fft求电压电流信号的频谱，可用于谐波分析-get frequce by fft

能测量频率，并且能测占空比10 90 ，还能产生1M 占空比10 的脉冲波- 能测量频率，并且能测占空比10 90 ，还能产生1M 占空比10 的脉冲波 Can measure the frequency, and can measure the duty cycle 10 90 , but also can produce the 1M duty cycle 10 of the pulse wave