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Don't COMmit same mistakes made with jitter

31 Mar 2016  | Ransom Stephens

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Channel operating margin (COM) combines multiple measurements into one signal-to-noise-like figure of merit, analogous to effective number of bits (ENOB), used to characterise analogue-to-digital converters. With COM, the greater the margin, the better the channel. Because COM is built from different measurements and includes the results of models, there are many ways for it to fail.

As we introduce this new observable, it's timely to look back on the legacy of how we handled a similar situation almost 15 years ago: jitter.

Compared to COM, jitter seems simple: the variation of the timing of signal transitions with respect to their ideals. It's easy to think of the distribution as a histogram of these timing variations. Figure 1 shows how an oscilloscope displays the jitter in an eye diagram now it uses a histogram to characterise the jitter.

Figure 1: Oscilloscopes can provide you with a jitter distribution (Source: Teledyne LeCroy).

The mistakes of total jitter
Peak-to-peak jitter, which has appeared on clock data sheets for generations, turned out to be inadequate. Jitter from random processes—predominantly from phase noise within the SerDes' reference clock—varies over time; the longer you measure peak-to-peak jitter, the larger it gets. At this point, those of us driving the standards for high speed serial data technology followed Yogi Berra's advice: "When you come to a fork in the road, take it."

To get away from poorly defined, impossible-to-reproduce peak-to-peak jitter, we made the well-reasoned choice to incorporate BER( bit error ratio) in the definition of a new quantity, TJ(BER) (total jitter defined at a BER). TJ measures the eye closure at a given BER, that is, if TJ(BER) is less than the bit period for the BER specified, then you have some jitter margin and you should be okay—which is the desirable feature of a peak-to-peak measurement.

Sounds great! Drinks all around, right?

Well, because the BERs we cared about were very low, 1E-12 to 1E-18, TJ(BER) turned out to take a very long time to measure and the only equipment that could measure it, BERTs (bit error ratio testers) were really expensive and not all that useful for diagnosing other problems in the lab. So, we developed techniques to estimate TJ(BER) from measurements that could be made quickly, but this led to a huge problem; what you might call a clusterjitter. The extrapolation techniques relied on the abilities of oscilloscopes to measure separate components of jitter: RJ, DJ, ISI, PJ, DCD, and several abbreviations that you either know or wish you could forget.

Different test-and-measurement companies developed different approaches and the measurements disagreed badly. From 2000 to well into 2006, equipment from differing T&M companies—companies you know and rely on and who make great equipment—differed by at least 30% and frequently more than 100%. It wasn't until 2004 that anyone assembled a system that could accurately distinguish which results were right and which were wrong.

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