[sdiy] SSM2164 state variable filter

[Tom Wiltshire]

David,

> Without damping, the circuit is an oscillator -- a very crappy one which
> won't give anything even resembling a sine wave, since the oscillations
> aren't filtered, as they would be in a 4P cascade filter.  The oscillations
> will look more like a square wave with slanty sides.

Yep! That's a good description of what I'm seeing! I was thinking of it more as a sine wave with the tops cropped, but it's the same thing.

> The limiting network (back-to-back zeners) must be placed across the
> first integrator cap (which generates the BP output).  This will give very
> nice sine waves during oscillation, and still allow nice resonance during
> filtering.

This works very nicely. The sine wave oscillation amplitude varies from about 1.5V at the lowest frequency to about 6.6V at the very highest. This is a considerable improvement over other places I've tried putting various types of clipping network.

> Placing the zeners across the second integrator cap (which
> generates the LP output) would still give sine waves during oscillation, but
> the filter won't work properly since the resonance will clip very close to
> the peak signal level.

This is Roman's solution. It works, but the oscillation amplitude varies enormously.

> Putting the zeners across the input summer's
> feedback resistor doesn't limit the oscillations.

No. This doesn't work at all. Been there, done that.

> Of course, when the input signal and the filter are tuned to the same
> frequency, the two will reinforce each other and then the BP output will
> clip fairly severely, but otherwise not.

In practice, this doesn't seem to be a problem. There is some distortion of the sine wave when the two are the same frequency, but this mostly looks like the input signal breaking through, rather than distortion. In any case, it doesn't suddenly introduce a lot of unwanted gritty harmonics, and it manages to retain something of the original signal's character.

> That's my take on the situation, based on simulation.

Your simulation is very close to the reality on my breadboard! Good work, and thanks.

Regards,
Tom