stefan.ballmer@LIGO.ORG - posted 18:40, Sunday 20 September 2015 - last comment - 20:17, Sunday 20 September 2015(21709)
Weird HF noise
We again have the non-stationary noise described in alog 21353 Interestingly, the noise also seems to move around in frequency. Attached is a power spectrum of 3 different UTC times, both of DELTAL_EXTERNAL and OMC_DCPD_SUM. (OMC_DCPD_NULL does not show it.)
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.. was due to to the RF45 EOM driver acting up. Attached are the same specra, from the same times, but including the H1:LSC-MOD_RF45_AM_CTRL_OUT_DQ channel, as well as its coherence with DCPD_SUM. H1:LSC-MOD_RF45_AM_CTRL_OUT_DQ is clearly elevated when we have this noise show up. As a temporary fix (because we are still in a CAMLAND alert) we lowered the modulation index by 1dB: H1:LSC-MOD_RF45_AM_RFSET was 23.2 is 22.2 and related (because this lowered the light entering HAM6) H1:ASC-ODC_AS_A_DC_LT_TH was -13000 is -11000 (this could just stay there - no need to revert) Remarks: - Currently the DOWN script will NOT set H1:LSC-MOD_RF45_AM_RFSET back on lock loss - This change in the SB power will lower the modulation index (normally ~0.3) by 1 dB. - Thus the carrier power (~(1-Gamma^2/2) will go up by about 1%. - Updated the guardian to reset H1:LSC-MOD_RF45_AM_RFSET to 23.2 in the DOWN state (ISC_LOCK.py, svn revision 11674) It was reloaded at 2:59 UTC.
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Tagging a few more subsystems so more people get the memo to help investigate, and note the change in configuration for this lock stretch.