HBM Genesis GEN5i User Manual page 124

GEN5i
As explained later in this chapter, a signal must actually cross the preset level.
This is to avoid erroneous triggering on a small amount of noise on the signal.
To make the trigger detector even more stable when noisy signals are used,
the single-level trigger detector has been expanded with a hysteresis. This
basically doubles the logic.
For the advanced trigger modes the single-level trigger detector with
programmable hysteresis has been implemented twice to provide a dual-
level trigger detector with selectable hysteresis on each channel. Levels are
usually referenced as primary trigger level and secondary trigger level.
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