Summary:

Today, I repeated the coil driver noise measurement. Now, the coil driver noise curve in the noise budget plot (Attachment #1) is the actual measurement of all 12 coils (made with G=100 SR560). I am also attaching the raw voltage noise measurement (input terminated in 50ohms, Attachment #2). Note that POX11 spectrum has now been re-measured with analog de-whitening engaged on all 3 optics such that the DAC noise contribution should be negligible compared to coil driver noise in this configuration. The rows in Attachment #2 correspond to 800 Hz span (top) and full span (bottom) on the FFT analyzer.

Details:

The main difference between this measurement, and the one I did middle of last year (which agreed with the expectation from LISO modeling quite well) is that this measurement was done in-situ inside the eurocrate box while last year, I did everything on the electronics benches. So I claim that the whole mess of harmonics seen in the raw measurements are because of some electronics pickup near 1X6. But even disregarding the peaky features, the floor of ~30nV/rtHz is ~6x than what one would expect from LISO modeling (~5nV/rtHz). I confirmed by looking that the series resistance for all 3 MC optics is 430ohms. I also did the measurement with the nominal bias voltages applied to the four channels (these come in via the slow ADCs). But these paths are low-passed by an 8th order low pass with corner @ 1Hz, so at 100 Hz, even 1uV/rtHz should be totally insignificant. I suppose I could measure (with EPICS sine waves) this low-pass filtering, but it's hard to imagine this being the problem. At the very least, I think we should get rid of the x3 gain on the MC2 coil driver de-whitening board (and also on MC1 and MC3 if they also have the x3 factor).