Last week I measured the AS RF72 WFS with and without whitening while the IMC was offline. The first attached plot compares the reference traces with the nominal one stage of whitening, and the live traces with no whitening. Based on that result, I determined that we probably should apply another stage of whitening to AS A RF72. Today I applied the second stage of whitening and saw that it makes a small improvement in the dark noise, again taken with the IMC offline. The second attached plot compares the reference traces with one stage of whitening and the live traces with two stages of whitening.
While changing the whitening, I noticed that the overall offsets of each segment change, each one by a different amount. I averaged the input signals over 30 seconds and used those values to update the dark offsets. Some segments are about the same, while others have changed. This SDF screenshot compares the old and new dark offsets for each segment. I also SDFed the second stage of whitening and anti-whitening compensation for all segments.
I also increased the whitening gain of AS A RF72 by 6 dB (from 12 to 18 dB). I compensated that gain change in the anti-whitening filter bank FM6 with a -6 dB gain filter. I'm less certain of the overall effect this will have, but I hope it will help amplify the signal further relative to any ADC noise (see my alog here for reference). We use very little range on these PDs in full lock and during acquisition, so I don't think this will impact locking.
I reran the dark offsets after this change, and have SDFed them below. These diffs appear in both the ASC and CS ISC models. There will be observing diffs that should be accepted!
