In favor of keeping the same servo gains, I tuned the digital demod phases for the POX and POY photodiode signals to put as much of the PDH error signal in the _I quadrature as possible. The changes are summarized below:
POX / POY demod phases
| PD |
Old demod phase [deg] |
New demod phase [deg] |
| POX11 |
79.5 |
-75.5 |
| POY11 |
-106.0 |
116.0 |
The old locking settings seem to work fine again. This setting isn't set by the ifoconfigure scripts when they do the burt restore - do we want it to be?
Attachments #1 and #2 show some spectra and TFs for the POX/POY loops. In Attachment #2, the reference traces are from the past, while the live traces are from today. In fact, to have the same UGF as the reference traces (from ~1 year ago), I had to also raise the digital servo loop gain by ~20%. Not sure if this can be put down to a lower modulation depth - at least, at the output on the freq ref box, I measured the same output power (at the 0dB variable attenuator gain setting we nominally run in) before and after the changes. But I haven't done an optical measurement of the modulation depth yet. There is also a hint of lesser phase available at ~100 Hz now compared to a year ago. |