Digital Circuit Boards: Mach 1 GHz

2.12 Ground/Power Planes

Ground and power planes were introduced to digital circuit board design to allow the operation of the circuits at higher clock rates. The improved performance can be related to the tight control of fields associated with the transport of logic signals. It is recognized that the capacitance between ground and power planes stores energy. This capacitance is in parallel with local decoupling capacitors. The energy in these parallel capacitances is available to drive transmission lines. Efforts have been made to increase the stored decoupling energy by using a higher dielectric constant material between the conducting planes. There is a time delay associated with retrieving this stored energy, which requires that decoupling capacitors must still be used. This topic is covered in the next section.

When a trace and a conducting plane are used as a transmission line, the fields that transport signal or energy are tightly confined. The field geometry for a wave is the same whether one of the planes is associated with ground (common) or with the power supply voltage.

N.B.

The average potential on a conducting plane does not affect the field patterns associated with a wave moving between two conductors. The waves in a bathtub are the same whether the bathtub is on the first or second story of a building.

If the conducting plane is floating, the fields are still confined. This is not a recommended practice, as there will be reflections at the plane edges and ...