Introduction to Conducted Emissions Testing

Conducted emissions testing is a critical aspect of electromagnetic compatibility (EMC) compliance for electronic devices. It involves measuring the unwanted high-frequency currents generated by a device that can propagate along power lines or other interconnecting cables, potentially causing interference to other equipment connected to the same power network.

To ensure that electronic products meet the relevant EMC standards and regulations, such as FCC Part 15, CISPR 22, or EN 55022, manufacturers must perform conducted emissions tests using specialized test equipment and follow guidelines to reduce emissions levels when necessary.

Conducted Emissions Test Setup

A typical conducted emissions test setup consists of the following components:

1. Device Under Test (DUT): The electronic device being tested for conducted emissions compliance.

2. Line Impedance Stabilization Network (LISN): A device that provides a defined impedance between the power source and the DUT, enabling consistent and repeatable measurements of conducted emissions. The LISN also isolates the test setup from external noise on the power lines.

3. Electromagnetic Interference (EMI) Receiver or Spectrum Analyzer: An instrument that measures the amplitude of the conducted emissions across a specified frequency range, typically 150 kHz to 30 MHz. EMI receivers offer better sensitivity and dynamic range compared to spectrum analyzers.

4. Transient Limiter: A device that protects the EMI receiver or spectrum analyzer from high-voltage transients that may occur during the test.

5. Coaxial Cables: Low-loss, shielded cables used to connect the LISN to the measuring instrument.

6. Ground Plane: A conductive surface, usually copper or aluminum, on which the DUT and LISN are placed to establish a common reference ground.

The conducted emissions test setup is typically arranged as follows:

Power Source -> LISN -> DUT -> Ground Plane
                  |
                  |-> Transient Limiter -> EMI Receiver/Spectrum Analyzer

Conducted Emissions Test Procedure

1. Set up the test equipment as described in the previous section, ensuring that all connections are secure and the DUT is properly grounded to the ground plane.

2. Configure the EMI receiver or spectrum analyzer:

3. Set the frequency range to 150 kHz – 30 MHz.
4. Select the appropriate resolution bandwidth (RBW) and detector type as specified in the relevant EMC standard.

5. Set the measurement time to ensure adequate data capture.

6. Power on the DUT and allow it to reach its normal operating state.

7. Measure the conducted emissions on both the live and neutral power lines using the EMI receiver or spectrum analyzer.

8. Record the emission levels at each frequency and compare them against the limits specified in the applicable EMC standard.

9. If the emissions exceed the limits, note the frequencies and amplitudes of the violations for further analysis and correction.

Conducted Emissions Reduction Techniques

If a device fails the conducted emissions test, several techniques can be employed to reduce the emissions levels:

1. Power Line Filtering: Install EMI filters at the power input of the device to attenuate high-frequency noise. Common filter topologies include LC, Pi, and T filters.

2. Ferrite Beads: Place ferrite beads on power cables or other interconnecting wires to suppress high-frequency currents. Ferrite beads act as high-impedance elements at high frequencies, effectively reducing conducted emissions.

3. Shielding: Enclose sensitive electronic components or high-frequency sources within shielded enclosures to minimize electromagnetic radiation that can couple onto power lines or cables.

4. Grounding and Bonding: Ensure proper grounding and bonding of the device’s enclosure, printed circuit boards (PCBs), and cable shields to provide a low-impedance path for high-frequency currents, reducing their ability to propagate as conducted emissions.

5. PCB Layout Optimization: Follow good PCB layout practices, such as minimizing loop areas, segregating high-frequency and low-frequency circuits, and using ground planes to reduce the coupling of high-frequency noise onto power traces.

6. Spread Spectrum Clocking: Implement spread spectrum clocking techniques in digital circuits to reduce the peak energy of clock harmonics, effectively spreading the emissions over a wider frequency range and reducing their amplitude.

7. Snubber Networks: Use snubber networks, such as RC or RCD circuits, across inductive loads (e.g., relays, motors) to suppress high-frequency ringing and reduce conducted emissions.

Conducted Emissions Test Equipment Selection

When selecting conducted emissions test equipment, consider the following factors:

1. Frequency Range: Ensure that the EMI receiver or spectrum analyzer covers the required frequency range for conducted emissions testing, typically 150 kHz to 30 MHz.

2. Dynamic Range: Choose an instrument with adequate dynamic range to accurately measure both low-level and high-level emissions.

3. Sensitivity: Opt for an EMI receiver or spectrum analyzer with high sensitivity to detect low-level emissions that may be close to the noise floor.

4. Detectors: Verify that the instrument supports the required detector types (e.g., quasi-peak, average, peak) as specified in the relevant EMC standard.

5. LISN Impedance: Select a LISN with the appropriate impedance (e.g., 50 Ω/50 µH) as defined in the EMC standard.

6. Transient Protection: Include a transient limiter in the test setup to protect the measuring instrument from high-voltage transients.

7. Software and Automation: Consider the software capabilities of the test equipment, such as the ability to automate measurements, generate reports, and perform data analysis.

Frequently Asked Questions (FAQ)

1. What is the purpose of conducted emissions testing?
   Conducted emissions testing is performed to ensure that electronic devices do not generate excessive high-frequency currents on power lines or interconnecting cables, which can cause electromagnetic interference (EMI) to other equipment connected to the same power network.

2. What is the frequency range for conducted emissions testing?
   The typical frequency range for conducted emissions testing is 150 kHz to 30 MHz, as specified in most EMC standards.

3. What is a Line Impedance Stabilization Network (LISN) used for in conducted emissions testing?
   A LISN is used to provide a defined impedance between the power source and the device under test (DUT), enabling consistent and repeatable measurements of conducted emissions. It also isolates the test setup from external noise on the power lines.

4. What are some common techniques to reduce conducted emissions levels?
   Common techniques to reduce conducted emissions levels include power line filtering, using ferrite beads, shielding sensitive components, proper grounding and bonding, optimizing PCB layout, implementing spread spectrum clocking, and using snubber networks.

5. What factors should be considered when selecting conducted emissions test equipment?
   When selecting conducted emissions test equipment, consider factors such as frequency range, dynamic range, sensitivity, supported detector types, LISN impedance, transient protection, and software capabilities for automation and data analysis.

Conclusion

Conducted emissions testing is a crucial step in ensuring that electronic devices comply with relevant EMC standards and do not cause electromagnetic interference to other equipment. By understanding the test setup, procedure, and reduction techniques, manufacturers can effectively measure and control conducted emissions levels in their products.

Investing in appropriate conducted emissions test equipment and following best practices for EMC design can help streamline the compliance process, reduce development time and costs, and ultimately bring products to market faster while meeting the necessary regulatory requirements.