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G.H.RAISONI COLLEGE OF ENGINEERING, NAGPUR

Department:-Electronics & Communication Engineering

Branch:- 7th

Semester[Electronics & Telecommunication]

Subject:- Television Engineering

List of Experiments

CYCLE -I

1. To study picture tube of monochrome television.

2. To study in detail, circuit of RF tuner section through various test points, the fault

simulation and step-by-step fault finding.

3. To observe the composite video signal at the output of VIF section

4. To study in detail, circuit of SIF and video section through various test points, the

fault simulation and step-by-step fault finding.

5. To study the Horizontal oscillator section through various test points and to study

the fault simulation and step-by-step fault finding.

CYCLE-II

6. To study the Horizontal output section and vertical oscillator section and to

through various test points and to study the fault simulation and step-by-step fault

finding.7. To study the detail circuit of video and chroma section through various test points

and to study the fault simulation and step-by-step fault finding.

8. To study the detail circuit of System Control Section through various test points

and to study fault simulation and step-by-step procedure.

9. To plot the radiation pattern of single dipole and folded dipole antenna using

antenna trainer kit.

10. (a)To setup an active satellite link and to demonstrate link fail operation.

(b) To establish an audio/video satellite link between transmitter and receiver.

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Experiment No.1

Aim: -To study picture tube of monochrome television.

Apparatus: -Monochrome Television trainer kit.

Theory: -

Picture tube is a transducer to convert electrical video signals varying with time intovariations of light in space to reproduce the original picture. It uses the phenomenon of

fluorescence and scanning.

Monochrome picture tube consists of an evacuated glass envelope, housing an electron

gun to produce a sharply focused electron beam and a fluorescent screen to produce light

when the electron beam strikes the phosphor element on the element. Also there are

devices to deflect the electron beam horizontally and vertically to enable it to scan thescreen through width and height to reproduce the original picture. The functions of

various of picture tube are discussed below:

Electron Gun:It produces a well-focused electron beam, using thermionic emission of electrons from a

Cathode and few grids for controlling, accelerating, and focusing the electrons.

Cathode:

It consists of a thoriated oxide of tungsten which is heated by a heater wire placed closedto the cathode but without touching it electrically. The material between the heater and

the cathode is a thermal conductor, but it is an electric insulator. Such a cathode known as

indirectly heated cathode.

Control Grid 1:The element next to cathode is the control grid. It is a metallic cylinder of nickel, and has

a small aperture, which allows the electrons to pass through. Control grid is kept at

negative potential with respect to cathode. The grid bias is 30 to 50V variable.Negative grid bias enables the control grid to controls the space charge of the electron

coming out of the cathode. This controls the brightness of the screen.

Screen Grid 2:It is a metallic cylinder of nickel, which is kept at positive potential with respect to

cathode. This grid accelerates the electron in the beam. It is therefore also called asaccelerating grid. The effect of voltages on the screen and the control grid is such a that

the electrons converges in the space between these two grids.

Focusing Grid:This grid provides the required the electrostatic field to prevent spreading of the electron

and help in producing a sharp spot on the screen.

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Final Anode:

To cause fluorescence in the phosphor material, the electrons should strike the screenwith great velocity. The required high velocity is imparted by the final anode, which is

coated to a long conducting coating of aquadag inside the wide bell of the tube. The

aquadag coating extends from the screen to the narrow neck. It is given a very high

potential, called EHT.

Fluorescent screen:It consists of rectangular faceplate made of an optically flat glass of high quality. The

glass is 1.5 cm thick to withstand the outside air pressure against the internal vacuum.

There is a very thin aluminum coating on the back of the phosphor surface. It improvesthe brightness by reflecting the light from back to front. Also the aluminum coating stops

the ions to reach the phosphor coating, which permanently damages the screen.

Deflection Coil:Electron beam is deflected by magnetic deflection. The saw tooth currents flow in the

horizontal and vertical deflection coils which produces the magnetic fields which deflectsthe beam in the picture tube. The mounting block containing deflection coils is called asyoke.

Result: - Cross-sectional view of the picture tube is studied.

Viva Question:-

1. Why electrostatic focusing is preferred over electromagnetic focusing?

2. What is the function of aquadag coating?

3. At what potential all the grids maintained?

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Experiment No: - 2

Aim: -To study detail circuit of RF tuner section through various test points and tostudy the fault simulation and step-by-step fault finding.

Apparatus: -Monochrome TV trainer kit, CRO, CRO probes, and multimeter.

Block Diagram: -

Theory: -This section consists of RF amplifier, mixer and local oscillator and is normally mounted

on a separate sub-chassis, called the Front end or RF tuner. The purpose of the tunerunit is to amplify both sound and picture signals picked by the antenna and to convert the

carrier frequency and their associated bands into intermediate frequencies and their

sidebands. The receiver uses the superheteordyne principle as used in the radio receivers.

RF tuner consists of RF amplifier, local oscillator and mixer. The function of tuner is to

select a single channel signal form the various signals picked up by the antenna, amplifyit and mix it with the continuous wave output of the local oscillator. The function of thelocal oscillator is to tune the desired station.

Adjusting the tuned circuits of all the three stages does channel selection. Each tunedcircuits consists of a coil and a capacitor. The resonating capacitance consists of a

distributed capacitance of the circuit and small fixed ceramic capacitors. The fine-tuning

control is varied to obtain exact picture & sound IF. The IF signal form the tuner iscoupled to the first picture IF amplifier through a coaxial cable. The AGC line is

connected from the main circuit to the tuner circuit. The tuner is enclosed in a compact

shielded box.

Balun Transformer:The tuner must have input impedance equal to the characteristic impedance of the

antenna for maximum signal transfer to avoid reflections on the line. The Baluntransformer is used to match the twin wire ribbon feeder impedance of 300 ohms to the

75 ohms input impedance of RF amplifier.

Balun Trap Ckt. RF

amplifier Mixer

Local Osc.

Picture

IF=38.9MhzSound

=

AFT signalAGC signal

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HP Filter and Trap circuits:

Unwanted signals which lie in the IF band of 32 MHz to 42 MHz are rejected by the twotrap circuits connected on either side of the HP filter. The HP filter has a cut-off

frequency of 45 MHz.

RF Amplifier:The RF amplifier provides adequate gain to weak signals. It maintains high signal to

noise ratio at the mixer. RF amplifier prevents radiation from local oscillator & rejectsimage frequency signals.

Local oscillator:The Local oscillator generates an unmodulated sinusoidal voltage in the mixer with the

incoming RF signal to affect its frequency translation to the IF band. The LO frequency is

changed whenever a different channel is selected to maintain difference equal to the IF

between the LO frequency & the tuned channel carrier frequency.

Mixer Stage:The function of the mixer stage is to convert the incoming RF signal frequency fromdifferent channels into a common IF pass band of the receiver. The mixer stage & LO in

combination can be considered as frequency converters.

Following are the factors on which design of tuner depend:

1. Choice of IF & local oscillator frequencies.2. Need for an RF amplifier stage.

3. Coupling networks.

There are two types of tuner: 1. VHF Tuner

2. UHF Tuner

Procedure: -(1) Study the circuit diagram of RF tuner.

(2) Observe the various signals at various test points provided on the kit.(3) Simulate the fault using the jumper arrangements provided on the kit.

(4) Go with the step-by-step procedure to detect the fault as given in the trainer kit

manual.

Result: -The RF tuner is the first block of the TV receiver which select a particularstation and rejects all the other stations. It converts the input signal into IF signal. It

receives the signals like AGC and AFT from other sections of the receiver.

Viva Question:1. What is the function of BALUN transformer?

2. What is the use of ganged tuning?3. How is combined IF of the signal?

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Experiment No. 3

Aim: - To observe the composite video signal at the output of VIF section.

Apparatus: -Monochrome TV trainer kit, CRO, CRO probes, and multimeter.

Theory: -

Composite video signals consists of a camera signal corresponding to the desired picture

information, blanking pulses to make the retrace invisible & synchronizing pulses tosynchronize the transmitter and receiver scanning. A horizontal synchronizing pulse is

needed at the end of each active line period whereas a vertical sync pulse is required after

each field is scanned. Horizontal and vertical sync pulses must have same amplitude toget higher efficiency of the picture signal transmission but their duration should be

different so that they can be separable at the receiver.

The level of the video signal in the composite video signal corresponds to the maximumwhiteness of the signal transmitted. This level is called as peak- white level, which is

fixed at 10 to 12.5 % of the maximum value of the signal, and the black level corresponds

to about 72%.

The sync pulses that are added at 75 % level called as blanking level. The white level is

referred to as pedestal. Thus the picture information may vary between 10% to 75 % ofthe composite video signal depending on the relative brightness of the picture at any

instant. The darker the picture, higher will be voltage within those limits. The lowest 10%

of the voltage range is not used to minimize noise effects. At the receiver, the picture tubeis biased to ensure that a received video voltage corresponding to about 10 % modulation

gives complete whiteness on the screen.

Composite video signal has an average value or DC component corresponding to the

average brightness of the picture. In the absence of DC component, the receiver can not

follow the changes in the brightness.

The composite video signal contains blanking pulses to make the retrace lines invisible

by raising the signal amplitude slightly above the black level during the time, the

scanning circuits produces retraces. The composite video signal contains the vertical andhorizontal blanking pulses to blank the corresponding retrace intervals. The repetition

rate of horizontal blanking pulses is equal to the line scanning frequency of 15625 Hz.

Similarly the frequency of vertical blanking pulses is equal to the field frequency of 50Hz.

The theory should cover following points,

Detail diagram of Composite video signal.

Detail description of each and every parts of composite video signal.

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Procedure: -1)Search the test point in the VIF section at which you can observe the composite

video signal.

2)Observe each and every part of composite video signal and draw it in thepractical file.

Result: -The composite video signal consists of video signal corresponding to picture details,

synchronizing pulses, and the blanking pulses. The horizontal and vertical sync. pulses

are used to achieve the horizontal synchronizing and vertical synchronizing between thetransmitter and receiver. The blanking pulses are used to blank the screen during

horizontal and vertical retrace.

Viva Question:1. Define pedestal height?

2. Give the significance of P/S ratio.3. Why sync pulses are added at the blanking level?

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Experiment No: -4

Aim: -To study the detail circuit of SIF and video section through various test pointsand to study the fault simulation and step-by-step fault finding.

Apparatus: -Monochrome TV trainer kit, CRO, CRO probes, and multimeter.

Theory: -

The SIF section consists of various subsections such as SIF amplifier section, FMdetector section, audio pre amplifier section, and electronics attenuator for volume

control. The SIF signal is separated from the composite video signal is given to the SIF

amplifier. The amplified SIF signal is given to the in built SIF detector (FM detector)section.

The main function of this section is to separate the carrier frequency from the SIF signal.

The carrier frequency SIF signal is termed as audio signal. This audio signal is then fed to

the audio amplifier section. The amplifier section amplifies the audio signal and passes itto the volume control sub section. The purpose of the volume control is to control the

audio signal. This controlled audio signal is given to the sound output section.

The sound output section consists of sound driver and sound output subsections. Thepurpose of the sound driver is to amplify the signal. The amplified version of the audio

signal is further sent to the audio output section. Audio output section amplifies the audio

signal up to peak level.

The theory should cover following points, Detail circuit of SIF and video section Description of the circuit in detail.

Procedure: -1) Study the circuit diagram SIF section and video section.

2) Observe the various signals at various test points provided on the kit.

3) Simulate the fault using the jumper arrangements provided on the kit.4) Go with the step-by-step procedure to detect the fault as given in the trainer kit

manual.

Result: - The SIF section consists of the SIF amplifier, FM detector, and audio preamplifier. In the video section the signal is processed before giving it as an input to the

picture tube for reproduction of the received signal.

Viva Question:1. Which IC is used in sound section of the monochrome TV trainer kit?2. Why FM is used for sound transmission?

3. List the audio amplifier IC known to you?

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Experiment No: -5

Aim: -To study the Horizontal oscillator section through various test points and to studythe fault simulation and step-by-step fault finding.

Apparatus: -Monochrome TV trainer kit, CRO, CRO probes, and multimeter.

Theory: -

The Horizontal section consists of following subsections, the sync amplifier, syncseparator, AFC section, and Horizontal oscillator amplifier section. The sync pulses

separated from the composite video signal are fed to the sync amplifier. The sync

amplifier amplifies the sync signal. The amplified sync signal is given to the sync

separator section, which separates the horizontal sync signal and vertical sync signal fromthe original amplified sync signal. After this separation, vertical sync signal is given to

the vertical section and simultaneously horizontal sync signal is given to the AFC section.

AFC section controls the phase level of the horizontal sync signal and converts it into DCcontrol voltage termed as AFC. This AFC signal flows to the horizontal oscillator

section. This section generates the horizontal line signal of 15625 Hz. The oscillated

signal is amplified by the horizontal amplifier and given to the driver section.The theory should cover following points,

Detail circuit of horizontal oscillator section.

Description of the circuit in detail.

Procedure: -1)Study the circuit diagram of horizontal oscillator section.

2)Observe the various signals at various test points provided on the kit.

3)Simulate the fault using the jumper arrangements provided on the kit.4)Go with the step-by-step procedure to detect the fault as given in the trainer kitmanual.

Result: -The horizontal section consists of sync amplifier, sync separator, AFC section,horizontal oscillator section. The horizontal section generates the saw tooth current of

frequency 15625Hz to move the electron beam in the horizontal direction.

Viva Question:1. Define: Front porch, Back porch.2. Whats the significance of front porch and back porch

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Experiment No: -6

Aim: -To study the Horizontal output section and vertical oscillator section various testpoints and to study the fault simulation and step-by-step fault finding.

Apparatus: -Monochrome TV trainer kit, CRO, CRO probes, and multimeter.

Theory: -

The vertical sync signal obtained from the sync separator section is given to the verticaloscillator section. This section now generates the vertical signal of 50 Hz. The oscillated

vertical signal of 50 Hz is now synchronized along with vertical sync signal. To maintain

the vertical signal of 50 Hz generated by vertical oscillator section, one control

mechanism is connected. This control mechanism is termed as vertical hold control.The horizontal output section is also called as EHT section. An anode voltage of the order

of 15 KV is needed for sufficient brightness in black and white picture tubes. This is

known as HV or EHT supply. To obtain such a high voltage by stepping up the mainvoltage with a transformer is almost impossible and prohibited in cost. A different

method of this high voltage generation is used. During the retrace intervals of horizontal

scanning, high voltage pulses of amplitude between 6 to 9 KV are developed across theprimary winding of the horizontal output transformer. These are step up by an

autotransformer winding of about 10 to 15 KV and fed to a high voltage rectifier. The

output of the rectifier is filtered to provide required dc voltage.The theory should cover following points,

Detail circuit of horizontal output and vertical oscillator section.

Description of the circuit in detail.

Procedure: -1) Study the circuit diagram of horizontal output section and vertical oscillator

section.

2) Observe the various signals at various test points provided on the kit.

3) Simulate the fault using the jumper arrangements provided on the kit.

4) Go with the step-by-step procedure to detect the fault as given in the trainer kitmanual.

Result: -The horizontal section generates the EHT voltage, which is given as the input to the

internal aquadag coating of the picture tube. The vertical oscillator section generates the

saw tooth current of frequency 50 Hz that is fed to the vertical deflection winding toachieve the vertical movement of the beam inside the picture tube.

Viva Question:

1. How much is the frequency of horizontal scanning & vertical scanning?

2. Specify the timing required for horizontal scanning & vertical scanning.

3. At what time the sync pulses are added during the scanning process?

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Experiment No.7

Aim: - To study the detail circuit of video and chroma section through various testpoints and to study the fault simulation and step-by-step fault finding.

Apparatus: -Colour TV trainer kit, CRO, CRO probes, and multimeter.

Theory: -The chroma section consists of chroma amplifier, colour oscillator, colour killer detector,

matrix, and luminance signal amplifier. From the composite video signal the Y signal isdetected and it goes to Y amplifier. The signal corresponding to red green and blue colour

is separated in this section.

Any colour has three characteristics. These are:

1. Luminance2. Hue or tint

3. Saturation

The hue and saturation of a colour together is called as chrominance.The theory should include following points

Detail circuit of video and chroma section.

Description of circuit diagram in detail.

Procedure: -(1) Study the circuit diagram of video and chroma section.(2) Observe the various signals at various test points provided on the kit.

(3) Simulate the fault using the jumper arrangements provided on the kit.

(4) Go with the step-by-step procedure to detect the fault as given in the trainer kitmanual.

Result: - The colour composite video signal, luminance signal and sub carrierfrequency is observed in the video and chroma section.

Viva Question:

1. Give the significance of chromaticity diagram.

2. Define Hue, Saturation and Luminance.

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Experiment No. 8

Aim: -To study the detail circuit of System Control Section through various test pointsand to study fault simulation and step-by-step procedure.

Apparatus: -Colour TV trainer kit, CRO, CRO probes, and multimeter.

Theory: -In almost all the TV receivers the circuits are provided to adjust volume, brightness,

contrast, channel selection etc. For various system control purposes either remote controlor front panel control is provided. The remote control enables the TV viewer to operate

from a distance. Most of the controls provided on the front panel are also provided on the

remote control.

The TV trainer has IC 901 FONDA 4ST for system control purpose. It is dual in linepackage 40 pins IC that controls all the sections of the TV trainer. Pin 1, 2 and 3 are used

for band switching purpose. These pin transfers the signal to the tuner section. The pin 5,

6, 7, 21 and 22 are the key matrix pins, which are connected with the keyboard. Keyboardis used for selecting one function from the front panel. Pin 11 is connected to the pin 5 of

the memory IC. Whenever we store any channel or put it into memory the memory

signals are stored in the memory IC with the help of these pins. The pin 28 is stand byinput pin power ON/OFF signals are obtained here. Pin 34 is infrared remote input pin

ON pressing the remote switches. The signals are obtained first at this pin for any

function from remote switches. The remote provides the infrared waves which falls onsensor EYE ON front panel. This eye recognizes the signal and amplifies it and gives to

pin 34. So signal work inside the IC for which remote key was pressed. Pin 36 is the

colour control pin. The pin 38 is brightness control pin. Pin 39 is volume control pin.The theory should include following points

Detail circuit of System control section. Description of circuit diagram in detail.

Procedure: -1) Study the circuit diagram of system control section.

2) Observe the various signals at various test points provided on the kit.3) Simulate the fault using the jumper arrangements provided on the kit.

4) Go with the step-by-step procedure to detect the fault as given in the trainer kit

manual

Result: -The system control section controls all the operations of the receiver such as channelselection, brightness control, contrast adjustment etc.

Viva Question:1. If the remote control is not working, then what may be the probable causes for

such condition?

2. What do you mean by chrominance?

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Experiment No. 9

Aim: -To plot the radiation pattern of single dipole and folded dipole antenna usingantenna trainer kit.

Apparatus:- Antenna trainer kit, single dipole and folded dipole antenna.

Theory:-

The antenna is a reciprocal device, means it radiates or receives electromagnetic energyin the same way. Thus although the radiation pattern is identified with an antenna that is

transmitting power, the same properties would apply to the antenna even if it was

receiving power. Any difference between the received and radiated powers can be

attributed to the difference between the feed networks and the equipment associated withthe receiver and transmitter. The antenna radiates the greatest amount of power along its

bore sight and also receives power most efficiently in this direction.

The radiation pattern of an antenna is peculiar to the type of antenna and its electricalcharacteristics as well as its physical dimensions. It is measured at a constant distance in

the far field. The radiation pattern of an antenna is usually plotted in terms of relative

power. The power at bore sight, i.e. at the position of maximum radiated power, isusually plotted at zeros degrees; thus, the power in all other positions appears as a

negative value. In other words the radiated power is normalized to the power at bore

sight. The main reason for using dB instead of linear power is that the power at the nullsis often of the order of 10,000 times less than the power on the bore sight which, means

that the scales would have to be very large in order to cover the whole range of power

values.The radiation pattern is usually measured in the two principal planes, namely, the

azimuth and the elevation planes. The radiated /received dB is plotted against the anglethat is made with the bore sight direction. If the antenna is not physically symmetrical

about each of its principal planes, than one can also expect its radiation pattern in theseplanes to be unsymmetrical. The radiation pattern can be plotted using the polar or the

rectangular / Cartesian co-ordinates.

Procedure: -1. Keep the main unit on the table and connect power cord. Check the mains voltage

and switch on the unit.

2. Mount simple dipole (/2) on the transmitting mast.3. Bring the detector assembly near to main unit and adjust height of both

transmitting and receiving antenna same.4. Keep detector assembly away from main unit approximately 1.5 m and align both

of them. Ensure that there are no reflector sort things in the vicinity of the

experiment such as steel structure, pipes, cables etc.

5. Keep the RF level and FS adjust to minimum and directional coupler switch toFWD.

6. Keep detector level control in the center approximately

7. Increase RF level gradually and see that there is deflection in the detector meter

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8. Adjust RF level and detector level so that the deflection in the detector meter is

approximately 30-35 uA.9. Align arrow mark on the disk with zero of the goniometer scale.

10. Start taking the reading at the interval of 5 or 10 degree.

11. Convert the uA readings of detector assembly in to dBs, with the help of the

conversion chart given at the end of this workbook.12. Plot the polar graph in degrees of rotation of antenna against level in the detector

in dBs.13. Calculate the following with the help of graph.

a) Beam width

b) Front / Back ratio.c) Gain of the antenna

Result:-

The radiation pattern of simple dipole antenna and folded dipole antenna is as shown in

graph.

Viva Question:-1. Which type of antenna is used for commercial purpose and military purpose?2. What is the significance of side lobes in the radiation pattern?

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Experiment No.10

Aim: - (a)To setup an active satellite link and to demonstrate link fail operations.(b) To establish an audio/video satellite link between transmitter and receiver.

Apparatus: -

Uplink Transmitter, Downlink Transmitter, Dish antennas, Connecting cables, SatelliteTransponder

Theory: -

Satellite communications is one of our most rapidly growing and evolving technologies

bringing with it a multitude of business opportunities in the decades to come.The satellite communication is essentially a microwave link repeater. It receives the

energy beamed up at it by an earth station and amplifies and return it to earth at a

frequency of about 2 gigahertz away, this prevents interference between the uplink and

the downlink. Communication satellite appears to hover over given spots above theequator. This is to say, they have same angular velocity as the earth, and so they appear to

be stationed over one spot on the globe.

The satellites are always stationed in round or elliptical orbit. Elliptical orbits are notgood because the speed gets varied. There are mainly two types of antenna on the

satellite, one for receiving and one for transmitting the signal. These satellites are

basically repeater stations. These stations are stationed at a specific angle and directionfrom the earth. The speed of the satellite is same as the earth. These satellites are also

known as Geosynchronous satellite.

The various application of Satellite are:Satellite Television

Direct Broadcast Satellites

Cable TVDirect Home Reception

Telephone Service via satellite

Data communication servicesData broadcasting using SatelliteInteractive data communication

Procedure: -1. Connect the satellite uplink transmitter to AC mains.2. Switch On the transmitter by mains switch and frequency display will come on.

3. The transmitting frequency can be selected by UP-DOWN switch. The frequency

can be changed from 1200-1250-1300 MHz.

4. The transmitter ON-OFF toggle switch will switch on off the transmission.5. Connect X1 antenna to uplink transmitter to maximum.

6. Set the o/p gain of uplink transmitter to maximum.

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7. Place downlink receiver at a convenient distance of 5-7 m.

8. Connect the downlink receiver to the AC main and switch it on by mains switch.9. The downlink receiver can be changed from 1100 1150 1200 MHz.

10. The downlink receiver also has tuning potentiometer, which can be used to tuned

any frequency from 950 1500 MHz.

11. Keep the tuning pot fully anti-clockwise12. The downlink receiver on-off toggle switch will switch on off the receiver.

13. Attach R2 antenna to the downlink receiver with BNC BNC lead.14. Align both the transmitter and receiver antennas in line.

15. Place a satellite transponder between transmitter and receiver at a convenient

distance of 5-7 m.16. Connect the satellite transponder to AC mains and switch it on by mains switch.

17. The receiver side of the satellite transponder has an on off toggle switch, which

will switch off the receiver of the satellite. Similarly on off toggle switch on the

transmitter side will switch off transmitter of satellite.18. Adjust transmitter uplink frequency to 1300 MHz and transponder receiver

frequency to 1300MHz.19. Keep downlink frequency of transponder to 1100Mz.20. Keep the downlink receiver to 1100MHz.

21. Now connect a tone output to tone input of the uplink transmitter by patch cord.

22. Keep the downlink voice switch in the on position & you will be able to hear tonein the speaker of receiver.

23. This is a test link for active satellite communication.

24. Connect any other audio signal to the audio II of uplink transmitter and you will

hear the music in the speaker of downlink receiver.

Result:-

The above shows that a successful satellite communication link has been set upbetween transmitter and receiver.

Viva Question:-1. What do you mean by passive satellite?

2. What is transponder? What are their different functions?3. Why microwave frequency is used for satellite communication?