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 a comparator and makes a pulse wave whose width depends on the reference voltage you set on the pulse width pot; and unlike for instance in 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 had good reasons, like perhaps to deal with supply voltage variations, or with frequency dependence of the ramp wave amplitude? Anyway, the PW voltage ranges from about -4.5 V to +4.5 V.
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 down, and that also depends on R68. And obviously if the output shuts down that sets the minimum width you get, but if you reach the pot limit before shutdown happens, then that sets the minimum width.
Things are a little frequency dependent too. But at 200 Hz, here’s what I see. With the pot at the low end you get a narrow “on” and wide “off” (or really positive and negative, but let me call them on and off), at the other end you get a wide “on” and a narrow “off”; I’m interested in the minimum “on” width at the low end and minimum “off” width at the high end.
| R68 | Low end shuts off? | Min “on” width | High end shuts off? | Min “off” width |
|---|---|---|---|---|
| 1M | Y | ~0.2% | Y | 20% |
| 2M | Y | ~0.2% | N | 12% |
| 3M | N | 2% | N | 11% |
| 10M | N | 5% | N | 10% |
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:
| R68 | Low end shuts off? | Min “on” width | High end shuts off? | Min “off” width |
|---|---|---|---|---|
| 1M | Y | ~0.1% | Y | 20% |
| 2M | Y | ~0.2% | Y | 11% |
| 3M | Y | ~0.1% | Y | 8% |
| 10M | Y | ~0.2% | Y | 2% |
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. In that case you presumably want R68 to be the value that’s on the boundary between reaching shutdown and not reaching it, which I find to be 2M. As a reminder, Wilson started with 10M and changed to 1M so he could reach the shutdown 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 shutdown.
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.
