什么是Trigger，它在示波器中的作用是什么？
先来看看别人是怎么说的。
The trigger event establishes the time-reference point in the waveform record. All waveform record data is located in time with respect to that point. The instrument continuously acquires and retains enough sample points to fill the pretrigger portion of the waveform record. That is the part of the waveform that is displayed before, or to the left of, the triggering event on screen. When a trigger event occurs, the instrument starts acquiring samples to build the posttrigger portion of the waveform record, that is, the part displayed after or to the right of the trigger event. After a trigger is recognized, he instrument will not accept another trigger until the acquisition is complete and the holdoff time has expired.
trigger事件是在被抓到的波形中建立一个时间参考点。所有的被抓到的波形以这个时间点来排序。设备在这个时间点到来之前（pretrigger）会一直抓取和保存数据，这些数据会被用来绘制trigger时间点之前的图；当trigger事件发生之后，设备在这个时间点到来之后（posttrigger）也会一直抓取和保存数据，这些数据会被用来绘制trigger时间点之后的图。
当设备识别到了一个trigger的时候，它在抓取完成之前和holdoff时间之前不会接受另外一个trigger。
The trigger event defines the point in time at which a repeating “window” of waveform information is stabilized for viewing. Imagine you are taking a trip in your car. You need to arrive at your destination in the minimum amount of time, but you want to photograph a particular landmark along the way. You know you can get to your destination quickly because you have a very fast car, but what’s your strategy for capturing the point of interest on film? One choice would be to randomly snap pictures on your camera as you drive and hope to capture an image of the landmark. Obviously this leaves too much to chance.
A more logical approach would be to give the driver instructions on where to stop so you can get a good clean picture of your point of interest.
trigger事件是用来定义一个时间，来让一个重复的信号可以被我们稳定的观测到。
想象一下，你在开车去一个地方，但是要求你必须在最短的时间内到达。但是你有像在这期间拍摄一张一个特定地点的地标照片，你怎么办？
一种办法是随机的拍照片，但是你的运气需要足够好，可能才能拍到你需要的照片。
一个更好的办法是：到达地标建筑的时候，把车停下来，拍一张照片，接着走。
可将示波器“触发”看作“同步图片获取”。当示波器捕获并显示重复输入信号时，每秒可获取输入信号的数万个图片。为了查看这些波形 （或图片），必须将图片获取与“某一刻”同步。“某一刻”是输入信号中的唯一时间点，或者在使用示波器的多个通道时，是基于输入信号的布尔组合的唯一时间点 （逻辑“码型 ”触发）
下图的左边是没有设置trigger的图，右边是设置trigger以后的图，可以看出图像已经没有叠影了。
示波器有很多的trigger类型。但是一些类型需要额外的硬件支持。例如Bus等。
下面是MSO4000B的Manual中支持的Trigger类型，很多我也没有使用过。
Reference
https://zhuanlan.zhihu.com/p/87293941
https://zhuanlan.zhihu.com/p/90091154
MSO4000B Manual