Measuring Reference-Level Crossings

NI High-Speed Digitizers Help (NI-SCOPE)

Edition Date: January 2017

Part Number: 370592AB-01

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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, but noise can create a higher number of level crossings. To minimize the impact of noise, you can create a hysteresis window around crosspoints. To count a crossing, the signal must start outside this window and then pass through the window to the crosspoint. A crossing is not counted again until the signal passes outside the window and then passes through the window to the crosspoint. The following figure shows an exaggerated example of digital hysteresis, where the dots are the real crosspoints after eliminating the spurious crosspoints caused by noise.

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