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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?

One question that often comes up is why worry about jitter if ultimately we're only concerned about the BER. The reason is that too much closes the eye (in width) which leads to errors. Jitter and noise analysis are simply tools that let you quickly predict and analyze problems in the BER. Ultimately, it is all about the errors, but eliminating those errors in a design requires insight into the cause or causes of excessive jitter.

The place to start is to gain an understanding of how the system performs from an overall BER perspective. The oscilloscope accomplishes this using eye diagrams and statistical analysis to create a bathtub plot, so named because of the shape of the resulting chart as the limits change. With the BERT instrument the result is a jitter peak graph resulting from an exact count of every bit. As shown in figure 2, the jitter peak from the BERT on the left and the oscilloscope jitter bathtub plot are nearly an exact equivalent.

Figure 2: Equivalent view of BER performance between BERT jitter peak on the left and oscilloscope jitter bathtub on the right.

Given the close alignment in results, the oscilloscope is a very useful complement to the BERT, since the measurement of TJ to the BER=10-12 can take hours using a BERT and the result doesn't reveal what kinds of problems are causing the jitter. The oscilloscope can measure a small amount of data in smart way and then can break the jitter into jitter components typically following the accepted jitter model shown in figure 3.

By making assumptions, the oscilloscope can make TJ@BER calculations that mirror the results obtained using the BERT in a fraction of the time C that is, if all the assumptions are true. All models of complex systems make assumptions and simplifications, so the fit between the model and the true system behavior will never be exact. As discussed in the remainder of this article, a particularly daunting problem to date has been crosstalk.

Figure 3: The industry's jitter model 2001-2010.

The crosstalk problem
To achieve performance targets, most serial systems use multiple lanes. As frequencies and data rates increase past 10 Gb/s, a small amount of crosstalk can eat up the jitter budget and create timing issues.

Crosstalk occurs when one signal is affected by a neighboring signal. At high data rates a signal propagates more like a guided wave than a simple DC current. The wave is guided by the conducting trace but radiates through the dielectric medium, typically FR4. When more than one signal is present, every conducting trace on the board includes artifacts of the signals on every other trace. The accepted terminology is to say that an aggressor signal causes crosstalk on a victim signal. Crosstalk occurs when the signal of an aggressor is picked up by the conductor guiding the victim signal. Unavoidable discontinuities in circuit layout, like connectors and vias, where capacitive coupling is greatest, are critical points that act like antennas in generating crosstalk.

Real-time sampling and equivalent-time sampling oscilloscopes use spectrally-based jitter analysis techniques to separate the various jitter components. On real-time sampling equipment, where the frequency components are not aliased, the jitter and voltage noise spectra have sub-harmonic peaks that, rather than appearing as sharp lines, are smeared into broad resonance shapes. On under-sampling equipment, like an equivalent-time sampling oscilloscope, where the spectrum is aliased, crosstalk appears as continuous noise.

In both cases, these spectrally-based jitter analysis techniques, which measure random jitter (RJ) by integrating the jitter spectrum continuum, overstate RJ with the crosstalk timing effects. This leads to an increase in RJ and an overestimation of TJ. Figure 4 shows oscilloscope measurements of jitter, in this example a DUT with a large amount of crosstalk.

Figure 4: TJ error in oscilloscopes compared to a BERT. (RTO = real-time oscilloscope, Sampling = equivalent-time oscilloscope).

Crosstalk appears to the oscilloscope as bounded uncorrelated jitter or BUJ since it follows a bounded distribution. The bounded nature of the distribution is obscured by the complexity of the data pattern. The seemingly random distribution of 1s and 0s causes different amounts of voltage noise to be transmitted on each aggressor-signal transition.

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