FFT. It doesn't seem to find the frequencies

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Maria Goncalves Møller 2018-11-15

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https://ww2.mathworks.cn/matlabcentral/answers/430019-fft-it-doesn-t-seem-to-find-the-frequencies

评论： Maria Goncalves Møller 2018-11-15

Hi, Im having a lot of issues turning my time domain into a frequency domain. What am I doing wrong? I expect some high frequency noise - also outside of 500 Hz, and the fft only shows frequencies around 0 Hz...

%% Load data
load('henrikrigtigforsoeg.mat')                             
data1 = data(1:20000)*0.0049;                     
fs  = 1000;                                                 % Sampling frequency
time = 1/fs;                                       
leng = 20000;                                           %length of signal
t = (0:leng-1)*time                                     % Time vector
%% Cut the data
kont_1 = data1(3000:7000);                          %Contraction 1
t_y1 = (0:4000)*time;                                    %time vector for contraction 1
leng_k1 = length(kont_1);                            %length of the contraction
%% FFT
Y1 = fft(kont_1);                                      % Lav en FFT på data
P2_y1 = abs(Y1/leng_k1);
P1_y1 = P2_y1(1:leng_k1/2+1);
P1_y1(2:end-1) = 2*P1_y1(2:end-1);
f_y1 = fs*(0:(leng_k1/2))/leng_k1;                         
%% Plot data
figure
subplot(2,1,1)                                          % Plot the raw data
plot(t_y1,kont_1)
title('Contraction in time-domain')
xlabel('Time in seconds')
ylabel('Magnitude in V')
subplot(2,1,2)                                         % Plot the Fourier transformation
plot(f_y1,P1_y1)
title('Single-sided power spectrum of first contraction')
xlabel('Frequencies in Hz')
ylabel('Power in V')