Butterworth filter: Number of coefficients?

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Max Behr
Max Behr 2020-5-19
评论： Star Strider 2020-5-19
Hello
I am trying to create a low pass filter. My device has a sample rate of 100 and the cutoff value should be 10 Hz.
But according to the manual the filter should also have a setting "number of coefficients". And this should be 40 in my case...
How can I implement the number of coefficients into the filter?
Thanks.
My Code:
Fs = 100;
Fco = 10;
Fn = Fs/2;
Wn = Fco/Fn;
[b,a]=butter(1,Wn,'low')
m_filtered=filter(b,a,m)

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Star Strider
Star Strider 2020-5-19
The first argument to butter is the filter order, and the number of coefficients in the transfer function vectors will be 1 less than that, so:
Fs = 100;
Fco = 10;
Fn = Fs/2;
Wn = Fco/Fn;
n = 40;
[b,a]=butter(n-1,Wn,'low');
figure
freqz(b,a,2^14,Fs)
should do what you want.
Note that this creates an unstable filter.
Are you absolutely certain that you want to use a IIR filter here? A filter order of 40 is more typical of a FIR filter, especially for a lowpass application such as this. (If you want a FIR filter instead, use the kaiserord and fir1 functions to create it.)
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