[sdiy] Help with a strange filter

Tim Stinchcombe tim102 at timstinchcombe.co.uk
Wed Jun 13 23:04:41 CEST 2012


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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