Re: Butterworth and critically damped filter reference

[Bob Fulks <bfulks@xxxxxxxxxxxx>]

At 10:55 AM 8/2/2004, Alex Matulich wrote:

>According to the derivation of the Butterworth coefficients
>at http://kwon3d.com/theory/filtering/fil.html any even-order
>Butterworth coefficients can be derived from cascading the
>calculations for a 2nd-order filter.  "For example, a 6th-order
>low-pass filter has 3 (6 divided by 2) elementary 2nd-order filters.
>Passing the data through these 3 2nd-order filters consecutively is
>the same to passing them through the 6th-order filter once."

I seem to remember that the poles of a Butterworth filter are equally distributed in a circle near the cutoff frequency. It would seem that you could not get that pattern by cascading identical 2-pole filters but I could do some checking.

Bob Fulks