Amplitude-Frequency response peaks at zero, how to fix?

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Chinwe Orie
Chinwe Orie 2018-7-9
评论： Star Strider 2018-7-10
I made an amplitude-frequency response curve but I have a peak at 0 Hz. I'm not sure why but I know for sure that this is definitely wrong. The peak should be around 125 Hz. How do I get rid of the zero peak? I have attached the code and the excel data file. Thanks!
f = xlsread(e);
%plotting the signal
t = f(:,1);
X1 = f(:,2);
data_d = detrend(X1,1);
subplot(2,1,1)
plot(t,X1);
hold on
plot(t,data_d, 'g')
hold off
%Some of the envelope code was not working properly because MATLAB read
%some lines of the excel file as infinite numbers. The envelope function
%does not take infinite numbers.
%Thus, the below code deletes the infinite numbers from the array. There
%are only 2 of them.
fin = isfinite(X1);
t = t(fin);
signal = X1(fin);
Fs = 5000;
%Since the signal is real-valued, we use only the positive frequencies from
%the DFT to estimate the amplitude. Scale the DFT by the length of the
%input signal and multiply all frequencies excep 0 and the Nyquist by 2.
xdft = fft(signal);
xdft = xdft(1:floor(length(signal)/2+1));
xdft = xdft/length(signal);
xdft(2:end-1) = 2*xdft(2:end-1);
freq = 0:Fs/length(signal):Fs/2;
subplot(2,1,2)
plot(freq,abs(xdft))
hold on
plot(freq,ones(floor(length(signal)/2+1),1), 'LineWidth', 2)
xlabel('Hz')
ylabel('Amplitude')
xlim([0 200])
ylim([0 0.02])
hold off

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采纳的回答

Star Strider
Star Strider 2018-7-9
The peak at 0 Hz is the direct-current (d-c) or constant offset. You can eliminate it by subtracting the mean of the signal before taking the Fourier transform.
xdft = fft(signal - mean(signal));
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Chinwe Orie
Chinwe Orie 2018-7-10
Ah yes, that's true. But doesn't it do the plot using normalized frequency? Is there a way to convert normalized to the regular frequency scale?
Star Strider
Star Strider 2018-7-10
The only ‘normalized frequency’ I’m aware of uses a scale from 0 to pi. The frequency otherwise goes from 0 to the Nyquist frequency (half the sampling frequency).

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