STUDY OF NEC TRANSMITTER

IN

HIGH POWER TRANSMITTER

DOORADARSHAN - VISAKHAPATNAM

SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR AWARDING THE DEGREE OF BACHELOR OF ENGINEERING

IN

ELECTRONICS AND COMMUNICATION ENGINEERING

BY

N.V.S.T SRI HARSHA (08A51A0475)

SYAMSUNDARA RAO (08A51A04A4)

GANESWARA RAO (08A51A0491)

SATISH KUMAR(08A51A04)

FROM

ADITYA INSTITUTE OF TECHNOLOGY AND MANAGEMENT

TEKKALI

ORIGIN OF DOORDARSHAN

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 station 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.

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.

DD - NATIONAL (CHANNEL-7) DD-NEWS (CHANNEL-9)

INTEGRATED ON 02-08-1986 01-04-2003

TRANSMITTER TYPE BEL NEC

CHANNEL VHF(band-III) 07- 09

VISION POWER 10KW 10KW

AURAL POWER 1KW 1KW

VISION FREQUENCY 189.2396 MHZ 203.26MHZ

AURAL FREQUENCY 194.7396 MHZ 208.76 MHZ

SATELLITE INSAT 3A INSAT 3C, INSAT 4B

LOOKING ANGLE 93.50 740, 93.50

AZIMUTH 1490 2200 ,1490

ELEVATION 670 63.20,670

DOWNLINK FREQUENCYREGIONAL : 3820MHZ, NATIONAL : 3725 MHZ NEWS 3725MHZ

MEAN SEA LEVEL350 METRES

350 METRES

FIG: FEATURES OF DOORDARSHAN KENDRA HPT

PROCESS GOING ON IN DOORDARSHAN

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

Satellite

Uplink

Antenna

Downlink

Antenna

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

Earth station

Terrestrial system

User

SatelliteSatellite

Uplink

Antenna

Uplink

Antenna

Downlink

Antenna

Downlink

Antenna

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. The signal is divided into visual signal

and aural signal. Demodulator separates audio and video signal. The video Signal is

further amplified to 1V peak to peak by Video Distribution Amplifier ( VDA) and

Stab Amplifier in Video Signal Chain where as the Aural Signal is amplified to 10

dbm by Audio Distribution Amplifier.

FIRST LET US CONSIDER VISUAL SIGNAL CHAIN :

This visual signal first undergoes the input monitoring section. Input

monitoring section consists of mainly three section.

Video distributor

Colour Stab Amplifier

Video Equilizer

VIDEO DISTRIBUTOR:

The video distribution amplifiers are employed in Transmitters to distribute

composite video signals to a number of units. This contains two identical

distribution amplifiers each providing five outputs. Here the input signal coming

from TVRO is amplified to 1V peak to peak.

COLOUR STAB AMPLIFIER:

So when the visual signal goes to this amplifier block, the signal is amplified,

and we get perfect 1v peak to peak signal. This is used with sync. Processor to

process the composite color video signal. The composite video signal is processed to

remove hum and noise from the timing components in the sync. Processor which

produces regenerated noise free sync and blanking signals. These noise free signals

are added to the composite color video signals in the color stabilizing amplifier.

VIDEO EQUALIZER:

The video equalizer is used to compensate for the video signal attenuation in

cables (OD lengths up to 300 mts ). The equalizing can be adjusted in 21 steps.

AUDIO DISTRIBUTION AMPLIFIER

In the similar way audio signal undergoes many process to get the signal in

the range of 0 to10 dbm. First the signal go through audio distribution block where

audio level correction is carried out. Then the signal undergoes pre-emphasis

technique to get perfect audio signal.

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.

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.

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.

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.

DD-NEWS: NEC TRANSMITTER

(CHANNEL-9)

The block diagram of the NEC transmitter is as shown in figure bellow.

NEC 10 KW Transmitter

Figure : Complete Block Diagram of a 10 kW TV Transmitter (Courtesy –NEC)

EXCITER UNIT:

The exciter section consists of various parts like

AD-DA converter

Visual modulator

IF corrector

Digital video compression

Synthesizer

Visual mixer

Aural modulator

IM corrector

Aural mixer

VISUAL

REF I/P

VIF O/P

10MHz O/P

600Ω AURAL O/P

75Ω

NICAM IN

A MOD MONITOR A AGC IN A O/P MONITOR

A/D-D/A CONVERTOR

VISUAL MODULATOR

IF CORRECTOR VISUAL

MIXER

VIDEO I/P

FEEDBACK IN

DIGITAL VIDEO COMPENSATOR

SYNTHESIZER

AURAL MODULATOR IM CORRECTOR AURAL MIXER

AURAL MODULATOR:

This unit generates a frequency modulated aural IF signal by modulating a

voltage controlled oscillator with an audio input. Two sets of audio inputs are

provided one for 600Ω balanced line and other 75Ω unbalanced line used for sound

multiplexer broadcasting. To fix the average frequency of modulator oscillator at the

reference input the automatic phase control (APC) circuit is provided. This unit is

applied to a VHF or UHF mixer circuit.

IM CORRECTOR:

This unit is used for dual sound having two carriers in which some correction

of IM due to the non-linearity of the power amplifier stage can be carried out. This

unit contains a low level and a high level correction circuit each having correction

circuits for amplitude and phase. In the low level circuit, correction can be

performed by combining a non-linear signal generated by class B amplifier with the

linear signal, as a result the phase combining of the linear signal with the non-linear

signal produces amplitude correction. Difference phase combining in the rated

phase difference produces phase correction. High level correction of amplitude is

performed by the use of a saturated class A transistor amplifier high level correction

of phase is performed by the use of a class C transistor amplifier.

AD-DA UNIT:

This unit has functions that converts video input signal supplied to the exciter

into PCM signal and sends the PCM signal to a unit for digital correction and which

converts the video PCM signal after the digital correction into analog video signal

and supplies the analog video signal to a visual modulator unit.

The functions exercised by the AD-DA unit are as follows:

1. Allows switching between input video signal from the main line and that from

a feedback line

2. Converts analog video signal fed to exciter into a PCM signal and supplies the

video data to DVC unit at a next stage.

3. Converts the 12 bit video data from DVC unit in to an analog video signal, and

supplies the signal to V Mod unit.

4. Clamps the pedestal potential of the video signal.

5. Carries out synchronous separation of the main input video signal, the

feedback input video signal and that of the input synchronous signal when

scrambling is used.

DIGITAL VIDEO COMPENSATOR:

This unit compensates by the use of digital signal processing

technology distortion of input video signals and different types of distortion

like linear and nonlinear distortions produced in transmitter and receiver. It

is composed of a non-linear distortion compensating circuit, linear

compensating circuit, control circuit, etc. It will receive the demodulated

output signals of the transmitter and automatically compensates for the di

The non-linear distortion compensating circuit (for differential gain,

differential phase and luminance linearity) compensates as an auxiliary

circuit to the IF corrector. The non-linear distortion is caused by the power

amplifier of the transmitter. Furthermore, the circuit is an APL follow-up type

that can compensate for variations in the characteristics of the power

amplifier by APL.

The linear distortion compensating circuit (pre-compensation

circuit for compensating, or the frequency and group delay characteristics of

the transmitter, and the group delay characteristics of receiver) performs

high accuracy compensation by using a 128 tap digital filter (for input signals

with the filter co-efficient of 12 bits) The non-linear and linear distortion

compensating circuits respectively can be bypassed.

The control circuit is provided with a DSP (digital signal

processor) that analyze demodulated output of the transmitter and

automatically computes the compensation values.

VISUAL MODULATOR:

This unit is intended to convert a base band video signal into a modulated IF

signal with ring modulator in which IF carrier is also phase modulated by a

processed video signal to pre-correct the incidental carrier phase modulation

(ICPM). The video signal for the IF phase modulation is arbitrarily sliced into three

regions of sync, black and white in which each signal is individually expanded or

compressed then summed into the processed video signal by which the carrier for

the ring modulator is phase modulated.

IF CORRECTOR UNIT:

This unit is generally used for correction of non-linear distortion generated in

the power amplifier stage enables correction of DG and DP characteristics of visual

signal. This also contains a means to combine two modulated IF carriers of visual

and aural allowing multiplex operation of the transmitter.

VHF MIXER UNIT:

In this unit the IF signal applied at input is converted to an RF signal and the

RF signal is then passed through filters to separate out only the specified band and

amplified to obtain an RF signal of +20dbm. By applying AGC to IF signal, the output

of the transmitter is maintained at a constant level.

SYNTHESIZER UNIT:

This unit generates signals at three frequencies visual IF and local frequency.

The exciter section is to receive two input of both video and audio incoming

to the TV transmitter. It contains same signal processing plug in board by which

signals are converted into IF modulator visual and aural TV signals. The visual and

aural outputs are then given to exciter switch. The modulated visual IF signal passes

through a VHF mixer unit and IF corrector unit and modulated aural IF signal passes

directly through a separate VHF mixer unit of same type, thereby producing VHF TV

signal on desired channel. The visual output power that is 100 mw is given to 2-way

divider block, which is used for dividing the signal to two transistor power amplifier

blocks and two transistor power amplifier in parallel. This two 50 mw power is then

given to 5-way divider block, where the power gets divided into five parts producing

1kw each which gets combined in a 5-way combiner resulting in a total power of

5kw. The two 5kw power (10kw) gets combined into 2-way combiner and results in

10kw of power which is then given to V/A combiner.

POWER DIVIDER

DESCRIPTION:

The block schematic diagram of the two way power divider is shown in figure.

J2 output-1(50Ω)

INPUT J1(50Ω)

J3 output-2(50Ω)

Block schematic diagram of 2-way power divider

2-WAY POWER DIVIDER

1. GENERAL: This 2-way power divider for dividing the signal to two transistor power amplifier blocks and two transistor power amplifiers is parallel

2. FEATURES:

This 2 way power divider has the following features

Wide frequency range Low transition loss Low input return loss(VSWR)

3. SPECIFICATION: The specification of this 2 way power divider is shown in table

3 WAY 0° 3DB

5-WAY POWER DIVIDER

1. GENERAL: This 5-way power divider for dividing the signal to two transistor power amplifier blocks and five transistor power amplifiers is parallel

2. FEATURES:

This 5 way power divider has the following features

Wide frequency range Low transition loss Low input return loss(VSWR)

3. SPECIFICATION: The specification of this 5 way power divider is shown in table

ITEM SPECIFICATION1.frequancey range 170-230MHZ2.VSWR ≤1.23.return loss ≥20DB4.insertion loss ≤1.0DB5.maximum input power rating 2mw6.input/output impedance 50Ω7.input/output terminal BNC-j8.ambient temperature -10 to +45°C9.dimensions W×H×D 600×57×22mm

ITEM SPECIFICATION1.frequancey range 170-230MHZ2.VSWR ≤1.23.return loss ≥20DB4.insertion loss ≤0.6DB5.maximum input power rating 10mw6.input/output impedance 50Ω7.input/output terminal BNC-j8.ambient temperature -10 to +45°C9.dimensions W×H×D 200×55×22mm

4. DESCRIPTION: The block diagram of the power divider is shown in figure.

J2 Output-1 50Ω

J3 output-2 50Ω

Input j1 j4 output-3 50Ω

50Ω j5 output-4 50Ω

J6 output-5 50Ω

3-way divider

2-way

0°3db

2-way

0°3db

2-way

0°3db

POWER AMPLIFIER:

OPERATING PRINCIPLES:

The drive PAI comprises two stage of power amplifier circuits containing PIN attenuator, a phase shifter, RF limiter, hybrid IC MC5388 and FET 2SK1543

A signal from J1(BNC-J) on the front panel passes through input and the PIN attenuator, Where the gain of the unit is adjusted. The phase shifter adjusts the phases between two units when this unit runs in parallel with another. The phase shifter can adjust the phase in a range within approximately 40 degrees. The RF amplifier protects the unit from overdrive input signal and has been adjusted so as to operate 1DB above the input rating.

The MC5388(IC 301) at the first stage, which is a class A hybrid IC, has a gain approximately 18DB. The next stage amplifier circuit having an FET 2SK1543(TR301) which is a single class AB has a gain of approximately 18DB. The output from the FET 2SK1543(TR301) passes through an isolator W1 and then is to the drive PA2.

DRIVE PA2 CIRCUIT:

The drive PA2 comprises one stage of power amplifier having a 2SK1543M. the drive PA2 is class AB operation with a gain of approximately 16DB. The output from the drive PA2 circuit passes through an isolator W2 and is distributed by a Wilkinson 3-way divider and then fed to the final PA.

FINAL PA CIRCUIT:

The final PA circuit is the final stage amplifier circuit supplied with six 2SK1543MPS which uses a pair of characteristic matched 2SK1543M in parallel and it comprises two stage of 2 way distribution circuits of distributed type and one stage of power amplifier using 2SK1543S as well as a circulator combiner. The signal from the drive PA is divided into six sub signals and distributed by the distributor and each final stage amplifier circuit. the final stage amplifier circuit of optimum class AB push pull design using 2SK1543MPs recurs a gain of approximately 16DB, maximum out of 250W or more and a drain efficiency of 70% or higher at the maximum output. The 2SK 1543s used in this units are developed

MOS FETs of a push pull constructions which are of a high output, a high gain a high reliability for use in particularly VHF band TV transmitter.

Six outputs of the signals amplified at the final stage amplifier circuit lead to a 6-way combiner through circulator.

6 WAY COMBINOR AND DIRECTIONAL COUPLER:

Each output of the six final PAS FPA1 to FPA6 is combined with 6-way combiner.

The combined visual signal forms the final output of the TR PA and is applied to terminate J2 on the rear panel at a level of 60.8DBM.

To monitor the output, two directional couplers are provided. One of the outputs among them is supplied to the terminal J4 on the front panel.

Two outputs of directional couplers are applied to the alarm circuit board.s

5-WAY POWER COMBINOR

1. GENERAL:

This 5-way power combiner is used for combining five transistor power amplifiers in parallel to obtain the required power.

2. FEATURES:This 5-way power combiner has the following features1) wide frequency range2) low insertion loss3) low input return loss(VSWR)

3. SPECIFICATION: The specification of this 5-way power combiner is as shown in table

DESCRIPTION: The type of this 5-way power combiner uses quarter wave impedance transforms into combiner More ever this 5-way power combiner is made of copper plate strip line with in aluminum case. The copper plate strip line is supported from the aluminum case by Teflon insulators.

2-WAY POWER COMBINOR

1. GENERAL: This 2-way power combiner is used for combining two transistor power amplifier blocks and two transistor power amplifiers in parallel to obtain the required power.

2. FEATURES:This 2-way power combiner has the following features1) wide frequency range2) low insertion loss3) low input return loss(VSWR)

3. SPECIFICATION:The specification of this 2-way power combiner is as shown in table

ITEM SPECIFICATION1.frequencey range 170-230 MHZ2.VSWR ≤1.23.return loss ≥20DB4.insertion loss ≤0.2DB5.maximum input power rating 1000W6.input/output impedence 50Ω7.input/output terminal W×-20D/W×-39D8.ambient temperature -10 TO 45®C9.dimensions W×H×D 613×79×420mm

4.

DESCRIPTION: The type of this 2-way power combiner uses quarter wave impedance transforms into combiner More ever this 2-way power combiner is made of copper plate strip line with in aluminum case. The copper plate strip line is supported from the aluminum case by Teflon insulators.

GENERAL: The power supply in a stabilized power supply of switching type that receives 200V AC and supplies 50V DC to loads. In design consideration of small size light weight and high performance this power supply is provided with three special built in A-D converters.

PerformanceTypeRating continuousCooling far external air cooled

5. INPUT Rated voltage: 200V AC, 3-phase

ITEM SPECIFICATION1.frequencey range 170-230 MHZ2.VSWR ≤1.23.return loss ≥20DB4.insertion loss ≤0.2DB5.maximum input power rating 100W6.input/output impedence 50Ω7.input/output terminal W×-20D/W×-39D8.ambient temperature -10 TO 45®C9.dimensions W×H×D 190×79×430mm

Voltage regulation ±15% Frequency: 50/60 HZ Frequency range: ±5HZ

OUTPUT:

ITEM OUTPUT

Output voltage +50V

Load currentMax 48A

0 AMin

Variable range of output voltage 40-50V

Output voltage setting accuracy

Voltage accuracy ±0.3V

24ASet current

Accuracy output voltage stabilization

To load fluctuation 0-100% : 400mv p-p or less

7 mv/dc or lessTemperature variation

Ripple voltage Hurn Fundamental wave spike

15mvp-p

600mvp-p

EFFICIENCEY: 85% or more at rated input and outputPOWEER FACTOR: 90% or more at rated input and output

DIELECTRIC STRENGTH AND INSULATION RESISTANCE:

MEASUREMENT POSITION DIELECTRIC STRENGTH

INSULATION RESISTANCE

Between a collective position On the primary input side and A collective position of the secondary output side

Required to be free ofAny abnormality when 1.5KV AC is impressedAcross the measurementPosition far one minute

Required to be 50MΩ orMore when measured 500VDC megger

Ambient temperature range

Performance assurance: -10°C to 45°C

Working assurance: -20°C to +50°C

AMBIENT HUMIDITY RANGE:

Performance assurance: 95% or less

RUSH CURRENT:

Not less than three times the peak value of rated primary input current.

DESCRIPTION OF CIRCUITS:

1. INPUT FILTER CIRCUIT: The input filter circuit supposes simultaneously with suppression of external noise the noise produced by the switching circuits to prevent the noise from being transmitted external circuits.

2. RECTIFIER CIRCUIT: The rectifier circuit is for rectifying the AC voltage to the DC voltage is of a capacitor input type which has a chock coil inserted for improving power factor.

3. RUSH CURRENT PREVENTIVE CIRCUIT: The rush current preventive circuit suppress the such current by means of a resistance inserted into the input line, and reduces the loss of the resistor by actuating the thyristor after the D/D converter is started.

4. D/D CONVERTER: The D/D converter converts the DC voltage into 50V DC output. This converter has been in double forward type using power mosfet in consideration to small size and high efficiency the working frequency of the converter is about 400KHZ.

5. AUXILARY POWER CIRCUIT: This power circuit supplies power to the protective and control circuits.

6. UNDER VOLTAGE INPUT PROTECTIVE CIRCUIT: This protective circuit detects continuously the primary input

voltage the input rectified voltage of D/D converter and interrupts the power supply to the converter, turning the output of the converter off. If the primary input voltage is abnormally low further more. If the normal primary voltage one of three wires becomes open the protective circuit turns the output of this power supply is off.

7. D/D CONVERTER CIRCUIT ON/OFF CONTROL UNIT: This control circuit controls the ON/OFF operation of the D/D converter at the time of under current input over voltage input, over voltage output , under voltage output or actuation of the overheat protective circuit or by an external PS CONT signal.

8. OVER VOLTAGE PROTECTIVE CIRCUIT:

This protective circuits detects continuously each secondary output voltage if the output voltage rises abnormally, the protective circuit makes notching this turning the output off.

9. OVER CURRENT PROTECTIVE CIRCUIT: This protective circuit detects continuously the primary switching current of each D/D module and makes notching if the output current exceeds the rating owing to such reason as overload thus turning the output off.

10. UNDER VOLTAGE PROTECTIVE CIRCUITS: This protective circuit detects consciously each output voltage when the output voltage rises normally the protective circuit sends out a PSON ANS signal.

11. NOTCHING ALARAM CIRCUIT: If the over voltage over current or under voltage protective circuit is actuated notching alarm circuit interrupts the operation of the D/D converter for about 4sec and the same time when the converter is stopped. This circuit makes the ALM the front panel light and sends out a PS FAULT signal The notching count time is about 30sec and then the notching is counted 4 the times the period of 30sec the notching alarm circuit actuates the NFB, interrupting the primary input voltage.

12. OVER HEAT PROTECTION CIRCUIT: If the internal temperature of the power supply rises abnormally far such reason discontinuation of the cooling the overheat protective circuit actuates the NFB interrupting the primary input voltage.

13. OUTPUT VOLTAGE MEETING CIRCUIT:

This meeting circuit detects each output voltage and send out the voltage for the meeting.

V/A COMBINER UNIT:

V/A combiner work in the similar fashion as CIN –diplexer in BEL transmitter,

and are used to combine the visual and aural outputs.

VISUAL I/P

AURAL I/P DL

O/P ANTENNA

V/A

COMBINER

HARMONIC FILTER

OUTPUT FILTER

CIBD UNIT:

CIBD stands for Constant Impedance Band Diplexer which is similar to CIN

Diplexer in BEL transmitter. The output filter and harmonic filter are used for

combining visual and aural signals of a VHF TV transmitter and attenuating spurious

and harmonics.

A simple explanation of the CIB Diplexer is as follows:

The signal of the aural transmitter applied at terminal (1) of the 3db coupler

H1 appears at terminals (2) and terminal (3) with the same amplitude but with a

phase difference of 900 (terminal (2) is 900 ahead in phase of terminal (3) , because

of nature of 3db coupler, no output appears at terminal (4).

The signals appearing at terminal (2) and terminal (3) then passes through

aural band pass filters respectively, and terminal (21) and terminal (31) of other 3db

coupler H2 still with 900 phase difference.

OPF

Vin

CIBD

1 2 A 21 11

Ain BPF1

H1 H2

4 3 BPF2 B 31 41 O/P

The signals appearing at 3db coupler H2 terminal (21) and terminal (31) with

900 phase difference are combined in the H2 into terminal (41) because signal at

terminal (21) has a 900 lead phase from that at terminal (31) but no output appears

at terminal (11). The signal appearing at terminal (41).

Mean while, the visual transmitter output is connected to terminal (11) of 3db

coupler H2. The visual signal entering this terminals does not appear at terminal

HF

(41), but at terminal (21) and terminal (31) with same phase difference of 900

(terminal (21) is 900 ahead in phase of terminal (31)).

The visual signals are reflected at points A and point B, then reaching

terminal (21) and terminal (31) of 3db coupler H2. Since the length of point A to

terminal (21) of H2 is equal electrically to that of point B to terminal (31) of H2, the

visual signal entering to terminal (21) of H2 is combined with that to terminal (31)

because of nature of 3db coupler. Then the combined visual signal appears at

terminal (41).

The CIB Diplexer has constant input impedance as viewed from visual and

aural input and allows sufficient isolation between visual and aural signals.

Accordingly, it can supply visual and aural signals to antenna without mutual

interference.

The absorbing resistor absorbs aural signal components reflected

by filters and the visual signal components passed through filters.

ADVANTAGES OF NEC TRANSMITTER OVER BEL TRANSMITTER

High reliability and low maintenance are two reasons NEC’s transmitters are

known for.

1. HIGH PERFORMANCE EXCITER:

Nonlinear distortions are corrected at the videos and IF stages. Among the

nonlinear parameters are luminance distortion, differential gain (DG) , differential

phase(DP) , and incidental carrier phase modulation(ICPM). All of these items are

individually corrected. Higher reliability, performance and maintainability can be

obtained. The exciter also employs a pedestal AGC circuit and a surface acoustic

wave vestigial side band filter.

2. Highly efficient transistor power amplifier unit

3. Compact components for ease of installation newly developed, greatly

maintained component parts are mounted on each cabinet in such a manner as

to facilitate ease of maintenance service.

4. EASE OF OPERATION:

The transmitter is equipped with a display panel for visually displaying all

operating and faults status for the operator in one convenience location.

Supplementing the EL display, the fault displaying functions distributed among

several units. in order to improve the operational flexibility of the television

transmitter and the expectancy of the visual and aural transistor power amplifier

unit, a wide variety of circuit design improvements have been incorporated.

FUNCTIONING OF CONTROL SYSTEM OF NEC TRANSMITTER:

CONTROL SYSTEM:

The transmitter can be controlled in three modes

Remote

Local

Maintenance

In either mode all controlling signals are processed in the transmitter control

which is also capable of controlling signals are processed in the transmitter control

which is also capable of controlling several equipments such as V/A combiner and

main blowers.

TRANSMITTER CONTROL SYSTEM:

This control system applies to TV transmitters. This system is composed of

different types of equipment that are described in the following section and which

respectively have special functions. The details of monitor and control by this

system are all displayed on a display unit, the EL display, where operators can read

the necessary information. Furthermore, an RS-232C connector for serial data

communication fitted to this system allows for connection of this system to PC, thus

enabling monitoring of information the same as is on EL display.

EL CONTROL:

This will control the display on the EL DISPLAY unit that receives information

signals from transmitter control and which makes display for monitoring the TV

transmitter. This EL control is mounted with a CPU and performs control operation

by means of control software. Furthermore, the EL CONTROL is provided with an

RS-232C serial interface connector to send out the data displayed on the EL

DISPLAY. This performs control of the display but has no relation to control of TV

transmitter.

TRANSMITTER CONTROL:

The transmitter control is a controller system for television transmitter

series. This controls and monitors the television transmitter and in the event of a

serious alarm in the transmitting system shuts off the transmitter in order to protect

it from the serious damage. All of the information monitored by the transmitter

control is displayed on the EL control. The information is also sent to the relay card

and transmitted to external equipment. To enable analogue values to be displayed

on the EL display, the output power of the television transmitter, reflective power

V/A combiner , absorbed power, information from transistor power amplifiers and

the output level and deviation of the mixer in the exciter are converted to digital

signals.

TX-PA MONITOR:

This TX-PA monitor is a part of monitors and control system of television

transmitters. This distributes the signals coming from transmitter control to

transistor power amplifiers (TR-PA’S) and power supplies for transistor amplifiers.

Furthermore this monitor samples output power , reflection power, temperature,

DC voltage, drain current, alarm etc in sequence and time division multiplexes and

sends this data through a data bus line to transmitter control.

TR-MONITOR:

This unit indicates the transmitter output power and detects any abnormal

condition

Hence the DD-NATIONAL and DD-NEWS channels are combined in kathrein

block through channel-7 and channel-9 producing the total output power of 20kw

which is again further divided into two parts each 10kw and is given to mast

antenna and finally from mast antenna consumer receives the channels.

LATEST ADVANCEMENT IN TRANSMISSION:

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 entertainment programmes over a single delivery platform. In addition,

since digital technology permits a highly efficient exploitation of the frequency

spectrum, the number of TV channels that 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 nice

markets with dedicated transmissions.

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

digitally compressed, encrypted and beamed from very high power satellites. The

programmes can be directly received at homes. This mode of reception facilitates

the use of small receiving dish antennas of 60 to 90 cm diameter installed at

convenient location in individual buildings without needing elaborate foundation

/space etc. 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

sharp contrast to Cable TV, lends itself to easy monitoring and control.

DTH is digital in nature hence more number of channels are available and

bandwidth is reduced. This works in Ku-band for up linking and down linking

process.

DEFINITION:

DTH is defined as the reception of satellite programmes with a personal dish

in an individual home. And an individual Set Top Box empowering you to pick &

choose you bundles of choice and pay for what you watch.

In DTH, TV channels will be transmitted from the satellite to a

small dish antenna mounted on the window or rooftop of the subscriber's home. So

the broadcaster directly connects to the user.

DTH can also reach the remotest of areas since it does away with the

intermediate step of cable and wires (cables) that come to your house. DTH offer

digital superior quality picture against cable TV today which is most analog.

DTH is an encrypted transmission that travels to the consumer directly

through a satellite. DTH transmission is received directly by the consumer at his end

through a small dish antenna. The encrypted transmission is decoded by an

individual STB at your home.

The other advantage of DTH is the availability of satellite broadcast in rural

and semi-urban areas laying of cable is difficult.Due to digital compression

techniques, many more niche channels are available than cannot be provided by

cable operators. DTH also offers digital quality signals which do not degrade the

picture or sound quality. It also offers interactive channels and program guides with

customers having the choice to block out programming which they consider

undesirable.