[sdiy] DC-1 MHz, through-zero FM, controlled, oscillator without core capacitor

[cheater cheater]

I've been watching this video by Mr. Carlson:

https://www.youtube.com/watch?v=9r6ntkwAadY

It shows an old school diffuse red LED being used in reverse polarity
as a varactor diode, and a setup using a potentiometer and that LED is
used together in the same way as a multi-plate, variable, air
capacitor such as a tuning capacitor taken out of a vintage AM radio.

The range of the simple circuit in the video is about 2 MHz. It occurs
to me that one should be able to use a setup with two such oscillator
cores, where one is being controlled, and the other one is at the
middle of its range, together with some static low pass filters to
extract only the fundamental sine waves, and a ring modulator, in
order to get A-B mixing products to extract the full range and to
center the range around 0 Hz and not around 27 MHz. Shaping to square
wave, triangle, and other synthesizer waveforms is possible starting
with a sine wave.

The low pass filters could probably require some form of capacitance,
but maybe inductors are better suited. I think there might exist
inexpensive premade brickwall filter modules for this sort of
frequency range, but I'm not a radio guy.

The circuit is demonstrated to be voltage controllable using light, by
using another LED, which can be fed constant current, dependent on
voltage input. This setup makes the control circuit electrically
decoupled from the oscillator and trivial to implement, essentially
building a vactrol.

The stability will probably suck, but if you want stability go play on a VST.

The 2 MHz range provides the capability to divide the frequency by 128
to achieve at least short-term stability, while numeric control could
provide correction current for long term drift. This could be achieved
by having a second copy of the oscillator running at a fixed
frequency, thermally coupled to the oscillator under control. The
second oscillator is input into a counter and control is exercised
using a PID loop in feedback. The oscillator core is low component
count and tiny in real estate, therefore it's easy to throw cores at a
problem until it's solved.

I hope someone plays around with this idea since I don't have a good
way of doing that myself at the moment.

I wonder if this would have a different sound than most typical VCOs,
especially if exposed to FM. If you make one, please post line-in
demos.

Cheers