Quote:
Originally Posted by dominicbeesley
I'm going to have to (try to) analyse how you got your NFB working! I never got mine to work that well, demodulating the signal. When I tried as I was struggling to get stability with all the various poles needed to remove the RF from the AF etc...
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Yes Dom, the poles are a right pain aren't they, easily upsetting stability.
Doing a rough count, from Q3 emitter, there's one due to the modulation choke L4 (plus one at a much higher frequency, due to C13 with L4). Then the RF tank circuit, considered as RF-envelope-as-output, adds 1 (which is Q-dependent) and the matching network adds another 1. Finally the demodulator's RF filter add a further 2. I wanted to use a single-stage RF filter here, but didn't like the amount of residual RF that this would leave on Q14 source, so in went the second stage...
[Feel free to disagree with the pole count here anybody]
With all these poles, I chickened out of analytical design and went for empirical methods, aiming as much as possible to have one dominant pole (R38 C26), such that by the time the plot of AF frequency rose to the point that other poles kicked in, the loop gain would have rolled off below unity. And because the modulation is linear, I didn't need oodles of feedback to crush distortion, and it worked.
The overall loop gain is highly dependent on accurate aerial tuning (C15) because if this is off tune, the RF output amplitude is low, so feedback is low. Which means the circuit is still stable, but off-tune the feedback virtually disappears!