As reported in elog 2258, the BS PSD showed large noise last Friday. I checked also today, the noise is still present. So we decide to replace it. As also mentioned in elog 2258, we have only a spare low-gain PSD. So we decide to use SR560 to amplify its output.

The only two SR560 could be used are the one used for CCSB lock and the one used for FC length control (feedback to END mirror). So I temporary took them and used to amplify the low-gain PSD. As shown in the first attached figure, there is a comparison of the PSD. It shows that resistors have a ratio of 100,000:500. Therefore, I set the gain of 200 for the two SR560 for PSD's pitch and yaw.

Then I compare the spectrum of this PSD with the old high-gain PSD. This is shown in the attached figure 2. It is clear that the gain of 200 is too much. I adjusted it to 50 and then, as shown in the attached figure 3, old and new spectrums match around several Hz. It is also shown in this figure that the noise floor of new PSD above 10Hz is generally smaller.

Then I checked the local control of BS also works well.