FFT issue

B T
2012 3 3
3 个回答
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1 个投票

When I attempt a Fourier Transform of my data, v, all the values of Y are NaN + NaNi
%v(t)
t=-1.5:0.001:1.5; %time, s
B=1.5*pi; %T
f0=20; %Hz
for i=1:length(t);
v(i)=(B*sin(2*pi*f0*t(i)))/(pi*t(i));
end
plot(t,v);
max(v) %Voltage,V
min(v) %Voltage,V
%Fourier Transform
Y=fft(v)
Any clues as to why this is occurring??
Thanks guys

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 采纳的回答

Walter Roberson
Walter Roberson 2012-3-3

0 个投票

Your time range includes time 0, and your v(i) for time 0 is 0/0 which is NaN. Once you have a NaN in your data, it is going to "pollute" everything else when you do the fft()

更多回答（2 个）

B T
B T 2012-3-3

0 个投票

%v(t)
t=-1.5:0.001:1.5; %time, s
B=1.5*pi; %T
f0=20; %Hz
for i=1:length(t);
v(i)=(B*sin(2*pi*f0*t(i)))/(pi*t(i));
end
plot(t,v)
display('Max V')
max(v) %Voltage,V
display('Min V')
min(v) %Voltage,V
%Fourier Transform
t=(0.01*10^-15):0.001:1.5; %time, s
B=1.5*pi; %T
f0=20; %Hz
for i=1:length(t);
v1(i)=(B*sin(2*pi*f0*t(i)))/(pi*t(i));
end
Y=fftn(v1);
f=0:(20/length(Y)):19.9867;
plot(f,Y)
So I am now getting values for the Transform, but the values seem to be almost...inverted. It should look half a square, ending at frequency, f, of 20. But instead it looks flipped, and all values are almost 0 until just before f = 20.
any suggestions?
http://farm8.staticflickr.com/7190/6802385642_1cf49a449e_o.png
^similiar to what I'm looking to get

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Walter Roberson
Walter Roberson 2012-3-4
Do you possibly need to fftshift() to get what you are looking for?
B T
B T 2012-3-4
Im probably not using it right right now, but I will look into it more tomorrow morning. Thank you though, I think this might lead to the solution.

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