FRF resonant frequencies

Thread Starter

j.gomez

Joined Apr 15, 2022
1
Hi,
I'm new here and relatively new to signal processing and analysis. My ultimate goal is to find the mode shapes of a system, but I'm currently working on building the FRF before I move into the next stage. When computing H=Sxy/Sxx (cross power spectrum over input spectrum), I'm getting a minimum instead of a maximum magnitude on my frequency vs magnitude graph, while the rest of the FRF is pretty much constant. I applied FFT to both the input and output, and both have their resonant frequencies or spectral line at the expected frequency (2.01 Hz). I assume the magnitude is minimum Since Sxx has a higher magnitude spectral line than Sxy, but I'm told the FRF should show a maximum since the test showed a mode at 2.01 Hz and should obtain the information from there.
What am I not understanding? Or what does it mean to have a minimum at the frequency of interest?
Input is in lbf and output is in g's
 
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