John Larkin said:

No, a Butterworth rings a bit for a step input. Critically damped is

closer to Bessel.

It's worth getting a good filter book, like Williams+Taylor, or even

the old classic "Simplified Modern Filter Design" by Geffe. They have

scads of normalized filter tables, response curves, and scaling rules.

Lancaster's Active Filter Cookbook is good to have around, too.

John

If I pick F=40KHz and R=8R then I get L=45uH and C=350nF, if I use the

(guessed)sums I've given above. In LTspice the AC filter response is

flat to crossover with no peaking and -3dB down at 40KHz.

If I do a transient response on it then the output overshoots and

recovers in about one cycle. That's what I thought critically damped

meant.

Overdamped would rise to the final value without overshoot. I think

I'm getting confused now, maybe critically damped rises to the final

value in the fastest possible time.

I started out by making the impedance of the L and the C equal to the

load resistance at the crossover frequency, 32uH and 500nF, and got a

peaked response.

Then I tried making them twice the load resistance, thinking that (in

ac terms) they appear in parallel to the load. That gave 64uH and

250nF and the response looked overdamped.

Then I made them equal to the load and scaled them by that root(2) and

1/root(2) and the response looked much nicer..... 11.3 ohms, 45uH and

350nF.

So I fiddled the values by root(2) and 1/root(2) and it got better.

Kevins software gives me 45uH and 700nF but uses equal source and load

resistances. If I make the input resistor small then the response

peaks at crossover.

Well, I think I am suitably lost now.

Zebedee