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

[rburnett at richieburnett.co.uk]

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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