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52 The PCB Design Magazine • March 2014 out tool. In this case, each address signal has four layer changes. The red and green traces are the top and bottom layers—which should be kept as short as possible—and the yellow and orange traces are inner layers embedded be- tween the planes. This was a particularly dif- ficult route as there were two DDR2 memory chips placed on both the top and bottom sides of the board, so each address signal had to go to four different chips and still maintain the cor- rect delay. The longest routes should be placed on the inner layers as this reduces electromagnetic ra- diation. With all other factors being equal, gen- erally, a trace routed on the inner stripline layer exhibits 4–10 dB less noise than a trace routed on the outer microstrip layer. Also, please note that there are more high harmonics on the top layer routing. The high-frequency components radiate more readily because their shorter wave- lengths are comparable to trace lengths, which act as antennas. Consequently, although the amplitude of the harmonic frequency compo- nents decreases as the frequency increases, the radiated frequency varies depending on the trace's characteristics. beyond design Matched Length Matched Delay continues Figure 2: electric (blue) and magnetic (red) field lines radiation from microstrip and stripline layers.