There are various active filter calculation programs or online calculators
on the net.
Immediately to mind are those from AD, LT, NatSemi, TI?, ...
Each has strengths and weaknesses.

This page from 'Okawa Electronic Design' (that just has to be [TM] :-) )
seems to be extremely good.

                  http://sim.okawa-denshi.jp/en/Fkeisan.htm

I think this reference came via a PICList post earlier today but if so I
can't find it. If so, many thanks.

So-far noticed  strengths are relative simplicity of use, reasonable
flexibility, & some good features.
Nice is the ability to specify Exx ranges for capacitors and resistor
values eg caps as E6, R's as E12.
Ability to spec (some) caps or leave choice to program is good. (* see
below)
A good range of plots

I haven't yet checked if it actually produces correct results but it
presumably does.

Also on the site are a range of other calculators but I'd see others as
less useful (to me anyway).

BUT

Electronic Training Course

On the same site is a wide ranging electronics tutorial with about 100
subject headings **BUT** ALL in Japanese.
GoggleTranslate makes it readable and somewhat comprehensible
(right click, "Translate to English" in Chrome.)

            http://okawa-denshi.jp/techdoc/

_________________________________________

* minimising component value.
I find a basic 3rd order low pass S&K unity gain filter a useful quick test
to see what a program produces.
(Good enough usually for a PWM filter)(Bessel can be surprisingly good
overall).
This program allowed 2 R values (3 Rs), 2 C values (3 Cs) + the unity gain
buffer
(Buffer: either 1x transistor if Vbe drop OK, or 2 transistors for almost
no DC offset, or 1 unity gain OA for good DC compliance. )
((When I looked at the same design elsewhere recently they managed to have
all Cs and al Rs different and non standard.))




                Russell
--=20
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