What is oscilloscope triggering? What is the role of oscilloscope triggering?

Any oscilloscope has limited memory, so all oscilloscopes must use triggers. A trigger is an event of interest that the oscilloscope should find. In other words, it is something that the user wants to find in the waveform. A trigger can be an event (i.e., a problem in the waveform), but not all triggers are events. Examples of triggers include edge triggers, glitch signal triggers, and digital pattern triggers.

The reason why oscilloscopes must use triggers is that their memory capacity is limited. For example, the Agilent 90000 series oscilloscope has a memory depth of 2 billion samples. However, even with such a large memory, the oscilloscope still needs some events to distinguish which 2 billion samples need to be displayed to the user. Although 2 billion samples may sound very large, it is still not enough to ensure that the oscilloscope memory can capture the event of interest.

The memory of an oscilloscope can be thought of as a conveyor belt. Whenever a new sample is taken, it is stored in memory. When memory is full, the oldest samples are deleted to allow the newest samples to be stored. When a trigger event occurs, the oscilloscope captures enough samples to store the trigger event in the required location in memory (usually in the middle), and then displays this data to the user.

Oscilloscope trigger function

The trigger function of an oscilloscope has two main functions: first, to isolate the event of interest, and second, to synchronize the waveform, or to stabilize the displayed waveform.

Isolating the event of interest means that the event isolated at the trigger point is the signal that meets the trigger condition. As shown in the figure below, the event isolated at the trigger point is the pulse width that is always less than 47.5ns or greater than 52ns. The pulse width is calculated by the time interval at the intersection of the pulse width waveform at the trigger point when the trigger level crosses the trigger point.

Isolate events of interest

To synchronize waveforms is to find a triggering method that stops the waveform from "shaking", that is, to find the regularity of the signal to synchronize the signal. As shown in Figure 2, there are four pulses in each data packet. These four pulses are not at equal time intervals. If the rising edge is used as the trigger source, the waveform cannot be synchronized and will "shake" visually. However, each data packet arrives at equal time intervals. If the rising edge of the first pulse of each data packet is used as the trigger source, the waveform can be displayed stably. Therefore, edge delay triggering can be used. After the previous rising edge arrives, the next rising edge is triggered after a delay. In the above example, the time required to be delayed is the blue time interval marked.

Synchronous signal enables stable waveform display