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

[cheater cheater]

It makes sense to mention that the LED would be sealed off from light
except for the control light source, like in any other vactrol

On Mon, Dec 1, 2025 at 4:12 AM cheater cheater
<cheater00social at gmail.com> wrote:
>
> 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