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July 2016 • The PCB Design Magazine 15 NEW SI TECHNIQUES FOR LARGE SYSTEM PERFORMANCE TUNING cally include additional route length, marked in Figure 1 as an "Extension" to the "Original Channel" at left. This additional length raises the question of the need for re-timers (green, at center) when combined with older SerDes (blue, at left). While the Tx in newer SerDes adds some new features, more significant changes have been made in Rx technology as CTLE (con- tinuous time linear equalization) and DFE (decision feedback equalization) implementa- tions continue to improve. As a result, it can be shown that the upper signal path in Figure 1 (old Tx driving new Rx) functions acceptably with the additional route length rendering the re-timers unnecessary in position "A." More challenging is the lower signal path (new Tx driving old Rx), thus reducing the design problem to assessing the need for the re-timer in position "B." Since older technology is involved, real-time auto-negotiation of Tx settings in the physical system is not an option and other forms of chan- nel optimization must be found. A design-time methodology, more fully described in the next section of this paper, is utilized to derive opti- mal performance given the available configura- tions and setting options in the SerDes involved. Specifically, the design task is to determine if the FFE (feed forward equalization) taps in the newer Tx can present an acceptable signal to the older Rx without the use of the re-timer in position "B." This analysis is performed using the system model previously described in [7,8] . The first, and most challenging scenario examined was the largest system which in- cluded thousands of serial links interconnected across dozens of PCBs. The plot at left in Fig- ure 2 shows simulated eye heights and widths for ~1,000 channels implemented without a re- timer in position "B" versus design targets (hor- izontal and vertical markers). For this analysis, the channels were "coded" with best-known SerDes configuration settings. As shown, chan- nels without the "extension" (red) perform ac- ceptably against the targets, while some chan- nels with the extension (green, with highlights in gold and black) are failing. Highlighted chan- nels (gold and black) below and near the eye height target are those with the lengths up to 25% longer than initial budgets. Failures of such a linear nature are typically related to ad- ditional length and loss. To address the failing channels, an initial implementation of channel optimization algo- rithms was tested with the results shown at right in Figure 2 (same color schemes). As shown, the performance of all channels exceeds design tar- gets with more than 60% improvement in eye height and width for the worst-case channels. Performance comparisons revealed improve- ment in more than 85% of channels using these initial optimizations, motivating further refine- ments in the algorithms. Most importantly, Tx Figure 2: Simulated results, best-known settings (left), optimized settings (right).