Matt, Sheila
We locked and saw that we have 3 orders of mognitude or so to much noise at DC readout. It seems this is related to PSL problems, the atached screenshot shows coherence between the ISS second loop PDs and DARM up to 100 Hz, as well as coherence between frequency noise and DARM. The pink trace shows the coherence between intensity and frequency noise. With this noise, we were able to power up to 10 Watts, but saturated the DC PDs.
We looked a little but at impementing a dither loop for SRM control. The alingment dither system currently would allow us to demodulate POP18, but POP90 has much better signal, as you would expect. The IPC for adding this is already in the models, so I've just added it to the matrix but didn't do the model restart yet (WP5840).
We then gave up on locking and went to the PSL racks to look at some signals at high frequencies. We saw a glitch that happens at a repetition rate of 37 kHz, and has frequency content of nearly a MHz, which shows up in the laser intensity noise. Matt has a picture of this in the ISS PD and the ref cav transmission. If we turn off the ISS this is still there (as expected since its above the bandwidth), but it is harder to see on top of low frequency intensity noise.
When the ISS is off, the ISS PDs wander from rail to rail, and oscillate only when they are near the upper rail.
These pictures show what we saw at the PSL rack. The first shows the o'scope: channel 1 is the ISS first loop DCPDA (out of loop, but B looks the same at this frequency), the second shows the PMC TRANS. (The second and third pictures are there to document where we connected the cables.)
There are noise bursts in the PMC TRANS which oscillate at ~500kHz and repeat at 37kHz. (For offline data mining: the o'scope photo was taken at 18:55:56 local time.)
Before to finding the problem shown in the previous comment, we noticed that the noise eater was oscillating. Sheila reset it and it stopped, but someone (Keita?) might want to look at the monitor channels to see how they behave in when oscillating (and when not oscillating). We first noticed this on the FSS fastmon at 18:31 (first photo), saw it again on the PMC trans signal at 18:38 (second and third photo, ~1MHz triangle with ~15% modulation of the power) and fixed it shortly after that. By 18:55 (previous comment) it was not oscillating. (All times are local.)
A little more info on:
"When the ISS is off, the ISS PDs wander from rail to rail, and oscillate only when they are near the upper rail."
While looking at the ISS first loop PDs, we noticed that if the loop is open there are large ~1MHz noise bursts. Going to DC coupling and zooming out, it seems that the PD signals oscillate when they approach the upper rail at ~14V (see photo). This may indicate that the load resistors on the opamps involved are too small, and so the opamps become unstable when outputting large voltages... or maybe it is something else. In any case, the ISS first loop should not be operated with large PD voltages (currently 1.8V).
The message: things are not as bad as they seemed yesterday, but we still have a problem with frequency and intensity noise.
I was confused yesterday, the guardian did not make the transition to DC readout because it checks that the ISS is on before making the transition, so we were actually still locked on RF when I thought we were on DC readout. Tonight I turned the ISS back on, and transitioned to DC readout without a problem. There is still a lot of coherence with the ISS and frequency noise. The IFO had no problem getting to 22 Watts, but we lost lock because of the HSTS coil driver switcing (which I had moved to just after the BS coil drivers, and have now moved to just before the BS coil drivers switching).
The first attached spectra (+coherences) was taken at 2 Watts, the second at 22 Watts.
The atteched trend of recent two days shows that the noise eater was bad from about Apr/24/2016 1:31:50 to 1:52:50 UTC (that's 18:31:50 to 18:52:50 Pacific time).
According to this wiki entry https://lhocds.ligo-wa.caltech.edu/wiki/SYS_DIAG%20Guardian#PSL_Noise_Eater the noise eater is monitored by H1:PSL-MIS_NPRO_RRO_OUTPUT and its nominally good range is -5852 +/- 50.
Matt and I were curious about the ISS inner loop PD circuit design, so I made a quick LISO model. This essentially reproduces the analysis given in T1000634, just showing a little more detail about which components are limiting the noise (mostly the transimpedance and TFIN resistors, R6 and R2)
The design looks OK as far as the linear modeling is concerned -- and as long as the dynamic range constraint plotted in figure 5 of T1000634 is maintained when the second and third loops are engaged.
Modeling files are in /ligo/home/christopher.wipf/Data/20160425_iss
