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EMI Types and Coupling Methods

Author Cadence System Analysis

Key Takeaways

  • When electromagnetic fields disturb the normal functioning of an electronic circuit, it is called electromagnetic interference (EMI). 

  • Depending on the path taken by the EMI to travel from the source to the target, the EMI coupling method changes.

  • According to the coupling paths through which EMI is transmitted, EMI coupling methods are divided into conductive coupling, inductive coupling, capacitive coupling, and radiative coupling.

EMI

All electronic circuits confront electromagnetic fields         

Any disruption to the normal functioning of a circuit due to either internally or externally generated electromagnetic fields is called electromagnetic interference. The EMI generated by a circuit affects its own operation as well as the electrical and electronic systems in its surroundings. There are different EMI coupling methods through which the electromagnetic fields travel from the source circuit to the target. The EMI can be coupled to the target via conduction, radiation, or inductive or capacitive coupling. The dominant EMI coupling method in a circuit is dependent on its frequency. At low frequencies, conducted EMI coupling is greater. As the frequency increases, other coupling methods become active. Radiation coupling is most active at higher frequencies.

Whatever the EMI coupling method is, it is always important to prevent EMI influence in electronic circuits. To do this, various EMI mitigation techniques can be used. The appropriate EMI mitigation system to be used depends on the EMI coupling method that is dominant in the circuit. That is why a good understanding of EMI coupling methods is essential for better electromagnetic compatibility (EMC) in electronic circuits.

The Types of Electromagnetic Interference

Here are a few types of EMI:

  • Man-made EMI: This type of EMI is generated by another electronic device. Examples of man-made EMI include when two signals are close to each other or multiple signals of the same frequency simultaneously exist in one device. 
  • Natural EMI: Natural EMI is generated from electrical storms, lightning, or cosmic noises.

Classifications of EMI based on duration:

  • Continuous EMI: Continuous EMI is generated from the source continuously and keeps on affecting the target. Continuous EMI can be a natural or man-made EMI. 
  • Impulse EMI: Impulse EMI is a pulsed electromagnetic signal that can be natural or man-made. 

The bandwidth of electromagnetic signals is considered for classifying EMI into:

  • Narrowband EMI: The EMI generated from an oscillator or noises due to distortions in a transmitter are examples of narrowband EMI, as they mostly comprise a single frequency. The mitigation of narrowband EMI is comparatively easier than broadband EMI.
  • Broadband EMI: When electromagnetic disturbances comprise multiple frequencies or cover a wide spectrum. Broadband EMI can be either man-made, as in arc welding, or natural, such as solar radiation-induced interference affecting satellite signals.

EMI Coupling Methods

The EMI coupling method changes depending on the path taken by the EMI to travel from the source to the target. Electromagnetic disturbances are transmitted from source to the victim through a variety of means, such as a common conducting path,  mutual inductance or capacitance, or air or vacuum. According to the coupling paths through which EMI are transmitted, EMI coupling methods are divided into four types:

  1. Common-Impedance Coupling or Conductive Coupling
    In electrical and electronic circuits, components are connected to each other over power supply lines or communication lines. The currents induced by EMI sources such as lightning or faults can travel via these common lines and produce undesirable voltages affecting the proper functioning of the circuit. Common lines are characterized by impedances producing the voltage drop across two points that were previously at equipotential. The transmission of EMI via common-mode impedance is called common-mode (common-impedance) EMI coupling or conductive coupling. Apart from common-mode, there can be EMI resulting from currents induced in two conductors, which is called differential mode EMI coupling.

  2. Inductive Coupling
    When the EMI from a source couples to a target over the magnetic field, then the coupling is inductive coupling. Inductive coupling utilizes the principle of electromagnetic induction to induce currents in the target due to the varying magnetic field between the source and target. The intensity of the coupling voltage is dependent on the current variations and mutual inductance between the source and target. 

  3. Capacitive Coupling
    The voltage variations and coupling capacitance between the source and target induce unintentional currents in the target, causing capacitively-coupled EMI in the target. The frequency, source-target distance, input impedance of the target, insulation of the target cables, and height of the cables from the ground plane are some of the parameters affecting capacitive EMI coupling. 

  4. Radiated Coupling
    When the source and target are separated physically from each other, the EMI travels via air or vacuum to reach the target. The electromagnetic interference is radiated in this type of EMI coupling. Radiated EMI mostly comprises higher-frequency signals in the microwave frequency range. Radiated EMI can also be referred to as radio frequency interference (RFI).

The mitigation techniques applied for RFI are entirely different from the counter-measures for EMI caused by conductive coupling. A clear understanding of EMI coupling methods is essential for designing efficient and effective EMI mitigation techniques. Cadence software can help you incorporate suitable EMI mitigation systems in your circuit design for EMC.

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