Problem to calculate the coherence (using NetworkFunction-VI) with only 1 row of data for each, the stimulus and response input

Question

Hello,
I am trying to calculate the coherence of a stimulus and response
signal using the Network Functions (avg) VI's. The problem is that I
always get a coherence of "1" at all frequencies. This problem is
already known (see KnowledgeBase document: Why is the Network Functions (avg) VI's Coherence Function Output "1"?).
My trouble is that the described solution (-> the stimulus and response input matrices need to have at least two rows to get non-unity coherence values) doesn't help me much, because I only have one array of stimulus data and one array of response values.
Thus, how can I fullfil the 'coherence-criteria' to input at least two rows of data each when I just have one row of data each?
Any hint or idea is very much appreciated. Thanks!
Horst

Answer 1

With this weird board layout, I'm not sure whether you were asking me, but, on the assumption that you were, here goes:
I found no need to use the cross-power spectrum and power spectrum blocks
1... I was looking for speed.
2... I already had the component spectral data there, for other purposes. From that, it's nothing but addition and multiplication.
3... The "easy" VIs, assume a time wave input, as I recall. Which means they would take the same spectrum of the same timewave several times, where I only do it once.
I have attached PNGs of my code.
The PROCESS CHANNEL vi accepts the time wave and:
1... Removes DC value.
2... Integrates (optional, used for certain sensors).
3... Windows (Hanning, etc. - optional)
4... Finds spectrum.
5... Removes spectral mirrors.
6... Scales into Eng. units.
7... From there, you COULD use COMPLEX-TO-POLAR, but I don't care about the phase data, and I need the MAG^2 data anyway, so I rolled my own COMPLEX-TO-MAG code.
The above is done on each channel. The PROCESS DATA vi calls the above with data for each channel. The 1st channel in the list is required to be the reference (stimulus) channel.
After looping over each channel, we have the Sxx, Syy, and Sxy terms. This code contains some averaging and peak-picking stuff that's not relevant.
From there, it's straightforward to ger XFER = Sxy/Sxx and COHERENCE = |Sxy|^2 / (Sxx * Syy)
Note that it uses the MAGNITUDE SQUARED of Sxy. Again, if you use the "easy" stuff, it will do a square-root operation that you just have to reverse - it is obtained faster by the sum of the squares of the real and imag parts.
Hope this helps.
Steve Bird
Culverson Software - Elegant software that is a pleasure to use.
Culverson.com
Blog for (mostly LabVIEW) programmers: Tips And Tricks
Attachments:
ProcessChannel.png ‏25 KB