Issue link: https://iconnect007.uberflip.com/i/1481368
28 DESIGN007 MAGAZINE I OCTOBER 2022 e electric fields surrounding the microstrip exist partially within the dielectric material(s) and partially within the surrounding air. Since air has a dielectric constant (Dk) of one, it will speed up the signal propagation compared to the stripline. Even if the trace widths are adjusted on each layer, so as the impedance is identical, the propagation speed of microstrip is always faster than stripline—typically by 13-17%. e speed of propagation of digital signals is independent of trace geometry and impedance. If you are aware of this issue, then the flight time (as shown in Figure 2) can be matched to compensate for the varying trace delays, so that at the nominal temperature, all signals running on either microstrip or stripline will arrive at the receiver simultaneously. Alterna- tively, many routers these days have matched delay routing which enables one to take the flight time variation between microstrip and stripline configurations into account. Note that matched delay is quite different from matched length routing which does not consider flight time. For the DDR3/4 fly-by configuration, for instance, it is best to route all the critical traces on two symmetrical paired layers. In this case, the paired layers are 1 and 12, 4 and 9, plus 6 and 7. Layers 4 and 9 are best, as they are embedded and close to the plane pairs and active devices of a 12-layer PCB. ere are 200- mil fanouts from the microstrip layer to these (not shown). ese two layers have identical delays of 319.50 ps and are symmetrical in the stackup embedded between planes. Figure 3 shows the routing directions of the data lanes (0-3) combined with the associated differential strobes and the address, control and command (ACC) signals combined with the differential clock. One does not need to worry about layer- induced flight time skew because layers 4 and 9 are identical. Figure 4 graphs the relative radiation between outer and inner layers. In this case, the trace routed on the inner layer 4 exhibits between 4 to 10 dB less noise than the trace routed on the top layer. Note that there are radiating har- monics above 40 dB on the top layer routing. Also, high frequency components radiate more Figure 2: Relative signal propagation of microstrip and stripline (simulated in iCD Design Integrity).