Butterworth filter: Number of coefficients?

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Max Behr 2020-5-19

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评论： Star Strider 2020-5-19

采纳的回答： Star Strider

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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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采纳的回答

Star Strider 2020-5-19

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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.)