Emission Sources

Digital systems generate electromagnetic interference through multiple mechanisms inherent to their operation. Understanding these emission sources is fundamental to designing products that meet regulatory requirements and coexist peacefully with other electronic equipment. Every switching event, clock transition, and signal edge contributes to the electromagnetic signature of a digital device, creating a complex spectrum of conducted and radiated emissions.

Identifying and characterizing emission sources represents the first step in any EMC design strategy. By understanding where interference originates, engineers can implement targeted mitigation techniques at the source level, which proves far more effective and economical than attempting to contain emissions after they have propagated throughout a system. This article examines the primary emission sources encountered in digital electronics and the physical mechanisms that drive them.

Clock Harmonics

Clock signals represent the single most significant source of emissions in digital systems. The periodic nature of clock waveforms concentrates their energy at discrete frequencies, creating strong spectral lines at the fundamental frequency and its harmonics. These emissions often dominate the measured spectrum and frequently determine whether a product passes or fails compliance testing.

Spectral Content of Clock Signals

An ideal square wave contains energy only at odd harmonics of the fundamental frequency, with amplitudes that decrease as 1/n, where n is the harmonic number. However, real clock signals deviate from ideal square waves, exhibiting finite rise and fall times, duty cycle variations, and asymmetries that modify the harmonic content.

The rise and fall times of a clock signal determine the bandwidth of its spectral content. Faster edges produce harmonics that extend to higher frequencies before rolling off. The envelope of the harmonic spectrum remains flat up to a frequency approximately equal to 1/(pi times the rise time), then decreases at 20 dB per decade. This relationship explains why faster logic families with sub-nanosecond edges generate problematic emissions at frequencies reaching into the GHz range.

Duty cycle asymmetry introduces even harmonics into the spectrum. A perfect 50% duty cycle produces only odd harmonics, but practical clocks with duty cycles that deviate from 50% exhibit significant even harmonic content. This effect becomes particularly problematic when even harmonics coincide with frequencies where emissions limits are most stringent or where the system has resonant structures that amplify specific frequencies.

Clock Distribution Networks

Clock distribution networks act as efficient antennas, coupling clock energy to the surrounding environment. Long clock traces behave as transmission lines that radiate electromagnetic energy proportional to their length and the current flowing through them. Each branch of a clock tree adds to the total radiating structure, often creating complex radiation patterns.

Impedance discontinuities in clock distribution networks cause reflections that increase overshoot and ringing, extending the effective bandwidth of the clock signal beyond what the edge rates alone would suggest. These reflections also increase current flow as the driver works to overcome the mismatches, elevating both conducted and radiated emissions.

Clock skew compensation circuits and delay-locked loops (DLLs) can introduce additional emission sources. The periodic correction signals generated by these circuits create modulation sidebands around the clock harmonics, spreading the spectral energy and potentially causing emissions at frequencies not directly related to the clock fundamental.

Spread Spectrum Clocking

Spread spectrum clocking intentionally modulates the clock frequency to reduce peak spectral emissions. By spreading the clock energy over a wider bandwidth, the peak amplitude at any single frequency decreases, potentially providing several dB of margin in EMC testing. However, this technique trades peak reduction for increased broadband noise floor.

The modulation profile significantly affects spread spectrum effectiveness. Triangular and Hershey-kiss profiles distribute energy more evenly than simple sinusoidal modulation. Center-spread modulation maintains the average frequency at the nominal value, while down-spread modulation keeps the maximum frequency at the nominal value, which may be important for timing-critical applications.

Spread spectrum clocking does not eliminate emissions; it redistributes them. Systems that are sensitive to broadband noise or that must operate near other frequency-sensitive equipment may find spread spectrum problematic. Additionally, spread spectrum can complicate analog measurements and may interfere with precise timing requirements in mixed-signal systems.

Switching Noise

Every logic transition generates transient currents that produce electromagnetic emissions. Unlike periodic clock signals, switching noise from data signals creates a broadband noise floor that spans a wide frequency range. The aggregate effect of thousands or millions of simultaneous switching events produces a complex spectral signature that can be equally challenging as discrete clock harmonics.

Transient Current Demands

When a CMOS gate switches states, it must charge or discharge the load capacitance connected to its output. This charging current flows through the power distribution network, creating voltage drops across parasitic inductances and resistances. The rate of change of current (di/dt) determines the magnitude of inductive voltage drops, which in turn affect both signal integrity and emissions.

The current waveform during a switching event resembles a narrow pulse, with duration roughly equal to the signal transition time. This narrow pulse contains significant high-frequency content, with the spectrum extending to frequencies well above the inverse of the transition time. Faster logic families generate higher-frequency spectral content for equivalent switching activity.

The peak current during switching depends on the output drive strength and load impedance. Large capacitive loads require high peak currents, while resistive loads create sustained current flow throughout the transition. Mixed loads exhibit complex current profiles that combine capacitive spikes with resistive sustained components.

Crosstalk-Induced Switching

Crosstalk between adjacent signal traces induces transient currents on victim lines, even when those lines are not intentionally switching. These induced currents contribute to the overall switching noise budget and can cause false switching in extreme cases. The coupled energy adds to conducted emissions on the victim line and radiates from both aggressor and victim structures.

Forward crosstalk, which travels in the same direction as the aggressor signal, creates a pulse whose duration depends on the coupled length. Backward crosstalk produces a pulse at the near end that persists for the round-trip delay of the coupled section. Both mechanisms inject energy into the victim circuit, contributing to the overall noise environment.

Differential signaling reduces crosstalk-induced emissions by ensuring that induced currents in the positive and negative conductors largely cancel. However, common-mode noise from imperfect balance can still couple to adjacent circuits and contribute to radiated emissions through cable connections and other common-mode paths.

Data-Dependent Emissions

The spectral content of data signals depends on the bit patterns being transmitted. Random data produces a continuous spectrum with energy distributed according to the data rate and transition density. Repetitive patterns concentrate energy at discrete frequencies related to the pattern repetition rate, potentially creating strong emissions at unexpected frequencies.

Idle patterns and training sequences used in high-speed serial interfaces often have repetitive characteristics that generate discrete spectral lines. These emissions may differ significantly from those produced during normal data transmission, requiring testing across multiple operating modes to fully characterize the emission profile.

Data encoding schemes affect emission characteristics by controlling transition density and pattern distribution. 8b/10b encoding, for example, guarantees specific run length limits and DC balance that shape the spectral content. Scrambling randomizes the data sequence to eliminate discrete spectral lines, spreading the energy more uniformly across the signal bandwidth.

Ground Bounce

Ground bounce occurs when rapid current changes through ground path inductances cause the local ground reference to shift relative to the system ground. This phenomenon affects both signal integrity and EMC performance, as the shifting ground reference appears as a voltage source that drives current through any available path, including those that couple to the external environment.

Mechanisms of Ground Bounce

When an output driver pulls its output low, current flows from the load through the driver to the ground pin. The inductance of the bond wire, package lead, and PCB trace connecting the driver ground to the power supply return impedes instantaneous current changes. The voltage developed across this inductance (V = L times di/dt) causes the local ground to rise above the system ground reference.

The magnitude of ground bounce depends on the current slew rate and the total ground path inductance. High-speed drivers with large output currents and fast edge rates produce the most significant ground bounce. Package inductance typically dominates in through-hole packages, while bond wire inductance becomes significant in surface-mount and fine-pitch packages.

Ground bounce can propagate to other circuits sharing the same ground path, causing noise injection into sensitive analog circuits and potentially triggering false switching in digital inputs referenced to the bouncing ground. The distributed nature of ground bounce makes it particularly challenging to measure and characterize at the board level.

Impact on Emissions

Ground bounce creates voltage differentials between different ground regions of a system. These differential voltages drive common-mode currents through any paths that connect the regions, including cables, chassis connections, and parasitic capacitances to the enclosure. Common-mode currents are particularly efficient radiators because they do not cancel as differential currents do.

The periodic nature of ground bounce associated with clock signals concentrates emission energy at clock harmonics. Data-dependent ground bounce adds broadband noise content to the emission spectrum. The combination of discrete and continuous spectral components creates a complex emission signature that varies with operating conditions.

Ground plane discontinuities near high-current switching circuits can channel ground bounce currents in unexpected directions, creating slot antennas and other radiating structures. Proper ground plane design with solid returns under high-speed signals minimizes these effects and contains ground bounce within localized regions.

Mitigation Approaches

Reducing ground path inductance provides the most direct mitigation of ground bounce. Multiple parallel ground pins, distributed throughout the package, reduce total inductance by providing parallel current paths. Modern IC packages include numerous ground connections specifically to minimize ground bounce effects.

Slowing edge rates reduces di/dt and proportionally reduces ground bounce magnitude. Many drivers offer selectable edge rate control or multiple drive strength options to allow optimization for each application. The trade-off between signal integrity margins and EMC performance must be carefully balanced.

Decoupling capacitors provide local charge storage that reduces the current flowing through package and board inductances during switching. Proper decoupling requires capacitors with low equivalent series inductance (ESL) placed as close as possible to the power pins. Multiple capacitors of different values address different frequency ranges in the emission spectrum.

Simultaneous Switching Noise

Simultaneous switching noise (SSN), also called simultaneous switching output (SSO) noise or delta-I noise, occurs when multiple outputs switch simultaneously, creating aggregate current demands that exceed the power delivery network capacity. The combined effect produces larger voltage disturbances than would occur from individual switching events.

Aggregation Effects

When N outputs switch simultaneously, the total switching current can approach N times the single-output current. The voltage disturbance scales proportionally with this aggregate current, potentially creating ground bounce and power supply droop many times larger than single-bit switching would produce. Wide data buses and memory interfaces are particularly susceptible to SSN effects.

The worst-case switching scenario depends on the circuit topology. For many designs, all outputs switching in the same direction (all rising or all falling) produces the maximum aggregate current. However, designs with significant output-to-output coupling may exhibit different worst-case patterns depending on the relative timing and direction of transitions.

Statistical correlation between data bits affects the probability of worst-case switching patterns. Random data produces statistically independent switching events, while structured data with high correlation increases the probability of many simultaneous transitions. Address buses incrementing through sequential values and data buses with patterned content exhibit higher correlation than truly random data.

Frequency Characteristics

SSN produces spectral content concentrated at frequencies related to the switching rate and the resonant frequencies of the power delivery network. The current transient excites resonances between plane capacitance and via inductance, between decoupling capacitors and their mounting inductance, and between package and board structures.

Anti-resonances in the power delivery impedance can amplify specific frequency components of SSN, creating emission peaks at unexpected frequencies. These anti-resonances occur where inductive and capacitive elements interact destructively from an impedance perspective but constructively from a noise perspective.

The broadband nature of the switching transient means that SSN contributes to emissions across a wide frequency range. Unlike clock harmonics, which are concentrated at discrete frequencies, SSN creates a noise floor that can raise emission levels throughout the spectrum.

Design Constraints

Package specifications typically define SSO limits indicating the maximum number of outputs that may switch simultaneously without exceeding acceptable noise levels. These limits depend on the package inductance, driver characteristics, and the target noise budget. Exceeding SSO limits can cause signal integrity failures and excessive emissions.

Staggering output switching times reduces peak aggregate current by spreading transitions over a wider time window. Some output drivers include intentional skew to prevent simultaneous switching, while system timing may naturally distribute transitions across multiple clock phases.

Power delivery network design must account for worst-case SSN scenarios. Target impedance methodology sets impedance limits across the frequency range of interest to ensure that current transients produce acceptable voltage disturbances. Meeting these targets typically requires careful coordination of decoupling capacitors, plane design, and package selection.

Power Supply Noise

Power supply noise encompasses voltage fluctuations on supply rails that result from switching activity and imperfections in the power delivery network. These fluctuations modulate circuit operation and create emission sources that can radiate directly or couple to other circuits and cables.

Switching Regulator Noise

Switching regulators generate emissions at their switching frequency and harmonics. The discontinuous current drawn from the input and delivered to the output creates high di/dt events that produce both conducted and radiated emissions. The switching frequency, typically in the range of 100 kHz to several MHz, places fundamental emissions in a frequency range where many EMC standards impose strict limits.

Input and output ripple currents flow through circuit traces that can act as loop antennas. The magnetic field created by these current loops couples to nearby circuits and radiates into the surrounding environment. Minimizing loop areas through careful layout significantly reduces switching regulator emissions.

Switch node ringing, caused by parasitic inductances and capacitances interacting with the rapid voltage transitions, can produce emissions at frequencies well above the fundamental switching frequency. Snubber circuits and gate drive optimization can reduce ringing, while spread spectrum modulation of the switching frequency reduces peak emissions.

Load Transients

Sudden changes in load current produce transient voltage deviations that propagate through the power distribution network. When a processor enters or exits a low-power state, or when a high-speed interface begins or ends activity, the step change in current demand creates voltage transients that affect all circuits sharing the power domain.

The power delivery network response to load transients depends on its impedance versus frequency characteristics. At low frequencies, the voltage regulator feedback loop maintains regulation. At higher frequencies, the regulator cannot respond quickly enough, and bulk capacitance provides current. At still higher frequencies, local decoupling capacitors and plane capacitance must supply the transient current demand.

Load transient events produce broadband spectral content that adds to the emission noise floor. Repetitive load transients, such as those associated with periodic processing tasks or communication frame structures, create discrete spectral components that may coincide with EMC measurement frequencies.

Power Plane Resonances

Power and ground plane pairs form resonant cavities whose resonant frequencies depend on the plane dimensions and the dielectric properties of the separating material. At these resonant frequencies, relatively small excitations produce large voltage variations that can dominate the emission spectrum.

The fundamental resonant frequency of a rectangular plane pair occurs when the plane dimensions equal half a wavelength. Higher-order modes create additional resonances at integer multiples and combinations of the fundamental frequencies in each dimension. Irregularly shaped planes have more complex resonant mode structures that may be difficult to predict analytically.

Plane resonances are excited by any switching activity that injects energy at the resonant frequencies. Once excited, the resonant modes persist until losses dissipate the energy, creating sustained oscillations that can last for many clock periods. Damping techniques, including resistive termination at plane edges and distributed resistive loading, reduce resonance amplitudes.

Cable Radiation

Cables connected to digital systems often serve as the primary radiating structures, particularly for frequencies below a few hundred MHz where PCB traces are electrically short. Common-mode currents flowing on cable shields and signal conductors radiate efficiently because of the cable length and their exposure outside the system enclosure.

Common-Mode Currents on Cables

Differential signals on cables produce magnetic fields that largely cancel in the far field because the forward and return currents are closely spaced and flow in opposite directions. Common-mode currents, however, flow in the same direction on all conductors, and their fields add constructively, creating significant radiated emissions even with modest current levels.

Common-mode currents develop from multiple sources: ground potential differences between connected equipment, unbalanced coupling to signal traces, and intentional signals that have common-mode components. Even a few microamperes of common-mode current on a meter-long cable can produce emissions exceeding FCC Class B limits.

The cable acts as a monopole antenna driven against the system ground plane. The radiation efficiency depends on the cable length relative to wavelength, reaching maximum when the cable length equals a quarter wavelength. Multiple connected cables create a complex antenna structure with radiation characteristics that depend on the relative lengths and orientations.

Shield Effectiveness

Cable shields provide common-mode rejection when properly terminated at both ends. The shield intercepts common-mode currents and returns them to the source through the shield-to-ground connection rather than allowing them to radiate from the inner conductors. Shield effectiveness depends on shield coverage, transfer impedance, and termination quality.

Shield terminations must provide low-impedance connections to the chassis or ground reference at all frequencies of concern. Pigtail connections that extend the shield conductor beyond the connector introduce inductance that degrades high-frequency performance. 360-degree terminations using appropriate connector designs maintain shield effectiveness to much higher frequencies.

Transfer impedance quantifies how effectively a shield prevents external fields from inducing voltages on internal conductors and vice versa. Braided shields have higher transfer impedance at high frequencies than solid shields because of coupling through the braid apertures. Combination shields using foil with an overlying braid offer improved high-frequency performance.

Cable Filtering

Ferrite cores placed around cables increase the common-mode impedance, reducing the common-mode current for a given driving voltage. The ferrite material provides frequency-dependent impedance that typically peaks in the range of 10 to several hundred MHz, depending on the material formulation.

Multiple turns through a ferrite core increase the impedance proportional to the square of the number of turns. However, parasitic capacitance between turns creates a self-resonance that limits effectiveness at high frequencies. Larger cores accommodate more turns but have lower self-resonant frequencies.

Cable filtering at the connector interface, using filtered connectors or common-mode chokes, addresses emissions before they can propagate along the cable. This approach is more effective than external ferrites because it prevents the common-mode current from exciting the cable structure in the first place.

Slot Radiation

Openings in enclosures and ground planes act as slot antennas that radiate electromagnetic energy at frequencies where the slot dimensions approach resonance. Even small slots can produce significant emissions when excited by internal fields, often dominating the emission spectrum at specific frequencies.

Slot Antenna Principles

A slot in a conducting surface radiates according to Babinet's principle, which relates slot antenna behavior to complementary dipole antenna behavior. A half-wavelength slot has characteristics similar to a half-wave dipole, including relatively high radiation efficiency and defined polarization.

The slot radiation pattern depends on the slot orientation and the field distribution along its length. Horizontal slots radiate primarily with vertical polarization, while vertical slots produce horizontally polarized emissions. The surrounding structure affects the pattern, causing directivity variations that depend on the observation angle.

Slot resonance occurs when the slot length approaches half a wavelength. At resonance, the slot presents a relatively low impedance to the internal fields, allowing efficient energy transfer from internal sources to radiated emissions. Non-resonant slots also radiate but with lower efficiency.

Common Slot Sources

Enclosure seams, particularly those between removable panels, form slots that can extend the full length of the seam. Even with apparent metal-to-metal contact, surface oxide layers and imperfect mating create high-impedance gaps that behave as slots at high frequencies.

Ventilation openings represent intentional slots that must balance thermal requirements with EMC performance. Large openings provide better airflow but also more efficient radiation. Honeycomb structures and waveguide-below-cutoff designs can provide ventilation while maintaining shielding effectiveness.

Cable entry points create slots when cables penetrate the enclosure through oversized holes or when multiple cables pass through the same opening with air gaps between them. Proper cable entry treatment using appropriate glands, grommets, or filtered connectors maintains enclosure integrity.

Mitigation Techniques

Reducing slot length below a quarter wavelength at the highest frequency of concern eliminates resonant radiation. Multiple short slots, each below the critical length, radiate far less efficiently than a single long slot with equivalent total area. This principle guides the design of ventilation patterns and seam treatment.

Conductive gaskets at seams maintain electrical continuity even when mechanical contact is imperfect. The gasket must provide low-impedance contact at the frequencies of concern, which typically requires proper selection of gasket material and compression to achieve adequate contact pressure.

Waveguide-below-cutoff principles apply to openings that must remain open, such as ventilation holes. When the opening dimensions are small compared to the wavelength, electromagnetic waves cannot propagate through, providing significant attenuation. Honeycomb structures extend this principle by creating many small waveguide sections in parallel.

Common-Mode Currents

Common-mode currents represent the sum of currents flowing in the same direction on all conductors of a transmission system. Unlike differential-mode currents that confine their fields to the immediate vicinity of the conductors, common-mode currents create far-field radiation efficiently and represent the dominant emission mechanism in many digital systems.

Origins of Common-Mode Currents

Common-mode currents arise from asymmetries in ostensibly balanced systems. Differences in trace routing, component placement, or termination impedances create imbalances that convert a portion of differential-mode energy to common-mode. Even small percentage imbalances can produce significant common-mode currents because the common-mode radiation efficiency far exceeds that of differential-mode signals.

Ground potential differences between different parts of a system drive common-mode currents through any available path. When two circuits reference different ground points that have voltage differences due to current flow, cables connecting the circuits carry common-mode current proportional to the voltage difference divided by the common-mode impedance.

Capacitive coupling from noise sources to cables and chassis creates common-mode currents that may be unrelated to the intended signal paths. Stray capacitance from switching regulators, high-speed logic, or other noise sources to nearby cables injects common-mode current directly onto the cable structure.

Common-Mode Rejection

Differential signaling provides inherent common-mode rejection when the receiver responds only to the voltage difference between the two conductors. This rejection protects signal integrity from common-mode noise but does not prevent the common-mode currents from radiating. Reducing common-mode current generation remains essential for EMC compliance.

Common-mode chokes present high impedance to common-mode currents while allowing differential-mode signals to pass unimpeded. The choke consists of two or more windings on a common magnetic core, wound so that differential-mode flux cancels and only common-mode flux magnetizes the core. This selectivity makes common-mode chokes effective filters without affecting signal integrity.

Balanced driver and receiver design minimizes common-mode generation by ensuring symmetric operation. Matched termination impedances, symmetric trace routing, and balanced component values all contribute to maintaining the differential-mode nature of the signal and minimizing common-mode conversion.

Measurement Considerations

Common-mode current measurement using current probes requires careful interpretation. The probe measures the vector sum of all currents passing through it, which represents the common-mode component when used on a cable bundle. Separating individual conductors allows measurement of each current, from which common-mode and differential-mode components can be calculated.

The relationship between measured common-mode current and radiated emissions depends on the cable configuration and frequency. Simple monopole antenna models provide order-of-magnitude estimates, but accurate prediction requires consideration of the cable's position relative to ground planes and the proximity of other conductive structures.

Diagnostic techniques using near-field probes can locate common-mode current sources before they propagate to cables. Comparing emissions with and without cables connected identifies whether cables or the product enclosure dominates the emission path, guiding mitigation efforts toward the most effective interventions.

Practical Considerations

Frequency Planning

Strategic selection of clock frequencies and their harmonics can avoid placing strong emissions at frequencies where limits are most stringent or where the system has resonant structures. Frequency planning requires knowledge of the emission spectrum, the applicable limits, and the frequency-dependent characteristics of the system's radiating structures.

EMC standards typically specify measurement bandwidths and detector types that affect how emissions at different frequencies contribute to measured results. Understanding these measurement parameters helps predict which emission sources will dominate test results and guides mitigation priorities.

Layer Stackup Design

PCB layer stackup significantly affects emission sources by determining loop areas, return path inductances, and parasitic coupling. Closely spaced power and ground planes reduce power delivery inductance and contain fields between the planes. Signal layers adjacent to reference planes minimize loop areas and provide well-defined return paths.

Interplane capacitance between power and ground planes provides distributed decoupling that supplements discrete capacitor placement. This capacitance is most effective at high frequencies where discrete capacitor inductance limits performance. Thin dielectric layers between planes maximize this beneficial capacitance.

Component Placement Strategy

Placing high-speed circuits and potential emission sources near the center of boards reduces the effective antenna length of traces that could radiate. Edge placement allows traces to couple to the board perimeter and connected cables, increasing radiation efficiency.

Separating analog and digital circuits reduces coupling of switching noise into sensitive measurement circuits. This separation also contains emission sources within defined regions where targeted mitigation measures can be applied without affecting the entire design.

Locating connectors and cable attachment points away from high-speed circuits reduces direct coupling of emissions to cables. When proximity is unavoidable, proper filtering at the connector prevents noise from reaching the cable structure.

Summary

Digital systems generate electromagnetic emissions through numerous mechanisms, each with distinct characteristics and mitigation approaches. Clock harmonics concentrate energy at discrete frequencies determined by the clock period and edge rates. Switching noise from data signals creates broadband spectral content that elevates the overall noise floor. Ground bounce and simultaneous switching noise produce voltage transients that drive common-mode currents through power distribution networks and cables.

Power supply noise from switching regulators and load transients adds both discrete and continuous spectral components. Cables serve as efficient antennas for common-mode currents, often dominating emissions in the frequency range below several hundred MHz. Slots in enclosures and ground planes radiate at frequencies where their dimensions approach resonance. Common-mode currents, arising from circuit imbalances and ground potential differences, represent the most efficient radiation mechanism in many systems.

Effective EMC design addresses emission sources at their origin rather than attempting to contain emissions after they have propagated through the system. Understanding the physical mechanisms that generate emissions enables engineers to make informed design decisions that balance EMC performance with other system requirements. The techniques described in this article provide the foundation for creating digital systems that meet regulatory requirements and coexist harmoniously with other electronic equipment.