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July 2016 • The PCB Design Magazine 25 We also desire that the precursor tap should zero out the intersymbol interference at the pre- cursor position. In other words, the signal con- tribution from the precursor tap at the precur- sor tap time (W[-1]V[0]) should cancel out the signal contribution from the main tap at the precursor tap time (W[0]V[-1]). Still keeping in mind that the main tap will be adjusting the amplitude of the main pulse, the desired condi- tion is (EQ3) Solving these ewo equations, (EQ4) (EQ5) Applying these equations to the values shown in Figure 7 produces the result shown in Figure 8. Note in Figure 8 that when the precursor tap response (in green) modifies the original pulse response (in red), the resulting pulse response (in blue) goes through zero at the precursor tap position. Note also that the amplitude of the main pulse has been reduced significantly, and that there was some equaliza- tion at the postcursor tap positions. The calculation of subsequent tap weights follows the same process; however, the equa- tions become much more difficult to solve alge- braically. Note the voltage V'[1] measured at the first postcursor bit position in Figure 8. For the first postcursor tap, the equations are (EQ6) (EQ7) (EQ8) and the resulting pulse response is shown in Figure 9. Figure 8: Pulse response with precursor tap applied. NEW SI TECHNIQUES FOR LARGE SYSTEM PERFORMANCE TUNING