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Revisiting the history of jitter: The early years

19 May 2015  | David Maliniak

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The fact is that in those early digital days, jitter just wasn't much of an issue. Even into the 1970s and 1980s, with parallel buses, data rates in the tens of megabit/s, and rise times of nanoseconds, jitter still did not raise many alarms. If a unit interval was that long with correspondingly long setup and hold times, then the thickness of the edge relative to the overall parameters made the likelihood of timing uncertainty causing a bit error extremely low (figure 3).

Figure 3: Remember the carefree 70s and 80s, when no one really cared very much about jitter?

But by the late 1990s, the scenario was very different with respect to jitter. The transition from parallel to serial data buses was well underway. Data rates had climbed into the gigabit/s range while rise times had dropped into the hundreds of picoseconds. As a result, a little fuzziness on a rising or falling edge had become much more significant with respect to the entire unit interval.

Thus, in the late 1990s, the question had become, "How do I characterise setup and hold times with any real level of certainty?" Which is to say, how much jitter is there? Ah, NOW it matters! One simplistic method that gained prevalence was to measure the peak-to-peak jitter on eight clock edges. Obviously, this is not a particularly accurate method, as there will be a good amount of variation on any eight given edges of a clock output. One thing had become clear: Jitter affects setup and hold margins. The longer we measure for, the shorter setup and hold times become, and the tighter the margins become.

Around this time, some advances in measurement technology arrived that allowed edge times to be analysed with a bit more detail. Stay tuned for subsequent posts that continue the story of jitter.

About the author
David Maliniak joined Teledyne LeCroy in 2012 as Technical Marketing Communications Specialist. He worked for over 30 years in the electronics OEM B2B press, most recently as Test and Measurement and EDA Technology Editor at Electronic Design magazine. Maliniak holds a BA in Journalism from New York University.

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