AGC
AGC
After trying several more elaborate AGC
circuits, I came up with the very simple circuit above. On a quiet frequency the
output to the IF amplifiers is about 6 volts. When the receiver is tuned to a
strong signal, the signal from the audio pre-amp is amplified by the first BC109
transistor, then converted to a slowly changing DC voltage by a voltage doubling
rectifier. This DC voltage is inverted by the second BC109. The voltage at the
collector of the second BC109 will decrease as the received signal gets
stronger. This voltage is applied to gate 2 of the IF amplifiers. The output is
also used to drive the signal meter. I used a 50 microamp moving coil meter. The
10K pot is used to zero the meter. The 100K pot sets the meter deflection. I set
mine so that very strong 100KW broadcast stations read full scale on the meter.
If you have an accurate signal generator, you can calibrate the meter to read S9
for 50 microvolts input. I will calibrate mine properly, one of these days......
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EI9GQ HOME BREW RADIO
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