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From: firstname.lastname@example.org (john jardine)
Subject: Re: Problem with Spice simulation of Linkwitz-Riley filter
Date: 19 Sep 2002 16:18:33 -0700
References: <email@example.com> <0E+YXIAkwWi9Ew7m@jmwa.demon.co.uk>
NNTP-Posting-Date: 19 Sep 2002 23:18:33 GMT
John Woodgate wrote in message news:<0E+YXIAkwWi9Ew7m@jmwa.demon.co.uk>...
> I read in sci.electronics.design that john jardine
> wrote (in <871568ec.0209181518.71687923@pos
> ting.google.com>) about 'Problem with Spice simulation of Linkwitz-Riley
> filter', on Wed, 18 Sep 2002:
> >My general apologies for overt sarcasm but my cat and me have
> >discovered that a C plus an R will filter high frequencies. We've
> >decided to call it the Spot-Jardi configuration. We feel this
> >arrangement may possibly also be configured to block low frequencies
> >and will henceforth, be looking for research funding to pursue the
> If by that you mean that cascading two Butterworth filters is hardly
> innovative, you have a point BUT the real point is that such a filter
> has very useful properties that were unsuspected before Linkwitz and
> Riley published their work. Naturally, people then looked at cascading
> other types of filter, but, AIUI, none of those have similar very useful
Yes, but ;-) ...
It is *not* a Linkwitz-Riley filter. (or even Butterworth's) It is two
CR lo-pass filters separated by a buffer op-amp.
It may just happen to have CR component value ratios calculated to
result in the particular useful response curve as described by
Linkwitz to his audio colleagues in 1976 but the circuitry is still
just two CR lo-pass filters separated by a buffer op-amp.
Carlos could equally as well have presented a 6th order filter
consisting of 3 Sallen-Key LP stages and also called it a
Linkwitz-Riley filter, or a Butterworth filter or Chebychev filter or
any one of the many described mathematical curve shapes out there.
He'd be incorrect. It would actually still be 3 off Sallen-Key LP
stages in series happening to have CR ratios optimised to give the
particular wanted overall response shape.
My grizzle is with the 'leakage'(contamination?) and resultant
confusion that happens in the description of filters. A filter should
only be ascribed the name(s) of the guy (it's always men) who invented
the electronic circuit arrangement, eg Mr Sallen, Mr Delyiannis, Mr
Antoniou et al. That's the clever bit. The response shape is a
secondary consideration, as it has a near infinite number of
variations and in many cases the preferred mathematical descriptions
come from outside the electronics area.
Most response curves shapes, also have guy's names attached and I feel
these people should not, by default, be allowed to claim 'ownership'
of a particular electronic filter. Some of their described curves are
useful, many are just me-too's and the large bulk are worthless.
I admit it's too easy to describe or specify a filter as a '3rd order
Cheb' or 8th order Cauer, I do it myself but do know why I am doing
it. Newcomers to the field though cannot easily make this distinction
and assume that the actual HP Sallen and Key circuits that they are
using to make their Linkwitz-Riley response (for example), cross-over
networks, were an actual electronic circuit arrangement invented by Mr
Linkwitz and Mr Riley. This is unfair to Mr Sallen and Mr Key and
detracts from the cleverness of their invention.
Digital filters can be named by the describer of their response shape.
Analogue filters should be named primarily by the inventor of the
structure through which the response curved is generated.
I got the impression from the way Carlos drew his the spice setup that
he ascribed the arrangement of CR's and op-amp, as an invention of
Linkwitz and Riley. (I maybe wrong)
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