How to design a 3rd order butterworth bandpass filter characteristic which will then be used for the dfilt.df1tsos function?

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Patrick Wiegel
Patrick Wiegel 2017-4-3
回答： Binaya 2024-10-10
I have to create a 3rd order butterworth bandpass characteristic [10 12] (Hz). Thereafter, I need to use the biquad second-order section filter function (dfilt.df1tsos) and filter the data with the filter function.
I do have the following code for a 2-order filter:
Fs = 5000; Fn = Fs/2; Lf = 10 Hf = 12
[z,p,k] = butter(1,[Lf Hf]/(Fn), 'bandpass');
[s,g] = zp2sos(z,p,k);
Hd = dfilt.df1tsos(s,g)
fvtool (Hd)
y = filter(Hd, data');
I am not sure how to handle the combination of bandpass butterworth characteristic and the biquad filter since I have little experience with filtering.
Would be great if anyone provides help.

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回答（1 个）

Binaya
Binaya 2024-10-10
Hi Patrick
Your approach is correct. To design a 3rd order Butterworth bandpass filter, you just need to modify the "butter" function of the MATLAB in your code to the following:
% Design a 3rd order Butterworth bandpass filter
[z, p, k] = butter(3, [Lf Hf] / Fn, 'bandpass');
Also note that the 3rd order Butterworth filter will also include the first-order and second-order characteristics in its SOS(second-order section).
You can refer to the following documentation links for more details:
  1. "butter": https://www.mathworks.com/help/signal/ref/butter.html
  2. "zp2sos" : https://www.mathworks.com/help/signal/ref/zp2sos.html
  3. "design", you can refer to 'design' to apply a design method to a filter specification object: https://www.mathworks.com/help/dsp/ref/design.html
Hope this helps.

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