Do filters add up??

[Martin Czech]

> I have a state-variable filter which has a 12db/oct low pass response,
> among others.  If I put two of these in series, with the same cutoff
> frequency and resonance, would that yield 24db/oct overall?
> 
Yes and No.

Yes, you'll get a 24 dB/oct slope.

No, the resonance will be different from a 24dB filter,
since you have two feedback loops instead of one.
To get the usuall 24dB feedback with oscillation, you have to feed 
the output of the last
filter back to the input of the first, with no resonance inside
the blocks, of course.
Or the other way arround:
A state variable is nothing more than a biquad block,
so it will emulate any *fixed* frequency response, if you
have enough filters to cascade (Tcheby, Bessel, Cauer, Butter, HP, 
LP, BP, Notch whatever you want).

But the pole shifting ways in the
s-plane will be different from a single feedback-loop design.

Therefore I provided a voltage controlled feedback input for my statevar
(simple n-Jfet-divider), with 0 and 180 deg phase shift switch.

m.c.