Electromagnetic Interference/EMI Shielding

Electromagnetic interference, abbreviated as EMI, refers to the effects of electromagnetism on electrical circuits. If left unaddressed, EMI can greatly worsen or even halt the function of circuits. Sources of EMI are widespread and include cosmic noise (similar to what causes TV static), mobile phone signals, power lines and lightning. In particular, interference originating from other wires is referred to as crosstalk [1]. 

EMI can be radiated or conducted. Conduction can only occur if there is physical contact or some conductive path between the source and the target (such as a wire). Radiation occurs when there is no contact and the two are separated by some distance [1].

Mitigation

Since sources of EMI are so common, finding ways to mitigate it is important. Shielded cables can be used for this purpose. Shielded cables are regular wires with an additional layer of conductive material underneath the insulating coat. The conductive material usually consists of a foil or a braid. Unlike foil, braids have gaps and therefore do not fully cover the wire they are protecting. However, braids are much more flexible than foils. Multiple layers of EMI shielding can be applied to a cable if so desired, such as a braid placed over a layer of foil [2]. In any case, they operate in the same way as a Faraday cage, where any excess charge is distributed evenly on the exterior layer, mitigating the effects on the interior. Shields typically need to be grounded in order to be effective, as doing so allows it to remove the noise [3].

For differential signaling, wire are usually arranged as a twisted pair, in which two of them are twisted around each other in a helix pattern. Twisting the wires means the distance between the source and a given wire varies over the length of the pair, which spreads the effect of EMI between the pair. As a result, the voltage difference between the two complementary wires will remain fairly close. Doing so also reduces the loop area which reduces the induced voltage as a result [3]. Examples of twisted pair cables include ethernet and USB cables, both of which use differential signaling [7].

Due to the cost, it is preferred to deal with EMI in other ways before resorting to shielded wires. Examples include storing equipment in metal containers or covering cables with metal conduits [2].

References

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