Filtering of 60 Hz signal - not working

Question

Chinwe Orie 2019-10-6

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评论： Chinwe Orie 2019-10-7

Following the example "Remove the 60 Hz hum from a signal", I tried to run a butterworth filter in order to filter out multiples of 60 Hz (60, 120, 180, 240, etc.) from my signal. However, in some of my signals, the filter actually amplifies those frequencies which I am trying to filter out. Here is some my code. All help is appreciated:

multiples60 = 60:60:2500;
for v=1:length(multiples60)
    halfpowerHz1 = multiples60(v) - 2;
    halfpowerHz2 = multiples60(v) + 2;
    d = designfilt('bandstopiir','FilterOrder',2, ...
               'HalfPowerFrequency1',halfpowerHz1,'HalfPowerFrequency2',halfpowerHz2, ...
               'DesignMethod','butter','SampleRate',Fs); %butterworth filter
    if v==1
        buttLoop = filtfilt(d,fsignal);
    else
        buttLoop = filtfilt(d,buttLoop);
    end
end

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Answer 1

Sulaymon Eshkabilov 2019-10-6

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Hi,

Here is what I have tested by changing in one point of your code and seen that it is working quite well now:

multiples60 = 60:60:2500;
for v=1:numel(multiples60)
    halfpowerHz1 = multiples60(v) - 3;
    halfpowerHz2 = multiples60(v) + 3;
    d = designfilt('bandstopiir','FilterOrder', 2, ...
               'HalfPowerFrequency1',halfpowerHz1,'HalfPowerFrequency2',halfpowerHz2, ...
               'DesignMethod','butter', 'SampleRate',Fs); %butterworth filter
        if v==1
        buttLoop = filtfilt(d,fsignal);
    else
        buttLoop = filtfilt(d,buttLoop);
    end
end
L = length(fsignal);
N = 4*2048; % block size
 f = Fs/2*linspace(0,1, N/2+1);
Y = fft(buttLoop, N)/L;
plot(f, Y(1:N/2+1)), grid on

Good luck

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Chinwe Orie 2019-10-7

Thank you! It works, but I'm wondering why just changing the code from 'length(multiples60)' to 'numel(multiplies60)' and changing the number substraction from -2 to -3 would cause it to work? Is there a chacne that -3 might be too high?

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