New-Tech Europe | November 2016 | Digital edition

is directly traced to microdefects in the PCBs and connectors used to manufacture very large, high performance systems, removing these signals from those PCBs and backplanes can solve the problem. This is not a new idea. If one looks back to several of the high performance computers designed by Cray Research and other companies in that market, all of the very high speed signals were moved from PCB to PCB over shielded twisted pairs or, in some cases, unshielded twisted pairs. This latter technique is how the Ethernet has been able to operate over long distances using ordinary phone wiring at data rates as high as 1 Gb/s. The first advantage of the cable method is the opportunity for crosstalk between signals to be eliminated. A second advantage the cable method has is the backplane can now be manufactured from standard PCB laminate material as its only task is to carry power to the modules plugged into it and to hold all of the connectors in a rigid structure. What about the problem caused by those plated through holes that are necessary to hold the connector pins in place as well as the plated through holes required to connect component pins to traces in the daughter cards? What has been demonstrated by simulations as well as by laboratory measurement is that when a signal travels the length of the plated through hole or via, the parasitic

capacitance of the hole is distributed along the length of the hole, rendering it virtually invisible. This leaves the task of tackling skew. When the differential pairs are connected with shielded pairs, such as twinax, the two sides of the differential pair travel in a very uniform dielectric that is common to both sides of the pair. The result is that skew or difference in travel time between the two sides of a pair can be made virtually zero. Figure 5 is an example of a design that uses this twinax method to implement a very complex high performance switch/router. Using this method, performance as high as 56 Gb/s can be achieved using ordinary PCB materials. This avoids the problem of designing very complex PCBs and then managing the supply chain process to insure that all of the complex manufacturing problems are kept under control. Figure 6 shows the loss vs. frequency of several 2.6m - 3m differential paths. The measured paths include two daughter cards connected through twinax cable as shown in Figure 5. The insertion loss is near ideal up to about 25 GHz. This demonstrates that this assembly is potentially capable of 50 Gb/s. A further advantage of implementing all of the high performance signals in twinax cable is that it is possible to make wiring changes at the backplane level by reconfiguring the twinax cables to implement a

new function not available when the original backplane design is done. This helps eliminate the “fork lift” upgrades often required with hard wired backplanes. Conclusion Advances in semiconductor technology are making it possible to connect components in products such as switches and routers at rates as high as 56 Gb/s. As these higher speeds are achieved, micro-scale variations in the materials used to fabricate PCBs and backplanes can significantly degrade signals. Among the problems encountered are loss, skew, crosstalk, and degradation due to the parasitic capacitance of the plated-though holes required to mount the connectors to the backplanes and daughter cards. By using twinax cables to make these connections instead of implementing them in PCBs and backplanes with traditional traces, skew, crosstalk, and degradation from the plated-though holes can be virtually eliminated. Due to the ultra-low loss of the twinax cables, path lengths can be longer, or the frequency of operation can extend much higher than is possible with the laminate systems currently available. Authors Ritchey & Knack are with Speeding Edge, McMorrow with Samtec division Teraspeed Consulting.

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