How to interpret FFT output signal?

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Melvin Corvers on 6 Aug 2020

Commented: Rena Berman on 9 Oct 2020

Hi,

I am analyzing a signal using FFT to obtain its frequency spectrum. Raw data is shown below.

The machine should measure at a frequency 0.316 Hz.

Code spectral analysis:

                         x = t2(:,3);
                        
                         %x = x-mean(x);
                        
                         n = length(x);
                        
                         dt = 3;
                        
                         fs = 1/dt;
                        
                         t = (0:n-1)*dt;
                        
                         y = fft(x);
                        
                         f = (0:n-1)*(fs/n);
                        
                         功率= abs (y)。^ 2 / n;
                        
                         figure()
                        
                         plot(f,power)
                        
                         xlabel('Frequency [Hz]')
                        
                         ylabel('Power')
                        
                         gridon

After spectral analysis I end up with two peaks: one at 0 Hz and one at 0.3316 Hz.

If you uncomment the second line you end up with this: a peak at 0.0017 and 0.3316 Hz.

My question is how to interpret the first peak? Has it a physical meaning? I've read somewhere that a large peak at zero frequency means that you have a massive DC offset. What does that mean? And could it be that through discretization the actual value (0.316 Hz) and found value (0.3316 Hz) are slightly different?

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Rik on 7 Sep 2020

Question posted by Melvin Corvers recovered from Google cache ( permalink , should be available after a few hours):

Hi,

I am analyzing a signal using FFT to obtain its frequency spectrum. Raw data is shown below.

The machine should measure at a frequency 0.316 Hz.

Code spectral analysis:

                               x = t2(:,3);
                              
                               %x = x-mean(x);
                              
                               n = length(x);
                              
                               dt = 3;
                              
                               fs = 1/dt;
                              
                               t = (0:n-1)*dt;
                              
                               y = fft(x);
                              
                               f = (0:n-1)*(fs/n);
                              
                               功率= abs (y)。^ 2 / n;
                              
                               figure()
                              
                               plot(f,power)
                              
                               xlabel('Frequency [Hz]')
                              
                               ylabel('Power')
                              
                               gridon

After spectral analysis I end up with two peaks: one at 0 Hz and one at 0.3316 Hz.

If you uncomment the second line you end up with this: a peak at 0.0017 and 0.3316 Hz.

Rena Berman on 9 Oct 2020

(Answers Dev) Restored edit

Answers (1)

Peng Li on 6 Aug 2020

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//www.tianjin-qmedu.com/matlabcentral/answers/575968-how-to-interpret-fft-output-signal#answer_476026

Based on your code, your sampling frequency is 1/3 Hz. There is no way you can detect a component of 0.316 Hz using 1/3 Hz sampling frequency.

The spectrum is symmetrical to 0 Hz. By default MATLAB gives a shifted spectrum that is symmetrical Fs/2. You can fftshift it or you can simply plot the first half. The second peak on the righthand side is not what you want to detect; it is simply a mirror of the peak on the left hand side.

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