MATLAB: FFT and plot amplitude spectrum

Question

hello everybody

I got stuck on this problem and really need your help

I must plot the amplitude spectrum of a data set. my data set is the attached “momentrate.mat”. The first column is ‘time(s)’ and the second one is ‘momentrate(Nm/s)’.these 2 columns give a plot like the left figure in attached pdf file.by

plot(momentrate(:,1),momentrate(:,2))

now I need to take the amplitude spectrum of the above plot, and it should be something like the right figure in attached pdf file.

I tried the following codes

y=momentrate(:,2);
t=momentrate(:,1);
dt=diff(t)
sample_frequency= numel(t)/(t(end)-t(1))
nfft=2^nextpow2(length(y));
Y=fft(y,nfft);
Y=Y(1:nfft/2);
m=abs(Y);
f=(0:nfft/2-1)*sample_frequency/nfft;
plot(f,m);

but this won't give me anything like what I suppose to have.

Can you please tell me what is wrong?

I would appreciate if you could explain to me with simple codes.

Answer 1

You might wanna use "logarithmic plot" instead of simple plot. So last line would be

 loglog(f,m);