扩频接收机中改进的锁频方案

An Improved AFC Scheme in Spread Spectrum Receiver

利用叉积自动鉴频进行锁频时,为了得到正确的频率误差需要消除基带信息翻转的影响。提出了一种频率锁定的改进方案,基于该方案提出了一种用相关能量进行误锁检测的方法。该方案的鉴频对象从相邻的两个伪码周期信号换成同一个伪码周期内相邻的半个伪码周期信号,无需利用点积项消除信息比特符号翻转的影响。利用Matlab对方案进行了仿真,分析了工作范围及抗噪性能,给出了基于FPGA的实现方案。实验数据表明：此方案的线性鉴频能力提高一倍,前端捕获间隔可以增大一倍,在同等频偏条件下,捕获时间缩短一半。频率误锁检测可在误锁发生过程中完成,并在2-3个信息比特内将载频调整至正确的频率上。

To achieve correct frequency lock while using cross product automatic frequency control （AFC） ,the effects of baseband data bit shift must be eliminated. An improved cross product AFC scheme is proposed, after analysis of the classic one. A method is given to detect false locks based on the scheme. To free from data shift, the pre-correlation objects are changed to two adjacent half PN - cycle signals between one PN- cycle. Simulation with MATLAB and implementation on FPGA of the scheme is also described. The experiment results show that the frequency detection range of improved scheme is twice of the classic one. The pre-capture time is shortened to an half in the same frequency range. The frequency step can be increased to shorten the frequency and PN code pre-capture time. The false locks detection method can modify the frequency to the right point within 2 - 3 baseband data bits.