Jitter separation: Where science meets art

-November 15, 2013

Jitter is one of the most widely used terms amongst engineers who design digital data links. Whether you design a new board or validate an old board, you'll run have to confront jitter and its components. The problem is how to separate jitter into random and deterministic components. Knowing those components can uncover the sources of jitter.

Jitter is simply the "wiggle" or time variation of digital clock's edge that each transition makes. That difference in timing is called TIE (time interval error). I'll use that as the basis for this discussion.

In an ideal world, rise and fall times would be infinitely fast and every edge of a digital stream would consistently fall the exact same time away from the last. The end result would be no jitter, no wiggling, and no eyes diagrams that suffer from being closed or close to closed. Unfortunately it is not a perfect world, and what once was conceived as an infinitely fast edge, becomes slower as tradeoffs occur as data rates increase.

A signal often must travel through inexpensive PCB material. That, combined with other factors, causes a signal to lose amplitude. Plus, a signal can couple with other signals, causing it to move from a perfectly timed edge to an edge that is now moving from the clock. That's the "wiggling" that Figure 1 shows. Because of these real-world conditions, digital eye diagrams will close, making it harder for a receiver to distinguish between a logic 1 and a logic 0.

Figure 1. Jitter occurs because the time between edges in a digital data stream will vary.

Jitter separation lets you learn if the components of jitter are random or deterministic. That is, if they are caused by crosstalk, channel loss, or some other phenomenon. Separating jitter enables engineers to better understand the systematic problems of their devices and to quickly find solutions to adjust for any errors in them. The jitter-separation concept seems simple enough. In an ideal world, all jitter separation techniques would work the same and give exactly the same answers. All the tools are looking at the same jitter.

Unfortunately this is not always the case. In fact, "answers" for different jitter separations can vary widely. The problem has become prevalent enough, that compliance tools can ensure that all designers use standardized separation techniques. What seems like a simple problem that can solved fairly simply is complicated by the fact the separation tool's answers will vary by test-and-measurement vendor and instrument.

So how does an engineer know which answer is correct? This is where science meets art and the user must use the tools available to him (the science) to decide which answer best represents what he or she is debugging (the art).

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