[Tanaka, Komori, Ushiba]

Summary:

Strange coupling seems to come from the LSC residual to ASC coupling.
So,we engged resonant gains at excitation frequencies, and then strange coupling becomes smaller.

Detail:

We found that DHARD_P signals, which can be seen in AS_RF17_QPDA{1,2}_{I,Q} signals, are completely different from the one measured the day before yesterday.
After several investigation, we found that it seems to come from DARM residual.
Figure 1 shows the spectra with (red) and without (blue) resonant gain at 13.125 Hz when exciting DHARD_P at 13.125 Hz.
As you can see, very large peak can be seen in DARM error signals if we didn't engage resonant gain.

Figure 2 shows the power spectrum with 30 cnts excitation of DHARD_P with and without resonant gain in DARM loop.
Large side lobe was disappeared when engaging resonant gain and amplitude of spectra are completely different.
So, large coupling proably LSC to ASC coupling.

Figure 3 shows the TF and coherence.
Coherence of the measurement improved and TF frm OpLev to WFS signals are completely different.