S. Dwyer, J. Kissel, D. Sigg

While finding out the best effective frequency noise estimate on the SQZ side of things, we found a VCO calibration in FM3 available on the fast path of the SQZ LO frequency servo (H1:SQZ-LO_SERVO_CTRL), which turns that out-of-loop, pick-off path into a diagnostic effective frequency noise estimate (in lambda = 1064e-09 [m] "red" light). See attached screenshot for how to find this filter bank.

While we can't quickly find any documentation/aLOG on this calibration, it makes sense to us that it has the same frequency response (a zero at ~40 Hz, and a pole at ~1.6 Hz) and roughly the same gain as the primary CARM / IMC Common Mode board servo's fast path (~237000 [Hz/V]).


    IMC VCO calibration     FM3 "VCO"   zpk([40],[1.6],237000,"n")         LHO:65175 (Updated Sept 2022)
    SQZ LO VCO calibration  FM3 "VCO"   zpk([39.72],[1.5292],260000,"n")   (Installed O3 era, we think)

I've accepted the VCO, FM3 filter as ON in the H1SQZ safe.snap SDF, and confirmed that it's either ignored or by default *also* saved to the OBSERVE.snap.