Electromagnetic Interference (EMI): What it is & How To Reduce it

What is Electromagnetic Interference?

Electromagnetic interference (EMI) is unwanted electromagnetic energy that degrades an electrical circuit or equipment function. It requires an emission source, a coupling path and a susceptible receiver. Coupling can occur through conductors, electromagnetic induction, electric fields or radiation.

A time-varying electric field and magnetic field can propagate together as an electromagnetic wave. Its speed is exactly 299,792,458 m/s in vacuum and is lower in material media.

The figure shows electromagnetic-spectrum bands by frequency and wavelength. More wireless and switching devices create more potential source-receiver combinations, but interference does not necessarily grow exponentially; compatibility depends on emissions, immunity, separation, spectrum use and installation.

EMI can occur across a wide frequency range, including radio and microwave bands. Devices with rapidly changing electrical currents can produce conducted or radiated emissions, but those emissions become interference only when they cause an unwanted effect.

Interference is defined by an unwanted effect at a receiver, not by two fields physically distorting each other. Energy can enter the receiver at the wanted frequency, at harmonics or through nonlinear mixing and overload. Audible radio noise or picture errors are possible effects on a received signal. EMI in the radio-frequency range is often called radio-frequency interference.

Current flow and switching electricity can create emissions. Energy from one device may travel through shared wiring or couple through surrounding fields into another device. It is EMI when the coupled energy degrades the second device’s operation.

What causes Electromagnetic Interference?

EMI sources include intentional transmitters, switching equipment, natural events and electrical faults.

Ambient EME

Common sources of ambient electromagnetic energy include:

• Television transmitters
• AM radio, FM radio and satellite transmitters
• Solar activity
• Lightning, with its high voltage and current transients
• Airport radar, electrostatic discharge and broadband noise
• Switch-mode power supplies
• Arc welders, motor brushes and switching contacts

Power Quality Degradation factors

Disturbances on a mains supply can also affect connected equipment:

• Power line faults
• Electrical fast transients
• Electrical noise superimposed on the supply
• Voltage surges, dips, spikes and sustained overvoltage or undervoltage

Railroad and Mass transit systems

Rail and mass-transit systems may contain these sources:

• Train signalling systems
• Emissions from train control equipment
• Track circuits

Medical Equipment Sources

Medical environments contain equipment that can emit or receive interference:

• Life-support equipment such as ventilators
• Defibrillators
• Infusion pumps
• Motorised wheelchairs
• Telemetry
• Imaging systems such as X-ray and MRI equipment

Types of Electromagnetic Interferences

Engineers classify EMI by source, duration and occupied bandwidth. Each view helps narrow the likely source and coupling path.

By source, EMI may be human-made or natural.

Human-made EMI

Human-made sources include transmitters, switching converters, digital clocks, motors, relays and electrostatic discharge. Interference can occur without equal frequencies because harmonics, modulation, intermodulation and receiver overload also create unwanted responses.

Natural EMI

Natural sources include lightning, atmospheric electrical activity, solar radio emissions and cosmic noise.

By duration, an interfering source may operate continuously or produce short impulses.

Continous EMI

Continuous EMI persists while the source operates and a coupling path exists. Examples include a clock oscillator, broadcast carrier or running motor drive.

Impulse EMI

Impulse EMI consists of short transients from sources such as switching contacts, electrostatic discharge or lightning. A short pulse has frequency content over a broad range and can disturb voltage, current or data signals.

By occupied bandwidth, EMI is described as narrowband or broadband.

Narrowband EMI

Narrowband EMI occupies a small frequency range relative to the measurement span. Oscillators, carriers and their harmonics are common sources. Its impact and difficulty depend on receiver bandwidth, amplitude, modulation and frequency separation.

Broadband EMI

Broadband EMI spans a wide frequency range. Fast switching edges, electrostatic discharge, lightning and arcing can produce broadband energy. Arc-welder emissions occur during arcing, while solar radio activity can raise receiver noise; “sun outage” more often describes the Sun entering a satellite antenna beam.

EMI Coupling Mechanisms

Troubleshooting starts by identifying the source, victim and coupling path. The main paths are conducted, radiated, capacitive and inductive coupling. A control can reduce source emission, interrupt the path or improve receiver immunity.

Conduction Coupling

Conducted coupling occurs when unwanted current travels through a shared conductor, power lead, signal cable or reference path between source and receiver. Conducted noise is commonly described in two modes:

Common Mode

Common-mode noise appears in the same direction on two conductors relative to a common reference, such as chassis or earth.

Differential Mode

Differential-mode noise appears between two conductors, with currents flowing in opposite directions around the intended circuit loop.

Radiation Coupling

Radiated coupling transfers energy through electric and magnetic fields without a shared conductor. Far-field wave behaviour applies at sufficient distance from the source relative to wavelength and source size; capacitive and inductive near-field coupling dominate many close-range equipment problems.

Capacitive Coupling

Capacitive coupling occurs through mutual capacitance between nearby conductors or structures. High dV/dt, large overlapping area, close spacing and high victim impedance increase coupled voltage.

Inductive Coupling

Inductive coupling occurs through mutual inductance. High dI/dt and large source or victim loop area increase the induced voltage; separation, loop-area reduction and routing can reduce it.

How to reduce EMI?

Industrial EMI sources include variable-frequency drives, soft starters, SCR heater controllers, switching contacts, AC and DC motors, AC and DC generators, switch-mode power supplies, radio transmitters, arc welding, electrostatic discharge and lightning. Poor control of these sources can corrupt measurement and control signals.

EMI requires a source, susceptible receiver and coupling path. Design controls include edge-rate control, filtering, physical separation, return-path design, bonding, shielding, transient suppression and improved receiver immunity. The correct combination depends on frequency and coupling mode.

Earth ground

Ground and bonding systems provide safety paths, lightning protection and circuit references. Signal and return currents flowing through their finite impedance create voltage differences, so safety grounding and signal-reference design must be coordinated.

Lightning and fault currents can create large potential differences. Plan grounding at the start of the design, meet the applicable safety rules and trace the actual current paths when diagnosing a noise problem.

Return current may not follow the path suggested by a schematic. Grounding depends on frequency, impedance, cable length, geometry and safety requirements.

A single-point reference can reduce common-impedance coupling in some low-frequency systems, as the figure illustrates. It is not a universal grounding rule: high-frequency systems often need short, low-inductance bonding and a ground plane. Avoid daisy-chained sensitive returns when shared impedance creates unacceptable noise.

In the figure, the ground potential of circuit 1 depends on its return current through impedance Z₁ and on the return currents from circuits 2 and 3 through that shared impedance. This effect is common-impedance coupling.

Separate return connections can reduce common impedance at low frequency, although they require more wiring. Practical systems often combine grounding topologies according to frequency, safety and layout.

Shielding

Shielding provides a conductive barrier that reduces electric-field, magnetic-field or radiated coupling. It must form part of a complete enclosure or cable system to meet the applicable emission and immunity limits. 

Shielding reduces coupling between circuitry and its environment. Performance depends on material, thickness, frequency, apertures, seams, bonding and termination. Reflection and absorption models alone do not capture leakage through practical openings and cable penetrations.

A shielded cable surrounds insulated conductors with a conductive braid, foil, tape or polymer layer. Its performance depends on coverage and termination, and installation must preserve shield continuity and the intended bonding method.

An unshielded twisted pair uses balanced signalling and regular twists to reduce loop area and make external coupling similar on both conductors. The receiver can reject much of that common-mode noise when the pair, connector and termination preserve balance.

Shielded and unshielded cable are not interchangeable fixes. Choose from the noise environment, frequency range, cable length, interface standard and available bonding. A poorly terminated shield or an unbalanced pair can provide less immunity than its cable label suggests.

How to reduce Conducted and Radiated EMI?

• Filter conducted noise at the source, receiver or interface.
• Use suitable cable shields with the specified terminations.
• Design PCB return paths and cabinet bonding for the relevant frequencies.
• Separate noisy power wiring from sensitive signal cables.
• Improve receiver immunity and reduce enclosure or cable coupling.

Which Cable is Not Subject to Electromagnetic Interference or Radio Frequency Interference?

Optical fiber cables use light and a non-conductive fibre, so the optical signal is immune to electromagnetic pickup and does not create a ground loop. A cable can still contain metallic strength members or armour, and its transceivers and power wiring remain susceptible to EMI.

Fibre reduces electromagnetic eavesdropping and electrical-noise coupling, but it does not make data secure by itself. Optical links still need authentication and encryption, and physical tapping is possible. Copper-link immunity depends on balance, twisting, shielding, termination and receiver design.