3.1 Coupling Mechanisms
Coupling is defined as the
means by which a magnetic or electric field produced by a circuit induces
a voltage or current in another circuit. Conductive coupling occurs when
the interfering and the interfered-with circuits are physically connected
with a conductor and share a common-impedance path. Free-space coupling
occurs when the aggressor generates an electromagnetic field that is ether
radiated and then received by the victim (far-field coupling) or that is
inductively or capacitively coupled (near-field coupling) to the victim.
3.1.1 Conductive Coupling
When the aggressor and the
victim share a common-impedance path, the voltage drop in the common impedance
can translate into interference - see Figure
Figure 3.1 Principle of conductive
coupling between circuits caused by common-return-path impedance: the impedance
Zc is common to both circuits; thus the voltage drop
in Zc generated by i1 ( i2
) effects also U2L ( U1L ).
Also practical reference planes
do not exhibit a zero impedance. Any currents flowing in a conductive plane
will produce potential differences between various points on the plane.
Interfacing circuits referenced to these various points can experience
3.1.2 Free-Space Coupling
Free-space coupling is the
transfer of electromagnetic energy between two or more circuits not directly
interconnected with a conductor. Depending on the distance between the
circuits, the coupling is defined as either near-field or far-field coupling.
Near-field coupling can be subdivided into inductive and capacitive coupling,
according to the nature of the electromagnetic field. In inductive coupling,
a magnetic field is generated. Capacitive coupling is produced by an electric
field between the aggressor and the victim. For far-field coupling electromagnetic
waves are the principle coupling mechanism.