Intel Embedded Intel486 Hardware Reference Manual page 310

EMBEDDED Intel486™ PROCESSOR HARDWARE REFERENCE MANUAL
Inductive coupling occurs when a current in one trace produces current in a parallel trace. This
current reduces with the distance from the source. Hence, closely spaced wires or traces incur the
greatest degree of inductive coupling. Both traces in this case act like normal conductors.
Radiative coupling occurs when two parallel traces act as a dipole antenna which radiates signals
that parallel wires can pick up. This results in the corruption of signal that is already present in
the trace. The intensity of this type of coupling is directly proportional to the current present in
the trace. However, it is inversely proportional to the distance between the radiating source and
the receiver.
10.3.3.2 Minimizing Electromagnetic Interference
When laying out a board for an Intel486 processor-based system, several guidelines should be
followed to minimize EMI.
One source of EMI is the presence of a common impedance path. Figure 10-22 shows a typical
layout which does not have the same earth ground or the signal ground.
V V
CC CC
GND
GND
Z
Z
4
2
Z
3
(Parasitic Parasitic
C C
Chassis Ground
Capacitance) Capacitance
Figure 10-22. Typical Layout
To reduce EMI, it is necessary to minimize the common impedance paths, which are Z , Z and
2 3
Z shown primarily as ground impedances. During current switching, the ground line voltage
4
drops, causing noise emission. By enlarging the ground conductor (which reduces its effective
impedance), this noise can be minimized. This technique also provides a secondary advantage in
that it forms a shield which reduces the emissions of other circuit traces, particularly in multi-
layer circuit boards.
10-26