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From: Phil Hobbs
Subject: Re: Asymmetrical frequency response of lumped element bandpass filters
Date: Fri, 18 Oct 2002 12:22:43 -0400
Organization: IBM T. J. Watson Research Center
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The Technical Manager wrote:
> > Well the main strength of the prototype concept is that computation is
> > alleviated and the design is reduced to a table look-up.
> That advantage may well have been useful 30 or so years ago when many engineers and
> virtually all hobbyists had no access to a computer but IMO nowadays is only a small
> advantage in a world when computers are cheap and ubiquitous. I consider it to be a
> "technicians workbench" method and quite bad design practice to use a standard lumped
> element filter response as a benchmark and don't think the deficiencies of this
> practice are publicised enough. This is suspicion but I reckon that a lot of books
> and papers written on filters using the lumped element prototypes as a benchmark are
> written by the older generation who grew up in a world of slide rules, mathematical
> approximations and taking pi as 22/7 so old habits die hard.
Well, I don't know about yours, but my inductors usually have tempcos of
between 30 and 300 ppm/K, so the 400 ppm difference between 22/7 and pi
amounts to about 1.3-13 degrees C. I also don't know where to get
inductors and capacitors that accurate. Do you?
More seriously, tuning a complicated filter is a high art, and at high
frequency it isn't trivial to take into account all the distributed
strays in a consistent manner. Thus in order to make your comment about
the inaccuracy of the classical method really bite, you'd have to have a
computer-controlled screwdriver adjusting the filters during
manufacture, as well as some pretty fancy electromagnetic simulation
code. Maybe you do, but you don't say so.
From my experience, the speed at which I can iterate to a good design
has little to do with the fine points of how I get my starting point, as
long as it's close. A physical prototype filter (not a low-pass
prototype design) is a necessity for high performance at high frequency.
Your scorn would be less misplaced if we were designing digital filters,
which can be made as accurate as desired, and are nearly trivial to
IBM T. J. Watson Research Center
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