Patent Publication Number: US-2015061698-A1

Title: Electromagnetic interference (emi) test apparatus

Description:
TECHNICAL FIELD OF INVENTION 
     This disclosure generally relates to a radio-frequency (RF) energy coupling apparatus for electromagnetic interference (EMI) susceptibility testing of a device, and more particularly relates to using a micro-strip transmission line to couple RF energy into a wire connected to a device under test. 
     BACKGROUND OF INVENTION 
     Electrical devices sold to vehicle manufactures are typically required to comply with the Electromagnetic Interference (EMI) or Electromagnetic Compatibility (EMC) specifications established by the vehicle manufactures. One test used to determine compliance with the vehicle manufacture&#39;s specification is the so-called Bulk Current Injection (BCI) test. During BCI testing, bundles of wires are encompassed by a current injection probe that is clamped around the wires. A relatively high power (up to 50 Watts) radio frequency (RF) signal is supplied to the current injection probe to inductively couple RF currents into the wires encompassed by the current injection probe. Because of the expense of generating the high power RF signal, and the expense of providing a RF screen room for the BCI test, the BCI test may not be readily available for use by engineers when automotive products are being developed. What is needed is a less expensive test method that can be used by engineers while developing new products, or while seeking to determine why a product did not pass a BCI test. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment, a radio-frequency (RF) energy coupling apparatus for electromagnetic interference (EMI) susceptibility testing of a device is provided. The apparatus includes a ground-plane, a micro-strip, a first dielectric layer, a coupling-strip, and a second dielectric layer. The micro-strip overlies the ground-plane. The first dielectric layer is interposed between the ground-plane and the micro-strip. The combination of the ground-plane, the micro-strip, and the first dielectric layer cooperate to form a micro-strip transmission line configured to propagate RF energy from a RF generator to a termination load. The coupling-strip overlies the micro-strip opposite the first dielectric layer. The coupling-strip is configured to couple RF energy from the micro-strip to a harness wire connected to the device. The second dielectric layer is interposed between the coupling-strip and the micro-strip. 
     Further features and advantages will appear more clearly on a reading of the following detailed description of the preferred embodiment, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention will now be described, by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic of a test system using a radio-frequency (RF) energy coupling apparatus in accordance with one embodiment; 
         FIGS. 2A and 2B  are top and end views, respectively, of a micro-strip transmission line that is part of the radio-frequency (RF) energy coupling apparatus of  FIG. 1  in accordance with one embodiment; 
         FIGS. 3A and 3B  are bottom and inverted-end views, respectively, of a coupling assembly that is part of the radio-frequency (RF) energy coupling apparatus of  FIG. 1  in accordance with one embodiment; and 
         FIGS. 4A and 4B  are top and end views, respectively, of the radio-frequency (RF) energy coupling apparatus of  FIG. 1  including RF connectors in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a non-limiting example of a test system  10  that includes a radio-frequency (RF) energy coupling apparatus, hereafter referred to as the apparatus  12 , for electromagnetic interference (EMI) susceptibility testing of a device  14 . The device  14  is labeled in  FIG. 1  as DUT to mean ‘device under test’. As described above, the known bulk current injection (BCI) test method is a relatively expensive test to perform as a high power radio frequency (RF) amplifier, a complicated current injection probe, and a RF screen room are needed to perform a BCI test. The test system  10  presented herein is a relatively low cost tool that can be used by engineers to, for example, pre-test the device  14  prior to submitting the device for BCI testing, or as an investigation tool when seeking to understand why the device  14  did not pass a BCI test. It is also recognized that the test system  10  may be used independent of BCI testing as part of a product development activity. 
     In general, the test system  10  includes an RF generator  16  configured to output RF signals at various frequencies and amplitudes. In contrast to the BCI test which may require up to fifty watts (50 W) of power to induce the desired current into a wiring harness connected to the device under test, the RF generator  16  only needs to output about one-hundred milli-Watts (100 mW or 0.1 W) to be effective to evaluate the device  14 . The test system  10  may also include a termination load  18 , for example a fifty Ohm (50Ω) load, to provide a proper termination of an RF signal  20  emitted by the RF generator  16  and propagating through the apparatus  12 . 
     The test system  10  may also include a controller  22 , such as an electronic control module (ECM), configured to operate the device  14  and provide a representative termination load of a wiring harness  24  that corresponds to whatever the device  14  is connected to during normal in-vehicle operation. Alternatively, the controller  22  may be a custom built device that merely mimics the input/output characteristics of whatever the device  14  is connected to during normal in-vehicle operation. In practice, each of the wires that form the wiring harness  24  may include connectors (not shown) installed in the wiring harness  24  so that each wire can be individually opened and connected to the apparatus  12 . Once connected, RF energy from the RF signal  20  can be coupled into each wire of the wiring harness  24  by the apparatus  12 , and thereby injected into the device  14  for testing the immunity or susceptibility of the device  14  to RF interference (RFI) or electromagnetic interference (EMI). 
       FIGS. 2A and 2B  illustrate, respectively, a non-limiting example of a top view and end view of a lower portion  26  of the apparatus  12  that includes a ground-plane  28 ; a micro-strip  30  overlying the ground-plane  28 ; and a first dielectric layer  32  interposed between the ground-plane  28  and the micro-strip  30 . By way of example and not limitation, the micro-strip  30  may have a strip-width  34  of three millimeters (3 mm) and a strip-length  36  of one-hundred-ten millimeters (110 mm). The ground-plane  28  may have a plane-length equal to the strip-length  36 , and a plane-width of sixty millimeters (60 mm). The first dielectric layer  32  may have the same length and width dimensions as the ground-plane  28 , and preferably has a substrate-thickness  38  of one-point-six millimeters (1.6 mm). 
     Preferably, the first dielectric layer  32  is formed of circuit board material such as the readily available FR- 4  material. Similarly, the micro-strip  30  and the ground-plane  28  are preferably formed of copper foil so that the lower portion  26  can be fabricated using conventional printed circuit board manufacturing techniques. The copper foil may be seven-hundred-ten micrometers (710 um) thick, which is commonly called two-ounce copper in circuit board manufacturing circles. However, it is recognized that other thickness of copper may be suitable. 
     Those skilled in the RF transmission arts will recognize that the combination of the ground-plane  28 , the micro-strip  30 , and the first dielectric layer  32  in the example dimensions suggested cooperate to form a micro-strip transmission line  40  configured to propagate RF energy from the RF generator  16  to the termination load  18 . Using a relative permittivity of four-point-three (4.3) for the first dielectric layer  32 , the characteristic impedance of the micro-strip transmission line  40  is calculated to be about fifty Ohms (50Ω). Accordingly, as suggested above, the impedance of the termination load  18  is selected to match the characteristic impedance of the micro-strip transmission line  40 . It is recognized that other combinations of dimensions and materials could be used to achieve a characteristic impedance of 50Ω. The example dimensions and materials presented herein were used to build a prototype of the apparatus  12  for evaluation in the test system  10 . 
       FIGS. 3A and 3B  illustrate, respectively, a non-limiting example of a bottom view and inverted end view (inverted relative to  FIG. 4B ) of an upper portion  42  of the apparatus  12  that includes a coupling-strip  44 . As shown in  FIG. 4 , when the apparatus  12  is assembled, the lower portion  26  and the upper portion  42  are arranged so the micro-strip  30  and the coupling-strip  44  are as close together as possible, but isolated electrically by a second dielectric layer  52  ( FIG. 4B ), the reasons for which will become clear in the further description of the apparatus  12  provided below. In other words, the coupling-strip  44  is configured to overlie the micro-strip  30  opposite the first dielectric layer  32 . The upper portion  42  may also include feed-strips  46 A and  46 B to provide an electrical connection from the coupling strip  44  to the edge of a third dielectric layer  48  for making an electrical connection to the wiring harness  24 . By locating the coupling-strip  44  close to the micro-strip  30 , the coupling-strip  44  is configured to readily couple RF energy from the micro-strip  30  to a wire  24 A of the wiring harness connected to the device. 
     Preferably, the coupling-strip  44  has a coupling-width  54  substantially equal to a strip-width  34  of the micro-strip  30 , 3 mm for example. Having the coupling-width  54  substantially equal to the strip-width  34  is preferred as doing so maximizes the RF energy coupling between the coupling-strip  44  and the micro-strip  30 . The coupling-strip  44  also has a coupling-length  56  of seventy millimeters (70 mm) that is the length of the coupling-strip that overlies the 110 mm length of the micro-strip  30 . The coupling-length  56  affects the efficiency of RF energy coupling, and can be selected to avoid resonance effects. While not subscribing to any particular theory, the coupling length  56  should be greater than one-tenth of a wavelength (λ/10) of the lowest frequency of the RF signal  20 . 
     The second dielectric layer  52  is generally interposed between the coupling-strip  44  and the micro-strip  30  for direct-current (DC) electrical isolation. However it is also desirable to have the second dielectric layer  52  as thin as possible so that the RF energy coupling between the coupling-strip  44  and the micro-strip  30  is maximized. The prototype tested used a fifty micrometer (50 um) thick layer of stationary cello-tape commonly used for, among other things, securing decorative wrapping paper about a wrapped gift. It is recognize that a wide variety of materials may be suitable for use as the second dielectric layer  52 , and the selection used for the prototype was only selected based on convenience. For example, the second dielectric layer  52  may be formed by the protective coating commonly placed over circuit traces on a printed circuit board to protect the metal used to form the circuit traces. 
     The upper portion  42  of the apparatus  12  illustrated in this non-limiting example includes the third dielectric layer  48  overlying the coupling-strip  44  opposite the second dielectric layer  52 . Preferably the third dielectric layer  48  is as thin as possible in order to minimize the effect the third dielectric layer  48  has on the characteristic impedance of the micro-strip transmission line  40 . The prototype of the apparatus  12  that was tested had a third dielectric layer  48  with a second substrate-thickness of eight-hundred-micrometers (800 um or 0.8 mm). However, the third dielectric layer  48  was only included to make it convenient to build a prototype. Preferably, an assembly technique is used that allows for the coupling-strip  44  to overlie the micro-strip  30  without having to rely on the third dielectric layer  48  to support the coupling-strip  44 . As such, the micro-strip transmission line  40  is characterized as having a characteristic impedance, the third dielectric layer  48  is characterized by a third dielectric thickness  58 , and the third dielectric thickness  58  is selected to minimize influence on the characteristic impedance of the micro-strip transmission line  40 . 
       FIGS. 4A and 4B  illustrate, respectively, a non-limiting example of a top view and end view of the apparatus  12  that includes a first connector  62  coupled electrically to the ground-plane  28  and the micro-strip  30  at a first end  64  of the apparatus, and a second connector  66  coupled electrically to the ground-plane  28  and the micro-strip  30  at a second end  68  of the apparatus that is opposite the first end  64 . It is noted that the first connector  62  and the second connector  66  are not shown only to simplify the illustration. The first connector  62  and the second connector  66  are preferably co-axial type connectors suitable for the frequency range of the RF signal  20  ( FIG. 1 ). A suitable connector is a Sub-Miniature version A (SMA) type connector available from a number of manufacturers. 
     It is noted that the third dielectric layer  48  has a substrate-length  60  shorter than the strip-length  36  so there is clearance to attach the first connector  62  and the second connector  66 . A suitable clearance for each connector is ten millimeters (10 mm) and so the substrate length of ninety millimeters (90 mm) was used for the prototype of the apparatus  12 . The width of the third dielectric layer  48  was selected to match the width of the first dielectric layer  32 , sixty millimeters (60 mm) for example, so support between the third dielectric layer  48  and the first dielectric layer  32  could be added if necessary to make the apparatus  12  robust. 
     The apparatus  12  may also include a third connector  72  coupled electrically to an input end  74  of the coupling-strip  44  via the feed strip  46 A; and a fourth connector  76  coupled electrically to an output end  78  the coupling-strip  44  via the feed strip  46 B. The third connector  72  and the fourth connector  76  are both preferably arranged along a first side  70  of the apparatus  12  adjacent the first end  64  and the second end  66  so the test system  10  can be neatly arranged as suggested in  FIG. 1 . 
     Accordingly, a test system  10 , a radio-frequency (RF) energy coupling apparatus (the apparatus  12 ) for electromagnetic interference (EMI) susceptibility testing of a device  14  is provided. The apparatus  12  provides for efficient coupling of RF energy into a wire  24 A of a wiring harness  24  connected to a device  14  that is being tested for EMI susceptibility. Due to efficient coupling of RF energy, a much lower power amplifier can be used to regenerate the RF signal  20  and no screen room facility is needed. The apparatus  12  has been demonstrated to be handy to easily debug BCI test failures in the device  14 . The coupling has been shown to be efficient for different configurations of the controller  22 . The prototype of the apparatus built to the dimensions described herein has a flat band response for frequencies from 50 MHz-1000 MHz. The test system  10  is able to replicate a BCI test anomaly without comprehensive EMC test facilities. 
     While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.