rich.abbott@LIGO.ORG - posted 11:31, Friday 08 February 2019 - last comment - 14:25, Friday 08 February 2019(46870)
More Changes to OMC Piezo Drive
Fil, Dean, Richard, Rich After removing the newly implemented revised piezo driver two days ago, some modifications were made to see if there was any improvement to the broadening of noise in DARM around 180Hz. We put the revised chassis back in and the DARM noise is back at 180Hz. Attached is a detailed description of the state of the changes including: 1. Noise spectra of the bias HV driver as measured on the bench 2. Power supply rejection vs frequency of the bias HV driver 3. Simplified description of the topology used. Koji reminded me that this type of power line harmonic noise associated with the OMC PZT driver has been seen at LLO, and mitigated by the addition of a ferrite toroid to the shutter trigger cable. There was also mention of unplugging the Picomotor driver while it was in an unused state. These changes apparently improved multiple lines in DARM. It's a good read. See: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=41374 This should be investigated at LHO prior to removing (again) the modified OMC Piezo driver.
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Noticed that at LLO they examined the HV monitors too. These should be checked for significant stuff at or around 180Hz. A comparison can be made to the length piezo vs the biasing piezo. Seems like a good diagnostic. Those signals are whitened too, so the SNR should be good.