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Calculating the RMS of Fluctuations about a Constant Signal

I'm trying to calculate the turbulence intensity of a wind tunnel using LabVIEW 7.1
My input signal is basically a DC signal with noise added
what i'm trying to do is to first work out what the Amplitude of the DC Signal, subtract that from the original signal so i'm left with just the noise and then get the rms of just the noise.
So far i've got my DC amplitude fine but i'm having trouble getting that to subtract from the original signal. Should i be able to feed the DC amplitude value straight into a subtract operator with the input signal to get the noise on its own or do i need to do something else.
The link below is to a powerpoint presentation which shows it done, but not how to do it.
look at the last 2 slides
Would be greatful for any help
Mark Thompson

Message Edited by MIncorporated on 05-14-2007 06:07 AM

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It's not clear what you mean by "trouble." Labview won't run? Answer isn't what you expect?

In any case I would consider using a high pass filter, then the RMS.
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Unfortunately, the last 2 slides don't show anything. All they show is a general, meaningless, equation, and a front panel with two pretty graphs. No code, so nothing to compare.

I'm not sure using a high-pass filter is a good idea, since that would cut out some of the noise if there's high-frequency components in it, and he's trying to get the noise.

It seems like you're trying to subtract a constant from all elements of an array? What form is your DC signal in? Is it an array, or a waveform type? Either way, the subtract node is polymorphic, so it will handle the subtraction properly:

Perhaps it would help if you posted your code, or a snippet of it.

Message Edited by smercurio_fc on 05-14-2007 09:52 AM

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Thanks for the help guys, I've managed to sort it out

Heres my block diagram for anyone whos interested.

Feel free to point out any ways i could have done it better. Its my first VI, so i'm looking for tips.


Cheers Mark



Message Edited by MIncorporated on 05-17-2007 07:37 AM

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