Den, Sheila, Evan
~> We tried reverting to the old QPD offsets (the ones we used throughout O1) for the soft loops and the PRM pointing loops. These seemed to make the recycling gain slightly worse, and did not improve the jitter coupling. This indicates that (as we had suspected) these offsets are no longer good to use.
~> We measured the 9 MHz oscillator phase noise coupling into DARM. This had been done previously (19911), but with a suspicious calibration and in a way that also drove the 45 MHz phase. This time, we used the OCXO to drive both the harmonic generator (bypassing the 9 MHz distribution amplifier) and to serve as a reference for an IFR/OCXO PLL with a >40 kHz bandwidth. The IFR is used to drive the 9 MHz distribution amplifier. The error point of the PLL was offset in order to maintain the relative time delay of the 9 MHz and 45 MHz signals. When we relocked, we found that the 18 MHz and 27 MHz signals had flipped sign, but otherwise the interferometer locked normally. However, there was a great excess of 45 MHz noise in DARM (worse even than before we installed the 9 MHz bandpass). Nevertheless, we were able to drive enough to see the effect of 9 MHz phase modulation in DARM. The coupling is roughly 2–4×10−6 mA/Hz above 100 Hz.
Also, twice last night (while locked on the IFR) we were battling a 900 Hz line in DARM that increased over the duration of the lock, and eventually caused EY to saturate.
We suspected PI, but this line was also present at 900 Hz in the DCPD IOP channels. So it is not folding around the 8 kHz Nyquist of the digital downsampling.
This line does not appear in the IOP channels for the end station QPDs. (2 W, dc readout)
It's the third harmonic of the beamsplitter violin mode. Den added a stopband filter to the BS M2 length drive, and the line went down.
Unclear why this had not been a problem before.