[sdiy] I've made a VCO... On accident...

[harrybissell]

Sounds like you removed the resonant feedback resistor.

To get a "Q" of 1, it should be equal to all the other resistors.
Larger values make the "Q" go higher. But it is unusual for a
SVF to go into full oscillation.  You should study it carefully.
It must be getting some positive feedback, somehow.

Anyway, its cool.  The question is will the next one do the
same thing ???

H^) harry

Jeff Farr wrote:

> Heh, I wish I could take a pic of my 'scope at the moment.  It is, as
> I suspected, removing the 100k resistor from the feedback between the
> second integrator and the summer that causes this.  By using the same
> taps from the filter arrangement I get a nice square, triangle, sine.
> All with perfect symmetry and even gain, no DC shift through their
> range, the sine in particular looks perfect.  There seems to be an
> upper limit to the range short of 20k, perhaps as low as 10k (my scope
> hasn't been calibrated since the early 90's at best).   The lower
> limit is LOW, nice for a LFO.  There is some clipping in the triangle
> and sine output above a given frequency, trimming affects this, but
> doesn't cure it.  In any case it seems I have a nice VCF/VC-LFO module
> now ;)  I have to look into if I screwed up the original feedback path
> from the schematic, the circuit was working with all filter responses
> before discovering this, so I assume it was correct.
>
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