Durring yesterday's maintence window I made some excitations on both transmons to investigate the noise peaks around 75-85 Hz. The main conculsions are that the coupling from TMS X L drive to DARM is bilinear while the coupling from TMSY is linear, and it seems likely that TMSX motion accounts for some significant part of the unexplained noise in the H1 noise budget (21162) even at frequencies where there is no coherence between DARM and the X QPDs.
In the first attached screen shot you can see the DARM spectra durring some of my excitations in the upper panel and TMS QPD spectra in the lower panel. In the QPD spectra, you can see that the X end QPDs have some excess noise compared to Y. These spectra were from a time when I had a TMSX longitudnal injection at 75 Hz (this is the same time as the yellow DARM trace). There is a narrow line in the X QPD spectra, but in the DARM spectrum the line that appears is about 1 Hz wide, indicating there is some bilinear coupling. The excitation either did not show up or was very small in the QPD sums.
I also made a few injections into TMSY longitudnal which produced only narrow lines in DARM, both X and Y injections are shown at 100 Hz for comparison.
We can attempt to make projections of this noise into DARM using the ratio or the injection line peaks or the rms of the injection peaks to estimte the coupling. I drove longitudnally and the only good witness sensor we have is an angular sensor. If the coupling mechanism is something like scatter off the QPDs that goes directly back into the arm, DARM would be mostly sensitive to the longitudnal TMS motion and if the QPDs are only seeing the unintentional length to angle coupling the projection from the normal level of the QPD spectra to the normal level of DARM could be an overestimate. This projection does show that this would be within a factor of 10 of DARM from about 100 Hz down to at least 75 Hz.
Perhaps when we get a chance we can try misaligning TMSX to see if we can reduce the noise in DARM this way.
AS Keita suggested, I checked if some of this noise (or at least the linear coupling seen for TMSY) could be simply that my drive on TMS moves the ISI and that motion propagates down the quad to the test mass. This seems to be much too small to explain the observed coupling.
Using the GS13s, and the calibration for them that Jim told me, the table motion caused by my 75 Hz drive to TMSX was 4.9 nm/rt Hz, while my 90 Hz drive to TMS Y caused the table to move 3.4 nm/rt Hz. Using the quad model, The table motion is attenuated by 281 dB at 75 Hz and 294 dB at 90 Hz. The test mass motion induced due to my TMSX drive would be 4e-23 m/rt Hz at 75 Hz, and 6.724e-24 m/rt Hz for TMS Y. (too small to explain the lines in DARM).
Of course it is possible that the coupling mechanism is through the motion of the ISI, not TMS.