GB2310727A

GB2310727A - Electromagnetic test cell

Info

Publication number: GB2310727A
Application number: GB9604317A
Authority: GB
Inventors: Richard Ian Wrigley; Brian Joseph Cahill; Andrew Charles Marvin; John Frederick Dawson
Original assignee: RAINFORD GROUP Ltd; RAINFORD GROUP PLC
Current assignee: RAINFORD GROUP Ltd; RAINFORD GROUP PLC
Priority date: 1996-02-29
Filing date: 1996-02-29
Publication date: 1997-09-03
Also published as: GB9604317D0

Abstract

An electromagnetic test cell comprises an electrically conductive enclosure 1 and an antenna 11 for receiving or transmitting electromagnetic radiation. The antenna 11 comprises a plurality of electrically conductive elements arranged to provide parallel electromagnetic emissions within the enclosure 1. The electrically conductive elements may be parallel flat strips of electrically conductive material. A power splitting and impedance matching arrangement may receive signals from or supply signals to one end of the strips via respective coaxial couplings. The other end of each element may include an impedance matching element prior to being connected to the conductive enclosure 1. The test cell may include plural antennae, with two or three antennae arranged mutually perpendicular. The test cell may be lined with electromagnetic energy absorptive material 6. The device under test 7 may be automatically guided into position, to form an electrical connection 13 within the test cell, ready for testing.

Description

Electromagnetic Test Cell This invention relates to an electromagnetic test cell.

Electromagnetic test cells are used in the electronics industry to test electromagnetic emissions and immunity to interference of electrical and electronic devices, assemblies and subassemblies, eg for compliance with international standards. In order to comply with such standards, electrical and electronic devices are tested in electromagnetic test cells to prove their Electromagnetic Compatibility (EMC). EMC refers to a maximum level of electromagnetic emission from an operating device measured over a wide band of frequencies, and a minimum level of immunity of the device operation to interference from external electromagnetic signals.

Electromagnetic test cells provide a controlled electromagnetic environment for testing and evaluation of electromagnetic emissions from electrical and electronic devices, and interference effects from pick up of radio frequency (RF) signals. Electromagnetic test cells usually comprise a conductive metal enclosure, an electromagnetic signal receiving/transmitting point (antenna) and a means of controlling the system resonances.

Known electromagnetic test cells are relatively large and expensive. Typical test cells are medium sized rooms, all the surfaces of which are covered with RF absorbing material. A device under test is positioned at one end of the room, and an antenna is situated at the other. The separation of the device under test and the antenna in the test cell is arranged to be sufficient that the signals incident on one from the other can be taken as approximations to planar wavefronts.

The size of currently available electromagnetic test cells is considerably greater than the volume available to accommodate devices under test. The provision of such large test facilities is expensive, and furthermore is undesirable for industrial operations with limited space availability. As a consequence oftheir size, test cells are commonly provided in a location remote from production lines, and a time and cost penalty is incurred transporting devices from the production line to the test cell.

Although there has been development of electromagnetic test cells in recent years, currently available test cells cannot be readily incorporated into production lines and processes without considerable cost and space penalties being incurred. Thus, there is a need for a compact and portable electromagnetic test device which can be quickly and easily incorporated into a factory production line.

There has now been devised an electromagnetic test cell which overcomes or substantially mitigates the above disadvantages.

According to the present invention there is provided an electromagnetic test cell comprising an electrically conducting enclosure, and an antenna for the detection or emission of electromagnetic radiation, the antenna comprising a plurality of electrically conducting antenna elements arranged to produce substantially parallel electromagnetic wavefronts within the enclosure.

The test cell according to the invention is advantageous, primarily in that it can be of much reduced size compared with known electromagnetic test cells. The cell can therefore be installed readily in production line-type environments, leading to time and cost savings. Providing the antenna as a set of elements rather than the conventional metal plate is advantageous because the flow of current transverse to the orientation of the elements is limited, reducing the occurrence of high frequency electromagnetic moding effects which ordinarily occur in metal enclosures.

The antenna elements are preferably strips of electrically conductive material. The strips are preferably flat and are arranged parallel to one another.

Suitably, a second set of two or more antenna elements may be provided, with an orientation different to that of the first set of antenna elements.

Most preferably, two sets of antenna elements are provided, and the orientation of the second set of antenna elements is normal to the orientation of the first set. Using both sets of antenna elements in combination will allow the position of a source of electromagnetic radiation to be determined in two dimensions.

A third set of two or more conducting strips may be provided to form a third set of antenna elements with an orientation different to that of the first and second sets of antenna elements.

Preferably, the orientation of the third set of antenna elements is normal to the orientation of the first and second sets of antenna elements. Using all three sets of antenna elements in combination will allow the position of a source of electromagnetic radiation to be determined in three dimensions.

Preferably, one or more of the sets of antenna elements is positioned adjacent to a portion of the enclosure that is at least partially covered by material which absorbs electromagnetic radiation.

Suitably, one end of each of the conducting strips is connected to the conducting enclosure, the other end being connected to a signal source or a signal receiver.

Preferably, internal surfaces of the cell are lined with material which absorbs electromagnetic radiation at the frequencies used in operation of the cell. Most preferably, in a rectangular cell three internal walls of the cell are lined with such material (eg. the top, rear and one side).

Preferably the conducting strips of at least one of the antennae are positioned adjacent to the material which absorbs electromagnetic radiation.

Suitably, the strips comprising an antenna situated adjacent to the material which absorbs electromagnetic radiation are spaced relative to each other such that electromagnetic radiation incident at the general area of the antenna will be at least partially absorbed by the material which absorbs electromagnetic radiation.

This arrangement of antenna strips and material which absorbs electromagnetic radiation is advantageous because it allows the absorption of incident electromagnetic radiation, reducing the interaction between the antenna and any object within the test cell and thereby suppressing high frequency electromagnetic moding effects.

Preferably, each of the conducting strips passes through the material which absorbs electromagnetic radiation prior to their connection to the conducting enclosure.

Preferably, each conducting strip has a non-conducting gap which is closed by resistors of an impedance which substantially matches the impedance of the conducting strips at low frequencies, the gap being preferably located adjacent to the point of contact of the conducting strip with the material which absorbs electromagnetic radiation.

Suitably, the electromagnetic test cell is of dimensions which allow it to be positioned at or close to a production line. Conveniently, a door is provided on the enclosure of the test cell to allow a pallet or other transport means to be positioned in the enclosure of the cell.

Preferably, the inside of the enclosure is provided with rails or other means which guide the pallet or transport means to and from a desired position within the enclosure.

Suitably, the inside of the enclosure is provided with electrical connectors which locate with corresponding connectors on the pallet or transport means when the pallet or transport means is correctly located in the enclosure.

Preferably, the door of the enclosure, when in an open configuration, provides rails or other means which guide the pallet or other transport means to and from the enclosure.

Suitably, dowels, plugs, pegs or other location means are provided on the pallet or transport means as an aid to the quick and accurate positioning of a device for testing on the pallet or transport means.

Preferably, the location means incorporates electrical connectors which connect to the device for testing, through which power may be provided to the device, and signals transmitted to and received from the device.

The device according to the invention is particularly suited for electromagnetic signals in the 20 MHz to 1.5 GHz frequency range.

The invention will now be described in greater detail, by way of example only, with reference to the accompanying drawings in which : Figure 1 a is a cross-sectional plan view of an electromagnetic test cell in a production line layout, Figure ib is a cross-sectional view from the side of the electromagnetic test cell in the same production line layout, Figure 2 is a cross-sectional plan view of the test cell with a door in an open configuration, and a device for testing positioned on a pallet in a loading position, Figure 3 is a view similar to Figure 2, with the device and pallet in a test position, Figure 4 is a cross-sectional view from the side of the test cell in the configuration of Figure 2, Figure 5 is a cross-sectional view from the side of the test cell with the device and pallet in the test position, and the door in a closed configuration, Figure 6 is a view from the front of the test cell with the door in an open configuration and the device and pallet in the loading position, Figure 7 is a view similar to Figure 6, but with the door in a closed configuration and the device and pallet in the test position, Figure 8 is a cross-sectional view from the side of the test cell showing an arrangement of two sets of antennae arrays and power splitters, Figure 9 is a cross-sectional plan view of the test cell showing the arrangement of one set of antenna arrays, and radio absorbing material, Figure lOa is a detailed view from below of two elements of an antenna array situated in the test cell, Figure lOb is a detailed view from the side of one element of an antenna array situated in the test cell, and Figure 11 is a view from above of one of the power splitters.

The absolute and relative dimensions of the test cell according to the invention are important to its operation. The scales marked in centimetres on each of the Figures show the dimensions of the specific embodiment described.

Referring first to Figure 1, an electromagnetic test cell comprises a metal rectangular box 1 with a large vertical door opening and hinged door 2 which opens and closes about a horizontal hinge 9 at the lower front edge of the box 1 to form a totally enclosed conductive box. A flexible metal braided RF gasket 16 is provided (see Figure 4) to maintain electrical continuity between the door 2 and the box 1 when the door 2 is closed. In the closed position, the door 2 is secured by latches 10.

The box 1 is lined with carbon loaded radio frequency absorbing material 6, such as that provided under the brand name "Eccosorb" AN79 by Emerson & Cummings Limited, on the top horizontal, back vertical and the right vertical face (as viewed from the door of the cell). This material reduces unwanted transverse currents induced in the metal cell walls of the box 1 when the test cell is in use.

Two antennae 11, 12 are mounted within the box 1 internally of, but not in contact with, the radio frequency absorbing material 6 (see Figure 8). Qne antenna 11 is mounted transversely at the top of the cell and the other antenna 12 is mounted transversely at the back of the cell.

The antenna 11 mounted at the top of the cell comprises an array of nine horizontal conductive brass strips equispaced by 8 cm (see Figure 8), the middle strip of the antenna 11 being located 420 mm from the front vertical face of the box 1. The antenna 11 is positioned just below the radio absorbing material 6. The antenna 12 at the back of the cell comprises an array of seven horizontal conductive brass strips, which are also equispaced by 8 cm, the middle strip being located 290 mm from the base of the box 1,just inside the radio absorbing material 6.

Referring now to Figure 10, one end of each of the brass strips 20 in the antennae 11, 12 is fixed by a conductive connection 24 to the side of the box 1. The ends of the brass strips 20 of the antenna 11 mounted at the top of the cell passes through, and is electrically in contact with, the radio frequency absorbing material 6 at the rear of the cell. The ends of the brass strips 20 of the antenna 12 mounted at the back of the cell similarly pass through the radio frequency absorbing material 6 on the side wall of the cell. The other end of each of the strips 20 is connected to a standard bulkhead 50 ohm BNC connector 25 by means of which the strip 20 may be connected to a signal source (not shown). It is important that the remainder of the brass strips 20 of each of the antennae does not come into contact with the radio absorbing material 6, therefore each brass strip 20 is bonded to a 5 mm thick, non-conductive / non-metallic strip 23 of similar width to the brass strips, which acts as an insulator. Each of the brass strips in the antennae 11, 12 is 0.5 mm thick and has a width of 20 mm.

Adjacent to the intersection of each brass strip 20 with the radio frequency absorbing material 6 the strips 20 are provided with a gap 15 which is connected by four parallel resistors 22 with a resistance chosen to match the low frequency (below 300 MHz) wave impedance. The resistors are mounted on a small PCB 21 which is electrically and mechanically connected, for example by soldering, across the gap in each brass strip 20.

Each of the horizontal brass strips 20 (Figure 10) of the antenna 11 is connected via the BNC connector 25 (reference points Al to J1 in Figure 8) by individual equal lengths of 50 ohm coaxial cable (not shown) to a separate connection point (reference points alto j 1 on Figure 8) on a nine way balanced resistive power splitter assembly 18. For example with reference to Figure 8, the first strip in this antenna is connected to point Al which is cabled to reference point al on power splitter 18. The second strip is connected to point B1 which is cabled to reference point bl on power splitter 18 and so on for each of the remaining strips in the array. Similarly, each of the vertical brass strips 20 (Figure 10) of the antenna 12 is connected via a BNC connector 25 (reference points A2 to G2 on Figure 8) by individual equal lengths of 50 ohm coaxial cable (not shown) to a separate connection point (reference points a2 to g2 on Figure 8) on a nine way balanced resistive power splitter assembly 17. For example, with reference to Figure 8, the first strip in this antenna is connected to point A2 which is cabled to reference point a2 on power splitter 17. The second strip is connected to point B2 which is cabled to reference point b2 on power splitter 17 and so on for each ofthe remaining strips in the array.

The power splitter assemblies 17 and 18 and the antenna connections Al to J1 and A2 to G2 and all interconnecting coax cables (not shown) are enclosed by a metal cover 15 which is fixed to the box 1 (see Figure 9).

An input signal is supplied to each of the power splitters 17, 18 via a single BNC connection point 28 (Figure 11). The signal is then split into nine tracks 26 of equal output signals matched to the impedance of the brass strips 20 of the antennae 11,12.

The braiding of the 50 ohm coaxial cable (not shown) interconnecting the brass strips 20 is electrically connected to the ground of the respective power splitter 17, 18 which in turn is electrically connected to the conductive metal box 1 of the test cell.

The test cell door 2 has a set of non-conductive / non-metallic rails 3 bonded to its internal surface. A similar set of non-conductive / non-metallic rails 3 is bonded to the internal surface of the horizontal base of the box 1 (Figure 4).

A non-conductive / non-metallic pallet 5 is mounted on the rails 3. The pallet 5 has two sets of runners to facilitate movement on the rails 3. The pallet 5 is designed to support a device under test (DUT) 7.

At the rear of the pallet 5 is located an array (not shown) of standard electrical and electronic connectors to enable the DUT 7 to be connected to analytical test equipment (not shown) located outside the box 1. The pallet connectors are wired internally within the pallet 5 in a non conductive block to a series of fixed brass contact pins, one for each separate electrical power and control signal. This contactor assembly also incorporates positional location plugs / dowels.

An assembly 13, similar to the pallet contactor assembly, is located within the metal box cell at the bottom rear of the box 1, the radio absorbing material 6 being locally removed from this small volume. Within this assembly a series of suitable standard spring contact test probes are positioned to make electrical contact with the pallet's fixed brass contact pins. This assembly 13 also incorporates positional location holes.

The assembly 13 is removable and provides the means for power and control signals from the DUT 7 on the pallet 5 to be routed outside the cell 1 for connection to analytical test equipment.

The assembly 13 is mounted through a rectangular hole in the box 1 by a suitable gland plate (not shown) and metal braided RF gasket (not shown).

The pallet 5 and DUT 7 are positioned within the box cell 1 for testing by the rails 3, pallet runners and the positional location plugs / dowels of the pallet contact assembly which locate into the spring probe fixed assembly 13 positional location holes to accurately locate the pallet 5 within the box cell 1 and electrically connect the DUT 7 to the outside analytical test equipment.

For a given DUT 7 a pallet location template block (not shown) is fixed using predefined location dowels to the pallet 5. This template block accurately and repeatably locates a given DUT 7 within the test cell as the pallet 5 itself is accurately located as described above. Furthermore, the template block incorporates defined cable routes /trunking which standardises for a given DUT 7 the test layout thereby increasing the repeatability and hence the accuracy of the test cell.

The door 2 when hinged down into the horizontal (open) orientation (Figures 2 and 4) extends the set of rails 3 thereby enabling the pallet carrying the DUT to move easily in and out of the box, thus enabling the invention to be easily incorporated into a production line. An example production line arrangement 8 is shown in Figures 1 to 7.

The box 1 is mounted on a stand 4 which also supports the door 2 when in the horizontal (open) orientation. At the base of the stand 4 adjustable feet 14 are incorporated to accommodate uneven floors.

For the specific example of test cell set out herein the DUT 7 has a maximum size as shown by the drawings which is 600 millimetres long, along the axis of the rails 3, by 400 millimetres wide by 300 millimetres in height.

In use, a device travelling along a production line 8 is selected manually or automatically and diverted from the production line 8 onto the pallet 5 situated adjacent to the test cell. Electrical plugs and sockets on the device under test are coupled to corresponding sockets and plugs on the pallet 5. The pallet slides along the guide rails 3 provided on the upper surface of the open door of the cell, and is guided into the cell by the rails disposed therein 3. The pallet is located in its correct position within the test cell by the contactor assembly 13. The assembly 13 electrically connects the pallet (and hence the device) to outside analytical test equipment. The door 2 of the test cell is closed and secured by latches 10, the RF gasket 16 providing electrical conductivity between the door 2 and the box 1.

The device is tested as in conventional electromagnetic test cells to assess its immunity to electromagnetic interference and / or the size and frequency range of its electromagnetic emissions.

Operation of the test cell requires that the R.F. voltage between the sets of antennae 11,12 and the metal cover 15 is measured. This voltage will vary considerably according to the layout and contents of the cell.

The width, density and spacing of the antenna strips 20 is determined as a compromise between having a suitable number and width of strips 20 to generate a uniform electromagnetic field distribution in the cell, and having a suitable proportion of space between the strips to allow the absorption of electromagnetic radiation by the radio frequency absorbing material 6.

Upon the conclusion of the testing procedure, the device is removed from the test cell 1 by opening the door 2 and guiding the pallet 8 back along the rails 3. Dependent upon the outcome of the electromagnetic tests, the device is either returned to the production line or detained for further investigation.

The antennae 11,12 of the invention are arranged to produce a substantially uniform electromagnetic field. In a test cell with dimensions different to the illustrated example, with a field distribution of similar uniformity, the arrangement of the antennae 11,12 would be correspondingly different.

Claims

1. An electromagnetic test cell comprising an electrically conducting enclosure, and an antenna for the detection or emission of electromagnetic radiation, the antenna comprising a plurality of electrically conducting antenna elements arranged to produce substantially parallel electromagnetic wavefronts within the enclosure.

2. A test cell as claimed in claim 1, wherein the antenna elements are strips of electrically conductive material.

3. A test cell as claimed in claim 2, wherein the strips are flat and are arranged parallel to one another.

4. A test cell as claimed in any preceding claim, comprising two sets of antenna elements, the orientation of the second set of antenna elements being normal to the orientation of the first set.

5. A test cell as claimed in claim 4, wherein a third set of antenna elements arranged normal to the orientation of the first and second sets of antenna elements.

6. A test cell as claimed in claim 2, wherein one end of each of the conducting strips is connected to the conducting enclosure, the other end being connected to a signal source or a signal receiver.

7. A test cell as claimed in any preceding claim, wherein the intemal surfaces of the cell are lined with material which absorbs electromagnetic radiation at the frequencies used in operation of the cell.

8. A test cell as claimed in claim 2, wherein the conducting strips of at least one antenna are positioned adjacent to material which absorbs electromagnetic radiation.

9. A test cell as claimed in claim 8, wherein the strips comprising an antenna situated adjacent to the material which absorbs electromagnetic radiation are spaced relative to each other such that electromagnetic radiation incident at the general area of the antenna will be at least partially absorbed by the material which absorbs electromagnetic radiation.

10. A test cell as claimed in claim 8 or claim 9, wherein each of the conducting strips passes through the material which absorbs electromagnetic radiation prior to their connection to the conducting enclosure.

11. A test cell as claimed in any preceding claim, wherein each conducting strip has a nonconducting gap which is closed by resistors of an impedance which substantially matches the impedance of the conducting strips at low frequencies, the gap being preferably located adjacent to the point of contact of the conducting strip with the material which absorbs electromagnetic radiation.

12. A test cell as claimed in any preceding claim, wherein the inside of the enclosure is provided with rails or other means to guide a pallet or transport means to and from a desired position within the enclosure.

13. A test cell as claimed in claim 12, wherein the inside of the enclosure is provided with electrical connectors which locate with corresponding connectors on the pallet or transport means when the pallet or transport means is correctly located in the enclosure.

14. A test cell as claimed in claim 12, wherein location means are provided on the pallet or transport means and the location means incorporates electrical connectors which connect to the device for testing, through which power may be provided to the device, and signals transmitted to and received from the device.

15. An electromagnetic test cell substantially as hereinbefore described and as illustrated in the accompanying Figures.