In order to understand the angle to length coupling, we made some A2A and A2L to DARM transfer function measurements, few weeks ago, using broadband excitations and noticed some phase mismatch between the measurement and the model as seen in the first attachement [AngletoDARM_20191126.png].
In order to make sure the phase mismatch, we observed, was not due to some non-linear coupling due to the high coherence excitations, we made additional measurements using sinusoidal exciations with few different amplitudes. The second and third attachment [ 2019-12-04_A2LEXC_DARM_38W.png and 2019-12-04_A2AEXC_DARM_38W.png] below show similar phase offset that we saw in our broadband injections ( ~10 deg compared to model) for both A2A to DARM and A2L to DARM measurement.
Since I've been poking around with pyDARM code for a little bit, I notice that there are a lot of corrections for delays. Perhaps the phase error you are seeing comes about from the combition of delays: totalUserDelay, DAC delay, actuation delay and a specific "unknownPUMDelay"... there are few more delays maybe.
Have a read in the
/ligo/svncommon/CalSVN/aligocalibration/trunk/Common/pyDARM/src/actuation.py (computeDARM function)
You can use output from that function (actProd), to spit out the transfer functions of the relevant delays, to evaluate at your data points and subtract away.