What is the different between power spectrum and power spectral density?

[wesley eng]

Hi,

I am quite confused about the function of power spectrum and power spectral density in FFT module. May I know what is the different between power spectrum and power spectral density and Magnitude (peak) and Magnitude (RMS) as well? Thanks.  

[DuffmanSays]

wesley eng,

These two VIs will allow you perform the two mathematically different calculations of Power Spectrum and Power Spectral Density of a time signal. I would suggest taking a look at the detailed help for both of these functions (linked above) for a detailed description of how they work.

Also, the RMS value of a signal is a statistical measure of the amplitude of that signal. Essentially is calculated as the square root of the mean squared value of a signal. The outputs of the FFT-based VIs will often be in units of RMS value. I hope this information is useful for you! Have a great day.

Mike D.

[Bauch@Berkeley]

Hello,

I am really confused too. I do not understand why there are two different VIs for Power Spectrum and Power Spectral Density in Labview. I have read all help information and wikipedia articles in german and english and can conclude: there is no difference.

The german wikipedia says "Power Spectrum" and "Power Spectral Density" is the same. The English Wikipedia has no note about a "Power Spectrum" as itself.

The Power Spectral Density is the squared Fourier Transformed F(w), which can be written as |F(w)|^2 = |F*(w)F(w) |. With the fourier convolution theorem
one can easily show, that this is the same like the fourier transformation of the autocorrelation function. So it is possible to measure the density in two ways. First calculating the fourier transformed and then multiply. Or second autocorrelate the function and than fourier transform.

The difference is, the fourier transformed of a signal does not have to exist to calculate the density with the second way.

But what is exactly the Power Spectrum???

NI says the power spectral density is the scaled (standardized) power spectrum. Nothing else?

Hope someone can help. Thx

[NoahR]

Hi Bauch,

Thanks for contacting National Instruments.  I have been looking into your question about the Power Spectral Density function and I think that I found some information that will help you out.  I have attached a link below that goes through some of the details about the LabVIEW Analysis Concepts.  In particular, I think you should take a look at Chapter 4, page 27.  I think this should answer some of your questions.  If you have any other questions about this issue, please feel free to post back and we can go from there.  Thanks!

http://www.ni.com/pdf/manuals/370192c.pdf

[nivea25]

hi NoahR

thanks for the link that you posted but I have few confusions i hope if you could help me.

In my project I have to calculate power spectral density of a signal in a certain frequency band. I have used FFT in matlab but it gives PSD of whole signal.