本系统用引脚可编程开关电容滤波器MAX264来实现程控滤波器，利用DDS产生精确外部时钟来实现fc在1KHz到40KHz步进可调。-The system uses pin-programmable switched-capacitor filter to achieve the programmed filter MAX264, use DDS to generate accurate external clock to achieve fc at 1KHz to 40KHz step adj