Question
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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Expert Answer
Kshitij Singh
PhD Expert
Answered Nov 20, 2025
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
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