[sdiy] VCPO Variable-Phase Oscillator

Thu May 23 10:52:25 CEST 2024

Might the Electronotes diagram have also 'lifted' from the 1974 source ?    I don't have the original schematic to see if it's the same component values or not.
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From: Synth-diy <synth-diy-bounces at synth-diy.org> on behalf of Phillip Gallo via Synth-diy <synth-diy at synth-diy.org>
Sent: 23 May 2024 01:04
To: synth-diy at synth-diy.org DIY <synth-diy at synth-diy.org>
Subject: Re: [sdiy] VCPO Variable-Phase Oscillator

Since it was lifted directly, a notice would have been appropriate.  Perhaps it's a noteworthy coincidence.

p

On Wed, May 22, 2024 at 5:31 PM Mike Bryant <mbryant at futurehorizons.com<mailto:mbryant at futurehorizons.com>> wrote:
Why would there need to be.  That technique was definitely in use in 1974, and probably long before that.

________________________________
From: Synth-diy <synth-diy-bounces at synth-diy.org<mailto:synth-diy-bounces at synth-diy.org>> on behalf of Phillip Gallo via Synth-diy <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>>
Sent: 23 May 2024 00:25
To: synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org> DIY <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>>
Subject: Re: [sdiy] VCPO Variable-Phase Oscillator

.... and NO reference within the ETI article to Electronotes Application Note #73. 1978.

p

On Wed, May 22, 2024 at 4:42 PM S Ridley via Synth-diy <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>> wrote:

Like this?  Page 71-74:

https://www.worldradiohistory.com/UK/Electronics-Today-UK/80s/Electronics-Today-1983-01.pdf

Steve

On Wed, 22 May 2024 at 19:38, Brother Theo via Synth-diy <synth-diy at synth-diy.org<mailto:synth-diy at synth-diy.org>> wrote:
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<mailto: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
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