Parameters required for designing Butterworth low pass filter

[Shivani Tiwari]

I have to design a butterworth filter for PPG signal. To calculate the order of the filter I need to know 4 values namely.

1. Passband frequency
2. Stopband frequency
3. Passband ripples
4. Stopband attenuation.

I have the values of passband freq and stop band freq. How can I determine the values of passband ripple and stopband attenuation??

 

[FvM]

A butterworth filter has a monotonous frequency response, no passband ripple at all.

It's usually chosen directly according to it's stopband slope, e.g. 2nd order has 12 dB/octave or 3rd order 18 dB/otacve.

 

[Shivani Tiwari]

A butterworth filter has a monotonous frequency response, no passband ripple at all.

It's usually chosen directly according to it's stopband slope, e.g. 2nd order has 12 dB/octave or 3rd order 18 dB/otacve.

What is chosen directly? Stopband attenuation? And actually I wanted to design this filter using matlab.. N there in order to calculate the order of filter...all 4 mentioned parameters were asked.

 

[FvM]

The parameters used to design a butterworth low-pass in Matlab are order and cut-off frequency. See Matlab help for butter() function.

 

[Shivani Tiwari]

[Moved]How do I design a Butterworth bandpass filter for PPG signal.

I want to design a Butterworth bandpass filter using MATLAB for Photolysthemograph signal. I know the bandpass freq that is .5-6 hz.. Can I design the filter using just passband freq (if yes, please tell me the steps) or do I need some other parameters as well.?

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Is there a specific way of choosing order of the filter or is it chosen randomly...n thanks for the help.

 

[FvM]

The 12:1 ratio of cutoff frequencies strongly suggests to design the filter as high-pass/lowpass combination. Still same topic.