Electromagnetic Interference (EMI) is disruptions or interference to an electrical and electronic device/system caused by unwanted electromagnetic energy/signal emission from external sources. The external source can be a man-made source (e.g., an electronic device) or a natural source (e.g., lightning).  

Usually, electrical and electronic devices emit unwanted electromagnetic signals/energy while operating.  This unwanted electromagnetic signal/energy may affect the performance or even damage the other nearby and connected electronic devices or systems. It is called Electromagnetic Interference (EMI). The EMI emitted from the source (e.g., an electronic device) reaches or affects other devices (or victims) in various ways such as the conductive way (i.e., via wire/cable), radiated way (via air), electromagnetic induction coupling, and electrostatic coupling. 

 Figure 1: Understanding Conducted EMI (propagate via power line) and Radiated EMI (unwanted electromagnetic energy emissions, propagate via air) 

The unwanted electromagnetic signals emitted from a source (e.g., an electronic device) when disturbed or interfere with the reception of radio signals is called Radio frequency interference (RFI).  Hence, electromagnetic interference (EMI) is sometimes also called Radio Frequency Interference (RFI). The term Radio Frequency Interference (RFI) is usually used when electromagnetic interference impacts the reception of radio signals.  

EMI may cause electrical and electronic device(s) malfunctions/damage, can prevent the effective utilization of radiofrequency (rf) spectrum, can ignite flammable or other hazardous atmospheres, and could also directly impact human tissue.  For example, computer-generated unwanted EM signals may reach the TV via a power line and/or radiated way and may interfere with TV operation.

EMI can be prevented or mitigated by properly designing and producing products, using EMI mitigation techniques such as power line filters, EMI shielding, proper PCB design, grounding, etc, and compliance with EMC standards.  

Note: EMI can be used intentionally in electronic warfare for radio jamming.

What are the causes of Electromagnetic interference (EMI)? 

EMI occurs due to both human-made and natural sources.  

Human-made EMI sources include ignition systems, cellular networks of mobile phones, power electronic converters, SMPS, motors, computers, LED screens, power grid switching, interruption of inductive loads/inductive load switching, relay contact bounce, AC/DC switching operation, workshop machine operation, automobiles & vehicles, malfunctioning or improperly designed electronic devices, etc. For example, In the case of inductive load switching, when current flow through an inductive load is disrupted suddenly (by switching operation), there will be transient voltage (in KV) developed across the inductive load. This can severely damage electrical/ electronic devices connected (&near) to the same line.  

Natural EMI sources include Lightning, solar storms, solar flares, cosmic radiation, and static noise.  For example, Solar flares (highly intense electromagnetic radiation released from the sun’s surface) can affect satellite communication systems and can also cause HF radio signals degradation or complete absorption, resulting in radio blackout.  Cosmic radiation may cause undesirable accidental effects on the aircraft's digital systems. 

Types of Electrometric interference (EMI): 

In addition to the man-made or natural EMI category, EMI can also be classified based on characteristics such as bandwidth, time duration, and EMI coupling mechanism.

Classification of EMI based on bandwidth:

• Narrowband EMI: This EMI occurs over a discrete frequency, often at a single frequency. It mainly affects communication devices, including radios, TVs, and mobile phones. The narrowband EMI can be caused by oscillators, communication transmitters, test equipment, and other causes. This type of EMI is typically observed as continuous sine waves and can be continuous or intermittent. Typically, this EMI does not cause permanent harm to the device or system. The mitigation of narrowband EMI is easier as compared to broadband EMI.  
• Broadband EMI: This EMI occurs over a broad frequency spectrum or comprises multiple frequencies. It occupies a large part of the electromagnetic spectrum and can cause permanent damage to the device or system.  The broadband EMI can be caused by both natural and manmade sources. Examples of manmade sources include arc welding, computers, switching circuits, SMPS, communication transmitters, and more. An example of a natural source is solar radiation, which can interfere with satellite signals.

Classification based on duration:

• Continuous EMI: This EMI is emitted from a source continuously and keeps interfering with the nearby device. This EMI can be caused by man-made or natural sources.
• Impulse EMI: It is a pulsed electromagnetic signal that occurs either within a brief period or sporadically. Impulse EMI can be caused by man-made or natural. For example, it can be caused by electrostatic discharges, lightning, current switching systems, and other causes. 

Classification based on EMI coupling mechanism:

Electromagnetic interference (EMI) occurs when there is a source, coupling path (s), and a receptor (or victim) is present. The EMI emitted from a source may reach or affect the victim basically via four coupling mechanisms. The four coupling mechanisms are conductive coupling, radiative coupling, capacitive coupling, and inductive coupling (magnetic coupling). The EMI from a source (e.g., computer) may travel via one or more of these coupling paths to reach/affect the receptor/victim (e.g., TV).   

Based on these coupling mechanisms, EMI can be classified as conducted EMI, radiated EMI, and coupled EMI. 

Figure 2: Four Electromagnetic interference (EMI) coupling modes  

Conducted EMI: 

Conducted EMI (also called conducted emission) occurs when there is a physical electrical path between the source and the receptor (Figure 2). Conducted EMI are the unwanted electromagnetic signals (EM signals) or noise currents at radio frequencies emitted from a source and propagated through the power cord or power line, connected cables, PCB traces, and associated circuits to reach/affect the victim device. Here, the EMI reaches/affects the victim device via the conducted power line, cables, or PCB traces, hence the name conductive coupling.  An example of conducted EMI is that the switching ON of a motor or clothes dryer may cause a computer on the same electrical line circuit to reboot. 

Note: In conducted EMI scenarios, common mode noise is the noise currents that flow in the same direction on both the conductors. The common mode noise currents complete the circuit through the parasitic capacitances and ground path. Differential mode noise is the noise currents that flow on the signal and return paths with equal magnitude and opposite directions (a 180° phase shift) to each other.  

Coupled EMI: 

Coupled EMI can be categorized into two types: capacitive coupled and inductive coupled EMI. 

Capacitive coupled EMI occurs when there is a time-varying electrical field between two adjacent conductors that are very close to each other, typically less than a wavelength apart. The time-varying electric field and the capacitive coupling between the conductors result in unintentional currents being induced in nearby conductors. These unintentional currents can then travel through the conductor, potentially causing interference in the connected victim device. 

Inductive coupled EMI occurs when there is a time-varying magnetic field between two adjacent conductors that are very close to each other, typically less than a wavelength apart. The time-varying magnetic field and the mutual inductance between the conductors result in unintentional voltage being induced along the nearby conductors, potentially causing interference in the connected victim device. The inductive coupled EMI occurs by the electromagnetic induction principle between the two conductors. 

Note: Capacitive and inductive coupled EMI primarily fall under the conducted EMI emissions category. Since, both capacitive and inductive coupling involve the transmission of unwanted signals through physical conductors like power lines, cables, or printed circuit board (PCB) traces.  
Note: For lower frequencies, EMI is due to conduction.  

Radiated EMI: 

Radiated EMI (also radiated emission) occurs when the source and victim device act as radio antennas. In the radiated EMI, the source emits or radiates unwanted EM signals or noises during its operation in the form of electromagnetic waves (at radio frequency). The waves are propagated through space (air) and are picked up or received by the victim device (Figure 2). This interference can manifest as disruptions, distortions, or malfunctions in the victim's device. Radiative coupling or electromagnetic coupling occurs when there is a large distance between the source and the victim. 

An example of radiated EMI is that radiated EMI from a computer can disturb a TV, audio equipment, or other devices in the same environment.
Note: For higher frequencies, EMI is caused by radiation.

How to prevent or mitigate Electromagnetic interference (EMI)? 

Mitigating EMI is crucial in maintaining the proper functioning of electronic devices and systems. Here, some commonly employed EMI mitigation techniques are provided: 

Proper PCB design: Properly designing the printed circuit board (PCB) and consideration of EMI is crucial when designing electronics and PCB. Careful design of the PCB can minimize signal loops and optimize grounding. PCB designers must consider component location and routing.  

Good quality electronics: Use good quality electronics from reputable manufacturers for product manufacturing. 

Grounding: Proper grounding provides a path for unwanted electrical currents to dissipate, reducing the risk of EMI. 

Filters: The use of filters on power lines or signal lines can suppress conducted EMI or high-frequency noise currents to a satisfactory and acceptable level. The filter prevents EMI from leaving an electrical and electronic device and also prevents the entering of EMI from external devices via the connected power/signal line. Hence, the filters limit the EMI emission from a device as well as enhance the immunity of the device against the conducted EMI from the external device.  

Ferrite beads and Ferrite cores: The use of ferrite beads on cables can mitigate conducted EMI. Also, placing ferrite cores around cables can absorb high-frequency noises on a power supply line/data line.  

Shielding: Shielding (i.e., placing conductive materials around sensitive components or devices) can suppress or block radiated EMI. Shielding acts as a Faraday cage and prevents radiated EMI from leaving a device and also prevents the entering of Radiated EMI from external devices. Hence, the shielding limits the EMI emission from a device as well as enhances the immunity of the device against the radiated EMI from the external devices. 

Isolation: Physically separating sensitive components or devices from potential sources of EMI. For example, the use of Isolation transformers, optical isolators, and physical distancing.  

Shielded and twisted pair cables: The use of shielded and twisted pair cables ensures higher signal integrity. 

Utilize fiber optic cable: Fiber optic cable can be used as an alternative to copper cables since the fiber optic cable is immune to EMI.  

Wireless network proper planning: Proper planning of wireless networks near high-power transmission lines minimizes EMI potential and ensures optimal performance of the wireless network. Proper planning includes antenna placement and other factors. 

Compliance with EMC emission and immunity Standards: Complying with Electromagnetic Compatibility (EMC) standards during the design and testing phases will ensure an electrical and electronic product will function properly in its intended electromagnetic environment and will not interfere with/disturb other devices in the same environment.  EMC means the ability of an electrical and electronic device to function satisfactorily in its intended electromagnetic environment without disturbing other nearby devices in the same environment.  

EMC emission standard ensures that the product will not emit more than the EMI level specified in the relevant emission standard document. The emission standards are available for both conducted and radiated EMI. EMC immunity standards ensure that the product can withstand some specified amount of EMI and will function properly while working in the intended real-time environment.  The immunity standards are available for both conducted and radiated EMI.  

Government regulatory bodies establish EMC standards and guidelines to minimize interference and ensure the coexistence of various electronic devices in the same electromagnetic environment. For example, FCC Part 15 is a United States EMC emission standard (conductive, radiated) for radio frequency devices (intentional and unintentional radiators). EN 55011 is a European Union (EU) EMC emission standard for Industrial, Scientific, and Medical (ISM) radio-frequency equipment.

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