产生信号，并显示信号的幅频特性曲线

xt=xtg产生一个长度为N，有加性高频噪声的单频调幅信号。能画出仿真图形-Generate signals and displays signal amplitude-frequency characteristic curve

 　xt = xtg produce a length N, a single frequency amplitude of the high frequency signal plus noise. Can draw simulation graphics