Crosstalk

Crosstalk is yet another major concern for PCB designers. Figure 8a shows the cross section of a PCB indicating three parallel traces and their associated electromagnetic (EM) fields. When the spacing between the traces is too narrow, the EM fields of the traces will interact and the signals on the traces become corrupted as shown below. This is called crosstalk.


Crosstalk can be corrected by increasing the spacing between the spacing between tracks. However, PCB designers are under constant pressure to shrink their layouts and hence reduce the gap between tracks. Also, there are times when a designer has no alternative but to wear some amount of crosstalk in their design. Clearly, PCB designers need a strategy of managing crosstalk.



Many ‘rules of thumb’ have been published over the years about what is an acceptable spacing between conductors. A common rule is the 3W rule where the spacing between traces must be at least three times the width of the trace.

However the reality is that ‘acceptable’ spacing between conductors depends upon the application, the environment and the design margins. The spacing between traces changes from one situation to another and must be calculated for each. Furthermore, there are times when crosstalk can’t be avoided and the impact of crosstalk must be calculated. In these situations there is no substitution for a computer simulation.

A good example of these issues is in high-speed, high-density connectors. Here the PCB designer may know that there is some amount of crosstalk between the conductors and he/she can’t do anything about it because the geometry of the connector is fixed. By using a simulator, the designer can determine the impact on the signal integrity and can evaluate the effects on the system.