Let's Design and Build a (mostly) Digital Theremin!

"Interesting articles.Amazing what people do in their kitchen !They can't be married ! About thermal drift, I made an experiment.I heated the coils with a hair dryer for several minutes. I only noticed a drift of one or two digits.From 0.970 mH to 0.971 mH and from 1.925 mH to 1,927 mH." -- André

Yes, I've done tests too from freezer to room temperature, and saw no drift with the air-cores.  Ferrites on the other hand drift a lot.  I turn the meter on for a while before testing to hopefully let it stabilize, who knows how stable these inexpensive things are.  I know the capacitance measurement of the meter I have drifts a lot, I assume due to the ferrite reference coil inside it.

"After using option "7. Factory Reset", the out-of-bounds values were gone and also the "ER!" message. Once it was working I used Quartus to program the FPGA configuration that I compiled and it still was working as with the original FPGA configuration."  -- gerbold

I thought that was probably the issue.

"When monitoring the pitch drive output of the AFE, I can see that the frequency raises up to 1.6 MHz, then breaks down to zero and rises again. I assume this is the effect of my poor inductance with the ferrite ring core. The volume does look pretty much the same."

If you have some small pF caps laying around, you might be able to get it to lock with your low Q inductors by plugging them into the C6 socket on the AFE board.  Maybe start with 10pF or so and stop at 100pF.  The goal is ~3Vpp or so at the "C divider" test point if you have a scope.  You won't need to do this with André's high Q coils.

Today I tried your suggestion of applying just a solvent on the coil.
I used acetone which does react with PVC.
It doesn't work because acetone does not penetrate between the wire turns !

I tried something else. I just coated the coil with a clear varnish in spray.
It's drying now. I'll tell you the result tomorrow when it's fully dry.

Thank you for experimenting and reporting your results André!