VCO breadboard part 2

OK, gonna write this complicated stuff up maybe more for my own benefit than anything else, but your thoughts welcome.

What happens in the pulse output is the ramp goes into an op amp wired as a comparator and makes a pulse wave whose width depends on the reference voltage you set on the pulse width pot. Unlike in, for instance, the YuSynth VCO, which uses fixed voltages for the two ends of that pot, Wilson sets those voltages using sample and holds of the extreme values of the ramp waveform. Which is a lot more complicated, but I suppose he felt he had good reasons, like perhaps to deal with supply voltage variations, or with frequency dependence of the ramp wave amplitude? Anyway, the PW pot voltage limits are about -4.5 V to +4.5 V.

With the pot at the low end you get a pulse wave with a narrow positive part and wide negative part, and at the other end you get a wide positive part and a narrow negative part. Taking the width of the positive part as the pulse width, I’m interested in the minimum value of the minimum width and the maximum value of the maximum width.

What happens at the extremes is complicated, because the pulse width vs. voltage behavior varies with the value of R68, but also there is some voltage value at either end where the pulse output shuts off, and that also depends on R68. And obviously if the output shuts down that sets the extreme of the width you get, but if you reach the pot limit before shutdown happens, then that sets the extreme width.

Things are a little frequency dependent too. But at 200 Hz, here’s what I see.

R68Low end shuts off?Min widthHigh end shuts off?Max width

That last line seems to accord with Wilson’s 5% to 90% width claim.

So with 1M you’re constrained by where the output shuts off, with 3M or 10M you’re constrained by the voltage range. With 2M you have one situation on the low end and the other on the high end. I decided to try increasing the voltage range, by giving the width pot fixed ±12 V limits instead of the sample and hold values. I got this:

R68Low end shuts off?Min widthHigh end shuts off?Max width

So here it becomes clear if the voltage range is wide enough, increasing R68 has no effect on the low end and benefits you on the high end. But with the more limited range, the pot limits produce worse minimum widths at the low end and little effect at the high end as you increase R68.

In that case you presumably want R68 to be the value that’s on the boundary between reaching shutoff and not reaching it, which I find to be 2M. Below that value the maximum width decreases, and above that value the minimum width increases. As a reminder, Wilson started with 10M and changed to 1M so he could reach the shutoff region, but 2M is better from what I see.

Or you want to say to hell with Wilson’s sample and hold, just use 10M and a fixed voltage range large enough to get you to shutoff.

I wish I knew the rationale and the quantitative observations that led to his using the sample and hold scheme so I could decide if I find it worth keeping.

It looks as though the voltages from the sample and hold are rock steady over a period of hours if nothing else changes. But I tried changing the frequency from 200 Hz to 2000 Hz and both voltages got larger in magnitude by about 60 mV. I didn’t time it but I think it actually took a couple of minutes or so to stabilize. (I don’t know if that was the ramp amplitude stabilizing or if for some reason it took the sample and hold that long.) So, apparently, yes, the ramp amplitude is frequency dependent and hence so is the pulse width if you use a fixed voltage threshold — but only at about the few percent level.

(And no, the reverse isn’t true — frequency doesn’t depend on the pulse width setting.)

My gut feeling is, unless there’s a larger effect on the ramp amplitude caused by something else, I’d be happier with a fixed voltage range.

Edit to add: I’ve just tried out a third option, which is to use Wilson’s sample and holds, but run their outputs through non inverting amps with a gain of 1.24. That, I can only presume, preserves at least some* of whatever benefits his design has, but makes the pot voltages large enough to reach shutoff on either end with R68 at 10M. So I get pulse widths from 0% to 98%. This just needs a TL072 and four resistors which can go on a tiny auxiliary stripboard. I mean, one could even dead bug it, really…

* I think it ruins exact cancellation of the amplitude fluctuations, but it’s better than no cancellation at all. It might in principle be better to add an offset to each voltage instead of amplifying it. Only then you’re sensitive to fluctuations in the offset voltage, so you’d need precision stable voltage references… and it gets more complicated than it’s worth. But the amplifier is easy and should be pretty stable.

Read the rest of the MFOS VCO build series:

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