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 13-9, 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. The points in a period are defined
to be the midpoint between the first point before crossing the middle
threshold to the last point after crossing the middle threshold.
Waveform period is measured at the middle threshold
Figure 13–9
Measurements
Finally, Rising and Falling Edges are Determined
13–12
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