Waveform measurements - info needed

[Thayer79]

I am fairly new to LabView and am trying to understand some of the vis in the Waveform Measurements and Analyze folders. Help with any of the following questions is appreciated.

1) What is the phase output of the Spectral Measurements Express vi? I've seen several examples using this function and none of them use the phase output. If I put in a simulated sine wave with no phase offset, the FFT phase output is -90 deg at the input freq.

2) What is the amplitude output of the Tone Measurements Express vi? It looks like average peak value over the duration of the signal, but I'd like to be sure.

3) If I get a value of 28.42f at the "detected phase" output of the Extract Single Tone Information vi, what does the "f" mean?

4) Is there any information about the details of these functions? The information in the Help window is scant, the tutorials provide a lot of theory, the examples don't always cover what I want to know. Is there somewhere that provides a fuller explanation of the inputs and outputs of the functions?

Thanks in advance,
Margaret

[Graziano]

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1) What is the phase output of the Spectral Measurements Express vi? I've seen several examples using this function and none of them use the phase output. If I put in a simulated sine wave with no phase offset, the FFT phase output is -90 deg at the input freq.

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I'll just answer this: when you do a spectral measurement, you have Magnitude and Phase.  Usually, the most used measurement regards Amplitude (FFT for someone...), but in some application, also phase matters (for example, to check if a system has a generalized linear phase).

   Hope this helps...

graziano

[Thayer79]

Thank you for your answer, but I still have the quesions. Perhaps I should elaborate.

I've inherited a data acquistion system. The incoming waveform currently finds
   1) amplitude from the Spectral Measurements (using the Get Amplitude function to get the amplitude of the FFT), and
   2) phase from the Tone Measurements vi

I have been putting a simulated sine wave into these functions to try and understand them better. I notice the following:

 -- The phase from Tone Measurements is 0, matching the input. The phase from the Spectral Measurements is -90 (at the freq of the input signal). Hence my first question, "What is the phase output of the Spectral Measurements Express vi?" Is it referring to the complex plane?

 -- When I take the amplitude from the Tone Measurements I get a better answer (closer to my simulated input) than the FFT in the Spectral Measurements gives, but I'd like to know more about Tone Measurements before I start using it for amplitude. Hence my second question, "What is the amplitude output of the Tone Measurements Express vi? It looks like average peak value over the duration of the signal, but I'd like to be sure."

-- When I put my sine into the Extract Single Tone function, the phase is 28.42f. My third question asked what the f at the end of the number means.

And finally, I'd really like to know if there is a resource for detailed information about inputs/outputs to these functions.

Thanks again,
Margaret

[Amanda_H]

Hi Margaret,

In response to question 1, the Spectral Measurements Express VI is used for FFT-based measurements, and the Tone Measurements VI is used to find the frequency or amplitude of a specific signal. Have you tried viewing the Detailed Help for these commands? These can be viewed by right-clicking on the command, choosing Control-H to bring up the Context Help, and clicking Detailed Help. This will allow you to view Examples and detailed information on the inputs and outputs.

Here is some detailed information about the Spectral Measurements Express VI from the Help:

FFT-based spectral computations assume that the finite block of signal data represents one period of a periodic signal. The computed spectrum of this effective periodically extended signal shows energy spreading into frequencies that were not present in the original signal. To reduce this spectral leakage, use smoothing windows to taper the sharp transitions in the effective signal. You do not typically use windows if you can acquire an integer number of cycles of each frequency component measured or if you are analyzing noise spectra.

In response to question 2, the amplitude of the Tone Measurements Express VI returns the amplitude of the detected single tone in Volts peak (Vp).

In response to question 3, what is the phase of the signal that you're measuring? I used this VI with a Simulate Signal VI and simulated a sine wave and was able to correctly use the Extract Tone VI for all phases. Have you tried doing this?

Also, here is the manual for the Spectral Measurements Toolkit. Let me know if you have any more questions.

[Thayer79]

Hello Amanda,

Thanks so much for your helpful answer. This ("FFT-based spectral computations assume that the finite block of signal data represents one period of a periodic signal.") was particularly useful, since it doesn't appear in my help window. I'm using 7.1 and it looks like most of the the answers to my questions are in the version 8 help.

I tried to follow the link at the bottom of your reply and it didn't go anywhere, but knowing that the information was out there, I searched through the NI website and found what I think you were referring to. Is this it?

http://www.ni.com/pdf/manuals/370355f.pdf 

It talks about Zoom FFT. Do your reply and the information in the pdf pertain to the 7.1 Spectral Measurements Express vi as well as the version 8 Zoom FFT?

Regards my question 3: I'll double check tomorrow when I return to my office (I'm in a lab in another town today).

Many thanks for your help. I can see why no one answered me - they know the information is in the help. But, I can't see the same help they are, because I'm using a different version of LabView.

Margaret

[Amanda_H]

Hi Margaret,

I'm glad I could help. Sorry for the broken link. The link you provided is correct. Yes, those responses apply to Zoom FFT as well. I hope you find the manual and information in the LabVIEW help to be useful to you. Have a great day!