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26 The PCB Design Magazine • July 2016 Several observations can be made from Fig- ure 9: 1. One could improve the equalization solu- tion by increasing the magnitude of the post- cursor tap by a little bit. Note that when doing so, the other two taps would have to be adjusted as well, both to maintain tap weight normaliza- tion and to maintain good equalization at the precursor tap position. 2. The amplitude of the main pulse has been reduced by approximately a factor of two. This is one of the factors to consider when choosing FFE taps: equalization costs amplitude, reducing the energy delivered to the receiver. 3. The intersymbol interference at later sym- bols has been all but eliminated. This is espe- cially true for a well behaved pulse response as shown in this example; however, the same phenomenon occurs for less well behaved pulse responses. In general, one FFE tap affects the in- tersymbol interference at multiple bit positions. Equation 1 states that the minimum eye height is the main pulse response amplitude mi- nus the sum of all the intersymbol interference amplitudes. FFE derives its effectiveness from the fact that even though it reduces the main pulse amplitude, it also reduces the intersymbol interference amplitude at many positions, thus gaining leverage from the amplitude that was invested in equalization. The general form of Equation 6 through Equa- tion 8 can be applied to estimate additional tap weights. However, for best results those tap weights should be adjusted manually after the equations have been used to provide the initial estimate. The overall procedure can be completed in less than an hour—practical for a few test cases but not for equalizing thousands of channels in a system. 3.4 Intersymbol Interference vs. Eye Height While minimizing intersymbol interference goes a long way toward improving channel per- formance, intersymbol interference as a perfor- mance measure is not the same as eye height or eye width. Furthermore, the optimum configu- ration for one performance measure will proba- bly not be exactly the optimum for another per- formance measure. Rather, the optimum con- figuration will be a function of the performance measure that was chosen. This section will con- centrate on eye height because it's almost always harder to maximize than eye width. In the case of the FFE tap weight calculations described above, the weight given to the equal- izing taps subtracts from the main pulse, and therefore subtracts from the eye height. There- fore backing off slightly from the tap weights Figure 9: Pulse response equalized by precursor tap plus postcursor tap. NEW SI TECHNIQUES FOR LARGE SYSTEM PERFORMANCE TUNING