Zero mean values but still dominant frequency is zero

Hello,

I have a series of numbers and I try to find the dominant frequency using FFT. I know that for the FFT to work properly my series should have a mean equal to zero. However even if I impose the mean to be zero, the dominant frequency still remains zero, which is of no interest. Here is my code(a is the array that consists of the series)

a=detrend(a,0);
Fs=67890;
T=1/Fs;
L=1000;
t=(0:L-1)*T;
Y=fft(a);
P2 = abs(Y/L);
P1 = P2(1:L/2+1);
P1(2:end-1) = 2*P1(2:end-1);
f = Fs*(0:(L/2))/L;
b=P1/max(P1);

When I plot(f,b) I take this image

which means that zero frequency is still dominant. What am I doing wrong? Or is it impossible to do anything?

Answer 1

[Walter Roberson]

You appear to be plotting on a truncated log scale. 0 does not appear on your x axis. Your peak is somewhere in the range 0 to 70 Hz.