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Block diagram of colour TV Receiver

Updated: Category: Education
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The Tuners used in colour TV are VHF/UHF type, now a days hyper band and super band tuners are used for cable channel reception. Varactor tuning is done by frequency synthesis technique or Phase Locked Loop(PLL) method.

Figure shows the block diagram of colour TV receiver. The RF requirement in colour TV is more critical than monochrome TV receiver. Output of tuner is given to Pre IF amplifier, which amplifies IF signal. In VIF section, SAW filter is used to get desired wave shaping and complete IF band pass response. In design of SAW filters, two inter digital electrodes mounted on piezo electric substrate. AFT control is design to maintain VIF 38.9 MHZ for its correct value. The 1st and 2nd VIF stages are AGC controlled. The sound IF detector is fed from output of 2nd VIF amplifier. The output of 3rd VIF stage feeds to video detector. 33.4MHz trap circuit (by pass filter) prevents passage of SIF & its side band to final IF amplifier. This prevents generation of strong dot patterning rate. In sound section, SIF diode detector is used where heterodyning process is done and inter carrier frequency 5.5 MHz is obtained. SIF trap is used for select wanted frequency. SIF amplifier is used to permit passage of only the inter carrier sound signal. The amplified SIF signal is processed through limiter cum FM detector in same way as monochrome TV receiver. FM detector is differential peak type or PLL type circuit. Audio power amplifier is used for final amplification of sound.

The tone control is used there for bass and treble. Volume control is used for set amplitude of sound. The output from power amplifier is given to loud speaker of IV receiver. From 3rd video IF amplifier, signal is given to video detector which employs diode as rectifier and filter circuit which grounds unwanted high frequency components. 5.5MHz filter is used to remove sound signal. The luminance signal is increased by 1st Video amplifier which operates as a grounded emitter. 2ndvideo amplifier is emitter follower.

3rd, 4th and 5th video amplifier constitutes luminance channels, where the contrast and brightness control is provided. To prevent colour sub carrier frequency 4.43 MHZ, trap circuit is inserted at the input of luminance channel. DC clamp circuit is used to restore dc signals. Contrast control is used to vary amplitude of video signal that feeds into cathode of the picture tube. The use of delay line ensures time co-incidence of chrominance signals. Comb filter is used to select frequencies that are to be passed 'without affect of bandwidth of the signal. The function of the colour decoder is to recover three colour signals from composite colour signal. The decoder is used to perform two functions, first Synchronous detection of quadrature modulated sub carrier, second to obtain R,G,B by matrixing the detected colour difference signals with Y signals. The out put of 2nd video amplifier fed to the three stage chroma band pass amplifier. The first stage is tuned to accept the chrominance signal. Delayed and suitably shaped line sync pulses are fed at the base of this amplifier transistor which is often connected as an emitter follower. The next stage is tuned amplifier having bandwidth of nearly 2 MHZ around 4.43 MHZ. This stage is also used as vestigial side band correction when unequal colour channel bandwidth is present.
The gain of band pass amplifier is controlled by ACC circuit to obtain a nearly constant output voltage. A saturation control is also used in band pass amplifier. The last stage is design to feed chroma signal to the two demodulator circuit. Last chroma band pass amplifier is also known as delay line driver because it feeds signal to demodulators through the delay line network. The other circuit is called colour killer which in turn is controlled by a 7.8 KHZ tuned amplifier and IDENT circuits. During reception of any colour transmission, a 7 .8 KHZ switching rate signal for (R—Y) is available at the APC (Automatic Phase Control] circuit of the reference oscillator. The 0° shift is given to (B-Y) demodulator and A 90° shift is given to (R—Y) demodulator. Then get (G-Y) and apply R-Y, B-Y, G-Y to drive amplifiers and after mixed with Y signal, amplified R, G, B is given to cathodes of picture tube.
Sync separator is used to separate the sync pulses from CVD (Composite Video signal). It is used to maintain 50 HZ for vertical oscillator and l5625HZ for horizontal oscillator for faithful reproduction of picture. SMPS(Switch Mode Power Supply) is used in TV receiver for higher efficiency, operating on lower voltage, miniature in size and constant output.

Thank You For Reading.


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