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What is the formula of Function 'Freq' and 'FFTmag' in HyperGraph

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My time data setting is as follows.


Time step is 0.001 sec, Time range for analysis is 0~1.5 sec.  (=> the number of time data is 1500)


I use the 'Freq' and 'FFTmag' function in HyperGraph.


The frequency data range is 0~499.51171875Hz and the number of the frequency data is 1024.


(I attached the time and frequency files in .csv format.)


According to the figure, 


Time step, Δt= 0.001      ->       Bandwidth or Max Freq , F_max = 500Hz

the number of time data, N = 1500  -> Spectral line, SL = 750


I think Frequency resolution(Δf) should be F_max / SL = 500/750 = 0.6667Hz.

But in my data, frequency resolution is about 0.488Hz.


I don't know why.


What is the formula of 'Freq' and 'FFTmag' in HyperGraph?





frequency.csv time.csv

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