We have been changing the digital SRCL offset to undetune ("zerotune"- Peter F.) the SRC. However, we are wary of saturating POP RF45, and also leaving enough ADC range to handle any glitches. We have been running with 21 dB of whitening gain on POP45. I just did a quick measurement while we are down to determine if we can change the whitening gain. We are shot noise limited on this photodiode, with about 1e-10 W/rtHz of shot noise. The attached screenshot shows three measurements: in lock with 35 mW incident, a dark measurement (IMC offline) with the whitening on, and a dark measurement with the whitening off. Jenne and I determined we can reduce the whitening gain by 6 dB. I made this plot showing the comparison of the three measurements.
I updated the whitening gain and the anti-whitening gain filter to be 15 dB and -15 dB respectively. These changes were SDFed by Jenne.
This will hopefully allow to increase the magnitude of the SRCL offset further.
I accepted these changes in SDF. The slow SDFs are same, so only one screenshot. For the FM change, I accepted in both safe and observe.
I forgot to make a corresponding -15 dB filter for the Q channel. I just corrected that now and SDFed in safe. We will need to SDF in observing when we get there.
I added a trace of POP45 back in full lock with the 6 dB less whitening gain. The incident power at the time of this new trace (blue) was about 33 mW. The green trace is from a previous full lokc with the full 21 dB of gain.
With the -200 digital SRCL offset, we now have about 11000-12000 cts on POP45 (versus the ~22000 cts from before), so we have much more room to increase this offset further. Jennie has tested that -240 is even better for undetuning SRCL.
