(Note to operators: Don't touch modulation depth, it doesn't do anything good while making it difficult for everybody to analyze the glitches.)
Summary
1. Output level of RF AM stabilization went back and forth between two levels since our last incursion to the PSL room.
2. Looking at RF AM glitch in LSC-ASAIR_B_RF90, LSC-ASAIR_A_RF45 and ASC-AS_RF45, it seems like the RF level going to EOM is constant but the modulation phase changes.
The first attachment shows a jumbo RF45 glitch reported in alog 23618 at around 11-21 2015 11:44 UTC. Nobody was touching the modulation depth slider.
Top left shows that the RF AM control signal jumped up by about 0.2% of its DC value and then came back after about 4 seconds. During this 4 second window, the signal was full of large glitches. This was felt by the control and error signal of the loop (top and middle, left ) but not by out-of-loop AC and DC (top and middle, middle row).
This was also felt by the spare unit in the CER (middle right) and the demodulator at the ISC rack that Evan installed (alog 23567, top right). For the CER unit, the jump was about 2E-5 of the DC level. For Evan't demodulator, it's AC-coupled, but using the gain of 1000 for SR560 and using 175mV DC level before amplification, the jump seems to be 5mV/175mV/1000~3E-5, which is consistent with the CER unit. Since these are much smaller than what the RF AM stabilization see (0.2%), it seems to me that the glitch in CER and ISC rack is a result of something bad in the EOM modulation path propagating back to the CER.
Interestingly, ASAIR_A_RF45_I_ERR jumps but Q_ERR doesn't (bottom, middle). The latter should be dominated by the DARM length offset. If the RF level coming out of the EOM driver jumps, I would expect both Q and I jump. This seems to me that the RF AM stabilization is working fine, but there's some phase jump associated with the AM glitch and the effect is more evident in the phase that has smaller signal.
Similarly, ASAIR_B_RF90_Q_ERR jumps but I_ERR doesn't (bottom, left), ASC-AS_A_RF45_I_SUM does but Q_SUM doesn't.
The second attachment shows the second jumbo glitch, but this was not reported by the operators because it happened when IFO is not locked. You can see that the LSC-MOD_RF45_AM_CTRL_OUT jumped by similar amount as the first event, but it never came back. No IFO signals were available of course, but you can see that it's not like there's a huge electronics coupling for LSC-ASAIR_B and ASC-AS_A_RF45.
It seems like there are two states, whatever they are, and the RF AM stabilization is responding to the transition from one to the other (or maybe the stabilization is going crazy). It could be an impedance change somewhere but I'm not sure.
Whatever the cause is, it cannot be after the EOM driver. If something is going on after the driver, the reflection should propagate back to the rms detector of in-loop and out-of-loop differently, and I cannot imagine the out-of-loop channels look flat as they do here.
So, something is going on, it could be anywhere between the distribution amplifier for this specific path in CER to the EOM driver itself, including these two end points. We could swap the distribution amplifier spigot of the spare and PSL in CER, I doubt that it does anything but we can further narrow down the trouble spot.
The third attachment shows that this going back and forth behavior was present before. This is from a week ago noted in Ed's alog. The amplitude was much smaller, but we can see that the output level jumped at around 77 seconds, then back at around 123 sec.
