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How Do the High-Speed Digitizer (Scope) Boards Calculate the Rise Time of a Signal?



Primary Software: Driver Software>>NI-SCOPE
Primary Software Version: 2.6.1
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:
How do the High-Speed Digitizers (scopes) calculate the rise time of a signal? Is it done in hardware or software?

Solution:
The digitizers do not have on-board processors, so the measurement will be calculated in software based on the acquired data. The rise time will only be calculated for the first edge present in the data that is fetched from the board.

This means that if you acquire a 2 MHz waveform for 1 ms at a sampling rate of 20 MS/s, you will have 20,000 data points representing 2,000 periods of the waveform. If all 20,000 data points are fetched from the board at once, the rise time will only be calculated for one edge of the waveform, rather than for all 2,000 periods. If you wish to average all the rise times of the acquired waveform, you will need to fetch smaller chunks of data (such as a single period) at a time and then average the measured rise times.

Also keep in mind that the resolution of the measurement will be dependent on the sample rate of the acquisition since the calculation is done by finding the number of samples present in the edge and comparing this with the sample period of the data. Thus the rise time measurement may be off by up to two full sample periods in the worst case (±1 sample period at the beginning and end of the edge).

Related Links:
KnowledgeBase 46DA70JX: Relationship Between Bandwidth and Rise Time
Specifications Explained: NI Oscilloscopes and Digitizers

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Report Date: 09/04/2001
Last Updated: 10/04/2017
Document ID: 2D39R50A

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