Digital Circuit Boards: Mach 1 GHz

3.11 Cross Coupling Continued

In a system of conductors, the propagation velocity depends only on the dielectric constant. A change in velocity does not change the rise time of an event. It only changes the distance a wave travels in a given time. In a given dielectric, the magnetic field and the electric field travel at the same velocity. The partitioning of fields that we have just used does not change this fact.

The voltage in the victim forward wave is the sum of the inductive and capacitive currents given by Equations 3.3 and 3.5 multiplied by the characteristic impedance or

3.6

The ratio C_M/C is approximately equal to L_M/L. To a first approximation, the terms cancel. For traces stacked one above the other, the capacitive term may dominate. For side by side traces, the inductive term may dominate. Because A_SF is proportional to t and inverse to τ_r, this mode of cross talk can be a problem when trace lengths are long and rise and fall times are very short. On outer traces, (microstrip) the ratio of C_M/C is reduced because part of the E field is in air. This usually results in an increased inductive coupling. This would suggest that to avoid cross coupling, longer traces be run as stripline.

N.B.

Forward wave coupling increases as rise time decreases.

The reverse coupled wave is the sum of Equations 3.2 and 3.4 or

3.7

There is no cancellation of terms in this equation. ...