Academy of EMC

Conductive Coupling / Common Impedance Coupling (galvanic coupling).

Conductive coupling happens e.g. when two circuits share a common path/trace to ground or to another reference plane. Why could this lead to EMC problems?
Here an example: If now one of these circuits experiences an ESD, burst, or surge pulse, a high current may flow for a short time through this common PCB trace and introduce a noise voltage to the second circuit is introduced. This is the reason why it is important having low-impedance ground planes to earth/chassis. Because with low-impedance ground planes, the conductive coupling can be minimized.
The circumstance of a common ground path or power supply path often leads to bad signal integrity as well. Let's assume the first circuit drives some power electronics and the second is a sensitive measurement circuit. The high currents of the power electronics (noise currents) introduce a noise voltage in the said common path/trace and will lead to interference on the measurement circuit.

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Capacitive Coupling (near-field coupling).

Capacitive coupling is a near-field coupling, meaning: the noise source coupling structure and the victim receiving structure are somehow close together (on a PCB or in a cable harness) compared to the wavelength of the interference signal. The field of concern for capacitive near-field coupling is E-field (electric field). The energy of the radiated E-field noise typically falls off with 1/r^3 or 1/r^2 (1/r3 if the E-field is predominant, 1/r2 if the H-field is predominant) in the near-field area, where r is the distance between the emitting noise source structure (typically cables or metal plates) and the receiving structure (typically cables or PCB traces) of the victim.
Generally speaking, capacitive coupling is an issue where you have noise sources (typically cables or metal plates) with fast transient signals or high-frequency signals and victims with high impedance circuits (e.g. Analog to Digital Conversion (ADC) inputs).
Why could capacitive coupling lead to problems during EMC testing? Here an example: several wires are together in the same cable which means that each wire is capacitively coupled to the other wires inside that cable (the capacitance is bigger the longer the cable and the closer the wires to each other). One of the wires drives the reset signal of your controller system. During EMC testing, an ESD pulse happens to a connector pin that is connected to a wire in that cable. If there are no measures against ESD at the connector pin, this pulse may couple capacitively into the other wires in the same cable and potentially reset your controller (in case the controller reset signal is not filtered properly).
Capacitive coupling is also a topic when it comes to signal integrity: A typical example of capacitive coupling is a clock trace (noise source) laying parallel over e.g. several cm to a sensitive analog sensor signal trace (victim). The shorter the rise- and fall-time of the clock signal, the better (worse for your circuit) the coupling.

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Inductive Coupling (near-field coupling).

Inductive coupling is a near-field coupling, meaning: the noise source coupling structure and the victim receiving structure are somehow located close together (on a PCB or in a cable harness) compared to the wavelength of the interference signal. The field of concern for inductive near-field coupling is H-field (magnetic field). The energy of the radiated H-field noise typically falls off with 1/r^3 or 1/r^2 (1/r3 if the H-field is predominant, 1/r2 if the E-field is predominant) in the near-filed area, where r is the distance between the emitting noise source structure (typically current loops, coils) and the receiving structure (typically loops) of the victim.
Generally speaking, inductive coupling is an issue where you have noise sources with high-current traces/wires or large loop areas and victims with low impedance or large loop area structures (e.g. sensor signal in a cable that builds a current loop with the chassis structure as the sensor signal cable is not laid close the chassis).
Inductive coupling could lead to issues during EMC testing, e.g. because of high currents flowing through cables and PCB traces during ESD testing or surge testing where currents up to several kA could occur.
Inductive coupling may also lead to bad signal integrity as well. A typical case is a high current Pulse Width Modulation (PWM) signal line (and therefore a high energy H-field), laid parallel to a sensor signal with 4...20mA-signaling, whereas the sensor signal and its return current path build a large loop structure (where the magnetic field of the PWM signal can couple into).

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Radiated Coupling (far-field coupling).

Radiated coupling is a far-field coupling, meaning: the noise source coupling structure and the victim receiving structure are located far away from each other compared to the wavelength. The field of concern for radiated coupling is the electromagnetic field (EM-field), where the H-field and the E-field energy both fall off with 1/r.
During radiated immunity/susceptibility EMC testing (IEC 61000-4-3), the testing equipment antenna radiates a predefined EM-field to the Equipment Under Test (EUT). When the EUT is tested regarding radiated emissions (CISPR 11, EN 55011), the EMC test equipment antenna is placed at a predefined distance (e.g. 3m, 10m) in order to measure radiated EM-field by the EUT.

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Coupling paths overview and summary.

For the quick reader, here is a summary of EMC coupling paths. For the quick reader, here is a summary of EMC coupling paths. More theoretical and practical knowledge about EMC can be found in our Knowledge Base.

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