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Ramping up the bandwidth

Posted: 22 Oct 2015 ?? ?Print Version ?Bookmark and Share

Keywords:data transfer? insertion loss? PTFE? cable? PCBs?

While everyone agrees that one of the most pressing needs in the technology space is "How do we get to next-generation high-speed data transfer rates?", there are differing opinions as to how to accomplish this. There are even differing opinions regarding where we currently are in this process. Some companies claim that they are struggling just to get to 28 Gbps products, others say they are comfortable with their 28 Gbps technology solutions, while still others claim they have left 28 Gbps in the dust and are (data) streaming along at 56Gbps. While there may not be an exact concurrence as to where we as a hardware industry stand relative to high-speed data transfer rates, there are some givens.

The first given is that even if we are successfully achieving information transfer rates of 28 Gbps, as an industry we have to accept that even with the best materials available today we can just barely get to 56 Gbps, which is the next level on the data transfer rate ladder.

For my own edification, I did insertion loss plots for a typical long-reach backplane with various materials, including PTFE (Teflon), which is about the best material we could ever hope to use for PCBs (figure 1). However, PTFE is so cost-prohibitive that it is not a viable solution for either the near or the long term of future generations of commercial hardware. The reality is that we have come a long way from FR-4 laminates to where we are now with far more sophisticated materials such as Isola's Tachyon 100G laminate. Materials such as Tachyon 100G have gotten us this far to 28 Gbps and will likely get us to 56 Gbps for short and mid-reach systems. But after that, we will have hit the limit as to what we can reasonably expect to deliver with products that require even faster information transfer rates.

Figure 1: PCB Material Loss in dB/inch for typical backplane traces.

The second given is that we cannot grow bandwidth without optics. Optical systems have nearly unlimited bandwidth, but the pure and simple matter is it is difficult, if not at times nearly impossible, to put the number of optical connections required on a printed circuit board to replace the aggregate bandwidth of which copper traces are capable. Embedded silicon photonics may be the answer for the future, but with silicon photonics everything mattersthe materials, the way engineers design boards and the way in which these boards are fabricated.

In about 20 years, I think we will have silicon photonics printed circuit boards in volume production, but probably not much sooner than that. And, as noted above, moving to silicon photonics is not an easy processeverything will have to change. What we as an industry have right now is a printed circuit board infrastructure that is paid forall the machines, all of the equipment, all of the materials and all of the manufacturability. Printed circuit boards with copper are really, really cheap. Optics currently are not.

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