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AUGUST 2021 I DESIGN007 MAGAZINE 25 NRZ's higher Nyquist frequency (the highest frequency that can be coded at a given sampling rate in order to be able to fully reconstruct the signal) results in higher channel-dependent loss, making PAM4 a more viable solution for high-frequency serial communications. Using ultra-low skew twin-ax cable to route signals over the PCB is a key performance enabler as signal range and integrity require- ments continue to become ever more impor- tant in high-speed applications. Fly-over tech- nology provides performance and cost advan- tages compared to lossy PCBs, with up to 112 Gbps performance at 150 mm cable length. Fly-over also allows designers to go from one board to another as a flexible backplane archi- tecture within a rack, as well as being used as a rack-to-rack interconnect. e traditional backplane architecture is retained, but the sys- tem offers lower PCB complexity while achiev- ing higher performance targets. However, with the growth of 5G data traf- fic and AI computing, there is a need for faster connectivity to meet the increasing band- width. Consequently, serial speed beyond 112 Gbps per lane is now required. If we follow the SerDes technology progression, by dou- bling the data rate per lane every two years, the next generation I/O data rate will be 224 Gbps. With the signaling rate increasing, electrical channels like PCB traces or copper cable both have bandwidth limitations over certain reach distances. Alternatively, opti- cal fiber cables can be used to transmit high bandwidth data over both short and long dis- tances. Also, there are new emerging technol- ogies such as chiplet and co-packaging optics (CPO) where most of the channel operates in the optical domain. Key Points • Applications such as PCIe-Gen 4 (16 Gbps) and higher may need high- performance materials for stackup construction. • Fiber weave skew has been dealt with using a variety of physical layout and laminate construction techniques while still using the more common existing PCB materials. Figure 3: NRZ vs. PAM4 encoding.