MATLAB: I’m a newbie. I have a data as shown below. How to do write MATLAB code for FFT of this kind of an arbitrary signal? Thanks.

fftMATLAB and Simulink Student Suite

Time(s) Amplitude 0.707 0.707 1.401 0.694 2.104 0.703 2.803 0.699 3.497 0.694 4.183 0.686 4.869 0.686 5.575 0.706 6.288 0.713 7.007 0.719 7.711 0.704 8.401 0.69 9.098 0.697 9.8 0.702 10.505 0.705 11.192 0.687 11.876 0.684

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    % Are you looking for this one
  amplitude=[ 0.707 0.694 0.703 0.699 0.694 0.686 0.686 0.706 0.713 0.719 0.704 0.69 0.697 0.702 0.705 0.687 0.684];
t=[0.707 1.401 2.104 2.803 3.497 4.183 4.869 5.575 6.288 7.007 7.711 8.401 9.098 9.8 10.505 11.192 11.876];
%nfft=; % Length of FFT);
subplot(211);plot(t,amplitude);
xlabel('Time'); ylabel('Amplitude');
k=fft([t,amplitude]);
subplot(212);plot(k);
xlabel('Frequency'); ylabel('Power');

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MATLAB: Could you please let me know how to write code to draw the power spectral density (PSD) vs frequency for the data? Thanks.

Hi Ray,

Your data are:

amplitude=[0.707 0.694 0.703 0.699 0.694 0.686 0.686 0.706 0.713 0.719 0.704 0.69 0.697 0.702 0.705 0.687 0.684];

t=[0.707 1.401 2.104 2.803 3.497 4.183 4.869 5.575 6.288 7.007 7.711 8.401 9.098 9.8 10.505 11.192 11.876];

The vector t values are non-uniformly spaced in time. I see two options how to calculate PSD:

1) You can use the Matlab function 'interp1' to interpolate data on uniform time grid and then apply the function 'fft' or any other Matlab function that calculates PSD. Code example:

T_mean=(t(length(t))-t(1))/(length(t)-1); %Calculate mean sampling period

amplitude_i=interp1(t,amplitude,t(1):T_mean:t(length(t))); %Interpolate data on uniform time grid

Psd_i=abs(fft(amplitude_i)).^2/(length(t)*T_mean); %Calculate PSD of interpolated data

semilogy(Psd_i); %Plot PSD on log scale as PSD has a large peak on zero frequency

2) Another option is to use the function 'nedft' downloadable on fileexchange:

https://se.mathworks.com/matlabcentral/fileexchange/11020-extended-dft

This function allows you to calculate the PSD directly from the non-uniform data (amplitude and t vectors) without interpolating them to uniform grid. Let me known if you are interested in applying it.

MATLAB: How to convert a char into double

Note that there are not only numbers in that string, it also contains 'S' characters apparently at random, e,g.:

>> f(8480:8500)
ans =
3 -118 S 623 1623 -71
%      ^ not digit character!

One simple and efficient solution is to remove those 'S' characters before using sscanf:

v = sscanf(strrep(f,'S',''),'%d');

This correctly returns all 4297 numbers in the string, and can be easily adapted (e.g. using regexprep to remove specific types of characters, e.g. if it is possible that other alphabetic characters might occur in the string).

The simplest and most efficient solution by far is this:

v = sscanf(f,'%d%*[S ]');

Nothing will beat that simple command in terms of simplicity or speed.

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MATLAB: Could you please let me know how to write code to draw the power spectral density (PSD) vs frequency for the data? Thanks.

MATLAB: How to convert a char into double

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