说明：  以一个具体的随机信号为例，简单介绍了周期图法、平均周期图法、修正的平均周期图法以及BT法的基本原理，并对这些方法的性能进行分析。
(Taking a specific random signal as an example, this paper briefly introduces the basic principles of periodic graph method, average periodic graph method, modified average periodic graph method and BT method, and analyzes the performance of these methods.)