FFT and plot amplitude spectrum

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

[Shubham Gupta]

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

 loglog(f,m);

[Samaneh Arzpeima]

yes! loglog sounds about right.Thank you so much.

Do you mind if I ask another question,

the figure looks really different when I use different numbers for FFT

nfft=1024; %length of FFT
%or
nfft=length(x); %this one will give me 101

which do you think would be okey? I also found a

nfft=2^nextpow2(length(x))

[Shubham Gupta]

How to choose the value of nfft in Matlab?

You can give it a read, I hope it helps.