As speeds reach 25 Gbit/s, wired backplanes may replace etched PCBs.
In one of my DesignCon presentations, I showed what I self-proclaimed as "The Most Important Slide You Will See at DesignCon." I described in a very "graphical" way, the limitations with conventional backplanes using copper and laminates, and showed by comparison, how important it will be to break out of our "copper" backplane rut, and how to do it.
One of my most popular events at DesignCon 2013 was “Speed Training, a Potpourri of SI Puzzlers.” If you missed this talk, you can download a copy of the slides from my website. Among the topics I covered was forensic analysis using S-parameters.
I showed the comparison of the measured insertion loss of three different interconnects. Two of them were 24 inches long backplane channels, one using FR4, and one with Megtron6. On the same scale, it's clear that Megtron6, with its lower dissipation factor, offers lower loss than FR4 material. The difference, however, is less than an order of magnitude. Even if you had an air for the dielectric, you will still have the copper losses.
The use of conventional backplanes requires equalization techniques for data rates above 10Gbit/s no matter what laminate materials used. There is a limit to how high you can compensate for an attenuation leveraging even the best equalization. And with equalization comes the higher power consumption to fuel the equalization circuitry. Getting to 28Gbit/s will be tough. What to do?
The third interconnect I showed was the measured insertion loss of a 36 inch good-quality coax cable. Figure 1 compares the measured insertion loss of these three interconnects. It clearly shows the advantage of interconnects that look like coax cables over interconnects that look like traces on a board.
Figure 1. Measured insertion losses of three interconnects, taken from a slide in my speed training talk. The complete talk can be downloaded from BeTheSignal.
This article originally appeared on The Connecting Edge.
— Eric Bogatin is Signal Integrity Evangelist, Teledyne LeCroy and writes BeTheSignal blog on EE Times