Design007 Magazine


Issue link:

Contents of this Issue


Page 55 of 117

56 DESIGN007 MAGAZINE I JUNE 2020 Particular care needs to be taken with oscil- lators and switch-mode power supplies that may generate high-frequency electromagnetic fields. If space permits, keep these circuits at least 250 mils from any critical signals to avoid parasitic coupling. Route fences (a thin elongated keep-out), rather than regional keep-out areas, are a use- ful tool to control the routing. The ground plane should not be split; rather, a pass-through gap is left in the fences so that data and control sig- nals can enter and leave that area, as seen in Figure 1. At high frequencies, return current follows directly under the signal trace on the reference plane, and as such, will follow the trace through the pass-through gap. 2. Bridging Planes of Different Potential These days, it is quite common to have mul- tiple power supplies on a board. Instead of allocating one or two power supplies per plane, it is best to use the dual-stripline lay- ers to provide mixed-signal/power pours and reduce the layer count. This also prevents the overlapping of electromagnetic fields, which creates crosstalk. This is particularly critical with dual asym- metric stripline configurations where one or two signal layers are sandwiched non-uni- formly between two planes, as in Figure 2. It is important to have a clearly defined return current path and to know exactly where the return current will flow. The question is not, "Which plane does the return current flow on?" but, "How is the current distributed on each plane?" Also, if a return path discontinu- ity exists, then the current tends to divert in- creasing the loop area, inductance, and delay. A via that provides the connection between signal layers, referenced to planes of different DC potential, creates return path discontinui- ties. In other words, the return current has to pass between the planes to close the current loop, which increases the inductance, affecting the signal quality. This return current can also excite the parallel plate resonance mode, caus- ing significant electromagnetic radiation from the fringing fields. If the reference planes are at the same DC potential, then they can be directly connected by stitching vias near the signal via transition to provide shorter paths for the return current. Figure 2: Return-path current density for a dual asymmetric stripline.

Articles in this issue

Archives of this issue

view archives of Design007 Magazine - Design007-June2020