Coupling Decoupling Networks (CDNs)
for EMC Conducted Immunity Testing

1. Introduction

Electromagnetic immunity testing evaluates whether a product continues to operate correctly in the presence of radio-frequency disturbances that couple into its cables. Unlike radiated immunity (where the EUT is exposed to a free-space RF field), conducted RF immunity applies the disturbance directly to a cable connected to the Equipment Under Test (EUT) — simulating the real-world mechanism by which RF fields from nearby transmitters (broadcast, cellular, Wi-Fi, two-way radio, industrial ISM equipment) are received by cables acting as unintended antennas and conducted into the product.

The governing international standard is IEC 61000-4-6 (identical to EN 61000-4-6 in European harmonization), which defines the methodology from 150 kHz to 80 MHz (extendable to 230 MHz for small equipment). Three RF coupling methods are recognized: the Coupling Decoupling Network (CDN), the Bulk Current Injection (BCI) probe, and the Electromagnetic (EM) Clamp. Of these, the CDN method is the preferred method because it provides the lowest uncertainty of applied stress and the best decoupling of auxiliary equipment.

The Com-Power CDN product line covers every IEC 61000-4-6 cable category with 21 models across four series. This application note explains the theory of operation, the IEC 61000-4-6 test methodology, the calibration procedure, and the practical selection of the correct CDN for each cable type — with real-world use cases drawn from medical, industrial, EV, broadcast, IT, process-control, and telecom product development.

2. Why the CDN Method Is Preferred Over BCI and EM Clamp

IEC 61000-4-6 lists three coupling methods, but the standard itself designates the CDN as preferred when a CDN exists for the cable type being tested. The reasons are measurement uncertainty, efficiency, and repeatability:

3. Theory of Operation: How a CDN Works

A CDN is a passive network of resistors, capacitors, and inductors arranged to perform two simultaneous functions: coupling RF from the test generator into each conductor of the EUT cable, and decoupling that RF from the AE and the building mains. The design achieves these functions while presenting a standardized 150 Ω common-mode impedance at the EUT port across 150 kHz – 80 MHz (and typically up to 230 MHz).

3.1 The 150 Ω Common-Mode Impedance

IEC 61000-4-6 specifies 150 Ω ± 20 Ω common-mode impedance at the EUT port. This represents the typical impedance presented by a long cable in free space when excited by RF fields — the physical situation being simulated. The CDN establishes this impedance with a combination of internal resistors and the 50 Ω source impedance of the RF generator:

3.2 Circuit Elements

3.3 Voltage Division Factor (VDF)

Because the CDN splits the RF generator's voltage between its internal 100 Ω coupling network and the EUT-side 150 Ω common-mode impedance, a defined voltage division factor of 9.5 dB exists between the open-circuit generator voltage (U_o) and the voltage actually appearing at the EUT. This factor is characterized during calibration and applied during test-level setting (see Section 6.2).

4. IEC 61000-4-6 Test Methodology

The IEC 61000-4-6 test sweeps an AM-modulated RF signal across a defined frequency range while monitoring the EUT for performance degradation. Key parameters of the test:

4.1 Test Levels and Typical Applications

5. CDN Categories: Four Series, 21 Models

Com-Power offers four CDN series matched to the four IEC 61000-4-6 cable categories. Every model operates from 150 kHz to 230 MHz, presents the standardized 150 Ω common-mode impedance, uses a 50 Ω BNC RF injection port, and ships with NIST-traceable calibration.

6. Calibration and Test Level Setting

Before any CDN-based conducted immunity test is performed, the applied test level must be calibrated using a defined procedure that removes the EUT from the signal path and substitutes a calibration adapter. This is the single most frequent source of error in field conducted immunity testing, and every Com-Power CDN ships with the necessary calibration information and is compatible with the standard calibration accessories.

6.1 Required Calibration Accessories

• 150 Ω to 50 Ω Adapters (ADA-515-X series)Convert the CDN's 150 Ω EUT port impedance to 50 Ω for connection to the measuring receiver or power meter. Required for level-setting measurement.
• Shorting Adapters (ADA-X series)CDN-specific adapters that short the EUT port cable to the CDN case ground during calibration, presenting the correct common-mode impedance to the RF generator.
• 6 dB AttenuatorPlaced between the RF power amplifier and the CDN; protects the amplifier from reflected power during calibration and improves impedance match.
• Measuring Receiver or Power MeterRequired to read the calibrated voltage (U_mr) at the 150 Ω to 50 Ω adapter output during level setting.

6.2 The Calibration Voltage Relationship

Because the CDN has a 9.5 dB voltage division factor and the 150 Ω to 50 Ω adapter introduces an additional 6 dB (a factor of 6 when adapter loss and the 50 Ω load are combined), the voltage measured at the adapter output during calibration relates to the required open-circuit test voltage (U_o) as follows:

6.3 Calibration Procedure Overview

1. Connect the RF amplifier output through a 6 dB attenuator to the CDN's BNC RF input. Ensure all cables are 50 Ω coaxial with N or BNC connectors as appropriate.
2. Install the shorting adapter (ADA-X) on the CDN's EUT port. The shorting adapter connects all EUT-side conductors together and to the CDN case, presenting the correct common-mode termination.
3. Connect the 150 Ω to 50 Ω adapter (ADA-515-X) between the shorting adapter's monitor output and a 50 Ω measuring receiver or power meter.
4. Set the RF generator to an unmodulated carrier at each required frequency — typically stepping through the test range (150 kHz, 300 kHz, 500 kHz, 1 MHz, 3 MHz, 10 MHz, 30 MHz, 80 MHz, and extended frequencies if required).
5. Adjust the RF amplifier level until the measuring receiver reads U_mr = U_o/6 for the required test level. Record the amplifier setting.
6. Repeat for every frequency in the test plan. The result is a table of amplifier settings vs. frequency that produces the calibrated test level at the EUT port.
7. Remove the shorting and ADA-515-X adapters and install the EUT. Switch the RF carrier to 80% AM 1 kHz modulation and apply the recorded amplifier setting at each frequency during the actual immunity test.

7. M-Series (Power) CDNs — Ten Models

Product line: com-power.com/products/cdns/power-series

The M-Series CDNs are designed for AC and DC power supply lines of the EUT. They inject RF common-mode disturbance onto every conductor of the power cable and decouple that disturbance from the building mains, preventing it from being fed back into other equipment in the test environment. The M-series is specific to the number of conductors in the power cable — using a 3-conductor CDN on a 2-conductor cable (or vice versa) does not produce the correct per-conductor stress level.

7.1 Complete M-Series Specifications

All M-series models share: 150 kHz – 230 MHz frequency range; 150 Ω common-mode impedance per IEC 61000-4-6; 9.5 dB voltage division factor; 50 Ω BNC (female) RF input; 40 Vrms maximum RF input; NIST-traceable calibration; IEC 61000-4-6 and CISPR 16-1-2 compliance.

7.2 Real-World M-Series Use Cases

Figure 1: Medical device test setup with CDN-M325E for single-phase AC mains

EUT Performance Criteria: Monitor displays for false-alarm triggers, ECG waveform artifacts, SpO₂ reading discontinuities, communications to central monitoring station, and any operator-interface lockups. Medical product families typically accept "no change in specified function" as Criterion A per IEC 60601-1-2.

Figure 2: Three-phase VFD test setup with CDN-M525E

EUT Performance Criteria: Monitor for motor speed regulation errors, fieldbus communication faults (Profibus, Profinet, EtherCAT), safety-trip false triggers, and fault-code events. VFDs typically must continue to operate with no trip during the test (Criterion A).

8. S-Series (Screened) CDNs — Five Models

Product line: com-power.com/products/cdns/screened-series

The S-Series CDNs are designed for shielded and coaxial cables where the RF disturbance is coupled to the cable's shield rather than to individual inner conductors. Because signal is injected onto the shield, S-Series CDNs are not specific to the number of inner conductors — the same CDN-C50E handles any 50 Ω coaxial cable regardless of how many conductors are inside the shield. Three S-series models (CDN-T8SE, CDN-C50E, CDN-C75E) are designed as "dual-threat" products: they function either as a CDN for IEC 61000-4-6 conducted immunity or as an ISN (Impedance Stabilization Network) for CISPR 22/32 conducted emissions measurement. A single S-series unit can therefore cover both immunity and emissions on the same cable type — a significant cost savings for labs with both capabilities.

8.1 Complete S-Series Specifications

All S-series models share: 150 kHz – 230 MHz frequency range; 50 Ω BNC (female) RF injection port; 40 Vrms max RF input; NIST-traceable calibration; IEC 61000-4-6 and CISPR 16-1-2 compliance. CDN-T8SE, C50E, and C75E additionally meet CISPR 22/32 for ISN use.

8.2 Real-World S-Series Use Cases

Figure 3: Broadcast video encoder test setup with CDN-C75E

EUT Performance Criteria: Monitor video output for sparkles, color shifts, frame drops, audio/video sync loss, and encoder lockup. HD-SDI at 1.485 Gbps is sensitive to cable impedance mismatch — the 75 Ω CDN must be carefully impedance-matched to avoid corrupting baseline video quality.

Figure 4: USB printer test setup with CDN-USB-BE

EUT Performance Criteria: Monitor for USB enumeration failures, print-job corruption, lost characters on labels, device disconnection and re-enumeration, and host PC error reporting. Verify barcode scan quality after test using a reference scanner — partial corruption may produce visually acceptable labels that are unreadable.

9. T-Series (Telecom / Balanced) CDNs — Three Models

Product line: com-power.com/products/cdns/telecom-series

The T-Series CDNs are designed for unshielded balanced lines — twisted-pair Ethernet, telephone lines, and multi-pair data cables. Balanced signalling relies on equal and opposite currents in two conductors of a twisted pair; common-mode noise (which the CDN injects) affects both conductors equally and should be rejected by the balanced receiver. The T-series CDN tests whether the EUT's actual common-mode rejection is adequate at all frequencies in the band.

Each T-series model is specific to the number of balanced pairs in the cable, because the CDN injects equal-amplitude common-mode signal onto every pair simultaneously.

9.1 Complete T-Series Specifications

9.2 Real-World T-Series Use Cases

Figure 5: VoIP phone + PoE test setup with CDN-T8E

EUT Performance Criteria: Monitor audio quality (clicks, dropouts, echo, garbled speech) using standardized VoIP metrics: Mean Opinion Score (MOS), jitter, packet loss. Monitor display artifacts, call-setup failures, PoE negotiation stability. Verify with a live call through the test session.

Figure 6: DSL modem test setup with CDN-T2E

EUT Performance Criteria: Monitor for DSL sync loss, negotiated speed degradation, CRC error rate, FEC correction rate, and outright connection drops. VDSL2 spectrum extends to 30 MHz and may interact with upper-band conducted immunity test frequencies — careful test-frequency planning may be needed.

10. AF-Series (Unscreened Unbalanced) CDNs — Three Models

Product line: com-power.com/products/cdns/af-series

The AF-Series CDNs are designed for unscreened, unbalanced cables carrying low-current signals — sensor inputs, 4-20 mA process loops, strain-gauge and thermocouple wiring, and multi-conductor industrial I/O. Connection is via 2 mm shrouded banana sockets, allowing direct connection of individual conductors or modular wiring assemblies that don't fit standard connector-based CDNs.

Because the signals carried by these cables are small (often millivolts or microamps), they are particularly sensitive to RF injection, and the AF-series is where conducted immunity often makes the difference between a product that passes certification and one that fails.

10.1 Complete AF-Series Specifications

10.2 Real-World AF-Series Use Cases

Figure 7: Temperature controller test setup with CDN-AF2E

EUT Performance Criteria: Monitor displayed temperature for spikes, offsets, noise exceeding specified accuracy band. Monitor PID output for oscillation. Check for false alarm triggers. Thermocouple millivolt-level signals are extremely sensitive to common-mode RF — even small injection levels can cause measurable temperature reading error.

Figure 8: 4-20 mA current-loop test setup with CDN-AF2E

EUT Performance Criteria: Monitor loop-current deviation — NAMUR NE 21 specifies < 1% of span deviation under Level 3 stress. Monitor HART digital communication (if present) for CRC errors. Verify no false alarm or fault indication. Process-control accuracy requirements are strict: even small common-mode injection can cause measurable DC offset in the loop current.

11. General EUT Test Procedure

The following procedure describes a typical IEC 61000-4-6 conducted immunity test session using a Com-Power CDN. Specific product-family standards may require variations — always consult the applicable standard for the final test plan.

1. Identify every cable of the EUT and select the correct CDN for each. Power cables use an M-series CDN matched to conductor count and current. Data cables use a T-series (balanced), S-series (shielded/coax/USB), or AF-series (unbalanced I/O) CDN matched to connector and conductor count. Use the CDN Finder Wizard if uncertain.
2. Calibrate each CDN for the required test level. Use ADA-X shorting adapters, ADA-515-X 150 Ω to 50 Ω adapters, and a measuring receiver or power meter per Section 6. Generate the table of RF amplifier settings vs. frequency producing U_mr = U_o / 6 at each point.
3. Position the EUT on a ground reference plane with 10 cm insulating support, per IEC 61000-4-6 geometry. Route CDN-to-EUT cables 30 mm to 50 mm above the ground plane. Keep the total CDN-to-EUT cable length to between 0.1 m and 0.3 m.
4. Connect every EUT cable through its corresponding CDN. Power through an M-series CDN to the AE (building mains). Data cables through the appropriate data CDN to their AE (switch, host PC, DCS, etc.). Terminate any unused AE ports per standard.
5. Power the EUT and bring it into its operating state(s) to be tested. Multi-mode products (printers printing, cameras streaming, VFDs running a motor) require operation in all relevant modes during test.
6. Apply the calibrated test signal at each frequency step through the band. Step size ≤ 1% of the preceding frequency (logarithmic sweep). At each frequency, modulate with 80% AM at 1 kHz and hold for the required dwell time (typically 3 s or longer).
7. Observe EUT performance during each dwell. Record any symptom: reading error, false alarm, output fluctuation, reboot, communication loss, display artifact, audible anomaly. Relate symptoms to frequency for root-cause analysis if failures occur.
8. Repeat the sweep for every CDN (every cable) of the EUT. Each cable must be separately stressed; some product-family standards additionally require combined-cable testing.
9. Classify each observed result per the applicable performance criterion. IEC 61000-4-6 itself defines Criterion A (normal operation), B (temporary degradation, self-recoverable), C (temporary degradation, operator recoverable), and D (damage or permanent loss of function). Product-family standards specify acceptable criteria.
10. Document the test: amplifier settings table, CDN models and calibration certificates, EUT operating modes, observed symptoms, pass/fail classification, test environment conditions. Retain for certification audit.

12. Common Measurement Pitfalls and How to Avoid Them

13. Building a Complete Conducted Immunity Test System

A CDN is one element of a complete IEC 61000-4-6 test system. Com-Power offers the Conducted Immunity Test System as a pre-integrated package including all required components:

14. Selection Guide: Which CDN Do I Need?

15. Complementary Products

16. Frequently Asked Questions

16.1 What frequency range does IEC 61000-4-6 cover?

The standard range is 150 kHz to 80 MHz. Product-family standards may extend to 230 MHz for small EUTs (physical dimension smaller than λ/4 at the upper frequency). All Com-Power CDNs support the full 150 kHz to 230 MHz range.

16.2 Why does the conductor count matter for unshielded CDNs?

Unshielded CDNs (M-, T-, AF-series) inject the RF disturbance onto every conductor inside the cable in parallel. A CDN designed for 3 conductors and applied to a 2-conductor cable would deliver only 2/3 of the intended per-conductor stress level. Match CDN conductor count to the cable exactly.

16.3 Do I need the ADA-X and ADA-515-X calibration adapters?

Yes, for any formal IEC 61000-4-6 test. The shorting adapter (ADA-X, CDN-specific) and 150 Ω to 50 Ω adapter (ADA-515-X) are required to set the test level using the U_mr = U_o/6 procedure before applying the test to the EUT. Com-Power recommends ordering the adapters with the CDN.

16.4 What's the difference between a CDN and an ISN?

A CDN (Coupling Decoupling Network) injects RF common-mode disturbance for IEC 61000-4-6 conducted immunity testing. An ISN (Impedance Stabilization Network) measures RF common-mode emissions for CISPR 22/32 conducted emissions testing. Com-Power CDN-T8SE, CDN-C50E, and CDN-C75E function as both — saving labs the cost of two separate units on the same cable type.

16.5 When should I use BCI or EM Clamp instead of CDN?

Use BCI or EM Clamp only when: (a) no CDN exists for the specific cable type; (b) the cable cannot be disconnected from the EUT for practical reasons; or (c) the product-family standard specifically calls out one of those methods. For every other case, the CDN is the preferred method with the lowest measurement uncertainty.

16.6 What RF amplifier power do I need for a Com-Power CDN?

For Level 3 (10 V) testing across 150 kHz to 230 MHz, a typical requirement is 75 W for most M/T/AF-series CDNs. Higher-loss cables (shielded, coax at high frequencies) may benefit from 150 W. Size for worst-case CDN insertion loss at the highest test frequency plus amplifier margin.

16.7 How often should I recalibrate my CDN?

Annual recalibration is standard; ISO/IEC 17025 accredited calibration is available on request. High-volume production labs may recalibrate every 6 months. Check CDN operation at each test setup by verifying the level-setting calibration — unexpected changes from the reference calibration data can indicate drift.

16.8 My EUT has multiple cable types — do I need multiple CDNs?

Yes, one CDN per cable type. An EUT with a power cable, a Gigabit Ethernet port, and a USB Type-B port needs three CDNs (e.g., CDN-M325E + CDN-T8E + CDN-USB-BE) for a complete test. Each cable is tested separately; some product standards additionally require combined-cable testing.

16.9 What about USB 3.0 and beyond?

Com-Power offers USB 3.0-compatible models (CDN-USB3-A-E and CDN-USB3-B-E). For USB 3.1/3.2/USB-C applications, contact Com-Power engineering for current product availability and custom configurations.

16.10 Where can I find the CDN Finder Wizard?

The interactive CDN Finder Wizard is hosted at com-power.com/tech-notes/cdn-finder-wizard. It includes all 21 Com-Power CDN models, Quick-Start presets for common cable types, cascading filters, active filter chips, Compare Mode for side-by-side specifications, and direct links to each CDN's product page and quote form.

17. Conclusion

IEC 61000-4-6 conducted RF immunity testing is a compliance requirement for the vast majority of products entering global markets — from medical patient monitors to industrial VFDs, from residential DSL modems to high-bay commercial lighting control, from EV chargers to process-control transmitters. The CDN coupling method is the preferred approach, delivering the lowest measurement uncertainty and best reproducibility of the three IEC 61000-4-6 injection methods.

The Com-Power CDN product line covers every IEC 61000-4-6 cable category with 21 models across four series: ten M-series power CDNs from 25 A to 100 A across single, dual, three-phase, split-phase, and DC configurations; five S-series shielded CDNs for Ethernet, coax, and USB (three of which double as ISNs for CISPR 22/32 emissions); three T-series balanced-data CDNs for telecom and Ethernet; and three AF-series unshielded unbalanced CDNs for industrial sensor and I/O cables. Every CDN operates from 150 kHz to 230 MHz, meets the 150 Ω common-mode impedance requirement, uses a 50 Ω BNC RF input, and ships with NIST-traceable calibration. ISO/IEC 17025 accredited calibration is available on request.

Whether the product under test is a consumer electronic device, a networked medical instrument, a three-phase industrial drive, a DC-fast-charging station, a broadcast video encoder, or a process-control transmitter, selecting the correct CDN — and calibrating it per IEC 61000-4-6 before applying the test — is the single most important preparation step for a valid conducted immunity measurement. Use the interactive CDN Finder Wizard to identify the right model for each cable, and consult Com-Power Applications Engineering for complete test-system design assistance.

References

Additional Com-Power resources: CDN Finder Wizard | Antenna Finder | Antenna Kit Selector | All Application Notes | Calibration Services

• IEC 61000-4-6:2013+A1:2017, Electromagnetic Compatibility (EMC) — Part 4-6: Testing and Measurement Techniques — Immunity to Conducted Disturbances, Induced by Radio-Frequency Fields, International Electrotechnical Commission. The governing international standard for conducted RF immunity testing using CDNs, BCI probes, and EM clamps.
• EN 61000-4-6:2014+A1:2019, European harmonized version of IEC 61000-4-6, referenced for CE marking under the EU EMC Directive.
• CISPR 16-1-2:2014+A1:2017+A2:2018, Specification for Radio Disturbance and Immunity Measuring Apparatus and Methods — Part 1-2: Coupling Devices for Conducted Disturbance Measurements, IEC. Defines the CDN and ISN specifications that the IEC 61000-4-6 test method relies on.
• CISPR 22:2008 and CISPR 32:2015, Information Technology Equipment — Radio Disturbance Characteristics — Limits and Methods of Measurement, IEC. Emissions-side standards for which certain Com-Power S-series CDNs (T8SE, C50E, C75E) also serve as ISNs.
• IEC 61000-6-2:2019, Electromagnetic Compatibility (EMC) — Part 6-2: Generic Standards — Immunity Standard for Industrial Environments. Specifies Level 3 (10 V) conducted immunity for industrial-environment products.
• IEC 60601-1-2:2014+A1:2020, Medical Electrical Equipment — Part 1-2: General Requirements for Basic Safety and Essential Performance — Collateral Standard: Electromagnetic Disturbances, IEC. Conducted immunity requirements for medical equipment, typically Level 2 (3 V) with Level 3 for special environments.
• NAMUR NE 21, Electromagnetic Compatibility of Equipment for Process and Laboratory Technology, NAMUR (User Association of Automation Technology in Process Industries). Specifies conducted immunity requirements and strict accuracy criteria for process-control instrumentation.
• SAE J1113-21, Electromagnetic Compatibility Measurement Procedure for Vehicle Components — Immunity to Electromagnetic Fields 10 kHz to 1 GHz — Absorber Lined Shielded Enclosure. Automotive conducted and radiated immunity methodology referenced for EV and vehicle-component testing.
• IEC 61851-21-2, Electric Vehicle Conductive Charging System — Part 21-2: Electric Vehicle Requirements for Conductive Connection to an AC/DC Supply — EMC Requirements for Off-Board Electric Vehicle Charging Systems. Emissions and immunity requirements for EV charging infrastructure.
• OIML R76, Non-Automatic Weighing Instruments, International Organization of Legal Metrology. Conducted immunity criteria for legal-for-trade weighing equipment.
• IEEE 802.3at-2009 (PoE+) and IEEE 802.3af-2003 (PoE), Power over Ethernet standards relevant to CDN-T8E and CDN-T4E test setups for PoE-powered networking equipment.
• ISO/IEC 17025:2017, General Requirements for the Competence of Testing and Calibration Laboratories, ISO. Defines calibration traceability and laboratory accreditation requirements applicable to CDN calibration.
• Sroka, J., "Practical approach to IEC 61000-4-6 testing," peer-reviewed publication on conducted immunity methodology and uncertainty analysis.
• Joo et al., "Simulation-Based System-Level Conducted Susceptibility Testing," Electronics, MDPI, 2019. Simulation and measurement comparison for IEC 61000-4-6 test setups.
• Com-Power Corporation, CDN Product Line, Costa Mesa, CA. Includes M-series (power), S-series (screened), T-series (telecom/balanced), and AF-series (unscreened unbalanced) CDN datasheets and technical documentation.
• Com-Power Corporation, Conducted Immunity Test System, Costa Mesa, CA. Pre-integrated IEC 61000-4-6 test system including CDN, RF power amplifier, directional coupler, attenuators, signal generator, and calibration adapters.
• Com-Power Corporation, CDN Finder Wizard, Costa Mesa, CA. Interactive online tool for selecting the correct CDN from all 21 Com-Power models by cable type, conductor count, current, voltage, connector, and industry.