AC/DC estimator - where does it come from ?

[matmix1]

In my program, I'm using AC & DC estimator block, in order to indicate RMS value of the signal. However, looking under his VI, I found interesting code for RMS value estimate, but I don't know, where does it come from ? Is there any general formula to be found somewhere in literature for such estimator ?

[MoReese]

Is this a function of LabView or OpenG?  I can't find anythig called RMS estimator.  This may be your term.  Can you post a VI?

[JÞB]

it requires the advanced analisys package (Not in base) but its on the signal operation palatte

[MoReese]

Thanks Jeff. 

The information is buried in a Call Library function, which I can't access.  Perhaps someone a bit more inquisitive can answer this.  But no, no formula, at least on the surface.  It's probably buried in the .dll that's being called.

[Dennis_Knutson]

Do you need more than what is in the help?

AC & DC Estimator Details 

You often need to measure the AC and DC content of signals you digitize. This VI applies the Hanning window, which is sufficient for separating most of the AC energy from the DC bin. If you have a periodic signal and you do not have an integral number of periods in the acquisition, spectral spreading occurs regardless of whether you perform an FFT later. The coherent gain and equivalent noise bandwidth constants for the Hanning window that this VI uses compensate for windowing effects on the gain and bandwidth.

LabVIEW uses the efficient mean and variance computations instead of Fourier analysis to determine the total AC energy and the true DC level.

[matmix1]

For those, who can't find this VI, I attacked below the "body" of this function. The desription in the help is ok, however I need to know, where the coherent gain (cg) and equivalent noise bandwidth (enbw) constants comes from, i.e. 0.5 and 1.5. Can I find this values and formulas somewhere ? or is it well known that such values are connected with properties of Hanning window ?

[matmix1]

Ok, I've found an answer to my question in first post. Topic can be closed.

[crossrulz]

What did you find?  Care to share?

[MoReese]

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@matmix1 wrote:

Ok, I've found an answer to my question in first post. Topic can be closed.

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Also, please mark the solution for you problem so others may be helped in the future.  Thanks.

[matmix1]

http://www.bores.com/courses/advanced/windows/files/windows.pdf

http://zone.ni.com/reference/en-XX/help/371361E-01/lvanlsconcepts/dc_rms_error_sources/

http://www.noise.physx.u-szeged.hu/DigitalMeasurements/AcquisitionAndProcessing/PowerSpectrum.pdf , pages 20-21.

In this documents, I've found information about coherent gain and equivalent noise bandwidth of particular window. Additionally, in the last document, there is described relation between variance, standard deviation and rms value of AC signal. The only problem is such, that I haven't found any exact formula connecting window and variance of the signal. Binding this information altogether, it only can be assumed that square-root of variance divided by coherent gain squared times equivalent noise bandwidth gives estimator of rms value of AC signal.