How to add automatic frequency control to varicap voltage?
This is how I understand AFC: -

Typically, an AFC circuit works by examining the IF frequency for being exactly 10.7 MHz or 455 kHz (AM). The IF frequency is the result of mixing the local oscillator with the incoming received RF and if the local oscillator drifts off the IF frequency changes and the FM demodulator will suffer and you'll hear audio distortion.

Because the local oscillator isn't fixed (it has to be at least as wide in range as the incoming RF) it is prone to drift however, if the IF output is not (say) 10.7 MHZ, this can be detected by a phase comparator that has a fixed 10.7 MHz oscillator reference.

The phase comparator will not have a dc output - it will rise and fall with the modulation but its average value will represent the "error" of the local oscillator from 10.7 MHz. This error is low pass filtered (sub 5Hz e.g.) and amplified and fed to the varactor device(s) that control the local oscillator.



Hence, once an RF station is close to being acquired (by course tuning), the AFC kind of takes over and nudges it into near-perfect "lock".

I really can't see how your AFC is intended to work.