Applying Two Filters Simultaneously

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Soeun Lee on 19 Jul 2021
Commented: Yazan on 20 Jul 2021
I'm trying to apply notch (stop) filter and highpass filter simultaneously for my data. Below is the code that I have so far.
Fs=24414;
t=0:1/Fs:435.13;
X=RawData;
Wn=10; % high cutoff
[b,a]=butter(5,Wn/Fs,"high");
Wn_2=[60 60]; % notch filter
y=filter(b,a,X);
[b,a]=butter(5,Wn_2/Fs,"stop");
z=filter(b,a,y);
plot(z);
What I'm trying to do is first apply highpass filter to my RawData (X) to obtain filtered data y, and then apply notch filter to y to obtain z.
Below is the graph that I'm getting. Please let me know what is wrong with my code. Thank you in advance for your help!
3 CommentsShow 1 older commentHide 1 older comment
Soeun Lee on 20 Jul 2021
Yazan on 20 Jul 2021
Provide what frequencies you are trying to filter. Do you realize that the cutoff frequency of your butter filter is specified as 10/Fs, which is 3.3554e-08 the Nyquist rate (Fs/2)?

Chunru on 20 Jul 2021
Fs=24414;
t=0:1/Fs:435.13;
X=rand(size(t))+2; % random noise + DC %RawData;
Wn=10; % high cutoff
[b,a]=butter(5,Wn/(Fs/2),"high");
% Filter response (Problem: very low cut off and very high Fs)
figure
freqz(b, a, 1024*64, Fs);
xlim([0 200]);
% DC will be filtered out by HPF
figure;
y=filter(b,a,X);
plot(y);
% Notch filter was not properly designed (againg Fs/F0 is very big)
% Wn_2=[60 60]; % notch filter
% [b,a]=butter(5,Wn_2/(Fs/2),"stop");
notchSpecs = fdesign.notch('N,F0,Q,Ap',6,60,10, 1, Fs);
notchFilt = design(notchSpecs,'SystemObject',false);
freqz(notchFilt, 1024*64, Fs);
Error using dfilt.basefilter/freqz (line 17)
This functionality is not available on remote platforms.
figure
z=filter(notchFilt,y);
plot(z);

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