DEMODULATOR circuit

Posted on Feb 7, 2014

amplifier, with bias at cutoff. Q5 acts much like a grid-leak detector. Without the subcarrier input, the demodulator cuts off and prevents signals from passing. Of course, the chroma signal must be at a low level. Otherwise, chroma might gate the demodulator on. Positive excursions of the reference subcarrier are strong enough to gate the demodul

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ator on. These positive excursions cause Q5 to sample the chroma signal at the reference subcarrier frequency. Whenever the reference subcarrier swings positive, it gates the demodulator on. Whenever the reference subcarrier swings negative, it gates the demodulator off. When the demodulator is off, no signal passes through it. In the demodulator, the subcarrier and chroma mix. The output is only positive subcarrier excursions that coincide with the chroma signal phase. Nothing else passes. Rectification. Only positive chroma excursions survive the trip. That is, the demodulator rectifies the chroma and the reference. During rectification, the demodulator eliminates the lower chroma sideband. A typical AM detector rectifies in the same way. Because the high-frequency subcarrier remains in the output, we need to filter out the subcarrier remnant. Otherwise, it might appear on the TV screen. The low-pass circuit. Notice capacitor C32 in the transistor Q5 collector circuit. This capacitor and collector resistor R22 form a low-pass filter. The filter attenuates the 3. 58 MHz signal. Attenuation keeps the burst and regenerated subcarriers off the picture tube. Unlike the original Col-R-Tel filter, this filter requires no peaking. After filtration, the demodulator`s output bandpass covers the span from zero to 500 kHz. Saturation Potentiometer VR2 controls how much color that demodulator Q5 passes to succeeding stages. By...

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