X-axis for Fast Fourier Transformation

Hello everyone,

I have a set of .csv files containing about 20,000 lines each. The first column is a time interval (10ms each) and the second, third and forth columns are for X, Y, and Z axes data respectively.

I've been having problems with the x-axis of my intended outcome. Have been getting warning message: Warning: Imaginary parts of complex X and/or Y arguments ignored

I've tried having the x-axis as 1/t, 1/10 and quite some other inputs but am still not able to get what i hope to get.

Hope that I could get some guidance on this.

Thank you in advance! :)

if true
  % code
end
t=Data1(:,1);
x=Data1(:,2);
y=Data1(:,3);
z=Data1(:,4);
XX=fft(x);
YY=fft(y);
ZZ=fft(z);
DataFFT(:,1)= t;
DataFFT(:,2)= XX;
DataFFT(:,3)= YY;
DataFFT(:,4)= ZZ;
csvwrite(fullfile(folderSource,'FFT.csv'), DataFFT);
subplot(3,1,1);
plot(t,XX);
title('X-Axis');
subplot(3,1,2);
plot(t,YY);
title('Y-Axis');
subplot(3,1,3);
plot(t,ZZ);
title('Z-Axis');
end

Answer 1

[Star Strider]

I am not certain what you are doing, or what you want.

I usually create frequency vectors (the x-axis for a Fourier transform plot) this way:

Ts = mean(diff(t));                                     % Sampling Interval
Fs = 1/Ts;                                              % Sampling Frequency
Fn = Fs/2;                                              % Nyquist Frequency
Fv = linspace(0, 1, fix(numel(t)/2)+1)*Fn;              % Frequency Vector
Iv = 1:numel(Fv);                                       % Index Vector (For One-Sided FFT Plots)

The plots would then be:

plot(Fv,XX(Iv))
title('X-Axis')

or:

plot(Fv,abs(XX(Iv))*2)
title('X-Axis')

depending on what you want, and similarly for the others.

See the documentation on fft (link) for details.