What is Electromagnetic Coupling?

Key Takeaways

• Electromagnetic coupling in transformers makes them electrically isolated and magnetically coupled.

• In radiated coupling, the source and receiver or victim are separated by a distance. The source and victim are more like radio antennas. The electromagnetic signal radiated by the source is received by the victim and interferes with the normal operation of the victim and the circuit where it is placed.

• When the components of a circuit are magnetically coupled, it is a form of inductive coupling. Parasitic inductance plays an important role in interference caused by inductive coupling. 

The phenomenon of electromagnetic coupling can be either good or bad news in regards to the performance of electronic circuits. If electromagnetic coupling is utilized in transformer action, then aim for tight coupling. However, if electromagnetic coupling is the cause of interference, then methods must be introduced in the circuit to loosen the coupling.

In this article, we will discuss the types of electromagnetic coupling and the systems this coupling affects. 

The Advantages of Electromagnetic Coupling

When an electromagnetic field in a circuit is capable of inducing an electric charge in another, then this phenomenon is called electromagnetic coupling. One of the advantages of electromagnetic coupling can be cited in transformer action. In transformers, alternating voltage applied to the primary winding creates an alternating magnetic field. The secondary winding is placed in the alternating magnetic field and is magnetically coupled to the primary winding. An electromotive force is induced in the secondary winding due to electromagnetic coupling. The electromagnetic coupling in transformers makes them electrically isolated and magnetically coupled.

Unintentional Electromagnetic Coupling

Unintentional electromagnetic coupling is troublesome. It can result in electromagnetic interferences—both external and internal interference and crosstalk in electronic circuits.

In electromagnetic coupling causing interference, the source components conduct or radiate electromagnetic signals to interfere with components in the same circuit or neighboring circuit. In DC circuits and low-frequency circuits, the effects of unintentional electromagnetic coupling are comparatively less compared to microwave and RF circuits. At high frequency, the source and victim components act like antennas and radiate electromagnetic signals. The transmission lines act as a path for conducting unwanted electromagnetic signals to the components nearby. 

Types of Electromagnetic Coupling

Electromagnetic interference can be from a radiated signal or conducted signal, depending on the coupling. It is important to have some knowledge of the coupling before planning methods to mitigate or shield it.

The types of electromagnetic coupling are:

1. Radiated coupling - In radiated coupling, the source and receiver or victim are separated by a distance. The source and victim are more like radio antennas. The electromagnetic signal radiated by the source is received by the victim and interferes with the normal operation of the victim and the circuit where it is placed.

2. Conducted coupling - When there is a path for unintentional electromagnetic signals to travel between source and victim, it introduces conducted coupling and interference. Transmission lines are the main conducting paths in electronic circuits. Conducted coupling can be of two types:

3. Common-mode coupling - When devices are connected by conductors and cables which share a common-mode impedance, then voltage drops can interfere with the electronic components and cause interference. At high frequencies, common-mode coupling is more active.

4. Differential-mode coupling - In the differential mode of coupling, the current flowing in two conductors connecting the source and victim will be in phase opposition. The electromagnetic disturbances are weakly coupled in differential mode coupling.

3) Capacitive coupling - The coupling capacitance between the source and victim is the reason behind capacitive coupling. Electromagnetic disturbances depend on the coupling capacitance and the voltage variations.

4) Inductive coupling - When the components of a circuit are magnetically coupled, then it is a form of inductive coupling. Parasitic inductance plays an important role in inference caused by inductive coupling. 

Sources and Victims

So far, we have discussed sources and victims in electronic circuits. The table below lists some of the common sources and victims that are electromagnetically coupled.

Various EMI shielding techniques, such as guard traces, via fences, and Faraday cages, are introduced in RF and microwave circuits to reduce electromagnetic coupling between circuit components. Cadence’s PCB design software supports the process of electromagnetically-compatible circuit design.

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