Xpander VCF

[Harry Bissell]

Exactly what I see...

the simulation gets increasing resonance... which suddenly drops off when
the
circuit begins to oscillate.

I usually use EWB which allows changing parameters during the simulation
run.
I can use the scope to verify that oscillation has begun...

BTW when I first simulated the Moog Ladder, it took a few hours to find
out...
now where did that resonance suddenly go ???

H^) harry

"jh." wrote:

> > I tried a spice simulation of this, using several models for the
> > 4-pole filter as a basis (including the JH 2040 clone). I did not
> > note where self oscillation occurred.
>
> Finding a self oscillation point is something that's extremely
> difficult in Spice. When you're running an AC simulation, you
> can see a resonant peak rising with increasing feedback gain, but
> you will never see an "infinite" peak which would be self oscillation.
> To make things worse, if you're further increasing feedback gain,
> the simulated resonance peak will get smaller again !
> You can only know if you're in the valid range if you're making a lot of
> AC runs with parameter sweep and see where the peak will stop
> increasing and apparently start decreasing again.
>
> It's good to have some theorethical calculations to estimate
> the self oscillation point, and then just verify with Spice.
>
> JH.