problem with fft periodogram

[Travis E. Oliphant]

aitor69gonzalez at gmail.com wrote:
> Hello,
> 
> I am ploting a fft periodogram of my data using a script (found in
> internet: http://linuxgazette.net/115/andreasen.html ) that gave me
> good results before. There should be a periodicity of >160 in the data
> as can be seen by eye. However, this script now says that there is
> periodicity of ~9. Can somebody explain me what's wrong? Thank you in
> advance.

The units are wrong on your period axis.  Right now you have them in 
units of "sample-spacing".  So, the plot is telling you that you have a 
periodicity of ~9 sample spacings.   To get it in unites of minutes you 
need to multiply period by the difference in minutes

period_in_minutes = period * (minutes[1] - minutes[0])

Then, plot period_in_minutes versus power.  I see a peak around 180 
minutes in your data.



-Travis