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Measurement of Differential Skew for State of the Art Applications
Keywords: Differential, Skew, Measurement
Within pair differential signal skew is increasingly a concern for link designers. At 1 Gbps the bit unit interval is 1000 ps, but at 10 Gbps it is only 100 ps. Within pair skew on the order of 100 ps is only 0.1 UI at 1 Gbps, but consumes the whole UI at 10 Gbps. This will cause complete conversion of the signal from differential to common mode transmission. Whether the differential transmission line is a pair of coaxial cables, a twin-axial cable or a PCB trace pair, determining the differential skew is necessary for ensuring signal integrity. While traditionally within pair skew has been measured in the time domain, the use of mixed mode S-parameters can shed some light on the effects of skew on signal integrity as well as potential EMI impact. Analysis of even and odd modes of propagation shows the sources of skew and suggests the most effective means of measurement. Sources of differential skew are discussed with examples of the effects on the differential signal. A time domain method is presented that takes into consideration the characteristics of the signals used in the intended application to measure within pair skew in a meaningful way. Comparisons to alternative time domain methods are presented as well as corresponding frequency domain parameters.
James R. Broomall, Senior Singal Integrity Engineer
W.L. Gore & Associates, Inc.
Elkton, MD

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