[Koji, Alastair, Zach]

In taking measurements to make a current noise budget for the gyro, we noticed that the primary error signal spectrum had a suspiciously similar shape to the gyro noise at low frequencies. We realized that we could now be sitting on the "spillover noise" that was less than an order of magnitude below our sensitivity before the upgrade. As a reminder, this is common-mode noise that is not fully cancelled by the primary loop (since it doesn't have infinite gain) and thus enters as residual noise into the secondary loop, looking like a differential (i.e. gyro) signal. It is calculated by simply taking the primary error signal and converting into angular velocity noise by dividing by the optical gain and multiplying by the standard Hz -> rad/s factor.

We decided that we should try adding low-frequency gain using two SR560s (one as a one-pole LPF and the other flat gain, summed together). We increased the gain by a factor of ~100 in the gyro band, but only observed an improvement of ~10 in both the gyro noise and error signal spectra. Below are plots of each. This is strange, but it does suggest that the limiting gyro noise source comes from somewhere in the primary loop, so that narrows some things down. Figuring that our SR560 rig was too hacky, we tried reinstalling my second boost switch by changing R30 to a 110k resistor and then putting a switchable leaded 1k resistor in parallel. The transfer function was what it should be, but the noise improvement we saw was not as good as the SR560 approach.

After taking loop transfer functions to determine the optical gain, I calibrated the primary error spectrum into equivalent spillover noise, both in the standard configuration and with the SR560s in for extra gain. They are plotted below:

This points out something interesting: even though the extra LF gain has roughly the same effect on both the gyro noise and primary error spectra, the spillover noise does not appear to be high enough to explain the gyro noise at low frequencies. The agreement at a few hundred Hz convinces me that the calibration is about right. The similarities in the LF spectra are strong enough that I am fairly certain the gyro noise comes from the same original disturbance as seen in the error spectrum, only it can't couple in via the spillover noise pathway. There must be some other coupling, and perhaps the fact that the extra LF gain has the observed effect on the gyro signal is a good place to start thinking about what it is.