(Sheila, Alexa, Rana)
- Note to red team: I have misaligned ITMY.
- Set PDH RF frequency via ifr to 24.407393MHz
- Adjusted REFL_A_RF9 offset (I:143, Q:100) and RD rotation (90 deg). Q signal sent to input 2 of CM board.
- PDH servo board changes: Input 1 gain to -7dB and Boost 2 ON (Boost 2 keeps getting turned off by autolocker when we loose lock, but does not get turned back on; this needs to be changed).
- COMM Handoff sucessful with the original script; the crossover is at 13Hz with a phase margin of 27deg (see attached picture).
- We saw some red flashes with H1:LSC-TRX_A_LF reaching 6000 counts, but were uable to lock it. We have more noise in the arm than previously seen.
During the afternoon, the locking of Green PDH was quite unstable. We suspected that there were some oscillations of the NPRO PZT and/or accidental HOM resonances (since the mode-matching / clipping is so bad).
* Sweeping the NPRO PZT with a low bandwidth PLL lock, found no substantial features in the neighborhood of the peak (~27.4 kHz). Even though there's no resonances in the TF, the peak dominates the RMS of the PDH error signal. We thought that this could perhaps be coming from an oscillation of the PSL FSS, but tweaking the FSS Fast gain doesn't change the peak frequency.
* We tried a few different modulation frequencies for PDH (23.4, 23.9, and 24.4 MHz). These were calculated to make the upper SB be at ~0.3-0.4 of an FSR. As expected we saw a big dip in the PDH loop in the 10-15 kHz range for these different modulation frequencies. These dips were not very stationary - we guessed that this was due to the alignment fluctuations.
* Daniel turned on the 1000:100 Boost in the servo board after awhile and this greatly helped the stability. At the best of times, the green arm power fluctuations were ~10%. At the worst of times, it was more like 50% and the mode would hop between 00 and 01. We had mixed results with the dither alignment and its not always working for both DOFs.
* We should use a directional coupler to check that we're at the peak frequency for the EOM.
Some observations: After reverting to the original sideband frequency we had a hard time locking. The behaviour was similar to what we experienced in the past when we had a lot of alignment fluctuations. We would stay "locked' but switch between 00 mode and a higher order transverse mode without loosing a step. In the past the transition was to a 10 mode whereas yesterday it was to a second order mode. The locking was better when we switched back again to the frequency that is 1MHz off. It turned out that the sidebands were coincidentally set near the second order transverse mode spacing. Using a frequency near nominal with the same tuning worked as well. However, it turned out the real problem was a lack of low frequency gain. With the standard network compensation we just have a pole near 1.6Hz, With the boost turned on the lock is a lot more stable. This seems especially important during elevated wind.