%load the file
disp=load('dowling.dat');
% Sampling frequency is 512
SF=512;
%calculate the PSD of the signal
PSDdisp = sqrt(psd(disp));
ndata=length(PSDdisp);
i=1:ndata;
j=(SF/2)/ndata*i;
figure(1)
"semilogy(j,PSDdisp,'b')"
xlabel('Frequency (Hz)');
ylabel('Power spectral density (log)');
axis tight

%calculate the FFT of the signal
FFTdisp = fft(disp);
%Calculate the square amplitude of the signal (power spectrum)
PS=FFTdisp.*conj(FFTdisp);
ndata=round(length(PS)/2);
i=1:ndata;
%the first PS number is the average
PSpositive(i)=PS(i+1);
%construct the frequency axis - up to half the sampling frequency
j=(SF/2)/ndata*i;
figure(2);
"semilogy(j,PSpositive,'b')"
xlabel('Frequency (Hz)');
ylabel('Power spectrum (log)');
axis tight