[sdiy] Help with a strange filter

[Tom Wiltshire]

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