What is EMC?
Electromagnetic compatibility describes a device's ability to function in its environment without unduly disturbing other devices and without being unduly disturbed itself. This article explains the basic terms with examples.
The definition in one sentence
Electromagnetic compatibility is achieved when a device works satisfactorily in its intended environment without itself causing electromagnetic disturbances that would be unacceptable for other devices in that environment.
Two directions follow from this, and in practice they are measured separately: what the device puts out – and what it can withstand.
The key terms
Each term with a short example from everyday development work.
Emissions
- Everything a device unintentionally puts out as electromagnetic energy – along cables or through the air.
Immunity
- How well a device keeps working when electromagnetic energy acts on it from outside.
Conducted and radiated
- Two routes disturbances travel: along cables, or as a field through space.
Coupling path
- The route from the source to the victim. Without a coupling path there is no EMC problem – even if a source is present.
Near field and far field
- Close to the source, the electric and magnetic fields have to be considered separately; from a few wavelengths away they behave like a wave.
Limit
- The maximum permissible disturbance level defined in a standard – depending on the type of product, its environment and the frequency.
Source, coupling, victim
Every EMC problem needs three ingredients. If one of them is missing, the disturbance does not occur. In practice that means there is almost always more than one place where something can be improved.
arises
Source
Switching components, clocks, motors, converters. Steep edges and high currents create broadband disturbances.
transfers
Coupling
Cable routing, ground planes, connectors, openings in the enclosure. This is where the most can usually be achieved with the least effort.
is disturbed
Victim
Sensitive inputs, sensors, communication interfaces. Filtering and decoupling make them more robust.
The legal framework
In the European Union, the EMC Directive 2014/30/EU governs the electromagnetic compatibility requirements for equipment. In Germany it is transposed into national law by the EMVG.
Compliance with harmonised standards gives rise to the presumption of conformity. Which standards apply to a product, and which evidence is required, depends on the type of product, its environment and the target market.