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The eyes have it

High speed serial communications links need to be designed and tested carefully. By Graham Pitcher.


It wasn’t long ago that data communications was a relatively simple design task. Today, all that has changed. To meet the demand for higher and higher data rates, designers are moving to serial data links. Along the way, they are having to deal with some fundamental physical issues.
Yves Braem is a signal integrity engineer with Tyco Electronics’ circuit and design department (www.tyco.com). He said: “With higher data rates, the problems you encounter become more and more significant. As frequency increases, transition times get smaller and the electrical waveform ‘sees’ the transmission path in ever more detail. This means you have to design carefully.”
In Braem’s opinion, high speed data links bring an increasing number of signal integrity problems as data rates move beyond 2.5Gbit/s. “Designers not only have to select reliable and first class components and interconnections, they also have to consider the interfaces between the different ‘building blocks’ in the system.” In his view, these interfaces used to be electrically short because the signal wavelength was much bigger than the component itself. “Now,” he continued, “the interface itself can jeopardise the whole interconnection quality.”
He believes three aspects need to be considered when designing high speed backplanes: pcb traces; connector/board interfaces; and board to board connectors. “The throughput of signal traces in pcbs is limited by trace losses, consisting of skin effect and dielectric losses,” he noted. These losses combine to limit the total trace length in the system and, therefore, system size. Dielectric loss can be reduced by specifying low loss pcb materials, whilst skin effect losses are reduced through the use of wider traces. But designers may not be able to specify wider because of space constraints and other factors.

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Vanessa Knivett

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