Electromagnetic interference (EMI), also known as radio-frequency interference (RFI) when it occurs in the radio frequency range, is a disruption caused by outside sources that affects electrical circuits. This disruption can happen through electromagnetic induction, electrostatic coupling, or conduction. These effects may make the circuit work worse or stop it from working. In data transmission, these issues can cause more errors or even complete loss of data.
Both human-made and natural sources create changing electrical currents and voltages that lead to EMI. Examples include car ignition systems, mobile phone networks, lightning, solar flares, and auroras (northern or southern lights). EMI often affects AM radios. It can also impact mobile phones, FM radios, televisions, and scientific observations related to radio astronomy and weather studies.
EMI can be used intentionally for radio jamming, such as in electronic warfare.
History
Since radio communications began, problems caused by unwanted signals from both planned and accidental transmissions have been noticed. This led to the realization that managing the radio frequency spectrum was necessary.
In 1933, a meeting of the International Electrotechnical Commission (IEC) in Paris suggested creating the International Special Committee on Radio Interference (CISPR) to address the growing issue of electromagnetic interference (EMI). CISPR later developed technical guides about testing methods and set limits for how much electromagnetic energy devices can emit and how well they can resist interference. These standards have been updated over time and now form the foundation for many global electromagnetic compatibility (EMC) rules.
In 1979, the Federal Communications Commission (FCC) in the United States introduced legal limits on electromagnetic emissions from digital devices. This was done because more digital systems were causing interference with both wired and radio communications. The test methods and limits used by the FCC were based on CISPR guidelines, even though similar rules had already been in place in parts of Europe.
In the mid-1980s, European Union countries adopted several "new approach" rules to standardize product requirements and remove technical barriers to trade within the European Community. One of these rules was the EMC Directive (89/336/EC), which applies to all equipment sold or used in the EU. This directive covers all devices that can cause electromagnetic interference or whose performance might be affected by such interference.
This was the first time laws required devices meant for the general public to meet standards for both how much electromagnetic energy they emit and how well they can resist interference. While meeting these standards might increase costs for some products, it also improves their perceived quality because they can function reliably in the complex electromagnetic environment of modern life with fewer issues.
Today, many countries have similar rules requiring products to meet certain levels of electromagnetic compatibility (EMC) standards.
Types
Electromagnetic interference is grouped into different types based on its source and how the signals behave.
The source of interference, often called "noise," can come from human activities (artificial) or natural causes.
Continuous interference, also called continuous wave (CW) interference, happens when a source sends out signals constantly over a range of frequencies. This type is further divided into subcategories based on the frequency range. It is sometimes called "DC to daylight," which refers to a wide range of frequencies from direct current to visible light. A common way to classify this type is by whether the frequency range is narrow (narrowband) or wide (broadband).
An electromagnetic pulse (EMP), also known as a transient disturbance, occurs when a source releases a short burst of energy. This energy is usually spread across a wide range of frequencies. However, it often causes a specific type of response in the affected device, which is a damped sine wave with a narrow frequency range.
Sources of interference are broadly divided into two groups: isolated events and repetitive events.
Examples of isolated EMP events include:
Examples of repetitive EMP events, which may occur as regular pulses, include:
Conducted electromagnetic interference happens when conductors (like wires or cables) are physically connected, unlike radiated EMI, which happens through induction (without direct contact). When electromagnetic disturbances in a conductor's field are not confined to its surface, they spread outward. This occurs in all conductors, and the interaction between electromagnetic fields in two conductors can cause EMI.
Some technical terms may have different meanings depending on the context. However, the terms used here are widely accepted and consistent with other scientific sources.
The basic setup of electromagnetic interference includes a noise source, a path that carries the interference, and a receiver (or victim). The source and receiver are usually electronic devices, though the source can also be natural, such as lightning, static electricity, or even the Big Bang.
There are four main ways interference can travel: conductive, capacitive, magnetic (or inductive), and radiative. A single path can involve more than one of these mechanisms. For example, a path might use inductive, conductive, and capacitive coupling together.
Conductive coupling happens when the source and receiver are connected directly through a conductor, such as a wire, cable, or metal enclosure. Conducted noise can appear differently on various conductors.
Inductive coupling occurs when the source and receiver are close to each other (usually less than a wavelength apart). There are two types of inductive coupling: electrical induction (often called capacitive coupling) and magnetic induction (often called inductive coupling).
Capacitive coupling happens when a changing electric field exists between two nearby conductors (less than a wavelength apart), causing a voltage change in the receiving conductor.
Inductive or magnetic coupling happens when a changing magnetic field exists between two parallel conductors (less than a wavelength apart), causing a voltage change in the receiving conductor.
Radiative coupling, also called electromagnetic coupling, happens when the source and receiver are far apart (usually more than a wavelength). In this case, the source acts like a radio antenna, sending out electromagnetic waves that travel through the air and are picked up by the receiver, which also acts like a radio antenna.
ITU definition
Electromagnetic interference (EMI), also called radio-frequency interference (RFI), is defined by Article 1.166 of the International Telecommunication Union's (ITU) Radio Regulations (RR) as "the effect of unwanted energy from one or more sources, such as emissions, radiation, or induction, that harms the ability of a radio communication system to receive signals properly. This can cause problems like making it harder to receive signals clearly, misunderstanding messages, or losing information that would normally be understood."
This definition is used by frequency administrators to assign specific frequencies to radio stations or systems and to check how different radio communication services work together without causing problems.
According to ITU RR (Article 1), types of interference are grouped into different categories as follows:
Conducted interference
Conducted electromagnetic interference (EMI) occurs when electrical conductors touch each other. Radiated EMI happens without physical contact, instead caused by induction.
At lower frequencies, EMI is typically caused by conduction. At higher frequencies, EMI is usually caused by radiation.
Electromagnetic interference can also occur through the ground wire in electrical systems.
Susceptibilities of different radio technologies
Interference is more common in older radio technologies, such as analog amplitude modulation, because these systems cannot tell unwanted signals from the intended signal. They also use omnidirectional antennas, which pick up signals from all directions. Newer radio systems have improvements that help them better select the correct signal. Digital radio systems, like Wi-Fi, use error-correction methods to reduce interference. Both analog and digital systems can use spread-spectrum and frequency-hopping techniques to improve resistance to interference. A highly directional receiver, such as a parabolic antenna or a diversity receiver, can focus on one signal while ignoring others.
The most extreme example of digital spread-spectrum signaling is ultra-wideband (UWB), which uses large parts of the radio spectrum at low amplitudes to send high-speed digital data. If used alone, UWB could use the spectrum very efficiently. However, users of non-UWB technology are not ready to share the spectrum with UWB because it might cause interference for their receivers. The rules and regulations about UWB are covered in the ultra-wideband article.
Interference to consumer devices
In the United States, the 1982 Public Law 97-259 gave the Federal Communications Commission (FCC) the authority to control how well consumer electronic equipment can resist interference from electromagnetic sources.
Common sources of radio frequency interference (RFI) and electromagnetic interference (EMI) include devices such as transmitters, doorbell transformers, toaster ovens, electric blankets, ultrasonic pest control devices, electric bug zappers, heating pads, and touch-controlled lamps. When multiple cathode ray tube (CRT) computer monitors or televisions are placed too close together, they may cause a "shimmy" effect because of the electromagnetic fields from their picture tubes, especially when one of their de-gaussing coils is activated.
Electromagnetic interference at 2.4 GHz can come from wireless devices like 802.11b, 802.11g, and 802.11n networks, Bluetooth devices, baby monitors, cordless telephones, video senders, and microwave ovens.
Switching electrical loads, such as electric motors, transformers, heaters, lamps, ballasts, and power supplies, often create electromagnetic interference, especially when currents exceed 2 A. To reduce this interference, a common method is to connect a resistor and capacitor in series across a pair of contacts. However, this method is only effective for very low currents and does not work well for currents above 2 A with electromechanical contacts.
Another way to reduce EMI is by using ferrite core noise suppressors, also called ferrite beads. These are inexpensive devices that attach to the power cord of the device causing interference.
Switched-mode power supplies can produce EMI, but this issue has become less common as design improvements, such as integrated power factor correction, have been implemented.
Most countries require electronic and electrical equipment to meet electromagnetic compatibility standards. These rules ensure that devices function properly in the presence of EMI and do not emit interference that could disrupt other equipment, such as radios.
Radio frequency signal quality has decreased by about one decibel each year since the start of the 21st century due to increased use of the spectrum. This has created a situation where the mobile phone industry must constantly build more cellular towers at new frequencies to manage interference, which leads to higher costs and frequent updates to mobile phones.
Standards
The International Special Committee for Radio Interference, known as CISPR (French for "Comité International Spécial des Perturbations Radioélectriques"), is a group within the International Electrotechnical Commission (IEC). This committee creates international standards to control radio interference caused by electrical devices. These standards apply to non-military areas such as homes, businesses, factories, and vehicles. These rules serve as a foundation for other standards used in different countries or regions, including the European Norms (EN) developed by CENELEC (European committee for electrotechnical standardisation). In the United States, organizations like the Institute of Electrical and Electronics Engineers (IEEE), the American National Standards Institute (ANSI), and the US Military (MILSTD) also create related standards.
EMI in integrated circuits
Integrated circuits can create electromagnetic interference (EMI), but they usually need to transfer their energy to larger objects, such as heatsinks, circuit board planes, or cables, to produce significant radiation.
To reduce EMI on integrated circuits, important methods include placing bypass or decoupling capacitors near each active device (connected across the power supply), controlling the speed of high-speed signals with series resistors, and filtering the power supply pins of the IC. Shielding is typically used only after other methods fail because it adds cost and requires components like conductive gaskets.
The effectiveness of radiation depends on the height of a conductor above the ground or power plane (at radio frequencies, these are similar) and the length of the conductor compared to the signal’s wavelength (including the fundamental frequency, harmonics, or transients like overshoot or ringing). At lower frequencies, such as 133 MHz, radiation mostly happens through I/O cables. Radio frequency (RF) noise enters power planes and is transferred to line drivers through the VCC and GND pins. The RF noise then travels through the cable as common-mode noise. Shielding has limited effect in these cases, even with differential pairs, because the noise is common-mode. The RF energy is transferred through capacitance from the signal pair to the shield, and the shield itself radiates the noise. A solution is to use a braid-breaker or choke to reduce the common-mode signal.
At higher frequencies, usually above 500 MHz, traces become longer and higher above the plane. Two methods are used: shaping signals with series resistors and placing traces between two planes. If these steps still leave too much EMI, shielding such as RF gaskets or copper/conductive tape can be used. Most digital devices use metal or conductive-coated plastic cases.
Unshielded semiconductors, such as integrated circuits, can act as detectors for radio signals found in homes (like those from mobile phones). These signals can be converted into low-frequency sounds, causing unwanted noise in audio devices like microphones, speakers, car radios, or telephones. Adding EMI filters or using special layout techniques can help reduce interference or improve resistance to RF signals. Some ICs, such as LMV831-LMV834 or MAX9724, are designed with built-in RF filters or special layouts to reduce signal interference.
Designers often test parts for RF immunity using special equipment. These tests are usually done in an anechoic chamber, a room with a controlled RF environment that mimics real-world conditions. Test signals create an RF field similar to what would be encountered in actual use.
RFI in radio astronomy
Radio interference, or RFI, is any signal that is not from space but comes from Earth. These signals can be much stronger than the weak signals from space, making RFI a major problem for radio astronomy.
Some important frequencies used in radio astronomy, like the 21-cm HI line at 1420 MHz, are protected by rules. However, modern observatories, such as VLA, LOFAR, and ALMA, can observe over a wide range of frequencies. Because there are limited radio frequencies available, some of these bands overlap with other uses, such as FM radio (88–108 MHz). This means radio telescopes must deal with RFI in these areas.
To handle RFI, scientists use methods like filters in equipment and software tools. One way to stop strong signals is to block their specific frequency completely. For example, LOFAR filters out FM radio signals between 90 and 110 MHz. It is important to remove these strong signals early because they can overwhelm the sensitive receivers used in telescopes. However, blocking a frequency means those frequencies cannot be studied with the instrument.
Another method is using software to detect and remove RFI within the observed frequency range. This software identifies parts of the data that are affected by interference and excludes them from analysis. This process is called data flagging. Most interference sources, like CB radios, are not always active, so most data remains usable. However, data flagging cannot solve problems caused by continuous signals, such as those from windmills or digital transmitters.
A way to reduce RFI is to create a radio quiet zone (RQZ). An RQZ is an area with rules that limit signals to help radio astronomy. These rules may include limits on signal strength, control of devices that emit signals, and management of things like aircraft and power lines. The first RQZ, the United States National Radio Quiet Zone (NRQZ), was created in 1958.
RFI on environmental monitoring
Before Wi-Fi was introduced, one of the most important uses of the 5 GHz frequency band was the Terminal Doppler Weather Radar. The choice to use the 5 GHz spectrum for Wi-Fi was decided at the World Radiocommunication Conference in 2003. However, weather monitoring groups were not part of this decision. Later, poor setup and incorrect configuration of a feature called DFS caused major problems for weather radar systems in several countries. In Hungary, the weather radar system stopped working for over a month. Because of the serious interference, weather services in South Africa stopped using the C band and changed their radar network to the S band.
Signals sent on frequency bands next to those used by passive remote sensing, such as weather satellites, have caused interference, sometimes very serious. There is worry that using 5G without proper rules could lead to major interference problems. Strong interference can harm weather prediction models and cause economic and safety issues. These worries led US Secretary of Commerce Wilbur Ross and NASA Administrator Jim Bridenstine to ask the FCC in February 2019 to stop a planned spectrum auction. The FCC did not agree to this request.