May 1, 2009 -- As multi-gigabit interfaces, like PCI Express, have continued to advance in frequency for each generation (PCI Expression Generation 2 operates at 5 Gbps), digital designers are being pushed into the RF realm of microwave theory. Once only reserved in the toolbox of RF engineers, S-Parameters are now being widely used on high-speed designs to analyze the passive interconnects in a system. S-Parameters are ideal for digital designers because they can be used as the behavioral model of a network, translated into terms of reflection and loss.

For passive interconnect structures, like transmission lines and vias, as the frequency increases, so does the dependency on loss. Standard methods of analyzing and modeling these types of structures are based upon certain assumptions of Maxwell’s equations that no longer hold true when considering a wide range of frequency dependent phenomena.

The advantage of using S-Parameters is that they describe how the structure behaves in the frequency domain, since they contain data taken across a wide frequency range. Understanding how to properly generate S-Parameter data and use the data to represent a structure in simulations will provide designers with a new way to tackle high-speed designs.

By Timothy Coyle. (Coyle is principal consultant and owner at LLC.)