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

[Donald Tillman]

That video is kind'a goofy.

Yes, the capacitance of a reverse-biased diode decreases with the reverse voltage.  "Varactor" diodes are optimized specifically for that, but any diode will do.

Two high frequency oscillators driving a modulator for an audio difference frequency is exactly how a Theremin works.  The frequencies are usually around 100Khz to 200Khz, and hand capacitance changes the tuning of one of the oscillators by a few percent.

It's also exactly how Morse code radio is received.  (Morse code radio does not transmit beeps.  The beeps are at the receiver where the local oscillator mixes with the received RF signal.)

And it's the basis for the quadrature thru-zero local oscillator in the Moog/Bode Frequency Shifter.

The 28 MHz this guy has is way, way too high for any useful audio work.  It covers the complete audio range in like a 0.04% RF frequency change.

  -- Don
--
Donald Tillman, Palo Alto, California
https://till.com

> On Nov 30, 2025, at 7:12 PM, cheater cheater via Synth-diy <synth-diy at synth-diy.org> 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
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