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Explore new approaches to high speed signal design

Posted: 07 Oct 2015 ?? ?Print Version ?Bookmark and Share

Keywords:printed circuit board? PCB? fabrication? electromagnetism?

Today, printed circuit board design engineers face challenges that, not so long ago, would have only been faced by scientists designing the very latest super computers: speed. Specifically, faster data rates, higher clock speeds and steeper rise/fall edges are making it more difficult to develop PCBs that meet today's proposed specifications. With wired signal speeds pushing into the GigaHertz range, real-world effects such as design variation in the PCB fabrication process can have an unknown impact on propagation delays.

The industry isn't blind to this challenge, and there are numerous solutions being proposed, particularly in the area of jitter and skew on Gbit/s differential signalsthe 'sweet spot' of high speed design. Where there's a problem there is always an opportunity, which can unfortunately also introduce confusion. Of course, the best solution will likely prevail eventually, but can you risk choosing the wrong solution for your next design?

Integrity concerns
For many designs, PCB engineers may not be concerned or even aware of the variations that exist within the laminate process used to build the PCB substrate, much less the potential effect those variations may have on signal integrity. But as speeds increase, it is those variations that can create integrity issues in the shape of skew and jitter.

The problem exists because of the first principles of electromagnetism, and their fundamental impact on transmission signals. The properties of the materials used to construct PCBs combine to create inductive and capacitive elements; the very core of transmission theory and, ultimately, the physical properties that underpin all electronic systems. It is little wonder, then, that the unchecked distribution of unwanted inductors and capacitors across a PCB carrying high speed signals has a detrimental impact on performance.

For differential signals the problem manifests itself as skew between the two signals at the receive end, caused by variations between the two signal paths, introduced by the interplay of glass, resin and copper in the substrate. As speeds increase, the skew becomes more pronounced until, ultimately, signal integrity is lost. Reducing skew and jitter can be addressed in a number of ways, however, and now is the time for engineers to become more familiar with both the problem and those potential solutions.

Very high speed protocols, such as XAUI, Hypertransport, PCI Express, Infiniband, SATA, SSCSI and others are now so prolific that there may soon be no modern design that doesn't make use of one or more in some way. Increasingly, component manufacturers are turning to high speed protocols such as these in order to meet demand for higher performance, meaning design team will no longer have the option not to implement one or more of them in their designs.

Laminates and proximity
It may come as a revelation for some that the materials used to make the substrate can vary, which introduces the need to select the right materials for a given design. For high speed design, a glass weave of 1080 glass is often chosen, almost by 'default'. However, the nature of this option can introduce inadvertent stray capacitance and inductance, simply by the way the copper track is distributed; the orientation and proximity of the copper to the glass fibres results in variations in the dielectric constant of a track. Understanding this effect in detail can significantly improve the integrity of a circuit's layout.

However, to really address the problem, engineers must take that understanding and apply it, employing both design tools and manufacturing technologies appropriate to their specific design. For example, selecting a different glass weave with an increased thread density or different thread profile will result in different results; whether those results are beneficial would require greater investigation and understanding, of course.

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