J. Kissel, H-Y Huang

[1] Actual Board S/N SN 02
[2] In-vac "QPD" Enclosure S/N S2100832
[3] Transimpedance Circuit Schematic D2000592
[4] Transimpedance Design Document T2000698
[5] Benchtop Measurement and Fitting Results T2100403
[6] Whitening Chassis interface D2200215 (Whitening Chassis S/N S2300003)

After repeating the remote, DAC driven measurement of the entire OMC DCPD sensing electronics chain (LHO:67730), and comparing it against the electronics chain with the transimpedance amplifier and whitening responses bypassed (LHO:67760), it's clear that a lot of the low-frequency wiggles -- when the TIA and Whitening chassis are included -- are a result of poor / out-of-date compensation of the transimpedance amplifier response, which hasn't been measured and fit since 2022-04-19 :: LHO:62696 (and prior to that, just after install in 2022-02-15 :: LHO:61810).

The exciting thing about today's measurement is that we got to use the D2200215-style whitening chassis user interface which is *MUCH* nicer and easier to use than the former D2100630-style chassis that was used in the 2022-02-15 and 2022-04-19 measurements. 

Namely, with the D2200215 chassis,
    - we don't have to unplug the DB25 cable that goes to the TIA in-vacuum *at all* during this measurement. That means that we don't have to power down the DCPDs nor the chassis to hook in and take the measurement. *That* means the the trans-impedance amplifier should be in the same thermal state it's "always" in -- on and having been running for months.
    - the electrical ground, 0V plane is well defined through-out the measurement, now that the chassis's 0V plane is both (a) available to clip lead into on the "From AI / DAC Test Input" DB9 port in the back, on pin 5, and (b) clearly connected to the "preamp output monitor" BNC spigots in the front.

I'll compare the results between 2022-02-15, 2022-04-19, and 2023-03-06 in due time, but for now, I show just show a quick plot of the results to show the data's good.

Attachments are:
    - 2023-03-06_H1_DCPDTransimpedanceAmp_MeasurementSetup_Pics.pdf: pictures of the connections and set-up,
    - D1900027-v4_aPLUSO4_D2200215-v1_OMCDCPDTransImpAmp_TransImpAmpSetup.pdf: a diagram of all the connections made during the setup,
    - 2023-03-06_H1OMCDCPD_TIA_MeasurementNotes.txt: text notes, mostly to detail the SR785 setup
    - 20230306_DCPDTransimpedanceAmp_OMCA_DCPDAB_tf.pdf: quick plot of the results.

Note: in our haste, we neglected to gather transfer functions of the measurement setup. We usually take a final measurement, with the "device under test" bypassed, capturing the transfer function of the SR785 and the SR785 accessory box. This way we can divide out any nonsense from the measurement test suite that we don't want polluting the transfer functions of the "device under test" we really want to measure (in this case, the TIA). 
I'm hoping that, since we've used the same serial number SR785 accessory box, and the data is on the same frequency vector as the 2022-04-19 data set, we can "just" divide that out instead. 
We'll never know the impact without just retaking the data with the complete suite of measurements. 
... next time.