FFT issue

Question

B T 2012-3-3

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/31010-fft-issue

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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Answer 1

Walter Roberson 2012-3-3

0
链接

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https://ww2.mathworks.cn/matlabcentral/answers/31010-fft-issue#answer_39485

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()

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B T 2012-3-3

Thank you.

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Answer 2

B T 2012-3-3

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/31010-fft-issue#answer_39486

在 MATLAB Online 中打开

%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