J. Kissel, B. Ratto
WP 10390

This morning, we repeated a measurement of the H1 OMC DCPD signal chain described in LHO:62653 -- first taken on March 16 2022 18:36 UTC (LHO:62250) -- because L1's OMC DPCD signal chain -- most prominently its transimpedance amplifier -- has (LLO:59540).

This measurement, sadly, confirms that response of the signal chain has changed by 1% around 80 Hz. 

I show these results in two ways:
    (a) 2022-04-19_H1_TIAxWC_OMCA_S2100832_S2101608_DCPDAandV_RemoteTestChainEXC.png shows the data exact how Daniel plotted it, showing the change as a function of time in the left two panels, and on the right two panels, I show how the DCPD B / DCPD A ratio has changed over time.

    (b) On the other hand, 20220419_H1_TIAxWC_OMCA_S2100832_S2101608_DCPDAandB_RemoteTestChainEXC_timeratios.pdf shows the same data, but in the right panels I show the ratio of the response in time -- i.e. 2022-04-19 / 2022-03-16 for DCPD A, and 2022-04-19 / 2022-03-16 for DCPD B. 

The second plot, (b), better conveys that both chains have changed, and crazily enough, they've changed in the same way, such that the B / A ratio remains roughly consistent in time.

We'll now head to the racks to repeat the SR785 driven measurement that isolates the transimpedance amplifier (a la LHO:61810) confirm that all this change is indeed from the trans impedance amplifier alone.

As an assist to support or refute any theory about what's going on, I also attach a trend of 
    - the HAM6 vacuum pressure, 
    - the "Zone 4" LVEA air temperature (the average of the LVEA wall sensors that are near HAM6, in both deg C and deg F), and 
    - the vertical position of the OMC suspension, which I use as a proxy for the temperature *in* the chamber.
One can see that all of these parameters that one might initial point to as effecting the big inductors inside the channels in a common way -- air pressure or air/vacuum temperature -- actually have not changed substantially between the 2022-03-16 and the 2022-04-19 measurement.

BLORP. Stay tuned as the investigation continues...