Jitter and timing analysis in the presence of crosstalk
By Dean Miles .Tektronix
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.”
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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:
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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
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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
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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 undersampling 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.(note: RTO = real-time oscilloscope, Sampling =
equivalent-time oscilloscope)
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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 overreport 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.
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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 equivalenttime 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.
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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.
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About the author
Dean Miles is a Senior Technical Marketing Manager at Tektronix responsible for Tektronix High
Performance Product Portfolio. Dean has held various positions with Tektronix during his more than 20
years with the company, including Global Business Development Manager for Tektronix RF
Technologies and Business Development Manager for Tektronix’ Optical Business Unit. Dean has
presented Tektronix’s Technologies in more than 80 Countries around the world and met with more
than 10,000 Engineers.
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