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Re: power spectrum/freqs.

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  • Subject: [mg15338] Re: power spectrum/freqs.
  • From: jim leddon <jleddon at>
  • Date: Fri, 8 Jan 1999 04:15:25 -0500
  • Sender: owner-wri-mathgroup at


I have a question regarding calculating the power spectrum and 
frequencies of a set of intensities values. The data set is a small 
one(see attached file-Mathematica 3.0).

In the attached file, I have calculated and plotted not only a power 
spectrum but also a power spectrum versus frequencies of a data set of
 intensity values.I am trying to determine if there are some dominant
 frequencies or if the power spectrum has a chaotic type behavior
shape.  set of values and converts these to a frequency domain set of
values. It  seemed reasonable that once I calculated the FT, I could
account for the  aliasing by removing the second part of the spectrum
and then use the  remaining FT values to get the power spectrum which
is by definition the  absolute value of the the FT(remaining values)
multiplied by it's  conjugate (remaining values)FT - divided by the
square root of the the  length of the data list. I then calculated the
frequencies based on the  FT, although I've noticed that  alot of
people who have used Mathematica  will use the "InverseFourier"
function to calculate the power spectrum  and frequencies. My question
is; in looking at the attached program, am  I correct about how to
calculate the power spectrum and frequencies? Is  it also correct to
use the entire FT spectrum times its' conjugate FT to  calculate the
power spectrum?

I also would like to ask if there is a way to use the "Frequencies" 
function under the "Statistics`DataManipulation`" package in order to 
calculate the frequencies. I have used this function before, however, 
there must be some way of ordering the output to correspond with a
power  spectrum if one wanted to plot both the power spectrum vs.

Thank you for your time and consideration in reading this email message.
 I'd look forward to hearing from you if you can help.

Debbie Leddon

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