Trigger holdoff
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Trigger holdoff is used in oscilloscopes to define a certain period following a trigger during which the scope will not trigger again. This makes it easier to establish a stable view of a waveform with multiple edges which would otherwise cause another trigger.
[edit] Example
Image the following repeating waveform:
The greenline is the waveform, the red vertical partial line represents the location of the trigger, and the yellow line represents the trigger level. If the scope was simply set to trigger on every rising edge, this waveform would cause three triggers for each cycle:
Assuming the signal is fairly high frequency, your scope would probably look something like this:
Except that on the scope, each trigger would be the same channel, and so would be the same color.
What we want to do is set the scope to only trigger on one edge per cycle, so we need to set the holdoff to be a little less than the period of the waveform. That will prevent if from triggering more than once per cycle, but still allow it to trigger on the first edge of the next cycle.
