请教PWM，PDM，Segma-delta Modulate

表述有误，pwm就是pdm，你说的是ppm?
ptm（就是pwm和ppm了），缺点，很明显，需要的带宽太大了，这个用时频反比粗估都可以
得到了。优点很明显啊，noise性能好，因为仅仅需要检测脉冲位置参量，产生也很容易，
模型用某种波形发生器+比较器描述即可，当然检测器也是非常简单了。

google!!
thanks!
all of these are basic modulation schemes ..
you definitely can find answers using google

Pulse-Width Modulation
Pulse Density/Duration Modulation看来这个缩写有两种意思，我以为是前一种

I have search it in google but hav't got what I want.
May I did it in a wrong way.
Anyway, Thank you!