说明：  PWM脉宽调制，是靠改变脉冲宽度来控制输出电压，通过改变周期来控制其输出频率。而输出频率的变化可通过改变此脉冲的调制周期来实现。
(PWM pulse width modulation, the pulse width is controlled by changing the output voltage is controlled by changing its output frequency cycle. The output frequency can be changed by changing the pulse modulation cycles to achieve.)