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Digital Circuit Boards: Mach 1 GHz by Ralph Morrison

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4.13 Split Ground/Power Planes

Circuit board layout involves interconnection of many components. These interconnecting traces must have a specified width and spacing to control characteristic impedance and limit cross talk. The number of traces involved and the component density determines the number of board layers that are needed. If board size is not an issue then most boards can be designed using four layers.

The cost of the board is related to the number of layers that must be used. One way to add a few traces without adding layers is to break one of the conducting planes into two or more islands. Traces can then be run in the open area between islands. It is preferred to design boards so that traces maintain the same characteristic impedance over their entire length. If traces running between two conducting planes cross a gap on one of the conduction planes, the characteristic impedance at the gap will change. For many designs this may not cause a problem. For very short rise times, the resulting reflection at the gap can be an issue.

N.B.
When a trace crosses a gap on an outer ground or power plane, the return current must follow the edges of the gap. This is a classic radiating antenna structure. This is a bad practice for all logic. A path for the return current can be provided by adding a ground jumper next to the crossing trace. This is an improvement but the characteristic impedance will still not be well controlled in this area.

Ground and power planes provide ...

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