Controlling electromagnetic interference (EMI), also referred to as radio frequency interference (RFI), and minimizing electromagnetic susceptibility (EMS) are two important aspects of maintaining electromagnetic compatibility (EMC). Ensuring electromagnetic compatibility at a specified level and under specific operating conditions is a common demand in the design of almost all electronic systems.

Common terms used when discussing EMC

• Electromagnetic Compatibility (EMC) – the ability for a device to operate as specified even when subjected to various forms of EMI that are within specified levels. Maintaining defined level of system performance when operated within a specified environment of electromagnetic interference.
• Electromagnetic Interference (EMI) – electrical energy from an external source that has the potential to negatively impact the performance of an electronic system. It can take the form of conducted, radiated or coupled emissions.
  • Conducted emissions – unwanted electromagnetic energy carried into an electronic system via wires, PCB traces, etc. It can take for form of common-mode or differential-mode (also called normal-mode) energy.
  • Radiated emissions – unwanted electromagnetic energy that is radiated through space from the source of the interference to the electronic system.
  • Coupled emissions – unwanted electromagnetic energy that is capacitively or inductively coupled from the source of the interference to the electronic system.
• Electromagnetic susceptibility (EMS) – the degree to which electronic equipment malfunctions or breaks down when subjected to varying levels of EMI. Sometimes referred to as the degree of immunity from negative performance impacts of EMI.

 

Sources of EMI

To achieve a desired level of electromagnetic compatibility it is necessary to consider both natural and man-made or artificial sources of EMI. Natural sources of electromagnetic interference include solar flares, cosmic radiation and lightning and other atmospheric phenomena such as high winds and storms. Most natural EMI sources produce radiated interference. Lightning can be a source for both radiated and conducted EMI.

Man-made or artificial sources of EMI are more numerous and varied, they include both intentional and unintentional sources. Intentional sources of EMI include all types of wireless transmissions such as satellites, Wi-Fi, Bluetooth, RFID readers, radio and television transmissions, and so on. Unintentional sources of EMI are even more numerous and range from automotive ignitions, to electric motors, lighting systems, switch-mode power converters, relays, consumer electronics, etc.

EMI characteristics

In addition to the differentiation between natural EMI sources and man-made sources, there are several ways that EMI can be classified, such as between narrowband and broadband or between continuous and impulse interference.

Broadband EMI can be generated by a variety of sources including natural sources such as solar activity and man-made sources such as arc welders and spread-spectrum systems such as some Class D amplifiers and switch-mode power supplies and CDMA mobile telephony. The most-common sources for narrowband EMI are man-made and include various transmitters such as in Wi-Fi networks, mobile telephony (other than CDMA), even the clock signal in digital systems, and so on.

Continuous and impulse EMI sources can also be man-made or natural. Continuous interference can be caused by a wide range of sources from 50/60Hz hum from electrical transmission lines and transformers to high-frequency interference from switch-mode power supplies, industrial and medical equipment, and microcontrollers. Sources of impulse interference include lightning, switching relays, electrostatic discharges, etc.

ITU definition of interference levels

The International Telecommunication Union (ITU) is the United Nations specialized agency for information and communication technologies (ICTs). The ITU allocates global radio spectrum and satellite orbits, and develops the technical standards that ensure networks and technologies seamlessly interconnect. ITU Radio Regulations (RR1.166 to RR1.169) define interference as follows:

• Interference: the effect of unwanted energy due to one or a combination of emissions, radiations, or inductions upon reception in a radiocommunication system, manifested by any performance degradation, misinterpretation, or loss of information which could be extracted in the absence of such unwanted energy.
• Permissible interference: Observed or predicted interference which complies with quantitative interference and sharing criteria contained in these Regulations or in ITU-R Recommendations or in special agreements as provided for in these Regulations.
• Accepted interference: Interference at a higher level than that defined as permissible interference and which has been agreed upon between two or more administrations without prejudice to other administrations.
• Harmful interference: interference which endangers the functioning of a radionavigation service or of other safety services or seriously degrades, obstructs, or repeatedly interrupts a radiocommunication service operating in accordance with Radio Regulations.

References

Discussion of Electromagnetic Interference, Boyd Corp.
Electromagnetic Compatibility, Wikipedia
Harmful Interference and Infringements of the Radio Regulations, International Telecommunication Union