifiers is fed to the mixer, is brought out to the socket. The

output of the VCO is frequency modulated by the audio signal from the audio amplifiers by injecting

the audio signal at the cathode of the Varactor diodes through an RF choke. The DC bias current for

the diodes is provided through the resistor. The output level of the VCO is about 0dBm.

6.1.14 Mixer:

The visual IF signal from the IF oscillator (38.9MHz) and the aural IF signal from the VCO

(33.4MHz) are injected in the base of the mixer transistor. Coil and capacitor are tuned to 33.4MHz to

provide isolation between visual IF port and VCO input port. The mixer output is 5.5MHz. This signal

is amplified by a common emitter amplifier, the output of which is transformed into a square pulse.

By the square pulse, pulse shaping network formed by diodes. This square signal is further frequency

divided by a chain of dividers to give an output square pulse at 537 Hz which is fed to the APC. This

division is carried out to minimize the phase error caused by the frequency modulation of audio signal

in the VCO.

6.1.15 Automatic Phase Control:

The APC is a sample and hold circuit using a CMOS analog gate for sampling. The square

wave signal from divider chain of mixer is transformed into a triangular wave by means of transistors

and the output is taken through a source follower. The output from the source follower is capacitive

coupled to the analog gate is derived from which is taken through an emitter follower and rectified

and applied to the analog gate. When the phase gate locked loop is unlocked the resulting AC signal

from the analog gate is amplified and rectified and the output of the rectifier operates a transistor

switch.

6.1.16 Power Combiner Unit:

Combiner unit is essentially a wide band amplifier of 32-42 MHz linearly combining the

vision IF and sound IF signals, maintaining sufficient isolation between vision and sound signals. It

consists of two pads of 16dB in the vision and sound inputs followed by a 6dB hybrid resistance

combiner. The combiner signal is fed to the base of the transistor which is biased for linear operation

and for wideband operation. The amplifier offers a gain of 22dB such that the input and output levels

are the same. The nominal output is +10dBm for vision and 0dBm for sound.

6.1.17 Driver Unit:

The up converted signal from the exciter is fed to a co-axial attenuator which is placed at the

front panel for adjusting the input level suitably. The signal is amplified using class a driver stages.

The amplifier consists of a driver amplifier and driver amplifier preceded by a step attenuator. The

26

overall gain of the amplifier can be adjusted by the front panel attenuator controlled to be about 33dB,

such that 25W/30W will available for channel 9 and 10 transmitter and 10W/15W (sync) will be

available channel for 11 and 12 transmitters at the output of driver unit. The output of the amplifier is

fed to a directional coupler where in samples of transmitted and reflected power is obtained and fed to

metering unit which detects the signal and feeds suitable 18 voltages to a DC meter placed at front

panel. The three position switch on the front panel selects the parameters to be monitored, e.g. vision

power, aural power, and reflected power. Readings are to be read with black picture signal. Aural

indication is valid for black picture only a separate exhaust fan operating at 230V AC is provide for

blowing of air in the driver unit to control the temperature rise for operation of the driver amplifier.

A portion of output power is taken to the back panel of the driver unit for monitoring purpose.

The front panel output called “ALC” can be fed to exciter “ALC IN” to keep the driver output to

constant at the level. The availability of the input power „28W‟ to the unit is indicated through a green

LED on the front panel.

6.1.18 +12V Regulated Power Supply:

The rectifier part of this power supply consists of a step-down transformer, a bridge rectifier

and an electrolytic capacitor for smoothing. The output of the rectifier is regulated using an integrated

circuit regulated ICI. A sample of the output voltage of the regulator obtained from the potential

divider formed by resistor and potentiometer PS compared with the built-in-stable 1.6V references in

ICI. The difference between the reference's and sensed voltage produces a voltage at pin 2 of ICI

which controls the series pass transistor. Thus, the output voltage is maintained constant at 12V, 1V

(both) as said by the sensing potential meter.

27

CHAPTER-7

UHF TRANSMITTER

In this transmitter the frequency range is from 564-574MHz. it requires 500W power. DD

news is broadcasted in channel 33. This transmitter is manufactured by Bharat Electronics (BEL).

Fig 7.1: Block diagram of UHF transmitter

7.1.1 Linearity corrector:

Linearity corrector operates in the UHF TV band of 470-600MHz and its function is to

correct the non-linearity that occurs in power amplifiers operated in this band. Non linearity in TV

amplifiers is measured in terms of 3-tone IMD and differential gain. The linearity corrector is a pre-

distorter circuit that is placed ahead of the power amplifier and pre- corrects the above mentioned

distortion so as to reduce them at the power amplifier output.

7.1.2 Base Band Corrector Unit:

The unit accepts the video signals and introduces the required pre-connection in

differential phase, differential gain and luminance and non-linearity in order to compensate

for the non-linearity encountered in the power amplifier without introducing any frequency

response fall and group delay

7.1.3 Exciter:

The audio and video outputs from audio-video switcher unit are fed to exciter unit.

The audio input is fed directly to the aural modulator while the video is passed through a low pass

filter before being fed to its respective modulator. The audio is frequency modulated using 33.4MHz

IF. While video signal is amplitude modulated using 38.9MHz IF. The modulated signals are

28

combined and then up converted to the desired transmitted channel frequency. The video output

power level after vestigial sideband filter and mixer is 10MW synchronous peak while audio is 1mW.

ALC (automatic level control) input is available on VSBF mixer unit which can be fed from P.A.

stages to keep the overall transmitter power output constant. The power supply need +16V and +28V.

For the unit is supplied by P.S.U. In addition one +12V supply kept constant „ON‟. Feeds power to

oven controlled oscillator in VSBF and mixer unit, in order to maintain high frequency stability.

7.1.4 Up- convertor:

The up-convertor unit combines modulated vision IF an aural IF signals and translates to

respective channels frequency suitable for transmission. The unit has in-built power supply. The status

and fault information are displayed on front panel of the unit.

29

7.2 CONSOLIDATED CCIR SYSTEM STANDARDS

The main characteristics of the CCIR System-B for mono-chrome Television adapted in India

are given below:

Video Characteristics:

Number of lines per picture : 625

Interface ratio : 2:1

Field frequency : 50 fields/sec

Picture frequency : 25 pictures/sec

Line frequency tolerance : 1562 lines/sec±0.1%

Aspect ratio : 4:3

Scanning frequency for lines : left to right

Scanning frequency for fields : top to bottom

Video bandwidth : 5MHz

Approximate gamma of picture signal : 0.5

30

7.3 THE FCC CHANNELS FREQUENCY IN SYSTEM

(VHF BANDS)

Channel number frequency range picture carrier sound carrier

(MHz) (MHz) (MHz)

1 54-60 55.25 59.75

2 60-66 61.25 65.75

3 66-72 67.25 71.75

4 76-82 77.25 81.75

5 82-88 83.25 87.75

6 174-180 175.25 179.75

7 180-186 181.25 185.75

8 186-192 187.25 191.75

9 192-198 193.25 197.75

10 198-204 195.25 203.75

11 204-210 205.25 209.75

12 210-216 211.25 215.75

Tab 7.1: FCC channels frequency in system

31

CHAPTER-8

TELEVISION, THE MAGIC LANTERN

The radio signals received by the TV antenna are normally weak; therefore they have

to be first amplified using a RF (radio frequency) amplifier. After amplification the signals

are demodulated, this is done through a tuner. A tuner can produce electric currents

(electromagnetic waves) having same frequencies as the carrier waves used to broadcast

various channels. Therefore when we select a particular band on the TV set, an electric signal

having a particular frequency is mixed with the amplified signal received through the antenna

to produce a demodulated signal. From the tuner, the television signal goes through

complicated electronic circuits in the set. These circuits further process the signal to separate

the audio and video portions of it. The audio signals are changed into sound waves by the

speaker; the video signals go to the picture tube where they recreate the picture. The picture

tube transforms the video signal into patterns of light that duplicate the scene in front of the

camera at the time of the broadcast or when the programme was recorded. One end of the

picture tube is rectangular and nearly flat which makes up the screen of the TV set. At the

other end the picture tube tapers off to a narrow neck. The neck of the picture tube holds

three electron guns. One each for

1. Red,

2. Blue, and

3. Green signals.

The tube of a black and white TV set has only one gun. Each electron gun in a colour

picture tube shoots a separate beam of electrons at the screen. The screen of most colour

tubes is coated with more than 300,000 tiny phosphor dots made-up of coating of

phosphorescent materials which emit light when exposed to some radiation such as light or

electrons. They continue to emit light for some time even after the source of radiation is

turned off. This explains why the TV screen glows after a torch light is switched off.

The dots on a colour TV picture tube are grouped in triangular arrangements of three

dots each one red, one blue, and one green. These dots glow with their respective colours

when struck by an electron beam. A metal plate perforated with thousands of tiny holes lies

about 13 millimetres behind the screen of a colour picture tube. This plate, called the shadow

32

mask, keeps the beams from hitting any other colour dots but their own. When the television

set shows a colour program, the neuron signals produced by light emitted from the three

coloured dots blend together in the viewer‟s brain to produce the perception of all the colours

of the original scene. In case of a black and white program the dots appear to produce

different amounts of white light. The picture on the TV screen is produced by the process of

scanning the electron beam horizontally on the screen at a very fast rate. The beam is guided

by the magnetic field produced by the signal electric current fed into coils which are located

around the neck of the picture tube. The electron beam scans the screen much as a person

reads from left to right, top to bottom. The scanning pattern for the normal TV sets used in

India is made up of 625 lines. In a high definition TV it is made up of 1125 lines.

As electrons constantly strike the screen, it acquires static electric charge, which can

induce electric charge on our hairs on our hands/arms and make hair stick to the screen.

A magnet distorts the path of the electron beams used to create the picture and hence

the picture formed on the screen gets distorted when a magnet is brought close to it.

Transmitting or receiving video signals without modulation/demodulation is indeed possible

for short distances it is used in a close circuit television setup. A video tape recorder records

and replays the video signal and the audio signals on a magnetic tape very much like an audio

tape recorder records audio signal on audio cassettes.

A normal TV receiver can receive transmitted TV on fourteen different channels. The

frequency of these ranges from54, 000,000 Hz or54 MHz to216, 000,000 Hz, and 216 MHz.

The bandwidth of each of these channels is6 MHz. Signals transmitted on such frequencies

are known as VHF range, or very high frequency signals. TV signals can also be broadcast on

frequencies between 470 MHz and 890 MHz known as the UHF range.

Doordarshan in India broadcasts TV programs on the national channel (DD1) in VHF

range. Both VHF and UHF signals act much like light, not bending much around the

curvature of Earth and pass through the atmosphere. They are also blocked by structures and

hills. An airplane coming in the way of the signal and the receiver therefore disturbs TV

reception. Television broadcasting antennas are usually placed on tall towers standing on

high ground, so that the radio signal which carries the television programme may travel as far

as possible. But still the maximum range of a TV broadcast signal is in between 100 to250

km. Television signals are therefore sometimes broadcast via satellites to reach an audience

33

farther off. The signals sent to the satellite are in the UHF range and are sent back towards

the ground by the satellites. These signals are received by cable operators using large dish

antenna. Often there is a limitation posed by the design of older television sets, which allots 6

MHz bandwidth to each channel. In order to overcome this limitation cable operators/recent

television sets use single sideband (S Band) technology to modulate/demodulate the signals.

Using this technology a larger number of TV signals can be distributed through a cable

network. Such signals have bandwidth less than 6 MHz‟s.

8.1 Antenna Section

An antenna is a piece of a conducting material which facilitates the resonance

between the receiver and transmitted electromagnetic waves. It can be in the form of a length

of a wire, a number of metallic rods, a coil or a dish. The dimensions and the design of an

appropriate antenna for a receiver located at a particular location depends both on the strength

of the signal in that area as well as on the electronic design of the receiver instrument. While

in the neighbourhood of a radio or TV transmitter a piece of wire may be often sufficient to

produce good reception, in far off places one need an antenna designed for better reception of

the signal. To be highly efficient, an antenna must have dimensions that are comparable with

the wavelength of the radiation of interest the wavelength of a 66-72 MHz‟s wave (the

frequency of channel 4 on which DD1 is transmitted) is about 4 meters, A conductor having

this length is often sufficient to receive strong TV signals. The folded rod in the middle of a

common television antenna is also about this length the other rods essentially serve as

reflectors to boost up the signal. Radio signals are usually strong. We therefore can pick up

signals from a local radio station even without an antenna. Sometimes a long metallic rod

which has length equal to a near fraction of the wavelength (one tenth or a quarter of the

wavelength of the transmitted radiation) or a coil of wire wound on a ferrite core can be

sufficient provided the receiver has been adequately designed to receive and process weak

signals. A radio frequency signal is often much stronger in a certain direction (the direction in

which the transmitter is located) than others. Also some antennas are directional they are

more effective in a particular geometry e.g. when the rods of a TV antenna are aligned

perpendicular to the direction in which transmitter is located the antenna is more effective.

Similarly a radio antenna of an AM radio receiver (made up of a wire coil wound on a ferrite

rod) if aligned toward the transmitter yields a higher output signal. The changing intensity of

the volume of a radio program broadcast on a medium wave band is due to such directional

34

characteristic of its antenna. The antenna of a FM radio set is often in the form of a

stretchable metallic rod, which normally points towards the sky, but one does sometimes

experience changes in the sound intensity in an FM set. This is due to the fact that the

wavelength of the electromagnetic waves used to carry FM signals is in the range of a few

meters the dimensions of our usual rooms. These waves are reflected from the walls of the

room and can be absorbed by objects coming in between. Hence sometimes signals become

very weak or very strong when the receiver is placed pointing to a certain direction or

someone comes in between.

The metro channel of Doordarshan is transmitted on a frequency band ranging from

174- 180 MHz; hence an antenna having a dipole about 1.3 meter length is sufficient

provided the signal does not get attenuated by the time it reaches the location. You may

recall, we have said earlier that as the frequency of the signal increases, it is more susceptible

to be absorbed by objects in the way. The metro channel therefore can be easily received only

in the cities where it is broadcast, or through the satellite. The signals transmitted from a

satellite are also in high frequency range. Ordinary rod antennas cannot pick up such signals,

one needs specially designed dish antennas linked to appropriate tuners to pick up and

process signals from them. The energy in an electromagnetic wave is easily dissipated by

inducing electric current (motion of electrons) in a closed loop of metallic conductors. A

receiver enclosed in a cage like structure made up of a metal (popularly known as the

Faraday‟s cage) is therefore unable to receive electromagnetic signals, because there is no

electromagnetic energy left. One can therefore understand why our transistor radio cannot

receive radio programs inside a train or a bus. Such a cage also acts a no entry zone for

electromagnetic waves. This also explains the use of shielded wires used for transmission of

TV programs by cable networks.

A metallic shield around the metallic wire carrying the signal prevents outside

interferences as well as attenuation of the signal by leaking of waves to neighbouring

locations. But then cable operators often have to compromise with low cost joins between

two cables, which are often sufficient for neighbouring TV sets to pick up which explains the

reception of cable programs even when you may not have a cable connection.

TV signals are easily reflected by huge buildings in the neighbourhood of a

transmitter; the reflected signal has a slightly different phase than the original signal and

gives rise to „ghost´ images.

35

CHAPTER-9

CONCLUSION

We would like to conclude that this training is a very great and enriching the

experience to learn about the low power TV transmitter. The transmitter service involves

great equipment that deals with monitoring section exciting system and we learn about the

above equipment of the Doordarshan relay centre and its working. We also learned about the

procedure of transmission, reception. And strengthening of the signal and retransmitting the

signal into space for the broadcast around the range of propagation.

36

REFERENCES

http://www.google.com/dhoordharshan/

http://www.ddindia.gov.in

Antenna Wave And Propagation – K.D. Prasad

Analog Communication – Sanjay Sharma