An analog signal is composed of four basic components: DC and AC magnitudes, frequency, and phase.模拟信号由直流和交流分量、频率和相位四部分组成。
By filtering the pulses, we obtain an analog value proportional to the PWM input.想办法对输出进行滤波, 就可以在输出端得到与数字输入对应的模拟信号.