Coupling Decoupling Networks
What are Coupling Decoupling Networks (CDNs)?
Coupling Decoupling Networks, commonly called CDNs, serve the purpose of transferring energy or disturbances (coupling) and also removing energy or EMI/noise (decoupling). These are commonly used during EMC immunity testing as a method of injecting noise which the EUT/DUT will be exposed to, then subsequently removing unwanted interference.
The design and capabilities of each network will differ depending upon the application, associated equipment, and underlying standard. The two main advantages of CDNs are:
- Low uncertainty of applied EMI/stress
- Decoupling of the auxiliary equipment (AE)
The associated video helps to show the setup of conducted RF CDNs including the calibration with the Teseq NSG 4070 test system
Where CDNs are used
There are a variety of different applications where these type of devices are used, the two most common being conducted RF (continuous) and transient immunity tests. Both types of EMI tests are commonly conducted on a variety of different power and data (signal) lines requiring the use of CDNs during testing.
The design of these CDNs differ based upon both application, as mentioned above, and type of line. For most applications, with the exception of power mains transient tests, the networks can be used interchangeably regardless of the manufacturer of the generator.
The associated image on the left shows many of the common immunity testing and emissions testing applications that use CDNs as well as the associated frequency ranges.
Conducted RF Immunity CDNs
CDNs designed for conducted RF testing are most commonly used for testing to IEC 61000-4-6 and are the preferred injection method. They are also the most efficient injection device used for conducted RF immunity testing. RF CDNs most commonly designed in accordance with IEC 61000-4-6 and have a common mode impedance of 150 Ohms.
The associated diagram illustrates the concept of how CDNs apply the associated disturbance generated by an RF test system on the associated cabling.
Coupling decoupling networks are the most efficient injection method, as opposed to EM Clamps or BCI Probes providing the highest test levels with least power required. The associated chart illustrates how with both 40 watt and 80 watt power RF amplifiers substantially higher levels are able to be achieved with CDNs.
These networks are cable specific, which can lead to multiple being used during testing. The below diagram references the Teseq CDN guide, which can be accessed by clicking here.
These CDNs can be used interchangeably with a variety of test systems allowing for mixing and matching of equipment. The most common manufacturers include:
Types of RF CDNs
The image below illustrates the generic types of RF CDNs required by the standard. These types are also broken down further into subcategories for specific connection/cable types (IE Coaxial, Unscreened, Etc.).The M Type series are often the most commonly used given they are designed for power lines. Three phase M Type CDNs cannot typically be used for single phase EUT/DUT applications despite having enough connections for the associated lines.
Selection Guides/Application Notes
The below selection guides and application notes provide information on both purchasing the correct CDNs as well as how to conducted testing.
Testing using RF CDNs
Testing for IEC 61000-4-6 is done using the substitution method regardless of the injection device used. This method uses the calibration data as a main factor in determining the associated level used during testing. The below image illustrates how the adapters and termination load from the calibration are replaced when preforming the test.
CDN Calibration Setup
During the calibration process the RF systems sweeps the associated frequency levels making the necessary adjustments to the test levels to meet the associated test level. Once the level is achieved the data is stored to be used later during the testing process. Additional information on conducted RF immunity testing can be found here.
The calibration setup for IEC 61000-4-6 using the CDN injection method can be seen to the left.
- 1) Conducted RF Test System (NSG 4070)
- 2) 100Ω Adapters
- 3) 6 dB Attenuator
- 4) Coupling Decoupling Network (CDN)
- 5) 50 Ω Termination Load
Setup using Modular System
The setup using a modular system for test level calibration will require connections of individual components and is typically controlled with software. The below common calibration setup configurations using external equipment and a Com-Power LISN.
The 50 Ohm to 150 Ohm adapters provide the 100 Ohm impedance required for IEC 61000-4-6 and are often called just 100 Ohm adapters. The shorting bars provide the connections from the CDN to the adapters and are typically purchased with the associated network. Shorting adapters, also called mating adapters, corresponding the line configuration on the network and cannot typically be used interchangeably.
The 100Ω (50 to 150Ω) adapters can be purchased from both Com-power as well as Fischer Custom Communications.
The associated image shows how the shorting and 50 Ohm to 150 Ohm adapters are connected to the CDN.
- 1) Shorting Adapter 5 Lines
- 2) Associated 5 Line CDN
- 3) 50 to 150Ohm Adapters
- 4) Shorting adapter connected
Transient Immunity CDNs
The most common transient pulses that require coupling decoupling networks are combination wave surge (IEC 61000-4-5) and electrical fast transients(EFT)/Burst (IEC 61000-4-4). The requirements for CDNs are different for both and surges require special consideration given the pulse line sync requirements.
Automatic & Manual Couplers
Both automatic and manual couplers are commonly used for surge testing for power mains testing above 16amps three phase equipment. Typically below 16 Amp single phase applications the CDNs are built into the systems.
As the names imply automatic CDNs provide adjustments up to a given threshold and coupling configuration, whereas manual couplers require adjustment to meet the same criteria. Often the manual systems do not offer different threshold selection severely limiting the capabilities to a much smaller subset of current options.
Combination Wave Surge CDNs
Surge generators use CDNs partly as a method to inject the transients directly onto different types of lines, most commonly power mains. Given the phase angle requirements at the output of the CDN of 0, 90, 180, 270 degrees (IEC 61000-4-5) it is crucial that line synchronization is maintained.
Communication between the generator with the corresponding CDN allows for the pulse to be placed on the selected phase angle. This is a unique requirement of the surge testing and the image show how the placement could potential look on a typical AC mains sine wave.
Electrical fast transients (EFT) are commonly tested to IEC 61000-4-4 which includes both data and power mains line testing. This type of EMI event is often called burst as well, given the fast repetition of quick pulses. This series of pulses doesn't have phase angle requirements, allowing for mixing and matching of equipment.
It is common to see both surge and EFT capabilities in the same CDNs allowing for an easy solution for testing both. This is most typically seen with automatic CDNs allowing for time effective testing rather then swapping out couplers and disconnecting the equipment under test.
Capacitive Coupling Clamps (CCLs)
CCLs provide an excellent means by which to couple EFT/Burst pulses onto data or communication without concerns of matching cable types. These devices can typically be used interchangeable so long as the correct connection is made with the associated transient generator. Coupling clamps only couple the pulses onto the data lines and do not provide any decoupling capabilities.
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