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Measuring Jitter in Signals (Jitter Analysis Toolkit)

LabVIEW 2012 Jitter Analysis Toolkit Help

Edition Date: June 2012

Part Number: 373270B-01

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When you measure jitter in signals, you typically perform a common set of tasks to acquire and analyze the signal data. The sections that follow describe steps in a basic jitter-measurement application and the VIs the Jitter Analysis Toolkit provides to perform the steps.

The following block diagram shows a common workflow for using the Jitter Analysis Toolkit to analyze jitter in a waveform.

Acquiring Waveform Data

If you want to perform automated testing of signals, stream data from hardware using drivers appropriate for your device. If you want to perform measurements on signal data saved to disk, you can read the data from file using functions appropriate for the file format you are using. For example, you can use the TDM Streaming VIs and functions to read waveform data from a .tdms file. You can use the Storage/DataPlugin VIs to read measurement data from a variety of file formats.

Managing Memory and Performance

Depending on how you acquire waveform data, the Y data values that are part of the waveform data type might be double-precision, floating-point numeric values or 8-bit signed integers. Analyzing waveforms represented by integers reduces the size of the data you must analyze, which requires less memory. Many VIs in the Jitter Analysis Toolkit can operate on the waveform data type when it contains double-precision floating-point data or 8-bit signed integers.

Measuring State and Reference Levels

After reading in waveform data, establish state and reference levels that allow you to identify repeating waveform features. For example, after you set a low reference level, you can identify each waveform sample that falls at that voltage level and the time at which it occurs in the waveform. High and low state levels define the voltages at which the signal is at its highest and lowest states. Reference levels fall within the state levels, often as percentages of the state-level amplitude, and allow you to identify features of waveform transitions. The high, mid, and low reference levels are typically 90%, 50%, and 10%, respectively, of the state-level amplitude.

You can use the State Levels VI with the Reference Levels VI to measure these levels for a waveform, or you can use only the Reference Levels (waveform in) polymorphic instance to measure all levels.

Identifying Transitions and Level Crossings

As mentioned previously, high and low reference levels identify transitions in a waveform between its high and low states. Transitions are also the waveform features in which jitter, deviation from the clock signal, is typically measured. When you identify transitions, you also reduce the total data you must analyze, which requires less processor resources.

After isolating the transitions in a waveform, it is useful to further reduce the data with which you are working to a single, common voltage value within each transition so you can measure time differences at that one point. This common voltage value is referred to as a crossing level. Applications typically use the 50% mid reference level as the value at which to identify level crossings.

Use the Level Crossing VI to measure transitions in a waveform and return the locations at which level crossings within the transitions occur.

Recovering the Clock Signal

With level crossings identified in each transition, you must compare their locations in time to the times at which the corresponding crossings occur in the reference clock waveform. The reference clock triggers the transitions in the measured waveform, so you can consider the clock signal to represent the ideal state of the waveform. Use the Clock Recovery VIs to recover the level crossings in the reference clock waveform from a measured waveform.

Measuring and Analyzing Jitter

After you identify level crossings in both a waveform and the corresponding reference clock waveform, you can use Jitter VIs to measure the following types of jitter:

  • Time interval error—Time differences of level crossings in a waveform and the level crossings from the reference clock.
  • Period jitter—Differences between cycle durations in a waveform and the corresponding cycle durations from the reference clock.
  • Cycle-to-cycle jitter—Differences between the amounts of period jitter in successive cycles of a waveform.

The previous VIs return jitter sequence arrays, where each element is a jitter value at a level crossing. You can use the Probability & Statistics VIs with the jitter sequences that many of the VIs on this palette return to perform probability, descriptive statistics, analysis of variance, and interpolation functions.

Separating Jitter Components

With jitter sequences, you then can calculate the amounts of random jitter, deterministic jitter, and total jitter in the signal as a whole, rather than jitter at each level crossing. You also can separate deterministic jitter into periodic jitter and data-dependent jitter. The Jitter palette contains VIs that separate components of jitter.

Creating Eye Diagrams

Use the Eye Diagram Measurements VIs to construct an eye diagram that displays information about jitter in a waveform signal. In eye diagrams, corresponding segments of a waveform are superimposed upon each other to illustrate variations in the time at which features in the waveform occur.


 

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