Electromagnetic Compatibility

The term electromagnetic compatibility (EMC) is used in the field of electrical engineering and is the ability of an electrical device to work satisfactorily in its electromagnetic environment without influencing the surrounding devices.
:As a consequence The Two main aspects of EMC arise:

::*Electromagnetic emission (Radiated or Conducted Emission)
::Devices have to be designed in a way that emitting electromagnetic disturbances do not interfere with the operation of radio and telecommunication and other devices in accordance with their purpose.

::*Electromagnetic interference (Radiated or Conducted Susceptibility)
::Devices have to be designed in a way that they have adequate immunity to electromagnetic interferences and operation in accordance with their purpose is possible.

Coupling Path

When investigating EMC there is always a noise source which affects a receptor (sink) via the coupling path as shown the figure to the right. Source and sink are usually electronic hardware devices that are conductively, capacitively, inductively, or radiatively coupled.

Conductive Coupling

Conductive coupling occurs when the coupling path between the source and the receptor is formed by a conducting body (for example a PCB trace) and occurs in two forms:
:*Capacitive coupling is applied when an electrical (E-) field, e.g. between two parallel conductors or PCB traces less than a wavelength apart, interact.

::*Inductive coupling or magnetic coupling is given when an electrical (H-) field induces voltage into loops closer than a wavelength of a noise signal.

Radiative Coupling

Radiative Coupling or electromagnetic coupling is given when noise source and sink are not directly connected and more than a wavelength apart. The electromagnetic field influences the receptor through the open space.

EMC Disturbances

To this point just the coupling paths were described. If analysed closely EMC disturbances can be split up into transient and radiated disturbances.

Transient disturbances are directly coupled to the supply or signal lines.
:Two types are distinguished:
:*Differential-Mode-Distortion appears on the signal line and the ground in the opposite direction to each other.

::*Common-Mode-Distortion appears on all lines in the same direction.

Radiated disturbances are coupled by the E/H-field electromagnetically. At the sink the disturbance is picked up. A wire, trace or conductance acts as antenna.

Common EMC Basics

Electromagnetic noise is produced in the source due to rapid current and voltage changes. When fast changes dU/dt or dI/dt spread via the coupling mechanism described earlier.
For this reason the main directive in EMC circuit design is to avoid unnecessary switching operations. Necessary switching should be done as slowly as technically possible. Noisy circuits, circuits with a lot of switching activity, should be physical separated from the rest of the design. To keep the unavoidable noise local the use of shielding, housings, choking coils, or RC elements is advisable.
At the noise source EMC aware design is as important as at the receptor.
The usage of the following measures is recommended.

::*Harmonic Wave Filters
::*Shielded Lines
::*Decoupled Cable Entries (Line filter, Signal filter)
::*Shielded Housings
::*Avoidance of Antenna Structures in PCB Design
ˆ The origin of noise at the source can be man made or natural. Examples are:
::*Lightning Electromagnetic Pulse (LEMP)
::*Nuclear Electromagnetic Pulse (NEMP)
::*Electrostatic Discharge (ESD)
::*High Frequent Circuit Signals (For example microcontroller activity)
ˆ

Legal Requirements

By the European law manufacturers of electronic devices are advised to run EMC tests in order to compulsory CE-labeling. Undisturbed usage of electric devices for all customers should be ensured and the electromagnetic field strength should be kept on a minimum level. The EU directive 89/336/EEC on EMC announces the rules for the distribution of electric devices within the European Union. A good overview of EME limits and EMI demands is given in List of EMC directives.

See also

• EMC Aware Programming
  
• Emission Aware Programming
  
• Immunity Aware Programming
  
• Electromagnetic compatibility
  
• List of EMC directives