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Analyzing jitter, timing in the presence of crosstalk

Posted: 28 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:Serial data standards? bit error ratio? jitter?

The vulnerability to crosstalk-induced BUJ differs between measurement systems. Oscilloscope measurements or extrapolations of jitter pessimistically bundle BUJ or NP-BUJ into RJ, and then over-report TJ as well. Jitter results (RJ, TJ) depend strongly on aggressor pattern complexity, with PRBS31 being the worst. PRB7, on the other hand, typically does not cause a large error. In the case of real-time oscilloscopes, RJ and TJ results also depend on record length, and longer record length provides more sample points to depict better separation. The exact mechanism of the problem is also implementation dependent.

BUJ measurement solutions
Currently there are a number of approaches to jitter analysis on signals where crosstalk is suspected, but none of them provide one-button push results similar to what oscilloscopes can provide for DDJ and PJ. One clue is if the jitter analyzer reports an inordinately large RJ measurement. It is rare that thermal effects, the ultimate cause of RJ, manage to conspire to greater than 3 ps RMS. If the RJ reported is larger than 3 ps then it's likely that crosstalk is causing problems.

Other tricks to identifying crosstalk require more control over the aggressor channel. For example, if it's possible to turn off the suspected aggressor signal, then you can compare the RJ measurement with and without a signal on the aggressor. If RJ-with aggressor > RJ-without then the problem is crosstalk. A work-around is to use the measurement of RJ with the aggressor off and the measurement of dual-Dirac DJ with the aggressor on in the Dual-Dirac model to estimate the Total Jitter of the system at the BER of interest. The problem with this approach is that it requires control of the aggressors which is not always possible. Another issue is that it is invalid in nonlinear systems (which most transmitters are), and is optimistic toward errors since some of the crosstalk is unbounded.

A more advanced approach would be to be to implement BUJ-aware jitter analysis algorithms. These would involve an additional step in jitter analysis after separation of DDJ and PJ to separate NP-BUJ from RJ as shown in figure 5. A key advantage is that this will work in every scenario since no control over the aggressor is necessary and nonlinear TX does not present a problem. Further, unbound crosstalk components would be correctly recognized as unbound. The downside to this approach is some pessimism remains in the result.

Figure 5: BUJ-Aware jitter analysis and the resulting jitter decomposition map.

To test the ability of a jitter analysis algorithm to accurately separate BUJ from other random jitter sources, we repeated the test shown in figure 4, but with the addition of the results from an equivalent-time sampling oscilloscope using a BUJ-aware jitter analysis algorithm. The result indicated by the dashed line in figure 6 still shows some pessimism compared to the BERT. Results with a real-time oscilloscope were slightly more pessimistic. That said reported TJ error accuracy is dramatically improved, making it possible to trust oscilloscope TJ measurements even in designs where you suspect crosstalk may be a source of jitter and noise related errors.

Figure 6: BUJ-aware jitter analysis (dashed line) algorithm shows greatly improved accuracy on a DUT with large crosstalk.

Summary
As data rates continue to increase, jitter has become a significant percentage of the signaling interval, making it increasingly important for designers to fully understand the types and sources of jitter in their designs. Since most high-speed serial design now involve multiple lanes, crosstalk is a nearly unavoidable consequence that must be factored into the jitter budget.

But to date, measuring the effects of crosstalk-induced jitter, or bounded uncorrelated jitter has been notoriously difficult using jitter separation techniques. Because BUJ has not been accounted for in the jitter algorithms it has been lumped with RJ leading to pessimistic total jitter results compared to the result obtained from a BER tester.

In recognition of this growing problem, particularly for data rates above 10 Gb/s, the jitter model is expanding to include BUJ with the addition of BUJ-aware algorithms. In tests involving a large amount of crosstalk, the new models have proven effective at delivering TJ results on real-time and equivalent-time sampling oscilloscopes consistent with those from a BERT. It also allows for a more thorough analysis of jitter problems in a design, including jitter induced by crosstalk.

About the author
Chris Loberg is a Senior Technical Marketing Manager at Tektronix responsible for Oscilloscopes in the Americas Region. Chris has held various positions with Tektronix during his 13 years with the company, including Marketing Manager for Tektronix' Optical Business Unit. His extensive background in technology marketing includes positions with Grass Valley Group and IBM. He holds an MBA in Marketing from San Jose State University.

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