Matt when scanning circuit change frequency of the PLL - then PLL seeks for max amplitude of signal and lock to this frequency of max amplitude ?
I'm not implementing the scanning circuit at this point. From what I can see so far, the PLL does everything we need. If the PLL is configured to have its capture range anywhere around the actual resonant frequency point, the PLL will find this frequency and track it automatically. All that is needed is to adjust the PLL center frequency so that it's somewhere close to the actual resonant frequency of the VIC and WFC. I'm currently looking at how to best adjust the capture range so the PLL doesn't attempt to lock on a harmonic, which can happen if the center frequency is set too high.
The PLL itself doesn't look at amplitude at all. It finds the edge of the signal, typically at the zero crossing. Hence, it's called a phase lock loop, because phase is what it is tracking. The means it uses to do this is by adjusting its internal VCO frequency until the phase angles line up or match. The output of the VCO drives the VIC and the feedback into the PLL phase detector determines how good a match it has and adjusts the frequency of the VCO accordingly. What you get in the end is exactly like you said--maximum amplitude or a resonant condition with voltage and current exactly 90 degrees from each other. This is the crux of impedance--a condition where at maximum voltage, there is zero current, which is why the voltage rises--there's nothing to stop it. Resistance can "impede" current, but it cannot do anything to voltage. So with no current, no resistance is felt by the tank circuit. Keep in mind this is only true precisely where the current is zero; at any other phase angle this is no longer true--regular Ohm's Law rules apply.
Where I'm a little concerned about all this is the talk of frequency doubling. What this is, is a wave traveling through the coils and bouncing back in such a way where you have two waves superimposed on each other--one came from the driver and the other is the reflected wave. I suspect the feedback may see both of these waves; if it does, it will try to speed up the VCO to match the phase, which is not what we want. I'm not real sure yet how Stan handled this unless by purely using a filter. It should be possible to take the signal exiting the feedback circuit and divide this by two before sending it to the PLL phase detector. I can't see that Stan did this or if he did, I'm not understanding how without using a D flip-flop or something similar. Maybe based on where the pickup coil is positioned, the feedback circuit never sees the reflected wave. Just not real certain at this point, but we will find out.