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All scalar measurements involving time use the concept of reference level crossings, which occur when the waveform voltage crosses the reference level. Linear interpolation accurately estimates the crosspoint times. In some cases, a digital hysteresis is used, where the window size is the vertical range times the value of hysteresis percent divided by 100. The waveform must be outside the window before a crossing is counted. If the waveform starts above the voltage reference level plus the hystersis window size and crosses the voltage reference level, a negative sloped crossing is counted. Another crossing is not counted until the waveform passes below the reference level minus the hysteresis window size. See the following figure for a hysteresis example. The figure shows an exaggerated example of digital hysteresis, where the dots are the real crosspoints after eliminating the fake crosspoints due to noise.
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