1. EMI vs EMC
Electromagnetic Interference (EMI) and Electromagnetic Compatibility (EMC) are two sides of the same coin. EMI refers to unwanted electromagnetic energy that disrupts the operation of electronic equipment. EMC is the ability of a device to function correctly in its electromagnetic environment without introducing intolerable disturbances to other devices.
Every electronic device is both a potential source of EMI (through emissions) and a potential victim of EMI (requiring immunity). The goal of EMC engineering is to ensure that all devices in a system can coexist without mutual interference. This requires controlling both emissions (what your device radiates or conducts) and immunity (what your device can withstand).
| Aspect | EMI (Interference) | EMC (Compatibility) |
|---|---|---|
| Definition | Unwanted EM energy that disrupts operation | Ability to operate without interference |
| Perspective | Source of the problem | Solution to the problem |
| Testing | Emissions measurements | Emissions + Immunity tests |
| Goal | Reduce to acceptable levels | Meet all regulatory limits |
| Standards | CISPR, FCC Part 15 | IEC 61000 series, CE Mark |
2. Emissions vs Immunity
Emissions testing measures the unintentional EM energy your device generates. This includes both conducted emissions (noise on power and signal cables) and radiated emissions (EM fields propagating through space). If your device exceeds the regulatory limits, it may interfere with nearby equipment such as radio receivers, medical devices, or aircraft navigation systems.
Immunity testing verifies that your device continues to operate correctly when exposed to external EM disturbances. These disturbances include electrostatic discharge (ESD), electrical fast transients (EFT), surge pulses, conducted RF, and radiated RF fields. Immunity ensures your product is robust in real-world environments.
Emissions
- Conducted Emissions (CE) — 150 kHz to 30 MHz
- Radiated Emissions (RE) — 30 MHz to 6+ GHz
- Measured with LISN, antennas, EMI receivers
- Limits set by CISPR, FCC, industry standards
- "What your device puts out"
Immunity
- ESD (IEC 61000-4-2)
- EFT/Burst (IEC 61000-4-4)
- Surge (IEC 61000-4-5)
- Conducted Immunity (IEC 61000-4-6)
- Radiated Immunity (IEC 61000-4-3)
3. Conducted Emissions
Conducted emissions are unwanted electrical signals that propagate from your device back onto power cables, signal cables, and interconnects. These emissions travel as voltage and current noise on conductors and can interfere with other equipment sharing the same power distribution system. They are measured using a Line Impedance Stabilization Network (LISN) which provides a defined impedance (typically 50Ω) and isolates the measurement from mains supply variations.
The frequency range for conducted emissions is typically 150 kHz to 30 MHz (CISPR 22/32). Common sources include switched-mode power supplies (SMPS), PWM motor drivers, digital clock circuits, and high-speed data buses. The switching frequency and its harmonics create spectral peaks, while broadband noise from fast edges creates an elevated noise floor.
Design Thumb Rules for Conducted Emissions
- Add input EMI filter — Common-mode choke + differential-mode capacitors at power entry
- Minimize loop area — Route power and return traces adjacent, use ground planes
- Decouple at source — Place bulk + ceramic caps directly at switching regulator input/output
- Control dI/dt — Use soft-start, slew rate control, snubbers on switching nodes
- Separate noisy and quiet circuits — Physical separation and filtered interconnection
SMPS Conducted Emissions Failure
A 48V-to-5V buck converter in an industrial controller failed conducted emissions at 500 kHz (switching frequency) and its harmonics. The 150 kHz to 1 MHz range showed peaks 12 dB above CISPR Class B limits.
Root Cause
Missing common-mode choke on input. Hot loop area between input cap and MOSFET was 15 cm² — far too large.
Fix Applied
Added 2.2 mH CM choke + 2× 100nF Y-caps at input. Reduced hot loop with tighter layout. Result: 18 dB margin.
4. Radiated Emissions
Radiated emissions are electromagnetic fields that propagate through free space from your device. Every current loop is a potential magnetic loop antenna, and every conductor segment is a potential electric dipole antenna. When these structures resonate or carry high-frequency currents, they radiate energy that can interfere with nearby equipment.
Radiated emissions are measured in an anechoic chamber or at an Open Area Test Site (OATS) at 3 m or 10 m distance, typically from 30 MHz to 1 GHz (CISPR 22) or up to 6 GHz (CISPR 32). The antenna rotates between horizontal and vertical polarization, and the DUT turntable rotates 360° to find the maximum emission direction.
f = frequency (MHz), A = loop area (m²), I = current (A), r = distance (m)
Design Thumb Rules for Radiated Emissions
- Continuous ground plane — No splits, slots, or gaps under high-speed signals
- Short traces for clocks — Keep high-speed clock traces as short as possible
- Filter I/O cables — Ferrites, common-mode chokes, or filtered connectors
- Shield the enclosure — Metal enclosure with proper gasketing at seams
- Control rise time — Don't use faster edges than needed; series resistors on clocks
5. ESD Immunity (IEC 61000-4-2)
Electrostatic Discharge is a rapid transfer of charge between two objects at different electrical potentials. A human body can accumulate 15-25 kV of static charge and discharge it in nanoseconds, producing current peaks up to 30-60 A with sub-nanosecond rise times. ESD is the #1 cause of field failures in consumer electronics.
IEC 61000-4-2 defines two discharge methods: Contact discharge (direct contact with conductive surfaces, more repeatable) and Air discharge (approaching the device until spark-over, higher voltage but less repeatable). Test levels range from ±2 kV to ±8 kV contact, and ±2 kV to ±15 kV air.
| Test Level | Contact Discharge | Air Discharge |
|---|---|---|
| Level 1 | ±2 kV | ±2 kV |
| Level 2 | ±4 kV | ±4 kV |
| Level 3 | ±6 kV | ±8 kV |
| Level 4 | ±8 kV | ±15 kV |
ESD Protection Design Rules
- TVS diodes on all external ports — Low capacitance TVS (< 1 pF for high-speed) at connector pins
- Guard ring on PCB edge — Grounded copper pour around board perimeter
- Keep ESD path short — TVS directly at connector, short trace to ground via
- Series resistance on I/O — 22-100Ω series resistor limits current into IC
- Avoid floating metal — All exposed metal must be grounded or have a discharge path
- Multi-layer PCB — Ground plane directly under I/O traces provides low-impedance return
6. EFT & Surge Immunity
EFT/Burst (IEC 61000-4-4)
Electrical Fast Transients simulate the noise produced by switching of inductive loads (relays, contactors, motors) on the same power distribution network. The EFT burst consists of groups of very fast pulses (5/50 ns rise/fall, 15 ms burst duration, 300 ms period) with amplitudes from ±0.5 kV to ±4 kV. These transients couple primarily through parasitic capacitance in cables and connectors.
Rise time: 5 ns | Pulse width: 50 ns | Repetition: 5/100 kHz | Burst duration: 15 ms | Burst period: 300 ms
Surge (IEC 61000-4-5)
Surge pulses simulate lightning-induced transients and power switching transients. The combination wave generator produces a 1.2/50 µs open-circuit voltage waveform and an 8/20 µs short-circuit current waveform. Test levels range from ±0.5 kV to ±4 kV line-to-ground and ±0.5 kV to ±2 kV line-to-line.
A ±2kV surge into 2Ω produces ~1kA peak current (8/20µs waveform)
Protection Components
| Component | Response Time | Energy Handling | Best For |
|---|---|---|---|
| TVS Diode | < 1 ns | Low-Medium | ESD, EFT, signal lines |
| MOV (Varistor) | ~25 ns | High | Surge on power lines |
| GDT (Gas Tube) | ~1 µs | Very High | Telecom, high-energy surge |
| Thyristor (TSS) | ~5 ns | Medium-High | Telecom lines |
7. Conducted & Radiated Immunity
Conducted Immunity (IEC 61000-4-6)
Conducted immunity tests the device's ability to withstand RF energy coupled onto cables in the frequency range 150 kHz to 80 MHz. The RF is injected using a CDN (Coupling/Decoupling Network) or EM clamp. This simulates the effect of nearby RF transmitters inducing current on cable runs. Test levels: 1V, 3V, or 10V RMS (0 dBV to 20 dBV).
Radiated Immunity (IEC 61000-4-3)
Radiated immunity exposes the device to a uniform RF field from 80 MHz to 6 GHz (or higher). The signal is amplitude-modulated at 80% AM, 1 kHz. Field strengths range from 1 V/m to 10 V/m (or 30 V/m for automotive). The test verifies the device continues to function correctly under RF illumination.
| Test | Standard | Frequency Range | Typical Level |
|---|---|---|---|
| Conducted Immunity | IEC 61000-4-6 | 150 kHz – 80 MHz | 3V / 10V RMS |
| Radiated Immunity | IEC 61000-4-3 | 80 MHz – 6 GHz | 3 / 10 V/m |
8. Noise Sources & Coupling Paths
Understanding EMI requires identifying the three elements of every interference problem: the source (where noise originates), the coupling path (how it gets to the victim), and the receptor/victim (what is affected). Breaking any one of these three links eliminates the interference.
Common Noise Sources
Switching Power Supplies
MOSFET switching creates broadband noise from kHz to hundreds of MHz. Hot loop radiation and conducted harmonics dominate.
Digital Clocks
Clock signals have harmonic content to 0.35/tr. A 100 MHz clock with 200 ps rise time has energy to 1.75 GHz.
Motors & Relays
Brush arcing, commutation noise, relay contact bounce create broadband conducted and radiated transients.
High-Speed Data Buses
USB, PCIe, DDR interfaces generate data-dependent spectral content across wide frequency ranges.
Coupling Paths
There are four fundamental coupling mechanisms. See Track 2: Coupling Mechanisms for a deep dive.
- Conducted — Through shared wires, traces, planes
- Capacitive (E-field) — Through parasitic capacitance between conductors
- Inductive (H-field) — Through mutual inductance between loops
- Radiated (EM wave) — Through free-space propagation in the far field
9. Compliance Overview
| Standard Body | Key Standards | Region | Scope |
|---|---|---|---|
| IEC / CISPR | CISPR 32, IEC 61000-4-x | International | Emissions + Immunity |
| FCC | Part 15 (Class A, B) | USA | Emissions only |
| CE Mark | EMC Directive 2014/30/EU | Europe | Emissions + Immunity |
| VCCI | CISPR-based | Japan | Voluntary emissions |
Class A devices are for commercial/industrial environments (relaxed limits). Class B devices are for residential use (stricter limits, typically 10 dB lower). If your product may be used in homes, it must meet Class B.
10. Real Failure Case Studies
IoT Gateway Exceeds FCC Part 15B at 240 MHz
An IoT gateway with a 48 MHz crystal oscillator failed radiated emissions at 240 MHz (5th harmonic) by 8 dB. The oscillator output trace ran 4 cm across a slot in the ground plane, creating a slot antenna resonant near 240 MHz.
Root Cause
Ground plane slot directly under clock trace created a slot antenna. The 5th harmonic of 48 MHz coincided with the slot's resonant frequency.
Fix
Routed clock trace away from slot, added 33Ω series resistor to reduce edge rate, stitched ground plane with vias. Passed with 6 dB margin.
USB Port Latch-up Under ±8kV Contact Discharge
A consumer tablet experienced system latch-up when ±8 kV contact ESD was applied to the USB-C connector. The USB controller IC would stop responding, requiring a power cycle to recover (Performance Criterion C — fail).
Root Cause
No TVS protection on USB data lines. ESD path went directly through USB controller's I/O protection diodes, exceeding latch-up threshold.
Fix
Added low-capacitance (0.5 pF) TVS array at connector. Shortened TVS-to-ground-via distance to 2mm. Added 22Ω series resistors. Result: passed ±15 kV air, ±8 kV contact (Criterion A).
LED Driver Fails CISPR 15 at Switching Harmonics
A 150W LED street light driver failed conducted emissions at 300 kHz and harmonics up to 5 MHz. Peaks exceeded CISPR 15 quasi-peak limits by up to 15 dB.
Root Cause
PFC boost converter's input filter was undersized. CM choke was saturating at full load. Y-cap values too small for required attenuation.
Fix
Upgraded CM choke from 3.3 mH to 10 mH (larger core to avoid saturation). Added second-stage LC filter. Increased Y-caps from 2.2 nF to 4.7 nF. Passed with 8 dB margin.