peter.king@LIGO.ORG - posted 09:21, Wednesday 28 November 2018 - last comment - 13:47, Wednesday 28 November 2018(45547)
FSS inspection
Following on from Keita's alog entry from yesterday this morning I looked at the mixer monitor signal. The MEDM screen screen indicates that the mixer output is typically ~0.5 V. With a digital multimeter I measured 1.2 mV when the MEDM screen indicated that the voltage was ~0.7 V. An oscilloscope trace of the signal is shown in tp3_1.png. CG20FG9.tif shows the measured open loop transfer function from this morning, with the marker placed at the UGF. The transfer function does not appear as smooth as ones I can recall from before. Phase margin looks okay. Whilst I was in the enclosure, I tweaked the alignment to the reference cavity a little and re-centered the beam onto the locking photodiode. The FSS still has some trouble locking even when the transmission voltage indicated on the MEDM screen exceeds the resonant threshold. I do not know why this would be, although sometimes either the runtime model and/or guardian takes over the common gain slider and moves it at what I think is an inappropriate time - like when there is no light flashing in the reference cavity, for example.
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The discrepancy in the mixer signal is not explained by the schematics of the TTFSS field box. Next step is to look at the front panel output of the field box.
A few moments ago (~1:35 pm local) I measured the mixer monitor voltage at the front panel of the TTFSS field box.
It measured -0.1 mV. Coincidentally I looked at the value on the MEDM screen and it reported a value consistent
with the voltmeter. Pulling up the trend data for the past 10 hours, the mixer voltage dropped close to 0 V since
about 8:10 am local.
Not sure what caused the big change.