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The Successful Design of High-Speed Serial Backplanes
April 15, 2009 |Estimated reading time: 2 minutes
Designers accustomed to creating serial backplanes operating at or above 1 Gb/s face a host of challenges and obstacles that at lower frequencies can be ignored.
The visual cue of "G" as in Gb/s or GHz doesn't signal "Gee, now what am I to do?" No, this is the microwave domain, where all components interact and a designer's choice to ignore these interactions will likely result in a defunct design.
Fret not. The design flow for a 1 Gb/s high-speed (high data-rate) backplane is much the same as that of its lower-frequency counterparts. However, an expanded simulation arsenal is now required; the channel performance must be verified through microwave simulation well before the design is committed to manufacturing.
This verification includes:
- Time-domain analysis for evaluating transient effects on the source signal and its arrival at the receiver, and
- Frequency domain analysis that shows how the interconnect channel responds over a wide range of frequencies.
So, what are some of these microwave eccentricities that defy accommodation by EDA PCB tools designed for lower frequency circuits? Or rather, why should I use a traditional microwave design tool for the successful design of high-speed serial backplanes?
There are many reasons, but one - dispersive interconnects - illustrates why high-frequency electromagnetic simulation is essential for high-speed circuits. Up to a few hundred megahertz, the current on an IC, package, or PCB metal interconnect behaves itself. Its distribution across the conductor's cross-section remains constant, making it very easy to model short coupled lines with resistors and capacitors. Make the lines a little longer and this modeling approach can be extended to inductors as well. For standard EDA tools, we're OK. So far.
However, at higher frequencies, wire loss is no longer constant, and the current, rather than being evenly distributed throughout the wire, begins to crowd toward the surface at a frequency-dependent rate.
Coupled with dielectric losses that vary with frequency, this results in nondispersive interconnect properties and the breakdown of lumped element techniques. Maxwell's equations, the physics of signal propagation, simplify to make RLC applicable at lower frequencies, but get complicated at higher frequencies.
RF and microwave designers are well versed in this. Over the years, they have perfected dispersive transmission line models such as microstrip, stripline, and coplanar waveguide that incorporate the frequency-dependent characteristics of the lines in a single model that covers DC to hundreds of megahertz. These capabilities, which microwave engineers take for granted, are essential ingredients in creating high-speed serial backplanes that work.
If I've piqued your interest thus far, click here to read a great Xilinx white paper on the topic. It explores the key elements to successful high-speed serial backplane design as detailed by Bill Dempsey of Red Wire Enterprises. He designed a backplane and daughter card for the Xilinx Virtex 4 evaluation kits.
The paper explores both simulated (frequency and time) and measured examples with the goal of providing valuable information aimed at helping designers sort out issues concerning interconnects, connectors, PCB materials, simulation, measurements, and the inevitable trade-offs required to achieve the desired "G" performance results.
Dr. Michael C. Heimlich is director of technical marketing for AWR Corporation. He can be reached at mike.heimlich@awrcorp.com.