A Beam Phase and Energy Measurement System Based on Direct RF Signal IQ Under- sampling Technology

Abstract-A diagnostic system being designed to measure the beam phase and energy of the Drift Tube Linac (DTL) in the Proton Accelerator of China Spallation Neutron Source (CSNS) is described and the characterization of the prototype is presented. The signals received from Fast Current Transformers (FCTs) are Radio Frequency (RF) signals with the frequency up to 350 MHz and a dynamic range of -30 dBm to 3.5 dBm. The RF signals are converted to orthogonal streams directly with the In-phase and Quadrature-phase (IQ) undersampling technique based on high-speed high-resolution A/D conversion. Thus a high quality sampling clock system is indispensable. Two different clock systems are implemented and tested for comparison. All Digital Signal Processing (DSP) algorithms are implemented in one single FPGA, meanwhile a Nios II embedded system is also integrated in it for data transfer through the Ethernet. This system achieves a phase resolution better than 0.07 degree over the input signal amplitude range of -41 dBm to 7 dBm.