Serial data standards continue to proliferate, providing dramatic improvements in PC and Server system performance. Testing these higher speed standards for evidence of jitter is critical for long-term stability and to achieving the objective of a good Bit Error Ratio (BER) in the design. Effective analysis begins with selecting the right instruments and have a good understanding of instrument noise, rise time and factors such 3rd, 4th, 5th harmonic performance.
But it’s more than just taking the measurement -- the proper instruments need to be paired with the proper analysis tools. And other factors such as jitter separation, and de-embedding/embedding are also important considerations when testing serial data rates beyond 8 Gb/sec. For this article, we will focus on a new approach to jitter separation in the presence of crosstalk, a growing problem as the number of lanes increases to boost computing system throughput.
All electrical systems that use voltage transitions to represent timing information have timing jitter. Historically, electrical systems have lessened the ill effects of timing jitter (or, simply jitter) by employing relatively low signaling rates. As data climb above 8 Gb/sec., jitter has become a significant percentage of the signaling interval, and understanding the types and sources of jitter is vital to successfully deploying high-speed serial technologies.
At its simplest, jitter is a deviation of an edge from where it should be as shown in Figure 1. As the ITU defines it, jitter is “short-term variations of the significant instants of a digital signal from their ideal positions in time.”
Figure 1: Jitter is the deviation of an edge from where it should be.
There are several ways in which jitter can be measured on a single waveform including period jitter, cycle-cycle jitter, and time interval error (TIE), and the design will often dictate which measurement is appropriate.
In the case of a stand-alone oscillator, the signal is a clock and it can be hopping or swept. Here period jitter is an appropriate measurement. In the case of a transmitter for a serial data stream, the signal is a data stream and ISI (inter-symbol interference) is a key problem. Here TIE jitter is the appropriate measurement.
The engineer on the prowl for jitter issues has a number of instruments available, each with unique strengths and weaknesses:
• A real-time digital storage oscilloscope (DSO) recovers the whole waveform and can measure anything and can be used for TIE, cycle-to-cycle and period jitter measurements. It has limitations, however, around frequency (or bit rate) and resolution of spectra, minute jitter and multi-level modulation.
• A BER Tester (BERT) is well suited for TIE jitter, particularly TJ or total jitter, a form of TIE. The advantage of the BERT is that is counts every bit, but the tests can be time consuming to perform.
• A real-time spectrum analyzer (RTSA) can be used for cycle-to-cycle and period measurements with complex modulations for mobile devices, looking at clocks, PLLs and understanding their dynamic performance. Limitations include span (sub-100 MHz) and bandwidth signals with large modulation spectrum.
Equivalent-time sampling oscilloscopes offer the best bandwidth and can be used for all jitter measurements for serial data. Currently, these are the only instruments with noise analysis and a BER eye. Limitations include no real-time capture and can only be used for repetitive patterns and some jitter spectra are aliased.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 – 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.
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 non-linear 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 non-linear 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.
Fig 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.
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.
-Article Courtesy Test & Measurement DesignLine