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Rise Time and Fall Time Measurements Calculation Using the NI-Scope Soft Front Panel



Primary Software: Driver Software>>NI-SCOPE
Primary Software Version: 2.0.1
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:
I am using the NI-Scope Soft Front Panel for a rise time measurement. I am performing the measurement by capturing two or three rising edges, but the results are not what I am expecting.  How does the Soft Front Panel calculate the rise time and fall time of my signal?

Solution:
To achieve an appropriate Rise Time or Fall Time calculation, you should have at least one occurrence of the type of edge (rising or falling) of interest.  This can be either one edge for a single edge measurement or a series of edges for an average Rise Time or Fall Time calculation.

The NI-Scope Soft Front Panel creates a histogram of the data you acquire. It uses reference levels to determine what voltage thresholds define the High, Low and Mid-Range bins. In general, these reference levels are calculated in terms of percentage of the waveform acquired. The NI-Scope Soft Front Panel records the calculation's low level as 10% of the waveform and the high level as 90% of the waveform by default.

When you use only a few edges of interest, there is a possibility for two types of errors: 
  1. With only a few edges acquired, there is more room for error since each edge has a more significant impact on what is seen by the Soft Front Panel as high or low. For example, if one rising edge has a much higher maximum and you ask for a fall time, the soft front panel will report an error because the falling edge did not drop to the newly calculated 10% of the waveform. In other words, the Soft Front Panel creates a histogram and finds the 10% and 90% points, and if the falling edge doesn't cross these points, it is not considered a legitimate edge.

  2. Because the high or low reference levels may not be what you are expecting them to be, you might receive a rise/fall time measurement that does not reflect the information you wanted to analyze.
The best solution is to make sure you are either acquiring only one edge or make sure to capture enough edges to reflect a suitable average. Both of these strategies should result in more accurate high or low reference level calculations, which will result in better Rise Time/Fall Time measurements.

Related Links:
NI High-Speed Digitizers Help: Time Measurements
NI High-Speed Digitizers Help: Rise Time
Documentation: Getting Started with NI-SCOPE
KnowledgeBase 2D39R50A: How Do the High-Speed Digitizers (Scopes) Boards Calculate the Rise Time of a Signal?

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Report Date: 02/25/2002
Last Updated: 10/04/2017
Document ID: 2IOFR3WZ

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