This is a recovered file. The images in this post may be out of order, and there may be duplicates.
Auto regen "AGC" - The RadioBoard Forums by sixtynine » Wed Feb 08, 2017
Finally got some time over our long weekend to impliment an idea for simply AGCing a regen RX. My interest is in SW AM, and my old ears are allergic to large and annoying exursions in volume. The idea I think has merit and works very well given it uses three components only. Ill get a video up in the next few days to demonstrate it working. I decided to try to vary the gain of the regenerative amplifier, rather than controlling the input signal level. Looking at the schematic is a bog standard mosfet regen, TR1, is an RF buffer. Voltage amplification is performed in the coil due to transforrner action. There are two feedback loops, one is the source to tap via C2, the other lesser known is the C1 Drain to tank, normally thought of as a "bypass". C1 effectively connects the drain to the bottom of the tank as in fig 1. We apply initial manual gain adjustment via gate 2 voltage to just below oscillation. To automatically vary the feedback, we have inserted our controversial old friend, a lightly forward biased diode in series with C1. The diode being DC coupled to the 4.5 v at the drain, and grounded via a 470k resistor gives a current of a few microamps putting the diode conduction point right at the "knee". Therefore, as the RF voltage excursions modulate this standing bias, C1 attains an average DC voltage level dependent on signal strength, varying the voltage across the diode, therefore varying the current through the diode , thus varying the dynamic resistance of the diode. This in turn varies the feedback from TR1 to the tank, both D1 and C1 being in series. Large RF signals decrease the voltage across the diode as C1 charges, increasing the dynamic resistance, thus reducing RF feedback, reducing regeneration and keeping the volume level constant. The diode conducts on every RF peak, effectively adjusting the feedback on a cycle by cycle basis..See fig 2 The diode also performs as a very sensitive rectifier for AM signals. The secret here is to use the lowest tapping and C2 to be as large as possible. This effectively bypasses the source to RF, giving TR1 useful RF output at the Drain. The downside is the radio will not efficiently demod CW/SSB, although will do so at much reduced volume. The prototype is stable, and very easy to listen to, certainly comparable to a normal superhet, this example tunes 4.5 to 12 megs. The video here is a quick demo of the regen receiving a moderately weak signal, the scope is monitoring the DC level at the diode cathode. This DC level is suoerimposed on the recovered audio, but you get the idea. the DC level varies in response to the RF level together with the rectified audio . The regen setting is very smooth given the "negative" DC feedback in the control loop. Heres a quicky bahdscan demonstrating the stability and constant volume obtainable with crappy signals here early in the evening. Hard to tune a large 6 MHz range without a slowmo drive. the signal is compressed to roughly the background noise level off station. https://youtu.be/QgnKZ9CR6NM And here you will see the derived "AGC" voltage superimposed on the audio from the rectified regen stage. https://youtu.be/tVq7EW1ZysU Ill now attempt to repair my scanner and upload the schematic so all this makes sense.