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1.1 Introduction: Doordarshan is the national service of India and is also one of the largest broadcasting organizations in the world. A network of three nationals, two special interest channels; 10 regional language channels, 4 state network and an international channels. Through a network of 868 terrestrial transmitters of varying powers it makes available television signals for over 87% of population. 300 million viewers in their homes watch Doordarshan programmes. Television sets established under various schemes in community centers in villages for a total number of 450 million viewers (India, 1998). The countrywide class room on national network is aimed to reach quality education of students in small villages. Television in India has been in existence for decades now. India did not begin till September 15, 1959 with a small studio. The service was called Doordarshan for the first 17 years, it spread haltingly and transmission was mainly in black and white. Doordarshan was established as a part of AIR, until 1976, it consisted of one national network and seven regional networks. In 1992 there were sixty three high power television transmitters, 369 medium power transmitters, 76 low power stations and 23 transposers. Regular satellite transmission began in 1982. Television has come to the forefront only in the past 21 years and more so in past 13. There were initially two ignition points, the first in the 80s 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.


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STUDY OF LOW POWER TRANSMITTER 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 100s in the eighties to more than 20,000 in the nineties. Presently Doordarshan operates 19 channels, two All India Channels, 11 regional languages satellite channels (RLSC), four State Networks (SN) an international channel and a sports channel. Regular satellite transmission began in 1982. Now more than 87% of population of the country can receive Doordarshan programmes through a network of nearly 1044 Terrestrial Television Transmitters. About 46 Doordarshan studios are producing television software.

The T.V. transmitters can be classified into three types basing on their coverage area, radiated power. High Power Transmitters Low Power Transmitters Very Low Power Transmitters TYPE High Transmitters Low Transmitters Very Low Power Transmitters 10 km 5-10watt Power 15-20 km 100watt Power COVERAGE 80-120 km POWER >1kwatt

T1.0Various types of power transmitters &their coverage


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STUDY OF LOW POWER TRANSMITTER 1.2 What is need of LPTS? HPTS can cover the 80-120 k.m. if the area is flat area i.e. without any obstruction. If any highest buildings or hills exists then coverage capacity reduces because of the transmitted

signals absorbed by them and they prevents the signal to go.At this time the transmission system fails to cover some areas which are behind the hills or the tallest buildings, this lead to the establishment of LPTS to transmit over a small distances of 15-20 km. BLOCK DIAGRAM OF LPT:


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1.3 TV Programmes Transmission Using Satellite:

Fig1.0 uplink and downlink process of signal TV Signals from studio are processed and up-linked to the satellite where these signals are further processed and then down linked to the Terrestrial T.V Transmitters with the help of transponders of the satellite. The signal received by the parabolic dish antenna is sent to the TVRO of input output chain with the help of coaxial cable and then it is further processed and modulated and transmitted into the space.


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2.1 Introduction: Although there are many ways of communication in practice

today,e.g.,linetelegraphy,linetelephony,visual signaling etc.but the radio communication (also called wireless communication),is the most fascinating and popular system of communication these days.The essential requirements of any communication system are fundamentally same eg.means of transmitting the message or intelligence,means to receive the

message.Inotherwords a Transmitter,a medium to carry the message or intelligence from one place to another and a receiver. Radio frequency waves, produced at transmitter, are carried by transmission lines to an antenna or aerial which radiate it in the form of electromagnetic waves into the space.let us define an antenna. 2.2 Antenna Antenna is a metallic device for radiating or receiving radio waves. So antenna in other words is a transitional structure between a free space and a guiding device. And this guiding device may take the form of a coaxial cable or any other transmission line which transports EM energy from source to antenna. 2.2.1. Antenna Parameters: 1) Power gain 2) Directivity 3) Polarization 4) Radiation pattern 5) Beam width 6) Band width 7) Radiation resistance POWER GAIN: The power gain of an antenna is a ratio of the power input to the antenna to the power output from the antenna. This gain is most often referred to with the units of dbi, which is logarithmic gain relative to an isotropic antenna. An isotropic antenna has a perfect spherical radiation pattern and a linear gain of one. DIRECTIVITY:

STUDY OF LOW POWER TRANSMITTER Directivity of a antenna is defined as the ratio of antenna intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power radiated by antenna divided by 4. POLARIZATION: The polarization of antenna is the polarization of the wave radiated by the antenna ie. the polarization describes the orientation of electric field into the space. It can also be defined as " the property of an electromagnetic wave describing the time varying direction relative magnitude of the electric field vector. Magnetic fields surround the wire and perpendicular to it, it implies that the electric field is parallel to the wire. So, this configuration is applied even after radiation of waves from the wire. So there arises the need for polarization of a radiating element. Polarization of an antenna refers to the orientation of radiated electromagnetic waves in the space. Classification of Polarization: Polarization is classified based on the orientation of electric field as linear, circular and elliptical polarization.

Fig2.0 classification of polarization Linear Polarization: An electromagnetic wave is said to be linearly polarized if all the cycles of the wave has same alignment in the space. Horizontal polarization:


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STUDY OF LOW POWER TRANSMITTER If an electric field vector E lies in horizontal plane then the wave is said to have horizontal polarization. Vertical polarization: If an electric field vector E lies in vertical plane then the wave is said to have vertical polarization... Circular/Elliptical Polarization: If two linearly polarized waves which are simultaneously produced in the same direction and are mutually perpendicular to each other with a phase difference of 90 degrees, then we get a circularly polarized wave. Circular polarization may be right -handed or left-handed depending upon the sense of rotation i.e., phase difference is positive or negative. Circular polarization results only when the amplitudes of the two linearly polarized waves are equal. If the amplitudes of the two linearly polarized waves are not equal then we result in elliptical polarization. We yet have another polarization called cross polarization which is basically the radiation in the undesired direction. At the TV broadcasting frequencies we generally use horizontal polarization as a standard. At microwave frequencies the polarization we use depends on the type of application. In any case the polarization of both receiving and transmitting antennae should be equal.

RADIATION PATTERN: A graphical representation of radiation properties of antenna as a function of space coordinator is termed as radiation pattern. BEAM WIDTH: The beam width of an antenna is the angular separation between the two half power points on the power density radiation pattern. It is also, of course, the angular separation between the two 3-db down points on the field strength radiation pattern of an antenna. BAND WIDTH: It refers to the frequency range over which operation is satisfactory and is generally taken between the half power points. There are actually two separate requirements for large bandwidth (in excess of 10%)

STUDY OF LOW POWER TRANSMITTER from antennae. The first is for antennae which could be narrowband but which are required to operate at a number of separate frequencies within a fairly large range. High frequency antenna's are often of this type, in which the required operation is helped by the fact when the antenna is switched to a new frequency, compensating circuits can be switched in also. RADIATION RESISTANCE: The input impedance 'Zin' of an antenna is the ratio of voltage to current at its input terminals where the power is fed to the antenna. Zin = Ra+jXa Ra = radiation part of impedance Xa = reactance part of impedance 2.2 TYPES OF ANTENNAS: Based on these there are different types of antennae. But basically used types of antennae are as follows: 1) Folded dipole antenna 2) Parabolic dish antenna 3) Yagi-uda antenna 4) Horn antenna

2.2.1 Folded dipole antenna: A variation of the dipole can be a solution to the problems caused due to dipoles, offering a wider bandwidth and a considerable increase in feed impedance. The folded dipole is formed by taking a standard dipole and then taking a second conductor and joining the two ends. In this way a complete loop is made as shown. If the conductors in the main dipole and the second or "fold" conductor are the same diameter, then it is found that there is a fourfold increase in the feed impedance. In free space, this gives a feed impedance of around 300 ohms. Additionally the RF antenna has a wider bandwidth. 2.2.2 Parabolic Dish Antenna: A dish antenna works on the same way as a reflecting optical telescope. Electromagnetic waves, light or radio, arrive on parallel paths from a distance source and are reflected by a mirror to a common point, called the focus. When a ray of light reflects from a mirror or flat surface, the angle of the path leaving (angle of reflection) is the same as the

STUDY OF LOW POWER TRANSMITTER angle of the arriving (angle of incidence).If the mirror is a flat surface, then the two rays of light leave in parallel paths; however if the mirror is curved, two parallel incident rays leave at different angles. If the curve is parabolic(y=ax) then all the reflected rays meet at one point as shown in figure.

F-focus Fig2.1 parabolic dish antenna

A dish is a parabola of rotation, a parabolic curve rotated around an axis which passes through the focus and center of the curve. The received signal can be transmitted to the low noise block converter(LNBC) through feed horn antenna. 2.2.3. YAGI-UDA ANTENNA: A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a directional antenna system consisting of an array of a dipole and additional closely coupled parasitic elements (usually a reflector and one or more directors). The dipole in the array is driven, and another element, 10% longer, operates as a reflector. Other shorter parasitic elements are typically added in front of the dipole as directors. This arrangement gives the antenna directionality that a single dipole lacks. Yagis are directional along the axis

STUDY OF LOW POWER TRANSMITTER perpendicular to the dipole in the plane of the elements, from the reflector through the driven element and out via the director(s).Directional antennas, such as the Yagi-Uda, are also commonly referred to as beam antennas or high-gain antennas.

Fig 2.2yagiuda antenna

2.2.4 HORN ANTENNA: A wave guide is capable of radiating radiation into open space provided the same is excited at one end and opened at other end. The radiation through this feared out wave guide is more than through that of a transmission line.

Fig2.3 horn antenna


STUDY OF LOW POWER TRANSMITTER 2.2.5 V-antenna: Its an omni directional antenna fabricated from the light weightAluminium tubes and flats..The electronic wiring is concealed within the tabular structure of the antenna and is sealed to prevent moisture penetration.A clamp is provided between the feed for clamping the feeder cable to prevent strain on the cable connector due to swaying of the cables.Arubberhood just above the feed point protects the connector from the direct rain. Average gain of V antenna is 4.3 dbm and power is 100watt,frequency range 174MHZto 232MHZ in UHF band.

Fig 2.4 v-antenna


3.1 Introduction: The signal which is transmitted to the terrestrial system by the transponders in the sattellites to be received by a parabolic dish antenna.The PDA consists of chicken mesh which decreases the pressure on Antenna.The signal which is received by the PDA is made to pass through a horn antenna later it connects to the LNBC (low noise block converter).LNBC is an amplifying device used to downlink the signal from GHZ range to

STUDY OF LOW POWER TRANSMITTER MHG range using a local oscillator frequency of 5150 MHZ. After down converting the signal from GHZ range to MHZ range it is allowed to pass through a RF cable to sent the received amplified signal to satellite system unit. 3.2 Parabolic Dish Antenna: A dish is a parabola of rotation, a parabolic curve rotated around an axis which passes through the focus and center of the curve. The received signal can be transmitted to the low






noise block converter(LNBC) through feed horn antenna.

The azimuth and elevated angles and the direction of the parabolic dish are different for different satellites. They can be varied by using horizontal and vertical motors from the control room. The azimuth and elevated angles can be given by the doordarshan organization.The following table mention the polarization, azimuth and elevated angles of Dish, and the down linked signal information for different satellites. T.3.1 TABULARFORM



93.5 E

93.5 E

74 E

93.5 E


7.5 mt

6.3 mt

3.6 mt

6.2 mt

















Regional Signal

National and News


Standby mode National

The unlinking and down linking can be done in any one of the frequency bands available Cband,X-band,KU- band etc.Fore.g.the 100LPT at Srikakulam operated in C-band.The below tabular form shows the unlinking and down linking frequency ranges of various bands T.3.2 DIFFERENT BANDS AND THEIR FERQUENCY RANGES






Fixed, point to point


5.9 6.4

3.7 4.2

ground nonmilitary.




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7.9 8.4

7.25 - 7.75

aircraft).Radio military only. Broadcast and



fixednon 500MHz


14 14.5

11.7 12.2

point military


3.3 Low Noise Block-Down Converter:

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

A low noise block-down converter (or LNB) is the receiving device of a parabolic satellite dish antenna of the type commonly used for satellite TV reception. The device is sometimes called an LNA (for low noise amplifier), LNC (for low noise converter) or even LND (for low noise down converter) but as block-down conversion is the principal function of the device, LNB is the preferred term, although this acronym is often incorrectly expanded to the incomplete descriptions, low noise block or low noise block converter It is functionally equivalent to the dipole antenna used for most terrestrial TV reception, although it is actually waveguide based. Inside the LNB waveguide a metal pin, or probe, protrudes into the waveguide at right angles to the axis and this acts as an aerial, collecting the signal travelling down the waveguide.


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Fig 3.2 LNBC block diagram The LNB is usually fixed on the satellite dish framework, at the focus of the reflector, and it derives its power from the connected receiver, sent "up" the same cable that carries the received signals "down" to the receiver. The corresponding component in the transmit link uplink to a satellite is called a Block up converter (BUC).


4.1Introduction: The amplified downlink signal from the LNBC is given to the satellite system unit through a cable of impedance 75 and 6MHZ bandwidth. The satellite system unit protects the remaining system components from lightening and thunders. The received signal can be

STUDY OF LOW POWER TRANSMITTER given to the exciter through the different stages. They are integrated receiver decoder (IRD) s, audio programming amplifier, video programming amplifier TV demodulator and monitoring amplifier. A distortion/noise level meter is there to show the signal level. 4.2 IRD: IRD means an integrated receiver to decoder, which can be used to decode the original baseband signal from the received one. In the past analog receivers are used but at present analog systems are replaced with the digital receivers as they are more advantageous than the analog receivers. It takes the received signal as input and seperatesaudio and video signals from it. It has two output ports one is audio signal output and another is video signal output. The audio and video signals are given to the audio programming amplifier and video programming amplifier for correction purpose respectively.

Fig4.1 Integrated receiver decoder

4.3 AUDIO & VIDEO PROGRAMMING AMPLIFIER: The unit I designed using latest devices and technology. This unit provides distortion less audio output it has wide bandwidth and excellent signal to noise ratio(S/N ratio).This unit has both line and microphone inputs. A VU meter is present in the unit for monitoring and several other features such as short circuit protection in audio outputs and power supply. The applications are in audio signal processing and used in sound monitoring applications in studio, broad casting networks etc. The function of video programming amplifier is to correct the video signal.


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STUDY OF LOW POWER TRANSMITTER 4.3.1 AUDIO/VIDEO SWITCH: The audio switcher/electronics patch panel is used in monitoring and test equipment rack for a LPT.It receives the video signals and audio signals from different sources and selects any one of them for further processing and transmission, with minimum deterorationin the characteristics of the signal. Its special feature is that we can monitor any input channel been selected for transmission without disturbing normal transmission. The main use of the audio/video switcher is channel selection. 4.3.2 TV DEMODULATOR: It is precision monitoring equipment for checking the quality of a TV transmitter in the VHF and UHF bands. It reconstructs original baseband signal and connected to the exciter stage. 4.3.3 TV PATTERN GENERATOR: This unit consists of several LED lights .the signal from the receiver connected to the pattern generator. to check whether the circuit good or not. when we connect the signal to TV pattern generator ,then at the output(on TV set) we can observe the color pattern lines. 4.4 SYSTEM UNIT: This unit consists of two switches which are used to operate two motor is for adjusting the vertical angle of PDA. Another for adjusting the horizontal angle of PDA. 4.5 VESTIGIAL SIDE BAND TRANSMISSION: In the 625 line TV system, frequency components present in the video signal extend from 0 Hz to 5 MHZ. A double side band AM transmission would occupy a total bandwidth of 10 MHz. To reduce the channel bandwidth and power, Vestigial sideband Transmission is in practice. In the video signal very low frequency modulating components exist along with rest of signal. These components give rise to sidebands very close to carrier frequency which are difficult to remove by physically realizable filters. Suppressing one complete sideband also not possible. The low video frequency contains the most important information of picture

STUDY OF LOW POWER TRANSMITTER and any effort to completely suppress the lower sideband results in objectionable phase distortion at these frequencies; it will look in the picture as smear. Therefore only a part of lower side band is suppressed and radiates signal with full Upper Side Band together with carrier and vestige of the partially suppressed Lower Side Band. This is called V.S.B or A5C transmission. In the 625 line system, frequencies up to 0.75 MHz in the lower sideband are fully radiated. So it is a double sideband transmission for lower video frequency.

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

(Fig 4.2.1 F Curve of TV setTheoretical)


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4.5.1 RECEPTION OF VESTIGIAL SIDE SIGNALS: Because of filter design difficulties it is not possible to terminate the bandwidth of signal abruptly at the edges of sideband therefore attenuation slope covering 0.5 MHz is allowed at either end.

(Fig4.2.2 Response for VSB reception) Now these visual and aural signals are given to the exciter for further processing. In the exciter stage, blocks like video processing unit , diode bridge modulator , delay equalizer , V.S.B filter , video up converter , linear amplifier , power amplifier and diplexer and frequency multiplier process the video and audio signals. The combined visual and aural signal after arriving the diplexer block is transmitted to mast antenna.


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5.1 EXCITER: The audio and video output of the program to be connected to the Exciter unit. The audio input is feed to the aural modulator through blank module, while the video is passed through a video processor unit to its respective modulator. The audio is frequency modulated using 33.4MHZ IF while video signal is amplitude modulated using 38.9MHZ IF. The modulated signal are combined in combiner and then up converted to the desired transmitted channel frequency in IF channel convertor unit.


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Fig5.1 exciter The video output power level is 10MW synchronous peak while audio is 1MW. In this automatic level is available on real panel of the exciter, which can be feed from driver unit. Exciter needs 28V for operation, and its supplied from 28V, 25A PSU units which was connected in parallel as shown in block diagram.

A DC-to-DC converter provided in the exciter derives +5V to +15V to be supplied to individual sub systems in the unit.


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Exciter internal Block diagarm:

Video in 1 Vp-p Video 1 Vp-p 75 Processor 50 +3dbm 50 Vision Modulator IF Correction +3dbm 0dbm 50

IF Combiner -7dbm 50 Aural Modulator

IF +10dbm Channel Converter

Audio in

46dbm 600

Driver unit 25/30 W 50

Divider & Combiner unit 50 50W RF PA1 RF PA2 50 50W

fig 5.1.1.internal block diagram of exciter

Exciter is functionally divided into seven modules namely:


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STUDY OF LOW POWER TRANSMITTER 1) 2) 3) 4) 5) 6) 7) Video processor. Vision modulator. IF corrector. Aural modulator. IF combiner. IF channel converter. DC to DC converter.

5.1.1 VIDEO PROCESSOR: The externally applied video signal is feed to the video input amplifier. This amplifier has high input impedance in order to provide the high ohmic loop through facility. The amplified video signal is feed to the potentiometer and to the synchronous separator. The potentiometer control the amplitude of the video signal from the input amplifier and there by it determines the modulation depth. This signal is applied to switch directly as well as through receiver pre corrector. The group delay equalizer introduces a pre distortion of the video signal. The synchronous separator consists of detector and a comparator. The peak level of the synchronous pulse is applied to detector. The output from detector is used as reference level for the comparator, which means that the synchronous pulse is sliced at 50% of the synchronous peak level. The back porch of the synchronous pulse initiates the clamp pulse generator. During this clamp pulse period the transistor is off and saturated, where by the video signal is clamped to zero in the junction. The clamp pulse does not affect the 4.43MHZ band stop filter ensures that the color burst in the video signal. In the same period the synchronous separator is also clamped to zero.

STUDY OF LOW POWER TRANSMITTER When no synchronous is applied, the DC restore is switched. In this case the peak value of signal is maintained. A part of the video signal is also applied to the video level detector via low pass filter. The video level detector consists of a peak detector followed by a DC amplifier. The DC output goes to the meter in IF combiner unit.

5.1.2. VISION MODULATOR: The vision IF 38.9MHZ is generated by a TCXO and split in to three identical signals by means of power splitter, The IF carrier is feed to the vision modulator which is doubled balanced ring modulator. In order to obtain a carrier as well as the side bands from the modulator the applied video signal is properly clamped to zero in video processor unit. The white level is adjusted by means of R8. The capacitive balance is adjusted by means of C1 and C28. The VISION IF AMPLIFIER which is a wide band amplifier amplifies vision modulated. Amplifier further amplifies the vision IF to +10dbm. A SAW VSB filter performs VSB filtering of the side bands with minimum group delay and high reliability. Amplifiers compensate the insertion loss of VSB filter and finally band pass amplifier amplifies the vision IF to a level to 10dbm. The power splitter splits the amplified vision IF from amplifier to two identical signals, one for combiner and other for monitoring. 5.1.3 IF CORRECTORS:


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Fig5.2 IF corrector

Correctors are of two types. They are amplitude linearity and phase linearity corrector.
1)Amplitude linearity corrector: The transistors Q1 and Q2 form a linear amplifier, input to which is controlled by potentiometer RV1. Transistors Q3 and Q4 form a non-linear amplifier input to which is controlled by potentiometer RV2. Adjust RV1 for minimum input to transistor Q1. Check for amplified input signal at collector of Q4. Vary RV3 and check for appearance of distortion at Q4 output, because of non-linearity. Set RV3 to position where distortion just appears. Adjust RV2 for minimum signal at Q3 base. Check for distortion less amplified IF input at collector of Q2. Next adjust RV1 and RV2 approximately for 50% signal passage, and check for availability of amplified, IF at collector of Q5. Demodulate the IF output available at TP3 through a TV demodulator and measure the synchronous picture ratio. Vary RV2 and RV1 and ensure that the synchronous picture ratio can be varied by +/5%. Feed multi burst pattern from VTSG and adjust the frequency response to best possible extent by varying VC1 and VC2.

STUDY OF LOW POWER TRANSMITTER 2) Phase linearity corrector: The input signal is splits into 2 branches. Signal passing through Q8 and Q9 stage is delayed using delay line and the other signal is processed through a non-linear amplifier combining these two signals introduces phase difference in the output signal. Disconnect the cable from TP1 and TP2 and solder it on TP5 and TP6. Adjust RV7 for minimum signal at Q8 base and RV5 for maximum signal at Q6 base. Check for amplified IF signal at collector of Q7 and introduction of distortion by RV6. Set RV6 to position, where it just introduces non-linearity. Adjust RV5 for minimum signal at Q6 base and RV7 or maximum signal at Q8 base and check for the same IF signal at emitter of Q9. Adjust RV5 and RV7 for approximately equal signal at base of Q8 and Q6,and check for amplified IF signal at collector of Q10.

5.1.4 AURAL MODULATOR: Aural modulator unit consists of the following sections in a single PCB. 1) 2) 3) 4) 5) 6) Audio amplifier 1KHZ oscillator VCO Mixer APC Meter circuit


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Fig5.3 Aural modulator Audio amplifier: The balanced audio signal at a level of +6dbm from the TVRO is converted in to unbalanced signal by the op-amp U8. The output of U8 is taken through a front panel potentiometer R83 to the audio amplifier U7. The audio amplifier uses a hybrid micro circuit BNC 1003. A 50sec pre emphasis is provided, which can be adjusted by the potentiometer R87. The output of the audio amplifier is applied to the varactor junction of VCO to frequency modulate the VCO signal. 1 KHZ oscillator: The 1KHZ oscillator is consists of two operational amplifiers U9 and U10. The frequency of the oscillator can be adjusted by R94 and distortion in output signal is minimized by R92, oscillator is powered with negative supply only when front panel of audio selection switch is in INTERNAL position. The output of the oscillator is level adjusted by R99 for 50 KHZ deviation. VCO: The voltage controlled oscillator is a varactor tuned oscillator, the frequency of which can be varied manually by the coil L5. Transistor Q16 forms the oscillator. The output of the oscillator is taken through a buffer Q18, amplified by two stages Q19 and Q20. The output of one of the amplifiers Q19 is feed to the mixer in the modulator unit and the output

STUDY OF LOW POWER TRANSMITTER from the other amplifier Q20 goes to the IF combiner unit through front panel BNC connector. The output of the VCO is frequency modulated by the audio signal from the audio amplifier/internal 1KHZ oscillator, the DC bias current for the diodes is provided through the resistor R48. The output level of the VCO is about 0dbm. Mixer: The visual IF carrier from the vision modulator unit and the aural IF signal from the VCO are injected at the base of the mixer transistor Q1. The mixer output is at 5.5MHZ. This signal is amplified by a common emitter amplifier Q2, the output of which transformed into square pulses by the pulse shaping network formed by diodes D1 and D2. This square wave signal is further frequency divided by a chain of dividers to give an output square pulse at 537HZ which is feed to the APC. The division is carried out to minimize the phase error caused by the frequency modulation of audio signal in the VCO. APC: The automatic phase control circuit is a sample and hold circuit using a CMOS analog gate for sampling. The square wave signal from the divider chain of mixer is transformed into a triangular wave by means of transistors and the output is taken through a source follower. The output from the source follower is capacitive coupled to the analog gate. The sampling of reference pulses for the analog gate is derived from a temperature controlled crystal oscillator at 1.1MHZ, the output of which is frequency divided by a chain of dividers. The frequency if the signal from the divider chain is equal to that of the divided signal from the mixer under the locked condition of the phase locked loop. The analog gate acts as an error detector and the output of the gate is applied to the varactor junction of the VCO, to correct the frequency through a low pass filter formed by R31, R33, C18 and C30. Metering:


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STUDY OF LOW POWER TRANSMITTER The meter circuit consists of an AC amplifier and a peak detector followed by a DC amplifier. The DC output is feed to the modulation meter in IF combiner unit via back panel wiring. 5.1.5 IF COMBINER: It combines vision IF with sound IFS and composite IF. IF is automatically level controlled with feed back from IF channel converter. Vision IF is detected for presence of synchronous pulse and inter carrier is sensed to

declared system health.

Fig5.3 Block diagram of IF combiner 5.1.6 IF CHANNEL CONVERTER:


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STUDY OF LOW POWER TRANSMITTER This Units Converts the IF input to the channel frequency and attenuator at the input is used for setting the output level to the nominal level of 10dbm. A wide band double balanced mixer is used for frequency conversion. A TCXO provides LO input to the mixer and the up converted signal is filtered in a helical band pass filter. The signal at the output of the filter is further amplified using two hybrid amplifiers. A directional coupler is used for providing a monitor output at the rear of the module. The main output passes through another band pass filter to reduce further unwanted signals. The final output is available at the front. LO monitoring is provided on the front panel. This is achieved using a resistive divider network. 5.1.7 DC TO DC CONVERTER: DC-to-DC converters with very low input respectively output voltage ratings have, due to the high current ratings, a relatively low efficiency. Therefore, it is necessary to operate several converter stages in parallel to achieve an acceptable total efficiency. Here, a possible solution for such a converter is presented. The input voltage of a (e.g. a solar buffered) battery (12 V or 24 V) has to be converted into a DC-voltage of 350 V, e.g. for a power transmitter. The total power to be managed is 1 kW. In the case of a single stage inverter, this leads to about 100 A input current (causing peak values in the power switches of up to 200 A). The resulting component stress is very hard and the design is also difficult to handle. To overcome this problem a topology was chosen, which uses several converter stages operating in parallel. All these stages operate at the same transformer leading to an optimal flux exploitation of the core. In the case of the 1 kW converter described here, four stages are used. 5.2 PRE-EMPHASIS &DE-EMPHASIS In processing audio signals, pre-emphasis refers to a system process designed to increase, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.


De-emphasis is a process designed to decrease, within a band of frequencies, the magnitude of some frequencies ( usually earlier pre-emphasised ) with respect to the magnitude of other frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation differences or saturation of recording media in subsequent parts of the system. It is the mirror of preemphasis, 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.


{channel selected according to site requirement}


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3.2 3.3 4 4.1 4.2 5 5.1 5.2 6 6.1 6.2 7 7.1 7.2 8 9 Aural Colour VIDEO INPUT Level Return Loss CARRIER FREQUENCY Vision Carrier Aural Carrier IF FREQUENCY Vision IF Aural IF OUTPUT Level Aural to Vision Ratio INTER-MODULATION DISTORTION SPURIOUS -5.5 MHz +11 MHz 10 OUTPUT (Peak Power) STABILITY -55 dBc OR Better -55 dBc OR Better 0.3 dB OR Better 0 TO +10dBm 1 : 10 dB. -60 dBc OR Better 38.9 MHz 40 Hz. 33.4 MHz 40 Hz. Channel Frequency 150 Hz Channel Frequency 150 Hz 0.5V TO 1.5V p-p / 75 OHMS. -30 dB OR Better (0 TO 5 MHz) FM F3E PAL B


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6.1 Driver unit: The upconverted signal from the exciter is fed to an attenuator which is placed at the front panel for adjusting the input level suitably the signal is amplified using class A driver stages. The overall gain of the amplifier can be adjusted by the front panel attenuator control to be about 33db such that 25/50W (sync peak) will be available at the output at driver unit. The output of the amplifier is fed to a direction coupler where in samples of transmitted and reflected power is obtained and fed to metering unit which detects the signal and suitable voltage to a dc meter placed at the front panel. The S portion switch on the front panel selects the parameter to be monitored viz,vision,power,aural power & reflected power readings are to be read with black picture an


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STUDY OF LOW POWER TRANSMITTER aural power indication is valid along with black picture only.A portion of the output power is takeof the front panel if the driver unit for monitoring purposes. The back panel output called ALC can be fed to exciter EXTALC is at rear panel to keep the driver output constant.The availability of the output power 28v to the unit is indicated through a seen LED an the front panel.DC Check facility in provided to moniter currents of four stages of power amplifier by indicating a chord to DC Check meter on divider/combiner unit

Block diagram of Driver Unit:

Antenn a
Video in Excite Audio in r RF RF out Drive ou r unit t A L C

Feed back signal

Divider & Combin er unit RF RF ou ou t t PA PA 1 2

RF in R F in

Fig 6.1 driver unit 6.2 Divider and Combiner Unit: This unit caters for three purposes:

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STUDY OF LOW POWER TRANSMITTER 1)To divide the power from driver unit which is to be fed to two final power amplifier. 2)To combine the output power from the 50w power amplifier to obtain 100w sync peak power and make available at antenna port. 3)To provide filtering for spurious sidebands using notch filters of frequency -5.5MHZ & fs+5.5MHZ[Abstract] Monitor unit detects the samples of forward and reflected power from the power amplifier &feedssuitable DC voltage to front panel meter for monitoring vision,reflected and aural power.the current/voltmeter and rotator switch provided at the top front panel are used to monitorthe current drawn by various transistor stages of dividerunit and power amplifier unit. 6.3 Power Amplifier Unit: This unit comprises of two similar 50w power amplifier modules.RF power is fed to driver unit and if divider into two signals and feed it to divider/combiner and then to each 50watt power amplifier. Each power amplifier is fed with power input which is amplified to 50 watt(sync peak) by four class A parallel power amplifier stages with a gain at approximately 10dbm.This output is fed to a direction coupler to obtain samples of forward & reflected power for monitoring purposes for the control unit. Thermistor serves the temperature at heat assembly & provides DC voltage green led is mount over the front panel to indicate normal when FWD power reflected power & heat sink assembly temperature are within satisfactory limits.where any of three red LEDs indication denotes the parameter exceeds beyond the limits the control with cutsoff the transistor bias from bias assembly placed in power amplifier unit, then tripping off the power amplifier.The system can be restored back after rectifying the fault & restoring the indications in the front panel using the push button switch RESET. Seperate power supply available for each power amplifier(28V,20A) placed at bottom portion of chauin assembly.A DC voltage is

proportional to current drawn by each at the transistors in power amplifier is available from bias unit on DC check connector placed over the front panel. 6.4 VSWR Measurement :

STUDY OF LOW POWER TRANSMITTER After installing the antenna panel, junction box and U links it is verified whether the panels are mounted as required. It is also verified if the correct lengths of branch feeder cables are used for each face. The d.c resistance is measured by using a bridge connecting it between inner and outer conductor of the U link or junction box. The d.c resistance is calculated as follows: If R is the d.c resistance from U link or junction box

R = a + b + c + d + e milliohms N Where, a = Antenna pannel ( about 5 m ) b = Branch feeder cable ( depends on the cable size and length and is measured separately) c = Contact resistance of the junction box terminal, 1m d = Junction box, 1m e = U link If the d.c resistance is very much higher than the calculated value it may be due to bad contact of the branch feeder cable with the junction box or poor contact of other matting surface. If the d.c resistance is higher than the nominal value, tap all the contact portions with a rubber hammer observing the meter in measuring instrument. The meter needle will kick back and forth while tapping the portion where the contact is poor and so the defect will be rectified. Measure the VSWR at the U link or junction box and by terminating the other U link with 50. Repeat the same procedure at the other U link input end. The VSWR should be less than or equal to 1.05 at vision carrier frequency and 1.1 at other frequencies in the channel. If VSWR is high it can be improved by slightly adjusting the pannels. Improper fixing of branch feeder cables can also deteriorate the VSWR . A platform is provided at the U link level ( JB level ) to keep the measuring instruments to perform tests.Thus VSWR measurement is carried out at doordharshan.


This report is a case study on the 100 watt low power transmitter that how the signals are receiving from the satellite ,their processing and transmission in the terrestrial system. The transmitter service involves the great equipment that deals with monitoring section exciting system and we learn about the equipment of the doordarshan center and its working We learned about the procedure of transmission, reception and strengthening of the signal and retransmitting the signal into the space for the broad cast around the range of propagation.


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