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

[Chromatest J. Pantsmaker]

I think you would implement the PLL circuitry *before* the audio-rate
modulation.

On Mon, Dec 1, 2025, 9:22 AM cheater cheater via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Interesting! I had no idea that was how a Theremin worked. Those can
> keep the pitch really well, actually.
>
> A PLL would mean completely digital control, wouldn't it?
>
> Do PLLs like that respond well to audio rate modulation?
>
> The idea with a second core was to let the oscillator drift a little
> bit owing to its analog nature, while also correcting for the gross
> drift. Do you think that would keep things close enough in tune to be
> musically useful?
>
> Low bass on a Theremin sounds pretty good.
> https://www.youtube.com/watch?v=CgPgasgoVbU
>
> I think a large portion of the sound is a question of wave shaping.
>
> On Mon, Dec 1, 2025 at 9:59 AM <rburnett at richieburnett.co.uk> wrote:
> >
> > It will work.  Varactor (varicap) diode VCOs are nothing new, although
> > they usually use a diode designed specifically for the job instead of an
> > LED! Likewise the hetrodyning (RF product "mixing") is a well
> > established technique in radio transmitters/receivers for shifting
> > frequencies up or down.
> >
> > The downsides of varicap VCOs are drift and poor linearity (MHz/volt) of
> > the control law. Both of these issues are typically solved in a modern
> > radio transmitter or receiver by making the VCO be part of a PLL
> > structure. This locks the VCO frequency to any one of a number of
> > multiples of an accurate (crystal) reference frequency. Then you get the
> > best of both worlds: adjustable frequency with perfect control
> > linearity, combined with crystal accuracy and low drift.
> >
> > If you mix together two RF signals up in the MHz to produce an audio
> > tone the resulting pitch will be *incredibly* sensitive to tiny
> > percentage changes in either of the RF oscillator's frequencies, and
> > things like phase noise. Whether the result sounds to your liking is
> > another matter!  I'd expect it to sound like a Theremin because that's
> > essentially the mechanism that it uses to generate audio.
> >
> > -Richie,
> >
> >
> > On 2025-12-01 03:31, cheater cheater via Synth-diy wrote:
> > > 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
> > >
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