Output unit of Power Spectrum.VI

I'm using LabVIEW5.1 in Windows95.
According to the VI Reference Manual, the unit of the Auto Power Spectrum.VI
is V^2rms. But the output of the Power Spectrum.VI is not specified.
1. I want to know the output unit of the Power Spectrum.VI
2. How can I change the output unit of the Auto Power Spectrum.VI. That is
from V^2rms to V^2/Hz.
The second question is more important to me.
Hyun-ho Lee
[email protected]

Spectrum Unit Conversion.vi in Signal Processing -> Measurement
"Hyun-ho Lee" <> wrote in message news:8ipfet$i5b$[email protected]..
> I'm using LabVIEW5.1 in Windows95.
>
> According to the VI Reference Manual, the unit of the Auto Power
Spectrum.VI
> is V^2rms. But the output of the Power Spectrum.VI is not specified.
>
> 1. I want to know the output unit of the Power Spectrum.VI
>
> 2. How can I change the output unit of the Auto Power Spectrum.VI. That is
> from V^2rms to V^2/Hz.
>
> The second question is more important to me.
>
> Hyun-ho Lee
> [email protected]
>
>
>
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What is the power spectrum unit for Unevenly Sampled Signal Spectrum.vi if the input signal unit is Volts?

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Hi,
The power spectrum will be in units of Vrms^2 which are power units. There is a very usefull VI that you can use to easily transform the units of your spectrum; the VI is called Spectrum Unit Conversion.vi. You select the type of spectrum that you have and the units that you want it to be converted to.
Here is a good document on spectral measurements in genereal:
frequency spectrum
I hope this helps.
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Power Spectrum no longer working correctly after upgrading to 8.0.1

Recently, I installed Labview 8.0.1 to correct a problem I was experiencing in 8.0. The installation resolved my problem, but now the Power Spectrum no longer gives the correct result. My program has not changed other than the update. I checked my circuit and everything is functioning properly. Before changing to 8.0.1 the power spectrum peaked at 40Hz and 80Hz, which was correct, and now it peaks at 0Hz and 10Hz. On top of everything, the peaks are much higher than they should be. I expect peaks with a max of 2.5millivolts and now they are 1volt. I don't understand what is going on. Nothing changed, but the software version. Can someone please help me?

In addtion to Altenbach's comments -
I'd recommend trying another VI function to get the power spectrum for
your signal to see if the behavior is the same. You can find some
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Finder.
You can go to LabVIEW's Example Finder by going to Help >> Find Examples.
Then search for FFT. There should be some results that include FFT and
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Power Spectrum with RFSA

Hi all,
I have some question about fetching power spectrum with RFSA drivers.
when i set span and resolution bandwidth as 100M and 1M respective in ni5660 Configure for Spectrum,and use ni5660 Read Averaged Power Spectrum to fetch the power spectrum,does averaged power spectrum output the 100M signal once?
if so,how can i draw the spectrum in Waveform Chart as RFSA demo show us?RFSA demo did use Waveform Chart to draw spectrum,did't it?
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Hello Liu Yuan,
If you take a look at ni5660 Power in Band example in the LabView Example Finder ( Help -> Find Examples ), you will notice it uses a Waveform Graph. The Read Averaged Power Spectrum vi is contained within a while loop which means that it will continuously output the averaged power spectrum. If you want to use the RFSA drivers, I suggest looking at the RFSA Getting Started Spectrum example in the example finder. This example only reads the spectrum once, but you could put a while loop around the read vi to get continuous spectrum output. If you have any more questions feel free to post back.
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What are the units for the power spectrum output? (units in)^2/Hz?

I can't seem to find the answer in the manual that came with the software or anywhere online. Maybe I am just looking in the wrong places.
Thanks
David

Hi David,
I assume you are referring to the step, "Power Spectrum", in Analysis»Frequency Domain Measurements
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Also, in the configuration tab of the step, for magnitude you can select either dB or linear.
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I am currently post processing time domain accel data. I run it through the Complex FFT VI in order to transfer it to the frequency domain, which works wonderfully. Unfortunately, I'm still a bit confused as to what the output units of the plot are...
Any help would be appreciated.

First of all you don't have to use the Complex FFT, the Real FFT would work fine because your input time domain data are real-data. Both Real and Complex FFT VIs return a complex spectrum output.
The complex spectrum data represent the magnitude and the phase of your signal at the different frequencies. So if you use the 'complex to polar' primitive in your Numeric>>Complex palette you can convert your complex spectrum to magnitude and phase (radians) signals. Next you need to scale your magnitude for 'half the length of your FFT' (that is divide by N/2) in order to get the correct 'amplitude' of the different tones. Finally you need to correct your value at 'dc' with a SQRT(2) factor ...
When you are done with all this and optionally also have corrected
for the noise bandwidth of your window, the unit of your magnitude will be the same as your input signal (m/s^2 or g I assume).
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Basic accelerometer-power spectrum readings

Hi all,
I'm new to Labview and am trying to create a simple power spectrum logger. I've had a look at multiple tutorials but am stuck getting to places (most of the tutorials require sound and vibration toolkit, are really old and incompatible with Labview 2011 etc). The program I'm trying to do is:
1) Collect signals from accelerometer (4kS/s)
2) Generate power spectrum
3) Save to disk (on board the cRIO)
My setup is:
- cRIO 9014
- NI 9234
- Labview 2011
So far, I've managed to get the signal readings (XYZ axis) and merge them, put them into a 'Collector' block and then output them to the power spectrum block. I've very crudely used a 'Time Delay' block to set the sample rate (which I know is probably not accurate, but I cannot figure out how else to do it). I'm having a problem figuring out how to input the sample rate into the collector, which is why my power spectrum is giving me 0 to 0.5Hz (it's likely assuming the sample rate is 1Hz). I've attached my .vi.
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Attachments:
accelero.vi ‏93 KB

I think what you want is the CompactRIO vibration data logger. It does not require the S&V measurement suite, though you will find it very useful.
Check this out!
http://zone.ni.com/devzone/cda/epd/p/id/6388
It has a waveform acquisition template for the FPGA. It has a data logging state design.
You can see a Video of it working here
http://www.ni.com/mcm/wind/
Let us know if this helps.
Preston Johnson
Principal Sales Engineer
Condition Monitoring Systems
Vibration Analyst III - www.vibinst.org, www.mobiusinstitute.com
National Instruments
[email protected]
www.ni.com/mcm
www.ni.com/soundandvibration
www.ni.com/biganalogdata
512-683-5444

What is the equation for a Low-Sidelobe Window Function (in Power Spectrum VI)?

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Power Spectrum

Ok here's the deal.. I acquire data (of type numeric array) from an acuqisition unit (the specifics of it aren't important for the problem) at a known sampling rate and wish to perform a power spectrum on this data. I've used the build waveform function to build a waveform from the 1D array of data with the addition of the dt value (1/Fs) and a timestamp and fed this waveform into the power spectrum vi. The spectrum is then plotted on a waveform chart.
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Another possible issue is with the timestamp but I think I've accounted for that.
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Strokes
Attachments:
Power Spectrum.vi ‏19 KB

Strokes,
you listen to mp3 music from time to time? Maybe, you have configured your application playing those mp3 to display something called "Bars and Waves" (at least, Microsoft calls it like this in Media Player). This display is comparable to an equalizer view.
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Hi,
I have a simple .vi that is attempting to calculate the total PSD over a 500Hz bandwidth. My question pertains to the Windowing function and
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thanks for any help
Attachments:
PSD of PDM.zip ‏127 KB

You may want to try "FFT Power Spectrum and PSD.vi". it takes care of the compensation for the window effect automatically.
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Attachments:
psd.vi ‏20 KB

Power Spectrum peak calculator

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Intern NSWCCD Carderock.

Yes, I did try that, but the problem is I don't understand the inputs required for it as it requires
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any help.
Intern NSWCCD Carderock.

Power Spectrum vi

Hi,
Can anaybody tell me the maximum size of an array that can be accepted by
the Power Spectrum vi in the signal processing toolset?
I find that it does not produce an output if an array size of over 5 million
elemnts is used, however it produces the required output when the an input
array of 4 million elemnts is used.
Additionally, when an array of 5 million element and over is used as the
input:
1. No error message is produced.
2. This occurs even when averaging is activated.
Thanks,
Luca

I am not sure which VI you were using exactly. Is Power Spectrum.vi on the palette Function>>Analyze>>SIgnal Processing>>Frequency Domain? I have tried this VI and do not have the problem you mentioned.
Would you attach some testing VIs?

What amplitude on power spectrum if analog input is 7.3V,sine,80hz?

I have an analog input of 7.3V, sine wave at 80 Hz feeding thru PXI-4475 (I have a PXI-1042 Chasis and PXI-8176 Controller and I am programming my application in Lab Windows-CVI). I see a sine wave with an amplitude of 7.3V in the time domain but I see an amplitude of 13.3V at 80Hz in the power spectrum. Should I see 7.3V in power spectrum display? My HP FFT Analyzer showed 7.3V at 80 Hz in power spectrum display. Please help. My email: [email protected] or my phone: 215-897-8766/7370. Thanks in advance.

There would appear to be a mathematical relationship between the 7.3 V sine wave and the 13.3 V amplitude as follows:
a) rms value of a sine wave at 7.3 V = 5.1619 V
b) square that result = 26.645
c) half the result = 13.32
The power spectrum should be expressed in terms of voltage squared. The power spectrum should produce a result that is a second order function of the initial amplitude. Are you sure the settings on the HP were for "power" when you read the output of 7.3V? The 13.3 V number appears to be the better of the two.

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.
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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

What is the difference between power spectrum and PSD?

Hi all,
I input a time domain signal to the FFT power spectrum VI and FFT PSD VI seperately, and got two outputs in frequency domain with similar shapes but different magnitudes.
I refer to some signal and system textbook, and find PSD(w)=1/pi*Power Specrum or equivalently PSD(f)=2* Power Specrum.But as I observed from the LabVIEW outputs, this relation is not correct. I input a 1KHz sin wave with amplitude=1, Fs=512KHz, #s=2048 , the PS peak is 0.5 and the PSD peak is around 0.013.
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Sunny

PSD is related to spectrum resolution, thus related to sample rate.
df= Sample rate/ #samples
I think they follow this relationship:
Amplitude FFT power spectrum = Amplitude PSD * df * 1.5

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