# [Ifeffit] dk setting in Athena

Kelly, Shelly D. SKelly at anl.gov
Mon Apr 24 11:15:24 CDT 2006

```Hi Geoff,

I don't think that I understand your definition of dk, so I am going to
give you mine.  A Fourier transform is a complex transformation that
requires the data that is being transformed to be defined from -infinity
to +infinity.  Since our data does not cover this range, we pad the data
set with zeros for most of the transformed data range.  At the edges in
between the actual data range and all the zeros there is a step
function, as the data goes from zero to some non-zero number abruptly.
A Fourier transform (FT) of a step function is a bunch of signals at all
R-values called a FT ripple.  This is not a good think to have in your
FT.  So we spread out the step function by applying a window.  The
length of the window is defined by dk.  In general dk should not be
zero.

See this presentation for some more discussion of dk.
http://www.mesg.anl.gov/Presentations/basics%20of%20exafs%20analysis%202
003.pdf

HTH
Shelly

> -----Original Message-----
> From: ifeffit-bounces at millenia.cars.aps.anl.gov [mailto:ifeffit-
> bounces at millenia.cars.aps.anl.gov] On Behalf Of Geoff Eldridge
> Sent: Monday, April 24, 2006 10:29 AM
> To: Ifeffit List
> Subject: [Ifeffit] dk setting in Athena
>
> Hello everyone,
>
> I've just started doing EXAFS in my research and I have found all your
> posts to be very helpful.
>
> My question is about the dk setting in Athena.  I know from earlier
> reading that dk affects where the window for the FT is set in k-space
> (i.e. a dk setting of one moves the window out further than the k-min
> and k-max settings).  I believe the purpose of this is to prevent
> cutting the tops of peaks out of the FT window.  If this is the case,
> would a dk setting of 0 then be appropriate provided no parts of peaks
> are clipped as shown in a k-space plot with the window shown?
>
> Thanks,
>
> Geoff
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```