We changed the RF14 QPD, adding 2 stages of whitening. This gave about a factor of 20 improvement in noise around 700Hz. There are no whitening filters on the DC channels. The RF channels had about the same SNR as the DC channels for jitter noise.
The standard whitening filter settings were copied from K1:IMC-REFL_QPDA1_DC to 1:IMC-REFL_QPDA1_RF14_I/Qs filter banks. Previously they were populated with 2-4 gains, they were anti-whitening filter gains but they were not consistent.
We turned off the IMC input alignment controls by turning off the input to IMC ASC DOF4 and DOF5 pitch and yaw. We added one whitening stage, took references then added a second stage. The resulting noise spectra of the QPD segments can be seen int he attached figure. Black trace is one segment with one stage of whitening. Otherwise reference traces are no whitening and live traces are 2 stages of whitening The traces with only on whitening filter stage on were taken without the input alignment on, but that did not affect the spectrum. Something seems misaligned on QPDA2 resulting in 2 QPD quadrants have a lot lower signal compared to the other 2.
2 stages of whitening has been left on. IMC input alignment was put back on the for final measurements and appears unaffected.
Upon Ushiba's suggestion we looked at the raw inputs the the REFL QPDs. Sure enough the whitening stage on QPD2 segment 3 and 4 in both I and Q channels are not engaged. See the attached plot.
Red green and blue on the top two plots show expected behaviour with the raw channel (IN1_DQ) being boosted at high frequency with whitenning. In the lower 2 plots red, green and blue (quadrant 4) are all overlapped showing no analogue whitening was engaged. The same is true for quadrant 3.
It is strange that segments 3 and 4 on both I and Q channels did not engage, we will check tomorow.
