Xpander VCF

[Martin Czech]

Open loop transfer function and stability criteria a la Bode?

m.c.

:::From: jhaible at t-online.de (jh.)
:::To: <harrybissell at prodigy.net>
:::Cc: <synth-diy at node12b53.a2000.nl>
:::Subject: Re: Xpander VCF
:::Date: Thu, 26 Oct 2000 23:37:22 +0200
:::X-Sender: 320011350262-0001 at t-dialin.net
:::
:::> 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.
:::