Auto Tune

[Dr Vel]

I am wanting to add auto-tune (PB Tune in schematic below) to an earlier model Bobcat which lacks the feature. It is a very similar circuit yet different enough I cannot simply add the missing parts. I need to fully understand what they were doing in order to design a similar function for the older model. I am not clear on exactly what the function is doing in terms of circuit theory. Can anyone provide an explanation of this function in terms of the circuit below or at least how auto-tune functions in general terms?

[DavidB]

Take a look at the Tesoro Aztec schematic. You can do similar and add an op amp from the output of your first 358 stage after the demodulator fet switch to the non- inverting input. Your detector only has one channel but this will still function like the re-tune circuit used in some all metal channels. If you notice most motion detectors have 2 channels with one demodulator reference at 0 and the other at 90 (phase quadrature). You will also notice that the motion filters are A/C coupled so they respond as you pass over a target. The non motion all metal channel is D/C coupled so they use a feedback circuit to adjust the level back to zero in the auto-tune mode. The Aztec is a quadrature non motion machine that uses this method in both channels.