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Radiated EMI Coupling Mechanisms and Measurement Setups

Key Takeaways

  • There are four EMI coupling mechanisms: conductive coupling, capacitive coupling, inductive coupling, and radiated coupling.

  • Radiated EMI coupling is the most commonly experienced and popular form of EMI coupling.

  • Common radiated EMI measurement setups are OATS, anechoic chambers, GTEM cells, and reverberation chambers.

 Anechoic chamber

For smaller electronic circuits, anechoic chambers are used for radiated emissions measurements

Designing compact electronic circuits that are electromagnetically compatible is one of the biggest challenges for engineers. With the increasing demand for high power density designs and circuit miniaturization, circuits are downsized by placing active and passive components in close proximity. This compact arrangement of components makes them viable to electromagnetic interference (EMI) coupling mechanisms such as conducted EMI, capacitively or inductively coupled EMI, or radiated EMI.

Radiated EMI is the most common EMI in electronic circuits. The electronic circuit of interest can be either a source or victim of radiated EMI, or both simultaneously. To estimate the impact of radiated EMI on an electronic circuit, it is essential to measure radiated emissions.

In this article, we will discuss different EMI coupling mechanisms while emphasizing the measurement setups for radiated EMI.  

EMI Coupling Mechanisms in Electronic Circuits

Electromagnetic interference is any unwanted signal that is injected into an electronic circuit from a source and impacts the normal workings of the circuit. An electronic circuit is a victim of EMI when its working is affected by EMI. The same circuit can act as a source, thereby disrupting the operation of nearby devices and equipment.

The EMI can be coupled from source to victim in different ways. EMI coupling mechanisms describe the way EMI travels from source to victim or affected devices. The coupling path differs in each EMI coupling mechanism, and it is important to understand EMI traveling routes and ways for addressing measurement techniques and mitigation methods.

EMI coupling mechanisms are classified into four types: conductive coupling, capacitive coupling, inductive coupling, and radiated coupling. 

Conductive EMI Coupling

In electrical and electronic circuits, conductive coupling happens when EMI emissions are transported from the source to the victim via conductors, wires, or cables. The coupling path in conductive coupling is characterized by impedance and is responsible for the voltage drop associated with conducted emissions.

There are two types of EMI conductive coupling:

  1. Common-mode conductive coupling, where the EMI present in the coupling paths is in-phase.
  2. Differential-mode conduction, where the EMI in the conductors is out of phase. 

Capacitive EMI Coupling  

When two devices or electronic circuits are connected and there is a varying voltage source, the charge is capacitively transferred from the source to the victim. This is called capacitive coupling

Inductive Coupling   

When the EMI from a source couples to the victim over the magnetic field, then the EMI coupling is inductive coupling. Inductive coupling utilizes the principle of electromagnetic induction to induce currents in the victim due to the varying magnetic field between the source and victim.   

Radiated EMI Coupling

When the source and victim are separated physically from each other, the EMI travels via air or vacuum to reach the victim. The electromagnetic interference is radiated in this type of EMI coupling, and is called radiated EMI coupling. Radiated EMI mostly comprises higher-frequency signals in the radio frequency or microwave frequency range. Radiated EMI is otherwise called radio frequency interference (RFI).

Radiated EMI coupling is the most commonly experienced and popular form of EMI coupling. As there is no physical connection required for radiated EMI coupling, it is likely to affect most electronic circuits. It is important to estimate the radiated EMI coupling in electronic circuits as a part of ensuring electromagnetic compatibility (EMC). In the upcoming section, we will discuss the measurement setups made for determining radiated emissions in electronic circuits. 

Measurement Setups for Radiated EMI Emissions 

Typically, radiated emissions are in the frequency range of 30 MHz and 1 GHz. To keep the electromagnetic environment clean and safe for other equipment, it is advisable to measure the radiated EMI from the device of interest. For estimation of radiated EMI from a device (source of EMI), measurement setups are made. Common radiated EMI measurement setups are discussed below.

Open Area Test Sites

Open area test sites (OATS) are common measurement setups made for estimating the radiated EMI from large equipment. They typically consist of an infinite metallic receiving antenna connected to a spectrum analyzer as well as cables to connect the spectrum analyzer and equipment under test (EUT). The EUT and receiver antenna are kept at a distance of 3m or 10m, and the emissions received by the antenna are measured using the spectrum analyzer.

Anechoic Chambers

For smaller electronic circuits, anechoic chambers are used for radiated emissions measurements. Anechoic chambers are fully or partially covered with microwave absorbers to avoid errors in measurement. 

GigaHertz Transverse Electromagnetic Cells

Other than anechoic or semi-anechoic chambers, GigaHertz transverse electromagnetic (GTEM) cells can be used to conduct radiated emission testing. A GTEM cell is a tapered double conductor transmission line with a constant characteristic impedance of 50 Ω. The inner walls of a GTEM cell are covered with microwave absorbers. The smaller end of a GTEM cell acts as an input-output port and is connected to the spectrum analyzer. Compared to OATS and anechoic chambers, the GTEM cell is a compact, affordable, and appropriate option for 0 Hz to 20 GHz. 

Reverberation Chambers 

A high-quality factor (Q factor) metallic enclosure where the transmitting antenna, receiving antenna, and EUT are placed forms a reverberation chamber (RC). An inhomogeneous electric field is created inside the RC due to the standing waves arising from the metallic boundary. Metallic paddles, called stirrers, placed inside the RC can modify the boundary condition of the chamber. The RC is accurate and reliable compared with its counterparts and gives results in a shorter period.

Radiated and conducted EMI coupling mechanisms play a critical role in the EMC performance of electronic circuits. As EMC factors into how competitive an electronic circuit will be on the market, it becomes increasingly important to estimate conducted and radiated emissions. Cadence software offers simulation tools to estimate radiated and conducted emissions from electronic circuits during the beginning design stages.

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