peter.king@LIGO.ORG - posted 16:38, Friday 06 May 2016 - last comment - 19:19, Friday 06 May 2016(27054)
ISS
In trying to figure out what's happening with the power stabilisation, I looked at the "dark" noise
of the power stabilisation photodiodes. pda-dark1.png shows the measured "dark" noise for both the
AC and DC output of the photodiode. Both are the same. I suspect that both photodiodes are damaged.
The ISS will lock but is very temperamental and depends on the set point (reference level). When
running the diffraction level should be between 12-14%. There is a problem with the reported DC output
and that measured at the photodiode. Often they do not agree. For example the MEDM screen will report
that the DC output is ~11V but measured at the photodiode it is ~7V. Whilst the reason might be related
to how the DC value is calculated from the AC value of the photodiode, it still is not consistent with
the measured value.
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When so-called analog "AC" signal (which is in reality a whitened signal with DC gain of 0.2) hits the rail, digital DC on the MEDM wouldn't agree with analog DC (first attachment).
Out of curiousity, I turned on the 1st loop and increased the diffraction to 16% by moving the REFSIGNAL from -2.3V to -1.9V, and ISS has been running OK in the past 40 minutes. Digital DC is a factor 5 larger than the REFsignal, this makes sense because the DC of the whitened analog signal that is actually used for the ISS is a factor of 5 smaller than the analog DC.
1st loop PDs as well as downstream of ISS 1st loop (FSS transmission) look about the same or maybe somewhat worse than before (e.g. 26758).
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