J. Kissel, H.-Y. Huang

We're continuing to try to validate the high-frequency portion of our pyDARM parameter set in different ways, in hopes to keep up with all the changes that have been happening to the OMC DCPD Sensing Chain.

To help isolate the electronics, digital filtering, and computational delays in the sensing chain alone (rather than using a PCAL driven full-IFO sensing function measurement), we've repeated the remote excitation, DAC driven measurements of the OMC DCPD sensing chain, as has been done before in LHO:62903 and LHO:66989. This data comes after (we thing / hope) the dust has settled on all the recent changes to the chain since the last time this measurement (2023-Jan-24) was taken. Those changes include:
 - 2023-Jan-31 the OMC whitening chassis change from S2300002 to S2300003 (LHO:67119),
 - 2023-Feb-15 the RCG upgrade such that the 524 kHz h1iopomc0 is no longer using a custom version of the code (LHO:67433),
 - 2023-Feb-21 The final configuration of the timing system and delays withing the h1iopomc0 code LHO:67522 and LHO:67693

We also enhance the measurement by expanding the frequency range of the data from 2000 Hz all the way out to 7444 Hz (the limit of the 16 kHz system), and in doing so use 251 point instead of 121, in order to better resolve the features of the anti-imaging, anti-aliasing filters, and computational delays. I also, for the first time grab the DCPD SUM, which is the "final answer" used for DARM_ERR, rather than just the individual DCPD responses.

The First attachment shows DCPDA and DCPDB's response to the exitation, as well as the ratio of the two channels.

The second attachment shows the "sum" of the two channels, OMC-DCPD_SUM_OUT_DQ, and then divided by 2 such that it's on the same scale as the individual channel responses. Note that this sum still used quite old balance matrix values of A = 1.01039 and B = 0.989671 that were determined in Mar 2019 (LHO:47228) well before the we replaced the entire signal processing chain.

Hsiang-yu and I will work together to understand the remaining differences between his model of this measurement and the data (as he's been doing in LHO aLOGs LHO:67194, 67336, and LHO:67333).

One thing we note, however, at the low-frequency end of the measurement -- we continue to see evolution of the trans-impedance amplifier's complex pole pairs at 15-20 Hz, whose frequency is defined by giant, ~2.6 mH inductors. We've already noted this evolution in LHO:62696 and LHO:62676, but perhaps now, as we get closer to O4, we should really start paying attention to this time-dependence. Recall that we're still compensating with poles and zeros derived from a fit of data taken during the original install, (LHO:61810), where it wasn't well documented how thermalized the giant inductors were during the time of measurement.
This may be the source of *some* (albiet a small component) of the poorly understood wiggles in the sensing function after a SRCL offset is applied (see LHO:67613).