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December 2014 • The PCB Design Magazine 49 beyond design SIGNAL INTEGRITY, PART 3 continues of signal layers. Figure 5 shows part of the stack- up cross-section in the ICD Stackup Planner. In this case, both signal layers are referenced to the GND planes on layer 3. Recently, I analyzed a design where the lands on the top layer connected to a WiFi module and the signal on layer 2 injected a ran- dom pulse into the module via this close cou- pling. The symptom was that the product ran for about an hour then all of a sudden missed a beat and had to be rebooted. Once this issue was fixed, the product then ran reliably for days without failure. Simple fix, but hard to find! Crosstalk is also typically picked up on long parallel trace segments. These can be on the same layer or may also be broadside coupled from the adjacent layer. Fortunately, source synchronous busses have a unique immunity to crosstalk, provided that the ringing has settled by the time the bus is sampled by the clock. So there are two issues here: 1. Keep parallel trace segments as short as possible to reduce coupling (crosstalk) unless you are using a synchronous bus. Space the groups of signals (e.g., address and data) by three times the trace width. 2. Always route the clock (or strobe) to the longest delay of the group of signals. This allows the data to settle before it is read by the clock. In a previous column, Matched Length Does Not Always Equal Matched Delay, I discussed matched length routing and how matched length does not necessarily mean matched de- lay. Flight time (propagation delay) varies de- pending on the dielectric material that the sig- nal propagates in. eq. 1 Where c is the speed of light and "Er eff" is the effective dielectric constant. In a microstrip configuration, the effective dielectric constant is dependent on the FR-4 Figure 5: layers 1 and 2 have only 3.4 mil separation and are prone to coupling. Figure 3: Broadside coupling (top) compared to edge coupling (bottom). Figure 4: Buildup microstrip layers—layers 1 and 2.