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 Z_c can translate into interference - see Figure 3.1.

**Figure 3.1** Principle of conductive coupling between circuits caused by common-return-path impedance: the impedance Z_c is common to both circuits; thus the voltage drop in Z_c generated by i₁ ( i₂ ) effects also U_2L ( U_1L ).

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 conductively-coupled interference.

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.

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