Re: [NEWBIE] what's the relationship between the units of data before and after FFT?

From: lucy (losemind_at_yahoo.com)
Date: 08/17/04 

Date: Tue, 17 Aug 2004 15:11:10 -0700

"Brien Alkire" <brien@rand.org> wrote in message
news:cftugk$988$1@lumberjack.rand.org...
> > Now I do discretization, take stepsize to be 0.01mm, the sigma of the
> > Gaussian function is 0.05mm. [SNIP]
> > But what are the frequency units for the frequency plot? How much is the
> gap
> > between the grids in this frequency plot after FFT?
> >
>
> I belive the units will be radians per mm.
>
>

But I was asking how much is the gap between the grids... i.e. what does one
grid represent?

Take stepsize to be 0.01mm, the sigma of the
Gaussian function is 0.05mm. Now I take grid to be [-150 to 150, -150 to
150], so totally there are 301 x 301 samples... representing the physical
region from [-1.5mm to 1.5mm, -1.5mm to 1.5mm].

try1=mygaussian([301, 301], 0.01, 0.05);
try1f=fftshift(fft2(try1));
mesh(try1f);

I know that before FFT2, each grid is 0.01mm, since I have set the stepsize
to this value...

But after FFT2, there are still 301x301 grids, but what does each grid in
this case represent?

Suppose we say the unit in spatial frquency is "cycle per m",

then how much "cycle per m" is one grid in the 301x301 grids after FFT2?

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