# How to find parameters of Butterworth low pass filter?.

Discussion in 'General Electronics Discussion' started by Shivani tiwari, Sep 8, 2015.

1. ### Shivani tiwari

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Sep 8, 2015
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??

2. ### AnalogKid

2,498
718
Jun 10, 2015
A Butterworth filter does not have passband ripple, which is why it is called a maximally-flat topology. The two parameters usually used to specify one are the corner frequency (frequency at which response id down 3 dBv), and either the attenuation slope (in dB/octave or dB/decade) or the attenuation at a particular frequency below the cutoff. For example, to keep AC power line hum out of a long-wire intercom you might want a cutoff of 300 Hz and a minimum attenuation of 30 dB at 60 Hz. When starting a filter design, these parameters come from the application requirements. They are used to determine the kind of filter to be used and its general parameters. Then, once the filter is designed, you can go back and calculate the specific performance at the same parameters to see if the filter matches or exceeds the requirements.

ak

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3. ### Shivani tiwari

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Sep 8, 2015
I wanted to design a filter using Matlab
And I came across this formula..which was mentioned in the site and I'd also mentioned in many books.so I wanted to know how to calculate passband ripple and stopband attenuation.

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4. ### Shivani tiwari

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Sep 8, 2015
I want to filter the photoplethysmogram(image attached) signal from Butterworth bandpass filter using MATLAB.the bandpass frequency is .5 Hertz to 6 Hertz. Can I design a filter just using bandpass frequency (if yes,please tell me the steps) or do I need know some other parameters as well.??I do not know how to calculate the order of filter either. sorry I am new to filter designing.

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5. ### AnalogKid

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Jun 10, 2015
Are you designing an analog filter circuit (opamps, capacitors, etc.) or a digital filter? The two are massively different. The Matlab program is for a digital filter. That particular program is not for filter design. It is for evaluating a set of filter requirements to determine how complex a filter is needed. It is an analysis tool to be used before starting a design.

ak

6. ### Shivani tiwari

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Sep 8, 2015
I wanted to design a Butterworth low pass filter for PPG signal(image attached) using matlab. It requires 4 parameters for determining the order of filter 1. Passband cutoff 2. Stopband cutoff. 3. Passband ripple. 4. Stopband attenuation. I have the cutoff frequencies..How can I chose the value of passband ripple and stopband attenuation for this particular signal(image attached). I know that the passband ripples ranges from 0-1dB and stopband attenuation >40 db(correct me if I am wrong)..so are these values chosen randomly from the given range or is there a specific way for doing it.

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7. ### Shivani tiwari

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Sep 8, 2015

Yes I wanted to determine the order of the digital filter using MATLAB. Sorry for using wrong terminologies, I am new to this. So how do I choose the values of passband ripples and stopband attenuation for a signal like this(image attached) . I know that passband ripples ranges from 0-1dB and stopband attenuation >40 (correct me if I am wrong) so are these values chosen randomly from the range or is there a specific way of doing it...and thanks

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5,165
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Dec 18, 2013
Hello
All of these questions are design specific. Only the designer can determine that, you have to find out what you need then maybe we can help design a filter. Also does the filter need to be active or passive, this makes quite a difference.

9. ### Shivani tiwari

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Sep 8, 2015
I need to determine the area under the curve and local maxima and minimas of the signal given in the image..and I want to design a digital filter. Thanks in advance.