Yoichi, Rana

Here is the open loop gain of the XARM loop.

The reference is from the pre-upgrade era. We get the extra phase delay because we have two anti-aliasing filters. One is the hardware filter at about 7kHz for 16kHz sampling. This filter should have been replaced to the one for 64kHz sampling but it has not yet happened. The second one is the software anti-aliasing filter applied when down sampling from 64kHz to 16kHz. So we have double AA filters, which are the culprits for the extra phase delay.

We should either replace the hardware AA filter to the 64kHz one (preferred way), or change the software AA filter to a less aggressive one (easier temporary fix).