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Electromagnetic interference (EMI)

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There are three different, fundamental mechanisms whereby interference can penetrate a cable:


Coupling with an ohmic resistor

Coupling with an electric field

Coupling with a magnetic field


If a cable is electrically connected with a source of interference, this can interfere with signal transmission. This is called ohmic coupling. The greater the coupling resistance, the smaller the coupling.


If a cable is located sufficiently close to a source of electric power, its electric field can transfer a noise signal to the signal wire.  This is called a capacitive coupling. The further away the signal wire is from the noise source, the smaller the coupling. Electrically conductive, grounded surfaces - a shield - can be used to divert electric fields and reduce any coupling.


If the cable forms loops, interference can be induced by a changing magnetic field. The smaller the surface of the loop, the smaller the coupled noise voltage.


Due to these properties of noise coupling, it is possible to reduce such interference through design and planning counter-measures.