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

[Martin Czech]

> 	>And I think that the SCR method is no good idea for a vco at all
> 	>(not stable).
> 
> I tend to agree, because some simulations I've done say
> the same thing. But I'm still wondering about all these
> Korg synths that use them - they are not exactly known
> as not stable. Rather the contrary. It's still a mystery to me.
> 
> JH.
> 

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" ? If we compare it with tempco
effects it may look not so important, because tempco is put into some exp
function, whereas trigger level and leakage are more "linear" effect,
but I don't want to call them "second order effects".
At low frequencys the leakage should be noticeable. 

m.c.