Power spectrum density question: One sided convert

Hi, there
I am using the PSD function. In order to get the RMS noise, I integrated the PSD data and doubled the result because I have two sides in positive frequency range and negative frequency range. Then my result is 1.414 (or Sqrt(2)) times higher than my RMS data measured in time domain.
Then I looked into the PSD fuction and found out that in the function of "Convert to on-sided spectrum", the amplitude was doubled. But it is not meantioned in the help file.
My thinking is: either change the help file of PSD function and mention that it is a doubled one-sided output or do not double the amplitude when convert the spectrum from two-sided to one-sided output.
best regards
Chengquan Li
CQ

Dr. Chengquan Li,
I know sometimes the help can be hard to navigate, but I think I found the help documentation that address this issue. It can be found here: Power Spectrum.
Under the heading "Converting a Two-Sided Power Spectrum to a Single-Sided Power Spectrum", you can find the following:
"A two-sided power spectrum displays half the energy at the positive frequency and half the energy at the negative frequency. Therefore, to convert a two-sided spectrum to a single-sided spectrum, you discard the second half of the array and multiply every point except for DC by two, as shown in the following equations."
This is why you noticed that the block diagram of "Convert to One-Sided Spectrum" had the output doubled. I hope this helps clear up the issue.
Regards,
Elizabeth K.
National Instruments | Applications Engineer | www.ni.com/support

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Message Edited by Yardov on 08-17-2007 06:50 PM
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