Peter, Alexa
Summary
We measured the digital delay from the IR TR PDs at the end station to the CARM slow path at the corner to be ~50 deg at 300 Hz. This delay will include an end station model delay and two LSC delays. We have two LSC delays because TR CARM is in the LSC model, as well as the digital slow path after the IFO common mode board. Our measurement is reasonable given Chris's delay plot (LLO#15933).
Details
We turned off the LSC-X_TR_A_LF_OUT (IR TR PD at END X) input and sent in an excitation with an amplitude of 100 cts. We turned off any filters along the TR CARM path (i.e. LSC-TR_CARM --> LSC-REFLBIAS --> ALS-C_REFL_DC_BIAS path), modulo some gains, and measured the transfer function at LSC_REFL_SERVO_SLOW_OUT. See LHO#15489 for the path.
So the above measurements only accounts for some of the phase delay we see in the CARM TF . We realized that there was margin for improvement in the compensation filter for the transmission signals. We made another filter for LSC-TR_CARM (FM8, 35:3000) that gives us some phase back. With respect to the one currently used (FM9, 35:1000^2), we removed one of the poles @ 1 KHz, and moved the other 1 kHz pole to 3 kHz. At 200 Hz we get back about 20 degrees of phase. We will test this filter as soon as we can (another earthquake now..).
We tested FM8, and this was indeed a good change. Attached is the sqrt(TRX+TRY) CARM OLTF with FM9 (green trace) and FM8 (brown trace). As designed, we get more phase with FM8 on. This is now implemented in the guardian.
Note: ignore the peak at about 50 Hz ... when we made this measurement we were father away from resonance than when we normally transition to sqrt(TX+TRY) so our z at 35 Hz was not properly compenstating for the cavity pole properly. This goes away and flattens as we reduce the offset.
