I'm trying to learn some of the basic MATLAB filter design functions and have been experimenting with the butter
function. This function calculates the transfer function coefficients of a Butterworth filter given a desired filter order and normalized cutoff frequency.
I have previous experiences with designing prototype filters and then modifying them, however, I'm having a difficult time understanding MATLAB's definition of normalized frequency.
I'm used to designing Butterworth filters with a normalized frequency of $1 \textrm{ rad/s}$, yet MATLAB insists that the normalized frequency has something to do with a sample rate? I assumed the butter
function was used for designing analog filters?
Furthermore, when I plot the frequency response of the function, the cutoff frequency I specify (Wn
parameter) never seems to correspond to the $-3\textrm{ dB}$ mark in the plots.
Any thoughts on these?
Wn
parameter is defined. Note that aWn
value of1.0
corresponds to half the input sample rate. There is often a factor of 2 error here for first-time users of MATLAB's filter design functions. $\endgroup$