基于TDC的二级频率标准驯服算法

Algorithm of disciplined secondary frequency standard based on TDC

摘要:

利用全球定位系统(GPS)接收到的秒脉冲(1PPS)，对常见的二级频率源温补晶振(TCXO)和相干布局囚禁(CPT)原子钟驯服开展研究。设计了硬件锁相环的驯服方案，利用时间数字转换器(TDC)测量本地分频1PPS与GPS接收机收到的1PPS时间差，实现本地信号相对GPS时间信号的锁定。锁定之后，TCXO实现了万秒稳定度为 8.5 × 10 - 12 ，驯服后3.5×10 ⁴ s的平均频率准确度提升至5倍以上。此外，深入研究了CPT原子钟的噪声模型，在Matlab上对其进行仿真，建立起频率白噪声和频率随机游走噪声在阿伦方差曲线上的对应关系，对比了平均滤波和平均滤波+卡尔曼滤波2种滤波测频方案对CPT原子钟的驯服效果，频率稳定度在5×10 ⁴ s时有一个数量级的提升。

Abstract:

By using the 1 Pulse Per Second(1PPS) received from Global Positioning System(GPS), the disciplining method is studied for common secondary frequency source Temperature Compensated Crystal Oscillator(TCXO) and Coherent Population Trapping(CPT) atomic clock. A disciplining scheme of phase-locked loop is designed. The time difference between 1PPS from local frequency division and that received by GPS receiver is measured by Time-Digital Converter(TDC) to realize the locking of local signal to GPS time signal. TCXO has achieved frequency stability of 8.5 × 10 - 12 at 10 ks, and the average frequency accuracy at 35 ks after disciplining has been improved by more than 4 times. In addition, the noise model of CPT atomic clock is deeply studied and simulated through Matlab. The corresponding relationship between white frequency noise and random walk frequency noise on Allan variance curve is established. The results after disciplining by average filtering and Kalman filtering on CPT atomic clock are compared, and the frequency stability increases by an order of magnitude at 50 ks.