For usage in Robert Schofield's magnetic coupling measurements and general optical lever optimization, we were playing around with the whitening chassis to optimize the QPD signals going to the ADC.

The optical lever QPD electronics have the ability to engage up to 3 levels of 10:1 whitening; all with a pole@10Hz and a zero@1Hz.  These filters are turned on via a daughter board connected to the whitening chassis (D1001530), which are exactly the same as ISC's whitening chassis.

Figure 1 shows the spectra of the three levels of whitening to make sure the electronics work the way we think they do, notice that we only go up to two levels of whitening since engaging all three levels completely saturates the ADC.  At two levels of whitening, we see that the higher frequency (~50Hz)amplitude is comparable to the low frequency (~.6HZ) amplitude but not yet dominating the RMS.  Although after monitoring the optical lever for a few hours with two levels of whitening engaged, we saw that the ADCs railed briefly about once an hour, not good.  So in the end, one stage of whitening was optimal and of course, a de-whitening filter was also implemented in digital land.

Figure 2 shows the whitened+de-whitened signal and compares it against the unfiltered signal to make sure we didn't screw up the digital de-whitening portion.

Figure 3 and 4 compare the dark noise currents of one stage of whitening engaged vs. no stage of whitening engaged.  There is about a magnitude of difference in the signal to noise ratio between the two set-ups at >10Hz, as we expected.