# FFT and plot amplitude spectrum

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Samaneh Arzpeima el 6 de Dic. de 2018
Comentada: Shubham Gupta el 10 de Dic. de 2018
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.
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Shubham Gupta el 7 de Dic. de 2018
You might wanna use "logarithmic plot" instead of simple plot. So last line would be
loglog(f,m);
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Samaneh Arzpeima el 7 de Dic. de 2018
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 el 10 de Dic. de 2018
You can give it a read, I hope it helps.

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