Understanding and Characterizing Timing Jitter Primer

Timing jitter is the unwelcome companion of all electrical systems that use voltage transitions to represent timing information. Historically, electrical systems have lessened the ill effects of timing jitter (or, simply “jitter”) by employing relatively low signaling rates. As a consequence, jitter-induced errors have been small when compared with the time intervals that they corrupt. The timing margins associated with today’s highspeed serial buses and data links reveal that a tighter control of jitter is needed throughout the system design.

As signaling rates climb above 2 GHz and voltage swings shrink to conserve power, the timing jitter in a system becomes a significant percentage of the signaling interval. Under these circumstances, jitter becomes a fundamental performance limit. Understanding what jitter is, and how to characterize it, is the first step to successfully deploying high-speed systems that dependably meet their performance requirements.

A more thorough definition will be introduced in Section 2, but conceptually, jitter is the deviation of timing edges from their “correct” locations. In a timing-based system, timing jitter is the most obvious and direct form of non-idealness. As a form of noise, jitter must be treated as a random process and characterized in terms of its statistics.

If you have a way to measure jitter statistics, you can compare components and systems to each other and to chosen limits. However, this alone will not allow you to efficiently refine and debug a cutting-edge design. Only by thoroughly analyzing jitter is it possible for the root causes to be isolated, so that they can be reduced systematically rather than by trial and error. This analysis takes the form of jitter visualization and decomposition, discussed in detail in Sections 3 and 4.

Although there are many similarities between the causes, behavior and characterization of electrical and optical jitter, the equipment used to measure jitter in optical systems differs from that used in electrical systems. This paper focuses primarily on jitter in electrical systems.

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