Please help me understand the problem with my theremin design

And as for the linearization coil, I guess my understanding has been too simplistic. But at least with my previous model, the theremin worked as inteded for various different L values. It just changed the linearity of the pitch field.

I will try to couple the oscillators using capacitors though.

Or here's another way I view the situation:
As long as both oscillators oscillate (which they do), there should be an output wave on the mixer, given the sum of the two oscillations will produce beats. It should not matter what kind of linearization coil there is.
One thing I have not tried yet is to substitute one of the oscillators with a sine wave generator. I will try that when I get the chance.

"As long as both oscillators oscillate (which they do), there should be an output wave on the mixer, given the sum of the two oscillations will produce beats. It should not matter what kind of linearization coil there is."  - Shweik

Well, if the beat is ultrasonic, then that 3rd order low pass filter on the output of the mixer will likely attenuate it quite a bit.

"As long as both oscillators oscillate (which they do), there should be an output wave on the mixer, given the sum of the two oscillations will produce beats. It should not matter what kind of linearization coil there is."  - ShweikWell, if the beat is ultrasonic, then that 3rd order low pass filter on the output of the mixer will likely attenuate it quite a bit.

No, that is certainly not the case. I can tune the trimmer capacitors to change the reference frequency and I'm very certain the frequencies are close enough to produce beats in the audible range.

Hello, so I did a bit of further testing yesterday.
I removed the linearization coil, but as I expected, it didn't change much. As I said, there is some reasonable audio output when the reference oscillator has lower frequency than the variable oscillator (ie. the wrong way). The resulting waveform looks like this:


As you can see, there is this spike at the top of the waves, so something is likely wrong already. What it is I have no idea.
When I tune the oscillators the right way (f[sub]var[/sub] 0cm there is also an increase in pitch.

So I tried to substitute the 330 kOhm resistors which isolated the oscillators from the mixer for 10 pF capacitors. Mind that 10 pF has about 30kOhm impedance at the oscillators' frequency, so that's quite a significant decrease. As expected, the resulting amplitude is much higher, but the behaviour of the theremin is much the same - here's a recording:
https://drive.google.com/file/d/1nj3-rZS-EUnb7kFxmSSujWRq34LRZEKd/view?usp=drive_link

So I suppose I will try to change some L C values in the oscillators next. But I won't have much time in the coming weeks, so don't expect any updates soon. I will still be grateful if you get any ideas on what might be causing these issues.