I reconfigured the transmission demod and PLL setup today, so that I could compare the gyro noise as measured in the two signal candidates (AOM feedback and PLL feedback). As detailed in the previous post, the DAQ has taken a big stinky poop and will require Alex's wizardry to reemerge from utter chaos. To do today's analysis, I hooked up an SR560 with one pole at 0.1 Hz to use as the slow feedback filter. I used the Pomona-encased voltage divider to get the gain low enough. The gyro seemed to stay locked indefinitely.
To get better low-frequency data, I took spectra down to a 25-Hz span. This doesn't get us that much more information than with 100 Hz given the smooth shape of the spectrum at low frequency, but anything helps. We will be able to get lower-frequency data once CDS is back up.
Below are three traces, all calibrated to angular velocity noise:
The third trace---which was inspired by Koji and Frank's elegant work last week---I took because I noticed that the low-frequency part of (1) and (2) looked qualitatively the same. Since we hypothesize that noise in the input optics (e.g. from air) will appear in (1) but not in (2), I suspected that covering the transmission optics in foil to reduce air noise would bring the low-frequency junk down, perhaps to or near the level seen by Koji and Frank when they covered the input optics. The foil "enclosure" is seen in the second picture.
As can be seen in the plot, (2) and (3)---which are all but identical---are noisier in broadband than (1), save for some resonances which suggest that the calibration is about right (the calibration for each is simply the appropriate VCO gain in Hz/V times the common gyro factor lambda*S/4A). What I think is that there is some non-ideality in the transmission setup that is contributing some excess noise somehow. I am not yet ready to abandon the conviction that IO noise should be suppressed by the loop gain in the PLL signal, as the data do not show that definitively.