[sdiy] SSM2164 state-variable filter

[Neil Johnson]

Hi Tom,

As I'm just finishing up the design of an SSM2164-based 3-pole state  
variable filter I think I can offer some thoughts...

> Firstly, the resonance varies with cutoff frequency, being more  
> pronounced at higher frequencies (in fact, it'll oscillate at high  
> frequencies when it won't when you tun the cutoff down. Is this  
> normal? Do all SVF designs do this?

I think in theory resonant fequency *should* be independent of Q.   
However my suspicion is that various non-linearities and other  
frequency-dependent effects (op-amp slew rate, limited GBW, etc)  
change the behaviour sufficiently that there is a connection between  
the two.

This is one aspect of my design that I am currently experimenting with.
An early draft of the schematic is here:
    http://www.njohnson.co.uk/pdf/MSM/msm-vcf.pdf

[Note: there are some errors in that version - a revised version will  
be appearing sometime soonish once I'm happy with the core design.]

> Secondly, the resonance increases as the resonance CV increases.  
> Whilst this is logical enough, the CV serves to increase the  
> attenuation of the VCA, so increasing the CV actually decreases the  
> output from the resonance VCA. Does this mean this VCA is being  
> used to cancel out the effect of the other feedback loop? (through  
> R6 or R53 on Roman's schematic)

Yes - in reality what you are changing is the 'damping'.  At 0V the  
gain cell has 0dB attenuation, so you are at maximum damping (lowest  
Q).  As the control voltage increases so does the attenuation (at  
33mV/dB), so there is less damping (higher Q) and the filter starts  
to ring - eventually going into self-oscillation.

> Thirdly, what is the function of the 5V6 zener diodes on the final  
> integrator? I haven't put them in yet, and it seems to work.

They limit the loop gain when in self-oscillation, otherwise you can  
end up slamming against the rails.

Cheers,
Neil
--
http://www.njohnson.co.uk