[Ifeffit] parseval's theorem and athena

kirshstein at googlemail.com kirshstein at googlemail.com
Thu Feb 17 08:23:09 CST 2011


Dear Matt and dear other readers,

I tested the method Matt suggested to make Athena put out the epsilon_k
value for my data.
As of itself, the methods works (I get a value), but this value is similar
but not exactly the same value as I obtain from calculating by myself
according to parseval's theorem.
Which makes me think that the small discrepancy might be due to the fact
that Athena considers a different R-range from what I used (all other
parameters being the same)?
I used R 15 to 30 Å. Does someone know what R-range Athena uses?
Also, just to be sure: so the  normalization used for FFT by Athena is
sqrt(delta_k / Pi)?

thank you for your kind help,

regina


On Fri, Feb 4, 2011 at 8:34 PM, Matt Newville <newville at cars.uchicago.edu>wrote:

> Regina,
>
> On Fri, Feb 4, 2011 at 10:02 AM,  <kirshstein at googlemail.com> wrote:
> > Hello,
> >
> > I would like to use parseval's theorem (as described in a document
> called:
> > Error Reporting Recommendations: A Report of the Standards and Criteria
> > Committee, Adopted by the IXS Standards and Criteria Committee July 26,
> 2000
> > ) to quantify the statistical noise in my spectra.
> >
> > In section 3 (as shown in the attachment), it says that for the formula
> to
> > work, the forward FT has to be normalized by  sqrt(deltak/Pi). (delta k
> is
> > the spacing of points in k space)
> >
> > What is the normalization used for FFT in athena? if it is not
> > sqrt(deltak/Pi), what is it? how does the parseval theorem formula need
> to
> > be modified as a function of FT normalization?
> >
> > Many thanks for answers!
> >
> > Regina
>
> It's even easier than that.  Ifeffit / Artemis do (and Athena can)
> report the epsilon_k and epsilon_r as defined in that report.  The
> 2000 report actually codified work done with the earlier feffit code
> to estimate the noise in the data in both k and R space.  Again,
> Artemis does this automatically, and you can see the values for
> epsilon_k and epsilon_r when looking at fit results.     Athena
> doesn't directly do this calculation, but if you open the Ifeffit
> Buffer (Edit -> Display Ifeffit Buffer), you can do the following:
>
> 1. choose FFT parameters in the Forward Fourier transform part of the
> main Athena window.
>
> 2. Hit the [R] Plot button for the group of data you're looking at to
> make sure the FFT parameters are up to date.
>
> 3. Open up the Ifeffit Buffer and look for the "Group Name" (Athena
> uses a 4 letter sequence which looks random). You'll see something
> like
>    newplot(jukc.k, "(1*jukc.chi*jukc.k^2)+0.0000", ....)
>
> All you're looking for is the 4-letter "Group Name" / prefix for the
> data set: in this case "jukc"
>
> 4.  In the Ifeffit> input line at the bottom of the buffer, type
>         chi_noise(jukc.chi)
> 5.  Then type
>         show epsilon_k, epsilon_r
>
> This will show the values estimated for the noise in chi(k) and chi(R)
> using the FFT parameters you input.  For most data and FFT parameters,
> epsilon_k should not change significantly, though epsilon_r will
> change significantly with the k-weight.
>
> Cheers,
>
> --Matt
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
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