[sdiy] Help with a strange filter

Tom Wiltshire tom at electricdruid.net
Thu Jun 14 01:03:40 CEST 2012

This is an interesting read on the topic too:


The gist of it is that for filters with gain=1, the sensitivity is significantly reduced. 


On 13 Jun 2012, at 22:04, Tim Stinchcombe wrote:

> Hi Tom,
> 	It puts in a brief appearance (one page) in a couple of old books I
> have (one is Active & Passive Filter Design, Huelsman, McGraw Hill, 1993),
> which give resistor ratios for equal Cs for a Butterworth response, and then
> proceeds to show how awful its sensitivities are with respect to a mere 5%
> change in gain (you could sim that in LTspice...).
> It also briefly appears in here (Fig 3):
> http://downloads.hindawi.com/journals/apec/1980/941601.pdf
> Ref 3 in that paper almost certainly mentions it too, as I have what may be
> its first outing academically (in a 'reprint' type book):
> 'Single amplifier, minimal RC, Butterworth, Thomson, and Chebyshev filters
> to sixth order',
> RS Aikens and WJ Kerwin, Proc. Int. Filter Symp., Santa Monica, Cal., Apr
> 1972, pp81-82
> - the 'editor's comments' on that paper in the book say that ref 3 above
> comments on the sensitivities of these higher order filters. The paper
> itself gives a handful of recursive equations, and some small tables for
> equal Cs and equal Rs parameter values for orders 3, 4, 5, 6.
> If you had access to a citation search engine, you might be able to find a
> more 'meaty' paper which references it.
> It also _might_ get a mention in 'The Design of Active Crossovers', by
> Douglas Self - go to Google books:
> http://books.google.co.uk/books?id=E01DvO2nsH0C&printsec=frontcover#v=onepag
> e&q&f=false
> and in Chp 8, circa p221 and you'll see some very similar structures (some
> pages are not there though, so it could actually be in there...).
> I could probably scan the few pages of the paper and the Huelsman book if
> you so wanted!
> Tim
> __________________________________________________________
> Tim Stinchcombe 
> Cheltenham, Glos, UK
> email: tim102 at timstinchcombe.co.uk
> www.timstinchcombe.co.uk
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl 
>> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom 
>> Wiltshire
>> Sent: 13 June 2012 11:31
>> To: synthdiy diy
>> Subject: [sdiy] Help with a strange filter
>> 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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