5.9 Cross Coupling Problems

In high speed logic, a major consideration is cross coupling. This topic was covered in Chapter 3. For logic signals with short rise and fall times, the major contributor to cross talk is the backward coupled wave. This wave lasts for the full round trip of the culprit wave and can cause the victim's logic level to be outside of its operating envelope. To do a full analysis of cross talk, a field solver may be needed.

Every logic transition requires the supply of field energy. As rise and fall times get shorter and clock speeds rise, the need to reduce power consumption follows. Reducing operating voltages reduces power consumption and makes it possible to raise the clock speeds. Lowering the operating voltage means smaller logic swings. This, in turn, reduces the available noise margin for the logic. Unfortunately, the need for noise margin increases with shorter rise and fall times. At shorter rise times, there is increased cross talk and greater signal loss caused by skin effect. Another element in the error budget that may have to be considered is dielectric loss. This error can be reduced by using a higher grade of board material.

The simplest way to reduce cross talk is to increase the spacing between traces. The simplest way to limit skin effect losses is to widen the traces. Both of these changes are in the direction to increase the size of the board. To avoid making a larger board, the only approach left is to increase the number of trace layers. Adding trace layers is in itself not sufficient. It is necessary to have a conducting plane associated with each added trace layer or it is impossible to limit crosstalk and control the characteristic impedance of the traces. Obviously, these added layers increase the cost of the product. Fortunately, there is another factor that should be considered in the cost equation. When more functions are handled by the ICs, the effect is to reduce trace count and board size.