低信噪比下锁频锁相器性能分析及改进

摘要

针对锁频锁相器(Phase and Frequency Detector,PFD)应用于低信噪比、大频偏的条件,通过理论分析和仿真验证阐述了窗口类型对系统频偏捕获速度,范围、噪声门限及相位噪声抖动的影响机理.推导出等效相位噪声功率谱密度的表达式.证明了大窗口具有更低的噪声门限和更小的稳态相位抖动,但捕获速度较慢.为了提高捕获速度,对鉴相器输出值取极性运算得到改进的PFD算法.新算法不仅能增加鉴相增益提高捕获速度;还可以减少等效噪声功率谱密度降低相位抖动;同时新算法不需要乘法器便于硬件实现.最后新算法的性能通过仿真得到了验证.%Under the condition of low Signal to Noise Ratio ( SNR) andlarge frequency offset, the impact mechanism of window type on acquisition time,frequency range,noise threshold and phase jitter in Phase and Frequency Detector (PFD) is presented by theoretic analysis and simulations. Closed-form expression of equivalent noise Power Specmran Density (PSD) is derived.The results show that the bigger window has a lower noise threshold and less phase jitter, but acquisition rate is slower. Modified algorithm replaces the phase detector output with its polarity. The new algorithm not only improves the performance of acquisition time and the phase jitter by increasing the gain of phase detector and reducing equivalent noise Power Spectrun Density respectively, but also reduces hardware implementation complexity. In last, the new algorithm is verified by simulations.