Electronics Guide

Digital Circuit Emissions

Digital circuits represent the most ubiquitous source of radiated emissions in modern electronic equipment. Every logic transition involves charging and discharging capacitances, drawing current pulses from the power supply system. The aggregate effect of millions of transistors switching creates substantial high-frequency energy that can couple to radiating structures throughout the system.

Clock signals are typically the dominant emission source in digital systems because they provide the most energetic and predictable switching activity. Clock distribution networks carry high-frequency signals across circuit boards to multiple destinations, with each trace potentially acting as a radiating antenna. The clock frequency and its harmonics appear prominently in emission spectra, with harmonic content extending to frequencies many times the fundamental depending on signal edge rates.

The spectral content of digital signals depends primarily on the rise and fall times of signal transitions. Faster edge rates contain higher frequency components according to the Fourier series of the trapezoidal waveform. A signal with 1 nanosecond edges contains significant spectral content beyond 300 MHz regardless of the fundamental frequency. Modern digital circuits with edge rates measured in hundreds of picoseconds generate spectral content extending well into the gigahertz range.

Data signals contribute to radiated emissions with characteristics that depend on data patterns. Random data creates noise spread across a range of frequencies without the discrete spectral lines of clock signals. Repetitive data patterns can create narrowband emissions at frequencies related to the pattern repetition rate. The combination of clock and data emissions produces complex spectra that may vary with equipment operating conditions.

Power Electronics Sources

Switching power converters generate radiated emissions through the rapid switching of power transistors and diodes. The fundamental switching frequencies of DC-DC converters typically range from tens of kilohertz to several megahertz, with harmonic content extending throughout the radiated emission measurement range. The high currents and voltages involved in power conversion create strong electromagnetic fields near the converter circuits.

The primary radiation mechanism from power converters involves common-mode currents driven by the rapid voltage changes on switching nodes. When a power transistor turns on or off, the drain or collector voltage changes by tens or hundreds of volts within nanoseconds. This dV/dt couples through parasitic capacitances to grounded structures such as heat sinks and enclosures, driving common-mode currents that can radiate from attached cables or structure dimensions.

Transformer-isolated converters can radiate from the interwinding capacitance of the transformer, which couples switching noise between primary and secondary. This coupling creates common-mode currents on secondary-side circuits and cables. Transformer construction techniques that minimize interwinding capacitance and include Faraday shields reduce this coupling, but elimination is not practical in most designs.

Motor drives and inverters present particularly challenging emission sources due to their high power levels and long cable connections to motors. The PWM switching of inverter outputs creates rapidly changing voltages on motor cables that can act as efficient radiating antennas. The cable lengths involved, often many meters, approach resonant dimensions at frequencies where switching harmonic content remains significant.

Cable and Interconnect Emissions

Cables are frequently the most efficient radiating structures in electronic systems. External cables connecting equipment to peripherals, networks, or power sources typically measure meters in length, making them effective antennas at frequencies above a few tens of megahertz. Internal cables connecting circuit boards or assemblies can also radiate, particularly when they carry high-frequency signals or common-mode currents.

Common-mode currents on cables are responsible for most cable-related emissions. These currents flow in the same direction on all conductors in a cable, using the cable as a whole as an antenna rather than the differential signal path between conductors. Common-mode currents are induced by ground potential differences between connected circuits, by coupling from internal emission sources, and by cable shield currents that fail to remain on the outer shield surface.

The radiation efficiency of a cable depends on its length relative to the wavelength at the frequency of interest. A cable becomes a particularly efficient radiator when its length approaches a quarter or half wavelength, creating resonant conditions that amplify radiation. At 150 MHz, a wavelength is 2 meters, so cables of 0.5 to 1 meter length are resonant and radiate efficiently at this common emission frequency.

Differential-mode signals on cables can also radiate, particularly when cable length becomes significant relative to wavelength. While differential-mode radiation is typically less efficient than common-mode radiation from the same cable, high-speed differential signals on long cables can produce substantial emissions. Proper cable shielding and termination techniques reduce both differential-mode and common-mode radiation.

PCB Trace and Plane Emissions

Printed circuit board traces act as small antennas, with radiation efficiency increasing as trace length approaches a significant fraction of the signal wavelength. At lower frequencies where trace lengths are electrically short, radiation efficiency is low. However, at frequencies above several hundred megahertz, even short traces can radiate measurably, particularly if they carry high-current or high-frequency signals.

Return current paths critically affect PCB radiation. When the return current flows directly beneath a signal trace on an adjacent ground plane, the opposing currents create a low-impedance transmission line with minimal radiation. When the return path deviates from this ideal, due to plane gaps, vias, or connector transitions, the increased loop area creates a magnetic dipole that radiates efficiently.

Slots and gaps in ground planes are particularly problematic as they can act as slot antennas. A slot cut across a return current path forces current to flow around the slot, creating a large loop area. At frequencies where the slot dimensions approach a quarter wavelength, resonance amplifies the radiation. Even small slots or splits in planes can be significant emission sources at higher frequencies.

High-speed signal vias create vertical current paths that contribute to radiation. The transition from one layer to another involves current flow through the via barrel and between reference planes, creating opportunities for radiation. Via stitching around high-speed signals and careful management of reference plane transitions minimize via-related emissions.

Enclosure and Structure Emissions

Equipment enclosures can act as radiating antennas when driven by internal emission sources. Metal enclosures develop surface currents from conducted noise on internal wiring, from coupling through apertures, and from direct connection to noisy circuits. These surface currents can radiate from enclosure edges, corners, and apertures.

Apertures in shielding enclosures allow electromagnetic fields to leak out, with radiation efficiency depending on aperture size and shape relative to wavelength. A slot aperture becomes an efficient radiator when its length approaches a half wavelength. At 300 MHz, a half wavelength is 50 centimeters, so even modest ventilation slots or cable exit holes can radiate significantly at common emission frequencies. Multiple apertures can couple to create resonant array effects.

Seam integrity affects enclosure radiation performance. Poor contact between mating surfaces allows currents to flow across the joint only at intermittent contact points, creating multiple slot-like radiators. Proper gasket selection, surface preparation, and fastener spacing maintain continuous electrical contact that prevents seam radiation.

Heat sinks attached to switching transistors often couple to the switching node voltage through the thermal pad or mounting hardware. This makes the heat sink an extension of the switching node, with its larger dimensions creating a more efficient radiating structure than the transistor package alone. Thermal management designs that minimize this coupling or provide proper shielding reduce heat sink-related emissions.

Intentional RF Circuits

Equipment containing intentional radio transmitters or receivers includes additional emission sources beyond the unintentional digital and power electronics emissions. Oscillators, amplifiers, mixers, and other RF circuits can produce spurious emissions outside their intended operating frequencies. While these circuits are designed for RF operation, unintended harmonics, intermodulation products, and oscillator leakage can create emission problems.

Local oscillators in receivers and transmitters produce fundamental and harmonic radiation that can violate emission limits if not properly shielded. The oscillator signal distributes throughout the RF section and can couple to antennas, cables, and enclosure structures. Shielding of oscillator circuits and filtering of oscillator distribution paths control this leakage.

Digital control circuits within RF equipment can modulate the RF signal path, creating spurious emissions at frequencies related to the digital switching. Clock signals, digital bus activity, and processor operations can couple to RF circuits through power supply connections, ground paths, or direct radiation. Isolating digital and RF sections through shielding and filtering addresses this interaction.

Harmonic and Intermodulation Products

Nonlinear circuit elements generate harmonic distortion products at integer multiples of the fundamental frequency. Switching transistors, diodes, magnetic components operating near saturation, and any circuit with nonlinear transfer characteristics produce harmonics. These harmonics can extend emission spectra far beyond the fundamental operating frequencies of the circuits.

Intermodulation occurs when multiple signals interact in a nonlinear element, producing new frequencies at the sums and differences of the original frequencies and their harmonics. In complex systems with multiple switching frequencies, oscillators, and signal sources, intermodulation can produce emission peaks at unexpected frequencies that may fall on sensitive radio services or regulatory measurement points.

The amplitude of harmonic and intermodulation products depends on the degree of nonlinearity and the signal levels involved. Higher-order products typically decrease in amplitude but may extend to frequencies where radiation efficiency is higher, potentially creating emission problems far from the fundamental operating frequencies. Linearizing circuits where possible and filtering outputs reduce harmonic and intermodulation emissions.

Source Identification Techniques

Identifying the specific source of observed radiated emissions requires systematic investigation using near-field probes, current measurements, and correlation analysis. Near-field magnetic and electric probes can locate areas of strong field intensity on circuit boards and around cables, pinpointing emission hot spots. Current probes on cables measure common-mode and differential-mode currents that may be driving emissions.

Frequency correlation relates observed emissions to known internal frequencies. Clock harmonics appear at integer multiples of the clock frequency. Switching converter harmonics relate to the converter operating frequency. By comparing emission frequencies to internal frequency sources, engineers can identify which circuits are responsible for specific emissions.

Configuration testing isolates emission sources by selectively enabling or disabling system functions. If emissions change when a particular circuit is powered down or when a specific cable is disconnected, that circuit or cable is implicated in the emission. Systematic configuration testing efficiently narrows the search for emission sources.

Time-domain analysis can distinguish between different source types based on their temporal characteristics. Continuous emissions suggest clock or oscillator sources, while emissions that correlate with data activity indicate data bus sources. Emissions that vary with power supply load suggest power converter involvement. This temporal signature information guides investigation toward the responsible circuits.

Source-Level Mitigation

Addressing emissions at their source is typically more effective and economical than relying solely on shielding and filtering. Source-level mitigation reduces the electromagnetic energy that must be contained, potentially reducing requirements for shielding enclosures, filtered connectors, and other costly containment measures.

Edge rate control reduces the high-frequency content of digital and switching signals. Adding resistance in series with gate drivers slows transistor switching, reducing the amplitude of high-frequency harmonics. The trade-off is increased switching losses and potential thermal challenges. The optimal edge rate balances emission reduction against efficiency requirements.

Spread spectrum techniques modulate operating frequencies to spread emission energy across a wider bandwidth, reducing peak levels at any single frequency. Spread spectrum clocking of digital systems and frequency dithering of power converters can significantly reduce peak emissions while maintaining functional performance. The modulation parameters must be chosen to avoid creating new problems such as audio frequency noise.

Layout optimization reduces the coupling between emission sources and radiating structures. Keeping high-frequency circuits away from cable attachment points, minimizing loop areas in return current paths, and maintaining continuous reference planes all reduce the efficiency with which generated noise reaches antennas. These layout practices are most effective when incorporated from the initial design phase.

Summary

Radiated emissions originate from numerous sources within electronic systems, including digital circuits, power converters, cables, PCB structures, and enclosures. Understanding the mechanisms by which each source type generates and radiates electromagnetic energy enables effective emission control. The combination of source identification techniques and targeted mitigation strategies produces systems that meet regulatory requirements while optimizing cost and performance.

Effective EMC engineering addresses emissions at multiple levels, from reducing noise generation in source circuits through proper layout that minimizes coupling to radiating structures and finally to shielding and filtering when needed. This comprehensive approach ensures that radiated emissions remain within acceptable limits across the full range of operating conditions and configurations.