Looking better. I'm curious about what the existing loop shapes are. The old FSS hardware is designed to drive an NPRO + 1 EOM. Is that the existing layout?

However, I think its not designed to drive an EOM. The PDH box should be able to drive the VCO if we replace the output OP27 with a TLE2027. The main point is that the refcav only provides a pole @ 40 kHz and we need the electronics to be 1/f below there. The old FSS board used to do this by the combination of the series resistor and the NPRO PZT capacitance. The VCO is not a capacitive load. I guess that Tara is working on a Simulink model of this whole setup?

I kind of doubt that we will have success without using resonant RFPDs. In most of the PSLs we use a pretty large modulation depth and its necessary to really tune the 2f trap to get rid of the J1^2 term.

------

Different idea: why not just use the VCO/AOM to lock to the ACAV? Then if we pick off the beam for the RCAV after the AOM, the feedback to the NPRO can be used as the differential cavity signal. In this way, we are not sensitive to the VCO calibration issues since its squashed by the ACAV loop.

Idea #2: Just use the transmitted light from the cavities and beat them. Its only phase detection, but in principle, this is easily good enough to detect what we want. Also RF sidebands are gone and all we need is an 1811 or such to detect the beat signal.