Tektronix DPScope II, User Manual

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2.5.3 Noise Reject
If there is noise on the signal (and there is always some amount) this can cause false triggering.
E.g. if triggering on a rising edge, the signal may actually be falling at a given instant, but a short
little positive noise spike right when the signal has crossed the trigger threshold from above will
look like a rising edge to the comparator and cause a trigger even though it is not a true positive
edge. As a result the oscilloscope will trigger on a falling edge instead of a rising edge.
The “Noise Reject” feature avoids this by requiring that the signal must remain above the
threshold (in the case of a rising edge trigger; below for falling edge) for at least half a division
(i.e. 5 samples). If it fails to do so (e.g. because the edge really was just a short noise spike as
discussed above), the trigger event will get suppressed and the DPScope II will keep waiting for
a trigger.
This feature is especially valuable for slowly varying signals. Note however that if your signal
period becomes shorter than about one division, activating the noise reject feature may prevent
the DPScope II from triggering at all. In this case either turn the feature off, or increase the
sample rate so the period becomes a larger fraction of the screen width.
HINT: Similarly, it is recommended to turn off Noise Reject in Equivalent Time Sampling Mode.
Using this mode means that you want to look at a quickly changing waveform anyway, and
since the true sample rate is limited to 1 or 2 MSamples/sec (with a corresponding noise reject
interval of 5

sec or 2.5

sec, respectively) chances are high the noise reject feature more often
than not will prevent signal acquisition in this mode.
2.6 Levels