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22 The PCB Design Magazine • July 2016 making the two mechanisms almost inter- changeable. The low frequency gain of a given CTLE configuration determines how desirable that configuration is compared to a similar re- sult obtained using FFE. • FFE vs. DFE trade-offs: In some ways, the effect of DFE can be very similar to the effect of FFE; however, there are also some important differences that affect the way the two mecha- nisms should be used together. 3.2 Clock Recovery An examination of the threshold crossing time distribution due to intersymbol interfer- ence for a number of different examples led to the conclusion that these threshold crossing time distributions are always symmetrical, or nearly so. This makes sense in that the effect of an intersymbol interference contributor on the transition time of a data edge is reasonably close to linear. That is, if a given intersymbol interference contributor delays the data transi- tion by a specific amount when the interfering bit is a "one," then the data transition will be advanced by almost exactly the same amount when the interfering bit is a "zero." Therefore the effect of all the interfering bits balances out (or nearly so) and both the median and mean transition times are deter- mined by the primary transition from a "zero" in the preceding bit to a "one" in the bit to be detected, and back to a "zero" in the bit fol- lowing the bit to be detected. In other words, the recovered clock timing will be half way be- tween the transition times for a 010 data pat- tern in isolation. Equivalently, we can examine the pulse re- sponse, looking for two non-zero values which are exactly one bit time apart and equal to each other. The recovered clock will be half way be- tween those two samples. We call this algorithm the "hula hoop" algorithm because if one imag- ines the pulse response to be a solid object (such as a length of bent wire) and one were to drop a ring with one UI diameter (the "hula hoop") on that object and level it, the center of the ring would be at the recovered clock time. Figure 6 illustrates the procedure as applied in a waveform viewer. This procedure only takes a minute or two, and is quite precise. 1. The user starts by placing two vertical markers exactly one UI apart in a position that straddles the main pulse. 2. The user places a horizontal marker that is centered between the points where the vertical markers intersect the pulse response. 3. The user shifts both vertical markers to approximately the intersection of the horizon- tal marker with the pulse response, while keep- ing them exactly one UI apart. 4. Steps 2 and 3 are repeated until both the vertical markers and horizontal marker inter- Figure 6: Hula hoop algorithm as implemented in a waveform viewer. NEW SI TECHNIQUES FOR LARGE SYSTEM PERFORMANCE TUNING