Electronics Guide

Conducted Emissions

Conducted emissions travel along cables and conductors connected to equipment:

• Power line emissions: Noise currents injected back into the AC mains, typically measured from 150 kHz to 30 MHz using a line impedance stabilization network (LISN)
• Signal line emissions: Unwanted signals appearing on communication, control, or sensor cables
• Harmonic currents: Low-frequency current distortion from nonlinear loads, regulated separately from RF emissions
• Voltage fluctuations and flicker: Variations in supply voltage that can affect lighting and other equipment

Conducted emissions are often easier to address than radiated emissions because filtering can be applied at cable entry points. However, filtering must be appropriate for the frequencies and impedances involved.

Radiated Emissions

Radiated emissions propagate through space as electromagnetic waves:

• Near-field emissions: Magnetic field (H-field) and electric field (E-field) components close to the source where wave impedance varies
• Far-field emissions: Plane wave radiation measured at distances where E and H fields maintain constant ratio
• Unintentional radiators: Emissions from circuits not designed as transmitters but containing oscillating currents
• Clock and switching noise: Harmonic-rich signals from digital circuits and switching power supplies that create broadband emissions

Radiated emissions result from current loops acting as antennas. Reducing loop areas, slowing edge rates where possible, and providing low-impedance return paths minimize radiation.

Emission Sources in Analog Circuits

While analog circuits are often victims of EMI, they can also be sources:

• Switching regulators: The most common emission source in analog systems, with fundamental and harmonic energy from switching transitions
• Oscillators: Crystal oscillators, VCOs, and PLLs can radiate if not properly shielded and filtered
• Class D amplifiers: Pulse-width modulated outputs contain significant high-frequency energy
• Data converters: Clock signals and digital outputs create emissions requiring careful management
• Op-amp instability: Oscillating amplifiers can radiate across a wide frequency range

Conducted Susceptibility

Disturbances entering via connected cables can disrupt operation:

• Power line disturbances: Voltage dips, surges, transients, and high-frequency noise on supply lines
• Radio frequency injection: RF energy coupled onto cables acting as receiving antennas
• Electrical fast transients (EFT): Bursts of fast pulses from switching inductive loads
• Surge immunity: High-energy transients from lightning and switching events
• Common-mode interference: Disturbances appearing between cables and ground reference

Radiated Susceptibility

Equipment must withstand electromagnetic fields from various sources:

• RF immunity: Continuous or modulated RF fields from transmitters, typically tested from 80 MHz to several gigahertz
• Magnetic field immunity: Power frequency and pulsed magnetic fields from transformers and industrial equipment
• Electrostatic discharge (ESD): High-voltage transients from charged personnel or objects
• Pulse immunity: Fast transients and surges representing lightning and switching events

Analog Circuit Vulnerability

Analog circuits present particular susceptibility challenges:

• High-impedance inputs: Sensor interfaces and instrumentation amplifiers with high input impedance readily pick up interference
• Wide bandwidth: Circuits designed for wide bandwidth cannot easily filter out interference within their passband
• Low-level signals: Millivolt or microvolt signals are easily overwhelmed by interference
• Rectification effects: RF interference can be rectified by semiconductor junctions, creating DC offsets and errors
• Intermodulation: Multiple interfering signals can mix to produce new frequencies

Differential-Mode Noise

Differential-mode (DM) noise appears between signal conductors:

• Signal degradation: Directly adds to or subtracts from the desired signal
• Current flow: Noise currents flow in opposite directions on signal and return conductors
• Loop radiation: Creates magnetic field proportional to loop area between conductors
• Filtering: Series inductors and parallel capacitors between signal lines attenuate differential-mode noise

Differential-mode noise is often the most directly harmful to signal integrity because it appears as a signal that the receiving circuit cannot distinguish from the intended signal.

Common-Mode Noise

Common-mode (CM) noise appears equally on all conductors relative to a reference:

• Ground reference shift: Represents potential difference between ground references at different points
• Current flow: Noise currents flow in the same direction on signal and return conductors
• Conversion to differential: Asymmetries in circuit or cable can convert common-mode to differential-mode
• Cable radiation: Common-mode currents on cables cause them to radiate as antennas

Common-mode noise is often the dominant EMC problem because it can be converted to differential-mode noise by imbalances, and because it drives cable radiation.

Mode Conversion Mechanisms

Understanding mode conversion helps prevent common-mode problems from corrupting signals:

• Impedance imbalance: Different impedances to ground on signal and return paths convert CM to DM
• Capacitive imbalance: Unequal stray capacitance from conductors to ground creates conversion
• Inductive imbalance: Unequal inductance in signal and return paths causes conversion
• Layout asymmetry: Physical asymmetries in PCB traces or cable construction enable conversion

Maintaining balance in signal paths, both electrically and physically, minimizes mode conversion and improves immunity to common-mode disturbances.

Common-Mode Rejection

Differential circuits inherently reject common-mode interference:

• Instrumentation amplifiers: Provide high common-mode rejection ratio (CMRR) for sensor interfaces
• Differential signaling: Signal information encoded as difference between two conductors is immune to common-mode shifts
• Balanced circuits: Matched impedances on both signal legs maximize common-mode rejection
• Common-mode chokes: Provide high impedance to common-mode currents while passing differential signals

Power Supply Filtering

Power supply inputs are a primary path for both emissions and susceptibility:

• Input filter stages: Pi or T filter configurations using inductors and capacitors to attenuate conducted emissions
• X capacitors: Connected line-to-line to attenuate differential-mode noise; require safety rating for AC mains
• Y capacitors: Connected line-to-ground to attenuate common-mode noise; limited in value for safety leakage requirements
• Common-mode chokes: Wound with opposing windings to present high impedance to common-mode currents
• Ferrite beads: Provide frequency-dependent impedance to attenuate high-frequency noise

Filter design must consider source and load impedances, which vary with frequency and operating conditions.

Signal Line Filtering

Filtering signal lines requires balancing noise attenuation against signal bandwidth:

• RC filters: Simple first-order lowpass filtering for slow signals; resistor prevents filter ringing
• LC filters: Higher-order filtering for signals requiring wider bandwidth; need damping to prevent resonance
• Feedthrough capacitors: Provide effective high-frequency bypass in shielded enclosures
• Ferrite filtering: Beads and cores provide lossy attenuation without resonance problems
• Differential filtering: Matched components on both signal legs maintain balance

Filter Placement and Implementation

Where and how filters are placed significantly affects their performance:

• Point of entry: Filter immediately where cables enter enclosure, before noise can couple to internal circuits
• Close coupling: Minimize lead length between filter components to prevent inductance from degrading high-frequency performance
• Ground connection: Filter ground must connect directly to enclosure or ground plane with minimum impedance
• Isolation: Separate filter input and output to prevent bypassing through parasitic coupling

Filter Component Selection

Component characteristics significantly impact filter performance:

• Capacitor ESL: Equivalent series inductance limits high-frequency bypass effectiveness; use low-ESL types
• Capacitor ESR: Equivalent series resistance affects filter damping and losses
• Inductor saturation: Core saturation reduces inductance under high current or DC bias
• Self-resonant frequency: Components become ineffective or opposite in function above self-resonance
• Temperature stability: Filter parameters should remain stable over operating temperature range

Shielding Principles

Shields attenuate electromagnetic fields through multiple mechanisms:

• Reflection loss: Impedance mismatch at shield boundaries reflects incident energy; dominant for electric fields and high-frequency plane waves
• Absorption loss: Energy dissipated as heat within shield material; increases with frequency, conductivity, permeability, and thickness
• Multiple reflections: Energy reflecting between shield surfaces can add constructively or destructively
• Skin effect: High-frequency currents concentrate at shield surface, requiring only thin shields at high frequencies

Shield Material Selection

Different materials offer different shielding characteristics:

• Copper and aluminum: Excellent conductivity provides good reflection loss and high-frequency absorption
• Steel: Magnetic permeability improves low-frequency magnetic field shielding; lower conductivity reduces high-frequency performance
• Mu-metal: Very high permeability for low-frequency magnetic shielding; expensive and mechanically delicate
• Conductive coatings: Spray, paint, or plated finishes provide shielding for plastic enclosures
• Conductive composites: Carbon-filled or metal-particle plastics offer moderate shielding with weight savings

Shield Integrity

Actual shielding effectiveness depends heavily on maintaining shield continuity:

• Seams and joints: Any gap or discontinuity degrades shielding; overlapping joints, conductive gaskets, or welding required for good performance
• Apertures: Openings for ventilation, displays, and controls limit shielding; keep dimensions small relative to wavelength
• Cable penetrations: Cables act as antennas coupling energy through shields; require filtering or shielded connectors
• Corrosion: Oxide layers at joints increase contact resistance; proper surface treatment and gasket selection essential

Shield effectiveness is limited by the weakest path for field penetration. A small gap or untreated cable can reduce a theoretically excellent shield to mediocre performance.

Shielding at Different Frequencies

Shielding requirements vary dramatically with frequency:

• Low-frequency magnetic fields: Require thick high-permeability materials or multiple nested shields; often the most difficult to address
• Low-frequency electric fields: Even thin conductive shields provide excellent attenuation due to reflection
• RF frequencies: Thin conductive shields effective due to skin effect; aperture control becomes critical
• Microwave frequencies: Any aperture larger than a fraction of wavelength severely compromises shielding

Ground Plane Functions

A properly designed ground plane serves multiple EMC functions:

• Signal return path: Provides low-inductance return for signal currents, minimizing loop areas
• Reference plane: Establishes common reference potential across the circuit
• Shielding: Contains electromagnetic fields between signal layers and ground plane
• Heat spreading: Distributes thermal energy from power dissipation

Ground Plane Implementation

Effective ground plane implementation requires attention to detail:

• Solid copper pour: Use continuous copper plane without unnecessary splits or slots
• Via stitching: Connect ground planes on different layers with multiple vias to reduce plane impedance
• Avoid slots and splits: Any gap forces return currents to detour, creating radiation and crosstalk
• Return current visualization: Consider where high-frequency return currents will flow; they follow the path of minimum inductance

High-frequency return currents flow directly beneath signal traces to minimize loop inductance. Any obstacle that forces current to deviate creates EMC problems.

Ground Plane Partitioning

Mixed-signal systems may benefit from strategic ground plane organization:

• Single-point connection: Separate analog and digital ground regions connected at one point to prevent digital return currents from flowing through analog ground
• Unified ground plane: Modern practice often favors continuous ground with careful component placement to achieve similar results without the drawbacks of splits
• Star grounding: Multiple separate grounds connected at a single star point; appropriate for low-frequency systems
• Hybrid approaches: Combine techniques based on frequency content and system requirements

Ground plane splits are controversial. While they can prevent unwanted current flow, they also force high-frequency return currents to detour around the split, potentially creating worse problems than they solve.

Grounding Strategies

Different grounding approaches suit different frequency ranges:

• Single-point ground: All returns connect to one point; prevents ground loops at low frequencies but fails at high frequencies where connection inductance dominates
• Multi-point ground: Multiple short connections to ground plane; essential for high-frequency systems
• Hybrid grounding: Single-point at low frequencies transitioning to multi-point at high frequencies, often using capacitors
• Chassis grounding: Connecting circuit ground to equipment chassis for shielding and safety

Cable Types and Shielding

Different cable constructions offer different EMC characteristics:

• Unshielded twisted pair: Twisting reduces magnetic field coupling and provides some differential-mode noise rejection
• Shielded twisted pair: Adds shield for electric field protection while maintaining twist benefits
• Coaxial cable: Provides excellent shielding with the shield serving as signal return
• Triaxial cable: Double shield with inner shield as signal return and outer shield for noise
• Ribbon cable: Convenient but poor EMC unless ground conductors interspersed with signal

Shield Termination

How cable shields connect to equipment determines their effectiveness:

• 360-degree termination: Shield connects circumferentially to connector shell for best high-frequency performance
• Pigtail termination: Wire connection from shield to ground; inductance limits effectiveness above a few megahertz
• Drain wire: Uninsulated wire in contact with foil shield; easier termination but similar limitations to pigtail
• Both-end grounding: Shield grounded at both ends provides best RF shielding but can create ground loops at low frequencies
• Single-end grounding: Prevents ground loops but allows shield to act as antenna at high frequencies

Connector Selection

Connectors must maintain the EMC integrity of cables and enclosures:

• Shielded connectors: Metal shell provides shield continuity through the connection
• Filter connectors: Incorporate filters directly in connector for convenient cable filtering
• Backshell design: Proper backshell provides 360-degree shield termination
• Contact plating: Gold or silver plating maintains low contact resistance and prevents corrosion
• EMI gaskets: Conductive gaskets maintain shield continuity in connector mating surfaces

Cable Routing

Physical cable placement significantly affects EMC:

• Separation: Keep sensitive analog cables away from noisy power and digital cables
• Crossing angle: When cables must cross, perpendicular crossing minimizes coupling
• Length minimization: Shorter cables act as less effective antennas
• Bundling: Group cables by noise category; avoid bundling quiet and noisy cables together
• Chassis bonding: Route cables along grounded surfaces to reduce radiation

Emissions Testing

Emissions tests measure unwanted electromagnetic energy from equipment:

• Conducted emissions: Measured using LISN (Line Impedance Stabilization Network) on power lines; results in dBuV versus frequency
• Radiated emissions: Measured in anechoic chamber or open area test site (OATS); results in dBuV/m versus frequency
• Quasi-peak detection: Weighted detector that responds more strongly to repetitive than random signals
• Average detection: True average of emissions over measurement time
• Peak detection: Captures maximum instantaneous value; useful for identifying intermittent issues

Immunity Testing

Immunity tests subject equipment to specified disturbances while monitoring operation:

• Radiated immunity: Expose equipment to calibrated RF field while monitoring for malfunction
• Conducted immunity: Inject RF energy onto cables using coupling/decoupling networks
• ESD testing: Apply electrostatic discharge to accessible surfaces and nearby objects
• Electrical fast transients: Apply bursts of fast pulses to power and signal lines
• Surge testing: Apply high-energy transients simulating lightning and switching events
• Power quality: Test immunity to voltage dips, variations, and interruptions

Test Facilities

Specialized facilities are required for accurate EMC measurements:

• Anechoic chamber: RF-absorbing room that simulates free-space conditions for radiated measurements
• Semi-anechoic chamber: Absorbing walls and ceiling with reflective floor; most common for compliance testing
• Open area test site (OATS): Outdoor ground plane in radio-quiet location; reference environment for chamber correlation
• Shielded room: Prevents external interference from affecting measurements; may have reflections
• Reverberation chamber: Uses mode stirring to create statistically uniform field for testing

Pre-Compliance Testing

Testing before formal compliance reduces risk and cost:

• Near-field probing: Identify emission sources using magnetic and electric field probes close to circuits
• Current probe measurements: Measure common-mode currents on cables to predict radiated emissions
• Spectrum analyzer surveys: Quick scans to identify major emission frequencies
• Immunity pre-testing: Apply simplified immunity stresses to identify vulnerabilities before full testing
• Design reviews: Examine schematics and layouts for EMC issues before hardware is built

Pre-compliance testing costs a fraction of formal compliance testing and allows issues to be found and fixed early in development when changes are less expensive.

Regulatory Requirements

Different markets impose different EMC requirements:

• FCC Part 15: United States requirements for unintentional radiators
• CISPR standards: International standards adopted by many countries; basis for CE marking
• CE marking: European conformity marking requires compliance with EMC Directive
• Industry-specific: Automotive (CISPR 25), medical (IEC 60601-1-2), and military (MIL-STD-461) have additional requirements
• Class A versus Class B: Industrial (Class A) versus residential (Class B) limits differ significantly

Schematic-Level Considerations

Good EMC begins at the schematic level:

• Input protection: Include ESD protection and filtering on all external interfaces
• Bypass capacitors: Provide adequate high-frequency bypass at every power pin
• Slew rate control: Limit edge rates where speed is not critical to reduce high-frequency content
• Differential signaling: Use differential interfaces for improved noise immunity
• Power supply filtering: Include filtering at power supply outputs and sensitive circuit inputs

Layout Guidelines

PCB layout profoundly affects EMC performance:

• Continuous ground plane: Maintain unbroken ground plane beneath all signal traces
• Short return paths: Place bypass capacitors close to IC power pins
• Signal routing: Keep high-frequency and sensitive traces short; avoid routing over ground plane gaps
• Component placement: Locate noisy and sensitive circuits in separate board areas
• Layer stack-up: Use adjacent signal and ground layers to minimize loop areas

System-Level Guidelines

EMC must be addressed at the complete system level:

• Enclosure design: Plan for shielding, filtering, and proper cable entry from the start
• Cable management: Specify cable types, shield termination, and routing in system design
• Grounding architecture: Define system grounding strategy including safety, signal, and chassis grounds
• Interface definitions: Specify filtering and protection requirements at every system interface
• Test planning: Plan for EMC testing throughout development, not just at the end

Emissions Problem Diagnosis

Identifying emission sources and paths:

• Frequency analysis: Emission frequencies often reveal the source; clocks appear at fundamental and harmonics
• Near-field probing: Locate radiation sources using close-proximity magnetic and electric field probes
• Current measurement: Common-mode current on cables indicates cable radiation
• Selective disabling: Temporarily disable circuit functions to isolate the source

Immunity Problem Diagnosis

Finding susceptibility paths and vulnerable circuits:

• Cable identification: Remove cables systematically to find coupling paths
• Shielding tests: Temporarily add shielding to isolate radiated versus conducted coupling
• Frequency correlation: Malfunction frequencies indicate vulnerable circuits based on their operating frequencies
• Injection testing: Apply interference directly to circuits or cables to confirm susceptibility path

Common Fixes

Solutions for typical EMC problems:

• Add filtering: Ferrite beads, feedthrough capacitors, or filter networks at cable entries
• Improve shielding: Add shields, improve gaskets, address apertures
• Fix grounding: Improve ground plane continuity, add ground stitching, review return paths
• Reduce source: Slow edge rates, reduce clock frequencies, add source filtering
• Cable changes: Add shielding, improve termination, change routing