Conducted Emissions Test Equipment and Reduction Guidelines

 

Introduction

Electromagnetic compatibility (EMC) is a critical aspect of electronic product design and manufacturing. One key component of EMC is conducted emissions, which refer to unwanted electromagnetic energy that propagates along power lines and other conductors. To ensure compliance with regulatory standards and maintain product quality, manufacturers must conduct thorough testing and implement effective reduction strategies. This comprehensive guide explores the equipment used in conducted emissions testing and provides detailed guidelines for reducing these emissions in electronic devices.

Understanding Conducted Emissions

What Are Conducted Emissions?

Conducted emissions are electromagnetic disturbances that travel along power lines, signal cables, and other conductive paths. These emissions can interfere with the proper functioning of other electronic devices connected to the same power source or nearby equipment. Conducted emissions are typically measured in the frequency range of 150 kHz to 30 MHz.

Types of Conducted Emissions

1. Common Mode (CM) Emissions: These emissions occur when current flows in the same direction on all conductors, including the ground.
2. Differential Mode (DM) Emissions: These emissions result from current flowing in opposite directions on different conductors.

Regulatory Standards

Several international and regional standards govern conducted emissions limits for various types of electronic equipment. Some of the most widely recognized standards include:

• CISPR 11 / EN 55011: Industrial, scientific, and medical equipment
• CISPR 22 / EN 55022: Information technology equipment
• FCC Part 15: Radio frequency devices (United States)
• MIL-STD-461: Military and aerospace equipment

Conducted Emissions Test Equipment

To accurately measure and analyze conducted emissions, specialized test equipment is required. The following sections detail the essential components of a conducted emissions test setup.

Line Impedance Stabilization Network (LISN)

The LISN is a critical component in conducted emissions testing. It serves several purposes:

1. Provides a stable, known impedance to the device under test (DUT)
2. Isolates the DUT from the power source
3. Couples the conducted emissions to the measurement equipment

Types of LISNs

LISN Type
Application
Frequency Range
50Ω/50μH
General purpose
9 kHz - 30 MHz
5Ω/50μH
Automotive
100 kHz - 108 MHz
150Ω
Telecom equipment
150 kHz - 30 MHz

EMI Receiver or Spectrum Analyzer

An EMI receiver or spectrum analyzer is used to measure the amplitude of conducted emissions across the frequency range of interest. Key features to consider include:

• Frequency range (typically 9 kHz - 30 MHz for conducted emissions)
• Resolution bandwidth (RBW) settings
• Detector types (peak, quasi-peak, average)
• Measurement speed
• Built-in EMC-specific features (e.g., limit line testing, report generation)

Transient Limiter

A transient limiter protects the sensitive input circuitry of the EMI receiver or spectrum analyzer from high-voltage transients that may occur during testing.

Test Software

Specialized EMC test software can automate the measurement process, apply correction factors, and generate comprehensive reports. Key features may include:

• Test sequence automation
• Real-time limit line comparison
• Data logging and report generation
• Remote instrument control

Ancillary Equipment

Additional equipment that may be required for conducted emissions testing includes:

• Power supplies
• Isolation transformers
• RF cables and connectors
• Attenuators and preamplifiers

Conducting Emissions Test Procedures

Test Setup

1. Connect the LISN between the power source and the DUT
2. Connect the EMI receiver or spectrum analyzer to the LISN's RF output
3. Configure the test software and measurement equipment
4. Ensure proper grounding of all equipment

Measurement Techniques

Peak Detection

Peak detection measures the maximum amplitude of emissions at each frequency. This method is fast but may overestimate the impact of short-duration transients.

Quasi-Peak Detection

Quasi-peak detection applies a specific charge and discharge time constant to the detector, providing a weighted measurement that correlates better with the subjective effect of emissions on analog communications systems.

Average Detection

Average detection measures the mean value of emissions over time, which can be useful for assessing the impact on digital communication systems.

Data Analysis and Reporting

1. Compare measured emissions against applicable limits
2. Identify frequencies with the highest emissions levels
3. Analyze the nature of emissions (narrowband vs. broadband)
4. Generate comprehensive test reports

Conducted Emissions Reduction Guidelines

Reducing conducted emissions is crucial for ensuring EMC compliance and improving overall product performance. The following guidelines provide strategies for minimizing conducted emissions at various stages of product development.

Circuit Design Techniques

Power Supply Design

1. Use linear regulators: While less efficient, linear regulators generally produce lower emissions compared to switching regulators.
2. Optimize switching frequency: Choose a switching frequency that balances efficiency and EMI performance.
3. Implement soft-switching techniques: Techniques like zero-voltage switching (ZVS) and zero-current switching (ZCS) can reduce emissions from switching power supplies.
4. Employ spread spectrum modulation: This technique spreads the emissions over a wider frequency range, reducing peak amplitudes.

Digital Circuit Design

1. Minimize clock speeds: Use the lowest clock frequency that meets performance requirements.
2. Implement clock gating: Disable clocks to unused circuit blocks to reduce overall emissions.
3. Use differential signaling: Differential signals generate less common-mode noise compared to single-ended signals.
4. Employ edge rate control: Slow down signal rise and fall times to reduce high-frequency harmonics.

Analog Circuit Design

1. Use balanced circuits: Balanced designs help cancel out common-mode noise.
2. Implement proper grounding: Use a star-ground topology to minimize ground loops.
3. Optimize circuit layout: Keep sensitive analog circuits away from noisy digital and switching circuits.

PCB Layout Techniques

1. Separate noisy and sensitive circuits: Physically separate high-speed digital and switching circuits from sensitive analog circuits.
2. Use ground planes: Implement solid ground planes to provide low-impedance return paths for high-frequency currents.
3. Minimize loop areas: Keep current loop areas as small as possible to reduce emissions.
4. Implement controlled impedance routing: Use proper trace widths and spacing to maintain consistent impedance for high-speed signals.
5. Use guard traces: Implement guard traces around sensitive signals to reduce crosstalk and emissions.

Filtering and Shielding

Power Line Filtering

1. Common-mode chokes: Use common-mode chokes to attenuate common-mode noise on power lines.
2. X and Y capacitors: Implement X capacitors (line-to-line) and Y capacitors (line-to-ground) to filter differential-mode and common-mode noise, respectively.
3. Ferrite beads: Use ferrite beads to suppress high-frequency noise on power and signal lines.

Signal Line Filtering

1. Series inductors: Add series inductors to signal lines to attenuate high-frequency noise.
2. Shunt capacitors: Use shunt capacitors to create low-pass filters on signal lines.
3. Common-mode filters: Implement common-mode filters on differential signal pairs to reduce common-mode emissions.

Shielding Techniques

1. Enclosure shielding: Use conductive enclosures to contain electromagnetic emissions.
2. Cable shielding: Implement proper cable shielding and grounding to reduce radiated and conducted emissions.
3. Board-level shielding: Use local shielding on PCBs to isolate noisy components or protect sensitive circuits.

Grounding and Bonding

1. Implement a proper grounding scheme: Use a single-point ground or multi-point ground strategy as appropriate for the application.
2. Minimize ground impedance: Use wide traces or planes for ground connections to minimize impedance.
3. Separate analog and digital grounds: Keep analog and digital grounds separate and connect them at a single point.
4. Proper bonding techniques: Ensure good electrical contact between shielding components and the main ground.

Component Selection and Placement

1. Choose low-EMI components: Select components with inherently low emissions, such as low-EMI microcontrollers and power supplies.
2. Use decoupling capacitors: Place decoupling capacitors close to IC power pins to reduce high-frequency noise.
3. Implement proper termination: Use proper termination techniques for high-speed signals to minimize reflections and emissions.
4. Consider component orientation: Orient components to minimize coupling between noise sources and sensitive circuits.

Advanced Techniques for Conducted Emissions Reduction

Spread Spectrum Clock Generation

Spread spectrum clock generation (SSCG) is a technique that modulates the clock frequency of digital systems to spread the energy of emissions over a wider frequency range, reducing peak amplitudes.

Benefits of SSCG

• Reduces peak emissions levels
• Can improve EMC compliance without significant hardware changes
• Minimal impact on system performance

Implementation Considerations

• Choose appropriate modulation parameters (modulation rate, deviation)
• Ensure compatibility with system timing requirements
• Consider potential impacts on sensitive analog circuits

Active EMI Cancellation

Active EMI cancellation involves sensing conducted emissions and generating an out-of-phase cancellation signal to reduce overall emissions.

Key Components

1. Sensing circuit
2. Signal processing unit
3. Cancellation signal generator
4. Injection circuit

Advantages and Challenges

Advantages
Challenges
Can achieve significant emissions reduction
Complex implementation
Adaptable to changing operating conditions
Potential for system instability
Effective across a wide frequency range
Additional power consumption

Power Supply Modulation Techniques

Advanced modulation techniques for switch-mode power supplies can help reduce conducted emissions.

Frequency Modulation

Modulating the switching frequency of the power supply spreads emissions over a wider frequency range.

Pulse Width Modulation (PWM) Dithering

Introducing small variations in the PWM duty cycle can help reduce harmonic content in the emissions spectrum.

Randomized Switching

Implementing pseudo-random variations in switching patterns can break up periodic emissions and reduce peak levels.

Electromagnetic Band-Gap (EBG) Structures

EBG structures are engineered materials or designs that exhibit unique electromagnetic properties, including the ability to suppress electromagnetic wave propagation within specific frequency bands.

Applications in EMC

1. Power plane decoupling: Implementing EBG structures in power distribution networks to reduce plane resonances and emissions
2. Filter design: Creating compact, high-performance EMI filters using EBG concepts
3. Package-level EMI suppression: Integrating EBG structures into IC packages to reduce emissions at the source

Design Considerations

• Frequency band of interest
• Physical size constraints
• Manufacturability and cost

Conducted Emissions Testing Best Practices

To ensure accurate and repeatable conducted emissions measurements, consider the following best practices:

Test Environment

1. Use a shielded room: Conduct tests in a shielded room to minimize external interference.
2. Control ambient conditions: Maintain consistent temperature and humidity during testing.
3. Minimize ground loop effects: Use proper grounding techniques for all test equipment.

DUT Configuration

1. Test in worst-case operating mode: Configure the DUT to operate in modes that generate the highest emissions.
2. Use representative cabling: Test with cables that match the intended product configuration.
3. Consider multiple power configurations: Test all relevant power supply configurations (e.g., AC, DC, battery).

Measurement Accuracy

1. Perform regular calibration: Ensure all test equipment is properly calibrated.
2. Apply correction factors: Account for LISN insertion loss and cable losses.
3. Use appropriate detectors: Choose peak, quasi-peak, or average detectors as required by the applicable standard.

Documentation and Traceability

1. Maintain detailed test records: Document all test configurations, equipment settings, and environmental conditions.
2. Use revision control: Implement a system to track changes in DUT hardware and firmware versions.
3. Retain raw measurement data: Store original measurement data for future analysis or auditing.

Troubleshooting Conducted Emissions Issues

When faced with conducted emissions compliance challenges, a systematic approach to troubleshooting can help identify and resolve issues efficiently.

Step 1: Analyze the Emissions Spectrum

1. Identify the frequencies with the highest emissions levels
2. Determine if emissions are narrowband or broadband
3. Look for harmonic relationships or patterns in the spectrum

Step 2: Isolate the Source

1. Disable or remove subsystems to identify the primary emissions source
2. Use near-field probes to locate specific components generating emissions
3. Analyze the relationship between device operating modes and emissions levels

Step 3: Implement Mitigation Techniques

1. Apply appropriate filtering based on the emissions characteristics
2. Modify PCB layout or component placement to reduce coupling
3. Adjust circuit parameters (e.g., clock frequencies, switching speeds) to reduce emissions

Step 4: Verify Effectiveness

1. Retest the device to confirm the impact of mitigation efforts
2. Ensure that changes haven't introduced new emissions problems
3. Validate that device functionality and performance haven't been compromised

Step 5: Iterate and Optimize

1. Fine-tune mitigation strategies based on test results
2. Consider trade-offs between emissions reduction and other design factors
3. Document successful techniques for future designs

Emerging Trends in Conducted Emissions Testing and Mitigation

As technology evolves, new challenges and opportunities arise in the field of conducted emissions testing and mitigation. Some emerging trends include:

1. Higher frequency testing: As devices operate at higher frequencies, there's a growing need to extend conducted emissions testing beyond the traditional 30 MHz upper limit.
2. Time-domain EMI (TDEMI) measurement: TDEMI techniques offer faster measurement times and provide insight into the time-varying nature of emissions.
3. Artificial intelligence in EMC: Machine learning algorithms are being developed to predict EMC performance and optimize mitigation strategies.
4. Wireless power transfer: The growing adoption of wireless charging systems introduces new conducted emissions challenges and testing requirements.
5. EMC for electric vehicles: The automotive industry's shift towards electric propulsion systems demands new approaches to conducted emissions testing and mitigation.

Conclusion

Effective management of conducted emissions is crucial for ensuring electromagnetic compatibility and regulatory compliance of electronic products. By understanding the fundamentals of conducted emissions, utilizing appropriate test equipment, and implementing comprehensive reduction strategies, engineers can design products that meet EMC requirements while maintaining optimal performance.

As technology continues to advance, staying informed about emerging trends and continuously refining testing and mitigation techniques will be essential for addressing the evolving challenges in the field of conducted emissions.

Frequently Asked Questions (FAQ)

1. Q: What is the difference between conducted and radiated emissions? A: Conducted emissions are electromagnetic disturbances that propagate along power lines and other conductors, typically measured in the frequency range of 150 kHz to 30 MHz. Radiated emissions, on the other hand, are electromagnetic disturbances that propagate through space as electromagnetic waves, usually measured at frequencies above 30 MHz.
2. Q: How often should EMC test equipment be calibrated? A: EMC test equipment should be calibrated regularly to ensure measurement accuracy. The calibration frequency depends on factors such as equipment usage, environmental conditions, and manufacturer recommendations. Typically, annual calibration is recommended for most EMC test equipment, but some critical components may require more frequent calibration.
3. Q: Can software techniques help reduce conducted emissions? A: Yes, software techniques can contribute to reducing conducted emissions. Some effective software-based approaches include implementing clock gating to disable unused circuit blocks, optimizing code to reduce processor activity, and using spread spectrum clock generation techniques. However, software techniques should be used in conjunction with hardware-based mitigation strategies for the best results.
4. Q: What are the key differences between quasi-peak and average detection in conducted emissions measurements? A: Quasi-peak detection applies a specific charge and discharge time constant to the detector, providing a weighted measurement that correlates better with the subjective effect of emissions on analog communications systems. Average detection measures the mean value of emissions over time, which can be useful for assessing the impact on digital communication systems. Quasi-peak measurements typically result in higher amplitude readings compared to average detection for the same signal.
5. Q: How do conducted emissions requirements differ for medical devices compared to consumer electronics? A: Medical devices often have more stringent conducted emissions requirements compared to consumer electronics due to the critical nature of their applications and the potential for interference with other medical equipment. Medical devices typically need to comply with standards such as IEC 60601-1-2, which may have lower emissions limits and require testing in more operating modes. Additionally, medical devices may need to meet specific requirements for use in healthcare environments, such as hospitals, where sensitive equipment is present.