BP

Following Friday night's lock, I looked at the spectrum and saw some regular structure, looking like a 2Hz comb at odd frequencies.This looks like a new comb. [figs 1&2]

As followe up, I ran coherence with all of the EBAY magnetometers and saw strong coherence is some places with this 2Hz comb, as well as the persisting 0.5Hz comb (see plots below)

0.5Hz comb: https://ldas-jobs.ligo.caltech.edu/~brynley.pearlstone/comb_investigations/May_2016_comb/H1:PEM-EX_MAG_EBAY_SEIRACK_Z_DQ_25_40Hz.png

0.5Hz comb + 2Hz comb: https://ldas-jobs.ligo.caltech.edu/~brynley.pearlstone/comb_investigations/May_2016_comb/H1:PEM-EX_MAG_EBAY_SEIRACK_X_DQ_25_40Hz.png

Note: These two are the same magnetometer (MAG_EBAY_SEIRACK), looking at 2 different axes. Both combs were also seen in other magnetometers.

Full list of plots: https://ldas-jobs.ligo.caltech.edu/~brynley.pearlstone/comb_investigations/May_2016_comb/

Previous efforts to mitigate the 0.5Hz comb was focussed on powering the timing card independently in the CS EBAY LSC-C! I/O chassis which handles DARM. This has not worked to eliminate the comb. I can't report any reduction yet, as Friday's lock was not sensitive at low (<100Hz) frequencies.

This 2Hz comb on 1Hz offset is the transform of a 1Hz square wave. The 2Hz comb might have to do with the 0.5s and 1s structure seen it Keith's data folding studies here: https://alog.ligo-wa.caltech.edu/aLOG/index.php