Since we will not have the new PDH servos ready by the time we are locking the mGyro, it is important for us to think about how we can minimize the gyro noise with the old boxes.
First of all, we will not be able to capitalize on the improved optical gain (from increased power on the new diodes) unless we can decrease the overall gain of the servo; otherwise, the loop will become unstable. We should figure out the best way to reduce the gain by an overall factor of ~40-50 dB, which is the expected enhancement from the new PDs.
Second, Koji will be pleased to hear that I think I have traced the excess low-frequency noise seen in this measurement to the AD8336 variable gain stage. It turns out that they have poor performance when used in low-gain configurations, leading to a likely input noise of greater than 400 nV/rHz in our current configuration, with an apparently high 1/f corner frequency of >10 Hz. By playing with the parameters a bit in LISO (the noise specifications in the data sheet are a bit esoteric), I am able to produce a noise plot that is quite like the one we measured in the above linked post.
I propose that we temporarily circumvent the variable gain stage altogether and reduce the gain of some or all of the stages, leaving a trimpot somewhere so that we can tune it as necessary. The box will of course be reverted for universal use once we get our new boxes. This combination should allow us to reach our estimated unavoidable displacement noise level in broadband above ~10 Hz. The hope is that this unavoidable displacement noise level is actually lower than we think at low frequencies (assuming that most of the low-frequency noise is caused by the air), so that using the new boxes to juice up our gain at low frequencies will put us below the requirement in the operational band. For reference, the most recent gyro noise budget is here.