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> A unique property of the Butterworth filter is that a single
> even-order (2n-pole) Butterworth is identical to a cascade of n
> 2-pole Butterworths. So you can construct a high-order filter using
> the basic 2-pole filter. This isn't true for other filter types,
> but it works here.
Hmmmm, that may be true with your code but it's not true of Butterworth
filters in general. In the frequency domain, a Butterworth of any order
is by definition -3dB at Fc with Q=0.707. When you stack them, each
filter contributes -3dB at Fc and the Q factors multiply. So, 2 second
order Butterworths gives you a filter with Q=0.5 that is -6dB at at Fc,
and so on as you add more stages.
--
Dennis
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