HP 54540 Series - Page 242

Measurements
Determining Rising and Falling Edges
The oscilloscope defines the points on a rising edge as the last point before
crossing the lower threshold to the first point crossing the upper threshold.
The points on a falling edge include the last point before crossing the upper
threshold to the first point crossing the lower threshold. This will measure
the fastest rise and fall times.
As shown in figure 13-8, it is clear that measuring the rise time from the first
point crossing the lower threshold to the last point through the upper
threshold gives a different answer than the shaded line that uses the edge
definition given in the previous paragraph.
Figure 13-8
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Waveform period is measured at the middle threshold
13-11