8.5 Radiated emissions testing
8 min read•august 21, 2024
Radiated emissions testing is a crucial aspect of electromagnetic compatibility. It involves measuring and controlling unintended electromagnetic energy released by electronic devices to prevent interference with other equipment and ensure regulatory compliance.
This topic covers the fundamentals of radiated emissions, measurement techniques, testing procedures, and mitigation strategies. Understanding these concepts is essential for designing and certifying electronic products that meet EMC standards and perform reliably in real-world environments.
Fundamentals of radiated emissions
- Electromagnetic Interference and Compatibility studies focus on controlling unwanted electromagnetic energy, with radiated emissions playing a crucial role
- Radiated emissions testing assesses the electromagnetic fields emitted by electronic devices to ensure compliance with regulatory standards and prevent interference
- Understanding radiated emissions fundamentals forms the foundation for effective EMC design and testing strategies
Definition and importance
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- Unintended electromagnetic energy released into the environment by electronic devices
- Potential to interfere with other nearby electronic equipment, causing malfunctions or degraded performance
- Critical for ensuring electromagnetic compatibility and regulatory compliance in various industries (consumer electronics, automotive, aerospace)
- Impacts product reliability, safety, and market access
Sources of radiated emissions
- Digital circuits with high-speed switching generate
- Power supply switching noise contributes to both conducted and radiated emissions
- Cable and PCB traces act as unintentional antennas, radiating electromagnetic energy
- Oscillators and clock circuits produce at specific frequencies
- Electrostatic discharge events create impulsive broadband emissions
Regulatory standards overview
- regulates radiated emissions for devices sold in the United States
- /EN 55022 provides limits for information technology equipment in Europe
- MIL-STD-461 sets requirements for military and aerospace applications
- Automotive industry follows specific standards (CISPR 25, ISO 11452)
- Limits typically specified in or , varying by frequency and device class
Radiated emissions measurement
- Accurate measurement of radiated emissions requires specialized equipment and controlled environments
- Testing procedures aim to replicate real-world conditions while maintaining repeatability and reproducibility
- Proper measurement techniques are essential for identifying non-compliant emissions and implementing effective mitigation strategies
Test setup and equipment
- or measures emission amplitudes across frequency ranges
- Signal generators and tracking generators for system calibration and verification
- Preamplifiers enhance measurement sensitivity for low-level emissions
- Turntables rotate the device under test to capture emissions from all angles
- Automated test software controls equipment and logs measurement data
- Transient limiters protect sensitive measurement equipment from impulse events
Antennas for emissions testing
- Biconical antennas cover lower frequencies (typically 30 MHz to 300 MHz)
- Log-periodic antennas used for higher frequencies (typically 200 MHz to 1 GHz)
- Horn antennas provide high gain for microwave frequencies (above 1 GHz)
- Loop antennas measure magnetic field emissions at lower frequencies (below 30 MHz)
- LPDA (Log-Periodic Dipole Array) antennas offer broadband coverage for multiple ranges
Measurement distance considerations
- Standard distances include 3m, 10m, and 30m, depending on the test standard
- measurements (less than λ/2π) provide emission source localization
- measurements (greater than 2D²/λ) assess overall radiated field strength
- Distance correction factors apply when testing at non-standard distances
- Measurement uncertainty increases with distance due to ambient noise and reflections
Testing procedures
- Radiated emissions testing involves systematic procedures to ensure accurate and repeatable results
- Different test environments offer varying levels of control over ambient conditions and reflections
- Selection of appropriate test method depends on regulatory requirements, frequency range, and equipment size
Pre-compliance vs full compliance
- conducted during product development to identify potential issues
- Utilizes simplified test setups and less expensive equipment for quick assessments
- Full compliance testing performed in accredited laboratories for final certification
- Pre-compliance helps reduce costs by addressing EMC issues early in the design process
- Full compliance provides official documentation required for regulatory approval
Open area test site (OATS)
- Outdoor test environment with a large, flat ground plane (typically metal)
- Requires clear area free from reflecting objects and electromagnetic interference
- Weather-dependent, limiting testing availability and potentially affecting results
- Offers good correlation with real-world environments for larger equipment
- Challenging to maintain consistent ambient noise levels
Semi-anechoic chamber testing
- Shielded room with RF absorbing material on walls and ceiling
- Provides controlled environment independent of external factors
- Eliminates ambient noise and unwanted reflections for more accurate measurements
- Allows testing in all weather conditions and at any time
- Size limitations may restrict testing of very large equipment
Reverberation chamber testing
- Highly reflective shielded room with mechanical stirrers to create statistically uniform field
- Efficient for testing immunity and emissions of electrically large objects
- Provides high field strengths with relatively low input power
- Challenges in correlating results with other test methods
- Useful for testing multiple device orientations simultaneously
Frequency ranges and limits
- Radiated emissions testing covers a wide range of frequencies to address various interference mechanisms
- Regulatory limits vary based on frequency, device classification, and intended operating environment
- Understanding different emission types helps in identifying and mitigating specific interference sources
Low frequency emissions
- Typically range from 9 kHz to 30 MHz
- Often dominated by magnetic field components
- Common sources include switch-mode power supplies and motor drives
- Measured using loop antennas to capture magnetic field strength
- Limits usually specified in dBμA/m due to near-field characteristics
High frequency emissions
- Extend from 30 MHz to several GHz, depending on the standard
- Electric field components become more significant at higher frequencies
- Digital circuits, high-speed interfaces, and wireless transmitters contribute
- Measured using various antenna types (biconical, log-periodic, horn)
- Limits typically specified in dBμV/m for far-field measurements
Broadband vs narrowband emissions
- Broadband emissions span a wide frequency range (switching transients, ESD events)
- Narrowband emissions concentrated at specific frequencies (clock , oscillators)
- Measurement bandwidths affect the characterization of emission types
- Broadband emissions often require quasi-peak or average detection methods
- Narrowband emissions typically measured using peak detection for worst-case analysis
Data analysis and interpretation
- Proper analysis of radiated emissions data is crucial for identifying non-compliances and emission sources
- Interpretation techniques help in distinguishing between actual emissions and measurement artifacts
- Understanding measurement uncertainties and applying appropriate margins ensure robust compliance assessment
Peak vs average measurements
- Peak measurements capture maximum emission levels, useful for identifying transient events
- represent time-averaged emission levels, relevant for continuous signals
- weighs emissions based on their repetition rate and duration
- Some standards require multiple detection methods for comprehensive assessment
- Correlation between detection methods helps in characterizing emission types
Margin analysis
- Compares measured emissions to applicable limits, accounting for measurement uncertainty
- Positive margins indicate compliance, while negative margins highlight potential issues
- Typically aim for 3-6 dB margin to account for production variations and measurement tolerances
- Critical for identifying borderline cases that may require additional investigation or mitigation
- Helps prioritize EMC efforts by focusing on emissions closest to or exceeding limits
Identifying emission sources
- Analyze frequency content to correlate emissions with known internal signals (clock harmonics)
- Use near-field probes to localize emission sources on PCBs or cables
- Vary operating modes and loads to isolate emissions from specific circuits or components
- Time-domain analysis helps link emissions to specific events in device operation
- Compare emissions patterns with common EMI mechanisms (differential mode, common mode)
Mitigation techniques
- Effective EMI mitigation strategies address emissions at their source, along propagation paths, and at potential victims
- Implementing multiple mitigation techniques often provides the most robust EMC solution
- Balancing EMC requirements with other design constraints (cost, size, performance) is crucial
Shielding effectiveness
- Metallic enclosures attenuate radiated emissions through reflection and absorption
- depends on material properties, thickness, and frequency
- Proper design of seams, joints, and apertures critical for maintaining shield integrity
- Conductive gaskets and fingerstock improve shielding at removable panels and doors
- Shielding can be applied at various levels (entire device, subsystem, or component)
PCB layout considerations
- Minimize loop areas in high-speed signal paths to reduce antenna effects
- Implement proper ground plane design with minimal splits or gaps
- Use stackup optimization to control impedance and reduce crosstalk
- Place decoupling capacitors close to IC power pins to minimize current loop areas
- Implement guard traces and stitching vias to contain high-frequency signals
Cable and connector optimization
- Use shielded cables for sensitive or high-speed signals to contain emissions
- Implement proper shield termination techniques (360-degree termination, pigtail minimization)
- Filter connectors can attenuate high-frequency noise at I/O interfaces
- Ferrite cores on cables provide common-mode noise suppression
- Twisted pair wiring reduces differential mode emissions through field cancellation
Challenges in radiated emissions testing
- Radiated emissions testing faces various challenges that can impact measurement accuracy and repeatability
- Addressing these challenges requires careful test setup, calibration, and data interpretation
- Understanding limitations helps in developing robust test methodologies and interpreting results
Ambient noise management
- External RF sources (broadcast transmitters, cellular networks) can mask device emissions
- Shielded test environments (semi-anechoic chambers) minimize ambient interference
- Ambient scans performed before and after device testing to identify and account for background noise
- Time-gating techniques can separate device emissions from pulsed ambient signals
- Differential measurements compare emissions with device on and off to isolate contributions
Ground plane effects
- Ground plane size and composition influence measurement results, especially at lower frequencies
- Reflections from ground plane can cause constructive or destructive interference
- Standardized ground plane requirements ensure consistency between test sites
- Elevated ground planes used in some test setups to control reflections
- Computational models can help predict and account for
Near-field vs far-field measurements
- Transition from near-field to far-field occurs gradually, not at a fixed distance
- Near-field measurements provide better spatial resolution for source identification
- Far-field measurements assess overall radiated emissions for compliance purposes
- Correlation between near-field and far-field results not always straightforward
- Some standards require measurements in both regions for comprehensive assessment
Emerging trends and technologies
- Advancements in measurement techniques and computational capabilities are shaping the future of radiated emissions testing
- New technologies address limitations of traditional methods and provide deeper insights into emission mechanisms
- Emerging trends focus on improving test efficiency, accuracy, and applicability to complex modern devices
Time-domain EMI measurements
- Utilizes high-speed digitizers and FFT processing for rapid spectral analysis
- Captures transient and intermittent emissions often missed by traditional swept measurements
- Allows for time-frequency analysis to correlate emissions with specific device operations
- Reduces overall test time compared to stepped frequency domain measurements
- Challenges in dynamic range and amplitude accuracy compared to traditional receivers
Computational EMC modeling
- Finite Element Method (FEM) and Method of Moments (MoM) simulate complex electromagnetic environments
- Allows for virtual prototyping and EMC assessment before physical hardware is available
- Helps optimize shielding, filtering, and PCB layout for improved EMC performance
- Combines circuit-level and 3D field simulations for comprehensive analysis
- Validation of simulation results with measurements remains crucial for ensuring accuracy
Wireless device emissions testing
- Increasing prevalence of IoT and 5G devices poses new challenges for emissions testing
- Distinguishing between intentional and unintentional radiators in complex wireless systems
- Development of new test methods for assessing emissions during various operational modes
- Integration of over-the-air (OTA) testing techniques with traditional EMC measurements
- Addressing potential interactions between multiple wireless devices in close proximity
Key Terms to Review (33)
Ambient noise management: Ambient noise management refers to the process of controlling and reducing background electromagnetic noise that can interfere with the performance of electronic devices and systems. This is particularly important in environments where sensitive equipment operates, as excessive ambient noise can lead to degraded performance or erroneous readings. Effective ambient noise management involves identifying sources of noise, implementing shielding techniques, and optimizing system design to minimize susceptibility to interference.
Anechoic Chamber: An anechoic chamber is a specialized room designed to eliminate reflections of sound or electromagnetic waves, creating an environment that is acoustically and electromagnetically isolated. This controlled setting is critical for accurately measuring emissions and immunity of devices without interference from external signals or reflections, thus ensuring precise compliance with various standards.
Average measurements: Average measurements refer to the central value obtained by calculating the mean of a set of data points, representing a typical value that characterizes a dataset. In radiated emissions testing, average measurements are crucial as they help to determine compliance with regulatory limits and assess the overall performance of electronic devices under standard operating conditions.
Broadband emissions: Broadband emissions refer to the wide range of electromagnetic energy emitted by electronic devices across multiple frequencies simultaneously. This type of emission can affect various communication systems and is particularly significant in radiated emissions testing, as it can interfere with the operation of nearby electronic equipment, potentially leading to malfunction or reduced performance.
Cable and Connector Optimization: Cable and connector optimization refers to the process of enhancing the performance and reliability of electrical connections and transmission media in order to minimize signal loss and interference. This practice is crucial for ensuring that electronic devices operate efficiently, particularly in environments where radiated emissions can impact performance. By selecting the right cables, connectors, and configurations, designers can significantly improve the electromagnetic compatibility of systems.
Certification Testing: Certification testing is a process that evaluates and verifies whether a device or system meets specific regulatory standards for electromagnetic emissions and immunity. This process ensures compliance with established guidelines, such as those set by regulatory bodies, to minimize interference and ensure reliable performance in various environments. It is crucial in assessing radiated emissions, confirming that devices do not exceed allowable limits and that they function properly in the presence of electromagnetic interference.
CISPR 22: CISPR 22 is an international standard that outlines the requirements for measuring and limiting electromagnetic interference (EMI) caused by information technology equipment (ITE) through conducted and radiated emissions. This standard plays a crucial role in ensuring the compatibility of electronic devices with their environment and in minimizing potential disruptions to other electronic equipment.
Computational EMC Modeling: Computational EMC modeling refers to the use of computer simulations and numerical methods to predict and analyze electromagnetic compatibility (EMC) issues in electronic systems. This approach allows engineers to assess how devices interact in terms of radiated emissions and immunity, helping to identify potential EMC problems before physical testing occurs. By employing advanced modeling techniques, designers can optimize their products for compliance with EMC standards more efficiently.
Dbμa/m: dbμa/m, or decibels microamperes per meter, is a unit used to measure the strength of electromagnetic fields, specifically in terms of the electric field intensity of radiated emissions. This term connects closely with understanding how devices emit electromagnetic radiation and how these emissions can interfere with other electronic devices. It plays a crucial role in assessing compliance with electromagnetic compatibility standards and regulations during testing.
Dbμv/m: The term dbμv/m refers to decibels microvolts per meter, a unit of measurement used to express the strength of electric fields in the context of electromagnetic emissions. This measurement quantifies how much electromagnetic radiation is radiated by electronic devices and is critical for determining compliance with regulatory standards. It helps assess the potential interference that devices may cause in their surrounding environment.
EMI Receiver: An EMI receiver is a specialized instrument used to measure electromagnetic interference (EMI) signals in various environments, helping to identify sources of unwanted emissions and ensure compliance with electromagnetic compatibility standards. This tool is critical in evaluating radiated emissions from electronic devices, determining their impact on other equipment, and verifying that they meet regulatory requirements.
Far-field: The far-field region refers to the area far enough away from a radiating source where the electromagnetic waves can be considered to be in a plane wave form, meaning the wavefronts are essentially flat. In this zone, the effects of distance on the electromagnetic fields become predictable and consistent, allowing for accurate measurements and analysis of radiated energy. This concept is crucial for applications such as testing antennas and assessing radiated emissions.
FCC Part 15: FCC Part 15 refers to a set of regulations established by the Federal Communications Commission (FCC) in the United States that governs unlicensed radio frequency devices and their emissions. This regulation is crucial for ensuring that electronic devices do not cause harmful interference to licensed radio services, maintaining a balance between innovation and spectrum management.
Federal Communications Commission (FCC): The Federal Communications Commission (FCC) is an independent agency of the U.S. government that regulates interstate and international communications by radio, television, wire, satellite, and cable. The FCC plays a vital role in managing the spectrum of radio frequencies to ensure that different devices can operate without causing harmful interference, which is particularly important in the context of radiated emissions testing.
Ground plane effects: Ground plane effects refer to the influence of the ground plane on the performance of electronic devices, particularly regarding their radiated emissions and susceptibility to electromagnetic interference. The ground plane acts as a reference point for signal return paths, and its design can significantly impact the radiation patterns, impedance, and overall effectiveness of electronic systems in minimizing unwanted emissions.
Harmonics: Harmonics are integer multiples of a fundamental frequency, representing the different frequency components that can arise in a signal. In electrical systems, these harmonics can affect the signal integrity and performance, potentially leading to distortion and interference in digital signals. Understanding harmonics is crucial for analyzing how signals behave in printed circuit boards and during radiated emissions testing.
High Frequency Emissions: High frequency emissions refer to electromagnetic waves that occur at frequencies typically in the range of 3 MHz to 30 MHz. These emissions can be a byproduct of electrical devices, systems, or circuits and can interfere with the operation of nearby electronic equipment. Understanding and testing for high frequency emissions is essential in ensuring compliance with electromagnetic compatibility standards and maintaining the proper functioning of electronic devices.
Intentional emissions: Intentional emissions refer to the deliberate transmission of electromagnetic energy by devices for specific functions, such as communication or data transfer. These emissions are often designed to operate within regulatory limits and can serve various purposes, including broadcasting signals, enabling wireless communication, or facilitating remote control of devices. Understanding intentional emissions is crucial for ensuring compliance with electromagnetic compatibility standards and preventing interference with other devices.
International Electrotechnical Commission (IEC): The International Electrotechnical Commission (IEC) is a global organization that prepares and publishes international standards for electrical, electronic, and related technologies. Its standards promote safety, efficiency, and interoperability in various applications, including radiated emissions testing, which ensures that electronic devices operate without causing harmful interference to other devices or systems.
Low frequency emissions: Low frequency emissions refer to electromagnetic radiation that occurs at frequencies typically below 30 MHz. These emissions can originate from various electronic devices and systems, causing potential interference with communication systems, especially those operating at similar frequency ranges. Understanding and controlling these emissions is crucial in ensuring electromagnetic compatibility in various environments.
Margin Analysis: Margin analysis is the process of assessing the difference between the costs of producing a product or service and the revenue it generates. It helps in understanding how much profit a product contributes after covering its costs. This analysis is vital for making informed decisions about pricing, budgeting, and resource allocation, especially in evaluating the effectiveness of different strategies in reducing emissions and enhancing immunity against electromagnetic interference.
Narrowband emissions: Narrowband emissions refer to the transmission of electromagnetic energy concentrated in a small frequency range, typically less than 25 kHz wide. This type of emission is significant because it can lead to interference in communication systems, especially if the narrowband signal overlaps with the frequency ranges used by other devices. Understanding narrowband emissions is crucial for ensuring compliance with regulatory limits during radiated emissions testing.
Near-field: The near-field refers to the region close to a radiating source, where the electric and magnetic fields do not behave like plane waves and are more complex in nature. This area is critical for understanding how electromagnetic fields interact with nearby objects, making it essential in various applications such as anechoic chambers for accurate testing, radiated emissions testing to ensure compliance with regulations, and antenna design where characteristics like gain and directivity are influenced by proximity to the source.
Open area test site: An open area test site (OATS) is a specialized facility designed for measuring radiated emissions from electronic devices in an unobstructed environment. These sites are crucial for accurately assessing electromagnetic compatibility and interference, as they minimize reflections and other disturbances that could affect the test results. OATS are commonly used to evaluate compliance with regulations and standards related to electromagnetic emissions.
Pcb layout considerations: PCB layout considerations refer to the practices and principles involved in designing printed circuit boards (PCBs) to optimize performance, reduce electromagnetic interference (EMI), and ensure compliance with regulatory standards. Proper PCB layout can significantly affect signal integrity, power distribution, and overall device reliability, especially when dealing with radiated emissions testing.
Pre-compliance testing: Pre-compliance testing refers to a series of assessments conducted on electronic devices and systems to evaluate their compliance with electromagnetic compatibility (EMC) standards before formal certification. This process helps identify potential issues with radiated emissions and other compatibility problems early in the design phase, allowing engineers to make necessary adjustments and avoid costly redesigns or delays later on.
Quasi-peak detection: Quasi-peak detection is a measurement technique used in electromagnetic compatibility testing to assess the amplitude of electromagnetic emissions. This method is particularly important as it mimics the response of human perception to radio frequency signals, prioritizing signals that have a certain duration and level, thus providing a more realistic assessment of potential interference. In the context of conducted and radiated emissions testing, quasi-peak detection helps ensure that devices comply with regulatory limits, focusing on emissions that are more likely to affect sensitive electronic equipment.
Shielding Effectiveness: Shielding effectiveness refers to the ability of a material or structure to attenuate electromagnetic interference (EMI) from external sources or prevent emissions from internal sources. It is a critical factor in designing systems that minimize unwanted EMI, ensuring the reliability and functionality of electronic devices in various environments.
Signal-to-Noise Ratio: Signal-to-noise ratio (SNR) is a measure used to quantify the level of a desired signal to the level of background noise. A higher SNR indicates a clearer signal, while a lower SNR suggests that noise is interfering with the signal. Understanding SNR is crucial in various contexts, as it helps determine the effectiveness of communication systems, assess the quality of electronic devices, and evaluate electromagnetic compatibility in different environments.
Spectrum Analyzer: A spectrum analyzer is an electronic instrument that displays the amplitude of signals as they vary with frequency, allowing for the analysis of the frequency components of electrical signals. It plays a vital role in identifying and measuring electromagnetic interference (EMI) from various sources, including man-made devices, and evaluating the effectiveness of different filtering techniques.
Time-domain EMI measurements: Time-domain EMI measurements refer to the assessment of electromagnetic interference by analyzing signals in the time domain rather than the frequency domain. This approach provides insight into transient behaviors and pulse characteristics of emissions, allowing for a detailed understanding of how these signals can affect electronic devices and systems.
Unintentional Emissions: Unintentional emissions refer to the electromagnetic energy that is emitted by electronic devices without the intent to transmit information, often as a byproduct of their normal operation. These emissions can interfere with the performance of other electronic devices and are a key concern in ensuring electromagnetic compatibility. Understanding these emissions is crucial for compliance with regulatory standards and for mitigating interference issues.
Wireless device emissions testing: Wireless device emissions testing is the evaluation process that measures the electromagnetic radiation emitted by wireless devices to ensure they comply with regulatory standards. This testing is crucial for maintaining electromagnetic compatibility, as excessive emissions can interfere with other electronic devices, disrupt communication systems, and pose health risks.