Optimization of the sampling process for the close orbit data in the digital Beam Position Measurement for SSRF

Beam Position Measurement (BPM) system is an indispensable diagnosis instrument that is used to monitor the beam position information. A digital BPM (DBPM) has been designed for Shanghai Synchrotron Radiation Facility (SSRF). Among the different types of data that the DBPM provides, the Close Orbit (CO) data is an important one which is applied to beam monitoring, as well as feedback to keep the beam orbit stable. Therefore, great efforts are required to optimize its performance. We implemented an off-tuned sampling method by using a Digital Phase-Locked Loop (DPLL) based on Alexander phase detector, and an interleaved switching scheme to address variance in the long-term performance of the sampling channels. To evaluate these sampling methods, we conducted commissioning tests in the SSRF. Test results indicate that the position resolution of the CO data can effectively be enhanced from 3.62 μm to 0.16 μm by the interleaved switching scheme while no obvious performance difference was observed with or without the off-tuned sampling method applied which was probably due to good performance of the sampling channels in the DBPM electronics.