Beat signal from double cavities in the same chamber was measured. At DC to 100Hz, it seems to be dominated by seismic. Above 100 Hz, the frequency's laser noise is the limiting source. The setup has yet to be optimized. This is for a quick check to see how beat signal changes with the new seismic isolation setup.
I measured beat signal after Paul and Frank locked ACAV. At frequency above 100 Hz, beat signal changes with RCAV's gain setup. Error signal from RCAV's mixer out matches the shape of the beat signal ( I did not do the calibration with error signal slope, just observed the displays on SR785). However, at low frequency (DC - 100 Hz), the beat signal does not change with anything, RCAV gain, ACAV gain, PLL gain, so I'm quite certain that it's the real signal we see here.
Tuning range on PLL loop was 10kHz, the calibration is 7kHz/V. A peak close to 7 Hz might come from the beam line transverse mode of the stack.
At DC - 10 Hz, the noise is lower than before. This might be the result from common mode suppression. However, the two spacers are not identical. They have significantly different holes' sizes. This may explain why the seismic cancellation is not that good at higher frequency.
We might have to suspend the whole vacuum chamber to win against seismic.