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How to interpret FFT output signal?

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梅尔文·科弗斯（Melvin Corvers）上6 Aug 2020

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Commented: 瑞娜·伯曼（Rena Berman）上9 Oct 2020

Hi,

我正在使用FFT分析信号以获得其频谱。原始数据如下所示。

机器应以0.316 Hz的频率测量。

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')
                       
                        网格上

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 2020年9月7日

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

Hi,

我正在使用FFT分析信号以获得其频谱。原始数据如下所示。

机器应以0.316 Hz的频率测量。

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')
                             
                              网格上

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?

瑞娜·伯曼（Rena Berman）上9 Oct 2020

(Answers Dev) Restored edit

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Answers (1)

Peng Li 上6 Aug 2020

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//www.tatmou.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.

频谱对称为0 Hz。默认情况下，MATLAB给出了对称fs/2的偏移频谱。您可以fftshift，也可以简单地绘制上半场。右侧的第二个高峰不是您要检测到的。它只是左侧峰的镜子。

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