Re: Brushing up on theory: Butterworth LCR filter design?

On Sat, 17 Jan 2009 08:24:10 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@xxxxxxxxxxxxxxx> wrote:

On Fri, 16 Jan 2009 21:20:34 -0800, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Fri, 16 Jan 2009 20:26:19 -0800 (PST), "miso@xxxxxxxxx"
<miso@xxxxxxxxx> wrote:

On Jan 16, 3:10 pm, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 16 Jan 2009 23:17:31 +0100, Richard Rasker



<spamt...@xxxxxxxxxx> wrote:
Hi all,

I want to design a passive 3rd or 5th order lowpass filter with an fc of
500Hz or 1kHz, preferably a Butterworth configuration for a flat bandpass
response. So I dusted off ye 'ole filter design theory, set out to
calculate polynomials ... and realized that I somehow miss the connecting
step to real-world components -- at least, as far as the LCLR ladder
configuration is concerned:

      L1    L2    L3
   .--mmm---mmm---mmm--.
   |      |     |      |
Ri| |  C1=== C2===    | | Ro
  | |     |     |     | |
   |      |     |      |
   `-------------------'

I know how to calculate active filters in a Sallen-Key configuration, and I
looked up things likehttp://en.wikipedia.org/wiki/Butterworth_filter,
but I somehow fail to make the connection between the transfer function H(s)
and the first schematic shown, with actual component placements and values.

Can anyone enlighten me in this respect? For active filters, there are some
quite good step-by-step explanations out there (e.g.
http://www.maxim-ic.com/appnotes.cfm/an_pk/1795) but so far I found nothing
for the humble LCR stuff. And I'm afraid step-by-step is what I need here,
as I seem to have all necessary information without knowing what to do with
it ...

Thanks already,

Richard Rasker

Normally one looks up a prototype filter in tables in a book (Williams
and Taylor is excellent) and normalizes it for impedance and
frequency. Chapter 1 of Williams discusses filter synthesis; it can
get "laborious."

John

I'm with you here. I have a copy of Huelsman and Allen with the
tables, as well as Zverev.

We bought the Nuhertz passive-filter design software, and it's
amazing. It will design gorgeous filters using finite-Q,
standard-value parts. Trying to do that by normalizing textbook
filters can drive you crazy.

The guy who does this is very smart and very, very crabby.

John



And you ?:-)

...Jim Thompson


I'm not as smart as he is, but I'm a lot more cheerful. And I don't
send obnoxious, insulting emails to my customers.

John

.

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