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Test & Measurement World, July/August 2012

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OSCILLOSCOPES compliance testing requires, usually down to a BER of 10–12 . Plus, you're not likely to have that much control of individual channels. Rob Sleigh, product marketing engineer for sam- pling scopes in Agilent Technologies' Digital Test Division, said, "At these speeds, the connectors and ICs have much smaller geometries and we should expect crosstalk from vari- ous sources. Additionally, 100 Gigabit Ethernet gearboxes switch between 4x25 Gbps and 10x10 Gbps, and since you can't shut off individual lanes, it makes crosstalk isolation even more challenging." How Tektronix addressed the problem Perhaps the most annoying problem with crosstalk is that jit- ter-analysis software frequently mistakes crosstalk for RJ (ran- dom jitter). Since RJ contributes disproportionately to the estimate of total jitter defined at a given BER, crosstalk can appear as a far more egregious problem than it really is. In Zivny's view, "The main problem is to accurately predict total jitter and eye opening while providing a quantitative mea- sure of the crosstalk problem." Tektronix accomplished these goals by implementing an approach that Zivny conceived. In the early 2000s, manufacturers of test and measurement equipment were in a heated battle to determine the most ac- curate jitter-analysis technique (Refs. 2 and 3). At the time, measurements taken with different equipment often varied by more than 100%. One of the approaches that brought the problem under control was to use a spectral technique for measuring RJ. With this technique, engineers look at the spectrum of the timing of logic transitions; periodic compo- nents show up as spikes that can be removed. Figure 4 shows the effect that crosstalk has on the spec- trum of uncorrelated jitter. Note the higher noise level and additional spikes in Figure 4b that are missing from Figure 4a. The RMS value of the remaining smooth continuum is then identified with RJ. RJ is assumed to follow a Gaussian distribution, and the amount of eye closure at a given BER can be estimated using the dual-Dirac model (Ref. 4). Cross- talk shows up in that continuum and, thus, the spectral tech- niques mistake crosstalk for RJ and overestimate eye closure, sometimes by more than an order of magnitude. (a) 3.5 3 2.5 2 1.5 0.5 1 0 0500 1000 1500 2000 2500 3000 3500 4000 x1013 Zivny's idea was to first isolate RJ and crosstalk from the spikes of sinusoidal and periodic jitter in the spectrum. Be- cause this data set doesn't contain any elements that are cor- related to the data, like intersymbol interference, it's called uncorrelated jitter. The data is then transformed back into the time domain where the cumulative distribution function is used to isolate unbounded Gaussian RJ. The remaining bounded jitter is labeled NP-BUJ (nonperiodic bounded un- correlated jitter). Whatever crosstalk that exists in the jitter distribution is isolated in NP-BUJ. There is no a priori reason to believe that NP-BUJ is ex- clusively crosstalk. Engineers have a good idea of what's going on in their systems, so it's not much of a stretch to equate NP-BUJ with the horizontal eye-closure caused by crosstalk. This analysis is also performed in the vertical direction where noise plays the voltage equivalent role of jitter to get NP- BUN (nonperiodic bounded uncorrelated noise) (Ref. 5). Whether or not NP-BUJ and NP-BUN are exclusively caused by crosstalk doesn't affect the eye closure estimates at given BERs. LeCroy extracts crosstalk residuals At LeCroy, Marty Miller's approach is quite different. Miller said, "My belief is that the most important aspect an oscillo- scope brings to the table is noise analysis and, to first order, crosstalk is signal-to-noise degradation—radiative effects with variations and fluctuations that you can see on a scope." Mill- er's approach is to automate a detailed examination of the victim waveform. The approach starts with the test patterns. "It's important in any crosstalk analysis," Miller said, "that the patterns transmit- ted on the aggressors not only be distinct from the victim test pattern but have a different length." If the aggressor pattern is the same length as the victim pattern, crosstalk will appear to be correlated to the data. If the aggressor pattern is precisely the same pattern as the victim pattern, not only will the cross- talk be correlated, but the crosstalk and the signal can exhibit constructive and destructive interference, or beats. Miller likes a short test pattern on the victim and long pat- terns on aggressors. This makes it possible to fully evaluate (b) 3.5 3 2.5 2 1.5 0.5 1 0 0500 1000 1500 2000 2500 3000 3500 4000 FIGURE 4. A spectral plot of RJ and PJ shows that crosstalk adds noise and spectral peaks that are missing in (a) and present in (b). Test & Measurement World | JULY/AUGUST 2012 | www.tmworld.com –18– x1013

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