SVT 3.1 FFT Spectrum Magnitude VI output is RMS or Peak (db OFF)

Please help with my SVT problem:
1. With db OFF, what magnitude EU value does the SVT 3.1 FFT Spectrum VI output? More to the point, is the EU an rms value or peak value? I thought it was rms but now I'm not sure.
2. When I double integrate acceleration values to get displacement then feed the displacement waveform to this FFT Spectrum VI, I seem to get high displacement values compared to other portable analyzers in our shop. If the output of the FFT Spectrum VI is for example, mils rms (single sided) I have to convert this to peak-to-peak values. So I multiply the each spectrum value by 1.414 (to get peak) then by 2 for peak-to-peak. Is there a more accurate way to get peak-to-peak displacement values from an acceleration waveform?
thanks,
Glen

Hi Glen,
There are several FFT-based VIs in LabVIEW and the output units of each function is different.
The following table will give you a better idea of what each function does. In your case you might be better off using the FFT VI that comes with the Signal Processing toolkit since you are looking for Volts output instead of the Vrms given by the FFT Spectrum functions.
If you have doubts about the units returned on a specific function I recommend using the Context Help in LabVIEW. As shown on the image below this gives you more information about the function and the variables used/returned by it. You can open Context Help by going to Help>>Context Help in LabVIEW.
Hope this helps. Regards,
Tica T
Applications Engineer
Message Edited by Tica T on 04-04-2006 10:38 AM
Attachments:
units.JPG ‏50 KB
functions.JPG ‏85 KB

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What is the difference between SMT zoom FFT, FFT spectrum, SVT FFT spectrum, complex FFT vi, real FFT vi

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Hi Brandon,
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Attachments:
SpectrumAnalyzer.vi ‏104 KB

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Averaged_FFT.vi ‏51 KB

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Hi Massif,
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Message Edited by Kabul on 06-10-2008 01:09 PM
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Attachments:
VRms to Vpeak1.vi ‏13 KB

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block diagram.jpg ‏48 KB

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Dear Sir
I take FFT of my real time data that I collect from current probe. I successfully get the FFT spectrum. But my supervisor ask me to normalize the harmonic component present at 25 Hz in the FFT spectrum with fundamental component. Can you please guide me how I should do this task.
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Solved!
Go to Solution.
Attachments:
LabVIEW 6281-50.vi ‏61 KB

The problem is to know what to use as a reference for the normalization. The image below shows an expanded view of the Instantaneous Current Spectrum with the display style set to show the actual data points. I also moved the frequency indicator wired to the Tone Measurements VI into the image. The frequency as detected is not exactly 50 Hz. Note that the data points in the spectrum are not symmetrical about 50 Hz. This is another indication that the sampling is not perfectly synchronous. The result is that the spectrum contains 11 points fro 49 to 51 Hz which are within 60 dB of the highest point. If you want to normaize to the fundamental, you need to determine the total energy in the fundamental component. That probably includes the data points in that 2 Hz range or maybe slightly more.
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FFT RMS or Peak?

Hi,
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Solved!
Go to Solution.

Donners,
It can get complicated but I suspect that what you are seeing is a result of the frequency of the waves not matching exactly the frequency bins of the FFT. For example suppose that df = 1 Hz. Then each element of the FFT array represents the energy in a "bin" one hertz wide centered on integer frequencies. For a wave with most of its energy at 7.3 Hz the FFT will likely have non-zero values for both the 7 and 8 Hz bins. In real systems with noise and slight frequency variations it is not uncomon for the data to be spread over 10 or 20 bins. To get the fundamental amplitude of the signal you would need to combine the energy in all those bins. Since the FFT produces a complex output, the phase component also needs to be considered. Non-sinusoidal waveshapes mean that some of the energy is in harmonics. A narrow square pulse can have a quite large amplitude yet still have no large spectral components.
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Solved!
Go to Solution.
Attachments:
fft graph1.JPG ‏40 KB

I set the dt value to 0.002. For this I set sampling rate 2000hz and no.of samples 1000000. In this way I achieve frequency span of 250hz. But resolution is still 100hz (as shown in attach 1). My question is how can I set resolution to 10hz? I increase the number of samples but from doing this only the frequency span changes and resolution remain same as 100hz. Resolution become 10hz only when I set total frequency span to 50 hz (but My frequency span of intrest is 0~500hz or 0~250hz not below thos).
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FFT Spectrum from 2D array

Hello everyone,
I am trying to make the Spectrum analysis from a 2D array. In this case is Amplitude (Y) vs Depth (X). The problem is that the depth (X) doesnt have a constant dt so I cannot create a normal waveform as an input for the Power Spectrum analyzer or Spectral Measurements vi.
This is an example of some of the data that we have:
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8390.7
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8391.2
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6.8555
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8396.2
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3.3945
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Solved!
Go to Solution.

I gave you a portion of the signal that always have a constant dt. Sorry ... There is another part of the signal where there are different dt's :
8407.5
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8411.5
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25.7285
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12.4975
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Need PID vi that outputs analog instead of digital (on/off)

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SVT 3.1 FFT Spectrum Magnitude VI output is RMS or Peak (db OFF)

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