[sdiy] VCPO Variable-Phase Oscillator

[Brother Theo]

Hi, this looks like a lot of fun. Are you producing them? Have you thought
of adding one or two more phased stages?
--Tim Ressel

On Sun, May 19, 2024 at 7:12 PM Tim Parkhurst via Synth-diy <
synth-diy at synth-diy.org> wrote:

> Hi All,
> Here's a weird little something I came up with several years ago that I
> think might be fun. It uses two VCO cores and a special synchronization
> circuit that allows voltage control of the phase between the two cores
> while they track at the same frequency. It's something I haven't seen
> before, so I'm thinking it's a super-special invention I came up with. In
> any case, that's my story, and I'm sticking to it. The theory below is a
> bit of a read, but I've got a little more documentation to go with it if
> anyone is really interested.
> ============================
> The VCPO features two triangle core oscillators in a setup that allows one
> core to drive the second. The output from the second core is variable in
> phase relative to the first. Let's see if I can give it a decent
> explanation...
> ===
> The circuit uses two triangle core VCOs. It derives a signal from the
> first core (call it A) and uses that to drive the second core (B). In the
> schemo here, I'm using two VERY simple 2164 VCOs, but the same sync / drive
> scheme could be applied to other triangle cores. This particular setup
> doesn't quite track 1 v/oct, but that is just because I used the simplest
> setup I could come up with for this first shot. With some adjustments and
> component tuning, I think it could be made to track just fine. As an LFO
> though, it works nicely. You could probably build one of these that tracked
> pretty well with a 13700-based core, or a slightly more elaborate 2164
> circuit.
>
> Core A is based around U5, U6, and U7. U1 and U2 form the CV summer. The
> CV drives U6 (Core A) and U6 (Core B). Core A is just a basic, plain
> vanilla tri core, with U5 acting as the comparator, and U7 acting as the
> integrator.
>
> Core B is based around U8, U6, and U9. Core B is also fairly standard, BUT
> NOTE that there is no hysterisis feedback around the comparator, U8 (Core A
> uses R10). This is important, because the comparator gets an input signal
> that is a combination of the Core B integrator (through R21) and the Core A
> integrator (through R20). This is a big part of what keeps the two cores
> locked to the same frequency.
>
> Now here's the "secret sauce:" U4 and Q1 change the polarity of the
> reference voltage going to the comparator of Core B. Normally, if you have
> a triangle going to a comparator input and you vary the reference voltage,
> you'll get pulse-width modulation. However, what we do here is use Q1 and
> U4 as a sign flipper, so that the reference voltage to the comparator
> SWITCHES POLARITY depending on whether the triangle at the comparator input
> is on the up slope or the down slope. The result is that we get a variable
> PHASE square wave coming out of the comparator.
>
> Basically, we take the triangle from Core A, and compare it in U4 to a
> reference voltage that changes polarity with the slope of the triangle.
> This gives us a variable phase square, and we use THAT to drive the Core B
> integrator. R21 provides a 'weak feedback' that helps keep the Core B
> triangle levels correct. The sign flipper (Q1, U4) is driven by the square
> wave from the Core A comparator (U5).
>
> In the Phase CV Summer / Sign Flipper (U3, U4, Q1), I use R37 to
> compensate for a slight offset that was introduced into the Core B outputs
> (B1=Square, B2=Tri). A better method  might be to use R33 instead. I have
> not tested the R33 method, but you would adjust this until the Phase CV
> coming out of U4 was equal in magnitude when it flipped sign. Again, if you
> use R33, you might not need R37. D3 and D4 are also untested, but are meant
> to avoid having the Phase CV from U4 exceed the +/-5V limits of the
> triangle outputs. If this CV goes above +5 or below -5, a DC offset will be
> introduced into the Core B outputs.
>
> So basically, that's it. Two triangle cores, remove the hysterisis
> feedback from the B comparator, and use the sign flipper to derive a
> variable phase square wave from Triangle A to drive Comparator B. Again, my
> guess is that the U3, Q1, U4 circuitry could be applied to sync just about
> any two triangle core designs. Ideally, you want the two tri cores to track
> together fairly well to avoid introducing more level differences between
> the A and B outputs. This might work well with the two gain cells available
> in an LM13700. Also, the output current from the CV Summer would need to be
> doubled from that of a typical application, since it will be driving two
> VCO cores.
>
> I've tested this circuit on a breadboard and it's a lot of fun. The sign
> flipper scheme allows you to vary the phase of the Wave B outputs, but it
> avoids the waveform discontinuities you'd get with a traditional capacitor
> reset "sync" circuit.
>
> One limitation of this circuit is that it 'only' allows the B output to
> vary up to 180 degrees from the A output. This means that you can't quite
> get 'through zero' phase cancellation between the two outputs. To overcome
> this, I've thought of using the output of Core B to drive a third VCO core
> (C) utilizing another Phase CV Summer / Flipper. In theory, this would
> allow the C outputs to vary by up to 360 degrees from the A outputs.
> =============================
> Okay, there it is. Whaddaya think?
>
> Tim (going through a phase) Servo
> ---
> "Imagination is more important than knowledge." - Albert Einstein
> ________________________________________________________
> This is the Synth-diy mailing list
> Submit email to: Synth-diy at synth-diy.org
> View archive at: https://synth-diy.org/pipermail/synth-diy/
> Check your settings at: https://synth-diy.org/mailman/listinfo/synth-diy
> Selling or trading? Use marketplace at synth-diy.org
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20240522/5c2b5544/attachment.htm>