FFT is finding a max amplitude at 0 Hz

Can anyone explain why fft is giving a maximum amplitude at 0 Hz if the actual signal is pretty clear?

In my code, x is the signal as a function of time and r = length(x):

NFFT = 2^nextpow2(r);   
X    = fft(x,NFFT)/r;
Fs   = 1/dt;                        % sampling frequency
f    = Fs/2*linspace(0,1,NFFT/2+1); % frequency
A    = 2*abs(X(1:NFFT/2+1));        % ampitude spectrum

[Aimie]

I should mention that I do get a second peak in the frequency at around 200 Hz, which is exactly what I want. I just want to get rid of the maximum peak occurring at 0 Hz...

Answer 1

[Honglei Chen]

You need to remove the DC component first. Just do

x = x-mean(x)

or

x = detrend(x)

before applying FFT

[Aimie]

Thanks! This works great. I had tried to do something similar by using

x = x - x(1)

but using detrend was a much better way to go (but I didn't know about that function before).

[Iftakharul alam]

Dear Honglei Chen,

really appreciated to see you answer!

Thank you so much.