But I don´t understand what is the correlation between parameter "n" and my frequency, how do I know how many points to solve?
The key relationship involving n is,
n=1/(dF*dT)
where dT and dF are the time and frequency sampling intervals . The choice of n depends on what dF you have (in your case dF=0.03) and what dT you want. Also, n must be an integer.
When using fft(x,n) you are telling the code to append zeros to x making it length n. However, in applications to time-frequency analysis (like yours) I find it is often better just to manually pad x to the length that you want, because the padding has to be done in a very specific way in conjunction with fftshift.
I illustrate all this below:
Mreaction=readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1129380/Mreaction_damped0.xlsx');
dT0=0.01; %maximum desired time sampling interval
dF=Mreaction(2)-Mreaction(1); %given frequency sampling interval
hMoment=complex( Mreaction(:,2) , Mreaction(:,3) );
n0=numel(hMoment);
n=max( ceil(1/dT0/dF) , n0 ); %ensures dT<=dT0=0.01
dT=1/dF/n; %actual time sampling interval
nAxis=( (0:n-1)-ceil((n-1)/2) ); %normalized axis
freqAxis=dF*nAxis;%frequency axis
timeAxis=dT*nAxis;%time axis
leftside=flip(conj(hMoment));
hMoment=[leftside(1:end-1); hMoment]; %two sided spectrum
hMoment(end+1:n)=0;
hMoment=fftshift(circshift(hMoment,1-n0)); %zero padding circulantly is complicated
myTdata=fftshift(real( ifft( ifftshift(hMoment)) ))/dT;
figure
plot(freqAxis,abs(hMoment));
xlabel Frequency
figure
plot(timeAxis, myTdata)
xlabel Time
