Problem from perform FFT on a square pulse

[leave710584]

Hello, 

I have some projects about performing Fourier Transform from time domain to frequency domain, and I have learn a lot from the topic "How to perform FT on XY Graph data?"

https://forums.ni.com/t5/LabVIEW/How-to-perform-FT-on-XY-Graph-data/td-p/2206506?lang=en

After referred to above discussion I have successfully performed the transformation on cosine function by using the FFT.vi. 
However, an unexpected problem was occurred when I tried square pulse function. 
It should be a sinc function after transformation...


I can not figure out where the problem it. Any help will be greatly appreciated!!!

[Bob_Schor · Accepted solution]

There is no problem -- you are getting the correct answer.  If you expand the data at the origin, you will see that it is the right-half of your sinc() function, while the data at the "other end" of the graph is the left half.  You need to learn more about how FFTs work and what the data represents ...

Bob Schor

[altenbach · Accepted solution]

Your images of the transforms are dc centered, while LabVIEW does Nyquist centered by default. The information is the same. Start by trying to understand the difference in convention.