I used the XARM as a reference to measure the frequency noise after the MC. It's huge around 4kHz--hundreds of times larger than the frequency noise the MC servo is actually squashing. This presents a real problem for our noise performance.
An elog search reveals that this noise has been present (although not calibrated till now) for years. We're not sure what's causing it, but suspicion falls on the piezojena input PZTs.
I didn't bother too much about it before because we previously had enough common mode servo oomph to squash it below other DARM noises, and I didn't worry too much about stuff at 4kHz.. Now that we have a weaker FSS and thus much weaker CM servo, we can't squash it, and the most interesting feature of our IFO is at 4kHz.
I'll measure the actual voltage noise going to the PZTs. I remember doing this before and concluding it was ok, but can't find an elog entry. So this time maybe I'll do it right.