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How Does the NI Scope Fetch Measurement Frequency or Period Work?



Primary Software: Driver Software>>NI-SCOPE
Primary Software Version: 3.5
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem:
I want to know how the NI Scope Fetch Measurement Frequency or Period works and how I can improve the results.

Solution:
The Period measurement finds the time in seconds between the first and third mid reference level crosspoints. A hysteresis window is applied when finding crosspoints. By default the mid reference level is 50% of the signal using the Base-Top method.

The Frequency measurement is the inverse of the Period measurement.

The worst case error in either measurement is +/- 1 Sample Clock Period.


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One approach to improving the measurement is to use averaging. There are a built in Average Period and Average Frequency measurements. The waveform is acquired and up to 256 mid reference level crossings are found. The time difference between the last crossing and the first crossing is divided by the number of periods found in the waveform. The last crossing is defined as the last crossing in the waveform with the same slope as the first crossing, so an integer number of periods exist in the waveform. Any error in the measurement (up to +/- 1 Sample Clock period) will be averaged across many periods instead of one.

 
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Another improvement that can be made is to apply a better form of interpolation. By default, a linear interpolation is used to determine when the signal crossed the mid-level reference. The signal may not be linear between all of the samples, which means that a more accurate representation of the signal would be made if a polynomial interpolation was used. It would be beneficial to apply an order of interpolation that most closely represents your signal.

 
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In LabVIEW, to perform a Period/Frequency measurement or Average Period/Frequency measurement using the niScope Fetch or Read Measurement VIs.

 
If you want to add polynomial interpolation you need to use the niScope Add Waveform Processing VI and the Polynomial Interpolation Order property in combination with the niScope Fetch or Read Measurement VIs.



Related Links:
NI Developer Zone Community: NI Scope with Waveform Processing and Measurement
KnowledgeBase 42PAJAOY: Why Does the niScope Fetch Measurement.vi Take So Long to Process?
Specifications Explained: NI Oscilloscopes and Digitizers

Attachments:
AvgLinearInterpolationWaveform.PNG
AvgPolyInterpolationWaveform.PNG
SingleLinearInterpolationWaveform.PNG




Report Date: 10/01/2009
Last Updated: 10/04/2017
Document ID: 5209M5AA

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