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 Unit of Power as MSA		  New Topic  Reply to Topic
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Aike

Thailand
1 Posts
Posted - 01/30/2012 :  12:28:46 PM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
I calculate power spectrum using FFT tool on OriginPro 8.1. I plot a graph: the x-axis indicates frequency (Hz) and the y-axis indicates Power Spectrum (???). I don't know the what is the unit of Power Spectrum. My data is about electron content. The unit of electron content is TECU. Can I refer the unit of Power Spectrum as TECU^2/Hz?

Drbobshepherd

USA
Posts
Posted - 02/01/2012 :  10:56:19 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
If your time-domain function is in units of TECU, and the time is in units of seconds, then your power spectral density (PSD) should be in units of TECU^2/Hz; however, if the FFT did not normalize the spectrum properly, your PSD units may be proportional to TECU^2/Hz.

If I were you, I would use Parseval's theorem to test the results. Compare the total integral of your PSD function to the total integral of the square of your time-domain function. If the ratio is close to unity, everything is OK; if not, then the ratio is your proportionality constant.
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Sam Fang

293 Posts
Posted - 02/01/2012 :  9:19:38 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Origin's Power Spectrum supports three types of normalization. And its unit based on its expression:
MSA (Re^2+Im^2)/n^2 unit: TECU^2
SSA (Re^2+Im^2)/n unit: TECU^2
TISA dt*(Re^2+Im^2)/n unit: TECU^2/Hz

Note that Origin has shown units for FFT results if input data contain unit since Origin8.5.1 .

Sam
OriginLab Technical Services
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