Test & Measurement World, July/August 2012

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OSCILLOSCOPES FIGURE 5. A signal-centric contour plot contains a superposition of the trajectories of all symbols in a test pattern, including all sources of noise and lines of equal BER, here at 10–6, 10–9, 10–12, and 10–15, with crosstalk (left) and without crosstalk (right). each bit in the victim pattern subject to a representative sam- ple of crosstalk impairments. The next step involves evaluating the shape, or trajectory, of each symbol in the victim test pattern. By comparing each repetition of a symbol, Miller's technique separates the cross- talk impairment into a waveform of its own that he calls crosstalk residuals. The residuals are what you would see on the victim lane if you could turn off the victim signal and look at the crosstalk alone on that lane. The process is inde- pendent of any phase relationships between the victim and aggressors, so it doesn't matter whether or not the victims and aggressors are frequency locked. The final result is a novel construct Miller calls a "signal- centric contour plot" (Figure 5). This plot is a combination of the trajectories of every symbol in the test pattern with all the jitter and noise as well as the actual crosstalk distribution portrayed as a fluid cumulative distribution function from which the probability of a deviation from the mean waveform can be calculated. It's like a generalized BER-contour plot and provides eye-closure estimates to any BER, with crosstalk properly included in the calculation. Agilent opens the catalog Agilent applications engineer Stephen Didde commented that, "The error detector of a BERT [bit-error-rate tester] has 100% data capture. It's the only test equipment that actually measures eye opening at very low BER. No matter what sig- nal impairments there may be, crosstalk from different lanes, power-supply interference, or other unknown problems, the BERT measurement can be trusted." In addition, Agilent is bringing more of its test equipment to home in on the problem. Sleigh said, "You shouldn't rely on one instrument to identify and determine root causes for crosstalk. In addition to time-domain waveform analysis using an oscilloscope, other tools such as vector network analyzers and simulation tools such as ADS [Agilent's Advanced Design System software] can provide insight into crosstalk issues early in the design process." Mike Resso, signal integrity measurement specialist in Agi- lent's Component Test Division, added, "Crosstalk is ulti- mately a problem of common-to-differential mode conver- sion. The differential S-parameters of a victim lane indicate how susceptible it will be to electromagnetic radiation within the board, regardless of its source. In fact, not only can the magnitude of crosstalk be determined, but the position where crosstalk is picked up by the lane can be located" (Ref. 6). Performing a complete calculation of crosstalk would re- quire a scattering matrix with two elements for every trace, four for each differential pair of traces. A 10x10-Gbps differ- ential system would require a 40x40 scattering matrix, which would require 1600 S-parameter measurements. No system is yet capable of making these measurements at that scale in an automated fashion. Resso said that Agilent engineers can measure six complete channels, a 24x24 scattering matrix that can account for a victim and up to five neighboring aggres- sors. He added, "The calibration takes about 15 minutes as well as the measurement, and the resulting file is 37 mega- bytes. To do more than 24 ports is definitely feasible, but we would need to think about managing the huge data sets." Test & Measurement World | JULY/AUGUST 2012 | –20– Photo courtesy of of LeCroy.

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