Fourier Transform peaking at 0

I have the following code that finds the FFT of a complex, time-dependent astronomical signal. "FileArr" is an array I've used to store several signal files.

      NumVec = [69, 70, 86, 87, 88, 93, 98, 99]; %not important here - for naming figures only
        L = 4096; %number of entries in each raw signal
        NFFT = 1024;
        test_freq = 29.9396258
        fs = 1000; %sampling frequency
        t_int = 10.24; %time interval, in seconds
        time = (0:t_int:L*t_int - 1); %time axis

        for count = 1:length(NumVec)
            x = abs(FileArr(:,count)); 
            %plotting raw data against time
            subplot(2,1,1)
            plot(time,x); %plot graph
            fVals=fs*(0:NFFT/2 - 1)/NFFT; %time transform
            %plotting FT
            X = fft(x, NFFT);
            PX = X.*conj(X)/(NFFT*L);  
            subplot(2,1,2)    
            plot(fVals,PX(1:NFFT/2));  
            %saving graphs
            name = sprintf('AbsCombT486%d', NumVec(count));
            saveas(gcf, name);
        end

Every graph is peaking at 0 Hz and not ~30, as expected. Would be grateful for an explanation as to why this is happening, and a solution. Thanks.

Answer 1

[Star Strider]

The 0 Hz signal is a constant (or d-c) offset. Changing your fft call to:

X = fft(x-mean(x), NFFT);

will remove the constant offset. (This works for ‘x’ as a vector or matrix.)