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Unbalanced twisted pairs can give you the jitters!

Posted: 17 Feb 2009 ?? ?Print Version ?Bookmark and Share

Keywords:serial video digital? test method? cables differential?

The transmission of serial-digital video above 1Gbit/s (as required by the DVI, HDMI and DisplayPort video-interface standards) has elevated the performance requirements for the cables that connect PC and HDTV monitors. Consequently, the traditional suppliers of analog A/Vcable must now learn the same lessons that makers of datacom serial-digital differential cables learned for InfiniBand and PCIe at 2.5Gbit/s, CX4 at 3.125Gbit/s, and Fibre Channel at 4.25Gbit/s.

This article highlights the phenomenon of data jitter caused by conversions between differential- and common-mode components of the video signal. It also exposes myths about intrapair skew, and proposes cable tests for the purpose of predicting jitter. The article demonstrates that differential cables with good performance need not be expensive, just well balanced. The most common type of differential cable for digital-video signaling in the 0.25Gbit/s to 3.40Gbit/s range required by DVI/HDMI systems is 100W shielded twisted pair. 100 twin-axial (twinax) cable is an alternative and is a mainstay of datacom applications.

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