📡Electromagnetic Interference Unit 10 – EMI/EMC in Digital Systems

Fundamentals of EMI/EMC

• Electromagnetic Interference (EMI) occurs when unwanted electromagnetic energy disrupts the operation of electronic devices or systems
• Electromagnetic Compatibility (EMC) ensures that electronic devices can function properly in their intended electromagnetic environment without causing interference to other devices
• EMI can be classified as conducted (propagating through physical connections) or radiated (propagating through air or space)
• The frequency spectrum of EMI ranges from low frequencies (kHz) to high frequencies (GHz), with different coupling mechanisms and effects at different frequencies
• The three main components of EMI are the source (emitter), the coupling path (medium), and the receptor (victim)
• EMC design aims to minimize the generation of EMI, reduce the coupling of EMI, and increase the immunity of devices to EMI
• The consequences of EMI include malfunctions, data corruption, reduced performance, and even permanent damage to electronic components

Sources of EMI in Digital Systems

• Digital circuits generate EMI due to fast switching transitions and high clock frequencies, which create high-frequency harmonics
• Power supply noise, such as ripple and spikes, can propagate through the system and cause EMI
• Electrostatic Discharge (ESD) events can generate high-voltage and high-current transients that induce EMI
• Cables and interconnects act as antennas, radiating or picking up EMI signals
  • Common-mode currents on cables are a significant source of radiated EMI
  • Improper termination or impedance mismatch can lead to reflections and standing waves
• External sources of EMI include nearby electronic devices, power lines, lightning, and intentional radiators (radio transmitters)
• Digital interfaces, such as USB, HDMI, and Ethernet, can be sources of EMI due to their high-speed data transmission

EMI Coupling Mechanisms

• Conducted coupling occurs when EMI propagates through physical connections, such as power lines, ground planes, or signal traces
  • Common-mode coupling involves currents flowing in the same direction on multiple conductors, creating a larger loop area for EMI radiation
  • Differential-mode coupling involves currents flowing in opposite directions on signal pairs, which can cancel each other's magnetic fields
• Radiated coupling happens when EMI propagates through air or space, in the form of electromagnetic waves
  • Near-field coupling dominates at distances less than $\lambda/2\pi$, where $\lambda$ is the wavelength of the EMI signal
    • Electric field (capacitive) coupling is predominant in high-impedance circuits
    • Magnetic field (inductive) coupling is predominant in low-impedance circuits
  • Far-field coupling dominates at distances greater than $\lambda/2\pi$, where the electric and magnetic fields are in phase and propagate as plane waves
• Crosstalk is the unintended coupling of signals between adjacent traces on a PCB or between nearby cables
  • Inductive crosstalk is caused by the magnetic field of one signal inducing a voltage in another conductor
  • Capacitive crosstalk is caused by the electric field between two conductors inducing a current flow

EMC Design Principles

• Minimize the generation of EMI at the source by reducing the amplitude and rise/fall times of digital signals
• Use proper grounding techniques, such as a single-point ground or a ground plane, to provide a low-impedance return path for EMI currents
• Decouple power supplies using capacitors to shunt high-frequency noise to ground and prevent it from propagating through the system
• Filter EMI using passive components, such as capacitors, inductors, and ferrites, to attenuate high-frequency signals
• Separate sensitive circuits from noisy circuits, both physically and electrically, to minimize coupling
• Minimize the loop area of current paths to reduce the magnetic field and radiated EMI
• Terminate transmission lines properly to prevent reflections and standing waves
• Use balanced or differential signaling to cancel out common-mode EMI

Shielding and Grounding Techniques

• Shielding involves enclosing sensitive circuits or devices in a conductive enclosure to attenuate incoming or outgoing EMI
  • The effectiveness of shielding depends on the material (conductivity and permeability), thickness, and frequency of the EMI
  • Gaps or seams in the shield can compromise its effectiveness, so proper gaskets or conductive adhesives must be used
• Cable shielding helps to contain EMI generated by the signals within the cable and to protect the signals from external EMI
  • The shield should be grounded at one end (for low-frequency EMI) or both ends (for high-frequency EMI) to prevent ground loops
  • Pigtail terminations of cable shields should be avoided, as they have higher impedance at high frequencies
• Grounding is essential for providing a low-impedance return path for EMI currents and for establishing a common reference potential
  • A single-point ground (star ground) is used to prevent ground loops and to minimize the coupling of ground noise between circuits
  • A ground plane provides a low-impedance return path for high-frequency currents and helps to minimize radiated EMI
• Bonding ensures electrical continuity between conductive surfaces, such as enclosures, cable shields, and ground planes, to maintain a consistent reference potential

PCB Layout Strategies for EMC

• Proper component placement is crucial for minimizing EMI coupling and optimizing signal integrity
  • Place sensitive circuits away from noisy circuits, such as power supplies, digital interfaces, and high-speed clock sources
  • Group components based on their function and frequency to minimize the coupling between different sections of the board
• Route high-speed signals on inner layers, surrounded by ground planes, to minimize radiated EMI and crosstalk
• Use ground planes on adjacent layers to provide a low-impedance return path for high-frequency currents and to shield sensitive signals
• Minimize the loop area of current paths by placing decoupling capacitors close to ICs and routing power and ground traces in close proximity
• Avoid splitting ground planes, as this can create a dipole antenna and increase radiated EMI
• Use guard traces or copper pours to shield sensitive signals from adjacent noisy traces
• Terminate transmission lines properly using matched impedances and termination resistors to prevent reflections and standing waves
• Follow recommended trace widths and spacings for controlled impedance and to minimize crosstalk

Testing and Measurement Methods

• Conducted EMI measurements involve measuring the EMI currents or voltages on power lines, signal lines, or ground connections
  • Line Impedance Stabilization Networks (LISNs) are used to provide a standardized impedance and to isolate the Device Under Test (DUT) from the power line
  • Spectrum analyzers or EMI receivers are used to measure the amplitude and frequency of conducted EMI
• Radiated EMI measurements involve measuring the electromagnetic fields emitted by the DUT using antennas and spectrum analyzers
  • Measurements are typically performed in a shielded enclosure (semi-anechoic chamber) to minimize external interference and reflections
  • The distance between the DUT and the antenna, as well as the antenna polarization and height, are standardized according to the relevant EMC regulations
• Susceptibility testing evaluates the immunity of the DUT to external EMI sources
  • Conducted susceptibility tests inject EMI signals into the power lines or signal lines of the DUT and monitor its performance
  • Radiated susceptibility tests expose the DUT to electromagnetic fields of varying frequency and amplitude and monitor its performance
• Near-field probing is used to locate and diagnose EMI sources on a PCB or within a device
  • Magnetic field probes (loop probes) are used to measure the local magnetic field and identify current loops or traces with high EMI
  • Electric field probes (monopole or dipole probes) are used to measure the local electric field and identify voltage fluctuations or electric field coupling

Regulatory Standards and Compliance

• EMC regulations are set by various international and national organizations to ensure the compatibility and safety of electronic devices
  • The International Electrotechnical Commission (IEC) and the International Special Committee on Radio Interference (CISPR) develop international EMC standards
  • The Federal Communications Commission (FCC) regulates EMC in the United States, with Part 15 being the most relevant for digital devices
  • The European Union (EU) requires CE marking for electronic devices, which includes compliance with the EMC Directive
• Compliance testing involves subjecting the DUT to standardized EMI measurement procedures and limits
  • Conducted EMI limits are typically specified in terms of the maximum allowed voltage or current over a specific frequency range
  • Radiated EMI limits are typically specified in terms of the maximum allowed electric field strength at a given distance over a specific frequency range
• Product certification is required for many electronic devices before they can be sold in a particular market
  • Certification involves submitting the DUT to an accredited testing laboratory, which performs the required EMC tests and issues a compliance report
  • The manufacturer must provide a Declaration of Conformity (DoC) stating that the product meets the relevant EMC standards
• Labeling requirements often include the CE mark (for the EU), the FCC logo (for the US), or other marks indicating compliance with specific EMC standards