FFT - Matlab Code not correct

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]')

Answer 1

[dpb]

"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.