Design of the fully digital beam position monitor for beam position measurement in SSRF

Beam position monitor (BPM) is an important part of beam diagnosis for accelerators. BPM processes the voltages signals of position-sensitive detectors (PSD) to retrieve beam position information. A BPM has been designed for Shanghai Synchrotron Radiation Facility. By applying Software Defined Radio technology, a fully digital architecture is implemented. The four channels of periodic pulses from button-type PSDs are bandpass filtered, amplified to get 500 MHz sinusoids. Four under-sampling ADCs digitalize these sinusoids. The sampled IF signals are decimated by a ratio of 169 with digital down-converters (DDC) implemented in FPGA. IQ data of DDC are sent to cordic blocks and converted to amplitude and phase data. Amplitude data is x and y position of beams. The initial test result shows that amplitude RMS uncertainty is about -73 dB. It may results in -79 dB RMS uncertainty of beam position.