计算信号循环谱的函数： 这个函数用来计算数字调制信号的循环谱图，结果存放在sxf中，surf和shading interp这两个函数配合使用就可以得到信号的循环谱图-Calculate the signal cyclic spectrum function: This function is used to calculate the cycle spectrum of digitally modulated signals, the result stored in the sxf