document1

STUDY OF LOW POWER TRANSMITTER

1. ORIGIN OF DOORDARSHAN

1.1 Introduction:

Doordarshan is the national service of India and is also one of the largest broadcasting

organizations in the world. A network of three nationals, two special interest channels; 10

regional language channels, 4 state network and an international channels. Through a network

of 868 terrestrial transmitters of varying powers it makes available television signals for over

87% of population. 300 million viewers in their homes watch Doordarshan programmes.

Television sets established under various schemes in community centers in villages for a total

number of 450 million viewers (India, 1998). The countrywide class room on national network

is aimed to reach quality education of students in small villages.

Television in India has been in existence for decades now. India did not begin till

September 15, 1959 with a small studio. The service was called “Doordarshan” for the first 17

years, it spread haltingly and transmission was mainly in black and white. Doordarshan was

established as a part of AIR, until 1976, it consisted of one national network and seven regional

networks. In 1992 there were sixty three high power television transmitters, 369 medium

power transmitters, 76 low power stations and 23 transposers. Regular satellite transmission

began in 1982.

Television has come to the forefront only in the past 21 years and more so in past 13.

There were initially two ignition points, the first in the 80’s when color television was

introduced by state owned broad caster. Doordarshan (DD) timed with 1982 Asian games

which India hosted. It then proceeded to install transmitter nationwide rapidly for terrestrial

broadcasting. In this period, no private enterprise was allowed to set up television signals. The

second spark came in early nineties with the broadcast of satellite television by foreign

programmers like CNN followed by STAR T.V and a little later by domestic channels such as

ZEE T.V and SUN T.V into Indian homes. The number of Televisions sets in India increased

from around 500,000 in 1976 to 9 million in early 1987 and to around 47 million in 1994;

increases are expected to continue at around 6 million sets per year.

AITAM ,TEKKALI Page | 1


If all the doordarshan centres, Mumbai has the most acute language problem,

having to cater to a cosmopolitan and varied audience in Hindi, English, Urdu, Marathi and

Guajarati. In 1984, doordarshan introduced a second channel for the big cities and permitted

cable operators to transmit locally made programs to fill the gaps in the schedule when

doordarshan was not in air. These cable operators grew from a few 100’s in the eighties to

more than 20,000 in the nineties.

Presently Doordarshan operates 19 channels, two All India Channels, 11 regional

languages satellite channels (RLSC), four State Networks (SN) an international channel and

a sports channel. Regular satellite transmission began in 1982. Now more than 87% of

population of the country can receive Doordarshan programmes through a network of

nearly 1044 Terrestrial Television Transmitters. About 46 Doordarshan studios are

producing television software.

The T.V. transmitters can be classified into three types basing on their coverage area,

radiated power.

High Power Transmitters

Low Power Transmitters

Very Low Power Transmitters

TYPE COVERAGE POWER

High Power

Transmitters

80-120 km >1kwatt

Low Power

Transmitters

15-20 km 100watt

Very Low Power

Transmitters

10 km 5-10watt

T1.0Various types of power transmitters &their coverage



1.2 What is need of LPTS?

HPTS can cover the 80-120 k.m. if the area is flat area i.e. without any obstruction. If any

highest buildings or hills exists then coverage capacity reduces because of the transmitted

signals absorbed by them and they prevents the signal to go.At this time the transmission

system fails to cover some areas which are behind the hills or the tallest buildings, this lead to

the establishment of LPTS to transmit over a small distances of 15-20 km.

BLOCK DIAGRAM OF LPT:



1.3 TV Programmes Transmission Using Satellite:

Fig1.0 uplink and downlink process of signal

TV Signals from studio are processed and up-linked to the satellite where these signals are

further processed and then down linked to the Terrestrial T.V Transmitters with the help of

transponders of the satellite. The signal received by the parabolic dish antenna is sent to the

TVRO of input output chain with the help of coaxial cable and then it is further processed and

modulated and transmitted into the space.



2. TRANSMISSION AND RECEPTION ANTENNAS

2.1 Introduction:

Although there are many ways of communication in practice

today,e.g.,linetelegraphy,linetelephony,visual signaling etc.but the radio communication (also

called wireless communication),is the most fascinating and popular system of communication

these days.The essential requirements of any communication system are fundamentally same

eg.means of transmitting the message or intelligence,means to receive the

message.Inotherwords a Transmitter,a medium to carry the message or intelligence from one

place to another and a receiver. Radio frequency waves, produced at transmitter, are carried by

transmission lines to an antenna or aerial which radiate it in the form of electromagnetic waves

into the space.let us define an antenna.

2.2 Antenna

Antenna is a metallic device for radiating or receiving radio waves. So antenna in other words

is a transitional structure between a free space and a guiding device. And this guiding device

may take the form of a coaxial cable or any other transmission line which transports EM

energy from source to antenna.

2.2.1. Antenna Parameters:

1) Power gain

2) Directivity

3) Polarization

4) Radiation pattern

5) Beam width

6) Band width

7) Radiation resistance

POWER GAIN:

The power gain of an antenna is a ratio of the power input to the antenna to the power

output from the antenna. This gain is most often referred to with the units of dbi, which is

logarithmic gain relative to an isotropic antenna. An isotropic antenna has a perfect spherical

radiation pattern and a linear gain of one.

DIRECTIVITY:



Directivity of a antenna is defined as the ratio of antenna intensity in a given direction

from the antenna to the radiation intensity averaged over all directions. The average radiation

intensity is equal to the total power radiated by antenna divided by 4π.

POLARIZATION:

The polarization of antenna is the polarization of the wave radiated by the antenna ie. the

polarization describes the orientation of electric field into the space.

It can also be defined as " the property of an electromagnetic wave describing the time varying

direction relative magnitude of the electric field vector.

Magnetic fields surround the wire and perpendicular to it, it implies that the electric field is

parallel to the wire. So, this configuration is applied even after radiation of waves from the

wire. So there arises the need for polarization of a radiating element.

Polarization of an antenna refers to the orientation of radiated electromagnetic waves in the

space.

Classification of Polarization:

Polarization is classified based on the orientation of electric field as linear, circular and

elliptical polarization.

Fig2.0 classification of polarization

Linear Polarization:

An electromagnetic wave is said to be linearly polarized if all the cycles of the wave has same

alignment in the space.

Horizontal polarization:



If an electric field vector E lies in horizontal plane then the wave is said to have

horizontal polarization.

Vertical polarization:

If an electric field vector E lies in vertical plane then the wave is said to have vertical

polarization...

Circular/Elliptical Polarization:

If two linearly polarized waves which are simultaneously produced in the same

direction and are mutually perpendicular to each other with a phase difference of 90 degrees,

then we get a circularly polarized wave.

Circular polarization may be right -handed or left-handed depending upon the sense of rotation

i.e., phase difference is positive or negative. Circular polarization results only when the

amplitudes of the two linearly polarized waves are equal. If the amplitudes of the two linearly

polarized waves are not equal then we result in elliptical polarization.

We yet have another polarization called cross polarization which is basically the radiation in

the undesired direction.

At the TV broadcasting frequencies we generally use horizontal polarization as a

standard. At microwave frequencies the polarization we use depends on the type of

application. In any case the polarization of both receiving and transmitting antennae should be

equal.

RADIATION PATTERN:

A graphical representation of radiation properties of antenna as a function of space coordinator

is termed as radiation pattern.

BEAM WIDTH:

The beam width of an antenna is the angular separation between the two half power

points on the power density radiation pattern. It is also, of course, the angular separation

between the two 3-db down points on the field strength radiation pattern of an antenna.

BAND WIDTH:

It refers to the frequency range over which operation is satisfactory and is generally

taken between the half power points.

There are actually two separate requirements for large bandwidth (in excess of 10%)



from antennae. The first is for antennae which could be narrowband but which are required to

operate at a number of separate frequencies within a fairly large range. High frequency

antenna's are often of this type, in which the required operation is helped by the fact when the

antenna is switched to a new frequency, compensating circuits can be switched in also.

RADIATION RESISTANCE:

The input impedance 'Zin' of an antenna is the ratio of voltage to current at its input terminals

where the power is fed to the antenna.

Zin = Ra+jXa

Ra = radiation part of impedance

Xa = reactance part of impedance

2.2 TYPES OF ANTENNAS:

Based on these there are different types of antennae. But basically used types of antennae are

as follows:

1) Folded dipole antenna

2) Parabolic dish antenna

3) Yagi-uda antenna

4) Horn antenna

5)v-antenna

2.2.1 Folded dipole antenna:

A variation of the dipole can be a solution to the problems caused due to dipoles,

offering a wider bandwidth and a considerable increase in feed impedance. The folded dipole

is formed by taking a standard dipole and then taking a second conductor and joining the two

ends. In this way a complete loop is made as shown. If the conductors in the main dipole and

the second or "fold" conductor are the same diameter, then it is found that there is a fourfold

increase in the feed impedance. In free space, this gives a feed impedance of around 300 ohms.

Additionally the RF antenna has a wider bandwidth.

2.2.2 Parabolic Dish Antenna:

A dish antenna works on the same way as a reflecting optical telescope. Electromagnetic

waves, light or radio, arrive on parallel paths from a distance source and are reflected by a

mirror to a common point, called the focus. When a ray of light reflects from a mirror or flat

surface, the angle of the path leaving (angle of reflection) is the same as the angle of the



arriving (angle of incidence).If the mirror is a flat surface, then the two rays of light leave in

parallel paths; however if the mirror is curved, two parallel incident rays leave at different

angles. If the curve is parabolic(y=ax²) then all the reflected rays meet at one point as shown in

figure.

F-focus

Fig2.1 parabolic dish antenna

A dish is a parabola of rotation, a parabolic curve rotated around an axis which passes

through the focus and center of the curve. The received signal can be transmitted to the low

noise block converter(LNBC) through feed horn antenna.

2.2.3. YAGI-UDA ANTENNA:

A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a

directional antenna system consisting of an array of a dipole and additional closely coupled

parasitic elements (usually a reflector and one or more directors). The dipole in the array is

driven, and another element, 10% longer, operates as a reflector. Other shorter parasitic

elements are typically added in front of the dipole as directors. This arrangement gives the



antenna directionality that a single dipole lacks. Yagis are directional along the axis

perpendicular to the dipole in the plane of the elements, from the reflector through the driven

element and out via the director(s).Directional antennas, such as the Yagi-Uda, are also

commonly referred to as beam antennas or high-gain antennas.

Fig 2.2yagiuda antenna

2.2.4 HORN ANTENNA:

A wave guide is capable of radiating radiation into open space provided the same is

excited at one end and opened at other end. The radiation through this feared out wave guide is

more than through that of a transmission line.



Fig2.3 horn antenna

2.2.5 V-antenna:

It’s an omni directional antenna fabricated from the light weightAluminium tubes and

flats..The electronic wiring is concealed within the tabular structure of the antenna and is

sealed to prevent moisture penetration.A clamp is provided between the feed for clamping the

feeder cable to prevent strain on the cable connector due to swaying of the cables.Arubberhood

just above the feed point protects the connector from the direct rain. Average gain of V

antenna is 4.3 dbm and power is 100watt,frequency range 174MHZto 232MHZ in UHF band.

Fig 2.4 v-antenna

3. TELEVISION RECEIVE ONLY SYSTEM

3.1 Introduction:

The signal which is transmitted to the terrestrial system by the transponders in the

sattellites to be received by a parabolic dish antenna.The PDA consists of chicken mesh

which decreases the pressure on Antenna.The signal which is received by the PDA is

made to pass through a horn antenna later it connects to the LNBC (low noise block



converter).LNBC is an amplifying device used to downlink the signal from GHZ range to

MHG range using a local oscillator frequency of 5150 MHZ. After down converting the

signal from GHZ range to MHZ range it is allowed to pass through a RF cable to sent the

received amplified signal to satellite system unit.

3.2 Parabolic Dish Antenna:

A dish is a parabola of rotation, a parabolic curve rotated around an axis which passes

through the focus and center of the curve. The received signal can be transmitted to the low

noise block converter(LNBC) through feed horn antenna.

The azimuth and elevated angles and the direction of the parabolic dish are different for

different satellites. They can be varied by using horizontal and vertical motors from the control

room. The azimuth and elevated angles can be given by the doordarshan organization.The

following table mention the polarization, azimuth and elevated angles of Dish, and the down

linked signal information for different satellites.

T.3.1 TABULARFORM



PARAMETER INSAT 3A INSAT 4B INSAT 3C INSAT 2E

LOOKINGANGLE 93.5° E 93.5° E 74° E 93.5° E

DIAMETER 7.5 mt 6.3 mt 3.6 mt 6.2 mt

AZIMUTH 149° 149° 220° 180°

ELEVATION 67° 67° 63.2° 70°

POLARIZATION Vertical Vertical Horizontal Vertical

DOWNLINKED

SIGNAL

Regional

Signal

National and

News

National Standby

mode

National

The unlinking and down linking can be done in any one of the frequency bands available C-

band,X-band,KU- band etc.Fore.g.the 100LPT at Srikakulam operated in C-band.The below

tabular form shows the unlinking and down linking frequency ranges of various bands

T.3.2 DIFFERENT BANDS AND THEIR FERQUENCY RANGES

BAND FREQUENCY

UPLINK

BAND, GHz

DOWNLINK

MAJOR USES BAND

WIDTH

C-BAND 5.9 – 6.4 3.7 – 4.2

Fixed, point to point

ground stations,

nonmilitary.

500MHz



X-BAND 7.9 – 8.4 7.25 - 7.75

Mobile(ships,

aircraft).Radio relay,

military only.

500MHz

Ku-

BAND

14 – 14.5 11.7 – 12.2

Broadcast and fixed-

point service; non

military

500MHz

3.3 Low Noise Block-Down Converter:

Fig3.1 Ku-band LNB with both sides uncovered.

A low noise block-down converter (or LNB) is the receiving device of a

parabolic satellite dish antenna of the type commonly used for satellite TV

reception. The device is sometimes called an LNA (for low noise amplifier),

LNC (for low noise converter) or even LND (for low noise down converter) but

as block-down conversion is the principal function of the device, LNB is the

preferred term, although this acronym is often incorrectly expanded to the

incomplete descriptions, low noise block or low noise block converter

It is functionally equivalent to the dipole antenna used for most terrestrial

TV reception, although it is actually waveguide based. Inside the LNB waveguide

a metal pin, or probe, protrudes into the waveguide at right angles to the axis and

this acts as an aerial, collecting the signal travelling down the waveguide.



Fig 3.2 LNBC block diagram

The LNB is usually fixed on the satellite dish framework, at the focus of the reflector,

and it derives its power from the connected receiver, sent "up" the same cable that carries the

received signals "down" to the receiver. The corresponding component in the transmit link

uplink to a satellite is called a Block up converter (BUC).

4. SATELLITE SYSTEM UNIT

4.1Introduction:

The amplified downlink signal from the LNBC is given to the satellite system unit

through a cable of impedance 75Ω and 6MHZ bandwidth. The satellite system unit protects

the remaining system components from lightening and thunders. The received signal can be



given to the exciter through the different stages. They are integrated receiver decoder (IRD) s,

audio programming amplifier, video programming amplifier TV demodulator and monitoring

amplifier. A distortion/noise level meter is there to show the signal level.

4.2 IRD:

IRD means an “integrated receiver to decoder”, which can be used to decode the

original baseband signal from the received one. In the past analog receivers are used but at

present analog systems are replaced with the digital receivers as they are more advantageous

than the analog receivers. It takes the received signal as input and seperatesaudio and video

signals from it. It has two output ports one is audio signal output and another is video signal

output. The audio and video signals are given to the audio programming amplifier and video

programming amplifier for correction purpose respectively.

Fig4.1 Integrated receiver decoder

4.3 AUDIO & VIDEO PROGRAMMING AMPLIFIER:

The unit I designed using latest devices and technology. This unit provides distortion

less audio output it has wide bandwidth and excellent signal to noise ratio(S/N ratio).This unit

has both line and microphone inputs. A VU meter is present in the unit for monitoring and

several other features such as short circuit protection in audio outputs and power supply. The

applications are in audio signal processing and used in sound monitoring applications in

studio, broad casting networks etc.

The function of video programming amplifier is to correct the video signal.



4.3.1 AUDIO/VIDEO SWITCH:

The “audio switcher/electronics patch panel” is used in monitoring and test equipment

rack for a LPT.It receives the video signals and audio signals from different sources and

selects any one of them for further processing and transmission, with minimum deterorationin

the characteristics of the signal. Its special feature is that we can monitor any input channel

been selected for transmission without disturbing normal transmission. The main use of the

audio/video switcher is channel selection.

4.3.2 TV DEMODULATOR:

It is precision monitoring equipment for checking the quality of a TV transmitter in the

VHF and UHF bands. It reconstructs original baseband signal and connected to the exciter

stage.

4.3.3 TV PATTERN GENERATOR:

This unit consists of several LED lights .the signal from the receiver connected to the

pattern generator. to check whether the circuit good or not. when we connect the signal to TV

pattern generator ,then at the output(on TV set) we can observe the color pattern lines.

4.4 SYSTEM UNIT:

This unit consists of two switches which are used to operate two motors.one motor is

for adjusting the vertical angle of PDA. Another for adjusting the horizontal angle of PDA.

4.5 VESTIGIAL SIDE BAND TRANSMISSION:

In the 625 line TV system, frequency components present in the video signal extend

from 0 Hz to 5 MHZ. A double side band AM transmission would occupy a total bandwidth

of 10 MHz. To reduce the channel bandwidth and power, Vestigial sideband Transmission is

in practice. In the video signal very low frequency modulating components exist along with

rest of signal. These components give rise to sidebands very close to carrier frequency which

are difficult to remove by physically realizable filters. Suppressing one complete sideband

also not possible. The low video frequency contains the most important information of picture



and any effort to completely suppress the lower sideband results in objectionable phase

distortion at these frequencies; it will look in the picture as smear. Therefore only a part of

lower side band is suppressed and radiates signal with full Upper Side Band together with

carrier and vestige of the partially suppressed Lower Side Band. This is called V.S.B or A5C

transmission. In the 625 line system, frequencies up to 0.75 MHz in the lower sideband are

fully radiated. So it is a double sideband transmission for lower video frequency.

(Fig4.2 Theoretical representation of the side bands in VSB transmission )

(Fig 4.2.1 F Curve of TV setTheoretical)



4.5.1 RECEPTION OF VESTIGIAL SIDE SIGNALS:

Because of filter design difficulties it is not possible to terminate the bandwidth of

signal abruptly at the edges of sideband therefore attenuation slope covering 0.5 MHz is

allowed at either end.

(Fig4.2.2 Response for VSB reception)

Now these visual and aural signals are given to the exciter for further processing. In the

exciter stage, blocks like video processing unit , diode bridge modulator , delay equalizer ,

V.S.B filter , video up converter , linear amplifier , power amplifier and diplexer and

frequency multiplier process the video and audio signals. The combined visual and aural signal

after arriving the diplexer block is transmitted to mast antenna.



5. EXCITER

5.1 EXCITER:

The audio and video output of the program to be connected to the Exciter unit. The

audio input is feed to the aural modulator through ‘blank module’, while the video is passed

through a video processor unit 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 signal are combined in combiner and then up converted to the desired

transmitted channel frequency in IF channel convertor unit.



Fig5.1 exciter

The video output power level is 10MW synchronous peak while audio is 1MW. In this

automatic level is available on real panel of the exciter, which can be feed from driver unit.

Exciter needs 28V for operation, and its supplied from 28V, 25A PSU units which was

connected in parallel as shown in block diagram.

A DC-to-DC converter provided in the exciter derives +5V to +15V to be supplied to

individual sub systems in the unit.



fig 5.1.1.internal block diagram of exciter

Exciter is functionally divided into seven modules namely:

1) Video processor.



2) Vision modulator.

3) IF corrector.

4) Aural modulator.

5) IF combiner.

6) IF channel converter.

7) DC to DC converter.

5.1.1 VIDEO PROCESSOR:

The externally applied video signal is feed to the video input amplifier. This amplifier

has high input impedance in order to provide the high ohmic loop through facility. The

amplified video signal is feed to the potentiometer and to the synchronous separator.

The potentiometer control the amplitude of the video signal from the input amplifier and

there by it determines the modulation depth. This signal is applied to switch directly as well as

through receiver pre corrector. The group delay equalizer introduces a pre distortion of the

video signal.

The synchronous separator consists of detector and a comparator. The peak level of the

synchronous pulse is applied to detector. The output from detector is used as reference level

for the comparator, which means that the synchronous pulse is sliced at 50% of the

synchronous peak level.

The back porch of the synchronous pulse initiates the “clamp pulse generator”. During

this clamp pulse period the transistor is off and saturated, where by the video signal is clamped

to zero in the junction. The clamp pulse does not affect the 4.43MHZ band stop filter ensures

that the color burst in the video signal. In the same period the synchronous separator is also

clamped to zero.



When no synchronous is applied, the “DC restore” is switched. In this case the peak

value of signal is maintained.

A part of the video signal is also applied to the “video level detector” via low pass filter.

The video level detector consists of a peak detector followed by a DC amplifier. The DC

output goes to the meter in IF combiner unit.

5.1.2. VISION MODULATOR:

The vision IF 38.9MHZ is generated by a ‘TCXO’ and split in to three identical signals

by means of power splitter, The IF carrier is feed to the “vision modulator” which is doubled

balanced ring modulator. In order to obtain a carrier as well as the side bands from the

modulator the applied video signal is properly clamped to zero in video processor unit.

The white level is adjusted by means of R8. The capacitive balance is adjusted by

means of C1 and C28. The “VISION IF AMPLIFIER” which is a wide band amplifier

amplifies vision modulated. Amplifier further amplifies the vision IF to +10dbm. A SAW

VSB filter performs VSB filtering of the side bands with minimum group delay and high

reliability. Amplifiers compensate the insertion loss of VSB filter and finally band pass

amplifier amplifies the vision IF to a level to 10dbm. The power splitter splits the amplified

vision IF from amplifier to two identical signals, one for combiner and other for monitoring.

5.1.3 IF CORRECTORS:



Fig5.2 IF corrector

Correctors are of two types. They are amplitude linearity and phase linearity

corrector.

1)Amplitude linearity corrector:

The transistors Q1 and Q2 form a linear amplifier, input to which is controlled by

potentiometer RV1. Transistors Q3 and Q4 form a non-linear amplifier input to which is

controlled by potentiometer RV2.

Adjust RV1 for minimum input to transistor Q1. Check for amplified input signal at

collector of Q4. Vary RV3 and check for appearance of distortion at Q4 output, because of

non-linearity. Set RV3 to position where distortion just appears.

Adjust RV2 for minimum signal at Q3 base. Check for distortion less amplified IF input

at collector of Q2. Next adjust RV1 and RV2 approximately for 50% signal passage, and

check for availability of amplified, IF at collector of Q5. Demodulate the IF output available at

TP3 through a TV demodulator and measure the synchronous picture ratio.

Vary RV2 and RV1 and ensure that the synchronous picture ratio can be varied by +/-

5%. Feed multi burst pattern from VTSG and adjust the frequency response to best possible

extent by varying VC1 and VC2.



2) Phase linearity corrector:

The input signal is splits into 2 branches. Signal passing through Q8 and Q9 stage is

delayed using delay line and the other signal is processed through a non-linear amplifier

combining these two signals introduces phase difference in the output signal.

Disconnect the cable from TP1 and TP2 and solder it on TP5 and TP6. Adjust RV7 for

minimum signal at Q8 base and RV5 for maximum signal at Q6 base. Check for amplified IF

signal at collector of Q7 and introduction of distortion by RV6. Set RV6 to position, where it

just introduces non-linearity.

Adjust RV5 for minimum signal at Q6 base and RV7 or maximum signal at Q8 base

and check for the same IF signal at emitter of Q9. Adjust RV5 and RV7 for approximately

equal signal at base of Q8 and Q6,and check for amplified IF signal at collector of Q10.

5.1.4 AURAL MODULATOR:

Aural modulator unit consists of the following sections in a single

PCB.

1) Audio amplifier

2) 1KHZ oscillator

3) VCO

4) Mixer

5) APC

6) Meter circuit



Fig5.3 Aural modulator

Audio amplifier:

The balanced audio signal at a level of +6dbm from the TVRO is converted in

to unbalanced signal by the op-amp U8. The output of U8 is taken through a front panel

potentiometer R83 to the audio amplifier U7. The audio amplifier uses a hybrid micro circuit

BNC 1003. A 50μsec pre emphasis is provided, which can be adjusted by the potentiometer

R87. The output of the audio amplifier is applied to the varactor junction of VCO to frequency

modulate the VCO signal.

1 KHZ oscillator:

The 1KHZ oscillator is consists of two operational amplifiers U9 and U10. The

frequency of the oscillator can be adjusted by R94 and distortion in output signal is minimized

by R92, oscillator is powered with negative supply only when front panel of audio selection

switch is in “INTERNAL” position. The output of the oscillator is level adjusted by R99 for

±50 KHZ deviation.

VCO:

The voltage controlled oscillator is a varactor tuned oscillator, the frequency of

which can be varied manually by the coil L5. Transistor Q16 forms the oscillator. The output

of the oscillator is taken through a buffer Q18, amplified by two stages Q19 and Q20. The

output of one of the amplifiers Q19 is feed to the mixer in the modulator unit and the output



from the other amplifier Q20 goes to the IF combiner unit through front panel BNC connector.

The output of the VCO is frequency modulated by the audio signal from the audio

amplifier/internal 1KHZ oscillator, the DC bias current for the diodes is provided through the

resistor R48. The output level of the VCO is about 0dbm.

Mixer:

The visual IF carrier from the vision modulator unit and the aural IF signal from

the VCO are injected at the base of the mixer transistor Q1. The mixer output is at 5.5MHZ.

This signal is amplified by a common emitter amplifier Q2, the output of which transformed

into square pulses by the pulse shaping network formed by diodes D1 and D2. This square

wave signal is further frequency divided by a chain of dividers to give an output square pulse

at 537HZ which is feed to the APC. The division is carried out to minimize the phase error

caused by the frequency modulation of audio signal in the VCO.

APC:

The automatic phase control circuit is a sample and hold circuit using a CMOS

analog gate for sampling. The square wave signal from the 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.

The sampling of reference pulses for the analog gate is derived from a

temperature controlled crystal oscillator at 1.1MHZ, the output of which is frequency divided

by a chain of dividers. The frequency if the signal from the divider chain is equal to that of the

divided signal from the mixer under the locked condition of the phase locked loop. The

analog gate acts as an error detector and the output of the gate is applied to the varactor

junction of the VCO, to correct the frequency through a low pass filter formed by R31, R33,

C18 and C30.

Metering:



The meter circuit consists of an AC amplifier and a peak detector followed by a

DC amplifier. The DC output is feed to the modulation meter in IF combiner unit via back

panel wiring.

5.1.5 IF COMBINER:

It combines vision IF with sound IF’S and composite IF.

IF is automatically level controlled with feed back from IF channel converter.

Vision IF is detected for presence of synchronous pulse and inter carrier is sensed to

declared system health.

Fig5.3 Block diagram of IF combiner

5.1.6 IF CHANNEL CONVERTER:



This Units Converts the IF input to the channel frequency and

attenuator at the input is used for setting the output level to the nominal level of 10dbm. A

wide band double balanced mixer is used for frequency conversion. A TCXO provides LO

input to the mixer and the up converted signal is filtered in a helical band pass filter. The

signal at the output of the filter is further amplified using two hybrid amplifiers. A directional

coupler is used for providing a monitor output at the rear of the module. The main output

passes through another band pass filter to reduce further unwanted signals. The final output is

available at the front. LO monitoring is provided on the front panel. This is achieved using a

resistive divider network.

5.1.7 DC TO DC CONVERTER:

DC-to-DC converters with very low input respectively output voltage ratings

have, due to the high current ratings, a relatively low efficiency. Therefore, it is necessary to

operate several converter stages in parallel to achieve an acceptable total efficiency. Here, a

possible solution for such a converter is presented. The input voltage of a (e.g. a solar

buffered) battery (12 V or 24 V) has to be converted into a DC-voltage of 350 V, e.g. for a

power transmitter. The total power to be managed is 1 kW. In the case of a single stage

inverter, this leads to about 100 A input current (causing peak values in the power switches of

up to 200 A). The resulting component stress is very hard and the design is also difficult to

handle. To overcome this problem a topology was chosen, which uses several converter stages

operating in parallel. All these stages operate at the same transformer leading to an optimal

flux exploitation of the core. In the case of the 1 kW converter described here, four stages are

used.

5.2 PRE-EMPHASIS &DE-EMPHASIS

In processing audio signals, pre-emphasis refers to a system process

designed to increase, within a band of frequencies, the magnitude of some (usually

higher) frequencies with respect to the magnitude of other (usually lower) frequencies in

order to improve the overall signal-to-noise ratio by minimizing the adverse effects of

such phenomena as attenuation distortion or saturation of recording media in

subsequent parts of the system.



De-emphasis is a process designed to decrease, within a band of frequencies, the

magnitude of some frequencies ( usually earlier pre-emphasised ) with respect to the

magnitude of other frequencies in order to improve the overall signal-to-noise ratio by

minimizing the adverse effects of such phenomena as attenuation differences or

saturation of recording media in subsequent parts of the system. It is the mirror of pre-

emphasis, and the whole system is called emphasis. The frequency curve (response) is

decided by special time constants, from which one can calculate the cutoff frequency.

It may be recalled that 7 MHz bandwidth is provided in band 3in VHF range. At these

frequencies, propagation takes place by space waves limited by maximum line of sight

distance between transmitting and receiving aerials. The signal strength at any place in

the service area must be large enough to overcome noise at that place and provide

satisfactory picture. The radiated power of transmitter is usually expressed as effective

isotropic radiated power (EIRP). In a TV transmitter, amplitude modulation of picture

carrier by video signal can be carried out at high level or a low level modulation.

In early transmitter designs, direct modulation was used. The picture was directly

modulated by video signal. This can be done at a high level modulation in final power

amplifier or at low level RF driving amplifier. At present, I.F modulation at low level is

used.




T.5.1 TECHNICAL SPECIFICATION OF THE EXCITER

1 TV STANDARD CCIR B/G PAL 625

2 FREQUENCY470 – 860 MHz (BAND-IV/V)

channel selected according to site requirement

3 MODULATION

3.1 Vision NEGATIVE AM C3F

3.2 Aural FM F3E

3.3 Colour PAL B

4 VIDEO INPUT

4.1 Level 0.5V TO 1.5V p-p / 75 OHMS.

4.2 Return Loss -30 dB OR Better (0 TO 5 MHz)

5 CARRIER FREQUENCY

5.1 Vision Carrier Channel Frequency ±150 Hz

5.2 Aural Carrier Channel Frequency ±150 Hz

6 IF FREQUENCY

6.1 Vision IF 38.9 MHz ± 40 Hz.

6.2 Aural IF 33.4 MHz ± 40 Hz.

7 OUTPUT

7.1 Level 0 TO +10dBm

7.2 Aural to Vision Ratio 1 : 10 dB.

8INTER-MODULATION

DISTORTION-60 dBc OR Better

9 SPURIOUS

-5.5 MHz -55 dBc OR Better

+11 MHz -55 dBc OR Better

10OUTPUT STABILITY

(Peak Power)0.3 dB OR Better


6. DRIVER UNIT

6.1 Driver unit:

The upconverted signal from the exciter is fed to an attenuator which is placed at the front

panel for adjusting the input level suitably the signal is amplified using class A driver stages.

The overall gain of the amplifier can be adjusted by the front panel attenuator control to be

about 33db such that 25/50W (sync peak) will be available at the output at driver unit.

The output of the amplifier is fed to a direction coupler where in samples of transmitted and

reflected power is obtained and fed to metering unit which detects the signal and suitable

voltage to a dc meter placed at the front panel.

The S portion switch on the front panel selects the parameter to be monitored

viz,vision,power,aural power & reflected power readings are to be read with black picture an


ExciterDriver unit

Divider &

Combiner unit

Antenna

PA 1

PA 2

Block diagram of Driver Unit:

Video in

Audio in

Feed back signal

RF out RF out

RF out

RF out

RF in

RF in

ALC


aural power indication is valid along with black picture only.A portion of the output power is

takeof the front panel if the driver unit for monitoring purposes.

The back panel output called ALC can be fed to exciter EXTALC is at rear panel to keep the

driver output constant.The availability of the output power “28v” to the unit is indicated

through a seen LED an the front panel.’DC Check’ facility in provided to moniter currents of

four stages of power amplifier by indicating a chord’ to ‘DC Check’ meter on

divider/combiner unit

Fig 6.1 driver unit

6.2 Divider and Combiner Unit:

This unit caters for three purposes:



1)To divide the power from driver unit which is to be fed to two final power amplifier.

2)To combine the output power from the 50w power amplifier to obtain 100w sync peak

power and make available at antenna port.

3)To provide filtering for spurious sidebands using notch filters of frequency -5.5MHZ &

fs+5.5MHZ

Monitor unit detects the samples of forward and reflected power

from the power amplifier &feedssuitable DC voltage to front panel meter for monitoring

vision,reflected and aural power.the current/voltmeter and rotator switch provided at the top

front panel are used to monitorthe current drawn by various transistor stages of dividerunit and

power amplifier unit.

6.3 Power Amplifier Unit:

This unit comprises of two similar 50w power amplifier modules.RF power is fed to driver

unit and if divider into two signals and feed it to divider/combiner and then to each 50watt

power amplifier. Each power amplifier is fed with power input which is amplified to 50

watt(sync peak) by four class A parallel power amplifier stages with a gain at approximately

10dbm.This output is fed to a direction coupler to obtain samples of forward & reflected

power for monitoring purposes for the control unit. Thermistor serves the temperature at heat

assembly & provides DC voltage control.one green led is mount over the front panel to

indicate “normal” when FWD power reflected power & heat sink assembly temperature are

within satisfactory limits.where any of three red LEDs indication denotes the parameter

exceeds beyond the limits the control with cutsoff the transistor bias from bias assembly

placed in power amplifier unit, then tripping off the power amplifier.The system can be

restored back after rectifying the fault & restoring the indications in the front panel using the

push button switch ‘RESET’. Seperate power supply available for each power

amplifier(28V,20A) placed at bottom portion of chauin assembly.A DC voltage is

proportional to current drawn by each at the transistors in power amplifier is available from

bias unit on DC check connector placed over the front panel.

6.4 VSWR Measurement :



After installing the antenna panel, junction box and U links it is verified whether the panels are

mounted as required. It is also verified if the correct lengths of branch feeder cables are used

for each face. The d.c resistance is measured by using a bridge connecting it between inner and

outer conductor of the U link or junction box. The d.c resistance is calculated as follows:

If R is the d.c resistance from U link or junction box

R = a + b + c + d + e milliohms

N

Where,

a = Antenna pannel ( about 5 mΩ )

b = Branch feeder cable ( depends on the cable size and length and is

measured separately)

c = Contact resistance of the junction box terminal, 1mΩ

d = Junction box, 1mΩ

e = U link

If the d.c resistance is very much higher than the calculated value it may

be due to bad contact of the branch feeder cable with the junction box or poor contact of other

matting surface. If the d.c resistance is higher than the nominal value, tap all the contact

portions with a rubber hammer observing the meter in measuring instrument. The meter needle

will kick back and forth while tapping the portion where the contact is poor and so the defect

will be rectified.

Measure the VSWR at the U link or junction box and by terminating the

other U link with 50Ω. Repeat the same procedure at the other U link input end. The VSWR

should be less than or equal to 1.05 at vision carrier frequency and 1.1 at other frequencies in

the channel. If VSWR is high it can be improved by slightly adjusting the pannels. Improper

fixing of branch feeder cables can also deteriorate the VSWR . A platform is provided at the U

link level ( JB level ) to keep the measuring instruments to perform tests.Thus VSWR

measurement is carried out at doordharshan.



CONCLUSION

This report is a case study on the “100 watt low power

transmitter” that how the signals are receiving from the satellite ,their processing

and transmission in the terrestrial system.

The transmitter service involves the great equipment that deals

with monitoring section exciting system and we learn about the equipment of the

doordarshan center and its working

We learned about the procedure of transmission, reception and

strengthening of the signal and retransmitting the signal into the space for the

broad cast around the range of propagation.
