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

"Another possible method I can think of is to wind the coil using two (or more) wires in parallel. One makes up the coil, and the other spaces the windings out. The second wire can be made of any material. It can be removed after the winding is finished, or left there if it's not magnetic or conductive. This is probably more difficult than using a single wire, but should still be easier than basket/honeycomb winding. One potential problem is that the friction between the wire and the former is normally quite low, if we remove the second wire, there will be space for the first wire to slide. We may need to use some adhesive to stick it to the former (which also has a higher dielectric constant than air / vacuum, though)."  - Zhuoran

One could perhaps use thread to space things out?  If it's porous then it would provide a path to the coil former for varnish / dope, likely increasing physical stability and therefore potentially decreasing thermal drift.  It's something of a challenge winding anything finer than say 32 AWG with my crude winder, the prospect of adding thread to even finer gauges isn't super appealing.  It's not clear what the ideal spacing might be for a given mH and aspect ratio, so it would likely require some (painful) experimentation.  And it would make the coils physically larger, though I personally am OK with that.

Lately I'm worrying less about the dielectric constants of the former & dope.  There seems to be a ceiling of 200 or so beyond which Q can't be practically increased.  Not sure how much of a Q factor the intrinsic (self) capacitance of the antenna plays into this either.  Q is a direct multiplier of drive=>antenna voltage, frequency selectivity, and oscillator stability, so it's a triple-whammy type spec.

90 degrees phase shift in feedback current is a kind of surprise for me.
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Trying to replace BJTs with current feedback opamp IC.
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Simulation with LT1395 and LT6210 instead of discrete opamp shows that a trick with getting drive signal shifted by 90 degrees from negative input current will not work because these ICs are tuned to have minumum feedback current during operation.
Small current (10-40uA) is too hard to sense. So, to use normal opamp with small feedback current instead of discrete one (which is tuned to have high feedback current), different solution for 90 degrees shift should be found.
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Ideas for getting of 90 degrees shift at resonance.
* get drive feedback without shift; advantages: we can get rid of feedback comparator, and use something like self-biasing unbuffered inverter buffer instead.
* current sensing cirquit has to be reworked to provide additional 90 degrees shift (e.g. via additional integrator); possible advantage - integrated signal is naturally LP filtered and has less noise.

It seems that the phase shift is provided by the delay of the amplifier, which is mainly the delay of the "I-to-V converter" (Q93, Q94, Q98, Q103, C9). The "I-to-V converter" is basically an integrator that integrates the current (mirrored) from IN-. Since integrators shift the phase by 90°, it's not very surprising to see a 90° phase shift in this circuit. The problem is that the result depends on the speed/gain/bandwidth of the integrator, which, when using ICs, we don't always have much control of, and are likely too high for this purpose.

I think this is effectively using the CFOA as an integrator (to provide feedback in order to "solve for" I(IN-) such that ∫I(IN-)dt (+ C) = DRV_SINE and eventually creating a differentiator: I(IN-) = d/dt(DRV_SINE), the current at IN- is the derivative of the drive signal, and R30 converts it to voltage).
If we don't need to use the CFOA to provide the phase shift, we can simply replace it with a buffer.

Maybe instead of using CFOA, use an actual integrator/differentiator to provide the shift, or use a phase detector that does not require the 90° shift is more sensible?

It seems that the phase shift is provided by the delay of the amplifier, which is mainly the delay of the "I-to-V converter" (Q93, Q94, Q98, Q103, C9). The "I-to-V converter" is basically an integrator that integrates the current (mirrored) from IN-. Since integrators shift the phase by 90°, it's not very surprising to see a 90° phase shift in this circuit. The problem is that the result depends on the speed/gain/bandwidth of the integrator, which, when using ICs, we don't always have much control of, and are likely too high for this purpose.

I think this is effectively using the CFOA as an integrator (to provide feedback in order to "solve for" I(IN-) such that ∫I(IN-)dt (+ C) = DRV_SINE and eventually creating a differentiator: I(IN-) = d/dt(DRV_SINE), the current at IN- is the derivative of the drive signal, and R30 converts it to voltage).
If we don't need to use the CFOA to provide the phase shift, we can simply replace it with a buffer.

Maybe instead of using CFOA, use an actual integrator/differentiator to provide the shift, or use a phase detector that does not require the 90° shift is more sensible?

Updated model, with integrating OTA current sensor. (Available under the same link)

Here, phase shift is not a property of amplifier, it's a result of integration.
 
CFOA is not a requirement anymore. Any unity buffer should be ok.

Feedback output does not have phase shift anymore, implemented on two inverters.

 Only single comparator is now required.