Waveform measurements - info needed

Question

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

Answer 1

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.
Amanda Howard
Americas Services and Support Recruiting Manager
National Instruments