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Entry  Thu Jan 14 02:33:37 2016, ericq, Update, LSC, ALSX Noise still anomalously high  2016-01-14_ALSXspectra.pdfPDHsig.pdf
    Reply  Tue Jan 19 17:54:39 2016, gautam, Update, LSC, ALSX Noise still anomalously high  2016_01_19_ALS_OutOfLoop.pdfGreen_Locking_Trends.png
Message ID: 11931     Entry time: Thu Jan 14 02:33:37 2016     Reply to this: 11937
Author: ericq 
Type: Update 
Category: LSC 
Subject: ALSX Noise still anomalously high  

[ericq, Gautam]

We checked the UGF of the AUX X PDH servo, found a ~6kHz UGF with ~45 degree phase margin, with the gain dial maxed out at 10.0. Laser current is at 1.90, direct IR output is ~300mW.

We recovered ALS readout of IR-locked arms. While the GTRX seemed low, after touching up the beam alignment, the DFD was reporting a healthy amount of signal. ALSY was perfectly nominal. 

ALSX was a good deal higher than usual. Furthermore, there's a weird shape around ~1kHz that I can't explain at this point. It's present in both the IR and green beats. I don't suspect the DFD electronics, because the Y beat came through fine. The peak has moderate coherence with the AUX X PDH error signal (0.5 or so), but the shape of the PDH error signal is mostly smooth in the band in which the phase tracker output is wonky, but a hint of the bump is present. 

Turning the PDH loop gain down increases the power spectrum of the error signal, obviously, but also smoothens out the phase tracker output. The PDH error signal spectrum in the G=10 case via DTT is drowning in ADC noise a bit, so we grabbed it's spectrum with the SR785 (attachment #2, ASD in V/rtHz), to show the smoothness thereof.

Finally, we took the X PDH box to the Y end to see how ALSY would perform, to see if the box was to blame. Right off the bat, when examining the spectrum of error signal with the X box, we see many large peaks in the tens of kHz, which are not present at the same gain with the Y PDH box. Some opamp oscillation shenanigans may be afoot... BUUUUUT: when swapping the Y PDH box into the X PDH setup, the ~1kHz bump is identical. ugh

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