An oscilloscope is a very basic instrument for electrical engineers, but many people are unable to effectively use the trigger function. Triggers are often considered very complicated, so when engineers find problems, they usually ask experts in the laboratory to help set the trigger. The purpose of this article is to help engineers understand the basic concepts of triggering and the effective use of triggering strategies.
What is a trigger
No oscilloscope will have infinite memory, so all oscilloscopes must use triggers. Trigger refers to an event that the user cares about, and the oscilloscope must discover. In other words, it is a situation that the user wants to find in the waveform. Triggering may be an event that represents a problem in the waveform, but this is not necessarily the case. Examples of triggers include signal edges, glitches and digital patterns. The limited memory forces the oscilloscope to use triggers. For example, Dingyang SDS1000CML series oscilloscopes provide a memory depth that can hold 2M samples, but even so, the oscilloscope still needs some events to tell which 2M samples to display to the user. The 2M samples may seem huge, but they are not enough to ensure that the oscilloscope's memory actually captures the desired events. The oscilloscope's memory can be thought of as a conveyor belt. Whenever a new sample is acquired, it will be placed in memory. When the memory is full, the earliest acquired samples will be discarded, so the memory will only contain the latest samples. When a trigger event occurs, the oscilloscope will capture enough additional samples to place the trigger in the required memory location (usually in the middle), and then display the data to the user.