As described in elog2846, FC easily unlocks due to saturation of FC GR correction signal sent to laser PZT.

Fig. 1 shows the FC GR correction signal (CH1) and error signal (CH2) with FC servo gain of 0.8. The FC GR correction signal is half of the signal sent to laser PZT. When the FC GR correction signal reaches +/- 0.44V (signal sent to laser PZT reaches +/- 0.88V), the FC unlocks. This saturation can be relaxed by Z correction as shown in Fig. 2 and we could lock FC for a few minutes with Z correction today.

Fig. 3 shows the same measurement with FC servo gain of 2. The error signal becomes noisy because of too high gain, but the saturation does not occur at +/- 0.44V.

Fig. 4 shows the FC GR OLTF with servo gain of 0.8 and 2. UGF is 14kHz with gain of 0.8 and 23kHz with gain of 2.