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42 DESIGN007 MAGAZINE I MARCH 2022 Article by Steve Hageman ANALOG HOME Does putting a ground pour on PCB signal layers make the isolation better or worse? It can go either way, but with the proper knowledge and application, this technique will improve your designs. In this article, I'll discuss how to simulate trace-to-trace isolation with true electromag- netic simulation soware. We'll also cover a variety of rules of thumb that can help you stay away from trouble. Fact or Fiction? Recently an acquaintance told me, "I have heard that putting a copper pour on a signal layer between traces actually makes the isola- tion between the traces worse." I grabbed one of my RF boards (see Figure 1) and said, "If that is so, then how do all these RF boards that I have done with co-planar waveguide over ground manage to function? ey all have cop- per pours on the signal layer, and they work to very high frequencies." 1 Since co-planar waveguide over ground (CPWG), which is essentially "pouring copper on a signal layer," is used for a lot of RF work 2 , and is proven to work for very high-perfor- mance RF circuits, how did this contradictory opinion catch on in the industry? To investigate this, I used a one-inch section of 50-ohm microstrip consisting of an aggres- sor trace from ports 1 to 2 and a victim trace running in parallel from ports 3 to 4 as shown in Figure 2. I used typical values for the dimen- sions as might be on a real PCB. e trace width is 20 mils, with a spacing of 60 mils from Does Copper Pour on a Signal Layer Decrease Signal-to-signal Isolation? Figure 1: Many RF boards are built with CPWG construction because it offers much-improved EM performance. By improved, I mean smaller, less radiation, and better crosstalk performance than microstrip construction. This is essentially a copper pour on a signal layer and it works well, or legions of RF engineers would not be using it. Figure 2: This 1-inch, 50-ohm aggressor trace (port 1 to 2) and victim trace (port 3 to 4) structure was modeled in Sonnet to study the effects of coupling between the traces.

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