GB2338073A

GB2338073A - A coupling and decoupling network for testing immunity to electromagnetic fields

Info

Publication number: GB2338073A
Application number: GB9811781A
Authority: GB
Inventors: Richard Carlile Marshall
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-06-03
Filing date: 1998-06-03
Publication date: 1999-12-08
Anticipated expiration: 2018-06-03
Also published as: GB2338073B; GB9811781D0

Abstract

A coupling and decoupling network (CDN) comprises a radio frequency input port, an associated equipment port and an equipment under test port. A multiple conductor common-mode choke coil L1 is arranged between the associated equipment port and the equipment under test port and a plurality of resistors R3 - R6 are arranged for injecting the radio frequency signal into the equipment under test port. A variable connection system may be used to interconnect the common-mode choke and injection resistors to allow the use of one network for a variety of different cable types. At least part of this variable connection system may be located within cable connectors. Voltage monitoring means may be included in the network to improve the efficiency of power transfer.

Description

( 1)

TESTING IMMUNITY TO ELECTROMAGNETIC FIELDS

2338073 This invention relates to methods and apparatus for testing immunity to electromagnetic fields in which radio-frequency energy is introduced into a cable associated with the equipment under test.

Reference should be made to European standards ENV50141 and EN61000-4-61996, which explain the terminology of this technical field and give practical details of how to do such tests over the frequency range 0. 15MHz to 2601fliz. These documents also define "Coupling and decoupling networks (CDNs)" which are used respectively to introduce radio-fiequency energy in common mode to a cable and to prevent this energy flowing out into the general environment, whilst maintaining a specific common-mode impedance of 150 ohms for each cable.

These "Cl)Ns" are required to be put on every cable connected between the equipment under test ("EUT') and auxiliary equipment ("AW). Hitherto, different CDNs have been needed for each cable application, necessitating a considerable investment in rarely-used items. Furthermore a different design of CDN is required according to the number of cores in the cable, the presence or absence of shielding, and the current, voltage, and impedance levels of the power, signal, or data to be transmitted through the cable. Appendix D of EN61000-4-6 gives 6 different examples of such networks. Furthermore the CDNs described therein rely upon the test generator to provide 50 ohms of the total 150 ohms output impedance, and may themselves reflect an infinite load to the test generator. For both these reasons it is customary to include a 6d13 power attenuator as shown in fig pre 2b of EN61000-4-6, which necessitates a corresponding increase o power from the test generator. This increases the cost of the test generator beyond that which is theoretically needed and introduces a dependency of the calibration of the network upon the attenuator and test generator.

According to the present invention there is provided a coupling and decoupling network including a radio-frequency test signal input port, an associated equipment port, and an equipment under test port ikith a defined common-mode output impedance of between 50 and 600 ohms, and also includin - - a multi-conductor common mode choke coil and a plurality of injection resistors which may be variably connected to the cable between the equipment under test and the associated equipment according to the number of cores in said cable and the presence or absence of a shield on said cable.

A specific embodyment of the invention will now be described by way of example with reference to the accompanying drawing in which:- Figure 1 shows the circuit of a CDN with one pair of connecting cables that is appropriate for a single-phase three wire power cable; Fig i of a CDN with an alternative pair of connecting cables that is Ture 2 shows the circuit appropriate for a four-conductor unshielded signal cable; Figure 3 shows how a common mode choke coil with a toroidal core may be wound advantageously for this invention.

1 Figure 4 shows how the monitor circuit may be arranged so as to receive a voltage equal to the errif provided by the test generator before said emf is reduced by the output impedance of said test generator.

C-1) Referring to Figure 1 the EUT is connected to a supply source via the common-mode choke L 1. A source of radio frequency power is coupled onto the EUT wires by blocking capacitors C 13C 16 and impedance-defining resistors R3-R6, which in this application are linked within the free connector shell on the EUT side so as to distribute the injected power to the LINE, NEUTRAL, and EARTH conductors, with the earth getting a double share. This arrangement may be compared to figure D2 of EN61000-4-6. Provision is made for monitoring the voltage level of the injected signal by rectifier D 1 and filter R] 4/C 19 whose time-constant may be chosen to allow the modulation envelope to be reproduced faithfully. Capacitors C 17 & C 18 ensure that any de component to the rf voltage will not disturb the calibration of rectifier DI. Theattenuation provided by RI 1, R12, & R13 is chosen having regard to the ratings of D I and the desirability of an easy-to-remember scale factor for the monitor output, R7-RIO provide a nlhmum load for the RF input to limit the VSWR should the EUT present a very high impedance, as it lkq11 at certain frequencies or if the EUT is disconnected. Capacitors Cl to C8 and Cl 1 to C 12 are linked,%,,lthin the free connector shell on the AE side to form a low- pass filter Xvith Ll so as to reduce radiation of the test signal from the remote parts of the supply cable. It should be noted that these capacitors, together with C 13 to C 16, should be suitably rated as safety-affecting components when the network is used with a supply of more than 40 volts.

It can be understood that in the case of a screened or coaxial cable the screen may be connected in the manner of the protective earth in the above embodiment, producing an embodiment that may be compared with figure D 1 of EN61000-4-6. All of the injection resistors R3 to R6 may be linked in parallel to the screen, and the four conductors through the choke coil L 1 may be c parallelled to the extent needed to give the required number of circuit conductors.

Ficure 2 ditTers from Figure 1 only in the AE and EUT connecting cables and the linking within their free connectors. It may be compared to figures D3 and D6 of EN61000-4-6. 4-wire unscreened cables such as are shown are used in telephone circuits, and for the connection of measuring bridges such as strain Qauú_,es. In such cases the injection res,,stors R3 to R6 distribute the injected power uniformly to the four conductors, and on the AE side small values of decoupling capacitor are linked in. It might be that the circuit under test would not function properly even with these small capacitors. in that case they would not be so linked. It can be understood that in the case of a 2-wire cable a similar arrangement to Figure 2 may be used, but with red & blue paralleled and yellow & green paralleled, for example.

To achieve the maximum high-frequency bandwidth ftom this network the stray capacitance to earth from the EUT connections must be minimised by suitable layout and choice of connector type and pin assignment. It is also facilitated by the low-capacitance winding arrangement of L1 that is shown in figure 3, wherein the Winding crosses the toroidal core so as to shield the winding ends from each other. The number of turns shown is for illustration and is not limitincr to the invention.

cl) In all the embodiments described above resistors R3 to R6 acting in parallel provide the whole of the nominal output impedance of the network, which is for example 150 ohms. Therefore it is the voltage at the rf input connector which must be standardised to establish the desired test severity, and the monitor circuit has been shown connected accordingly. However, further economy in test power may be achieved if the test generator output impedance, which is for example 50 ohms, forms part of the output impedance. This may be achieved by the adaption of Figures 1 or 2 that is shown in Figure 4. Here, the monitor rectifier D 1 is connected to the secondary of an autotransformer 40, whose primary is connected across a resistor RA 41, so as to add to the voltage presented to the monitor a component preportional to the output current. If for example, the transformer has 1: 1 ratio and RB=Zo, where Zo is the source impedance of the test generator 42, then the rectifier will be presented with a voltage equal to the test generator emf Vo, 43 despite changes in the load impedance presented to the network. Both Zo and R-A then form part of the output impedance, so that RB, 44, which in this simplified diagram represents R3 to R6 in parallel, is reduced in value accordingly. With the exemplary impedances given above RB becomes 50 ohms.

It is to be understood that the above description is by way of illustration and does not limit the invention. For example, more than four current injection resistors may be provided, and more than four conductors may thread the common-mode choke Ll. Ll may include more than one core element. In Figure 4 the transformer ratio and hence the value of RA may be changed.

4.

Claims

1) A coupling and decoupling network including a radio-frequency test signal input port, an associated equipment port, and an equipment under test port with a defined common-mode output impedance of between 50 and 600 ohms, and also including a multi-conductor common mode choke coil and a plurality of injection resistors which may be variably connected to the cable between the equipment under test and the associated equipment according to the number of cores in said cable and the presence or absence of a shield on said cable.

2) Apparatus according to claim 1 in which a plurality of decoupling capacitors are also provided which may be variably connected to the cable between said common mode choke coil and the associated equipment.

Apparatus according to claims 1 or 2 tooether with a variety of pairs of cables for use W between said coupling and decoupling network and the equipment under test on the one hand and the associated equipment on the other, said cables being connected to said network by connectors whose free portions are variably linked so as to achieve at least part of the variable connection claimed above.

4) Apparatus according to any of the previous claims in which a rectifier circuit is provided c for the monitoring of the test signal that is applied to said injection resistors.

5) Apparatus according to claim 4 in which said rectifier circuit is connected via a c 1 1 transformer to a currentsensing resistor so that it receives a voltage equivalent to the emf Z__ provided by the test generator before said emf is reduced by the output impedance of said test P enerator.

6) Apparatus according, to any of the previous claims in which means are provided to reduce the maximum standing wave ratio upon the transmission line connected to said test signal input port, said means including a shunt-connected resistor.

7) Apparatus according to any of the previous claims in which said common mode choke Coil includes a toroidal core, the centre-part of whose winding is taken diametrically across the core so as to minimise the stray-capacitance between the ends of said winding.

8) Apparatus substantially as descilbed herein with reference to Figures 1-4 of the accompanying drawing.