The filter cavity GR lock's OLTF may differ with the filter cavity GR+IR lock's OLTF at low frequency. Therefore, we start to investigate GR OLTF's low-frequency part.

In the attached figure, there are four measurements. Their legends are listed in the sequence of time on 2020/12/07. We could see that:

1. All measurement shows flat gain at low frequency, which is different from what we expect.

2. Morning and evening measurements' magnitude are quite different at low frequency. The reason for this difference is still unknown.

3. Measurement phases are different with/without SR560 (just passing through without gain/filters). We could see that the phase margin is better if SR560 is used.