Matt, Evan
We were perplexed by the steep slope of IMC F below 100 Hz, paticularly since it seemed to vary with PMC gain in the same way as the flat part of the spectrum above 1 kHz.
The attached plot shows the IN1 readbacks (i.e., no digital filtering or compensation applied) for IMC F and IMC L. Evidently, they seem to have the same spectral shape above 10 Hz. Since IMC L does not have any analog whitening, this would seem to indicate that IMC F readback has no analog whitening applied (despite what is implied by the schematic for the MC board).
However, the IMC F filter module (which produces the calibrated IMC frequency control channel that we've been using to estimate the IMC control noise) has a filter consisting of two 10 Hz / 100 Hz p/z pairs, as if to compensate for some kind of analog whitening.
What is actually stuffed into the analog whitening for the IMC F readback on the MC board?
[Also, we don't claim to understand why the TF is 0 dB at the peaks but -5 dB everywhere else.]
According to the schematics both MC_L and MC_F have the same whitening: 10Hz/100Hz double zero/pole with DC gain of 1. MC_I has a simple gain of 100.
Yes, this was confusion on our part about the analog source of IMC L. Indeed, they both seem to have whitening installed (by comparison with IMC I).