Matlab FFT doesnt show right frequency with measured signal, Script tested with sinus signal workes

user_15593700 0
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Hei Guys I am having some trouble getting the right frequency in my FFT Matlab script. I tested the script with a sinus signal and it workes just fine. but somehow with my measured signal (put in a txt file, see below) doesn't give me the right frequency. In the first plot I am plotting my signal and there I measured and calculated from hand the should be frequency, which is 33.333 kHz

% Parameter
td = 100*4e-6;  % Duration of the measurement ( i honestly just put something here)
fs = 250e3;     % samplingfrequency

% Frequency to be detected (measured from data in plot)
freq_soll_20 = 1/(7.2e-5 - (4.4e-5 + 4e-5)/2);
% Samples calc

s_20 = readmatrix('fft_cut_20.txt');
t_20 = 0:1/fs:((length(s_20)-1)*1/fs);

%% ---- Triggerdata
figure(1)
subplot(3,1,1);
plot (t_20, s_20)
grid on;
xlabel('Zeit (ms)');
ylabel('ADC LSB');
title('20um');

h_20=fft(s_20).*1/length(s_20);
f_20=1/td*(0:(length(t_20)-1));

subplot(3,1,2);
stem(f_20,abs(h_20));
grid on;
xlim([min(f_20) max(f_20)]);
xlabel('Frequenz (Hz)');
ylabel('Amplitude');
title('Ausgabe des FFTs');

subplot(3,1,3);
f2_20=f_20-round(max(f_20)/2);
stem(f2_20, fftshift(abs(h_20)));
grid on; 
xlabel('Frequenz (Hz)');
ylabel('Amplitude');
title(['Ausgabe des FFTs nach sortieren mit FFTSHIFT mit soll frequenz:'...
    num2str(freq_soll_20) 'Hz']);

the data is:

11284, 11601, 12033, 12355, 12344, 11985, 11558, 11444, 11807, 12491, 13090, 13199, 12694, 11878, 11290, 11500, 12855

my FFT gives me the frequency 1250*10^4 Hz. I tested this code with a sinus with these parameter and it worked just fine:

%Parameter
fs = 250000;  % Abtastrate
td = 100*4e-6;    % Dauer der Messung
f1 = 30000;    % Frequenz

% Samples berechnen
t  = 0:(1/fs):td;
s  = sin(2.*pi.*f1.*t);

Am I missing something?

fft matlab
1 Answers

A correct formula for the frequencies would be:

f_20=(0:(length(t_20)-1))*fs/length(t_20);

Your formula f_20=1/td*(0:(length(t_20)-1)); would be equivalent whenever td corresponds to the total signal duration, which is the case for your sine wave. The same cannot be said of your other signal which just has 17 samples, whereas td=100*4e-6 with a sampling rate of 250kHz corresponds to 100 samples.

user_29945960
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