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54 The PCB Design Magazine • March 2014 to be employed close to the layer transitions. Decoupling capacitors are normally place near the processor and memory devices which help alleviate this issue. To prove my point that two matched length traces both of 2,900 mil do not have equal de- lay, I have simulated two address signals and the results are shown in Figure 4. The delay difference here is 60ps—but still within spec. This is not much admittedly, but they are not matched, as the matched length would suggest. So, although these two signals are exactly the same length, because of the different combina- tion of microstrip and stripline layers plus the fact that the signal propagates faster through serpentine routes, the delays are different. The slight deviation in the signal wave- forms, in Figure 4, is due to the impedance mismatch as the traces separate into two T-sec- tions. Matching the impedance of bifurcated traces is practically impossible with tight rout- ing constraints. However, this is not an issue in this case, since the waveform is stable and the variance does not appear near the trigger point (Vinh). Points to Remember • Matched length does not necessarily mean matched delay. • The longest routes should be placed on the inner layers, as this reduces electromagnetic ra- diation. beyond design Figure 4: Comparison of two matched length (2,900 mil) traces. Matched Length Matched Delay continues