Studying LOW POWER TV TRANSMITTER

7/27/2019 Studying LOW POWER TV TRANSMITTER

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ACKNOWLEDGEMENT:

I feel happy in doing my vocational training in Doordarshan Kendra, Bareilly. I am great full to allthe staff members of the Doordarshan Kendra who helped me to get the practical knowledge of

the transmission ofDDNational (DD1) and DD News (DD2).

I would like to express my sincere thanks to Mr. A.K. Pachauri, Mr. Dhruv Chatterrji, and Mr.A.K.

Awasthi, Mr. Gupta and to all the staff members of Doordarshan Kendra because, their combine

efforts help me out to learn something about the practical work done in the transmission for

broadcasting.

Here, I am submitting my report in which I have collected all the data which will represent what

I have learnt during my training session.

Any errors, suggestions for the improvement of the report, brought to my notice would be

gratefully and thankfully acknowledged.


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ABSTRACT

Studying LOW POWER TV TRANSMITTER in

DOORDARSHAN relay station. In this transmission of signals are possible

from cable TV and D.T.H service. Low power transmitter in Kakinada is of

two types. Very high frequency and ultra high frequency.

V.H.F (very high frequency) is used within channel 11 for DD1.

U.H.F (ultra high frequency) is used within channel 33 for DD NEWS.

And here we are studying about three sections respectively

Receiving section,

Transmission section and

Antenna section.


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INTRODUCTION

DOORDARSHAN:

The Director General, Doordarshan is responsible for the overall

administration of the Doordarshan network which consists of 60 Doordarshan Kendras /Production Centres, 126 Doordarshan Maintenance Centres, 194 High Power

Transmitters, 830 Low Power Transmitters and 379 very Low Power Transmitters as on31.12.2004. Doordarshan is presently operating 26 channels.

Doordarshan is divided into four wings: Programme, News, Engineering, and

Administration & Finance, Programme Wing deals with all aspects relating to

programme conception, production and procurement at the national, regional, and local

level. News Wing puts out news bulletins and other current affairs programmes at the

national and regional level. Engineering Wing deals with all the hardware requirements

of the entire network, including the space segment and the studios, transmitters etc.Administration & Finance Wing deals with the administrative and financial aspects

including general administration, personnel management, and budget and plan

coordination.

PRASAR BHARATHI:

PrasarBharati (Broadcasting Corporation of India) is the public service

broadcaster in the country, with Akashwani (All India Radio) and Doordarshan as its two

constituents. It came into existence on 23rd November 1997, with a mandate to organize

and conduct public broadcasting services to inform, educate and entertain the public and

to ensure a balanced development of broadcasting on radio and television.

OBJECTIVES:

The major objectives of the PrasarBharati Corporation as laid out in the

PrasarBharati Act, 1990 are as follows:

(i) Upholding unity and integrity of the country and the values enshrined inthe Constitution.

(ii) Safeguarding citizens rights to be informed on all matters of publicinterest and presenting a fair and balanced flow of information.

(iii) Paying special attention to the fields of education and spread of literacy,agriculture, rural development, environment, health & family welfare and

science & technology.


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(iv) Creating awareness about womens issues and taking special steps toprotect the interests of children, aged and other vulnerable sections of the

society.(v) Providing adequate coverage to the diverse cultures, sports and games and

youth affairs.

(vi)

Promoting social justice, safeguarding the rights of working classes,minorities and tribal communities.(vii) Expanding broadcasting facilities and promoting research and

development in broadcast technology.

(viii) Promoting national integration

BEGINNING:

Doordarshan had a modest beginning with the experimental telecast starting in

Delhi on 15th September 1959 with a small transmitter and a makeshift studio. Theregular daily transmission started in 1965 as a part of All India Radio. The television

service was extended to Bombay (now Mumbai) and Amritsar in 1972. Till 1975, seven

Indian cities had television service and Doordarshan remained the only television channel

in India. Television services were placed under the management of two separate Director

Generals in New Delhi. Finally, Doordarshan as a National Broadcaster came into

existence.

THE HISTORICAL DEVELOPMENT OF TELEVISION:The history of television technology can be divided along two lines: those

developments that depended upon both mechanical and electronic principles and those

which are purely electronic. From the latter descended all modern televisions, but these

would not have been possible without discoveries and insights from the mechanical

systems.

The word television is a hybrid word, created from both Greek and Latin. Tele- is Greek

for far, while Vision- is from the Latin Visio, meaning vision or sight. It is often

abbreviated as TV or the Telly.

1.2 Electromechanical television:

The origins of what would become todays television system can be traced back

to the discovery of the photoconductivity of the element selenium by Willoughby Smith

in 1873, and the invention of a scanning disk by Paul Gottlieb Nipkow in 1874.


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The German student Paul Nipkow proposed and patented the first

electromechanical television system in 1884. Nipkows spinning disk design is credited

with being the first television image rasterizier. Constantine Perskyi had coined the word

Television in a paper read to the International Electricity Congress at the International

World Fair in Paris on August 25, 1900.

However, it wasnt until 1907 that developments in amplification tube technology

made the design practical. The first demonstration of the instantaneous transmission of

still was by Georges Rignoux and A. Fournier in Paris 1909, using a rotating mirror drum

as the scanner and a matrix of 64 selenium cells as the receiver.

In 1911, Boris Rosing and his student Vladimir Kosma Zworykin created a

television system that used a mechanical mirror drum scanner to transmit, in Zworykins

words, Very crude images over wires to the electronic Braun tube (Cathode ray tube) in

the receiver. Moving images were not possible because, in the scanner, The sensitivity

was not enough and the selenium cell was very laggy.

In 1927 Baird transmitted a signal over 438 miles of telephone line between

London and Glasgoy. In 1928 Bairds company (Baird Television Development

Company / Cinema Television) broadcast the first transatlantic television signal, between

London and New York, and the fist shore to ship transmission. He also demonstrated an

electromechanical color, infrared (dubbed Noctovision), and stereoscopic television,

using additional lenses, disk and filters. In parallel he developed a video disk recording

system dubbed phonovision; a number of the Phonovision recordings, dating back to

1927, still exist. In 1929 he became involved in the first experiment electromechanical

television service in Germany. In 1931 he made the first live transmission, of the EpsomDerby. In 1932 he demonstrated ultra-short wave television. Bairds electromechanical

system reached a peak of 240 lines of resolution on BBC television broadcast in 1936,

before being discontinued in favor of 405 line all-electronic system.

1.3 Electronic television

In 1911, engineer Alan Archibald Campbell-Swinton gave a speech in London,

reported in The Times, describing in great detail how distant electric vision could be

achieved by using cathode ray tubes at both the transmitting and receiving ends. Thespeech, expanded on a letter he wrote to the journal Nature in 1908, was the first iteration

of the electronic television method that is still used today. Others had already

experimented with a cathode ray tube as a receiver, but the concept of using one as a

transmitter was novel. By the late 1920selectromechanical television was still being

introduced, inventors Philo Farnsworth and Vladimir Zworykin were already working

separately on versions of all electronic transmitting tubes.


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Networks(SN), an International channel, a Sports Channel and two channels(DD-RS &

DD-LS) for live broadcast of parliamentary proceedings.

On DD National (DD-1), Regional programs and Local programmes are carried

on time-sharing basis. DD-News channel, launched on 3rd

November 2003, which

replaced the DD-Metro Entertainment channel, provides 24-hour news service.

DOORDARSHAN CHANNELS:

DD National

DD News

DD Sports

DD Bharati

DD Gyandarshan

DD Rajya Sabha

DD Lok Sabha

DD India

DD Bengali

DD Chandana (Kannada)

DD Gujarati

DD Kashir

DD Malayalam

DD North East

DD Oriya

DD Podhigai (Tamil)

DD Punjabi

DD Sahyadri (Marathi)

DD Saptagiri (Telugu)


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STATE NETWORKS (8)

Rajasthan

Madhya Pradesh

Uttar Pradesh

Bihar

Jharkhand

Chattisgarh

Haryana

Himachal Pradesh

Information sheet on LPT (mainpuri)

GENERAL:1. Latitude (in deg/min/sec) : 162443

2. Longitude (in deg/min/sec) : 821450

3. Date of commissioning : DD-I on 07-12-1982

4. Name of the DMC : ETAH

5. District : mainpuri

6. Ownership of the building : govt. building

7. Complete postal address : K-105 near fire station

DM colony ,by pass road mainpuri(u.p)

Pin-205001

TECHNICAL:

(A)Power supply:

1. Maximum demand/capacity : 30KW


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2. Monthly average consumption : 6,000 units

3. Monthly average expenditure : Rs. 30,000/- per month

4. Comment on stability : Stable, unless a special power cut

Imposed

(B) Transmitter: DD1 DD News

1. Make and type of transmitter: GCEL-121

2. Power and channel number : 100W CH=11

3. Year of installation : 07-02-1982(GCEL 121)

4. Satellite orientated to : INSAT 3A INSAT 3C INSAT 4B

(D) Antenna and tower: DD1 DD NEWS

1. Make and type of antenna : GCEL-V type SCALA-PARASLOT

2. Year of installation : 2001 2002

3. Make, type & length of cable : 70mtrs 70mtrs

4. Type of connection (N/Flange) : Flange Flange

(E) DG Set:

1. Make, type & capacity : KIRLOSKAR, 35KVA

2. Year of installation : 18-05-2003

3. Average hourly consumption : 3.5 Liters / hr

SATELLITE COMMUNICATIONIn telecommunications, the use of artificial satellites to provide communication links

between various points on Earth. Satellite communications play a vital role in the global

telecommunications system. Approximately 2,000 artificial satellites orbiting Earth relay

analog and digital signals carrying voice, video, and data to and from one or many

locations worldwide.


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Satellite communication has two main components: the ground segment, which consists

of fixed or mobile transmission, reception, and ancillary equipment, and the space

segment, which primarily is the satellite itself. A typical satellite link involves the

transmission or uplinking of a signal from an Earth station to a satellite. The satellite then

receives and amplifies the signal and retransmits it back to Earth, where it is received and

re-amplified by Earth stations and terminals. Satellite receivers on the ground include

direct-to-home (DTH) satellite equipment, mobile reception equipment in aircraft,

satellite telephones, and hand held devices.

Figure 3.1-Satellite Communication


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3.1 Satellite communication

1. Started in 1960.2. Uses Geo Stationary Satellite.3. Operates in C-Band & Ku-Band.4. Started in India in 1975.5. First Indian Satellite INSAT launched in 1982.6. Gulf War brought satellite television to prominence.

Frequency band Up Link Down Link

C-band 6 GHz 4 GHz

X-band 8 GHz 7GHz

Ku-band 14 GHz 11 GHz

Ka-band 30 GHz 20 GHz

Satellite Transmission Frequency bands

TABLE-1


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C-band Ku-band

Frequency band 4/6 GHz 11/14 GHz

Receive dish size 2 -3 meter 0.6 meter

No. of dishes Multiple since received from

different satellites

One

Rain attenuation Low High

Individual direct

reception

Not so easy Very easy

PROPAGATION MODES

4.1 Ground-wave propagation4.2 Sky-wave propagation4.3 Line-of-sight propagation

FIGURE-4.1 GROUNGD WAVW PROPAGATION

FIGURE-4.2 SKY WAVE PROPAGATION


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FIGURE-4.3 LINE OF SIGHT PROPOGATION

THE EARTH STATION

5.1 EARTH STATION:Earth station is the main part which communicates with satellite in which up linking and

down linking the signal into/from the satellite.

Specifications of doordarshan earth station

Up link frequency - 5974.5MHz Down link frequency - 3749.5 MHz Symbol rate - 6.25MBPS Uplink polarization - horizontal Downlink polarization - vertical Satellite - insat-3A in geosynchronous orbit FEC - Compression format - 4:2:2 Coded standard - MPEG2

BASIC TELEVISION SYSTEMS


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1. Cable transmission2. Direct to home (DTH)3. Transmitter service1. CABLE TRANSMISSION :

In addition to wireless transmission by broadcast

stations, the cable TV system provides a distribution system with co-

axial cables. It is similar to wired telephone system but it is used for

TV programs. The RF carrier signal are supplied so that a tuner can be

used to select the desired channel cable TV has become popular

because more channels are provided and strong signals can be

supplied for areas in which the antenna signal is not good enough

cable television started as a means by providing signals tocommunities that could not receive broadcast stations, either because

of distance or shadow areas in which the signals was too weak.

Today cable TV has developed far beyond that into huge

systems that covers huge areas even for locations having good

reception the reason is that cable TV does not have restrictions of

channel allocation for broadcasting. It offers up to 36 channels so

many programs that not available on broadcast television reach the

cable operator via satellite transmission.

Points to be discussed in cable channel are:

Block Diagram of Cable TV Transmission

Cable Channels

The Head End

Cable Distribution2. DIRECT TO HOME (DTH):Satellite TV a direct to home (DTH) from the signals we

can receive directly from the satellite through set-top box, that means there

is no middle man (cable operator).so DTH puts an end to all the problems

like unreasonable charges, cable operators strike, power outages, not


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getting your favorite channels and channels shifting their channel number

positions.Points to be discussed in cable channel are:

What is D.T.H?

How It Works

Advantages of D.T.H

Block Diagram ofD.T.H

What is DTH?

Direct-to-Home (DTH) satellite television is becoming a buzzword

in the satellite broadcast industry due to the fact that DTH offers

immense opportunities to both broadcasters and viewers. Thanks to

the rapid development of digital technology, DTH broadcast

operators worldwide have been able to introduce a large number of

new interactive applications in the television market besides a

large number of entertainments programmed over a single delivery

platform.

In addition, since digital technology permits a highly efficient

exploitation of the frequency spectrum, the number of TV channelsthat can be broadcast using digital technology is significantly

higher than with analogue technology. The increased number of

television channels allows the operator to satisfy the demand of a

number of niche markets with dedicated transmissions.

In general, DTH service is the one in which a large number of

channels are digitally compressed, encrypted and beamed fromvery high power satellites. The programmed can be directly

received at homes. This mode of reception facilitates the use of

small receive dish antennas of 60 to 90 cm diameter installed at

convenient location in individual buildings without needing

elaborate foundation /space etc.


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Also, DTH transmission eliminates local cable operator

completely, since an individual user is directly connected to the

service providers. However, a digital receiver is needed to receive

the multiplexed signals and view them on a TV. DTH, in sharpcontrast to Cable TV, lends itself to easy monitoring and control.

Transmission in Ku band is most appropriate and widely used for

the purpose. As mentioned above, all the encoded transmission

signals are digital - thus providing higher resolution picture quality

and better audio than traditional analog signals. All the advantages

of the digital transmission, as applicable to the terrestrialtransmission are relevant in the satellite transmission also.

HOW IT WORKS:

In DTH you receive the signals from satellite to a small dish

antenna installed at the roof top of your house. This signal is decoded by a

set-top box, which is provided by the broadcaster and connects to the dishantenna directly with a cable. The set-top box in turn connects to your TV.

So you become the master of your entertainment and watch the channel you

wish and pay for only those channels which you wish to watch.

ADVANTAGES OF DTH TV:

Digital picture:

The picture quality in DTH is much better. The quality of

picture is uniform across all channels.

Digital audio:

You get the stereo phonic sound. So if you have got have got a

home theatre, connect it to your set-top box you will get better soundeffects.

Electronic program Guide (EPG):

Its an on-screen guide that shows the program schedule orlisting of all channels. So you can find out whats playing on any channel.

You can also set remainders for programs you wish to watch and getsynopses of the program.


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Payment modes:

You pay for only for what you watch. Payment is in advance.Recharge is easy with various sources like the internet, mobile phones,

vouchers from vendors etc.

BLOCK DIAGRAM OF DTH:

TRANSMITTERS:

Main and relay (repeater) transmitters

Transmitting stations are usually either classified as main stations or relay stations

(also known as repeaters or translators).

Main stations are defined as those that generate their own modulated output signalfrom a baseband (unmodulated) input. Usually main stations operate at high power and

large areas.

Relay stations take an already modulated input signal usually by direct reception

of a parent station (off-air) and simply shift (translate) its frequency before

rebroadcasting. Usually relay stations operate at medium or low power, and are used tofill in pockets of poor reception within, or at the fringe of, the service area of a parent

main station.

Note that a main station may also take its input signal directly off-air from another

station, however this signal would be fully demodulated to baseband first, processed, and

then remodulated for transmission.

in TV broadcast both the sound signal and the video signal are to be conveyed to the

viewer using radio frequency these two signals have very distinct features.the audio

signals is a symmetrical signal without continous current but the frequency does not

exceed 20Khz. The videosignal consist of a logical component, the sync and the field

sync and an analogue part according to the line picture scanning.the unsymmetrical signal

thus has a continous component. The frequency bandwidth also extend from 0 to

5Mhz.the two signal modulates the carrier waves frequencies and type of modulation are

as per established standards.these modulated carriers are further amplified and then

diplexed for transmission on the same line and antenna.this technique is used in high

power TV transmission on the same line and antenna.howevwr for LPTs i.e transmitter

operating at snyc peak power less than 1KW, both the signals are modulated separately

TV RECEIVVIDEORECEIVERLNBCFEEDDISH


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but amplified jointly using common vision and aural mplifiers.both of these systems have

merits and demerits.

In the first case special group delay equailisation circuit is needed because of errors

caused by diplexer while in the second case inter modulation products are more

prominent and special filters for suppressing them are required.hence ,techniques of jointamplification is suitable only for LPTs and not for HPTs. through frequency modulation

has certain advantage over amplitude modulation.its use for picture transmission is not

permitted due to large bandwidth requirement, which is not possible due to very limited

channel space available in VHF\UHF bands.secondly as the power of the carrier and side

band component go on varying with modulation in the case of FM,the signal with

frequency modulation after reflection from nearly structures at the recievings end will

variable multiple ghosts,which will we very disturbing.thus AM is invariably used for

picture transmission which frequency modulation is generally usedmin sound

transmission.

There are two types of modulation-

1) Positive modulation.2) Negative modulation.

In positine modulation the increase in picture brightness cuases increase in amplitude of

modulation envelope.while in negative modulation,the increase in picture brightness

causes reduction in carrier amplitude i.e the carrier amplitude will be maximum

corresponding to sync tip and minimum corresponding to peak white.negative

modulation are generally used.

Advantage of negative modulation:

1) Impulse noise peaks appearonly in black region in negative modulation. Thisblack noise is less objectionable compared to noise in white picture region.

2) Best linearity can be maintained for picture region and non-linearity aeffects canbe corrected easily.

3) The efficiency of transmitter is better as the peak power is radiated during syncduration only.

4) The peak level representing the blanking or sync level may be maintainedconstant,thereby providing a reference for AGC in the receiver.

5) In negative modulation ,the peak power is radiated during the sync-tip,as such incase f fringe area reception,picture locking is insured ,and deviation of inter

carrier is also ensured.

Vestigial sideband transmission:


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Another feature of present day TV transmitters is vestigial sideband transmission.if

normal amplitude modulation technology is used for picture transmission,the minimum

channel bandwidth should be around 11Mhz taking in the account the space for sound

carrier and a small guard band of around 0.25Mhz. using such large transmission BW

will limit the number in the spectrum allotted for TV transmission. Toaccomodate large

number of channel in the allotted spectrim reduction in transmission bandwidth was

considered necessary. The transmission BW could be reduced to around 5.75Khz by

using single side band AM technique,because in principle oneside band of the double

sideband (dsb)AM could be suppressed. Since the two sidebands have the same signal

content.

It was not considered fesible to suppress one complete side band due to difficulties in

ideal filter design in the case of TV signals as most of energy is contained in lower

frequencies contain the most important information of the picture. If these frequencies are

removed,it causes objectionable phase distortion at these frequencies which will effect

picture quality.thus as a compromise only a part of lower sideband is suppressed while

taking full advantage of the fact that-

a) Visual disturbance due to phase errors are severe and acceptable where larepictures areas are concerned.

b) Phase errors become difficult to see on small details in the picture.thus lowmodulating frequencies must minimize phase distortions as they are very difficult

to see.

c) The lower side bands contains frequencies upto 0.75Mhz with a slope of 0.5Mhzso that final cutoff is at1.25Mhz.

Reception of vestigial side band signals:

When the radiated signal is demodulated with an idealized detector the response is

not flat.the resulting signal amplitude during the double sideband portion of VSB is

exactly twice the amplitude during the SSB transmission.in order to equalize the

amplitude the receiver response is desined to have an attenuation characterstics over the

double sideband region appropriate to compensate for the two to one relationship.this

attenuation characterstics,called nyquist slope, is assumed to be in the form of a linear

slope over the 750Khz with the visual carrier located at the midpoint relative to SSB

portion of the band such a characterstics exactly compensates the amplitude response

non-symmetry due to VSB.TV receivers have nyquist characterstics for reception which

introducrs group delay errors in the low frequency region.notch filters are used in

receivers as aural traps in the vision IF and video amplifier stages.the group delay errors

are pre-corrected in the TV transmitters ,so that economical receiver filter design is

possible.


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Depth of modulation:

Care must be taken to avoid over modulation at peak-luminance signal values to avoid

picture distorsions and interruptions in vision carrier.the peak white levels when over-

modulated tends to reduce the vision carrier power and even cause interruptions of vision

carrier.the periodic interruptions due to accidental over modulation results in

interruptions of the sound carrier in in inter carrier receiver system which produces

undesired sound buzz in the receiver output.therefore to prevent this effect,the maximum

depth of modulation of the vision carrier by peak white ignal values is specified as being

87.5%.the 12.5% residual carrier is required because of the inter carrier sound methord

used in TV receiver.the depth of modulation is set by using a ramp signal or step signal as

given in the manual .it should be 87.5% for 100% modulation.

Inter carrier:

The TV receiver incorporate inter carrier principle.accorsing to or system,the intercarrier i.e the difference between the vision transmitter frequency is 5.5Mhz. hence, it is

to be ensured that even when the modulating video signal is at white peak,12.5% of

residual carrier is left so that sound can be extracted even at the peak white level,where

the carrier power is minimum.

Power output:

The peak power radiated during the sync tip or sometime the carrier powe

corresponding to black level in desiginated as the vision transmitter power.the power is

measured by using a thruline power meter after isolating the aural carrier.the power read

on thruline meter is multiplied by a factor 1.68 to get the peak power radiated.as

transmitter output is connected to an antenna,having a finite gain,the effective radiated


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power is obtained by multiplying the peak power by the antenna gain.hence a 100W LPT

using transmitting antenna having a gain of 3db with respect to a half wave dipole will

have an ERP of 200W or 53db.

In TV broadcasting, the sound signal is transmitted by frequency modulating the RF

sound carrier in accordance with the standards.the sound carrier is 5.5Mhz above theassociated vision carrier. the maximum frequency deviation is +/-50Khz which is defined

as 100 percent modulation in PAL-Bsystem. In the case of NTSC, the maximum

deviation permissible is +/-25Khz .

1.TRANSMITTER:

High power TV Transmitters:

All the TV transmitter have the same basic design.they consist of an exicter

followed by power amplifier which boost the exicter power to the required level.

Exicter:

The exicter stage determines the quality of transmitter.it contains pre-corrector

units both at base band as well as at IF stage, so that after passing though all subsequent

transmitter stages, an acceptance signal is available.since,the number and type of

amplifier stages,may differ according to the requirement output power ,the characterstics

of pre-corrector circuits can be varied over a wide range.

Vision and sound signal amplification:

In HPTs the vision and sound carrier can be generated,modulated and amplified

separately and then combined in the diplexerat the transmitter output.in LPTs

sound,vision are modulated seperatly but amplified combinedly.this is known as common

vision and aural amplification.a special group delay equalization circuit is needed in the


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first case because of error caused by TV diplexer.in the second case the intermodulation

products are more prominent and special filters for suppressing them is required.as it is

difficult to meet the intermodulation requirements particular at high power rating,saperate

amplification is used in HPTs through combined amplifications require fewer amplifier

stages.

If modulation:

It is used in correcting distortion:

Ease of correcting distortion Ease in vestigial sideband shaping. IF modulation is available easily and economically.

Video chain:The input video signal is fed to a video processor.in VHF transmitter

LPF,delay equalizer and receiver pre-correcor proceed the video process.

Low pass filter:it is used to limit incoming video signal to 5Mhz.

Delay equalizer:group delay introduced by LPF is corrected.it also pre-distorts the video

for compensate group delay errors introduced in the subsequent stages and diplexer.

VHL low power transmitter(VHF range-54 to 216 Mhz):


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new generation transmitter have 1+1 redundant exciter unit.the transmitter design

is based on solid state techniques and employs modular construction. the video and audio

signals are processed in the exciter electronics and modulated at low level, at if frequency

of 38.9mhz respectively. Now ,this if signal are combined and passed though if corrector

and vsb filter saw filter is used for vestigial sideband shaping .this signal is upconverted

to desired channel frequency and amplified in linear power amplifier to obtain 100w

visual power and 10w aural power .rf is finally routed at antenna through channel filter

and directional coupler .

1v peak to peak input video signal is limited to 5mhz in LPF and is compensated

for group delay in delay equalizer and receiver pre-corrector unit.the resulted is subjected

to dc restoration by clamping at back porch amplified and inverted in video processor.the

output of video processor is fed to visual modulater where the same is amplitude

modulated with negative polarity at 38.9mhz IF and amplified.in new generation

exicter,the lpf deq and video processosor is intregated resuting into a single and compact

video processing unit.the modulator used is a ring modulater instead of double diode

balance modulator .the video signal polaity is reversd and modulates the if frequency .the

audio signal is frequency modulated at if of 33.4 Mhz in a varactor VCO modulator.the

output signal is passed through balanced to unbalanced transformer and pre-emphasised

in a 50micro second network.the information contained in the amplitude variation of

audio is converted into frequency variation in the VCO.the centre frequency of VCO is

maintained at 33.4Mhz by the means of PLL.

a).Functioning of PLL :


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output fom VCO 33.4Mhz is mixed with vision IF of 38.9Mhz using a

transistorized mixer.the resultin difference signalof 5.5 Mhz is shaped for square pulse

and using suitable dividers the frequency is reduced to a low value.genrerally,dividers are

5,16,128.thus the 5.5Mhz is reduced to 537Hz.a crystal oscillator of suitable frequency

1.1Mhz is used as standard oscillato is also divided by 16 and 128 to make it similar to

above value i.e.537Hz.the above two signal are fed to comparator ckt. So the error signal

is generated when there is a drift of centre frequency. The error signal is used to kept

VCO frequency under control.

b).IF cominer ,IF corrector and up-converter :

the modulated aural IF and vision IF are combined in a wide band amplifier and

passed through if corrector for pre-correcting DP/DG and response error.the correcting

signal is fed to VSBF and mixer unit for up-conversion by mixing the same with local

oscillator signal.the VSBF and mixer network contains a SAW filter for VSB shaping and

ALC amplifier for automatic level control.

c).local oscillator:

local oscillator frequency required for up-conversion is given by Fo=Fc+Fvif.the

LO signal corresponding to a particular channel in old exicters is obtained by generating

a Fo/4 signal in Tcxo, multiplying the same by 4 in a harmonic multiplier and then

mixing the resulting signal siin a mixer with avision IF sample.the mixer output is passed

through a high pass filter and amplified.

d).10 watt driver amplifier:

the RF output from exicter is raised to 10W in driver amplifier.the power

amplification is achieved in 3 class A stages. Transistor biasing is used for operating the

RF device on constant current and constant voltage.the drive is capable of giving 25w

power output.step attenuator is used at the input to set the output level. Circulator is used


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at the output for protection against mismatch.the driver is protected against over

temperature by mounting a thermal switch on heat sink of RF transistor.switch cuts off

the bais supply in case the heat sink and hence the transistor temperature is beyond state

limit p/s is also cut off in case the heat sink temperature of BEL 100w PA is beyond sae

limit.

e).100 watt power amplifier:

splitting and combining technique using 3db,90degree hybrid coupler/coxial

transformer is employed on power amplifier as the amplifying capacity of single RF

device is limited at VF frequencies.these signals are amplified into two separate 50W

amplifier modules and then combined by a3db hybrid coupler to obtain 100W power.

In BEL power amplifier two dual pushpull balanced RF devices each capable ofgiving 50W RF power are mounted on a common PCB.the combined output is routed

through a circulator and direction coupler.circulator provides protection against

accidental short protected against over temperature by thermal switchmounted on heat

sink.

In GCEL power amplifier uses two separate 50W modules,each employing four

RF transistor.each 50W unit is protected for over driver/mismatch and over

temperature.this is accomplished by sensing forward reflected power and heat sink

temperature and deriving corres ponding voltages.if any of the parameter is out of

operating limit,the corresponding LED glows and a relay switch off the bias supply.

LNBC(low noise block convertor):

The synthesized TVRO system signal in the range of 2.5 to 2.7 Ghz and convert

them into 1150 to 950Mhz in the S-band low noise block convertor,mounted inside the

LNBC unit .the down converted signal is then fed to the satellite system unit ,where it get

the signal and further down convert and demodulates the signal to provide two video


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separate balanced audio output the low noise block converter receiver the RF signals of

the range of 2.5 to2.7Ghz from the feed and down converts the signal into RF range of

1150Mhz to 950 MHZ which is then fed to satellite system unit through a 75 m length RF

cable .the same RF cable provides the required DC voltage to (LNBC) from satellite

system unit .

AUDIO OSCILLATOR:

RADART type (opt.01) low distortion audio oscillator is a solid state portable

audio source with 0.5W output power at a very low distortion of 0.1% this is highly

stable and very accurate with digit display. It has multi impedance balanced output with

excellent frequency response.the 600 ohm unbalanced position ohas an attenuation to

obtain signals down to 62 ohms.

APPLICATIONS:

This is extreamly usefull in high quality acoustics and Hifi technology and

broadcasting .they are use to drive impedance bridge, acoustic transducer etc.they are

widely used in developed production, research and training of AF ,medical process

control processes.

AUDIO-VIDEO SWITCHER:

A audio video switcher is used inany video system to receive the video signal and

audio signals from different sources,selects any of them for transmission, further

processing with minimum distortion in the characterstics of the signal.for video

UHF low power transmitter(UHF band-479 to 890Mhz):

TV transmitter supplied by BEL differ from those working at VHF frequencies.in

a number of aspects like generation of video and sound.IF frequencies generation of local

oscillator frequencies,circuitary aspects for protection,control and monitoring and finl

power amplification.PARA SLOT ANTENNAS are used.


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Modulator module:

It is aIF modulator which receives the video and audio signal and generates a

modulated IF signal containing an amplitude modulated video carrier of 38.9Mhz and

frequency modulated audio carrier of 33.4Mhz.inputs are processed and carrier are

generated. AGC and muting control is introduced.modulation depth is determined offset

is programmed.vision carrier is produced by aVcxo which is phase locked to the 500Khz

reference signal from LO of up-converter module.

Up-converter module:

The up-converter module comprises of local oscillator module and IF\RF

converters to generate RF at channel frequency.in this unit IF gain and slope is set.group

delay equalization and amplitude precorrection of done,IF output level is set. Local

oscillator is mixed with IF to produce RF.this unit also provides 500Khz reference signal

to modulator unit.

Half watt amplifier:

RF is amplified to 500mv at three stages. It contains logic circuitary to provide

AGC/limiter output to modulator and drive to meter and LED.

Power supply and metering module:

This has 28v,4Amps switched mode power supply unit with a regulator to provide

DC power requirement of all the above modules and metering facilities for the above

unit.

Power amplifier:


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Power amplifier are used to amplify the power, so that the system is able to drive

the load connected to the system.following power amplifier are used :

B amplifier(linearity precorrector):

This unit generates over all distortion products of low power level of correct

phase and amplitude to cancel out those produced by the subsequent power amplifier

stages control PCB protects RF transistors against over voltage and over temperature.

Q amplifier:

This unit serve as a driver amplifier to S amplifier. This has a typical gain of

8db and can provide 15w into a 50 ohms load. RF transistors are protected against over

voltages and over temperature b control PCB.

S amplifier:

This has a typical gain of 16-18db. This unit can be used alone ar in parallel with

similar units of form a higher powered amplifier.the RF circuit of this amplifier uses two

dual transistor in first stage, driving a further four in the second stage eachRF transistor

are protected against over voltage and over temperature by control PCB.

Control and logic unit:

This unit provides automatic start-up an shut-down with manual control for test

purpose. The operational state is monitored by 8 LEDs mounted on front panel.it alsocontrols a output meter to indicate output power of the transmitter two sockets for

monitoring forward and reverse RF power are also provided.a resetswitch demotes the

amplifier by reseting the over tempraturelatch in the respective amplifier control circuit in

case any one of the same has operated and switched off the amplifier.if the power

deviates from the normal rated power(+/-3db aprox.) power alarm LEDglows after 1 to 2

minute delay and a sound alarm is alarm is also switched on.


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RECEVING SECTIONReceiving section of doordarshan relay center consists of:

1. Programme Amplifier2. 8W Monitoring Amplifier

3. Insat 3A

4. JVR, JVC5. DTH

6. 6X2 Audio video switcher7. IF Oscillator

8. Driver Combiner Unit

9. Power Amplifier 100W

10. Driver Unit

11. Exciter Unit12. DC Power Supply

1) PROGRAMME AMPLIFIER:

Programme Amplifier is 19 rack mounting equipment with one input andtwo independent outputs with presentable gain controls. A level indicator

(VU Meter) to show the output level is also provided. It consists of fourstages:

Input stage

Line output stageVU Meter stage

Power supply

The equipment is self contained and works on 230V, 50Hz mains supply.

Block Diagram of Programme Amplifier:

Circuit Description:At the input the 600 balanced signal input is converted into

unbalanced signal by IC U1. This unbalanced signal is distributed to two

amplifiers U2A and U3A. P1 and P2 acts as gain control. This amplifier


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unbalanced signal is again converted into balanced signal output. This

conversion is done by IC U2B, U4A, U5A, and U3B.

IC U4B, U5B are used to drive the voltmeter.

Zero in vu meter corresponds to +9dbm which is the normal input level tothe transmitter for 30 kHz deviation at 400Hz modulation frequency.

PROGRAM AMPLIFIER:

Output impedance : 600Nominal input level : +9dbm

Frequency response : |1db| from 30Hz to 15 KHz

Distortion :


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output of this LP filter and feeds the controlled audio signal to the power

amplifier stage.

ii) POWER AMPLIFIER STAGE:

This part of circuit uses all discrete components and semiconductors. Thepower amplifier employees a quassi complementary output stage driven by

class A driver stage and differential input stage. Q18, Q19 form inputdifferential amplifier. Q17 is class A driver power transistors Q1 and Q6.

R29 provides a feedback from output to the base of Q18.The ratio of R 29 toR 44 decides the gain of the power amplifier. Q7 is used to set the bias

current in output transistors to drive them into class AB operation. It can be

set by preset RT4. A short circuit protection is provided by transistors Q8and Q9 and associated components.

Diodes D7 and D11 are connected across collector-emitter of Q1 and Q6

respectively for surge current protection R1, C4 provide high frequencystability.

iii. POWER SUPPLY SECTION:A centre tapped secondary of mains transformer TX1 and a bridge

rectifier comprising of D1 to D4 along with capacitors C2 and C3 from a

dual raw DC supply. A auxiliary 15V regulator along with its bridge rectifierSmoothing capacitor is located on this PCB No: 960202 the secondary 2

from the mains transformer is connected to the bridge D5-D8. The raw DC

is then coupled to 3pin regulator IC Z3 which gives + 15V output on TB 1through fuse F2. It is rated for 1A max.

3)6x2 AUDIO- VIDEO SOLID STATE SWITCHER AVS 6X2 SL

GENERAL DESCRIPTION:6x2 audio video switcher unit is multipurpose solid state switcher

designed and developed by ARRVI enterprises.

This switcher has 6 independent video inputs and 6 audio inputs. Theyare located on rear panel of the unit. Two 75 video outputs and two 600

outputs are available. This unit can be fitted into 19 standard rack.SPECIFICATIONS:

MECHANICLE:

1. Horizontal mounting module2. Height : 88.1MM


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3 .width : 482.6MM

4. Depth : 425.0MM5. Weight : 5.00Kgs

ELECTRICAL:Power supply : 230V10% 50Hz

Input (video) : input (audio)Impedance: 75 :600

Video return loss: : better than 36dbDifferential gain : better than 0.5%

Differential phase : 0.34 degrees

Audio cross talk : better than 70db

OPERATION:

Power on the unit after ensuring the physical and electrical readinessof the unit. Connect the various input sources to the VIDEO INPUT with75 terminations and the AUDIO INPUT connectors located on the rear

panel of the unit. Output connectors are to be accordingly connected. When

any 1 channel selector push button switch is pressed, the correspondingvideo and audio output are available on the output sockets. The selected

input channel is well defined by particular LED of the push button switch.

When the function of the switch is interrupted due to the power failure ALLCLEAR CONDITION appears on the switcher. On restoration of power the

interrupted channel is to be reselected. A RESET MODE is also provided to

facilitate ALL CLEAR CONDITION of the switcher.

CIRCUIT DESCRIPTION:The main unit consists of Logic Circuits PCB. Video switching and

amplifier PCB (Audio PCB) and Power supply PCB.

LOGIC CIRCUIT:

Logic circuit enables selection of required channel using Flip-Flop

and TTL Gates from U2 to U8. The selected channel from Switches S1 to S6are inverted and fed to the gates U3 and U4 which in turn enables flip-flop

U7 and U8. The selected flip-flop also lights up selected indicator through

drivers U1.

AUDIO SWITCHING AND AMPLIFIER CIRCUITS:


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Audio switching is done by solid state switches U7 to U9. The

balanced input is converted as unbalanced by U1 to U6 and this unbalancedsignals are fed to solid state switches U7 to U9. The selected unbalanced

signal is split into two outputs by U10. These two unbalanced signals are

converted into two balanced signals and amplified by U11 to U12 in AudioPCB.

VIDEO SWITCHING AND AMPLIFIER CIRCUITS:

Video signals are pre amplified by video amplifier circuit consistingof transistors Q1 to Q6. These signals are fed to solid state switches U1 to

U3. The selected channel gain can be adjusted through presetPR1 and

voltage offset can be adjusted through PR2.The switching process takes place as follows:

On selection of a particular channel, when the button1 is pressed the selected

signal command goes to U3 through the inverter U2. Then U 3 will enablethe flip-flop U7 in logic PCB. The enabled flip-flop will in turn select therespective audio and video inputs through audio and video PCB respectively.

The selected audio input is amplified and then balanced by ICs U10 to U12

in audio PCB. The selected video input is amplified through transistors Q7to Q11. The selected channel is indicated by LED provided on the switch.

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

a part 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.

RECEIVING SECTION BLOCK DIAGRAM

Parabolic

dish antenna

Low noise

amplifier

Low noise block

converter

Digital video

broadcast receive


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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.

1.1.1 COMPONENTS:1. PDA(parabolic dish antenna).2. IRD(Integrator receiver decoder).3. LNA(Low noise amplifier)/ LNBC(low noise block converter).4. Multiplexer.5. Encoder.


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FIGURE 5.1-EARTH STATION

5.2 Antennas:

Antenna(oraerial) is atransducerthattransmitsorreceiveselectromagnetic waves. In other

words, antennas convert electromagnetic radiation into electrical current, or vice versa.

Antennas generally deal in the transmission and reception ofradio waves.

FIGURE-5.2 ANTENNA

Types of antenna:

Isotropic antenna (idealized) Radiates power equally in all directions

Dipole antennas Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi antenna)

Parabolic Reflective Antenna
http://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Transducerhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Radio_wavehttp://en.wikipedia.org/wiki/Electromagnetic_radiationhttp://en.wikipedia.org/wiki/Receiver_(radio)http://en.wikipedia.org/wiki/Transmitterhttp://en.wikipedia.org/wiki/Transducer


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5.3 AParabolic AntennaPDA is a high-gain reflector antenna used for radio, television and data

communications, and also for radiolocation (radar), on theUHFandSHFparts of the

electromagnetic spectrum. The relatively short wavelength of electromagnetic radiation at

these frequencies allows reasonably sized reflectors to exhibit the desired highly

directional response for both receiving and transmitting.

A typical parabolic antenna consists of a parabolic reflectorwith a small feed

antenna at its focus. To find the focus, reflect the light of a flashlight off of the dish.

When the reflected beam is parallel, the flashlight is at the focus.

The reflector is a metallic surface formed into a paraboloid of revolution and

(usually) truncated in a circular rim that forms the diameter of the antenna. This

paraboloid possesses a distinct focal pointby virtue of having the reflective property of

parabolas in that a point light source at this focus produces a parallel light beam aligned

with the axis of revolution.

The feed antenna at the reflectors focus is typically a low-gain type such as a half-wave dipole or

a small waveguide horn.
http://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Radarhttp://en.wikipedia.org/wiki/Ultra_high_frequencyhttp://en.wikipedia.org/wiki/Ultra_high_frequencyhttp://en.wikipedia.org/wiki/Ultra_high_frequencyhttp://en.wikipedia.org/wiki/Super_high_frequencyhttp://en.wikipedia.org/wiki/Super_high_frequencyhttp://en.wikipedia.org/wiki/Super_high_frequencyhttp://en.wikipedia.org/wiki/Parabolic_reflectorhttp://en.wikipedia.org/wiki/Feed_hornhttp://en.wikipedia.org/wiki/Feed_hornhttp://en.wikipedia.org/wiki/Paraboloidhttp://en.wikipedia.org/wiki/Focal_pointhttp://en.wikipedia.org/wiki/Parabola#Reflective_Property_of_Parabolashttp://en.wikipedia.org/wiki/Parabola#Reflective_Property_of_Parabolashttp://en.wikipedia.org/wiki/Low-gain_antennahttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Waveguidehttp://en.wikipedia.org/wiki/Waveguidehttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Low-gain_antennahttp://en.wikipedia.org/wiki/Parabola#Reflective_Property_of_Parabolashttp://en.wikipedia.org/wiki/Parabola#Reflective_Property_of_Parabolashttp://en.wikipedia.org/wiki/Focal_pointhttp://en.wikipedia.org/wiki/Paraboloidhttp://en.wikipedia.org/wiki/Feed_hornhttp://en.wikipedia.org/wiki/Feed_hornhttp://en.wikipedia.org/wiki/Parabolic_reflectorhttp://en.wikipedia.org/wiki/Super_high_frequencyhttp://en.wikipedia.org/wiki/Ultra_high_frequencyhttp://en.wikipedia.org/wiki/Radar


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FIGURE 5.3 PDA

5.4 LNBC(Low Noise Block Convertor):

A signal form satellite is a very low power signal. The satellite reception dish does

a first amplification by reflecting and concentrating the signal received into one focus

point. The LNB, mounted exactly at this point in front of the parabolic dish, further


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amplifies this signal because this is still weak. The amplified satellite signal cant be send

directly through a coax cable. Due to still very high frequency(10 to 13 Ghz)sending this

signal directly in to coax cable would result in very high signal loss.

This is why the LNB also converts the signal into a lower frequency. The LNB frequency

determines over how many Mhz the signal is converted downwards.

FIGURE 5.4- LNBC


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5.5 IRD (INTEGRATED RECEIVER DECODER):

An IRD is generally used for the reception of contribution feeds that are intended

for re-broadcasting. The IRD is the interface between a receiving satellite dish networks

and a broadcasting facility video/audio infrastructure.

5.5.1 FEATURES OF IRD:

1. Satellite distribution.2. Radio reception.3. Audio & video decoding.4. Frame synchronizationof digital video output to analogue input.5. Videotest patterngenerator.

FIGURE 5.5 IRD
http://en.wikipedia.org/wiki/Frame_synchronizationhttp://en.wikipedia.org/wiki/Frame_synchronizationhttp://en.wikipedia.org/wiki/Test_cardhttp://en.wikipedia.org/wiki/Test_cardhttp://en.wikipedia.org/wiki/Test_cardhttp://en.wikipedia.org/wiki/Test_cardhttp://en.wikipedia.org/wiki/Frame_synchronization


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6.14 DETAILED CIRCUIT THEORY

The composition of transmitter is as follows:

1.

Base Band Corrector2. Exciter consists of

a. IF unitb. Upconverter Unit

3. Linearity corrector unit4. 6-way splitter5. 150 W power amplifier6. Control unit7. 2-way combiner8. Output filter9. Dual direction coupler10.26V/33V SMPS Power supply units11.28V linear Power supply units12.Fan unit

Now we will analyze each part of the Transmitter in detail:

6.14.1 Base Band Corrector Unit

The unit accepts the video signal and introduces the required pre correction in the

differential phase, differential gain and luminance non linearity encountered in the power

amplifiers without introducing any frequency response group delay. Base Band Corrector

Unit consists of 2 PCBs. The video input is fed to corrector unit directly and enters to a

boot strap circuit to meet the input return loss specification. The video input is also fed to

a clamping stage which comprises of a sync separator and clamping transistor

6.14.2 (a) TV EXCITER (IF UNIT)

The IF unit comprises of the following PCBs:

1. video processor2. VSB modulator3. IF synthesizer4. ICPM corrector


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5. Rectifier and regulator board6.14.2.1 VIDEOPROCESSOR:

The video processor consists of four main sectionsinput buffer, peak white

limiter , transmitter delay equalizer and receiver pre- corrector. The video signal to the

video processor board first enters the input buffer for good input return loss and then

passes through a variable video amplifier. The video signal then enters the peak white

limiter section where it is split into 2 paths viz, the main signal path and the limiting

signal generator path. The limiting signal path takes the video through a gain stage where

in video is amplified. In case of absence of input e video, the sync separator will not

function.

6.14.2.2 VSB MODULATOR

The VSB modulator generates the amplitude modulated vision signal at the

IF of 38.9 MHz the IF synthesizer output is also fed to this video processor PCB as the

IF carrier. The video signal is first fed to the boot strap buffer circuit. The input video

signal is also fed to 1 MHz active low pass filter stage. The filtered video signal is fed to

a sync separator stage. The separated sync pulses are passed through a shaping circuit.

The IF signal at 38.9 MHz is fed from the the IF synthesizer PCB to the first section.

This stage acts as an amplitude limiter and maintains a constant output even with

variations in IF synthesizer output level. Following is a buffer amplifier. The balanced

modulator has the clamped video signal and the buffered IF carrier signal at its inputs.

The video signal is applied in parallel to the modulator while the IF carrier is applied in

push-pull

6.14.2.3 IF SYNTHESIZER

The IF Synthesizer is a multifunction PCB. It consists of the following

two sections:

a) 38.9 MHz IF synthesizer

b) 5.5 MHz sound inter-carrier Synthesizer and FM Modulator

6.14.2.3.1 MHz IF SYNTHESIZER


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It is locked to a 500 kHz signal derived from the upconverter unit. The IF

output can be changed by +/- 2/3 line frequency offset to provide channel frequency.

The PLL circuit consists of a voltage controlled oscillator, 38.9 MHz tuned amplifier,

frequency pre-scalar , PLL IC and low pass filter.

6.14.2.3.2 MHz SOUND INTERCARRIER FM MODULATOR

5.5 MHz FM modulator is based around a 5.5 MHz PLL. The 38.9 MHz

signal from IF synthesizer is also fed to the mixer.

6.14.2.4 ICPM CORRECTOR BOARD

This PCB pre-corrects the phase of the vision IF signal depending on the

video signal. It consists of following blocks:

a) Input video amplifierb) Sync separatorc) Slicer circuit-Sync leveld) Slicer circuit-white levele) Slicer circuit-black levelf) Video adder and phased modulator

The video signal before reaching The input video signal is amplified by the video

amplifier. The output of the video amplifier is split into 2 paths, one to the slicing circuit

white and the other to the slicing circuit black and slicing circuit sync. Slicing circuits

consist of an active reference voltage generator, a diode for clipping and a differential

amplifier for amplifying the sliced signal. The reference voltage generator provides for

variable reference voltage generation. It is an NPN transistor with variables bias, for

getting the variable reference voltage. The reference voltage determines the level at

which video slicing takes place and hence the type of correction. The gain of the

differential amplifier determines the level of video fed to the phase modulator and hence

the amount of correction. The sync and black slice circuit is inverted and then buffered.

The 3 sliced signals are added in the video adder and fed to the phase modulator. The

phase modulator consists of a 90 degree splitter. The video IF signal from IF signal from


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IF synthesizer is phase modulated here and then amplified to the required level of +13

dBi.

6.14.2 (b) UPCONVERTER UNIT

The up converter unit combines modulated vision IF(V.IF) and aural

IF(A.IF) signals and translates to respective channel frequencies suitable for

transmission. The unit has inbuilt power supply. The status and fault information are

displayed on front panel of the unit. The up converter consists of following:

(a) LO synthesizer PCB(b) Mixer PCB(c) Band Pass filter(d) Amplifier and coupler PCB(e) AGC PCB(f) Regulator PCB(g) Display PCB-1(h) Display PCB-2(i) +5 V regulator PCB(j) 28 V SMPS supply

The VIF & AIF signals are fed to up converter unit. The VIF & AIF are fed to a mixer

PCB. The VIF & AIF signals at output PIN switches are combined & are mixed with LO

signal fed from LO PCB. The LO synthesizer, VCO, divider loop filter & OCXO of 16

MHz . With the above circuitry PCB generates a stable LO signal required for upconversion. The frequency of this signal can be programmed by setting switches provided

on PCB. The generated signal is fed to mixer PCB. LO sample is brought to front panel

of unit for monitoring purpose. The mixer circuit is built with dual mixer IC to form a

balanced mixer. Its output is fed to channel filter. The mixer PCB sends out DC voltages

corresponding to VIF & AIF signals to front panel metering circuit. The mixer output


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which is fed to channel filter is rejected for its image frequency. The output filter is fed

to amplifier & coupler PCB. The sample of final RF signal is brought to front panel up

converter unit for monitoring purpose. The amplifier & coupler PCB also houses a strip

line coupler which couples a sample of RF & this sample is detected by the diode. The

detected DC is fed to AGC PCB. The AGC PCB compares this voltage with the

reference. AGC voltage is set by AGC potentiometer. The difference voltage is fed to

window comparator circuit whose upper & lower limits are set by potentiometer. If the

difference voltage is in the set limit, the comparator output is AGC control voltage. The

control voltage is applied to AGC/MGC switch. On front panel PCB, when this switch is

put in AGC position, the AGC control voltage is applied to PIN attenuator circuit of amplifier &

coupler. PCB which in turn controls the RF output of the upconverter in AGC mode. The AGC

PCB gives out AGC unlock signal when AGC voltage goes out of range. The AGC PCB gives

out metering signal corresponding to the RF output of the upconverter.

For PCBs Upconverter Unit:

The audio IF input is passed through AURAL level control PCB for its level adjustment.

The audio IF output is applied at connector of mixer PCB and the mixer PCB combines

the V.IF and A.IF signals aimed signals with and then mixes the combined signals with

local oscillator signals. The mixed RF is fed to filter and is rejected for its image

frequency. The amplifier and coupler PCB consisting of strip line coupler which samples

RF and detects it. The detected DC is fed to AGC PCB. This PCB compares the detected

DC voltages with reference voltage set. The difference voltage is compared in window

comparator circuit. The AGC PCB also sends out final power metering signal to front

panel. Local oscillator PCB gives KHz signals which is fed to IF unit as reference signal

to IF synthesizer PCB. The upconverter works with 230 V AC (single phase) which is fed

to connector.

Local Oscillator PCB-1

The Local Oscillator module is a fractional N Type synthesizer which is phase locked to

a 16 MHz crystal Oscillator

MIXER PCB


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The function of this PCB is to combine Vision IF and Aural IF signal, received from IF

unit. The combined signal is amplified and mixed with LO signal received from LO

PCB(suitable required channel) in a balanced mixer. The output of balanced mixer

consists of two side bands. Finally, this RF is made available. This PCB also provides

metering signals to front panel to indicate video IF and audio IF levels by selection of

switch. The Video IF and Audio IF input to this PCB can be selected independently by a

front panel vision carrier ON/OFF and aural carrier ON/OFF switch.

Aural Level Control PCB

The audio IF signal input is fed to upconverter unit is passed through this PCB. The A.IF

signal is passed through aural level control PCB.

AMPLIFIER & COUPLER PCB:

This PCB is housed with pin attenuator circuit & two low gain hybrid amplifier amplifies

the low level RF, received from band pass filter in upconverter. The amplified signal is

made available at the output of upconverter. This PCB also has a directional coupler &

detector circuit at the output circuitry. The RF signal is coupled by coupler & detected by

detector circuit. The detected dc voltage is fed to AGC PCB which in turn feeds back the

AGC control voltage onto pin attenuator circuit to have AGC control action for the gain

stages of this PCB.

AGC PCB:

It has an inbuilt AGC circuit which is interfaced with amplifier coupler PCB to keep the

upconverter power output constant. The PCB takes in detected dc voltages from amplifier

& coupler PCB & compares with reference. The difference voltage is fed to control the

PIN attenuator circuit on the amplifier coupler PCB. This in turn controls the input RF

level to gain stages in amplifier & coupler PCB & maintains the RF output of

upconverter constant. The detected dc voltage is also fed to window comparator circuit.

The comparator circuit gives the power normal high signal when the detected dc is within

the set limits. The AGC control voltage is applied to amplifier & coupler PCB, when

AGC switch on front panel of unit is in AGC position. When this switch is in MGC

position, the MGC control voltage is directly fed from front panel PCB to amplifier &


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coupler PCB. The +28 V dc is fed to this PCB from SMPS which in turn get converted to

+24 V required by circuitry. The detected dc from amplifier & coupler PCB is fed to

AGC PCB at E1 & is amplified using OPAMP U3/A. The output of this amplifier is split

into 3 paths.

POWER SUPPLY UNIT:

It consists of variable voltage regulator circuit which converts +28 V fed to this PCB

from SMPS into +18 V dc required by local oscillator PCB.

BAND PASS FILTER:

It is a tunable four element combination line filter, featuring additional trap resonators to

provide greater than 30dB rejection at Fv-5.5MHz & Fv+11MHz. Any 8MHz TV

channel within the UHF band, the combined frequency range (channels 21 to 68) can be

selected.

Cooling of final stages

Low-power transmitters do not require special cooling equipment. Modern

transmitters can be incredibly efficient, with efficiencies exceeding 98 percent. However,

a broadcast transmitter with a megawatt power stage transferring 98% of that into the

antenna can also be viewed as a 20 kilowatt electric heater.

For medium-power transmitter, up to a few hundred watts, air cooling with fans is

used. At power levels over a few kilowatts, the output stage is cooled by a forced liquid

cooling system analogous to an automobile cooling system. Since the coolant directly

touches the high-voltage anodes of the tubes, only distilled, deionised water or a special

dielectric coolant can also be used in the cooling circuit. This high purity coolant is in

turn cooled by a heat exchanger, where the second cooling circuit can use water of

ordinary quality because it not in contact with energized parts. Very high power tubes of

small physical size may use evaporative cooling by water in contact with anode. The

production of steam allows a high heat flow in a small space

TV demodulator:


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Tv demodulators is a precision monitoring equipment for cheching the quality of a TV

transmitter in VHF and UHF bands.it is similar to a precision TV receiver in manyrespects.the function of the TVDM is to recover the picture and sound signals from the

carrier output.

Salient features:

Selection of envelope or synchronous detector. Automatic and manual level control Zero reference pulse for checking depth of modulation. Indication of input voltage and sound deviation.

General applications:

Deviation measurement. Monitoring picture and sound quanlity. Signal to noise ratio measurement. Depth of modulation. Group delay.


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Envelope detector:

This is simplest type of envelope detector consisting of a rectifying diode driving

a parallel RC network.this circuit has a significant insertion loss there must be driven by

an addition amplifier with signal levels of several volts peak to peak in order to recover

1v of video.the inherent non-linearity of this circuit along with its large input signal leads

to several design problems and receiver performance deficiencies as shown below-

1).with full bandwidth signal present (33.4 to 38.9 Mhz) undesirable beat products can be

generated during the process(33.4 to 38.9Mhz) undesirable beat products can be

generated during the process of demodulation.one of the common spurious products is

produced by the combination of 33.4Mhz sound IF signal and the 34.47Mhz chroma sub

carrier,resulting in a 12 Mhz picture beat interference and sound buzz.in

monochrome,receiver the sound carrier is attenuated by more than 20db and and the color

sub-carrier is attenuated from 10 to 15db to alleviate these effects.

2).quadrature distortion caused by VSB nature of the signal and receiver nyquist slope

can cause a luminance shift towards black of up to 10% as well as asymmetric transient

response.this situation can also be helped by attenuating the sub-carrier by 6db or more

with respect to the top of the response.with all these deficiencies the diode envelope

detector has been used in a great many monochrome and color receivers.

Synchronous detector:

the synchronous demodulator in which the modulated carrier is sampled by a pure

unmodulated carrier generated by a VCO at 38.9Mhz.this approach gives the idealreference waveform,hence the most accurate recovery of the original modulating

waveform.major advantages of synchronous detectors are-

1. Higher gain than diode detector.2. Low level input considerably reduce undesired bea generation.3. Reduces IF harmonics by greater 20db.


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4. Less or no quadrature distortion depending upon the purity of the referencecarrier.an improved form of diode demodulator uses a balanced full wave

configuration with a like element in a feedback loop.excellent linearity and low

beat product equivalent to the synchronous demodulator tpes have been achieved.

ANTENNA SECTION

For the propagation the electrical energy is converted into electro-magnetic

wave. Where the VHF and UHF transmissions are different

Here the antennas are of two types where are propagation of the signal isdone. And the word mast means that a supporting structure.

1. Self Supporting Mast:


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It is 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.

2. Guided Wire Mast:The mast here is suspended from the ground and it is supported by

some wires so that it would withstand to the climatic conditions.

TYPES OF ANTENNAE:

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

used types of antennae are as follows:

Dipole antenna

Folded dipole antenna

Helical antenna

Parabolic antenna

Satellite dish antenna

Yagi-uda antenna

Horn antenna

DIPOLE ANTENNA:

A dipole antenna is an antenna that can be made by a simple wire,

with a center-fed driven element for transmitting or receiving radio

frequency energy. This antenna, also called a doublet. These antennae are

the simplest practical antennae from a theoretical point of view; the current


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amplitude on such an antenna decreases uniformly from maximum at the

center to zero at the ends. It is a straight electrical conductor measuring 1/2

wavelength from end to end and connected at the center to a radio-frequency

(RF) feed line. The dipole is inherently a balanced antenna, because it is

bilaterally symmetrical.

Ideally, a dipole antenna is fed with a balanced, parallel-wire

RF transmission line. However, this type of line is not common. An

unbalanced feed line, such as coaxial cable, can be used, but to ensure

optimum RF current distribution on the antenna element and in the feed line,

an RF transformer called a balun (contraction of the words "balanced" and

"unbalanced") should be inserted in the system at the point where the feed

line joins the antenna.

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.

HELICAL ANTENNA:

A helical antenna is an antenna consisting of a conducting wire

wound in the form of a helix. It is a simple way of obtaining high-gain and a


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broad band of frequency characteristics. A helical antenna radiates when the

circumference of the helix is of the order of one wavelength and radiation

along the axis of the helix is found to be the strongest. This antenna is

mainly directional. The radiation from a helical antenna is circularly

polarized, that is to say that the Electromagnetic field rotates about the axis

of the helix in the direction of the helix turn. Therefore, the radiation is

either circularly polarized clockwise or counter-clockwise. If one were to

explore the field from a helical antenna in the direction of maximum

radiation with a simple monopole or dipole antenna, one should find that the

strength of the signal will remain the same as long as the dipole is

perpendicular to the axis of the Helix. On the side of a helical antenna, the

field is elliptically polarized. Therefore, the horizontal and vertical portions

of the signal will not be of equal proportions. When using Helical Antennae

it is very important to make sure that both antennae have the same thread

orientation (i.e. both clockwise) otherwise the received signal will be

significantly decreased.

PARABOLIC ANTENNA:

A parabolic antenna is a high-gain reflector antenna used for radio,

television and data communications, and also for radiolocation (radar), on

the UHF and SHF parts of the electromagnetic spectrum. The relatively

short wavelength of electromagnetic (radio) energy at these frequencies

allows reasonably sized reflectors to exhibit the very desirable highly

directional response for both receiving and transmitting.

SATELLITE DISH ANTENNA:


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A satellite dish is a type of parabolic antenna designed to receive

microwaves from communications satellites, which transmit data

transmissions or broadcasts, such as satellite television.

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.

HORN ANTENNA:


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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.

Basic Horn Concept:

When a wave guide is terminated by a horn the abrupt discontinuity

that existed is replaced by a gradual transformation. All the energy

travelling forward in the wave guide will now be radiated, provided

impedance matching is correct

VHF TV LPT antenna:

Following types of LPT antenna are being used in doordarshan network:

a) Band3 BEL make, half wave dipole V antenna.b) Crossed folded dipole halios make band-3 antenna.c) Folded dipole scala make band-1 antenna.

Antenna panels are stacked vertically and mounted on a tower having some 30 meter

height.the transmitter power is first taken to a power is first taken to a power divider

through a low low loss feeder cabls. Branch feeder cables from driver feed te antenna

panels.the crossed folded dipoles in scala and halios antenna are required to be fed in

quardaturre as explained earlier for an omni-direction pattern in horizontal plane.this

maybe obtained b increasing the electrical lengh of on set of the branch feeder cable

with respect to that of other set by quarter wavelength.the BHEL V antenna consist of

four quadrant dipole arranged vertically in two stacks.a stack contains two quadrant

dipole spaced at half wave length on a common balance feeder line made by aluminium

tubes.the tubes are exended a quater wavelength beyond each stack and short circuited

resulting in a quaer wave stub appearing as high impedance at the eed points. Both the

stack are fed with equal amplitude and phase current by connecting the branch feeder

cable at the center of the stack.the resultant radiation pattern in horizontal plane is almostomni-directional as claimed by the manufacturer.the feed arrangement including branch

feeder cables is entirely concealed to prevent entry of moisture.

UHF LPT antennas:


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Slot antenna elements ,cut in metal sheet are used as electromagnetic waves

radiator at UHF frequencies. Asimilar half wavelength slot cut in a flat metal sheet,fed atcentre.the long side of the slot carry current of opposite phase and because current are not

only confined to the edges of slot but spread out over the sheet.power is radiated equally

from both sides of the sheet,if the slot is horizontal.the radiation is normal to the sheet

and vertically polarized.the slot antenna can be easily with a coaxial transmission line by

connecting the outer conductor to the sheet.inner conductor is connected to the centre of

the slot. The feed point impedance of such a antenna element is of the order of 50ohms.

UHF LPT paraslot antenna:


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the UHF LPT paraslot antenna manufactured by SCALA is an array of slot

antenna.slot windows are cut in a cylindrical,heavy make aluminium pipe and the same

from 24 feet to 30 feet which is mounted on a mast of 30m height.symmetrical parallel

feed system completely oused within the centre of the antenna is employed for feeding

the slots.the radiation pattern in horizontal plane is off set omni directional.maximum

radiation occurs in the direction that faces the slot area .

thruline power meter:

the thruline power meter is the most powerful meter for measuring RF powers in

doordarshan and AIR.the heart of the thruline meter is the directional coupler

transmission line assembly.it is connected in series with the measure both forward and

reserve power levels. A sample loop and diode element are contained within each plug in

element. The main RF barrel is actually a special coaxial line segment with a 50 ohms

characteristic impedance. The thruline sensor works due to the mutual inductance

between the sampling loop and the centre conductor of the coaxial elements.


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As it is using a special taking power measurement of TV signal the RF input to the

thruline power should be power during normal transmission will vary with video signal

and the meter indication not be taken as the real output power .it may also be noted that

audio drive is also present during normal transmission .to get the peak power value in

case of TV signals that is block power is to be multiplied by a factor of 1.674 the output

value from the sample(E)r is the sum of the two voltages Er andEm.voltage Er created by

the voltage divider action of R and C on transmission line voltage E.

CONCLUSION

We would like to conclude this training as 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 equipment of the

doordarshan relay center and its working.

We also learned about the procedure of transmission, reception and

strengthening of the signal and retransmitting the signal into space for thebroadcast around the range of propagation.

THANK

YOU


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CONCLUSION

Doordarshan, the national television service of India, is devoted to public service broadcasting. It

is one of the largest terrestrial networks in the world. In my Industrial training at Doordarshan

Kendra, Aligarh, I have gained useful knowledge which will surely be of great help in future. This

training gave me an opportunity to learn the practical aspects of the knowledge of my field of

interest


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Studying LOW POWER TV TRANSMITTER