During the low noise lock at 4:50 UTC today (Oct 11), there is a comb of lines in h(t) at 56.84 and multiples. They are quite narrow lines. The most interesting characteristic is that they have extreme amplitude modulation. The first attachment is the h(t) spectrum with the lines marked. The second shows narrow BLRMS around each of the lines. The fundamental at 56.84 Hz has a half-period of about 24 minutes. The amplitude goes very close to zero, and looks very periodic. Each multiple n has zeros in the same place, but the period is n times shorter. Maybe this could be all generated from the fundamental with the right kind of nonlinearity. I'm not sure what would give such a slow but extreme modulation - maybe some centering servo? The BRUCO results show a lot of channels at End-X coherent. Accelerometers on BSC9 seem to see it, but without amplitude modulation. PCal X sees it in the TX and RX PDs, but stronger in RX; but it doesn't seem anywhere near the level to get into h(t).
The 56.84 Hz comb is a blast from the past. Here is an entry from March 2013 concerning H2 one-arm data suggesting that the comb could be purely DAQ-related: https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=5806
This one is close enough to the 57Hz ETM HWS that we've seen in the past that I thought I should mention it here. However, we've recently overhauled that system and I've not confirmed what the new frame-rate is yet.
Here's the old analysis: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=14778
I Looked at an empty EX ADC channel (H1:PEM-EX_ADC_0_08_OUT_DQ) and found that the 58.64 Hz comb is there too, indicating that the comb comes from some source independent of the interferometer (i.e. in the DAQ or RF pick-up). I have attached ASDs of the empty ADC in 0.1mHz bins both for the entire spectrum (0-1024Hz) and near to the 14th 56.84 harmonic (~795.76Hz). I was slightly surprised that a second nearby line (that would explain the amplitude modulation) was not seen even with the 0.1mHz resolution.
Yes - the ETM HWS are running at 57Hz. This doesn't mean that this is the issue since it appears to be DAQ related, buuuut .....
aidan.brooks@zotws11:~$ caget H1:TCS-ETMX_HWS_SYNC_FREQUENCY
H1:TCS-ETMX_HWS_SYNC_FREQUENCY 57 Hz
aidan.brooks@zotws11:~$ caget H1:TCS-ETMY_HWS_SYNC_FREQUENCY
H1:TCS-ETMY_HWS_SYNC_FREQUENCY 57 Hz
I've searched EX channels and found some channels where amplitude modulation is seen: H1:SUS-ETMX_L3_DRIVEALIGN_L_OUT_DQ H1:SUS-ETMX_L3_ISCINF_L_IN1_DQ H1:SUS-ETMX_L3_LVESDAMON_LL_OUT_DQ H1:SUS-ETMX_L3_LVESDAMON_LR_OUT_DQ H1:SUS-ETMX_L3_LVESDAMON_UL_OUT_DQ H1:SUS-ETMX_L3_LVESDAMON_UR_OUT_DQ H1:SUS-ETMX_L3_MASTER_OUT_LL_DQ H1:SUS-ETMX_L3_MASTER_OUT_LR_DQ H1:SUS-ETMX_L3_MASTER_OUT_UL_DQ H1:SUS-ETMX_L3_MASTER_OUT_UR_DQ Attached plots are BLRMS and zoomed spectrum of H1:SUS-ETMX_L3_DRIVEALIGN_L_OUT_DQ as an example. The vertical line (orange, dotted) in the zoomed spectrum is at 568.404 Hz (10th harmonics).
NO
we moved the Hartmann sync frequency and the lines moved accordingly - its Hartmann, not DAQ.
Why does the Hartmann sensor so strongly couple to DARM?
With regards to the coupling, the ETM HWS are served by the same power supply as the ring heaters. It's a long shot but it might be worthwhile to try disconnecting the ring heaters from the driver while the to see if the coupling of the 57Hz to DARM is changed.
https://dcc.ligo.org/LIGO-E1100891
