Developing Copper Cable Assemblies for High-Speed Backplane Connector Systems

A t almost any data rate, system designers would like to make cable assemblies transparent and have the freedom to disregard all the "fun" effects such as ISI, crosstalk, reflections, EMI, and the overall signal loss introduced by long assemblies. Designers have long been battling these effects and have wished many times that "if we could only get out what we put in, then we could spread the budget out where it’s needed." Unfortunately, the laws of physics still prevail and work against the reality of a transparent cable assembly. Although there have been improvements in IC design to compensate for these effects, the battle has moved from 2.5 to 5 Gbs and now rages on at 10 Gbs, and the need for a complete cable link designed for high-speed is even more critical. Transmitting data at 10 Gbs not only requires a well designed cable that minimizes loss, but also in the case of backplane applications, a high-density connector system capable of minimizing reflections and crosstalk effects. Fortunately, the use of low-loss, lowskew, differential cable along with the industry’s leading backplane connector families is leading the way in meeting the needs of transmitting high-speed signals. Even before Moore’s Law was established as a data rate benchmark, system designers faced the ever increasing bandwidthdistance-density challenges of interconnecting shelves and cabinets within a system. The number of channels and size and length of the cable bundle all compete with the available real estate to limit the maximum data rate. At 5 to 10 Gbs, system risetimes are extremely fast and the challenges to transmit multiple signals over long distances are compounded by increased skin effect losses, increased crosstalk, and increased noise sensitivity. Designers typically choose to transmit differential signals over high-speed twinax cable to minimize noise and take advantage of lower voltage swings. Twinax cables constructed with a low dielectric constant material such as expanded polytetrafluroethylene (ePTFE) exhibit extremely low loss and can be designed to have excellent high-bandwidth capability. In addition to loss, high-speed cables are designed to have controlled impedance profiles down the cable length with negligible variation to ensure maximum signal transfer. Maintaining constant cable geometry and uniform material characteristics not only ensures consistent impedance but also minimizes differential skew, which left uncontrolled can generDeveloping Copper Cable Assemblies for High-Speed Backplane Connector Systems