To check for possible spectral lines in the H2 One Arm Test due purely to the aLIGO DAQ / electronics, I picked a couple of channels that appeared very quiet in Greg Mendell's Fscan plots for SUS channels and applied the same spectral averaging procedure as used in previous studies of the  OAT feedback channel.

Many detailed spectra are posted here, with a sampling attached to this report. As can be seen in the first column of plots at the above link, the first channel examined, H2_SUS-ETMY_L1_OSEMINF_LL_IN1_DQ, has an unsurprising comb of 60-Hz harmonics (indicated with 'A' in the plots), with odd harmonics much stronger than even harmonics. In addition, though, it has an even stronger comb of 56.8407-Hz harmonics (indicated by 'B' in the plots, with two aliased reflections in the 900-1000 Hz band indicated by 'R' in the plots). This latter comb was previously seen in the OAT feedback channel and showed correlation with EY seismometer, accelerometer, magnetometer and microphone channels.

I tried looking at the corresponding ITMY chanel, H2_SUS-ITMY_L1_OSEMINF_LL_IN1_DQ, and found the 60-Hz harmonic comb to be lower and the other comb to be absent. The 2nd column of plots at the above link shows the spectra for the ITMY channel using the same scale as for the ETMY channel. The 3rd column of plots shows the ITMY channel with a zoomed-in vertical scale to look for evidence of other spectral lines. With the exception of a few lines that appear only on August 25, no other sharp structure above 10 Hz is apparent (although there are broad bumps here and there, for example, near 400 Hz).

In the  table of plots, each spectrum for each channel is shown for three cases: 1) individually for the days of July 16, 20; August 4, 22, 25; 2) individually for the days of August 28, 31; September 7, 13; and 3) averaged together over all the days of July 16, 20; August 4, 22, 25, 28, 31; and September 7, 13. These are all the days for which enough "science mode" OAT data was produced to warrant Fscan SFT production. Click on thumbnails to get pdfs.

Jeff Kissel confirmed that both of these channels were indeed connected to OSEM sensors during the One Arm Test, but nominally the full band above 0.5 Hz should have been dominated by sensor noise, as shown in this plot he provided.

The spectra attached to this report show some examples of the above for the full 0-1000 Hz band, where approximately 200 half-hour SFTs (0.5 mHz binning) have been averaged together with inverse noise weighting.