[A. Pele, S. Biscans, J. Warner, H. Radkins, F. Clara, R. McCarthy, K. Kawabe, J. Kissel]

We've been chasing the source of an exactly-0.5 [Hz] oscillation (and subsequent higher-order harmonics) in the H1 ISI ITMX, that was first identified in LHO aLOG 9494. After testing the ISI in many different configurations (See LHO aLOGs 9546, 9951, 9585, and 9605), and ruling out capacitive position sensors -- already been shown to have *other* oscillations and frequency combs because an unrelated problem with beating oscillator sync frequencies, see LLO aLOG 10629 -- we narrowed down the source of the problem to the Y3 channel of the Corner 3 T240 on Stage 1. With a single channel to attack, we launched Filiberto on the problem. He identified and fixed the problem (as indicated in 9607). I attach some visual aides to describe the problem; see H1ISIITMX_CableProblem.pdf, and further details below. I also attach a comparison of the performance using the same metrics as in the original discovery, with the ISI in the same control system configuration, but now with all combs absent. We get an RMS of 70 [nrad] in Pitch, which appears to be "pretty good" for the green team. Nice work, gents!

The path forward (for this chamber): This ISI still does not have sensor correction running, LLO's really-low-frequency blend filters, nor has it had any attention paid to optimizing the rotational DOFs to minimize sensor-noise, tilt-horizontal-coupling. All of these things can improve the stage 1 performance between ~0.1 [Hz] and 1 [Hz]. Further, we can probably add more gain to the QUAD damping loop filters at 0.56 [Hz] (the remaining peak corresponding the first "pitch" mode). There is some resonant gain already present, but I can check if we can add more -- the filters were designed for a Monolithic QUAD, where ITMX is a Wire-Hang QUAD and they have different first pitch mode frequencies. Further, we have done any coil balancing or frequency-dependent alignment decoupling which may also help.

Note that we still have to solve the CPS Oscillator Beat Frequency Combs resulting from other chambers, but that's another problem for another day (Rich Mittleman lands tomorrow, and that's one of his primary tasks).

Details:
-------- 
The Problem (w/ visual aides):
- PG1: A screenshot of the BSC-ISI Wiring Diagram (D0901301-v10, pg 10), showing the big-picture of the T240 signal chain, from the actual sensor inside the pod, inside the ISI, inside the BSC chamber to the cable going from the T240 Interface Chassis to the Anti-Aliasing chassis. The problem was inside the cable-side of the connection at the T240 Interface chassis, on the rack side of the long cable run from the chamber flange to the rack, which I've circled in red.
- PG2: A zoomed in screenshot of the chassis. The busted pin, Pin 2, which carries the positive leg of either the Y / V DOF had been broken inside the backshell of the cable. I've again circled the problem in red. Fil originally identified the flaw by testing the continuity of the cable and found the dead short on that pin. Fil conjectures that the pin failed because, again inside the backshell, the male pin had been over-crimped and/or not seated well, such that repeated normal use of the connector and bending of the cable caused the connection to snap.
- PG3: An example of a male DB25 connector, looking at the side which would be enclosed inside the backshell, and an associated pin. The pin is crimped to the signal cable, and press-fit in the connector hole.
- PG4: An example of a male DB25 connector, looking at the external connection side. I've only pushed the pin half-way in, but one can see how the pin might be mistaken for functional, but still not be fully seated.

Current Configuration of the ISI in this measurement:
- ISI-ITMY and ISI-BS CPS OFF
- HPI-ITMX running Level 1 Isolation Filters, with a position sensor only blend filters on all DOFs.
- ISI-ITMX running Level 1 Isolation Filters, with "T100mHz_N0.44" blends on ST1 XY and "750mHz" on ST1 ZRXRYRX and all of ST2. Note with out the extra 0.5 [Hz] notch that was the temporary temporary solution from a few days ago.
- SUS-ITMX running Level 2.1 damping loops.

The History:
Jim recalled having problems with this exact channel just after the cartridge install in mid-November 2013: though they were able to find a configuration in which their spectra passed acceptance testing (see E1100848), they spent a few days trying to fix a low-gain issue with this same channel (exactly a factor of two). Though we could not find any associated aLOGs, or saved raw data, Jim did managed to find a few saved .pdfs of the problem, one of which I attach here (see ITMX_ASD_after_filter_reload_2013_11_15.pdf). At the time, Jim and Seb were focused on solving the fact that the channel had a factor-of-two lower gain that the other 8 T240 DOFs, and in the rush that was ITMX they didn't notice the minor bump at 0.5 [Hz]. Further, he and Sebastian spent several days on it, because the fact-of-two problem was intermittent, and found that jostling the cable at the flange (not at the rack) was the most effective way to mitigate it. It went further unnoticed until we began *using* the T240s in the ST1 isolation loops. In summary -- this cable problem has been present ever since its install, but due to the usual chaos of a chamber install, plus the holidays, plus the rotating door of seismic commissioners with higher priorities, it was missed. HOWEVER, I really don't put anyone nor any testing procedure at fault; this is a 1 and 1000, perfect storm of problems that we will continue to have for the next few years as we begin to push the aLIGO systems to their designed performance levels.