Using two periods of quiet time during the last couple of days (1381575618 + 3600s, 1381550418 + 3600s) I computed the usual coherences:
https://ldas-jobs.ligo-wa.caltech.edu/~gabriele.vajente/bruco_STRAIN_1381550418/
https://ldas-jobs.ligo-wa.caltech.edu/~gabriele.vajente/bruco_STRAIN_1381575618/
The most interesting observation is that, for the first time as far as I can remember, there is no coherence above threshold with any channels for wide bands in the low frequency range, notably between 20 and 30 Hz, and also for many bands above 50 Hz. I'll assume for now that most of the noise above ~50 Hz is explained by thermal noise and quantum noise, and focus on the low frequency range (<50 Hz).
Looking at the PSDs for the two hour-long times, the noise belowe 50 Hz seems to be quite repeatable, and follows closely a 1/f^4 slope. Looking at a spectrogram (especially when whitened with the median), one can see that there is still some non-stationary noise, although not very large. So it seems to me that the noise below ~50 Hz is made up o some stationary 1/f^4 unknown noise (not coherent with any of the 4000+ auxiliary channels we record) and some non-stationary noise. This is not hard evidence, but an interesting observation.
Concerning the non-stationary noise, I think there is evidence that it's correlated with the DARM low frequency RMS. I computed the GDS-CALIB RMS between 20 and 50 Hz (whitened to the median to weight equally the frequency bins even though the PSD has a steep slope), and the LSC_DARM_IN1 RMS between 2.5 and 3.5 Hz (I tried a few different bands and this is the best). There is a clear correlation between the two RMS, as shown in a scatter plot, where every dot is the RMS computed over 5 seconds of data, using a spectrogram.
DARM low frequency (< 4 Hz) is highly coherent with ETMX M0 and R0 L damping signals. This might just be recoil from the LSC drive, but it might be worth trying to reduce the L damping gain and see if DARM RMS improves
Bicoherence is also showing that the noise between 15 and 30 Hz is modulated according to the main peaks visible in DARM at low frequency.
We might be circling back to the point where we need to reconsider/remeasure our DAC noise. Linking two different (and disagreeing) projections from the last time we thought about this, it has the correct slope. However, Craig's projection and the noisemon measurement did not agree, something we never resolved.
Projection from Craig: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=68489
Measurement from noisemons: https://alog.ligo-wa.caltech.edu/aLOG/uploads/68382_20230403203223_lho_pum_dac_noisebudget.pdf
I updated the noisemon projections for PUM DAC noise, and fixed an error in their calibration for the noise budget. They now agree reasonably well with the estimates Craig made by switching coil driver states. From this we can conclude that PUM DAC noise is not close to being a limiting noise in DARM at present.
To Chris' point above -- we note that the PUMs are using 20-bit DACs, and we are NOT using and "DAC Dither" (see aLOGs motivating why we do *not* use them in LHO:68428, and LHO:65807, namely that [in the little testing that we've done] we've seen no improvement, so we decided they weren't worth the extra complexity and maintenance.)
If at some point there’s a need to test DAC dithers again, please look at either (1) noisemon coherence with the DAC request signal, or (2) noisemon spectra with a bandstop in the DAC request to reveal the DAC noise floor. Without one of those measures, the noisemons are usually not informative, because the DAC noise is buried under the DAC request.
Attached is a revised PUM DAC noisemon projection, with one more calibration fix that increases the noise estimate below 20 Hz (although it remains below DARM).
