Flyover cables vs PCB tracks

30 August 2023 Interconnection

Samtec’s Flyover cable systems are designed to get signals off the PCB to improve signal integrity, increase design flexibility, and optimise thermal performance. These products route high-speed signals through low-loss, low-skew twinax cable. They operate across distances that can be up to several metres long, while removing the need for clock data recovery circuits (CDRs) or re-timers.

Figure 1. Samtec Flyover cable system.

In a recently published white paper, Samtec Senior Field Applications Engineer, Anthony Fellbaum, provides measured data and examples to explain why a cable solution can be better in a high-speed design than a PCB trace. It also addresses concerns such as cable management and cost.

In the published analysis, Samtec’s Flyover cable demonstrated significantly lower loss than and PCB grade material. Figure 1 shows a comparison of the magnitude (in dB) of insertion loss for three typical PCB materials and two gauges (30 and 34 AWG) of Samtec Flyover cable. For easier scaling and comparison, a 10-inch length of cable was used.

The figure shows that for a high-speed signal running at 14 GHz, the traditional PCB material (shown in yellow) is very lossy. As can be seen, even the best PCB performance is well below the Samtec low-skew high-speed twinax cable (30 AWG shown in green, 34 AWG shown in blue).

Specifically, a 30 AWG cable demonstrated an insertion loss of 1,21 dB for 56 Gbps PAM4, or -1,8 dB for 112 Gbps PAM4. The 34 AWG cable exhibited an insertion loss of -1,9 or -2,9 dB for the same data throughput.

Also shown in figure 1 is the affect that temperature has on the stability of the signals. PCB materials have a significant drop in performance over temperature. By comparison, the twinax cable has a very stable performance, which is thanks to the low moisture absorption properties of the dielectric used in the cable.

The full white paper can be freely accessed in the Samtec technical library:


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