Coupler-Decoupler Networks

Overview

        The Coupling/Decoupling Network or CDN is sometimes called an impedance stabilization network (ISN).  It facilitates the testing of Electromagnetic Compatibility (EMC) by coupling interference directly to or from the cables associated with the Equipment under Test (EUT).

        EN 61000-4-6 describes a highly repeatable method for immunity testing over the range 150KHz to 230MHz.  The standard sets out a test method that simulates radiated field testing by injecting interference onto the associated cables by using Coupling-Decoupling Networks (CDNs)..  This has two key benefits.  The first is that testing may be undertaken upon a bench rather than in a calibrated test chamber and so costs much less and is more accessible to design modification.  The second is a consequence of the first in that reproduceability is much better because of the absence of the effects of room resonances and reduced sensitivity to the exact layout of the EUT and its cables. Versatile CDNs from Richard Marshall Limited embody many new user-friendly features that improve accuracy and reduce cost whilst retaining compatibility with both with this standard and with EN55022:1998 for emission.

Standard test methods

        According to EN 61000-4-6 a CDN network is fitted onto each of the cables connected to the EUT.  One network is used to couple interference power into the EUT from an amplifier, whilst any others decouple the EUT from its environment and establish the desired common-mode cable impedance. The EUT is supported above a ground plane to control the electric and magnetic fields applied during the test.

        EN55022 covers limits and methods of measurement for both radiated and cable-borne emissions.  The latter covers the frequency range from 150KHz to 30MHz  and,  whilst recommending the use of the CDNs described in EN 61000-4-6, sets out some further possibilities including more specific coverage of tele-communication ports including ISDN and LAN cables.

 

46 Series Features

        The 46 series is versatile.  Each member may be set up for use with a variety of shielded or unshielded cables and so replaces several of the basic networks given as examples in appendix D of EN 61000-4-6. This considerably reduces the stock of CDNs required to accommodate varied test situations and so overcomes the most widely recognised problem with the test method.  This breakthrough has been achieved by the use of specially-designed internal components and links in the input and output connectors.  Two family variants are available as follows;

        The ”standard” S46xx version combines the versatility described above with 100% compliance to the most strict application of the procedures in EN61000-4-6.  The generator source impedance forms a part of the EUT interface impedance, giving rise to the requirement for the traditional power attenuator and pre-test calibration procedure described in EN 61000-4-6 using our type 46ZZ impedance adaptors.

        The “Enhanced” E46xx version offers built-in voltage monitoring that makes it independent of the generator source impedance.  This improves accuracy and traceability by allowing continuous and direct calibration without need of off-line checks and freeing calibration from dependence on the characteristics of specific cable, attenuator and amplifier parameters.  Thanks to the voltage monitor the E46xx has no need of a defined rf power source impedance, avoiding the power attenuator required in other systems to stabilize this impedance.  This gives an immediate saving of cost and complexity whilst leading to a reduced power requirement for the drive amplifier that can further reduce system cost.  Immunity tests to the severe “Class A” level of 10volts with 80% superimposed amplitude modulation require an amplifier of only 5watts nominal rating.

        This enhanced version is particularly recommended as providing the highest performance at the lowest cost for pre-compliance and production testing.  The E46xx versions are supplied complete with the manufacturers calibration curves, but if desired they may be checked on-site using 46ZZ adaptors.

        Both variants share the same careful detail design that has extended the useful frequency range to 500MHz, which often allows the VCDN to replace radiated field testing completely for informal design or production testing.  They meet the requirements of CIS/F/434/FDIS for testing Luminaire emission up to 300MHz.

        The I/O cables use widely available connectors chosen so that they may be plugged directly together for initial system setup before the VCDN is inserted.  The connections are fully documented so that users may make further sets as required.

        Any S46xx network may be used to decouple any remaining cables to the EUT by terminating its rf port with 50ohms.  Also, it will directly support emission testing according to EN55022:1998.   Although the S46xx variant is most cost-effective in these applications, an E46xx network may be applied to the decoupling of remaining cables to the EUT by short-circuiting its rf port.  For emission testing E46xx may be used with the type 22L Emission Receiver Adaptor which secures appropriate impedance matching as in the example ISN shown as Figure D4 of EN55022:1998.

Note that since EN55022 requires the network to be 800mm from the EUT rather than the 300mm specified in EN61000-4-6,  extended EUT cables are required for formal emission testing.