Understanding Oscilloscopes (VII): Oscilloscope Triggering Functions

The trigger modes include Auto and Normal, and the most commonly used one is Auto.

In addition, we also need to be clear about the concept of "pre-trigger".

Automatic triggerAuto

The oscilloscope will force the acquisition of waveforms and display them even if no trigger event occurs. The automatic mode uses a timer that starts when the acquisition starts and the pre-trigger information is acquired. If no trigger event is detected before the timer times out, the instrument will force a trigger. The length of time to wait for a trigger event depends on the time base setting. When a forced trigger is performed without a valid trigger event, the automatic mode cannot be synchronized with the waveform on the display, and the waveform will scroll across the screen. If a valid trigger occurs, the display will become stable.

Normal trigger

In normal mode, the oscilloscope will start scanning and updating the waveform only when the trigger condition (such as trigger level) is reached. Otherwise, it will be blank or the last triggered waveform. The waveform will be updated once each time it is triggered.

Forced trigger

As the name implies, it forces a trigger once from the moment it is pressed. In normal trigger mode, if it has not been triggered, you can force a trigger once to check the current status.

Pre-trigger

The figure below is the trigger buffer of the oscilloscope, which can also be considered as the storage depth of the oscilloscope. The oscilloscope is always sampling data. We can think that the left side of the middle line of the oscilloscope screen (when Time Delay=0) has been repeatedly buffering data (FIFO) and looking for trigger conditions. Once the conditions are met, the oscilloscope starts buffering the data on the right side of the middle line of the screen. The left side is the pre-buffer and the right side is the post-buffer.

Trigger holdoff

Assume that the holdoff time is set to 20ns, and the timing starts after the trigger condition is met. Any trigger condition is ignored within 20ns, and the search for trigger conditions begins again after 20ns until the trigger occurs.

It is specially designed for large-cycle repetitions with many non-repetitive waveform points that meet the trigger conditions. It is usually used to isolate burst pulse trains, fix the trigger point at the same position of each frame of data, and achieve stable display of complex waveforms.