  A fully digital beam position and phase measurement (BPPM) system was
designed for the linear accelerator (LINAC) in Accelerator Driven Sub-critical
System (ADS) in China. Phase information is obtained from the summed signals
from four pick-ups of the Beam Position Monitor (BPM). Considering that the
delay variations of different analog circuit channels would introduce phase
measurement errors, we propose a new method to tune the digital waveforms of
four channels before summation and achieve real-time error correction. The
process is based on the vector rotation method and implemented within one
single Field Programmable Gate Array (FPGA) device. Tests were conducted to
evaluate this correction method and the results indicate that a phase
correction precision better than +/- 0.3 degree over the dynamic range from -60
dBm to 0 dBm is achieved.
