# How to computer a frequency spectrum and find its largest spectral energy

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PetronasAMG on 24 Sep 2021
Edited: PetronasAMG on 26 Sep 2021
I am given set of equation
I am suppose to plot x1 x2 and x3 as a 3d dimensional phase, but direction wants me to omit any possible initial transient trajectory.

Chunru on 24 Sep 2021
Edited: Chunru on 24 Sep 2021
fs = 10;
tspan = [0:1/fs:100]; % specify the time points
xint = [3.0;2.0;1.0];
[t,x]= ode45(@nonlin,tspan,xint);
% remove some trainsition part of x
t = t(100:end);
x = x(100:end, :);
subplot(211); plot(t, x)
subplot(212); plot(20*log10(abs(fft(x))))
function dxdt = nonlin(t,x)
e = 2.5;
dxdt = zeros(3,1);
dxdt(1) = -(x(2)+x(3));
dxdt(2) = x(1) + ((1/5)*x(2));
dxdt(3) = (1/5)+(((x(1)-e))*x(3));
end
PetronasAMG on 24 Sep 2021
how would you remvoe the transient part in the phase portrait? not queit understanding the solution. Could it be explained more indepth?
Chunru on 24 Sep 2021
In the original code above, I did not remove the transition part. Just kept them. I update the code to remove the transistion period.

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