TRANSMISSION PROPERTIES OF BALANCED INTERCONNECTIONS

With the development of fine line (width - 3 mils) printed conductors and surface mount technology, it is becoming practical to consider balanced interconnections on printed wiring boards to help reduce crosstalk, increase noise immunity, and eliminate ground noise. Balanced interconnections require two conductors per signal; whereas unbalanced interconnections require only one conductor per signal, since the return current is via the ground plane. This paper develops simple equations for computing the characteristic impedances and crosstalk of both unbalanced and balanced interconnections in a lossless, homogeneous, microstrip configuration (signal conductors above a ground plane). The equations are applied to compare the crosstalk resulting from both types of interconnections having 4-mil conductor widths when the packing density is three signals per 54 mils. For example, when both characteristic impedances equal 109 OMEGA using FR-4 dielectric, balanced interconnections reduce crosstalk relative to the unbalanced case by a factor of about six. Similar comparisons for other signal packing densities and impedances are also presented. It is shown that balanced interconnections are much more immune to noise pick-up. Also, they are free of ground noise and radiate relatively little. A theoretical transformation is developed in order to apply the equations to conductors having rectangular or circular crosssections. The equations yield values which agree well with experiment, and they can also be applied to help analyze the interconnections associated with fiber-optic transceivers, monolithic microwave integrated circuits, hybrid integrated circuits, and various multichip modules. The effect of the dielectric constant on crosstalk is also discussed.