New-Tech Europe | November 2016 | Digital edition

than 2 ps. This sounds like the perfect answer to the skew problem. It would be if the methods by which the glass is spread were uniform between glass weavers. Unfortunately, this has proven not to be true. The driver for spreading the glass weave is the cell phone manufacturers. Their reason for spreading the glass is to make laser drilling of blind vias more uniform, not to preserve signal integrity. At present, there is no sure way to guarantee uniform spreading of the glass, so controlling skew by manipulating PCB laminates is problematic. Dealing with signal-path loss As mentioned at the start of this article, advances in semiconductor technology have resulted in transceivers that can tolerate as much as 38 dB of loss in the signal path at 32 Gb/s. This has made it possible to design systems with large backplanes with plug in modules. When the move to 56 Gb/s is made the materials that are available as laminates no longer have loss values that allow the design of the very large routers required in server farms and large IT centers. Figure 4 is the loss vs. frequency for a variety of materials that might be used to create next generation high performance Internet products. Notice that the two curves labeled “cable” have far lower loss than any of the laminate systems used to manufacture current products. This loss is representative of what twinax cable can achieve. This solves the problem of how to achieve 56 Gb/s in large systems without the need to resort to optical interconnects. More reliable & economical than PCB laminates Since the signal integrity problem at high data rates in large systems

Figure 5. Backplane using twinax cabling in place of PCB traces

Figure 6. Differential cable insertion loss, 2.6-3m

on the fabrication panels which are rectangles and thus increases overall board cost. Neither of these methods are satisfactory for most products which must compete in a price-sensitive market. The second method employs a glass weave style that has the glass uniformly spread across the surface.

Figure 3 is a photo of 1067 glass weave cloth with a 4 mil (102 micron) trace running across it. As can be seen, there are no resin filled voids in the example. Because of this, the difference in travel time of the two sides of the differential pair is minimized. In fact, we have been able to build quite large systems where the skew over 30 inches (76 cm) is less

44 l New-Tech Magazine Europe