How to measure display jitter
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 8/1/2009 2:00:00 AM
All serial data streams and clocks have jitter (the difference between the arrival of an edge and the expected time of arrival). The jitter you see on an oscilloscope is a function of the signal's timing jitter and amplitude noise, plus the jitter and noise that the oscilloscope adds to the signal.
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The jitter that an oscilloscope adds to the signal you see (call it display jitter) is a combination of the instrument's trigger noise and trigger jitter. Noise on an input signal produces a timing error in the displayed signal. If, at the start of a rising edge, noise has pulled a signal's amplitude low, the rising signal will cross a threshold point later than it would in a noise-free signal.
Trigger noise will also add jitter because it affects the point where an edge crosses the trigger level. The amount that trigger noise contributes to display jitter depends on the input signal's slew rate. The greater the slew rate (faster rise or fall time), the less display jitter you will see.
Oscilloscopes have traditionally measured display jitter (but called it trigger jitter) using a histogram (figure). But in some oscilloscopes, hardware has improved to where trigger noise dominates display jitter, making the trigger jitter component negligible. In addition, some oscilloscopes can mathematically remove most of the instrument's display jitter, producing a waveform that appears to have no noise or jitter at the trigger point.
Because of those factors, the traditional measurement won't work. You can, though, use a different technique to measure an oscilloscope's display jitter. It involves using a sine wave and employing frequency-domain techniques. To learn how the technique works and how to apply it, download the paper “The evolution of trigger jitter” by Steve Draving, hardware engineer at Agilent Technologies.
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Tony Karavidas - 2009-3-8 13:39:00 EDT
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