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I'm playing with an idea for a VCO (Voltage Controlled Oscillator), and would like to make something a bit out of the ordinary. My idea is to run multiple sawtooth oscillators from a single control voltage, with the possibility of de-tuning the frequency of each individual Oscillator core to some degree.

I was hoping to make this quite simple, and was thinking of using a 40106 Hex Inverting Schmitt trigger for the oscillator cores, and a Voltage controlled Current Sink as the control for this (oscillator schematic in picture, where the sawtooth waveform is taken from the Schmidt trigger input).

Current Sink controlled Oscillator

Making a single such oscillator was not hard. However, once I want to combine 2 or more of these cores, I cannot figure out a reasonably simple way to share the current sink between them. As I would like to be able to slightly tune their individual frequency, just connecting them directly causes them to sync up to the fastest one.

I could of course create one Current Sink for each, but the circuit for this is quite complex and also temperature dependent. So it would be a lot simpler if I could share one sink for all cores.

Does anyone have any good suggestions? I am aware that this may be trivial (or impossible), but I am sadly not that fluent in electronics (especially analog circuits).

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  • \$\begingroup\$ Define I/O properties 1st as desired. Normally this is used for PWM but varicaps could be used . \$\endgroup\$ May 27, 2021 at 22:04
  • \$\begingroup\$ You might want to look at the venerable 555 timer ic. Should have all you need and costs cents. Some of the old video games used these for sound effect generation. \$\endgroup\$
    – Kartman
    May 27, 2021 at 22:08
  • \$\begingroup\$ how well do you need them matched? \$\endgroup\$
    – Pete W
    May 27, 2021 at 22:13
  • \$\begingroup\$ The general idea is that a 40106 can provide 6 oscillators with just a few external components for each. Using other chips are fine (like the 555), but would require a lot more components and PCB real estate. I was hoping to make something fairly simple. They do not need to be very well matched. The idea is to have a pot on each which can detune them, but currently I cannot make them not synced (as the reset from one inverter, resets all of the oscillators). \$\endgroup\$ May 28, 2021 at 6:57

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I would create one high quality current sink, then mirror them to each VCO. This should be far simpler than a new sink for every VCO.

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    \$\begingroup\$ Current mirrors are great in opamp ICs where all of the parts are physically and thermally matched. With discrete components, they do not track well at all. \$\endgroup\$
    – AnalogKid
    May 27, 2021 at 22:28
  • \$\begingroup\$ I already use a current mirror for my Voltage Controlled Current sink. But I am not familiar with how you can mirror the output of one sink to the output of multiple others? Do you have any examples (I am quite the noob :/). Also, how do you avoid the mirror mirrors to become affected by temperature? In my current sink implementation, most of the parts are there to insure stable operation independently of temperature (take a look at any Synth VCO design for some insight in what this is about). \$\endgroup\$ May 28, 2021 at 7:06
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What is the worst case frequency range for each oscillator "core"? And, what power supply voltages are available for the circuits? Depending on the answers ...

The lower transition voltage level for a 40106 is several volts above GND. If you replace the 10 uA current sink with a variable, regulated voltage source with current-sinking capability, and vary that output voltage between GND and something just under the gate's lower transition voltage, you will have a single, zero-ohm point that can drive a large number of single-gate oscillators. Note: The frequency will vary logarithmically with voltage.

Adjusting the 1 K resistor (reference designators - !!!) lets you de-tune each oscillator individually.

Note: As these are CMOS circuits, scaling the impedances up by 10x will increase the isolation among the circuits.

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  • \$\begingroup\$ Yeah, but the point of a current sink is to have a pure sawtooth. Once you change this to a voltage source, the sawtooth becomes very distorted. Also, the 1K resistor can be changed for tuning in the current design. The problem is that the oscillators sync up and all run at the same frequency; when one inverter resets the cap, it also resets all the other caps. This is what I need a way to avoid. \$\endgroup\$ May 28, 2021 at 7:01
  • \$\begingroup\$ I forgot to answer your first questions :/ The frequency range does not have a hard requirement, but use 50 Hz to 2kHz as a minimum maybe? The supply for the 40106 is +12V (I use what's known as the Eurorack standard). \$\endgroup\$ May 28, 2021 at 11:44

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