[sdiy] Design for a very strong filter?

[BRUCE DUNCAN]

Switch cap filters are great but you will have trouble finding 8 order 
versions.
Most have been obsoleted. The National ones at least. I guess you put 
multiple
4th or 5th order devices in series.
It would be interesting to try a "discrete" version using cmos and DG* 
analogue switches and
capacitors.

Bruce


>From: Tom Wiltshire <tom at electricdruid.net>
>To: Robert Lorentz <robert.lorentz at gmail.com>
>CC: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] Design for a very strong filter?
>Date: Fri, 6 Jun 2008 21:09:51 +0100
>
>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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>
>
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