Issue link: https://iconnect007.uberflip.com/i/1400005
24 DESIGN007 MAGAZINE I AUGUST 2021 and beyond require a rethink (Table 1). Or, in the case of Samtec's solution (Figure 1), a com- plete rerouting of the signals using Flyover TM cables that remove the signals off the back- plane entirely, from chip to off-system connec- tor. ey have developed cables and connec- tors specifically to handle high-speed signals that can pass above and across the board. Using cables for 56 Gbps signals cuts losses by about half compared to PCB traces. is saves PCB cost by reducing the layer count, the material and specification requirements while adding flexibility to high-speed interconnects. e role of fly-over cables is to isolate sig- nals from the limitations of the PCB materi- als. As signal speeds increase, the dielectric material's Df and Dk become an issue, and traces need to be shaped and routed perfectly, without skew, to avoid signal coupling, crosstalk, and electromagnetic compliance (EMC) issues. Fly-over cables are ideal for 28, 56 and 112 Gbps data rate serial links. Recently, the shi from non- return-to-zero (NRZ) to pulse amplitude mod- ulation 4-level (PAM4) encoding for leading- edge server backplanes has made it extremely difficult to meet jitter and noise requirements over any useful length of board, despite enor- mous advances in channel characterization and equalization. NRZ is a modulation technique that has two voltage levels to represent logic 0 and logic 1. While PAM4 uses four voltage levels to represent four combinations of 2-bit logic–11, 10, 01, and 00 (Figures 2 and 3). e goal of these protocols is to transmit data efficiently over co-ax, fiber, or PCB inter- connects, but each uses a different method and has its benefits and drawbacks. e well-estab- lished NRZ is good for short dis- tance runs, has a throughput of 1 bit per unit interval (UI), min- imizes current change, and has a signal and noise ratio (SNR) of 0 dB. On the other hand, PAM4 can transport twice the signal of NRZ (throughput of 2 bits per UI) because it operates on four levels (Figure 3). But this makes reflections three times worse than NRZ, resulting in an SNR of -9.54 dB. Unfortu- nately, lower-loss cables do not dampen reflections as well as those with higher loss. e increased reflections raise the noise floor, which is critical to PAM4 encoding, especially on shorter cable lengths. However, Table 1: Maximum trace length (mm) vs. bandwidth (Gbps). Figure 2: NRZ eye vs. PAM4 eye (Source: Xilinx).