Evan told me that there seems to be an analog pole of 40Hz or so that is unaccounted for if he makes the transfer function from H1:SUS-ETMY_L3_LOCK_L_OUT to the LVESD monitor whitening outputs ( L3_LVESDAMON_LL_OUT and such) after removing all known analog and digital poles/zeros.

I measured the transfer function for EX, not from LOCK_L but from ESDOUTF_LL_IN2 to LVESDAMON_LL_OUT and confirmed that the problem is also in EX driver.

In the attached, blue is the measured transfer function divided by the transfer function of the whitening that is supposed to be (z, p)=([2;2;19.3k;19.3k], [40;40;965;965]) Hz (https://dcc.ligo.org/LIGO-D1500389).

I also multiplied the bule with 42Hz zero and got the green one, it seems like there's really 42Hz-ish analog pole that is not accounted for. (There are also high frequency things I'm not worried at the moment.)

Eventually I remembered that, in the old version of LVESD that we are using, there is an LPF for monitor output which had a 42Hz pole though it was supposed to be 1kHz (the LPF in question is on page 11 of D1500016). Rich Abbott fixed all spares, but not the ones we're using. One mystery solved.

I talked with Richard and Filiberto, and since they have some PI-related task scheduled on next Tue., I hope this can be fixed at the same time by either modifying or swapping.