Tuesday, 6 May 2014

Monochrome TV Receiver

Monochrome TV Receiver:

Monochrome TV Receiver has the following blocks.

1. TV Receiving Aerial: the TV signal radiated by the transmitter has to be intercepted. For this an antenna with high gain, Broad Band, highly directional is used. Yagi-uda multi-element array is preferred. Impedance of the aerial should match the impedance of the transmission line. The aerial also selects the required signal and rejects unwanted signals.

2. Tuner: Also called RF Tuner/ Front end. Signals from Aerial are amplified, down converted to intermediate frequency. Band and channel selection is also carried out here. This is a separate, sealed and riveted unit mounted away from other circuits. Unit’s body is grounded to avoid interference by other strong signals. Delayed AGC is used at RF amplifier.

3. VIF Amplifier: The output of RF Tuner has two intermediate frequencies i.e. video IF (38.9 MHz) and sound IF (33.5 MHz), occupying a band width of 7 MHz. Most of the amplification and selection is done here. This is also a separate, enclosed unit. Keyed AGC is used at the base of 1st IF Amplifier.

4. Video Detector: The output of VIF section is around 2 to 5VPP. This is fed to video detector to separate various signals i.e., video, AGC, inter carrier SIF. A special diode driven into saturation has many harmonics at its output. Using LPF, signals above 5 MHz are filtered out. OA 79, Ge, heavily doped, point contact diode is used for detection.

5. Video Output Amplifier: Output of video detector which is around 2 to 5 VPP is amplified by a single stage broad band, voltage amplifier to give an output of 80 VPP. Wave Trap is used to prevent 5.5 MHz inter – carrier SIF (38.9 – 33.4 MHz = 5.5 MHz) and disturb the picture. Output of this amplifier is fed to picture tube.

6. Picture Tube: This is a special type of CRT, having widescreen with wide deflection angle. Electromagnetic deflection and electro-static Focusing is used. H and V deflection coils moulded into a single unit ‘Yoke’ is mounted on the neck. Final anode is applied with Extra high Tension (EHT) which is around 12 to 18 kV. Screen is formed by long persistence P4 phosphor, which is a mixture of cadmium tungstate and zinc sulphide. The light radiated is Yellowish white. With proper output from syn section, picture is displayed on the screen.

7. Sync Section: Output of video detector which is composite video signal has H and V sync pulses either at top or bottom edges. By feeding video signal to sync separator, sync pulses can be separated. We can use clipper circuit when the video signal is with negative sync. Further, by using LPF, V- sync can be separated. Using HPF, H sync pulses can be separated. These pulses are used to synchronize the H and V saw-tooth oscillators. Output of these oscillators are amplified and used to drive the deflection coils on the picture tube.

8. Inter Carrier SIF Section:  Output of detector has 5.5 MHz, which is the result of heterodyning between VIF and SIF (38.9 MHz – 33.4 MHz). This has frequency modulated Audio Signal. By using highly selective tuned circuit (sound-take-off coil), 5.5 MHz is separated. This is amplified and frequency demodulated to recover Audio signal.

9. Audio Section: Output from SIF section which is weak AF signal is amplified both in terms of voltage and power to drive the loud speaker. Finally, we get sound output.

10. Power Supply: Various levels of DC voltages are required for the operation of TV receiver. So, 230 V, 1 φ AC is rectified, filtered and regulated to provide ripple free steady voltage to various stages. However due to many advantages switch mode power supplied (SMPS) are widely used now-a-days. EHT and BHT required at picture tube is obtained from Auxiliary power supply using Line output transformer (LOT/EHT).

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