I was skeptical about the SRCL noise in the noise budget, so I checked it again. I think it is pretty close.
Its a simple method, but should be accurate. SRCL_FF left on in the Guardian set state in NLN.
- Turn on broadband noise excitation in SRCL2_EXC with AwgGui.
- Adjust calibration of SRCL_OUT in DTT until the SRCL_OUT trace matches DARM.
- turn off excitation to see how the ambient SRCL compares to DARM.
From this exercise I get the attached plot. Looks like its very close in the 40-90 Hz band. Options:
- better low pass filtering; Downside: this is hard and will make the loop more unstable.
- lower the SRCL gain: could be fine, but we don't yet know what the allowed SRCL error signal fluctuations are. i.e., how much can the SRC length fluctuate before it goes nonlinear or ruins the WFS signals?
- improve the sensing noise: more power, more modulation depth, less electronics noise. Do we have a SRCL noise budget?
Why does this look so different than the SRCL contribution in the noise budget in entry 45169 , which was made just after the SRCL and MICH feed-forward was improved? In that NB the SRCL noise goes below 1e-21 strain/rtHz just above 70 Hz, while in this one that doesn't happen until 300 Hz. Has something in SRCL changed (loop gain, filters?) or is the Nov 9 NB thought to be in error?
This is consistent with the BruCo projection based on coherence between DARM and SRCL. See https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=45782
And also consistent with the level of cancelation you can get by linear noise subtraction ( will post more later on this)
On Saturday night the DARM offset was changed and the SRCL FF wasn't re-tuned, so you would not expect this to match the noise budget. I don't know if these changes were in place last night.
The DARM offset during this measurement was 18.3 cts (~12 pm), giving 20 mA (23.8 mW) of power on the OMC DCPD SUM channel. Guardian is the one who controls the DARM offset during the INCREASE_POWER and ADJUST_POWER state, so it always sets it to whatever gives 20 mA on the DCPDs. It's possible the SRCLFF needs to be retuned after the vent. However, I doubt it will improve the SRCL to DARM by more than a factor of 2 or so (i.e. our current SRCLFF is decent).
Here is a noise budget where only the SRCL noise is updated, taken today. (With 20mA DCPD sum at 20 W input power). The SRCL FF hasn't been returned since the TCS changes, that seems to explain the larger coupling.
