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

[cheater cheater]

frequency modulation inputs can carry both DC and AC components in one signal.

On Mon, Dec 1, 2025 at 10:11 PM Chromatest J. Pantsmaker
<chromatest at chromatest.net> wrote:
>
> 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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