TV_Unit_4

DHANALAKSHMI SRINIVASAN

COLLEGE OF ENGINEERING (An ISO 9001:2008 Certified Institution)

Coimbatore-641 105

B.E./B.Tech. DEGREE EXAMINATION

VII Semester

Electronics and Communication Engineering

080290057 TELEVISION AND VIDEO ENGINEERING

QUESTION & ANSWER BANK

2013-14

VII Sem ECE 080290057 Television and Video Engineering

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UNIT IV

PART A (2 Marks)

1. What are the advantages of SAW filter? May/June 2013

The advantages of Surface Acoustical Wave filters (SAW filters) are,

They are bandpass filters having sharp cut-off.

They can achieve the desired shape of the frequency response curve before detection.

2. What is the purpose of an AGC circuit in a TV receiver?

Nov/Dec2012, May/June 2011

Automatic Gain Control (AGC) circuit controls gain of RF and IF stages. AGC enables to

deliver almost constant signal voltage to the video detector irrespective of changes in the signal

picked up by the antenna.

3. How pincushion error can be corrected in colour picture tube? Nov/Dec2012

Pincushion error can be corrected in colour picture tube by two methods:

E-W (horizontal) pincushioning

N-S (top and bottom) pincushioning

4. Why do we prefer horizontal polarization for Television receiving antennas?

May/June 2012

We prefer horizontal polarization for Television receiving antennas because it results in

more signal strength, less reflection and reduced ghost images.

5. Where can you employ indoor receiver antennas? May/June 2012

In strong signal areas, it is sometimes feasible to use indoor antennas provided the

receiver is sufficiently sensitive.


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6. What is meant by degaussing a colour TV picture tube? Nov/Dec2011

Degaussing is the process of demagnetizing iron and steel parts of the picture tube

mountings.

7. What is the function of RF amplifier in a TV receiver? May/June 2011

When relative weak signals are received, there may be problems such as less signal to

noise ratio, local oscillator radiation and image rejection. Therefore a stage of RF amplification

is needed ahead of the mixer.

8. What is EHT and why is it required? May/June 2010

EHT is Extra High voltage which is required in a TV receiver for the picture tube. Such

high voltages are required so that the electrons are able to strike the phosphor elements on the

fluorescent screen of the picture tube with sufficient force for comfortable viewing.

PART B

1. (a)(i)Draw a VHF/UHF tuner with AFT and explain its working. (8 marks)

May/June 2013

2. (b) Present the detailed block diagram of UHF tuner and describe the functions of each

section. Why is its output connected to UHF tuner mixer? (16 marks)

May/June 2012

The block diagram of VHF/UHF tuner is shown below.


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Separate antennas are used for VHF and UHF channels. The UHF tuner does not employ

an RF amplifier because the input signal strength being very low is comparable to the noise

generated in the RF amplifier. Therefore, the incoming UHF signal is directly coupled to the mixer

where a diode is used for heterodyning instead of a transistor or a special tube.

The reason for using a diode is two-fold.

Firstly, the local oscillator output at ultra high frequencies is too low to cause effective

mixing with an active device whereas it is adequate when a diode is employed.

Secondly the noise level with a diode is lower than when an active device is used. Since the

diode is a non-linear device it can produce beating of the incoming channel frequencies

with the local oscillator frequency to generate side-band frequencies.

The weak output from the UHF tuner is coupled to the VHF tuner where both the RF

amplifier and mixer stages acts as IF amplifiers to boost the signal to the level normally obtained

when a VHF channel is tuned.

3. (a)(ii) Draw the cross sectional view of a Trinitron picture tube and explain its working.

(8 marks)

May/June 2013

4. (b) Elaborate on the following with reference to television receiver systems.

(i) Trinitron (8 marks)

May/June 2012


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5. (b)(i) With the help of a neat diagram, explain the gun structure and focusing details of a

Trinitron colour picture tube. (8 marks)

Nov/Dec 2011

6. (b)(ii) Explain the working of colour picture tube. (8 marks)

May/June 2011

Trinitron colour picture tube is shown below. It employs a single gun having three in-line

cathodes. This simplifies constructional problems since only one electron gun assembly is to be

accommodated.

The three phosphor triads are arranged in vertical strips as in the P.I.L. tube. Each strip is

only a few thousandth of a centimetre wide. A metal aperture grille like mask is provided very

close to the screen. It has one vertical slot for each phosphor triad.

The three beams are bent by an electrostatic lens system and appear to emerge from the

same point in the lens assembly. Since the beams have a common focus plane a sharper image is

obtained with good focus over the entire picture area. All this simplifies convergence problems and

fewer adjustments are necessary.

Fig: Trintron (cathodes in-line) colour picture tube (a) gun structure (b) electron beams, vertical-

striped three colour phosphor screen (c) constructional, focus and convergence details.


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7. (b)(ii) Explain the automatic degaussing and pincushion correction operations of a colour

TV picture tube. (8 marks)

May/June 2013

8. (b) Explain the following

(i) Pincushion correction techniques (8 marks)

May/June 2010

Automatic Degaussing:

Automatic degaussing circuit uses a thermistor and a varistor for controlling the flow of

alternating current through the degaussing coil.

When the receiver is turned on the ac voltage drop across the thermistor is quite high (about

60 volts) and this causes a large current to flow through the degaussing coil. Because of this heavy

current, the thermistor heats up, its resistance falls and voltage drop across it decreases. As a result,

voltage across the varistor decreases thereby increasing its resistance. This in turn reduces ac

current through the coil to a very low value.

The circuit components are so chosen that initial surge of current through the degaussing

coil is close to 4 amperes and drops to about 25 mA in less than a second. Once the thermistor

heats up degaussing ends and normal ac voltage is restored to the B+ rectifier circuit.

Pincushion Correction

Figure below is the sketch of a raster with a much exaggerated pincushion distortion. The

necessary correction is achieved by introducing some cross modulation between the two deflection

fields.


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There are two tupes of pincushion correction techniques.

1. E-W (horizontal) pincushioning

2. N-S (top and bottom) pincushioning

1. E-W (horizontal) pincushioning:

To correct E-W (horizontal) pincushioning, the horizontal deflection sawtooth current must

be amplitude modulated at a vertical rate so that when the electron beam is at the top or bottom of

the raster, the horizontal amplitude is minimum and when it is at the centre of the vertical

deflection interval the horizontal sawtooth amplitude is maximum.


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2. N-S (top and bottom) pincushioning:

The top and bottom or N-S pincushion correction is provided by forcing the vertical

sawtooth current to pulsate in amplitude at the horizontal scanning rate. During top and bottom

scanning of the raster a parabolic waveform at the horizontal rate is superimposed on the vertical

deflection sawtooth. The parabolic waveform at the top of the raster is of opposite polarity to that

at the bottom since the raster stretch required at the top is opposite to the needed at the bottom.

9. (a)(i) Explain the requirement of video and sound IF amplifier circuit in a monochrome

TV receiver. (10 marks)

Nov/Dec 2012

Video amplifier requirements:

In order to produce a suitable image on the screen of picture tube, the video amplifier must

meet the following requirements.

(i) Gain:

The video signal must be strong enough to vary the intensity of the picture tube scanning

beam to produce a full range of bright and dark values on the screen. A video signal amplitude of

about 75 volts peak-to-peak is needed to obtain a picture with full contrast.


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(ii) Bandwidth:

Higher frequencies are needed to reproduce horizontal information of the picture. The

lowest frequency for picture information in the horizontal direction can be considered as 10 KHz

when the camera beam scans all white and all black lines alternately.

(iii) Frequency Distortion:

Excessive frequency distortion cannot be tolerated because it changes picture

information. If high frequency content of the video signal is lost due to poor high frequency

response the rapid changes between black and white for small adjacent picture elements in the

horizontal line cannot be reproduced.

(iv) Phase Distortion:

Phase distortion is not important in audio amplifiers, because the ear does not detect

changes in relative phases of the various frequency components present in a given sound signal.

But the resultant shift in relative positions of the various picture elements is detected by the eye as

distortion.

(v) Amplitude Distortion or Nonlinear Distortion:

If the operating point on the transfer characteristics of a device for a given load and

signal amplitude is not carefully chosen, amplitude distortion occurs where different amplitudes of

the signal receive different amplification. This can result in limiting and clipping of the signal or in

weak signal output.

(vi) Manual Contrast Control:

It should be possible to vary amplitude of the video signal for optimum setting of contrast

between white and black parts of the picture. Any control that varies the amount of ac video signal

will operate as a contrast adjustment.


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Sound IF amplifier requirements:

The amplitude of the inter-carrier sound signal at the output of the video detector is very

low and so at least two stages of sound IF amplification are provided before feeding it to the FM

detector. This second IF stage is also used as a limiter, if necessary.

Each IF stage is a tuned amplifier with a centre frequency of 5.5 MHz and a bandwidth of

over 150 KHz to provide full gain to the FM sidebands. The desired bandwidth, though large, is

easily attained because it is a relatively small percentage ( 2%) of the intermediate frequency.

10. (a)(ii) Draw neatly and mark the various blocks of a colour TV receiver. (6 marks)

Nov/Dec 2012

The block diagram of a PAL-D colour receiver employing touch control channel

selection is shown below.


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11. (b)(i) Explain the requirements of horizontal and vertical deflection systems. (10 marks)

Nov/Dec 2012

Requirements of the Vertical Deflection System:

The requirements of vertical deflection systems are,

(a) Vertical Yoke Drive:

A voltage stepdown transformer provides an efficient means of coupling the output stage to

the deflection coils. The output transformer matches the relatively high impedance of the tube to

the low impedance deflection coils.

(b) Vertical Deflection Coils:

The magnetic field for vertical deflection is developed by two coils mounted 180 apart on

the neck of the picture tube. Physically these coils are a part of the yoke assembly that also

contains horizontal windings.

(c) Vertical Linearity:

When the sawtooth wave in the raster is linear, the vertical scan allows equal spacing

between horizontal lines and there is no distortion. When the sawtooth wave tends to become flat

towards its close, the magnetic field increases at a reduced rate with the result that the horizontal lines

crowd together at the bottom of the raster.

(d) Suppression of Undesired Oscillations:

During retrace time when the current through vertical deflecting coils suddenly drops to

zero, energy stored in the collapsing magnetic field sets up high frequency oscillations. The

frequency of ringing thus produced, depends on the inductance and the distributed capacitance of

the deflecting coils. The effect of such oscillations, if not suppressed, would be to pull the beam

upwards and downwards alternately instead of the smooth motion downwards.


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(e) Height of the Raster:

To fill the entire raster from top to bottom of the screen, a definite amplitude of

sawtooth current must flow into the deflecting coils. The output amplifier is designed to meet this

requirement. In addition, a control is provided at the input of the amplifier to finally adjust the

height of the picture. This control is known as height control.

(f) Vertical Rolling of the Picture:

The rolling of the picture upwards or downwards on the screen occurs on account of

incorrect vertical scanning frequency

(g) Internal Vertical Blanking:

The voltage pulses produced during retrace intervals, in the vertical output circuit are

coupled to the picture tube to provide additional blanking during vertical retrace time. This is in

addition to the blanking voltage at the cathode or grid circuit of the picture tube, which is part of

the composite video signal.

Requirements of the Vertical Deflection System:

(a) Output amplifier:

The amplifier employs a beam power tube having high voltage and power ratings. It is

driven by a trapezoidal voltage of approximately 80 V (P-P) which is obtained from the horizontal

deflection oscillator.

(b) Output transformer:

The circuit employs an autotransformer because of its higher efficiency as compared to one

having isolated primary and secondary windings.


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(c) Damper diode:

As soon as the polarity of the oscillatory voltage reverses, the damper diode (D1) is

forward biased and it conducts to dissipate energy stored in the deflection circuit.

(d) Generation of High Voltage:

During flyback periods the magnitude of induced voltage across the primary coil is very

high. In typical vacuum tube circuits it varies between 3 to 6 KV.

(e) Monochrome Yoke:

Since the current in the scanning coils deflects the electron beam, the yoke is rated in terms

of deflection angle in addition to other essential parameters necessary for generating the required

magnetic field intensity

12. (b)(ii) Discuss briefly the colour killer and sync separator circuits of a TV receiver circuit.

(6 marks)

Nov/Dec2012

Colour killer circuit:

When a monochrome transmission is received there is no input to the color killer and no

positive voltage is developed. Therefore no input is given to the second chroma amplifier from the

color killer circuit ,it blocks the second chroma amplifier. Thus it prevents the color noise on black

and white picture.

When a colour transmission is being received, the colour burst signal is present which is

detected by the colour killer circuit. The detected output of the colour killer turns on the second

chroma amplifier. The amplifier then passes the chroma signal to colour detector stages and finally,

red, green and blue colour signals are produced for activating the colour picture tube.


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Sync separator circuit:

The synchronising pulses generally called sync are part of the composite video signal.

The sync pulses include horizontal, vertical and equalizing pulses. There are separated from the

video signal by the sync separator. The sequence of operations of sync separator circuit is

illustrated in the block diagram below.

The clipped line (horizontal) and field (vertical) pulses are processed by appropriate

line-pulse and field pulse circuitry. The sync output thus obtained is fed to the horizontal and

vertical deflection oscillators to time the scanning frequencies. As a result, picture information is in

correct position on the raster.

13. (a)(i) How EHT is developed from the horizontal output circuit of the sweep amplifier?

Explain with appropriate sketches. (8 marks)

May/June 2012

EHT is Extra High voltage which is required in a TV receiver for the picture tube. Such

high voltages are required so that the electrons are able to strike the phosphor elements on the

fluorescent screen of the picture tube with sufficient force for comfortable viewing.

During flyback periods the magnitude of induced voltage across the primary (windings

L1 to L3 in figure a ) coil is very high. In typical vacuum tube circuits it varies between 3 to 6 KV.

By placing a suitably wound coil (L4) in the magnetic circuit and connecting it to the primary


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winding in a series aiding fashion, an additional voltage of about 6 to 8 KV is developed by

autotransformer action.

The corresponding circuit diagram and associated waveshapes are shown in figure

below. This circuit arrangement results in a total voltage of 9 to 14 KV across two ends of the

combined winding.

By suitably controlling the distributed parameters of the primary and secondary circuits

and utilizing the technique of third harmonic tuning, the total voltage can be enhanced by about 10

to 15 percent. Thus a pulse of the order of 11 to 16 KV is generated. This is fed to the high voltage

rectifier D2 which conducts to charge the filter capacitor to provide EHT supply.

It may be noted that the high voltage rectifier conducts for a short time during the

flyback period because D2 is reverse biased at other times on account of 11 to 16 KV dc at its

cathode (filament). The conduction of D2 partially dicharges CB but it is soon made up when D1

conducts.


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14. (a)(ii) Discuss on the factors that influenced the choice of picture IF and sound IF in the

625 B monochrome television system. (8 marks)

May/June 2012

In the 625- B system adopted by India and several other countries the recommended IF

frequencies are : Picture IF = 38.9 MHz, Sound IF = 33.4 MHz.

The factors which influence the choice of intermediate frequencies in TV receivers are:

a) Image Rejection Ratio:

The image rejection ratio is defined as the output due to desired station divided by

output due to image signal.

b) Pick-up Due to Local Oscillator Radiation from TV Receivers:

If the output from the local oscillator of a TV receiver gets coupled to the antenna, it will

get radiated and may cause interference in another receiver. With higher IF there is a greater

separation between the resonant circuits of local oscillator and RF amplifier circuits. Thus lesser


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signal is coupled from the local oscillator through the RF amplifier to the antenna circuit and

interference due to local oscillator radiation is reduced.

c) Ease of Separation of Modulating Signal from IF Carrier at the Demodulator:

For ease of filtering out the IF carrier freuency, it is desirable to have a much higher IF

frequency as compared to the highest modulating frequency.

d) Image Frequencies Should Not Lie in the FM Band:

The FM band is from 88 MHz to 110 MHz. With IF frequency chosen close to 40 MHz,

the image frequencies of the lower VHF band fall between 121 to 148 MHz and thus cannot cause

any interference in the FM band. Higher TV channels are much above the FM band.

e) Interference or Direct Pick-Up from Bands Assigned for other Services:

Amateur and industrial applications frequency band lies between 21 to 27MHz. If the IF

frequency is chosen above 40 MHz, even the second harmonics of this band will not cause any

serious direct pick-up problems.

f) Gain:

Active devices can perform very well at high frequencies. The quality of components and

other techniques have also considerably improved. Thus the gain criteria is no longer a constraint

in choosing higher IF frequencies. The merits of having high IF frequency are thus obvious and

this has lead to the choice of IF frequencies close of 40 MHz.

15. (b)(ii) CCD techniques (8 marks)

May/June 2012

The Basic CCD Characteristic Principle CCD cameras are based on strange electronic

chips, called CCD sensors. These components provide a higher sensitivity to light than common films,

allowing it to store the pictures on computers.

A CCD chip, is an array of light-sensitive elements. These are, in fact, very small electronic

capacitors. These capacitors are charged by the electrons generated by the light. In fact, each light


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element (commonly called photon), that reaches the CCD array's atoms, displaces some electrons. This

displacement provides the current source. These current sources, are localized in small delimited areas

(the capacitors), called pixels.

Common CCD chips are composed of several pixels, for instance, 192*165, 512*512,

1024*1024, or much more for rich people! It's easy to understand that it is physically impossible to

access each pixel individually. In fact, a CCD sensor provides only one serial output which each

capacitor can be discharged through (each pixel can be read). CCD array can be read through only one

output.

The capacitors are discharged in lines and there are some control gates that allow the transfer

of one pixel line into the next one. The last line of the array can be transferred into a horizontal shift

register. This Shift register allows the transfer of one pixel to the next one, and the last pixel of this

horizontal register is connected to the output gate.

How to take a picture

1. Clean the CCD array by reading the picture.

2. Wait for a defined time ( the exposure time) to allow the light to charge the capacitors.

3. Readout the picture.

The output gate of the CCD array can either be connected to an analogue or to a digital

converter in order to digitize the picture, or it can provide a standard video signal if the clock's timing

is according to the video norms. If the image is digitized, it will be easy to store it in a computer

memory. So, its processing will be easy to perform.

Noise Reduction

If a CCD array is used in a warm environment (20 C), the photosensitive area atoms

produce a current, which produces noise on the picture. To reduce this noise, the CCD array atoms

should be cooled, so that their thermal excitation is reduced. In astronomical use, the CCDs are cooled

to temperatures like -50C or -70C.


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They are cooled by two main processes:

1. Electrically cooled

2. Chemically cooled

Cooling a CCD with liquid gas, is the most efficient method, but this is still reserved for

professional purposes. This is mainly due to the equipment needed to store and use such liquids. So,

the most commonly used process is the use of a thermoelectric cooler, also known as Peltier cells.

They are devices that work as a heat pump. The heat contained in one face of the cell, is transferred to

the other face. These cells, should be powered by direct current.

The efficiency of such cells can be improved by cooling their hot face. In order to provide a

great cooling to the CCD, it is possible to use 2 Peltier cells, plus a water or glycol cooling system. The

first cell, cools down the CCD, the second cools the first one, and the liquid circuit evacuates the heat

provided by the second Peltier cell.

16. (a) With a neat block diagram explain the features of a monochrome television receiver.

(16marks)

Nov/Dec 2011, May/June 2010

The block diagram of a monochrome TV receiver is shown below.


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17. (b)(ii) Describe with the help of a schematic diagram the working of an LCD TV display.

(8 marks)

Nov/Dec 2011

Liquid Crystal Displays of small to medium size have been developed for pocket TV

receivers in colour as well as monochrome.

A 2 inch LCD display consists of a P-channel MOS switching matrix of 240*240 pixels.

The gates of all transistors in a horizontal row are connected to each horizontal common bus,

while the drains of all the transistors in vertical column are connected to common vertical buses

as in figure below.


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The horizontal gate buses provides row addressing. They are driven sequentially from a 220-

stage shift register via a set of 240 sample and hold (S/H) stages.

The video information is placed on each column bus of drain connections, during the horizontal

scan. This creates a line sequential display.

The stray capacitance and the cross under-drain resistances associated with the drain buses

present loading problems for the S/H stages.

As only 220 lines are available for display instead of 625, both the interlaced fields are scanned

on the same rows, while the extra lines of each field are eliminated during the top and bottom

overscan.

18. (b)(i) Explain the working of a AFC circuit in a monochrome TV receiver. (8 marks)

May/June 2011

In order to ensure that the horizontal oscillator operates at the correct frequency, and is

basically immune to noise pulses, all horizontal deflection oscillators are controlled by some form

of a circuit known as the automatic frequency control circuit (AFC circuit).

The automatic frequency control circuit is generally called flywheel sync, sync lock,

stabilized sync or horizontal AFC, based on the technique employed to develop the control voltage.


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AFC Operation:

The block schematic arrangement of a frequently used AFC circuit for the horizontal

deflection oscillator in monochrome receivers is illustrated in figure below.

An integrating circuit is a low-pass filter and hence sharp noise pulses do not appear at

its output. However, the differentitator, being a high-pass filter, develops output in response to

noise pulses in addition to the spiked horizontal sync pulses. This results in occasional wrong

triggering of the horizontal oscillator which results in diagonal tearing of the reproduced picture.

To overcome this difficulty, a special circuit known as automatic frequency control (AFC)

circuit is employed. The AFC circuit employs a discriminator arrangement which compares the

incoming horizontal sync pulses and the voltage that develops across the output of the horizontal

deflection amplifier.

The AFC output is a dc control voltage that is free of noise pulses. This control voltage is

used to synchronize the horizontal oscillator with the received horizontal sync pulses.

19. (a)Explain the working of

(i)AGC circuit (8 marks)

May/June 2011

AGC circuit controls gain of RF and IF stages to deliver almost constant signal voltage to

the video detector, despite changes in the signal picked up by the antenna. The change in gain is

achieved by shifting the operating point of the amplifying devices (tubes or transistors) used in the

amplifiers.


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The block diagram of AGC system is shown below.

Sync level in the composite video signal is fixed irrespective of the picture signal

details. Hence, sync pulse tops represent truly the signal strength. A peak rectifier is used to

develop a control voltage which is proportional to the sync level.

The composite video signal to the peak rectifier in the AGC circuit is either obtained

from the output of video detector or after one stage of video amplification. The output is filtered

and the dc voltage thus obtained is fed to the input (bias) circuits of the RF and IF amplifiers to

control their gain. Decoupling circuits are used to avoid interaction between different amplifier

stages.

20. (b)(ii) Receiver antennas (8 marks)

May/June 2010

Television receiver antennas are as follows:

Antennas for VHF Channels

Although most receivers can produce a picture with sufficient contrast even with a weak

signal, but for a picture with no snow and ghosts, the required antenna signal strength lies between 100

and 2000V.

Yagi-Uda Antenna

The antenna widely used with television receivers for locations within 40 to 60 km from the

transmitter is the folded dipole with one reflector and one director. This is commonly known as Yagi-

Uda or simply Yagi antenna.


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Indoor Antennas

In strong signal areas it is sometimes feasible to use indoor antennas provided the receiver

is sufficiently sensitive. These antennas have selector switches which are used for modifying the

response pattern by changing the resonant frequency of the antenna so as to minimize interference and

ghost signals.

Fringe Area Antenna

In fringe areas where the signal level is very low, high-gain antenna arrays are needed. The

gain of the antenna increases with the number of elements employed.

Multiband Antennas

It is not possible to receive all the channels of lower and higher VHF band with one

antenna. As a result, antennas for both the VHF bands generally use either separate dipoles for each

band or a dipole for the lower VHF band modified to provide broadside unidirectional response in the

upper VHF band also.

Conical Dipole Antenna

This antenna consists of two half-wave dipoles inclined at about 30 from the horizontal

plane, similar to a section of a cone.

Parabolic Reflector Antenna

In this type of antenna the dipole is placed at the focal point of a parabolic reflector.

21. (a)(i) With necessary diagrams explain the delta-gun colour picture tube. Describe how

purity and convergence are achieved in it. (16 marks)

May/June 2010


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Delta-gun colour picture tube employs three separate guns, one for each phosphor. The

guns are equally spaced at 120 interval with respect to each other and tilted inwards in relation to the

axis of the tube. They form an equilateral triangular configuration.

Fig: Delta-gun colour picture tube (a) guns viewed from the base (b) electron beams, shadow mask

and dot-triad phosphor screen (c) schematic diagram showing application of Y and colour

difference signals between the cathodes and control grids.

Generation of Colour Rasters:

The overall colour seen is determined both by the intensity of each beam and the phosphors

which are being bombarded. If only one beam is on and the remaining two are cut-off, dots of only

one colour phosphor get excited. Thus the raster will be seen to have only one of the primary colours.


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Similarly, if one beam is cut-off and the remaining two are kept on, the rasters produced by

excitation of the phosphors of two colours will combine to create the impression of a complementary

colour. The exact hue will be determined by the relative strengths of the two beams.

When all the three guns are active simultaneously, lighter shades are produced on the

screen. The is so because red, green and blue combine in some measure to form white, and this

combines with whatever colours are present to desaturate them.

Primary Colour Signals:

The demodulators in the receiver recover (B Y) and (R Y) video signals. The (G Y)

colour video signal is obtained from these two through a suitable matrix. All the three colour

difference signals are then fed to the three grids of colour picture tube (fig (c)). The inverted

luminance signal ( Y) is applied at the junction of the three cathodes. The signal voltages subtract

from each other to develop control voltages for the three guns, i.e.,

VG1 Vk = (VR VY) ( VY) = VR

VG1 Vk = (VG VY) ( VY) = VG and

VG1 Vk = (VB VY) ( VY) = VB

In some receiver designs the Y signal is subtracted in the matrix and resulting colour

voltages are directly applied to the corresponding control grids. The cathode is then returned to a fixed

negative voltage.

Purity and Convergence:

While deflecting the three beams by vertical and horizontal deflecting coils it is necessary

to ensure that each beam produces a pure colour and all the three colour rasters fully overlap each

other.

For obtaining colour purity each beam should land at the centre of the corresponding

phosphor dot irrespective of the location of the beams on the raster. This needs precise alignment of

the colour beam and is carried out by a circular magnet assembly known as the purity magnet. It is

mounted externally on the neck of the tube and close to the deflection yoke. The tabs on the magnets


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can be moved apart to reduce resultant field strength. This is illustrated for a two pole magnet in figure

below.

As shown in the same figure, the tabs when moved together change the direction of

magnetic field. Two, four and six pole magnet units are employed to achieve individual and collective

beam deflections. Thus to affect purity and static convergence the beams can be deflected up or down,

right or left and diagonally by suitably orienting the purity magnets.

.

Fig: Two pole purity magnet assembly (a) strong magnetic field when tabs (A and B) are

nearly together (b) spreading the tabs reduces magnetic field (c) rotating the magnets

together rotates the magnetic field to cause change in the direction of beam deflection.

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