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FFT - Matlab Code not correct

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Mepe
Mepe on 9 Jul 2018
Edited: dpb on 9 Jul 2018
Hello,
By means of ultrasonic sensors I made some measurements and would like to evaluate them now via Matlab (data source "data_time_domain.csv". I used 5MHz sensors and also set this frequency as input to the Pulse Generator. The automatically performed FFT on the oscilloscope also shows a peak at 5MHz as expected (oscilloscope_original_frequency_plot.jpg).
Now I try to use the data from the time-domain to perform the FFT in Matlab shows another picture (Matlab_frequency_plot.jpg). Here, the peak is at a much lower frequency. There is no explanation but surely the error will be in my code. As a sampling frequency, I have chosen the frequency that I have also set the pulse generator (5Mhz).
Do you have any idea where the mistake lies?
Many thanks! Regards, Frank
L=2424; %length of the signal
Fs=5000000; %sampling frequency
Fn=Fs/2 % Nyquistfrequenz
N=2^nextpow2(L); %scale factor
t=(0:L-1)*10^-3; %time domain array
f=linspace(0,Fn,length(t)); %frequency domain array
figure(1)
grid on
plot(t(1:length(t)),ChannelA(1:length(t)),'b') %plotting of the time domain)
title('Time Domain')
xlabel('time [s]')
ylabel('amplitude [V]')
%%FAST FOURIER TRANSFORM
xFFT=abs(fft(ChannelB,N)/L); % Divide By ‘L’ To Scale For Signal Vector Length
figure(2)
grid on
plot(f,xFFT(1:2424)*2,'b'),title('Frequency Response') %plotting the frequency spectrum (half spectrum->double amplitude)
xlabel('Frequency [Hz]')
ylabel('amplitude [V]')

Answers (1)

dpb
dpb on 9 Jul 2018
Edited: dpb on 9 Jul 2018
"As a sampling frequency, I have chosen the frequency that I have also set the pulse generator (5Mhz)."
There's your problem; you must sample at least twice the highest frequency in the signal (look up "Nyquist frequency").
Commercial O-scopes sample at much higher multiples than that in their auto modes; the Fmax of the plot you show is 10 MHz; if the scope uses only a simple baseband FFT, then you can presume a sample rate of 20 MHz.
You should be able to also download the sampling parameters from the scope to confirm the settings besides just the time trace values; see the doc for the instrument for how...
Also there's some inconsistency in the frequency vector and normalization; the Matlab FFT is two-sided in frequency; see the example in
doc fft
to see the generic way to pick the positive frequency and to normalize the one-sided PSD as there is only a single DC and Fmax array element.

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