11.1 Electromagnetic interference (EMI)
12 min read•august 20, 2024
Electromagnetic interference (EMI) is a critical concern in electronic systems. It can disrupt device operation, degrade signal quality, and even cause component damage. Understanding EMI sources, coupling mechanisms, and mitigation techniques is crucial for designing reliable and compliant electronic systems.
EMI measurement, , and are key strategies for managing electromagnetic interference. Spectrum analyzers, EMI receivers, and specialized probes enable accurate EMI assessment. Proper shielding design and material selection, along with effective filtering techniques, help minimize EMI's impact on electronic devices and systems.
Sources of EMI
- Electromagnetic interference (EMI) is the disruption of the normal operation of electronic devices due to electromagnetic fields generated by other sources
- EMI can be caused by both natural phenomena and man-made devices, and its effects can range from minor signal degradation to complete system failure
Natural vs man-made sources
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- Natural sources of EMI include lightning strikes, solar flares, and cosmic radiation
- These sources can generate powerful electromagnetic pulses (EMPs) that can disrupt electronic systems over a wide area
- Man-made sources of EMI are more common and include a wide range of electronic devices and systems
- Examples include power lines, motors, switches, digital circuits, and wireless communication devices (cell phones, Wi-Fi routers)
Common EMI sources
- Some of the most common sources of EMI in electronic systems include:
- Switching power supplies, which generate high-frequency noise due to their rapid switching action
- Digital circuits, which generate high-frequency harmonics due to their square wave signals
- Electric motors and transformers, which can generate both low-frequency and high-frequency EMI
- Other common sources include fluorescent lights, microwave ovens, and medical equipment (MRI machines, electrosurgical units)
High-frequency sources
- High-frequency EMI sources are particularly problematic because they can couple efficiently into electronic circuits and cause significant interference
- Examples of high-frequency EMI sources include:
- Wireless communication devices operating in the GHz range (cell phones, Wi-Fi, Bluetooth)
- Radar systems, which use high-power microwave signals for detection and ranging
- Microwave ovens, which use high-frequency electromagnetic fields for heating food
Transient sources
- Transient EMI sources generate short-duration, high-energy pulses that can cause severe damage to electronic components
- Examples of transient EMI sources include:
- Electrostatic discharge (ESD) events, which can generate voltage spikes of several kilovolts
- Lightning strikes, which can induce large currents and voltages in nearby conductors
- Switching of inductive loads, such as motors and relays, which can generate voltage spikes due to the sudden collapse of magnetic fields
Coupling mechanisms
- EMI can be coupled into electronic systems through various mechanisms, which can be broadly classified as conducted and radiated coupling
Conducted EMI
- occurs when the interference is transmitted through physical connections, such as power lines, signal cables, or ground planes
- Common sources of conducted EMI include:
- Power line disturbances, such as voltage fluctuations, transients, and harmonics
- Ground loops, which can cause circulating currents and voltage differences between different parts of a system
- Crosstalk between adjacent signal lines, which can cause unwanted coupling of signals
Radiated EMI
- occurs when the interference is transmitted through electromagnetic fields that propagate through space
- Radiated EMI can be further classified into near-field and far-field coupling, depending on the distance between the source and the affected device
- Common sources of radiated EMI include:
- Antennas, which are designed to efficiently radiate electromagnetic energy
- High-frequency digital circuits, which can generate significant amounts of electromagnetic radiation
- Poorly shielded or unshielded electronic devices, which can both emit and receive electromagnetic radiation
Near-field vs far-field coupling
- Near-field coupling occurs when the distance between the EMI source and the affected device is much smaller than the wavelength of the interference signal
- In the near-field region, the electromagnetic fields are dominated by either the electric field (capacitive coupling) or the magnetic field (inductive coupling), depending on the source characteristics
- Far-field coupling occurs when the distance is much larger than the wavelength
- In the far-field region, the electric and magnetic fields are orthogonal to each other and propagate as plane waves
- Far-field coupling is more difficult to mitigate because the fields can propagate over long distances and through obstacles
Common-mode vs differential-mode coupling
- Common-mode coupling occurs when the interference signal appears as a voltage between a signal line and ground, with the same polarity on both conductors
- Common-mode EMI is often caused by ground loops or differences in ground potential between different parts of a system
- Differential-mode coupling occurs when the interference signal appears as a voltage between two signal lines, with opposite polarity on each conductor
- Differential-mode EMI is often caused by inductive or capacitive coupling between adjacent signal lines
- Common-mode and differential-mode EMI require different mitigation techniques, such as common-mode chokes and differential-mode filters
EMI effects on circuits
- EMI can have various detrimental effects on electronic circuits, ranging from minor signal degradation to complete system failure
Signal integrity issues
- EMI can cause signal integrity issues by introducing unwanted noise and distortion into the desired signals
- Common signal integrity issues caused by EMI include:
- Jitter, which is the variation in the timing of digital signals
- Bit errors, which occur when the noise level is high enough to cause incorrect interpretation of digital data
- Analog signal distortion, which can manifest as harmonic distortion, intermodulation, or phase noise
Electromagnetic compatibility (EMC)
- EMC refers to the ability of electronic devices to operate correctly in their intended electromagnetic environment without causing or suffering from unacceptable interference
- EMC involves two main aspects:
- Emissions, which refer to the electromagnetic energy generated by a device that can potentially interfere with other devices
- Immunity, which refers to the ability of a device to operate correctly in the presence of electromagnetic interference
- EMC standards and regulations, such as those set by the FCC and IEC, define limits for emissions and immunity to ensure compatibility between different devices
Crosstalk in PCBs
- Crosstalk is the unwanted coupling of signals between adjacent traces on a printed circuit board (PCB)
- Crosstalk can be caused by both capacitive and inductive coupling between the traces
- Factors that affect crosstalk include:
- Trace spacing and geometry
- Signal rise and fall times
- Dielectric properties of the PCB material
- Crosstalk can be mitigated through proper PCB layout techniques, such as increasing trace spacing, using guard traces, and controlling trace impedances
EMI-induced noise
- EMI can induce unwanted noise voltages and currents in electronic circuits, which can degrade signal quality and cause malfunctions
- Common types of EMI-induced noise include:
- Thermal noise, which is caused by the random motion of electrons in conductors and is dependent on temperature
- Shot noise, which is caused by the discrete nature of electric charge and is associated with current flow across potential barriers (p-n junctions, Schottky barriers)
- Flicker noise (1/f noise), which is characterized by a power spectral density that is inversely proportional to frequency and is commonly found in active devices such as transistors and op-amps
- EMI-induced noise can be mitigated through proper circuit design techniques, such as using low-noise components, minimizing loop areas, and using proper grounding and shielding
EMI-induced component damage
- In severe cases, EMI can cause permanent damage to electronic components, either through direct overstress or through secondary effects such as overheating
- Examples of EMI-induced component damage include:
- Electrostatic discharge (ESD) damage, which can cause dielectric breakdown, junction damage, or thermal damage in semiconductors
- Electrical overstress (EOS) damage, which can cause melting, vaporization, or thermal decomposition of materials due to high currents or voltages
- Latchup, which is a condition in CMOS integrated circuits where a parasitic thyristor structure is inadvertently triggered, causing a high-current state that can lead to thermal damage
- EMI-induced component damage can be prevented through the use of protective devices such as transient voltage suppressors (TVS), ESD protection diodes, and current-limiting fuses, as well as through proper circuit board layout and grounding techniques
EMI measurement techniques
- Accurate measurement of EMI is essential for assessing the electromagnetic compatibility of electronic devices and ensuring compliance with relevant standards and regulations
Spectrum analyzers
- Spectrum analyzers are instruments that display the amplitude of an input signal as a function of frequency
- They are commonly used for measuring the frequency content and power levels of EMI signals
- Key specifications of spectrum analyzers include:
- Frequency range and resolution
- Amplitude accuracy and dynamic range
- Sweep time and real-time bandwidth
- Spectrum analyzers can be used with various types of antennas and probes to measure both conducted and radiated EMI
EMI receivers
- EMI receivers are specialized instruments designed for measuring electromagnetic emissions from electronic devices
- They combine the functions of a , a preamplifier, and a quasi-peak detector
- EMI receivers are designed to meet the requirements of specific EMC standards, such as CISPR 16-1-1, which define the measurement bandwidths, detector types, and dwell times for emissions testing
- EMI receivers typically have higher sensitivity and dynamic range compared to general-purpose spectrum analyzers
Near-field probes
- Near-field probes are used to measure the localized electric and magnetic fields close to the surface of a device under test (DUT)
- They are useful for identifying the sources of EMI and for troubleshooting EMC issues
- Common types of near-field probes include:
- Magnetic field (H-field) probes, which are loops that measure the magnetic field component of the EMI
- Electric field (E-field) probes, which are monopoles or dipoles that measure the electric field component of the EMI
- Near-field probes are typically used in conjunction with a spectrum analyzer or an oscilloscope to visualize the EMI signals in both the frequency and time domains
Antennas for EMI measurement
- Antennas are used to measure the radiated EMI from electronic devices in the far-field region
- The choice of antenna depends on the frequency range of interest and the type of EMI being measured
- Common types of antennas used for EMI measurements include:
- Biconical antennas, which are broadband antennas that cover the frequency range from 30 MHz to 300 MHz
- Log-periodic antennas, which are directional antennas that cover the frequency range from 200 MHz to 2 GHz
- Horn antennas, which are high-gain directional antennas that are used for measurements above 1 GHz
- Antennas are typically calibrated using a reference signal generator to determine their antenna factor, which relates the received voltage to the incident electric field strength
Conducted EMI measurement
- Conducted EMI measurements involve measuring the that is present on the power lines, signal lines, or ground conductors of a device
- Conducted EMI is typically measured using a line impedance stabilization network (LISN), which provides a defined impedance to the device under test and allows the measurement of the EMI voltage across a specified frequency range
- The LISN is connected between the power source and the device under test, and the EMI voltage is measured using an EMI receiver or a spectrum analyzer
- Conducted EMI measurements are important for ensuring that electronic devices do not generate excessive noise on the power lines that could interfere with other devices
Radiated EMI measurement
- Radiated EMI measurements involve measuring the electromagnetic fields that are emitted by a device in the far-field region
- Radiated EMI measurements are typically performed in an anechoic chamber or an open-area test site (OATS) to minimize reflections and interference from external sources
- The device under test is placed on a non-conductive table and is rotated to determine the maximum emission levels in different orientations
- The radiated EMI is measured using an antenna that is connected to an EMI receiver or a spectrum analyzer
- The antenna is typically placed at a specified distance from the device under test (e.g., 3 meters or 10 meters) and is scanned over the frequency range of interest
- Radiated EMI measurements are important for ensuring that electronic devices do not generate excessive electromagnetic fields that could interfere with other devices or pose a health hazard to users
EMI shielding
- EMI shielding is the practice of enclosing electronic devices or circuits in conductive materials to reduce the emission or reception of electromagnetic interference
Shielding effectiveness
- Shielding effectiveness (SE) is a measure of how well a shielding material or enclosure attenuates electromagnetic fields
- SE is typically expressed in decibels (dB) and is defined as the ratio of the incident field strength to the transmitted field strength
- The SE of a shielding material depends on several factors, including:
- Conductivity and permeability of the material
- Thickness of the material
- Frequency of the electromagnetic field
- Higher conductivity, higher permeability, and greater thickness generally result in better shielding effectiveness
Shielding materials
- Various materials can be used for EMI shielding, depending on the specific application and frequency range
- Common shielding materials include:
- Metals, such as copper, aluminum, and steel, which provide excellent shielding effectiveness due to their high conductivity
- Conductive polymers, which are composites of polymers and conductive fillers (e.g., carbon, metal particles) that offer lighter weight and greater flexibility compared to metals
- Conductive coatings, such as conductive paints, sprays, or plating, which can be applied to non-conductive surfaces to provide shielding
- The choice of shielding material depends on factors such as the required shielding effectiveness, weight, cost, and environmental conditions
Enclosure design for EMI shielding
- Proper design of the shielding enclosure is critical for achieving effective EMI shielding
- Key considerations in enclosure design include:
- Material selection, based on the required shielding effectiveness and other factors such as weight, cost, and corrosion resistance
- Seam and joint design, to ensure continuous electrical contact between the shielding surfaces and minimize leakage through gaps or openings
- Aperture control, to limit the size and number of openings in the enclosure that could allow EMI to escape or enter
- Other design considerations include the use of conductive gaskets, EMI-absorbing materials, and proper grounding techniques
Cable shielding techniques
- Cables can act as antennas that pick up or radiate EMI, so proper shielding of cables is important for maintaining EMC
- Common cable shielding techniques include:
- Braided shields, which are woven from conductive wire and provide good flexibility and mechanical strength
- Foil shields, which are made from thin layers of conductive material (e.g., aluminum) and offer high shielding effectiveness and low weight
- Combination shields, which use both braided and foil layers to combine the advantages of both techniques
- The choice of cable shielding technique depends on factors such as the frequency range, required shielding effectiveness, and mechanical requirements (e.g., flexibility, durability)
Apertures and seams in shielding
- Apertures and seams in shielding enclosures can act as unintentional antennas that allow EMI to escape or enter the enclosure
- The size and shape of the apertures determine their effectiveness as antennas, with larger apertures generally being more problematic at lower frequencies
- Seams between shielding surfaces can also allow EMI leakage if they are not properly designed and implemented
- Techniques for minimizing EMI leakage through apertures and seams include:
- Using conductive gaskets or spring fingers to ensure continuous electrical contact between shielding surfaces
- Minimizing the size and number of apertures in the enclosure
- Using waveguide-below-cutoff designs for necessary apertures (e.g., ventilation holes)
- Implementing proper bonding and grounding techniques to ensure a low-impedance path for EMI currents
EMI filtering
- EMI filtering is the practice of using passive or active filter circuits to attenuate electromagnetic noise and interference in electronic systems
Passive vs active filters
- Passive EMI filters use combinations of passive components, such as resistors, capacitors, and inductors, to attenuate EMI signals
- Passive filters are generally simpler, more reliable, and less expensive than active filters
- However, they may have limitations in terms of attenuation, frequency range, and insertion loss
- Active EMI filters use active components, such as op-amps or transistors, in addition to passive components to provide greater attenuation and selectivity
- Active filters can achieve higher attenuation and better performance at lower frequencies compared to passive filters
- However, they are generally more complex, expensive, and power-consuming than passive filters
Common-mode filters
- Common-mode EMI filters are designed to attenuate common-mode noise, which is noise that appears equally on both signal lines with respect to ground
- Common-mode filters typically use common-mode chokes, which are transformers with high common-mode impedance and low differential-mode impedance
- The common-mode choke presents a high impedance to common-mode noise, while allowing the differential-mode signal to pass through unattenuated
- Common-mode filters may also include capacitors connected between the signal lines and ground to provide additional attenuation at high frequencies
Differential-mode filters
- Differential-mode EMI filters are designed to attenuate differential-mode noise, which is noise that appears between the two signal lines with opposite polarity
- Differential-mode filters typically use series inductors and shunt capacitors to
Key Terms to Review (16)
CISPR Standards: CISPR standards are a set of international guidelines developed by the International Electrotechnical Commission (IEC) for limiting electromagnetic interference (EMI) in electrical and electronic devices. These standards aim to ensure electromagnetic compatibility (EMC) by specifying the maximum allowable levels of EMI that devices can emit, as well as the minimum performance criteria they must meet to function properly in their intended environments. The standards play a crucial role in protecting devices from unwanted interference and ensuring they do not disrupt other equipment.
Conducted EMI: Conducted EMI refers to electromagnetic interference that travels through conductive pathways, such as wires and cables, impacting electronic devices and systems. This type of interference can affect the performance and reliability of devices by inducing unwanted signals, which can lead to malfunctions or degradation of functionality. Understanding conducted EMI is essential for designing systems that are resistant to interference, ensuring electromagnetic compatibility.
Dbμv: dbμv, or decibels microvolt, is a logarithmic unit used to express the ratio of a voltage level to a reference voltage of one microvolt (1 μV). This measurement is crucial in understanding electromagnetic interference (EMI) because it allows for the quantification of very small voltages that can affect electronic devices. By expressing voltages in decibels, it becomes easier to analyze and compare the strength of signals relative to the noise floor, particularly in environments where EMI may disrupt normal operation.
Electromagnetic noise: Electromagnetic noise refers to unwanted electromagnetic signals that can interfere with the operation of electronic devices and communication systems. This noise can arise from natural sources, like lightning or solar flares, as well as man-made sources, such as machinery or electrical equipment, disrupting the intended signals and causing issues in data transmission and processing.
Emc design principles: EMC design principles refer to the set of guidelines and practices aimed at ensuring electromagnetic compatibility (EMC) of electronic devices and systems. These principles help minimize electromagnetic interference (EMI) and enhance the performance and reliability of devices, making them compliant with regulatory standards. By incorporating these principles during the design phase, engineers can create devices that effectively coexist with other electronic equipment without causing or experiencing interference.
Emc testing: EMC testing, or electromagnetic compatibility testing, is a process that evaluates how electrical devices interact with their electromagnetic environment to ensure they do not emit excessive electromagnetic interference and can withstand external interference. This testing is crucial for ensuring that devices operate reliably in the presence of other electronic equipment and comply with regulatory standards.
EMI Filter: An EMI filter is an electronic circuit designed to reduce electromagnetic interference (EMI) in electrical systems. These filters work by attenuating unwanted high-frequency noise signals while allowing the desired signals to pass through, ensuring that devices function properly without disruption from electromagnetic disturbances.
EMI Susceptibility: EMI susceptibility refers to the degree to which an electronic device or system is affected by electromagnetic interference (EMI), which can disrupt its normal operation or performance. This concept is crucial as it determines how well a device can tolerate the presence of unwanted electromagnetic signals that may cause malfunctions or degradation in functionality.
FCC Regulations: FCC regulations are rules set by the Federal Communications Commission to manage electromagnetic spectrum use and ensure that devices meet standards for electromagnetic interference and compatibility. These regulations play a vital role in minimizing electromagnetic interference (EMI) among devices, allowing various technologies to operate together without disruption, while also promoting electromagnetic compatibility (EMC) to protect sensitive equipment.
Filtering: Filtering is the process of selectively allowing certain frequencies of electromagnetic signals to pass through while blocking others. This technique is essential for reducing unwanted noise and interference in electronic systems, ensuring that signals maintain their integrity and clarity. Effective filtering enhances the performance of devices by promoting electromagnetic compatibility and reducing the impact of electromagnetic interference.
Industrial environment: The industrial environment refers to the setting in which manufacturing and production activities take place, encompassing the physical space, machinery, and operational processes involved. This environment can influence various factors such as electromagnetic interference (EMI), which can disrupt electronic devices and systems essential for industrial operations. Understanding the industrial environment helps to mitigate issues related to EMI, ensuring smooth functioning of equipment and compliance with regulatory standards.
Radiated emi: Radiated electromagnetic interference (EMI) refers to the unwanted electromagnetic energy that is emitted from an electronic device and can disrupt the operation of nearby devices. This phenomenon often arises from high-frequency signals in circuits, leading to interference that can affect communication systems, consumer electronics, and various other applications. Understanding this concept is crucial for managing electronic device performance and ensuring reliability in complex systems.
Residential environment: A residential environment refers to the physical and social setting in which people live, encompassing factors like housing, infrastructure, and community interactions. This setting plays a significant role in how electromagnetic interference (EMI) affects the day-to-day lives of residents, influencing everything from the functionality of electronic devices to health and safety concerns.
Shielding: Shielding refers to the practice of protecting sensitive electronic devices from electromagnetic interference (EMI) by blocking or diverting unwanted electromagnetic fields. This technique is essential in ensuring that devices operate correctly and maintain performance, especially in environments with a high level of EMI. It also plays a crucial role in achieving electromagnetic compatibility (EMC) and mitigating the effects of electromagnetic pulses (EMP).
Spectrum analyzer: A spectrum analyzer is a device used to measure and visualize the frequency spectrum of signals, displaying their amplitude as a function of frequency. This tool is essential in identifying and diagnosing electromagnetic interference (EMI) by allowing users to observe unwanted signals that may affect the performance of electronic devices. By analyzing the frequency components, a spectrum analyzer can help in understanding the sources of interference and in designing solutions to mitigate its effects.
Transient Voltages: Transient voltages are short-lived spikes or surges in voltage that occur in electrical systems, often caused by sudden changes in current or electromagnetic interference. These voltages can disrupt the normal operation of electronic devices and power systems, leading to potential damage or malfunction. Understanding transient voltages is crucial for protecting sensitive equipment and ensuring the reliability of electrical systems.