# how to create magnitude spectrum signal based on data from excel

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shila kaimi 2019 年 11 月 22 日
Commented: Star Strider 2019 年 11 月 25 日
stepsize=0.001;
N=1;
t=0:stepsize:N;
x= dataset;
X=fft(x);
f=size(x,2)/2;
mag_spec=abs(x)/f;
freq=(0:499/2*f*stepsize);
plot(freq,mag_spec(1:500));
i already tried this code, but the error state :
'Error using plot
Vectors must be the same length.'

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### 採用された回答

Star Strider 2019 年 11 月 22 日
Your code needs a few slight improvements:
stepsize=0.001;
N=1;
t=0:stepsize:N;
x= dataset;
X=fft(x);
f=size(x,2)/2;
mag_spec=abs(X)/(2*f); % Normalise By The Length Of The Original Vector
freq=(0:499*(2*f)*stepsize); % Use Full Vector Length To Create A Compatible Frequency Vector
figure
plot(freq,mag_spec(1:500));
xlim([0 50])

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ahmad syaiful md subri 2019 年 11 月 25 日
From the amplitude spectrum diagram of the signal (codding that you have give), how to design the lowpass filter to extract low frequency signal using Butterworth approximation
Star Strider 2019 年 11 月 25 日
Choose the frequencies you want the filter to select, then follow the instructions in the documentation for buttord, butter, and zp2sos functions. Use filtfilt to do the actual filtering.

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