[sdiy] Design for a very strong filter?

[Tom Wiltshire]

Robert,

People are going to scream at me, but...your requirements sound like  
a switched capacitor filter!

They provide steep rolloffs (8th order is common) and are easy to  
make tunable by varying the clock frequency, and are simple to design  
with since most of the work is done for you on the chips.

The only question mark is over audio quality, which is why I'll get  
screamed at for suggesting it. However, it you make sure the clock  
frequency is well above the audio band and then follow the switched  
capacitor filter with a standard  fixed lower-order lowpass to remove  
any residual clock noise, I can't see why you shouldn't be able to  
make something that does exactly what you want.

Maxim have a load of useful application notes:

http://www.maxim-ic.com/appnotes10.cfm/ac_pk/4

App note 733, a filter primer finishes off with a introduction to  
switched capacitor filters. Some of the others deal with applications  
of them in more depth. Their actual product datasheets also give  
example circuits.

Hope this helps,
Tom

<dons tin hat and ducks into bunker>


On 6 Jun 2008, at 19:50, Robert Lorentz wrote:

> Hello,
>
> I feel that my studio is in great need of some very very strong LP and
> HP filters (and BP would be nice, but less essential).  I am ok as far
> as "musical" filters go in order to achieve interesting timbres.
> However, I'm looking for filters that would be adjustable cutoff and
> extremely strong, like 60db or even higher
>
> The idea here is that I'd be using it for synth patches for specific
> spectrum restriction of the instrument, and I logically see that as
> being separate from doing eq mixdown on my board.  Plus my board's
> eq's are not sweepable anyway
>
>
> What design is best for this sort of thing?  Looking around it seems
> like I need a high order filter (like 10th order), and that it
> probably has to be active since I require a tweakable cutoff (unless I
> can find a 10ganged pot....)
>
>
> Precise frequencies and even linearity aren't important here, and
> accepting CV isn't required or anything like that.  A main goal is
> simplicity though, as I'd like to build a bank of these but have a
> relatively small budget for it.
>
>
> Thanks,
>
> Robert
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