Television and video engineering

Television and video engineering

Unit-4 Television Receiver systems

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 complexities and essential requirements of generation and transmission of composite video and sound signals associated with the televised scene. As is logical, we should now turn our attention to the receiver. In effect, a television receiver is a combination of an AM receiver for the picture signal and an FM receiver for the associated sound. In addition, the receiver also provides suitable scanning and synchronizing circuitry for reproduction of image on the screen of picture tube. We shall confine our discussion to monochrome (black and white) receivers and basic principle and essential details of colour receivers are described. However, it may be noted that all the circuits for a black-and-white picture are also needed in a colour receiver. The colour television picture is just a monochrome picture with colour added in the main areas of 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 has same polarization as the transmitting antenna. All TV antennas are mounted in horizontal position for better reception and favorable signal to noise ratio. The need for good signal strength has led to the use of tuned antennas. For channels located in the VHF band, a half wave-length antenna is most widely used. Such antennas behave like low Q tuned circuits and a single antenna tuned to the middle frequency of various channels of interest can serve the purpose. Various antennas in use are of dipole type with reflectors and directors. A folded dipole with a reflector is used because its response is more uniform over a band of frequencies. A Yagi antenna, i.e., a dipole with one reflector and two or more directors, is a compact high gain directional array, and is often used in fringe areas. In areas where signal strength is very low, booster amplifiers with suitable matching network are used. On the other hand, in areas situated close to a transmitting antenna, where signal strength is quite high, various types of indoor antennas are frequently employed. Since it is not possible for one dipole antenna to cover both upper and lower VHF band channels

effectively; high and low band dipoles are mounted together and connected to a common transmission line. For channels in the UHF bound, where the attenuation is very high and the signals reaching the antenna are weaker, special antennas like fan dipole, rhombic and parabolic reflector type are often used. A transmission line connects antenna to the receiver input terminals for the RF tuner. A twin-lead is generally used. This type is an unshielded balanced line with characteristic impedance equal to 300 ohms. When there is a problem of interference, a shielded coaxial cable is used. This cable has high attenuation, especially at UHF channel frequencies. It has a characteristic impedance of 75 ohms. The current practice is to design input circuit of the TV receiver for a 300 ohm transmission line. It has been found that a 300 ohm transmission line used with a halfwave dipole produces a broad frequency response without too large a loss due to mismatching. A folded dipole has impedance close to 300 ohms at its resonant frequency, and a much uniform response is obtained with this antenna. Receivers designed to receive UHF channels have two inputs; one to match a 300 ohm transmission

line and the other for a 75 ohm coaxial cable. A signal strength of the order of 500 μv to 1 mv and a signal to noise ratio of 30 : 1 are considered adequate for satisfactory reception of both picture details and sound output. RF Section: This section consists of RF amplifier, mixer and local oscillator and is normally mounted on a separate subchassis, called the Front End or RF Tuner. Either tubes or transistors can be used. With tubes, local oscillator and mixer functions are usually combined in one stage called the frequency converter. The purpose of the tuner unit is to amplify both sound and picture signals picked up by the antenna and to convert the carrier frequencies and their associated bands into the intermediate frequencies and their sidebands. The receiver uses superhetrodyne principle as used in radio receivers. The signal voltage or information from various stations modulated over different carrier frequencies is heterodyned in the mixer with the output from a local oscillator to transfer original information on a common fixed carrier frequency called the intermediate frequency (IF). The setting of the local oscillator frequency enables selection of desired station.

The standard intermediate frequencies for the 625-B system are-picture IF = 38.9 MHz, Sound IF = 33.4 MHz. In principle an RF amplifier is not necessary and signal could be fed directly to the tuned input circuit of the mixer. However, the problems of a relatively weak input signal with low signal to noise ratio, local oscillator radiation and image rejection are such, that a stage of amplification ahead of the mixer is desirable. The tuning for different channels is carried out with a channel selector switch which changes resonant frequencies of the associated tuned circuits by varying either inductance or capacitance of these circuits. The RF section is shown separately in Fig, where the channel selector switch has been set for channel 4 (Band I), i.e., 61 to 68 MHz. The picture carrier frequency in this channel is 62.25 MHz and the sound carrier 67.75 MHz. The RF amplifier must have sufficient bandwidth to accept both the picture and sound signals. This is illustrated in Fig. The local oscillator frequency is set at 101.15 MHz. In the mixer, both sum and difference (sideband) frequencies are generated. The output circuit of the mixer is however, tuned to deliver

difference frequencies i.e., the intermediate frequencies and their sidebands. The required IFs are then produced as here: (Local oscillator frequency of 101.15 MHz) (Picture carrier frequency of 62.25 MHz) = Picture IF of 38.9 MHz, (Local oscillator frequency of 101.15 MHz) Sound carrier frequency of 67.75 MHz) = 33.4 MHz. The desired output response from the mixer is shown in Fig. 8.4. Notice that frequency changing process reverses the relative positions of the sound and picture signals. This is obvious, since the oscillator works above the signal frequencies, and difference frequencies produced, when

the picture and sound frequencies are substracted must give a higher IF from the lower frequency picture signal. It may be noted that picture and sound signals would remain in the same relative position, i.e., with sound carreir frequency higher than picture carrier frequency if local oscillator frequency is set below, instead of above the carriers. The local oscillator frequency is kept higher because of ease of oscillator design and several other merits. The ratio of highest to lowest radio frequency that the local oscillator must generate, when the oscillator frequency is chosen to be higher than the incoming carrier frequency, is much less than when the local oscillator frequency is kept below the incoming channel frequency. It is much easier to design an oscillator that maintains almost constant output amplitude and a sinusoidal wave shape when its overall frequency range is less. This justifies the choice of higher local oscillator frequency. IF Amplifier action: A short length of coaxial cable feeds tuner output to the first IF amplifier. This section is also called video IF amplifier since composite video signal is the envelope of the modulated picture IF signal. Practically all the gain and selectivity of the receiver is provided by the IF

section. With tubes, 2 or 3 IF stages are used. With transistors, 3 to 4 If stages are needed. In integrated circuits, one IC chip contains the entire IF amplifier stages. Essential functions of IF function: The main function of this section is to amplify modulated IF signal over its entire bandwidth with an input of about 0.5 mv signal from the mixer to deliver about 4 V into the video detector. This needs an overall gain of about 8000. This gain should be adjustable, by automatic gain control, over a wide range to accommodate input signal variations at the antenna from 50 μv to 0.5 V, to deliver about 4 V peak-to-peak signal at the input of the video detector. To achieve desired gain, atleast three stages of tuned amplifiers are cascaded and to obtain desired bandwidth the resonant frequencies of these stages are staggered. Such an arrangement provides desired gain and selectivity. VHF Tuner: Signal is fed to RF amplifier through Balun-matching transformer. Mixer is the main source of noise where

super heterodyne principle is used, which produces snow on picture. RF amplifier should function as low noise amplifier. So gain is controlled using AGC bias. It allows maximum gain on weak i/p signals and enough attenuation on high inputs to prevent noise. For channel 4 (BAND I) i.e. 61 to 68 MHz. Picture carrier is 62.25 MHz and sound carrier is 67.75 MHz. Local oscillator frequency is 101.15 MHz. Both sum and difference frequencies are generated in the mixer. The o/p of mixer is difference of frequencies Picture IF Sound IF = LOF PCF = 101.15 62.25 = 38.9 MHz = LOF SCF = 101.15 67.75 = 33.4 MHz. Fine tuning control is used for small adjustments of local oscillator frequencies. Picture and sound signals would remain in same relative position.

Factors influencing the choice of picture IF and sound IF in 625-B (CCIR) television system: Picture and sound carriers in any channel spaced by 5.5MHz. Accordingly, if IF is fixed at a certain frequency sound IF automatically gets fixed at a frequency 5.5MHz less than picture IF frequency. The factors which influence the choice of IF in TV receivers 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 can chosen as 40 MHz and the local oscillator frequency as 140 MHz. I/P MIXER IF O/P I/P MIXER IF O/P F C =110 MHz F C =140 MHz MIXER MIXER Desired signal = 100 MHz Desired signal = 100 MHz Frequency Frequency Image frequency = 120 MHz Image frequency = 180 MHz If another station is operated at 120 MHz, it will also beat with local oscillator frequency 110 MHz to produce 10 MHz. If a station operated at 180 MHz, it will also beat with local oscillator frequency 140 MHz to produce 40 MHz.

In each case, undesired signal which gets received is spaced at a gap of twice the If frequency and is known as image signal. Image rejection ratio is defined as the ratio of output due to desired station to the output due to image signal. Without RF amplifier, there is nothing to stop the reception of image signal if it is present. Thus RF amplifier helps in reducing interference due to image signals and a higher IF results in a very high image rejection ratio. By choosing IF to be greater than half the entire band to be covered it are possible to eliminate image interference. For lower VHF band (41-68MHz) 13MHz MHz. the IF is Upper VHF band (174-230 MHz) the IF is 28 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: ANTENNA RF AMPLIFIER MIXER IF OUT LOCAL OSCILLATOR If output from local oscillator of a TV receiver gets coupled to the antenna, it will get radiated and cause interference in another receiver. Higher the IF, there is a greater separation between resonant circuits of local oscillator and RF amplifier circuits. Thus lesser signal is coupled from local oscillator to antenna and interference is reduced. 3.IMAGE FREQUENCIES SHOULD NOT LIE IN FM BAND: FM band is 88MHz 110 MHz. With IF as 40 MHz, 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.

4. INTERFERENCE OR DIRECT PICKUP FROM BANDS ASSIGNED FOR OTHER SERVICES: Amateur and industrial application frequency band is 21-27 MHz. If IF is above 40 MHz, second harmonics of this band will not cause any serious direct pick-up problems. 5.GAIN: Amplifiers with large gain at relatively low frequencies can be easily build.40 years back, IF is 120MHz.Due to limitations of active devices available, IF is12mhz.now active devices and ICs can work well at high frequencies. Choice of IF leads to 40 MHz. In 625-B system, picture IF is 38.9 MHz 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 and

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 IF signal. With 0.5mv o/p from mixer, 4v is given to video detector which needs an overall gain of 8000. Gain is adjustable by automatic gain control. To achieve this gain, at least three stages of tuned amplifiers are cascaded. Staggered tuning is employed to obtain wide band characteristics. Each amplifier is tuned to a different centre frequency and quality factor Q is lowered to obtain broad response. IF response of monochrome and colour receivers must include rejection filters to suppress adjacent channel interference. IF AMPLIFIER: It is necessary to include the trap circuits for vestigial side band correction. And to prevent

adjacent channel interference. Staggered tuning is employed to obtain wide band characteristics. AGC Circuit: AGC circuit varies the gain of a receiver according to the strength of the signal picked up by the antenna. Useful signal strength at the receiver i/p terminals may vary from 50 µv to 1v or more. dc voltage proportional to i/p signal strength obtained by rectifying video signal as available after video detector. AGC bias is used to control the gain of RF and IF stages in receiver to keep o/p at video detector almost constant.

Output is filtered and the dc voltage obtained is fed to i/p circuits of RF and IF amplifiers to control gain. Decoupling circuits are used to avoid interaction between different amplifier stages. RF amplifier is not fed any AGC voltage till the signal strength attains a predetermined level. This is achieved by providing voltage delay circuit in AC line and such a provision is called delayed AGC The change in gain is achieved by shifting the operating point of transistors. Changing the gain changes the ß. When the gain is changed by shifting the operating point towards current cut-off, it is called reverse AGC. When gain is changed by moving the operating point towards collector current saturation, it is called forward AGC

NON-KEYED AGC: AGC voltage developed across the peak rectifier tends 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 during line sync pulse periods and ripple is not present. Noise effects are also minimized. VIDEO DETECTOR: Modulated IF signals after due amplification in IF section are fed to video detector. Detector is designed to recover composite video signal and to transform sound signal to another lower carrier frequency. This is done by rectifying input signal and filtering out unwanted components. A diode is used to rectify either positive or negative peaks of input signal. LC filter is used to avoid undue attenuation of video signal while filtering carrier components. Circuit bandwidth must extend from 0 to 6MHz to include sound inter carrier frequency of 5.5MHz.

The detector output must include dc level of the demodulated signal. The detector diode polarity is chosen to to fit the needs of video amplifier. Cathode drive at the picture tube is now almost universal. Therefore, the polarity of the video signal output is positive sync. The video amplifier itself is usually an inverting amplifier (emitter follower). Then Its input must be of negative sync polarity. An advantage of this polarity is that noise pulses that exceed sync peak level can be clipped off by adjusting cut-off level of the amplifier.

Video Amplifier: The primary function of video amplifier is to provide enough gain such that the output video signal attains amplitude to drive picture tube from cut off for blanking to practically zero grid cathode voltage for peak white. In (p-p) values, signal may vary from 30v for small picture tubes to 150v for large picture tubes. The o/p of video detector is about 2 to 3v which needs amplification through video amplifiers. Gain control of video amplifier constitutes contrast control and for background brightness. For correct reproduction, large BW is to be maintained. If RC coupling is used, dc component is lost. This can be restored by clamping techniques after amplification. In monochrome receivers, amplifier stages following video detector are called video amplifiers. In colour receivers, delay line is used usually referred as luminance or Y channel.

FROM LAST IF AMPLIFIER TO 5.5 MHz SOUND IF AMPLIFIER SYNC (+) VIDEO 38.9 MHz EMITTER VIDEO DETECTOR STOP FOLLOWER AMPLIFIER TRAP R L SYNC (-) CONTRA ST TO CATHODE OF 38.9 MHz STOP TRAP FILTER: It prevents the entry of IF signal in the video amplifier. EMITTER FOLLOWER: TO AGC CIRCUIT PICTURE TO SYNC SEPERATOR It is a buffer stage. It does not include the signal polarity. It is usually included to provide isolation and impedance match on either side. It has high i/p impedance to work into high impedance load. It also provides low source impedance as its o/p for feeding AGC, sync separator and to 5.5 MHz trap circuits. It reduces amplitude distortions during sync peaks.

VIDEO AMPLIFIER REQUIREMENTS: Amplitude and phase relations of components of video signal must be preserved for producing high resolution picture. so video amplifier should have linear frequency response from dc (0 Hz) to 5 MHz. This is possible 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 needs 60 to 80 volts of video signal. It is usually an inverting amplifier and needs a negative sync signal at its i/p to deliver positive going signal at cathode of picture tube. 2. AMPLIFIER BANDWIDTH: Higher frequencies are needed to reproduce horizontal information of the picture. Camera tube scans alternate black and white lines. Actual period is 52μs. To obtain round figure value, line period 50μs. For reproducing very minute details, a high video frequency would be necessary, channel BW has been fixed at 5 MHz.

Signal frequencies corresponding to picture information scanned in vertical direction is much lower compared to picture information within a line. When brightness varies, signal frequency is lower than 25 Hz. This changes dc level corresponding to change in brightness level becomes almost zero. Ideally, video amplifier response should be linear from dc to highest modulation frequency of 5 MHz. Possible only when video 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 ac coupling is used, gain decreases at IF. Inequality in gain at different frequency components of signal is called frequency distortion. Excessive frequency distortion cannot be tolerated because it changes picture information. If HF content of video signal is lost, horizontal line cannot be reproduced. Frequencies from 100 KHz to 25 Hz Main part of picture information.100 KHz to10khz black and white details in horizontal direction.10 KHz 25Hz changes of shading in vertical direction. If LF response is poor, 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. Phase shift implies time shift. Resultant shift in relative positions of various picture elements is detected by the eye as distortion. Therefore relative phases of all frequency components must have same time delay. Eyes are sensitive to time delay errors and appear as smear on picture. At high video frequencies, phase distortion is not as evident because time delay at the frequencies is very small. 5. AMPLITUDE DISTORTION OR NON LINEAR DISTORTION: If operating point and signal amplitude is not carefully chosen, amplitude distortion occurs. It results in clipping and limiting of signal. Some gain has to be sacrificed to avoid amplitude distortion.

SAW filter: The operation of saw filter is based on the phenomenon of surface acoustic waves. Surface acoustic wave is anon electromagnetic wave that travels along the surface of piezo electric substrate. When electric field is applied, physical changes occur. When stress is applied, a proportionate electric potential is developed. Transducers are provided at two ends of wafer to convert electric energy to acoustic wave energy and then back to electric energy. Input signal frequency is about several GHz but on conversion to saw, signal travels at the speed of sound. SAW FILTER FOR IF SECTION: Bandwidth and frequency characteristics of SAW filter depend on geometric structure of input transducer array. Filter can be designed to have any group delay characteristics by changing the distance between electrodes of transducer array. It consists of i/p and o/p transducer arrays separated by a multistrip coupler which prevents

distortion of surface wave transmission caused by reflection of energy. Input transducer is designed to obtain for IF band pass. Output transducer is designed to obtain a broad band response. Its function is to convert the signal from surface acoustic wave energy back to its electrical form. Summary: In this chapter we have looked into the requirements and design aspects of TV broadcast receivers. The functional block diagram of monochrome TV was present and explained briefly. TV tuner and IF subsystem was explained. This will be the basics for colour TV receiver and it will be explained in further chapters.