Common Sources of EMI

Why This Matters

Electromagnetic interference isn't just an abstract engineering problem—it's the reason your car radio crackles near power lines, your Wi-Fi drops when the microwave runs, and why aircraft passengers are asked to switch devices to airplane mode. Understanding EMI sources is fundamental to electromagnetic compatibility (EMC), which ensures devices can coexist without disrupting each other. You're being tested on your ability to identify emission mechanisms, frequency characteristics, and mitigation strategies for different interference sources.

Don't just memorize a list of noisy devices. Instead, focus on why each source generates EMI—whether through conducted emissions, radiated fields, switching transients, or arcing. Knowing the underlying mechanism helps you predict interference scenarios, select appropriate filters or shields, and answer design-focused questions. This conceptual framework turns a memorization exercise into genuine engineering intuition.

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Conducted Sources: Power System Noise

These sources inject EMI directly into electrical wiring, where it propagates to connected equipment. Conducted emissions travel through power lines, ground paths, and signal cables rather than radiating through air.

Power Lines and Electrical Grid Infrastructure

• High-voltage transmission lines create strong electric and magnetic fields—these 50/60 Hz fields can induce currents in nearby conductors and cables
• Harmonics from nonlinear loads (transformers, rectifiers, and switching equipment) pollute the grid with frequencies up to several kHz
• Corona discharge on high-voltage lines generates broadband RF noise, particularly in humid conditions or at damaged insulators

Switching Power Supplies

• Rapid transistor switching creates high-frequency noise—typical switching frequencies range from 50 kHz to several MHz, with harmonics extending much higher
• Common-mode and differential-mode currents propagate through input/output cables, affecting connected audio, medical, and communication equipment
• EMI filters at input and output stages are essential; look for LC filters, ferrite beads, and X/Y capacitors in compliant designs

Electric Motors and Generators

• Brush arcing in DC motors produces broadband noise spanning kHz to hundreds of MHz—brushless designs significantly reduce this emission
• Commutation transients and winding inductance create voltage spikes that conduct back into the power supply
• Variable-frequency drives (VFDs) add switching noise; motor cable shielding and output filters are standard mitigation approaches

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Intentional Radiators: RF Transmitters

These devices are designed to emit electromagnetic energy, but their signals can interfere with unintended receivers. The challenge is managing out-of-band emissions, harmonics, and proximity effects.

Radio and Television Transmitters

• High-power RF output (often kilowatts) can overwhelm nearby receivers through front-end overload, even outside the transmitter's assigned band
• Spurious emissions and harmonics must meet strict regulatory limits—second and third harmonics are common interference culprits
• Intermodulation products occur when multiple transmitter signals mix in nonlinear junctions, creating new interfering frequencies

Mobile Phones and Cellular Networks

• Transmit power up to 2W in handheld devices can disrupt sensitive equipment within centimeters to meters
• Multiple frequency bands (700 MHz to 6 GHz for 5G) create complex interference environments; frequency coordination is critical
• Base station density in urban areas means cumulative RF exposure; medical and avionics equipment require careful immunity design

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Switching and Arcing Sources: Transient Generators

These sources produce EMI through rapid current interruption or electrical discharge, creating broadband transients rich in high-frequency content.

Welding Equipment

• Arc welding draws hundreds of amperes with rapid arc ignition and extinction, generating transients from DC to tens of MHz
• Conducted noise travels through facility wiring—welding should be isolated on dedicated circuits with adequate impedance separation
• Radiated fields from welding cables act as loop antennas; keeping cables short and twisted reduces magnetic field emissions

Lightning and Electrostatic Discharge

• Lightning generates electromagnetic pulses (EMPs) with rise times under 1 μs and peak currents exceeding 200 kA—induced voltages can reach thousands of volts in nearby conductors
• ESD events discharge in nanoseconds, producing broadband interference from DC to beyond 1 GHz; human body model assumes 150 pF and 150 Ω
• Surge protection devices (SPDs) and transient voltage suppressors (TVS) are primary defenses; proper grounding provides the essential low-impedance path

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Unintentional Radiators: Digital and Lighting Systems

These devices don't intend to emit RF energy, but their internal switching operations create unintentional emissions that can radiate or conduct to other equipment.

Digital Electronics and Computer Systems

• Clock signals and high-speed data buses generate harmonics extending to GHz frequencies; a 100 MHz clock has significant energy at 300, 500, and 700 MHz
• PCB traces act as antennas—trace lengths approaching $\frac{\lambda}{4}$ (quarter wavelength) become efficient radiators
• Spread-spectrum clocking reduces peak emissions by distributing energy across a frequency band, helping meet regulatory limits

Fluorescent and LED Lighting

• Electronic ballasts and LED drivers switch at 20–100 kHz, producing conducted and radiated noise in that range and its harmonics
• Dimmer circuits using phase-cut control create steep current edges that generate broadband interference affecting AM radio and audio equipment
• Quality LED drivers include EMI filtering; cheap replacements often fail conducted emissions tests and cause noticeable interference

Microwave Ovens

• Magnetrons operate at 2.45 GHz ISM band—the same frequency range as Wi-Fi (802.11b/g/n) and Bluetooth
• Leakage through door seals can reach $5 \text{ mW/cm}^2$ at 5 cm (the regulatory limit), enough to disrupt 2.4 GHz wireless links in the same room
• Pulsed operation at 50/60 Hz (magnetron cycles with AC line) creates characteristic periodic interference patterns on affected devices

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Quick Reference Table

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Self-Check Questions

1. Which two sources generate EMI primarily through arcing mechanisms, and how do their frequency spectra differ?

2. A wireless router experiences periodic interference every few minutes. Which common household EMI source is most likely responsible, and what frequency band does it affect?

3. Compare conducted vs. radiated emissions from switching power supplies—which path typically dominates at lower frequencies, and why?

4. If an FRQ asks you to recommend EMI mitigation for a factory with welding stations and sensitive test equipment, what three strategies would you prioritize?

5. Why do digital systems with faster clock speeds generally produce more EMI, and what design technique spreads this energy to reduce peak emissions?