Bob Hult, our friend at Bishop and Associates and Connector Supplier, recently published High Speed Signals Take The High Road on the Connector Supplier website. Among other things, the article discusses the challenges of high-speed signals in printed circuit boards.
As always, Bob’s article is well-researched and worth the read.
But if you only have time for the Cliff’s Notes, I’ll try to summarize it here.
Bob begins by pointing out that “as data rates pushed into RF regions, signal modeling transitioned from simple DC resistance to transmission lines that exhibit a whole new set of characteristics. Factors including impedance, attenuation, crosstalk, skew, jitter, intersymbol interference, and reflections all influence required signal integrity of the channel and must be managed.”
Bob notes that PCB materials and technology are improving, which of course boosts bandwidth performance. Examples cited include lower losses thanks to reduced dielectric constant and dissipation factor, PCB thickness control, CTE, and moisture absorption.
Nonetheless, as PCBs continue to increase in complexity, “system architects are now considering an alternate approach that reduces the use of PCB traces to conduct high-speed signals.”
The article then elaborates on that approach, what Samtec termed “Flyover™.” This approach moves signals off of their traditional pathways—copper traces on boards—and they literally “fly over” the board using twinax cables and specially developed connectors to relay the signals.
In a typical flyover application, the cable goes from the microprocessors/ASIC/FPGA directly to the QSFP, or other I/O connector.
Bob points out that “As signal speeds increase, the practical length of a PCB-etched conductor becomes shorter. For instance, at 56Gb/s, signal degradation can become unacceptable in channels as short as 10”. Use of the flyover concept may well become the most cost-effective solution at lengths much beyond that point.”
And since this is a Samtec blog, here are some other links about Flyover that may be of interest to you: