Electronics Guide

Operating Characteristics

Vacuum tubes operated at relatively high voltages, typically hundreds of volts for plate circuits, but with limited switching speeds. Rise and fall times were measured in microseconds at best, limiting significant harmonic content to frequencies below a few megahertz in most applications. The tubes themselves were physically large, and the circuits built around them correspondingly spacious by modern standards.

The power supply requirements of vacuum tube systems created one class of EMC concerns. High-voltage power supplies generated interference from rectifier switching and filter resonances. The substantial power dissipation as heat required ventilation that created potential apertures in shielded enclosures.

EMC Challenges

Interference between vacuum tube systems was primarily a concern at radio frequencies, where the operating frequencies of transmitters and receivers created direct potential for interference. The broad spectral output of spark-gap transmitters, used into the 1920s, was a particularly severe interference source before continuous-wave transmitters became standard.

Conducted interference traveled through power lines and could couple between circuits sharing common power supplies. The audio frequency hum from inadequately filtered power supplies was a familiar problem in audio equipment. At radio frequencies, conducted interference on power and signal cables could cause reception problems in nearby receivers.

Radiated emissions from vacuum tube equipment included both intentional signals and parasitic oscillations. The high-gain amplifier stages possible with vacuum tubes were prone to self-oscillation if layout and shielding were inadequate. These parasitic oscillations could produce spurious emissions that interfered with communications.

Solutions and Practices

EMC techniques developed during the vacuum tube era established practices that would continue into later technology generations. Shielding of high-gain stages prevented feedback and contained emissions. Careful circuit layout, particularly attention to ground connections, minimized unintended coupling. Filtering of power supply connections reduced conducted interference.

The physical size of vacuum tube equipment allowed generous spacing between circuits, providing inherent isolation that later miniaturization would eliminate. This relative spaciousness made EMC somewhat easier to achieve than in more compact later systems, although the higher operating voltages created their own challenges.

New Device Characteristics

Transistors operated at much lower voltages than vacuum tubes, typically a few volts to tens of volts rather than hundreds. This reduced certain EMC concerns related to high-voltage switching but introduced others. Transistors could switch faster than vacuum tubes, pushing significant harmonic content to higher frequencies.

The much smaller size of transistors enabled more compact circuits, beginning the miniaturization trend that would accelerate through subsequent technology generations. Smaller circuits meant shorter interconnections and potentially smaller radiating structures, but also closer proximity between circuits that could couple to each other.

Transistors were also more sensitive to damage from transients and static discharge than vacuum tubes. While tubes could often survive voltage spikes that would exceed their ratings, transistor junctions could be permanently damaged by relatively modest overvoltage events. This introduced new susceptibility concerns that would grow more severe as device geometries shrank.

Circuit-Level Implications

The transition to transistors enabled new circuit topologies that had EMC implications. The much higher gain available from transistors made it easier to achieve high amplification, but also easier to create unintended feedback paths leading to oscillation. Careful attention to layout and grounding became essential for stable operation.

Transistor switching circuits introduced switching transients as a significant EMC concern. While transistors could switch faster than tubes, they also generated faster edges and correspondingly higher-frequency harmonic content. The switching transients from early transistor power supplies and motor drives produced interference at frequencies where vacuum tube circuits had been relatively benign.

System-Level Changes

The lower power requirements of transistor circuits enabled battery-operated and portable equipment that introduced new EMC scenarios. Portable radios, hearing aids, and other battery-powered devices created new interference susceptibility situations as electronic equipment proliferated into new environments.

The economics of transistor production eventually enabled mass-market consumer electronics on a scale impossible with vacuum tubes. This proliferation meant more potential interference sources and more potential victims of interference in any given environment, driving the need for systematic EMC requirements.

Integration Effects

Integrated circuits placed multiple transistors and passive components on a single semiconductor die, drastically reducing size and interconnection lengths. The internal connections within an IC were inherently well-controlled, with short lengths and close proximity between signal and return paths. However, the connections between ICs and the outside world became critical EMC factors.

The package leads and bond wires connecting an IC die to external circuits introduced inductance that affected both emissions and immunity. Fast-switching outputs driving inductive leads generated voltage transients and produced emissions. Inductive input leads made circuits susceptible to electromagnetic interference that could couple into the package.

Power supply connections to ICs became increasingly important. The simultaneous switching of multiple gates within an IC created current transients on power and ground leads, producing both conducted emissions and voltage fluctuations that could affect circuit operation. The power supply decoupling that had been optional for slower circuits became essential for reliable operation of faster ICs.

Digital Logic Evolution

The evolution of digital logic families through the IC era progressively increased switching speeds and their EMC implications. Early logic families like RTL and DTL operated at relatively modest speeds with rise times measured in tens of nanoseconds. TTL logic, introduced in the late 1960s, achieved nanosecond-scale transitions.

CMOS logic, which would eventually dominate digital electronics, initially operated more slowly than bipolar families but consumed far less power. As CMOS process technology improved, speeds increased dramatically while maintaining the power advantage. The combination of high speed and high integration levels in CMOS would eventually create the most challenging digital EMC scenarios.

Each logic family had characteristic EMC behaviors. TTL, with its relatively high power consumption and significant supply current transients, was prone to conducted interference on power supplies. Early CMOS was more susceptible to damage from electrostatic discharge due to its thin gate oxides. Understanding these family-specific characteristics was essential for effective EMC design.

Analog IC Considerations

Analog integrated circuits presented their own EMC challenges. Operational amplifiers and other linear ICs were susceptible to interference that could be rectified or demodulated by nonlinear effects in input stages. The high gain of analog ICs meant that even small interference signals could produce significant output effects.

Mixed-signal ICs combining analog and digital circuits on the same die created on-chip interference problems. The digital circuits' switching transients could couple to sensitive analog circuits through the common substrate, degrading analog performance. Techniques for isolating analog and digital portions of mixed-signal ICs became an important area of design practice.

Clock-Based Operation

Microprocessors operate synchronously, with activity coordinated by a clock signal. This clock and its harmonics became the dominant emission signatures of microprocessor-based systems. Unlike random noise, the concentrated energy at clock harmonics could produce narrowband interference that was particularly troublesome for radio reception.

Clock rates increased relentlessly as microprocessor technology advanced. The original Intel 4004 operated at less than 1 MHz. Within two decades, clock rates exceeded 100 MHz, and they have continued increasing to multiple gigahertz in modern processors. Each increase in clock rate pushed significant harmonic content to higher frequencies and demanded new EMC design approaches.

The distribution of clock signals throughout a microprocessor system created a network of potential radiating structures. Clock traces on printed circuit boards, clock leads on IC packages, and cables carrying clocked signals could all radiate at clock harmonics. Controlling clock distribution became a primary concern for digital EMC.

Data Bus Emissions

Microprocessors communicate with memory and peripherals over parallel data buses that carry signals switching at or near the clock rate. The simultaneous switching of multiple bus lines created current transients far larger than any single signal transition. This simultaneous switching noise (SSN) became a dominant EMC factor in microprocessor systems.

Bus widths increased from 4 bits in early microprocessors to 8, 16, 32, and eventually 64 bits in modern processors. Wider buses meant more simultaneous transitions and correspondingly larger transient currents. The power supply system and grounding architecture had to evolve to handle these increased transient demands.

System Architecture Effects

The flexibility of microprocessor-based systems meant that electromagnetic behavior could depend on software as well as hardware. Different software operations produced different current waveforms and emission signatures. This software dependence complicated EMC testing, as the worst-case conditions might depend on specific software patterns.

The proliferation of microprocessors into diverse applications created new EMC scenarios. Industrial controllers, automotive systems, medical devices, and countless other applications introduced microprocessors into environments with different interference sources and susceptibility requirements. EMC requirements became increasingly application-specific as microprocessors spread.

Proliferation of Intentional Transmitters

The growth of cellular telephony, WiFi, Bluetooth, and countless other wireless technologies has populated the electromagnetic spectrum with intentional transmissions to a degree unimaginable in earlier eras. A modern urban environment contains countless active transmitters, from cellular base stations to individual smartphones, from WiFi access points to Bluetooth peripherals.

This proliferation has two major EMC implications. First, wireless devices must coexist with each other, operating in adjacent or overlapping frequency bands without mutual interference. This radio coexistence has become a major engineering challenge, particularly for devices that incorporate multiple wireless technologies. Second, the ubiquitous presence of wireless signals creates an ambient electromagnetic environment that non-radio equipment must tolerate without malfunction.

Radio Coexistence

Modern wireless devices frequently incorporate multiple radio technologies that must operate simultaneously without mutual interference. A smartphone might include cellular, WiFi, Bluetooth, GPS, and NFC radios, all operating in close physical proximity. The transmitter of one radio can interfere with the receiver of another through various coupling paths.

Coexistence engineering has emerged as a specialized discipline within EMC. Techniques including careful frequency planning, time-domain coordination, antenna isolation, and filtering help enable multi-radio devices. The continuing addition of new wireless technologies and frequency bands ensures that coexistence challenges will continue growing.

Immunity Requirements

The prevalence of wireless transmissions has driven increasingly stringent immunity requirements for electronic equipment. Standards bodies have progressively increased the RF immunity test levels as the ambient electromagnetic environment has intensified. Equipment that could operate reliably decades ago might be susceptible to interference from modern wireless sources.

Specific immunity concerns have emerged for particular wireless technologies. The pulsed transmissions of GSM cellular phones were found to interfere with audio equipment through rectification effects. Medical devices have required special attention to immunity against wireless signals that might be carried by patients or visitors.

Analog-to-Digital Migration

Functions that were once implemented with analog circuits are increasingly performed digitally. Audio systems, video systems, control systems, and instrumentation have all undergone digital transformation. While digital implementation offers many advantages, it introduces the EMC characteristics of digital systems into applications that previously had quite different electromagnetic signatures.

Digital systems generate emissions at clock harmonics that can extend to very high frequencies. Where an analog audio amplifier might have significant emissions only at audio frequencies and their harmonics, a digital audio system generates radio-frequency emissions from its clock and data processing circuits. This frequency extension of emissions requires new mitigation approaches.

High-Speed Data Interfaces

Modern digital systems rely on high-speed serial interfaces for communication between chips, boards, and systems. Interfaces like USB, SATA, PCIe, and HDMI operate at multiple gigabits per second with correspondingly fast signal transitions. These interfaces present severe EMC challenges due to their bandwidth requirements and the cables that often carry their signals.

The signal integrity requirements of high-speed interfaces are closely related to EMC. The same controlled impedances and careful terminations that enable reliable signal transmission also help control emissions and immunity. High-speed interface design has thus become an area where signal integrity and EMC expertise overlap.

Software-Defined Systems

The trend toward software-defined functionality extends to electromagnetic characteristics. Software-defined radios can change their operating frequencies and modulation schemes under software control, with corresponding changes in EMC behavior. Systems that reconfigure their operating modes dynamically present new challenges for EMC testing and compliance demonstration.

Switching Frequency Increases

Power converters based on switching semiconductors have progressively increased their switching frequencies. Higher switching frequencies enable smaller passive components and faster dynamic response, but extend the frequency range of switching transients and their harmonics. Where early switching converters operated at a few kilohertz, modern converters commonly switch at hundreds of kilohertz, and developments in wide-bandgap semiconductors are enabling megahertz-range operation.

The high currents and voltages switched in power converters generate substantial transient energy. The fast transitions of modern power semiconductors, combined with the large currents being switched, create severe conducted and radiated emissions challenges. EMI filtering for power converters has become a sophisticated specialty.

Wide-Bandgap Semiconductors

Silicon carbide (SiC) and gallium nitride (GaN) power semiconductors offer dramatically faster switching than silicon devices, enabling higher efficiency and power density. However, these faster transitions also generate higher-frequency emissions and more severe transients. The EMC implications of wide-bandgap semiconductors are an active area of research and development.

The very fast switching of wide-bandgap devices creates common-mode emissions through parasitic capacitances that were negligible with slower devices. New circuit topologies and EMC mitigation techniques are being developed to harness the benefits of these semiconductors while managing their EMC implications.

Electric Vehicle Systems

The electrification of transportation is driving massive growth in power electronics, from vehicle drive systems to charging infrastructure. Electric vehicle power systems switch very large currents at high frequencies, creating substantial EMC challenges. The conductive and radiated emissions from EV drive systems and chargers require careful design and extensive filtering.

Electric vehicles also present unique susceptibility concerns. The safety-critical nature of vehicle systems demands high immunity to electromagnetic interference. Vehicles must operate reliably near powerful wireless transmitters, through electromagnetic fields from power distribution systems, and in other challenging environments.

Nanoscale Device Effects

As transistor feature sizes have shrunk below 100 nanometers and continue toward atomic scales, new physical effects become significant. The extremely thin gate oxides of modern transistors make them vulnerable to damage from even modest voltage transients. Electrostatic discharge protection has become increasingly challenging as device geometries have shrunk.

Operating voltages have decreased along with feature sizes, reducing noise margins and making circuits more susceptible to interference. A voltage fluctuation that would have been negligible noise margin loss in a 5-volt system becomes a significant fraction of the noise margin in a 1-volt system. This voltage scaling has increased susceptibility concerns even as signal levels have decreased.

On-Chip EMC

The integration of billions of transistors on a single chip has made on-chip EMC a critical concern. Digital switching noise coupling to sensitive analog circuits through the common substrate, power supply noise affecting circuit operation, and clock distribution challenges are all manifestations of on-chip EMC that affect modern integrated circuits.

On-chip EMC mitigation techniques have evolved to address these challenges. Substrate isolation techniques, on-chip filtering and decoupling, and careful floor planning to separate noisy and sensitive circuits have become essential design practices for complex mixed-signal ICs.

Nanoscale Materials

Nanotechnology has enabled new materials with electromagnetic properties useful for EMC. Carbon nanotubes and graphene offer the potential for lightweight, effective shielding materials. Nanostructured magnetic materials enable smaller, higher-performance inductors and filters. Research continues on nanomaterials that might address future EMC challenges.

Quantum Computing EMC

Quantum computers, which exploit quantum mechanical effects for computation, present extreme electromagnetic environment requirements. Quantum bits (qubits) are extraordinarily sensitive to electromagnetic interference, which can cause decoherence and computational errors. The electromagnetic shielding and filtering requirements for quantum computers far exceed those of conventional electronics.

The isolation requirements for quantum computing systems include multiple layers of shielding, extreme filtering of all connections, and operation at cryogenic temperatures that provide additional thermal noise reduction. These systems push EMC engineering to its limits.

Quantum Sensing

Quantum sensing technologies can detect extremely weak electromagnetic fields, enabling new measurement capabilities. Quantum magnetometers based on nitrogen-vacancy centers in diamond can detect fields far weaker than conventional sensors. These sensors may enable new approaches to EMC measurement and characterization.

Future Implications

As conventional semiconductor scaling approaches fundamental limits, alternative computing approaches may become necessary. Whether quantum computing, neuromorphic computing, or other paradigms emerge, they will bring their own EMC characteristics that will require new understanding and techniques. The EMC field will continue evolving to address whatever technologies the future brings.