AW: SCR VCOs (was: AW: 303 clone filter and VCO not reproducable

[Haible Juergen]

	>Not to boost with it, but I deal with this SCR stuff every day now.
	>For ESD-Protection devices. I've just measured about 40 of such
structures
	>@ 27C and @120C Tj. From this I can see that at higher temperature
	>the leakage gets very much worse for an SCR (implicit current gain,
	>this gain is quite high for currents in the leakage range). Also
the
	>threshold voltage for the snap-back (this is over head triggering,
not
	>gated trigger) varries widely with temperature. geometry and
parameters.
	>And even worse: aging. I mean these devices see a lot of current,
and
	>this causes parameter shift.
	>
	>Excactly the kind of stuff I don't want to have in my osc.
Principally
	>faulty.  Ok, Korg may have used them, but what is the meaning of
"they
	>are not exactly known as not stable"

What I meant is "what tricks did they use to make them useful
*despite* all these effects you mentioned". Their circuit is not
just a naked npn and pnp, but some passive circuitry around
that. 
It's clear that the 3rd transistor is a temperature compensation
of sort, but how good is it ?
They *could* compensate for constant leakage (even if its quite
high) in linear VCOs (where you need an offset trimmer anyway),
but not if ageing plays a role ...
I mean, I know that SCRs have some lousy specs, but I'm after
the design tricks to make them usable for such a delicate 
application as VCOs nevertheless. It's done, and not too bad,
and my aim is to understand why and how it works. 
(Sorry that I have been a little unprecise in my first mail.)

JH.