GB2375178A - Apparatus and methods for testing immunity to and emission of electromagnetic fields - Google Patents

Abstract

An electromagnetic compatibility test apparatus comprises a conductive boundary 2 arranged to enclose the equipment under test (EUT) 1. A cable 3 linked to the EUT 1 passes through a filter arrangement 4 before passing through the said boundary 3 as it approaches the EUT 1. Switch means 6 is arranged which can be activated to couple an element of the cable 3 to the said conductive boundary 2. The switching means 6 may then be controlled in such a manner as to quickly identify a cable connection condition which is near to that which provides the worst-case of electromagnetic coupling. The switching means 6 may be a fast acting PIN diode switch, an electromechanical relay or an electrically or optically activated transistor. The filter arrangements may involve ferrite sleeves, common mode choke arrangements, decoupling capacitor arrangements and/or one or more absorbing clamps.

Description

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TESTING IMMUNITY TO AND EMISSION OF ELECTROMAGNETIC FIELDS This invention relates to methods and apparatus for testing immunity to and emission of electromagnetic fields in which radio-frequency energy is coupled through electromagnetic radiation in the immediate environment of the equipment under test. Within this specification references to"radiation"should be taken as including both inward reception of radiation and outward radiation unless the context explicitly disallows this.

Reference should be made to European standards EN55022 and EN61000-4-3: 1995, which explain the terminology of this technical field and give practical details of how to do such tests. These referenced documents discuss the importance of precisely defining the cable lengths and layouts of the equipment under test (hereinafter"EUT"). This is important because these cables form an important part of the antenna system by means of which the EUT radiates or absorbs electromagnetic energy. As is well-known in the art of antenna design changes in dimensions of say, one-tenth of a wavelength in antenna element length or spacing can have a large effect upon antenna gain and directivity.

It was shown in UK Patent Application GB2 179 502 how to limit the antenna effect of cables by the use of ferrite-cored common mode chokes, and recently it has been described by E Ristig in the IEC Committee draft CISPR/A/251/CD how an"absorbing clamp" (which is a well-known contemporary common-mode ferrite device fully described in Appendix G to IEC standard CISPR16) may be used to limit the length of each EUT cable that might act as an antenna.

For the definition of tests of electromagnetic compatibility that can facilitate international trade it is desired to secure measurement conditions that are easily repeatable yet approximate to practical use. Accordingly reasonable worst-case conditions for such cable-antenna effects should be established and these effects should not be reduced too much by the configuration of the cables in the test set-up.

Whilst Ristig's proposal referenced above points towards a method for defining the effective radiating length of each cable leaving the region of the EUT, it does not address the question of how it may be ensured that the radiation from different portions of such a single cable, or from a number of such cables, is not subject to destructive interference and so cancelled out or reduced to an extent that allows the EUT to pass a test that it should reasonably have failed. In the standards referenced above it is taught that this should be done by moving the cables about on a trial-and-error basis until a worst-case is obtained. However this is time-consuming and leads to significant differences in measurement results because of the different cable layouts used adopted by individual test engineers.

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It is therefore desirable to adopt some method of identifying the effects of destructive interference between the various cables acting as antennae. It has been suggested to keep the cable positions strictly fixed in standardised positions and to employ common mode chokes on the cables selectively to secure emission free from cancellation effects. These chokes might either be positioned close to the EUT so as to decouple the cable from the EUT, or be positioned at a distance from the EUT where they act by damping or detuning the resonance of the cable.

Hitherto this has been an unattractive approach because of the time taken to identify and test to the worst case selection of choke positions, which may be different at different frequencies.

Accordingly our previous application GB0003434. 8 taught how to provide a two-terminal switchable cable decoupling device including a common mode choke coil with an auxiliary winding, said auxiliary winding being short-circuited upon demand by an electrical or electronic switching means. However it has not been considered previously how to ensure that radiation effects during testing are representative of those that would be experienced in practical use where the common-mode impedance of a cable approaching the EUT might be of any value.

According to the present invention there is provided apparatus for the testing of Electromagnetic Compatibility within a space defined by a conductive boundary wherein a cable from the Equipment Under Test passes through said boundary, which apparatus comprises firstly a series element that includes either choke coils in series with or magnetic sleeves acting upon each individual core of said cable or alternatively a common-mode choke coil or magnetic sleeve in series with or acting upon said cable as a whole, and secondly a parallel element including electrical or electronic switching means adapted to couple to said boundary upon demand an element of said cable, said coupling being made to said cable at a place nearer to said Equipment Under Test than is said series element.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which :- Figure I shows a test set up for an EUT together with a switchable cable decoupler for a three-core unshielded cable; Figure 2 shows a test set up for an EUT together with a switchable cable decoupler adapted for a two-core cable with a shield and embodying an electromechanical relay; and Figure 3 shows test set up for an EUT together with a switchable cable decoupler adapted for a two-core unshielded cable and embodying a semiconductor switching device.

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Referring to Figure I the EUT 1 is enclosed in a conductive chamber 2 as decribed in the standards referred to in our preamble together with an antenna or other coupling means connected to an rf power source or to a measuring receiver respectively for radiation testing. The invention is independent of these latter elements and they will not be discussed further. A single three-core unshielded cable 3 from the EUT is lead through the chamber boundary wall and a series element comprising choke coils 4a, 4b, and 4c in each core provide a high common-mode impedance to radio-frequency signals flowing upon the cable inside the chamber.

It is to be understood that such currents will flow particularly at frequencies such that the cable within the chamber is of a resonant length, and by providing a high impedance these choke coils will cause resonances to occur such that there is a voltage maximum at the chamber boundary.

However, capacitors 5a, 5b, and 5c are also provided whose capacitance and lead-length are chosen so that closure of the switch 6 results in a very low common-mode impedance at radio-frequencies so that there will then be a voltage minimum at the chamber boundary.

Since the physical length and disposition of the cable is unchanged during the test the difference in resonant condition in response to the operation of switch 6 will achieve the objectives of the present invention, that is to establish different test conditions so as to ensure that testing is not performed only under conditions where electromagnetic coupling is at a minimum due to the cancellation of effects from two or more sources, and to conduct testing with widely-varying common-mode cable impedances. Since the switch position that gives greatest emission or susceptibility will vary with frequency and cable layout tests must be made with the switch both open and closed. This may be achieved by remote control means 7, which may include an electronic timer, counter, and state decoder such as is described in our co-pending application GB0003434. 8.

In the case of an EUT with a plurality of connecting cables both positions of the switch associated with each cable must be explored in several, but not necessarily all, possible combinations so as to get sufficiently near the worst-case electromagnetic coupling without excessive test time.

It is not necessary for the chokes 4 to offer an inductive impedance: Indeed to provide a high impedance over a wide frequency range a substantially resistive choke action such as may be achieved with a suitable ferrite core may be preferred. If the cable 3 were a three-core single-phase power cable then one core might be a ground connection, in which case the appropriate blocking capacitor of the set 5a, 5h and 5c might be omitted.

Figure 2 shows an example of the application of the invention to a coaxial or multicore shielded cable. The desired high series impedance is provided by a common-mode choke 4 which may take the form of one or more ferrite sleeves of high-frequency lossy ferrite material such as Philips grade 4S2, which may be split lengthwise for convenient assembly around the EUT cable 3. Alternatively a length of the cable may be wound into an air-cored choke coil, or the cable may be passed through an"absorbing clamp"as described in the preamble. The switch 6 is directly connected to the cable shield and utilises the contact of an electromechanical relay whose coil 8 is energised by a wire connected to the remote control means 7. To minimise rf leakage though the chamber wall the relay coil circuit includes a decoupling lead-through capacitor 9 and utilises a ground return.

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In the case of unshielded cables carrying high-speed data the capacitors 5a, 5b, and 5c shown in figure 1 might produce unacceptable differential mode loading or impedance discontinuity. In such a situation they may be replaced by the distributed capacitance of a length of shielded cable outside the test enclosure as shown in figure 3. In this arrangement the shielded section 10 may act as a resonant coaxial stub whose end nearest the"associated equipment"is open circuit. To prevent this from reflecting a high impedance in series with the switch it is desirable to ensure adequate resistive loss in the shielded section by providing a lossy dielectric or by inserting resistors in series with the shield at intermediate points along its length.

Figure 3 also shows how the switch may be implemented by using a decoupling capacitor 5 and a PIN diode 11 which may be an Hewlett-Packard type HSMP3890. The PIN diode is biased by a dc potential supplied by the remote control 7. Resistor 12 might be chosen as 2,200 ohms so as to prevent the remote control circuit and the decoupling capacitor 9 from reducing unduly the impedance of the diode switch when this is"open". If the output of the remote control 7 is held negative by, say, 24 volts with respect to ground then the PIN diode is reverse biased and presents an impedance of some 0. 3pF so that the"switch"is effectively open. If on the other hand the output of remote control 7 is held positive with respect to ground then the PIN diode conducts and assumes a low resistance so that the"switch"is effectively closed. This PIN diode switch has the advantage of very fast operation but the disadvantage of limited voltageand current-handling capability. It may therefore be preferred for emission testing, whilst the electromechanical relay may be the better choice for immunity testing.

It is within the scope of the present invention to implement the switch by an alternative device such as a junction transistor or field-effect transistor with electrical or optical input.

For emission testing each switch state, or each combination of switch states, may preferably be maintained for approximately 2 milliseconds, which is just long enough for a quasi-peak detector according to IEC standard CISPR16 within the measuring receiver to register the strength of the emitted field. Alternatively a complete sweep of the frequency range may be accomplished for each switch state or combination of switch states in turn. In this case remote control of the switches is not essential.

The above description is by way of illustration and does not limit the invention. In particular it should be noted that the various elements of the invention are arbitrarily combined in the three figures, and all other combinations of these elements are within the scope of the invention.

Claims (1)

1. CLAIMS 1) Apparatus for the testing of Electromagnetic Compatibility within a space defined by a conductive boundary wherein a cable from the Equipment Under Test passes through said boundary, which apparatus comprises firstly a series element that includes either choke coils in series with or magnetic sleeves acting upon each individual core of said cable or alternatively a common-mode choke coil or magnetic sleeve in series with or acting upon said cable as a whole, and secondly a parallel element including electrical or electronic switching means adapted to couple to said boundary upon demand an element of said cable, said coupling being made to said cable at a place nearer to said Equipment Under Test than is said series element.

   2) Apparatus according to claim 1 in which said series element provides a common mode impedance of greater than 300 ohms.

   3) Apparatus according to claim 1 in which said coupling of said parallel element is provided by capacitors connected to said cores.

   4) Apparatus according to claim 1 in which said coupling of said parallel element is provided by direct or capacitive connection to a shield that ordinarily forms part of said cable.

   5) Apparatus according to claim I in which said coupling of said parallel element is provided by the capacitance from a shield provided around a portion of said cable.

   6) Apparatus according to claim 5 in which said cable and said shield acting in combination as a radio-frequency transmission line include sufficient electrical loss to present a substantially non-resonant impedance at the place of connection of said switching means 7) Apparatus according to any preceding claim in which said switching means includes a semiconductor switching device.

   8) Apparatus according to any preceeding claim in which said switching means is operated so that either each period of short-circuiting or each period of open-circuiting is of a duration between 100 microseconds and 100 milliseconds.

   9) Apparatus according to any preceding claim in which said cable is one of several cables connected to said Equipment Under Test, and a plurality of instances of said electrical or electronic switching means are provided and further adapted to operate in a predetermined sequence.

   10) Apparatus substantially as described herein with reference to Figures I to 3 of the accompanying drawing.