Detecting Resonance

[JulesP]

Hi all,

I am developing a type of Phase Locked Loop based power supply for testing the performance of a variety of different types of electrolysers and, while the key components of the PLL work fine in generating a gated pulse, I am having trouble detecting resonance of the circuit. Theoretically, the layouts shown in pic 1 should behave as an LC circuit, and this has in principle been verified using the simulation software LTSpice (see Pic 2) albeit at a lower frequency than my circuit will resonate at. Using distilled water the electrolyser behaves as a capacitor, although it is not straightforward to measure its value in regular ways so I have resorted to calculation methods.

So with the circuit shown in Pic 3, I have been trying to get the PLL circuit to detect when resonance occurs and then to lock onto that frequency but, despite using a suitable frequency detection range suited to my cell, have been unable to do so. In fact, when I use the detection coil shown wrapped around the bifilar coil, the HF component of the PLL circuit vanishes perhaps due to the coil presenting a short to the two inputs of the op-amp.

So my question is: how have others detected resonance in this type of circuit when using a PLL or similar system to lock onto it?

Thoughts welcomed.

Thanks

Jules