Trigger is a synchronization event which sets zero reference time value in the signal registration in digital oscilloscopes.
Trigger event (trigger) result is a signal registration and an oscillogram display. All signal registration data is ordered according to the time in regard to this trigger. The oscilloscope consistently gathers and maintains the number of samples specified in the pre-trigger interval to maintain the signal part before the trigger. On the oscillogram this signal part is displayed before (closer to the left) the trigger. During the trigger the device starts registering the samples to form the signal part in the post-trigger interval. This signal part is displayed after (closer to the right) the trigger. The trigger moment index is mostly displayed at the top of the oscillogram window as a vertical marker with T letter or as an arrow (or triangle) pointing down.
The picture below shows the sine signal registration oscillogram with rising edge triggering. One of the terms for the trigger forming is the trigger level excess by the input signal. The trigger position is pointed with a vertical marker with T letter.
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