[sdiy] Bandpass Filter Recommendations

[Tom Wiltshire]

Hi Scott,

I did some work towards a similar thing, but never got to a point I  
was happy with. I decided to use an SVF based on the SSM2164. I  
started out with Roman Sowa's design, but only used the filter part  
of his circuit:
http://www.sowa.synth.net/modular/vcf.html

This uses two OTA stages for the filter and one for the resonance. I  
used the spare stage for the gain control. This makes a convenient  
total of 1 SSM2164 and one quad op-amp per filter.

I had some difficulty with the resonance. You'll find the discussion  
on this in the archives. Ian Fritz and Neil Johnson gave me a lot of  
help, but I never did get it going how I wanted, although I think  
this was a fault, not the design per se. It's currently a project on  
my "things to go back to" shelf.

If you want more details, ask me and I'll mail you the schematic.

Hope this helps,
Tom


On 24 May 2009, at 19:23, Scott Juskiw wrote:

> I'm working on a programmable filter bank and am asking the group  
> for recommendations on a suitable bandpass filter. The filter  
> requirements are:
>
> 1. voltage controlled frequency
> 2. voltage controlled resonance
> 3. voltage controlled gain
> 4. analogue circuit
>
> Some solutions I've come up with:
>
> 1. state variable filter
> 2. cascaded lowpass/highpass filters (using cascaded OTA stages)
>
> #1 is relatively easy but I'm less than thrilled with the 6db/oct  
> slopes. #2 seems to be better for my needs since I can get 12 db/ 
> oct slopes.
>
> Are there any other filter topologies that anyone can recommend?  
> Hopefully something that won't require 400 components per filter  
> since I'd like to have a bank of these on one PCB. But maybe that's  
> asking for too much. I would also consider a digital filter  
> solution, if it would be easier to build and if (big IF) I could  
> figure out how to do it digitally.
>
> Thanks
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