[sdiy] Help with a strange filter

Tom Wiltshire tom at electricdruid.net
Wed Jun 13 13:27:59 CEST 2012


A few more details..this problem has finally persuaded me to learn how to find frequency responses in LTSpice.

	http://www.electricdruid.net/images/FrequencyResponse.png

(Cap and resistor values tweaked a bit)

The filter looks pretty good in the sim. The final rolloff does approach 24dB - about -22dB measured.

Thanks,
Tom

On 13 Jun 2012, at 11:31, Tom Wiltshire wrote:

> Hi all,
> 
> Can anyone tell me more about following filter design?
> 
> 	http://www.electricdruid.net/images/Filter.png
> 
> This is a 4-pole filter that I originally saw in a Maplin ADA Echo project from the 1980's. I've used it a few times since and tweaked the equal resistor values to adjust the cutoff. But I'd like to know more about this filter:
> 
> Is it any good?
> If it is, why doesn't everyone use 4-pole filters that only use a single op-amp? Dead handy for antialiasing filters and such like.
> How would I design a version with different caps?
> Is this some mutant offspring of the multiple-feedback filter topology?
> What type of filter is it? (Butterworth, probably, but I'd like to know)
> Is it genuinely a 4-pole active filter, or (as I suspect) in fact a 2-pole active filter with 2 extra passive poles stuck on it?
> 
> I came across this design all those years ago (almost thirty!) and I've never, ever seen anything like it since. It doesn't ever appear in textbooks or on webpages about filters.
> 
> Thanks,
> Tom
> 
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