基于可编程逻辑阵列的速调管预失真模型

Predistortion model of klystron based on Field Programmable Gate Array(FPGA)

合肥红外自由电子激光(IR-FEL)是一个工作在中红外和远红外波段的自由电子激光装置,为达到其设计指标,需要使用低电平系统(LLRF)对加速腔内加速场的幅度和相位进行监测和控制。但是速调管的输入输出非线性特性,使得近饱和区控制增益降低,导致了反馈效率的降低。设计了基于可编程逻辑阵列(FPGA)的预失真模型对速调管的幅度非线性特性进行修正,并且对2048节点直接查找表算法和32节点线性插值查找表算法进行了比较和在线实验。比较结果显示,在准确度满足要求情况下,直接查找表算法比线性插值查找表算法延迟减少25%,并且资源消耗量要少于线性插值查找表算法。采用基于直接查找表算法的预失真模块在东芝E3729型号速调管上进行了反馈效率的比较,添加预失真模块后反馈效率提高了43%。

Hefei Infrared Free Electron Laser(IR-FEL)is a free electron laser device operating in mid-infrared and far-infrared.To achieve its design targets,a low level radio frequency(LLRF)system is needed to monitor and control the amplitude and phase of the acceleration field in the accelerator cavity.However,the input-to-output nonlinear characteristic of the amplitude of the klystron decreases the control gain near the saturated region,which reduces the efficiency of feedback.In this paper,a predistortion algorithm based on FPGA is designed to correct the nonlinear characteristics of the amplitude of the klystron,and the 2048-node direct lookup-table algorithm and the 32-node lookup-table with linear interpolation algorithm are compared and tested online.The comparison shows that,in the condition of accuracy requirement,the direct lookup-table algorithm consumes 25%less clocks than the lookup-table with linear interpolation algorithm,with less total resource consumption.The feedback efficiency is compared on Toshiba E3729 klystron,and it is improved by 43%after adding the direct lookup-table algorithm based predistortion module.