fft of given data

I got data from a lab, and the FFT I get have a peak in zero Hz. But it is not right, it should have a peak at 50 Hz (european frecuncy). I just don't know what else to try, I'm without new ideas of way to continuo. Please give me an adavise!

The data is xlsx below.

Summering:

% same as in http://www.mathworks.es/es/help/matlab/ref/fft.html but with Fs=150, thus 150/2=75 so close to my 50 Hz, and y and t are vectors given.
y=v;

Fs = 150;                    % Sampling frequency
% T = 1/Fs;                     % Sample time
L = length(t);                     % Length of signal
% t = (0:L-1)*T;                % Time vector

NFFT = 2^nextpow2(L); % Next power of 2 from length of y
Y = fft(y,NFFT)/L;
f = Fs/2*linspace(0,1,NFFT/2+1);

plot(f,2*abs(Y(1:NFFT/2+1))) 
title('Single-Sided Amplitude Spectrum of y(t)')
xlabel('Frequency (Hz)')
ylabel('|Y(f)|')

[Juan]

Finally semi solved, from

http://stackoverflow.com/questions/12960306/given-an-array-of-data-extract-possible-frequencies-with-fft-how-to

x=i;

%//If youre getting a massive peak at zero that dwarfs everything else, you
%//probably have a large DC offset. Easily removed in the time domain using
%//the following ..
x = x-mean(x);

tAxis = t;
dt = diff(tAxis(1:2)); %//sample period from time axis
fs = 1/dt;%//sample rate from sample period

NFFT = numel(x); %//number of fft bins - change if you like
Y = abs(fft(x, NFFT)).^2; %power spectrum

%//Calculate frequency axis
df = fs/NFFT;
fAxis = 0:df:(fs-df);

%//Plot it all
figure; plot(fAxis(1:NFFT/2), Y(1:NFFT/2))
xlabel('Frequency in Hz')
ylabel('Power')
xlim([0,300]);

[Juan]

Note: for better view of my data

change:

NFFT = 1e6;

add:

xlim([0,300]);

PD: it seems like i didnt need help from here, the stackoverflow-given-an-array-of-data-extract-possible-frequencies-with-fft-how-to would have been enough, but I wont delete this. This may help others.

Answer 1

[Wayne King]

That indicates that the data has a non-zero mean. First, subtract the mean

y = detrend(y,0);

or simply

y = y-mean(y);

Then execute your commands above.

[Juan]

Thanks for the answer.

That solution I have already tried. It is written in: http://stackoverflow.com/questions/12960306/given-an-array-of-data-extract-possible-frequencies-with-fft-how-to

But that doesnt solve the problem. The DC offset or mean is so small that I think is not a problem:

>> mean(y)

ans =

     1.3573e-13

>> max(y)

ans =

    309.9389

[Wayne King]

which column in the excel file is the data you are trying to Fourier transform? what is the title of the column