Figure 1: EMC-conducted immunity test setup uses a coupling decoupling network (CDN)

A Coupling Decoupling Network (CDN) is a device used in conducted RF immunity testing performed according to IEC 61000-4-6. The CDN serves as an interface between the RF signal generator and the Equipment Under Test (EUT), allowing controlled injection of disturbance signals (i.e., RF test signals) into the power, communication, data lines, or other lines connected to the EUT while preventing (decouples) these injected disturbance signals from affecting the power source or other connected auxiliary equipment (AE).

Coupling Decoupling Networks (CDNs) are available for a wide range of applications, including RF-conducted immunity testing according to IEC 61000-4-6, surge immunity tests according to IEC 61000-4-5, burst or EFT immunity tests according to IEC 61000-4-4, and testing applications of other applicable standards. Additionally, standards like CISPR 15 and EN55022 employ CDNs for conducted EMI emissions measurement purposes. The design and capabilities of each CDN can vary based on the specific application, associated equipment, and underlying standard.

Figure 2: Shows some common immunity testing and emission testing standards that use CDN

What are conducted immunity testing and emission measurement testing for electromagnetic compliance?

EMC testing is a procedure used to determine whether an electrical or electronic product meets relevant EMC emission and immunity standards. When a product complies with these standards, it is considered EMC-compatible and can be sold in the market of a country. An EMC-compatible product will operate satisfactorily in its intended electromagnetic environment without emitting intolerable EMI that may disturb the operation of other nearby devices. The EMC testing process typically includes four types of tests: conducted emission test, radiated emission test, conducted immunity test, and radiated immunity test.

Conducted electromagnetic interference (Conducted EMI) is the unwanted electromagnetic signals or noise currents at radio frequencies that travel along power or signal lines, disrupting the operation of other connected/nearby electronic devices. The conducted EMI (RF noise currents on cables/power lines) is typically caused by switching circuits, power supplies, and external electrical noise, impacting the performance of other connected/nearby equipment.

Conducted emission testing is performed to measure the level of conducted EMI emission from a working product or EUT. The purpose of this test is to verify that the device meets the regulatory limits set for conducted EMI emissions as per EMC standards. This test ensures that the device will not emit intolerable conducted EMI while operating in its intended electromagnetic environment (so that the device will not interfere with other nearby/connected electronic devices or systems on the same line/cable).

Conducted immunity testing verifies the immunity of electrical and electronic products against the conducted EMI. During this testing, a signal generator generates RF test signals (RF voltage/current). The generated test signals (i.e., disturbance signals) are injected on power or signal or similar lines connected to the equipment under test (EUT). The test signals simulate the conducted EMI that the device may encounter while operating in its intended real-world environment.  This testing is performed to ensure an electrical and electronic product will operate properly and will tolerate the conducted EMI that the product may encounter in real-world environments during its operation. 

According to IEC-61000-4-6, the following three devices can be used to inject test signals on power or signal lines for RF-conducted immunity testing. These three devices are Coupling Decoupling Network (CDN), BCI (Bulk current injection clamp), and EM clamp (Electromagnetic clamp). Usually, the CDN is the preferred device over the BCI and EM clamp to inject the RF signal for immunity testing purposes (see Figure 1).  

Working principle of CDN:

A CDN (Coupling Decoupling Network) consists of coupling and decoupling circuits made of passive components such as resistors, capacitors, and inductors. The coupling circuit is formed by a resistor and inductor components. The decoupling circuit is formed by the common mode choke (inductor) and decoupling capacitors (Figure 3). 

Figure 3: CDN use in the single-phase line, showing common mode coupling (High and Low output of signal generator is connected to N and PE) 

A basic CDN concept and its function are shown in Figure 3. While applying RF disturbance signals, the coupling circuit capacitor and resistor provide a low impedance path at RF frequencies, thereby providing a path for injecting the RF disturbance signal on power or signal lines. On the other side, the common mode chokes (inductors) and decoupling capacitors (i.e., decoupling circuit) block or decouple the RF disturbance signal from entering the power supply or other auxiliary equipment. Thereby, the decoupling circuit ensures that the power supply or auxiliary equipment is protected from the disturbance signal. For obtaining reproducible results, the CDN device has to be close to EUT.

What is a coupling path?

A coupling path represents how/in which way the signal generator’s output terminals (High and Low) are connected to the EUT’s port. Two main types of coupling paths can be selected under the coupling decoupling network (CDN) configuration. They are differential mode coupling and common mode coupling. 

For example, consider a single-phase system that consists of three lines: Line (L), Neutral (N), and PE (Protective Earth). Since the disturbance signal from the signal generator is injected/applied between two lines or points, the possible coupling paths are L to N, L to PE, and N to PE. 

When connecting the signal generator output terminals (High and Low) to L and N lines, the coupling is known as differential mode coupling.  When connecting the signal generator output terminals (High and Low) to L and PE respectively, or N and PE, the coupling is known as common mode coupling (see Figure 3). As a general rule, the coupling is usually considered as a common mode if the Low terminal of the signal generator is connected to the Protective Earth (PE). The choice of coupling path/mode depends on the test requirements specified in the EMC immunity test standards.

Types of Coupling Decoupling Networks

Based on the usage/application,  the types of RF CDNs for conducted immunity testing include M series OR Power series (for power lines), S series OR Screened series (for shielded lines), T series OR Telecom series (for unshielded balanced lines), and AF series (unscreened unbalanced lines carrying low currents). Also, the CDNs used for burst and surge immunity testing (according to IEC 61000-4-4 and IEC 61000-4-5) belong to the CDN types. Let’s discuss RF CDN types. 

M-series CDNs (or Power series):  The M-series CDNs are designed specifically for conducted disturbance immunity testing on products that use power supply lines. Testing is performed according to the IEC 61000-4-6 standard. Several M series models are available with the 150 kHz to 230 MHz frequency range. They are ideal for use in all power supplies, including mains, unscreened lines, unbalanced lines, and AC or DC.

S-series CDNs: The S-series CDNs are designed for conducted disturbance immunity testing on products that use lines such as shielded lines or screened lines and coaxial cables, according to the IEC 61000-4-6 standard. Several S series models are available with the 150 kHz to 230 MHz frequency range for testing to IEC 61000-4-6 and other standards. Some S-series CDNs are available to use as the ISN (Impedance Stabilization Networks) for conducted emissions testing as per CISPR 22/32. 

T-Series CDNs (or Telecom series Coupling Decoupling Networks): The T-series CDNs are designed for conducted disturbance immunity testing on products that use lines such as unscreened lines, balanced lines, and unshielded twisted pairs for Telecommunication such as Ethernet. The T-series CDNs are used for conducted disturbance immunity testing as per the IEC 61000-4-6 standard. Several T series models come with a frequency range of 150 kHz to 230 MHz.

AF series CDNs:  The AF series CDNs are designed for conducted disturbance immunity testing on products that use unscreened, unbalanced cables/lines with two, four, and eight conductors carrying low currents. The AF-series CDNs are used for conducted disturbance immunity testing as per IEC 61000-4-6 standard. Several AF series models come with a frequency range of 150 kHz to 230 MHz.

How to select a CDN?

As per IEC/EN 61000-4-6 standard CDN selection rules, the following figure shows a quick guide to selecting a CDN for use. 

Figure 4: IEC/EN 61000-4-6 CDN selection rules Advantages of CDNs:

• Coupling Decoupling Network is more efficient than the BCI and EM clamp methods to inject test signals on power or signal/control lines for RF-conducted immunity testing. Since the CDN requires the least amount of power for the highest test level. 
• Ensures accurate controllable stress levels are applied to equipment under test. 
• Feature a common mode impedance of 150 Ohms.
• Helps to decouple the auxiliary equipment (AE) or power supply from disturbance signals.

Disadvantages of CDN use: 

• In unshielded applications, CDN that is designed for 3 conductors is not suitable for injecting noise into a cable with 2 conductors. 
• CDNs are also cable-type and interface-specific, which often necessitates the use of multiple CDNs for a single test setup.   For example, CDN-USB-AE is for EUTs utilizing USB cables, CDN-T8E is for EUTs utilizing balanced cables, etc.

With the help of a CDN, disturbance signals (test signals) from the signal generator are directed toward the Equipment Under Test (EUT), ensuring they do not reach the power supply or Auxiliary Equipment (AE), thereby protecting the power source or auxiliary equipment. They are ideal for use in EMC testing, especially in RF-conducted immunity testing of electrical and electronic devices used in various fields. They are also used in surge and burst immunity tests as well as in conducted EMI emission measurement tests.

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