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24 The PCB Design Magazine • July 2016 sect the pulse response while the vertical mark- ers have remained one UI apart. 5. The recovered clock time is half way be- tween the two vertical markers. This recovered clock time and times before and after that are an integer number of UI away are the times at which the intersymbol interference is to be evaluated. 3.3 Minimizing Intersymbol Interference Given the recovered clock time, the next step is to choose FFE tap weights that minimize the intersymbol interference. Note that chang- ing the FFE tap weights can affect the recovered clock time; so given the FFE tap weights that minimize intersymbol interference, it may be desirable to recover the clock time again and then fine tune the FFE tap weights. Figure 7 shows the same pulse response as in Figure 6, indicating voltage samples that are spaced an integer number of UI apart. The desired sample is V[0] and the remaining samples are intersym- bol interference. Since in a real data signal the adjacent bits can be either "zero" or "one," it's the absolute values of the intersymbol interference samples that matters. Assuming that the data signal is a balanced NRZ signal, the corresponding signal voltages are -0.5 and +0.5. The minimum eye height is then (EQ1) One of the most important characteristics of FFE is that the effect of an individual tap per- sists much longer than the time at which the tap was applied. Thus, when applying the man- ual procedure, it's important to apply the earlier FFE taps before the later ones because each tap affects the impact of subsequent taps. In the current example, the first tap is a sin- gle precursor tap. We will therefore determine its tap weight before addressing the postcursor taps. Suppose that the tap weight for the precur- sor tap is W[-1] and the tap weight for the main tap is W[0]. We have assumed that the sum of the absolute values of all the taps is one. Fur- thermore, we are assuming that the main tap is positive, and we can observe that the precursor tap is going to be negative. Therefore the tap weight constraint is (EQ2) Figure 7: Intersymbol interference samples. NEW SI TECHNIQUES FOR LARGE SYSTEM PERFORMANCE TUNING