Television andvideoengineering

Unit-4TelevisionReceiversystems

Objectives: To learn the requirements of TV

receiver Study of monochrome and Colour

TV receivers. To learn functions of Tuning circuits

and different amplifier circuits.

Introduction:

The preceding chapters were devoted to the complexitiesand essential requirements of generation and transmissionof composite video and sound signals associated with thetelevised scene. As is logical, we should now turn ourattention to the receiver. In effect, a television receiver isa combination of an AM receiver for the picture signaland an FM receiver for the associated sound. In addition,the receiver also provides suitable scanning andsynchronizing circuitry for reproduction of image on thescreen of picture tube. We shall confine our discussion tomonochrome (black and white) receivers and basicprinciple and essential details of colour receivers aredescribed. However, it may be noted that all the circuitsfor a black-and-white picture are also needed in a colourreceiver. The colour television picture is just amonochrome picture with colour added in the main areasof picture information.

Monochrome TV receiver:

Block diagram of Black & white Receiver

The various sections of a monochrome TV receiver are:

1. Tuner

2. IF Subsystem

3. Video Section

4. Audio Section

5. Deflection circuits

6. Power supply

Antenna Systems:

Strongest signal is induced in the antenna if it hassame polarization as the transmitting antenna. All TVantennas are mounted in horizontal position for betterreception and favorable signal to noise ratio. The need forgood signal strength has led to the use of tuned antennas.For channels located in the VHF band, a half wave-lengthantenna is most widely used. Such antennas behave likelow ‘Q’ tuned circuits and a single antenna tuned to themiddle frequency of various channels of interest can servethe purpose. Various antennas in use are of dipole typewith reflectors and directors. A folded dipole with areflector is used because its response is more uniformover a band of frequencies. A Yagi antenna, i.e., a dipolewith one reflector and two or more directors, is a compacthigh gain directional array, and is often used in fringeareas. In areas where signal strength is very low, boosteramplifiers with suitable matching network are used. Onthe other hand, in areas situated close to a transmittingantenna, where signal strength is quite high, various typesof indoor antennas are frequently employed.

Since it is not possible for one dipole antenna tocover both upper and lower VHF band channels

effectively; high and low band dipoles are mountedtogether and connected to a common transmission line.For channels in the UHF bound, where the attenuation isvery high and the signals reaching the antenna are weaker,special antennas like fan dipole, rhombic and parabolicreflector type are often used.

A transmission line connects antenna to the receiverinput terminals for the RF tuner. A twin-lead is generallyused. This type is an unshielded balanced line withcharacteristic impedance equal to 300 ohms. When thereis a problem of interference, a shielded coaxial cable isused. This cable has high attenuation, especially at UHFchannel frequencies. It has a characteristic impedance of75 ohms.

The current practice is to design input circuit of theTV receiver for a 300 ohm transmission line. It has beenfound that a 300 ohm transmission line used with a half-wave dipole produces a broad frequency response withouttoo large a loss due to mismatching. A folded dipole hasimpedance close to 300 ohms at its resonant frequency,and a much uniform response is obtained with thisantenna. Receivers designed to receive UHF channelshave two inputs; one to match a 300 ohm transmission

line and the other for a 75 ohm coaxial cable. A signalstrength of the order of 500 μV to 1 mV and a signal tonoise ratio of 30 : 1 are considered adequate forsatisfactory reception of both picture details and soundoutput.

RF Section:

This section consists of RF amplifier, mixer and localoscillator and is normally mounted on a separate sub-chassis, called the ‘Front End’ or ‘RF Tuner’. Either tubesor transistors can be used. With tubes, local oscillator andmixer functions are usually combined in one stage calledthe ‘frequency converter’. The purpose of the tuner unit isto amplify both sound and picture signals picked up bythe antenna and to convert the carrier frequencies andtheir associated bands into the intermediate frequenciesand their sidebands. The receiver uses superhetrodyneprinciple as used in radio receivers. The signal voltage orinformation from various stations modulated overdifferent carrier frequencies is heterodyned in the mixerwith the output from a local oscillator to transfer originalinformation on a common fixed carrier frequency calledthe intermediate frequency (IF). The setting of the localoscillator frequency enables selection of desired station.

The standard intermediate frequencies for the 625-Bsystem are-Picture

IF = 38.9 MHz, Sound IF = 33.4 MHz.

In principle an RF amplifier is not necessary and signalcould be fed directly to the tuned input circuit of themixer. However, the problems of a relatively weak inputsignal with low signal to noise ratio, local oscillatorradiation and image rejection are such, that a stage ofamplification ahead of the mixer is desirable. The tuningfor different channels is carried out with a channelselector switch which changes resonant frequencies of theassociated tuned circuits by varying either inductance orcapacitance of these circuits. The RF section is shownseparately in Fig, where the channel selector switch hasbeen set for channel 4 (Band I), i.e., 61 to 68 MHz. Thepicture carrier frequency in this channel is 62.25 MHz andthe sound carrier 67.75 MHz. The RF amplifier must havesufficient bandwidth to accept both the picture and soundsignals. This is illustrated in Fig. The local oscillatorfrequency is set at 101.15 MHz. In the mixer, both sumand difference (sideband) frequencies are generated. Theoutput circuit of the mixer is however, tuned to deliver

difference frequencies i.e., the intermediate frequenciesand their sidebands.

The required IFs are then produced as here: (Localoscillator frequency of 101.15 MHz)–(Picture carrierfrequency of 62.25 MHz) = Picture IF of 38.9 MHz,(Local oscillator frequency of 101.15 MHz)– Soundcarrier frequency of 67.75 MHz) = 33.4 MHz. The desiredoutput response from the mixer is shown in Fig. 8.4.Notice that frequency changing process reverses therelative positions of the sound and picture signals. This isobvious, since the oscillator works above the signalfrequencies, and ‘difference’ frequencies produced, when

the picture and sound frequencies are substracted mustgive a higher IF from the lower frequency picture signal.It may be noted that picture and sound signals wouldremain in the same relative position, i.e., with soundcarreir frequency higher than picture carrier frequency iflocal oscillator frequency is set below, instead of abovethe carriers. The local oscillator frequency is kept higherbecause of ease of oscillator design and several othermerits. The ratio of highest to lowest radio frequency thatthe local oscillator must generate, when the oscillatorfrequency is chosen to be higher than the incoming carrierfrequency, is much less than when the local oscillatorfrequency is kept below the incoming channel frequency.It is much easier to design an oscillator that maintainsalmost constant output amplitude and a sinusoidal waveshape when its overall frequency range is less. Thisjustifies the choice of higher local oscillator frequency.

IF Amplifier action:

A short length of coaxial cable feeds tuner output to thefirst IF amplifier. This section is also called video IFamplifier since composite video signal is the envelope ofthe modulated picture IF signal. Practically all the gainand selectivity of the receiver is provided by the IF

section. With tubes, 2 or 3 IF stages are used. Withtransistors, 3 to 4 If stages are needed. In integratedcircuits, one IC chip contains the entire IF amplifierstages.

Essential functions of IF function:

The main function of this section is to amplifymodulated IF signal over its entire bandwidth with aninput of about 0.5 mV signal from the mixer to deliverabout 4 V into the video detector. This needs an overallgain of about 8000. This gain should be adjustable, byautomatic gain control, over a wide range toaccommodate input signal variations at the antenna from50 μV to 0.5 V, to deliver about 4 V peak-to-peak signalat the input of the video detector. To achieve desired gain,atleast three stages of tuned amplifiers are cascaded andto obtain desired bandwidth the resonant frequencies ofthese stages are staggered. Such an arrangement providesdesired gain and selectivity.

VHF Tuner:

Signal is fed to RF amplifier through Balun-matchingtransformer. Mixer is the main source of noise where

super heterodyne principle is used, which produces snowon picture.

RF amplifier should function as low noiseamplifier. So gain is controlled using AGC bias. It allowsmaximum gain on weak i/p signals and enoughattenuation on high inputs to prevent noise.

For channel 4 (BAND I) i.e. 61 to 68 MHz. Picturecarrier is 62.25 MHz and sound carrier is 67.75 MHz.Local oscillator frequency is 101.15 MHz. Both sum anddifference frequencies are generated in the mixer.

The o/p of mixer is difference of frequencies

Picture IF = LOF – PCF

= 101.15 – 62.25

= 38.9 MHz

Sound IF = LOF – SCF

= 101.15 – 67.75

= 33.4 MHz.

Fine tuning control is used for small adjustments oflocal oscillator frequencies. Picture and sound signalswould remain in same relative position.

Factors influencing the choice of picture IFand sound IF in 625-B (CCIR) televisionsystem:

Picture and sound carriers in any channel spaced by5.5MHz. Accordingly, if IF is fixed at a certain frequencysound IF automatically gets fixed at a frequency 5.5MHzless than picture IF frequency.

The factors which influence the choice of IF in TVreceivers are:

Image rejection ratio.

Pick up due to local oscillator radiation from

TV receivers.

Image frequencies should not lie in FM band.

Interference or direct pick up from bands

assigned for other services.

Gain.

1. IMAGE REJECTION RATIO:For an input signal at 100 MHz, local oscillator

frequency is 110MHz IF frequency is 10 MHz

For same input signal at 100MHz, IF frequency canchosen as 40 MHz and the local oscillator frequency as140 MHz.

Desired signal = 100 MHz Desiredsignal = 100 MHz

Frequency Frequency

Image frequency = 120 MHz Imagefrequency = 180 MHz

If another station is operated at 120 MHz, it willalso beat with local oscillator frequency 110 MHz toproduce 10 MHz. If a station operated at 180 MHz, it willalso beat with local oscillator frequency 140 MHz toproduce 40 MHz.

MIXER MIXER

MIXER MIXER

I/PI/P IF

O/P

40MHzFC=110

MHz

FC=140

MHz

IF

O/P

10MHz

In each case, undesired signal which gets receivedis spaced at a gap of twice the If frequency and is knownas image signal.

Image rejection ratio is defined as the ratio ofoutput due to desired station to the output due to imagesignal. Without RF amplifier, there is nothing to stop thereception of image signal if it is present.

Thus RF amplifier helps in reducing interferencedue to image signals and a higher IF results in a very highimage rejection ratio.

By choosing IF to be greater than half the entireband to be covered it are possible to eliminate imageinterference.

For lower VHF band (41-68MHz) the IF is13MHz

Upper VHF band (174-230 MHz) the IF is 28MHz.

UHF band (470-582 MHz) the IF is 56MHz.

It must be less than 41 MHz.

2.PICK UP DUE TO LOCAL OSCILLATOR RADIATION FROM TV RECEIVERS:

If output from local oscillator of a TV receiver getscoupled to the antenna, it will get radiated and causeinterference in another receiver.

Higher the IF, there is a greater separation betweenresonant circuits of local oscillator and RF amplifier circuits.Thus lesser signal is coupled from local oscillator to antennaand interference is reduced.

3.IMAGE FREQUENCIES SHOULD NOT LIE IN FMBAND:

FM band is 88MHz – 110 MHz. With IF as 40MHz, image frequencies of lower VHF band fall between 121-148 MHz and thus no interference is caused in FM band.Higher TV channels are much above FM band.

RF

AMPLIFIER MIXER

LOCAL

OSCILLATOR

IF

OUT

ANTENNA

4. INTERFERENCE OR DIRECT PICKUP FROM BANDSASSIGNED FOR OTHER SERVICES:

Amateur and industrial application frequency bandis 21-27 MHz. If IF is above 40 MHz, second harmonics ofthis band will not cause any serious direct pick-up problems.

5.GAIN:

Amplifiers with large gain at relatively lowfrequencies can be easily build.40 years back, IF is120MHz.Due to limitations of active devices available, IFis12MHz.Now active devices and ICs can work well at highfrequencies.

Choice of IF leads to 40 MHz. In 625-B system,

picture IF is 38.9 MHz and

sound IF is 33.4 MHz

In USA,

picture IF is 45.75 MHz

sound IF is 41.25 MHz

In British 625 line system,

picture IF is 39.5 MHz

sound IF is 33.5 MHz

IF sub system of a monochrome television receiver.

The IF sub system includes

1. IF section.2. AGC circuit.3. Video detector.IF SECTION:

Main function is to amplify the modulated IFsignal. With 0.5mv o/p from mixer, 4v is given to videodetector which needs an overall gain of 8000. Gain isadjustable by automatic gain control.

To achieve this gain, at least three stages of tunedamplifiers are cascaded. Staggered tuning is employed toobtain wide band characteristics. Each amplifier is tunedto a different centre frequency and quality factor ‘Q’ islowered to obtain broad response.

IF response of monochrome and colour receiversmust include rejection filters to suppress adjacent channelinterference.

IF AMPLIFIER:

It is necessary to include the trap circuits forvestigial side band correction. And to prevent

adjacent channel interference. Staggered tuning isemployed to obtain wide band characteristics.

AGC Circuit:

AGC circuit varies the gain of a receiver according to thestrength of the signal picked up by the antenna. Usefulsignal strength at the receiver i/p terminals may vary from50 µv to 1v or more.

dc voltage proportional to i/p signal strengthobtained by rectifying video signal as available aftervideo detector. AGC bias is used to control the gain of RFand IF stages in receiver to keep o/p at video detectoralmost constant.

Output is filtered and the dc voltage obtained is fedto i/p circuits of RF and IF amplifiers to control gain.Decoupling circuits are used to avoid interaction betweendifferent amplifier stages.

RF amplifier is not fed any AGC voltage till thesignal strength attains a predetermined level. This isachieved by providing voltage delay circuit in AC lineand such a provision is called ‘delayed AGC’

The change in gain is achieved by shifting theoperating point of transistors. Changing the gain changesthe ß.

When the gain is changed by shifting the operatingpoint towards current cut-off, it is called reverse AGC.When gain is changed by moving the operating pointtowards collector current saturation, it is called ‘forwardAGC’

NON-KEYED AGC:

AGC voltage developed across the peak rectifiertends to increase during vertical sync pulses periods,resulting in a 50 Hz ripple over negative AGC voltage.

KEYED AGC:

AGC rectifier is allowed to conduct only duringline sync pulse periods and ripple is not present. Noiseeffects are also minimized.

VIDEO DETECTOR:

Modulated IF signals after due amplification in IFsection are fed to video detector. Detector is designed torecover composite video signal and to transform soundsignal to another lower carrier frequency.

This is done by rectifying input signal and filteringout unwanted components. A diode is used to rectifyeither positive or negative peaks of input signal.

LC filter is used to avoid undue attenuation of videosignal while filtering carrier components. Circuitbandwidth must extend from 0 to 6MHz to include soundinter carrier frequency of 5.5MHz.

The detector output must include dc level of thedemodulated signal. The detector diode polarity is chosento to fit the needs of video amplifier. Cathode drive at thepicture tube is now almost universal.

Therefore, the polarity of the video signal output ispositive sync. The video amplifier itself is usually aninverting amplifier (emitter follower). Then Its input mustbe of negative sync polarity.

An advantage of this polarity is that noise pulsesthat exceed sync peak level can be clipped off byadjusting cut-off level of the amplifier.

Video Amplifier:

The primary function of video amplifier is to provideenough gain such that the output video signal attainsamplitude to drive picture tube from cut off for blankingto practically zero grid cathode voltage for peak white.

In (p-p) values, signal may vary from 30v for smallpicture tubes to 150v for large picture tubes. The o/p ofvideo detector is about 2 to 3v which needs amplificationthrough video amplifiers.

Gain control of video amplifier constitutescontrast control and for background brightness. Forcorrect reproduction, large BW is to be maintained. If RCcoupling is used, dc component is lost.

This can be restored by clamping techniquesafter amplification. In monochrome receivers, amplifierstages following video detector are called videoamplifiers. In colour receivers, delay line is used usuallyreferred as luminance or Y channel.

38.9 MHz STOP TRAP FILTER:

It prevents the entry of IF signal in the videoamplifier.

EMITTER FOLLOWER:

It is a buffer stage. It does not include the signalpolarity. It is usually included to provide isolation andimpedance match on either side. It has high i/p impedanceto work into high impedance load.

It also provides low source impedance as its o/p forfeeding AGC, sync separator and to 5.5 MHz trap circuits.It reduces amplitude distortions during sync peaks.

VIDEO

DETECTOR

38.9 MHz

STOP

TRAP

EMITTER

FOLLOWER

VIDEO

AMPLIFIER

TO 5.5 MHz SOUND

IF AMPLIFIER

SYNC (-)

RL

TO AGC

CIRCUIT

TO SYNC

SEPERATOR

CONTRAST

SYNC (+)

TOCATHODE

OF

PICTURE

TUBE

FROM LAST IFAMPLIFIER

VIDEO AMPLIFIER REQUIREMENTS:

Amplitude and phase relations of components ofvideo signal must be preserved for producing highresolution picture. so video amplifier should have linearfrequency response from dc (0 Hz) to 5 MHz. This ispossible only if amplifier is required.

REQUIREMENTS:

1. GAIN:It has video pre-amplifier and deliver an o/p of 4 to

6v (p-p) of video signal. To obtain this voltage, it needs60 to 80 volts of video signal. It is usually an invertingamplifier and needs a negative sync signal at its i/p todeliver positive going signal at cathode of picture tube.

2. AMPLIFIER BANDWIDTH:Higher frequencies are needed to reproduce

horizontal information of the picture. Camera tube scansalternate black and white lines. Actual period is 52μs. Toobtain round figure value, line period 50μs.

For reproducing very minute details, a high videofrequency would be necessary, channel BW has beenfixed at 5 MHz.

Signal frequencies corresponding to pictureinformation scanned in vertical direction is much lowercompared to picture information within a line. Whenbrightness varies, signal frequency is lower than 25 Hz.

This changes dc level corresponding to change inbrightness’ level becomes almost zero. Ideally, videoamplifier response should be linear from dc to highestmodulation frequency of 5 MHz. Possible only whenvideo amplifier is directly coupled.

3. FREQUENCY DISTORTION:Gain at HF falls off because of shunting effect of

device’s o/p capacitance of picture tube. When accoupling is used, gain decreases at IF. Inequality in gainat different frequency components of signal is calledfrequency distortion.

Excessive frequency distortion cannot be toleratedbecause it changes picture information. If HF content ofvideo signal is lost, horizontal line cannot be reproduced.

Frequencies from 100 KHz to 25 Hz Main partof picture information.100 KHz to10KHz black andwhite details in horizontal direction.10 KHz – 25Hz changes of shading in vertical direction. If LF response ispoor, picture is weak with poor contrast.

4. PHASE DISTORTION:It is not so important in audio amplifiers because

the ear does not detect changes in relative phases. Phaseshift implies time shift. Resultant shift in relativepositions of various picture elements is detected by theeye as distortion.

Therefore relative phases of all frequencycomponents must have same time delay. Eyes aresensitive to time delay errors and appear as ‘smear’ onpicture. At high video frequencies, phase distortion is notas evident because time delay at the frequencies is verysmall.

5. AMPLITUDE DISTORTION OR NON LINEARDISTORTION:

If operating point and signal amplitude is notcarefully chosen, amplitude distortion occurs. It results inclipping and limiting of signal. Some gain has to besacrificed to avoid amplitude distortion.

SAW filter:

The operation of saw filter is based on thephenomenon of surface acoustic waves. Surface acousticwave is anon electromagnetic wave that travels along thesurface of piezo electric substrate.

When electric field is applied, physical changesoccur. When stress is applied, a proportionate electricpotential is developed. Transducers are provided at twoends of wafer to convert electric energy to acoustic waveenergy and then back to electric energy.

Input signal frequency is about several GHz buton conversion to saw, signal travels at the speed ofsound.

SAW FILTER FOR IF SECTION:

Bandwidth and frequency characteristics ofSAW filter depend on geometric structure of inputtransducer array. Filter can be designed to have anygroup delay characteristics by changing the distancebetween electrodes of transducer array.

It consists of i/p and o/p transducer arraysseparated by a multistrip coupler which prevents

distortion of surface wave transmission caused byreflection of energy.

Input transducer is designed to obtain for IF bandpass. Output transducer is designed to obtain a broadband response. Its function is to convert the signal fromsurface acoustic wave energy back to its electrical form.

Summary:

In this chapter we have looked into therequirements and design aspects of TV broadcastreceivers. The functional block diagram of monochromeTV was present and explained briefly.

TV tuner and IF subsystem was explained. This willbe the basics for colour TV receiver and it will beexplained in further chapters.