FFT

Question

Okan 2012-6-21

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/41702-fft

Hi,

I am trying to perform FFT for ambient vibration data and My code calculates such an absurd value like 6.2172e-17 for the first value of Fourier Amplitude Matrix. However, other amplitudes are reasonable and if I plot these amplitudes in loglog figure, I get meaningless plot.

for j=1:3
      a{j}=textread(names{j},'%f');
      a{j}=detrend(a{j},'constant');
      a{j}=detrend(a{j});
      if ((60*60)/length(a{j}))==0.02
      A{j}=fft(a{j},2^nextpow2(length(a{j})));
      A1{j}=abs(A{j}(1:2^nextpow2(length(a{j}))/2))*0.02;
      %A1{j}(1)=A1{j}(2);
      Fs=50;
      f=Fs*(1:2^nextpow2(length(a{j}))/2)/(2^nextpow2(length(a{j})));
      figure (j)
      loglog(f,A1{j},'b');
      title(names1{j});
      xlabel('Frequency (Hz)');
      ylabel('Fourier Amplitude');
      elseif ((60*60)/length(a{j}))==0.01
      A{j}=fft(a{j},2^nextpow2(length(a{j})));
      A1{j}=abs(A{j}(1:2^nextpow2(length(a{j}))/2))*0.01;
      %A1{j}(1)=A1{j}(2);
      Fs=100;
      f=Fs*(1:2^nextpow2(length(a{j}))/2)/(2^nextpow2(length(a{j})));
      figure(j)
      loglog(f,A1{j},'b');
      title(names1{j});
      xlabel('Frequency (Hz)');
      ylabel('Fourier Amplitude');
      end
end