[sdiy] VCPO Variable-Phase Oscillator

[S Ridley]

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> 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> 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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