[sdiy] Suitcase synth updates - 3819 VCO softsync

[Electronium]

Hi Neil,
Thanks for your feedback on the VCO circuit... very helpful.

Well, obviously, since the circuit uses two TL084s, it's certainly possible
for each VCO to reside on it's own chip.  It's just a matter of the PCB
layout really.  Unfortunately, I've already etched one PCB... but maybe I'll
redo the layout for the next one.

Best regards,
Mike


> Neil wrote:
>
> > > 3819 VCO -- it would be better to run each VCO on its own TL084,
> > > rather than have each VCO use two halves, otherwise you'll get
> > > soft-syncing between the two (caused by the common supply connections
> > > within each chip to the two oscillators, and susceptable to glitches
> > > on the output of the comparator).
>
> Tim wrote:
>
> > Would not some stout bypass caps eliminate most of this problem?? The TL
> > series are kinda low power, at least compared to LM324s.
>
> Neil wrote:
>
> The trouble is that you have two comparators running two different
> oscillators on the same chip sharing the same supply pins and tracks on
> the chip, so any supply rail glitch caused by one comparator swinging its
> output from -ve to +ve (or vice-versa) into its load capacitance will be
> seen by the other comparator, and _possibly_ cause it to sync, especially
> if they are both very close in frequency.  You'll notice this if you try
> to detune one oscillator by a small amount---it just won't as the slower
> oscillator will be soft-syncing to the faster one (or vice-versa
> depending on how the glitch affects the offsets within the comparator).
>
> As this is all on-chip there's not a lot you can do, other than physically
> place them on separate chips.