Hi Rob,



No. I meant impedance, the complex quantity Z = R + i(X_j - X_c). It is intuitively obvious, without the benefit of calculation, that the capacitive reactance component of the impedance in the plane becomes precipitously greater the farther one gets from the trace. In order to make the same point with the inductive reactance component of the impedance, one must show that the same formulae which calculate the inductance of the trace relative to the ground plane apply to the inductance of the plane relative to the trace. Given the nature of inductance, that is not so intuitively obvious.

In the meantime I stand by my original statement. If you're concerned with external interference getting into your board, putting the signal traces on the inner layers can be very effective. And yes, even on SMT boards. Of course, that presumes you know where to route your traces.

Joe