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Pump Charged Pulse Averaging FM Demodulation
The IF signal is further ampified by Tr1 and Tr2 and fed into the limiter Tr3 which produces a clean square wave to pump charge the 180 pF capacitor.
When Tr3 is not conducting the 180 pF charges up through the 470 ohm resistor and the diode.
One side of the capacitor holds a positive charge and the other a negative charge.
When Tr3 conducts the positve side of the capacitor is pulled to ground and negative side of the capacitor has a negative voltage less than ground.
The emitter of Tr4 is then less than ground and the base is positive with respect to the emitter.
Tr4 conducts for a short fixed amount of time until the 180pf capactitor is discharged.
These fixed length pulses can be averaged into a demodulated signal.
The circuit can also be used as a linear analog frequency meter.
Pulse averaging demodulator.
A: Input signal from the IF. B: The squared up limiter signal. C: Fixed length pulses created from the limited signal. D: Averaging (integrating) those fixed length pulses into a recovered signal.
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