kamenges
Member
Originally posted by Peter Nachtwey:
The noise above the cut off frequency is attenuated at -40 db per decade instead of -20 db per decade for a first order filter...
All second order filter will roll off at -40dB per decade, even two first order filters in series...sooner or later.
Originally posted by Peter Nachtwey:
...and the cut off between between the high and low frequencies is much sharper. Some people call the cut off frequency the corner frequency.
This is where the Butterworth design (or any other two-pole filter that allows placement of imaginary poles) excels over the two first order filters in series, at least the way I calculate filter constants. I don't know how to calculate coefficients for imaginary poles. You can adjust the sharpness of the roll-off by locating the poles differently.
Keith