[Camilla, Jenne, Rick]
As a follow-on to the FSS measurement last week (alog 54820), today we measured the IMC loop (IMC only, no IFO or CARM involved) out to high frequency to see how this feature in the FSS at ~780kHz affects the IMC. As was the status for the last measurement out to such high frequency that I found via a quick search from back in July 2016 (alog 28363) it is a total non-issue for the IMC.
The first figure shows the full IMC OLG measured today, plotted with the data from the text file in the July 2016 alog. It looks like the ~780kHz feature is quite similar, although we have implemented something like the pole that Jeff suggests in that alog.
The second figure shows a zoomed in measurement of the feature. In particular, since this is so far above the UGF of the IMC loop, it is much much lower than unity. However, as Rick noted out loud, when comparing this zoom to the FSS3.tif from last week's measurement, the phase and magnitude features look almost flipped (the mag here looks like the phase in the FSS measurement, and vice versa). I'm not sure what that means though.
We may consider measuring the IMC loop and the CARM loop in full lock, although I suspect that this feature isn't hurting either of those. We may want to continue thinking about the FSS loop where the feature does cross unity gain, but since the phase is entirely stable it's not immediately clear that there's any action that needs to be taken.
A reminder that I was working with Chris Whittle during the 2016-era look in to the IMC / CARM investigations, and he did the majority of the work after the aLOG you found.
Here're some of the more relevant aLOGs from him about this:
- LHO aLOG 29811
- LHO aLOG 29735
Hope that helps!