Discussion of Electromagnetic Interference

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Andy Chou is a Boyd Market Segment Manager with more than five years of experience working in the area of EMI solutions. He also leads Boyd’s New Product Introduction team in developing new and emerging EMI products and applications. With experience as a global technical engineer, key account manager and product manager, Andy is fervently attuned to the management of EMI in the electronic industry and attendant market trends.


Introduction


Electromagnetic interference (EMI) also known as radio frequency interference (RFI) when in radio frequency, is caused by undesirable radiated electromagnetic fields, or conducted voltages or current from an external source, which interferes with the safe and stable operations of an electronic device. This interference can come from any artificial or natural object that carries rapidly changing electrical currents, like an electrical circuit, the sun or the Northern Lights. This disturbance or interference may interrupt, obstruct, limit or degrade effective electronic device performance. These effects can range from simple performance degradation to a total loss of data or, at worst, device failure.

To avoid these potential performance issues, electronic device design commonly incorporates several materials that promote stable performance in the presence of EMI.

An illustration of this design is the specific installation of a shielding enclosure and grounding gaskets shown in Figure 1.

Origin of EMI

Electromagnetic interference can come from a number of different sources and be categorized in a number of different ways. EMI sources can be natural or man-made. They can be categorized by duration like continuous interference or impulse “noise” (noise meaning interference), or categorized by bandwidth (narrow or broad band). How does EMI happen? Normally EMI occurs from a coupling mechanism (or path) caused by one of the following models. The general theory of coupling is shown in Figure 2.

- Conduction by electric current effect: Conducted noise is coupled between two or more components through interconnecting wires like a power supply or ground wires. Common impedance coupling is caused when currents from two or more circuits flow through the same component or circuit. Most conducted coupling between pieces of equipment occurs through AC power lines.

- Radiation by electromagnetic effect: In the near field condition, E field (electric) and H field (magnetic) coupling are diagnosed separately (mainly for electric field issues). In the far field condition, coupling is treated as a plane wave (mainly for radiated electromagnetic field issues).

- Induction by electric field or magnetic field effect:
-- Inductive Coupling: Magnetic field coupling is caused by current flow in conductors. The coupling mechanism may be modeled by a transformer.

-- Capacitive Coupling: Electric field coupling is caused by a voltage difference between conductors. The coupling mechanism may be modeled by a capacitor.

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