EE Web – 112G Digs in at DesignCon 2019

112G serial links have moved out of the lab and onto the exhibit floor. 56G ramps up while 28G goes mainstream.

By Martin Rowe, EE Web, 

Last year at DesignCon 2018, we witnessed high-speed digital designs that moved past 56 Gbits/s (56G) and onto 112 Gbits/s (112G). This year, DesignCon 2019 brought numerous demonstrations of 112G as the connectors and cables caught up with the silicon. While still appearing in technical papers and panels, 112G has certainly moved into the exhibit hall. Meanwhile, 56G has matured and is now a complete ecosystem.

When it comes to signal integrity and high-speed signals, transmission lengths certainly matter, especially with electrical signals over copper connections. Yes, optical transmission is an option, but nobody wants to pay for it. At a panel session on Jan. 31, OIF board president Nathan Tracy presented the table shown in Figure 1 that describes five OIF standards for different electrical transmission lengths.

Figure 1: The Optical Internetworking Forum has created standards for 112-Gbit/s copper connections. (Source: Optical Internetworking Forum and DesignCon)
Given a rule of thumb of 0.1 dB/in./GHz of insertion loss (Figure 2), PCB traces of, say, 10 inches are simply too long. That’s where cables that jump over PCBs have become popular.

Figure 2: Cable assemblies jump over PCBs to reduce insertion loss. (Source: Optical Internetworking Forum, Broadcom, and DesignCon)
Connectors for 112G took several forms at DesignCon, depending on the length of the transmission. For chip-to-chip or chip-to-module distances, sending 112G four-level pulse-amplitude modulation (PAM4) signals (28 GHz) over PCB traces results in excessive insertion loss. To get past that restriction, several connector companies have developed cable assemblies that jump over sections of boards.

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