[sdiy] state variable filter oscillating

[Ian Fritz]

At 08:29 PM 6/19/2004, mark s wrote:
>hi, i've built a state variable filter with a TL074 and an SSM2164. it is 
>oscillating like crazy when i turn my frequency up above, oh i dunno 
>6000Hz *and* when the resonance pot it turned way up. without getting deep 
>into the circuit itself, are there some basic principles i should look for 
>in order to troubleshoot ?
>
>thanks!
>mark

Hi Mark,

Yes, this is a well known phenomenon, although, strangely, I don't remember 
seeing it discussed on this list before.

High-frequency Q enhancement is caused by non-ideal op amp response, 
specifically extra phase shifts at high frequency.  This was discussed in 
detail in the Electronotes newsletter.  There are basically two ways to go 
about fixing it:  (1) The most correct method, which involves performing a 
detailed circuit analysis and adding a correct compensating component.  (2) 
The quick-and-dirty method, which works well enough most of the time, which 
is to just throw some extra phase lead into the resonant loop and adjust it 
to get the best overall response.

For my SF filter I added small caps across the resistors feeding the (+) 
input of the OTAs.  These may just be two lengths of insulated wire twisted 
together, as only a small amount of compensation is required.  If you post 
a diagram I can be more specific.

Good luck!

   Ian