This independently R&D LLRF control system operating frequency is 162.5 MHz, developed by IMP-CAS, which consists of superconductive cavity resonance frequency control , cavity voltage amplitude stability control and phase stability control .The LLRF system is based on IQ quadrature demodulation techniques , which is completely digitized feedback control and close loop system .Clock jitter of the system is critically im-portant for the LLRF stability and performance .In the paper , clock jitter is measured online via direct measure-ment, meanwhile the LLRF stability and performance is tested from different clock system .The mechanism of the influence of clock jitter to LLRF system is analyzed .Finally, on the basis of experimental data and clock jitter analysis result , the reasonable technical specification for LLRF control system is put forward .%文中介绍的高频低电平系统（LLRF）工作频率是162．5 MHz，作为中国C-ADS注入器II预研系统。该LLRF主要实现超导腔谐振频率、腔压幅值稳定及相位稳定控制。 LLRF采用4采样的IQ正交解调技术构成的全数字闭环反馈控制；时钟抖动对LLRF系统的稳定度和性能至关重要。论文对时钟抖动采用了直接测量的方法，完成了不同时钟系统下LLRF稳定度和性能在线测试，分析了抖动对LLRF作用机理，并根据实验数据，就时钟抖动提出了合理的技术指标。
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