[sdiy] Designing 4-pole filters with identical 2-pole stages - why not?

[Neil Johnson]

Hi Tom,

> I've been designing 4-pole Butterworth filters using 2-pole Sallen-Key sections.

Sounds like fun!

> The usual way to do this is to set the Q of the first stage to 0.541, and the Q of the second to 1.307. Multiplying one by the other gives an overall Q of 0.707, which is our Butterworth response.

Hmm.... in my experience the "usual way" is to specify what you want,
then grab a filter cookbook like Williams and Taylor or the venerable
Zverev, read out the coefficients from the appropriate table,
frequency and impedance scale, and there's your answer.  Not much to
it.

Or am I missing something?

Neil

--
http://www.njohnson.co.uk