Fundamentals of Electromagnetic Interference and Compatibility


Whenever electricity is used to drive equipment, an electromagnetic signal ensues as well. These signals can be used to transmit information from one point to the next, or they can simply be a byproduct of the operation of equipment. Where the signals are unintended, we speak of electromagnetic noise. It is this noise that can cause equipment to malfunction, and manufacturers must therefore take steps to reduce the effects of noise.  One of the main sources of electromagnetic signals is electrically powered equipment. In this category we can identify two main sources of electromagnetic noise: power supplies and motor drive systems. On the other hand, we also purposely generate signals in order to transmit them over a distance, as in a TV station. The intended generation of signals for information transmission is not considered noise, but it does have an impact on the overall electromagnetic compatibility (EMC) of equipment.  

A key characteristic of electromagnetic noise is its frequency. The first important frequency range is the range around the power network frequency, which in Europe is 50 Hz. Most loads connected to the power network are non-linear loads, i.e., they draw a current that does not follow the sinusoidal voltage. These currents are called harmonics. The radio frequency (RF) range is generally split into a conducted and a radiated range. For the lower part of the RF range, noise is expected to travel along lines rather than radiate from the equipment. The main reason for this is that the physical size of most equipment is simply not sufficient to radiate low frequency noise. To summarize, we have the following ranges: 

Harmonics: [50 Hz–2/2.5 kHz]

LF range: [2/2.5 Hz–9 kHz] Unregulated range

Conducted RF range:

- [9 kHz–150 kHz] Unregulated range

- [150 kHz–30 MHz] Regulated range

Radiated RF range:

- [30 MHz–1/2/3 GHz] Unregulated range

- [Above 3 GHz] Regulated range

Electromagnetic compatibility

Electromagnetic phenomena and their effect on electric and electronic equipment have compelled many countries to implement measures that ensure the proper operation of equipment. Originally, most requirements came from military sectors and civil aviation, where the need for flawless operation of equipment is strongly related to the safety of people. The scope of the EMC directive is very general. All products likely to emit or be susceptible to electromagnetic energy are covered. To show compliance with the directive, products must not emit EM energy in an amount sufficient to affect other equipment. At the same time, each piece of equipment must be sufficiently immune against EM energy from other sources.


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