NAOJ GW Elog Logbook 3.2
I measured FDS with CCFC around optimal detuning (attached figure).
CCFC filter gain is 2000 with 30Hz LPF. CC2 mass feedback gain is 2.7.
The spectrum for squeezing quadrature (red curve) seems distorted by backscattering noise and the fitting result is not reasonable.
Now the problem is a bump around 100Hz. I tried the different LPF cut off frequency and gain of CCFC filter, but the 100Hz bump cannot be removed...
Marc, Michael, Yuhang
Yesterday morning we started the measurement of opto-mechanical transfer functions to check the AA filter design (results will follow in another entry)
During this activity we injected noise in input pitch and yaw using diagui.
In order to send these noises from diagui we had to close the damp loop of input.
When we tried to do the same for end mirror we first could not see any noise so we remove many filters (especially 50 and 100 Hz notches) from this loop and then had a FC unlock due to an extremely large offset that we forgot to remove.
I think it is the only modification between your last 2 measurements, but as this loop had a gain of 0, it might not affect the apparition of this peak...