Abstract:
A tester comprises a first antenna that includes a cylinder formed from a dielectric material, a ground plane covering at least a portion of the outer surface of the cylinder, and a conductor covering a portion of the inner surface of the cylinder and located at approximately the axial midpoint of the center. The tester also includes a test fixture for holding a device under test and interfacing it with the first antenna.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to air interface testing of wireless communication devices and more particularly to systems and methods for a closed field antenna used for such testing. 
     2. Background 
     A wireless communication device typically includes a Radio Frequency (RF) port, or RF connector, that can be used to test the RF performance of the device. When using the RF port to test the device, a “tester” is interfaced to the device via the RF port. RF signals are then sent to the device and are received from the device through the RF port. The RF port, therefore, typically bypasses the device&#39;s antenna, which is an important factor in the RF performance of the device. Therefore, to fully test the device&#39;s RF performance, the antenna must be taken into account. This is accomplished through testing known as “air interface testing.” An air interface test is designed to test the RF performance of a wireless communication device by sending RF signals to, and receiving RF signals from, the devices antenna. 
     Unfortunately, simulating the air interface in a manufacturing or test facility is a challenging task. First, the device under test (DUT) must be isolated from external, interfering signals that can cause erroneous test results. This is especially important in the factory, where many devices are being tested simultaneously. Second, the DUT must also be isolated from reflections of its own signals, which can cause self-interference, or self-jamming. In a confined environment, RF signals transmitted by a DUT can reflect off of near by objects and interfere with the test, causing erroneous result much like interfering signals from other devices. 
     In order to combat interference from externally generated signals, the DUT is typically placed inside a metal isolation box. The box prevents the externally generated signals from interfering with the air interface testing of the DUT. Unfortunately, the box makes the second problem, i.e., self-jamming, worse, because the inside of the box reflects the signals transmitted by the DUT. Therefore, the inside of the box is usually covered with RF absorbing material. But the RF absorbing material is very expensive and it is very difficult to achieve adequate absorption. 
     Another problem with conventional systems is a lack of reliability and repeatability of test results. This is partly due to the varying isolation and absorbing properties of different test boxes. It is also partly due to the alignment of the DUT within the box, which as a result of conventional test box designs is critical to achieving accurate results. In practice, however, it is very difficult to achieve accurate alignment tolerances with most conventional test boxes. Therefore, alignment issues and interference from internal and external signals makes it difficult to achieve repeatable, reliable test data that can be correlated from one test fixture to the next. 
     Due to the forgoing, conventional test fixtures/boxes tend to be large, expensive units that fail to produce reliable, repeatable, and comparable test data. Thus, correlation of test results for all DUTs across all test fixtures in a particular factory, for example, is difficult if not impossible. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention a tester useful for combating the above problems comprises a first antenna that includes a cylinder formed from a dielectric material, a ground plane covering at least a portion of the outer surface of the cylinder, and a conductor covering a portion of the inner surface of the cylinder and located at approximately the axial midpoint of the center. The tester also includes a test fixture for holding a device under test and interfacing it with the first antenna. 
     In one embodiment, the test fixture is configured to hold the device under test, which comprises a second antenna, and position the device under test such that the second antenna is substantially within the cylinder. 
     Other aspects, advantages, and novel features of the invention will become apparent from the following Detailed Description of Preferred Embodiments, when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present inventions taught herein are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings, in which: 
     FIG. 1 is a diagram illustrating an example embodiment of a closed field antenna in relation to a device under test in accordance with the invention; 
     FIG. 2A is a side view of a cross section of an example closed field antenna such as the antenna in FIG. 1; 
     FIG. 2B is a cross sectional view of the example closed field antenna of FIG. 2A; 
     FIG. 3 is a diagram illustrating an example embodiment of a closed field antenna with an example interface to a test system in accordance with the invention; and 
     FIG. 4 is a logical block diagram of an example test system that includes a closed field antenna such as the antenna in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the following discussion relates general to testing and test fixtures for wireless handsets, it will be apparent that the systems and methods described herein can be used to test over the air performance for any wireless transmitter. Implementations for different types of transmitters can require adjustments in the geometry and alignments of various components; however, these types of adjustments do not effect the basic operation or functionality of the systems and methods described herein. 
     The invention addressees the problems mentioned above using a closed field antenna to interface an air interface tester with the antenna of a DUT. Preferably, the closed field antenna comprises a cylinder, the outside of which is preferably plated or covered with a conductive material that acts as a ground plane. This ground plane reduces the need for absorbing material within a test box, for example, because the RF signals from the DUT are largely confined within the cylinder by the ground plane. Additionally, the ground plane also shields the DUT from external interference as well, thus eliminating the need for a test box entirely. As a result, a small, inexpensive test fixture can be designed using such a closed field antenna. Moreover, for reasons that will be explained more fully below, such a test fixture can provide repeatable, reliable test data that can be correlated from one test fixture to the next. 
     FIG. 1 is a diagram illustrating an example implementation of a closed field antenna  100  in accordance with the systems and methods described herein. As can be seen, closed field antenna  100  comprises a cylinder  110 . On the inside of cylinder  110 , there is a conductor  104 , which is configured to transmit RF signals to and receive RF signals from DUT antenna  108 . The outside of cylinder  1   10  is covered with a ground plane  102 . It is ground plane  102  that confines the signals transmitted and received by conductor  104  within cylinder  110 . 
     Antenna  100  is preferably configured so that conductor  104  is positioned approximately at the midpoint of DUT antenna  108 , when DUT  106  is positioned for testing. The width (d) of conductor  104  is a function of the wavelength (λ) of the signals transmitted and received by conductor  104 . For example, for an 800 MHz application, d can be chosen to be a length approximately equal to ¼λ at 800 MHz. Further, the length (l) of cylinder  110  is preferably an odd multiple of the width (d) of conductor  104 . In the embodiment illustrated in FIG. 1, for example, the length (l) of cylinder  110  is equal to three times the width of conductor  104 , i.e., l=3×d. Thus, there is a distance d from conductor  104  to either end of cylinder  110 . DUT antenna  108 , however, radiates along its whole length. Therefore, it may be necessary to extend the length (l) of cylinder  110  to the length of antenna  108  so that ground plane  102  will adequately contain the radiating energy and provide adequate shielding from external interference. If the length (l) of cylinder  110  is extended, then the length of cylinder  110  is preferably still an odd multiple of d, such as 5d, 7d, etc. 
     Cylinder  110  is constructed from a dielectric material. Thus, the dielectric separates ground plane  102  from conductor  104 . The thickness (t) of the dielectric material should be sufficient to result in a very low capacitance between ground plane  102  and conductor  104 . The low capacitance enables broadband operation such that multiple ranges can be tested, such as both ranges in a dual mode application. For example, a conductor with a length (l) equal to ¼λ at 800 Mhz is also approximately equal to {fraction ( 5 / 4 )}λ at 1.9 GHz. Such a conductor  104  can be used for a dual mode 800 MHz1.9 GHz application if a thickness (t) is selected for the dielectric material that allows conductor  104  to operate over the range from 800 MHz to 1.9 GHz. 
     FIG. 2A illustrates a side view of a cross section of a closed field antenna  200  and is useful to illustrate the impact of the thickness (t) of the dielectric material. As can be seen, conductor  202  of length (d) is separated from ground plane  206  by cylinder  204 , which is made of a dielectric material. The view shown in FIG. 2A makes it easy to see that conductor  202 , cylinder  204 , and ground plane  206  form a capacitor, wherein conductor  102  and ground plane  206  act as the capacitor plates. The capacitance of a capacitor is affected by the area of the plates that form the capacitor, the distance between the plates, and the ability of the dielectric to support electrostatic forces. Larger plates provide greater capacity to store electric charge. Therefore, as the area of the plates increases, capacitance also increases. This means that the length (l) and width (d) of ground plane  206  and conductor  202 , respectively, will have a direct impact on the capacitance between the two. Thus, the effect of these dimensions on the capacitance must be taken into account for each application and can vary depending on the implementation. 
     Capacitance is also directly proportional to the electrostatic force field between the plates. This field is stronger when the plates are closer together. Therefore, as the distance between the plates decreases, i.e., (t) is decreased, capacitance increases. Therefore, to obtain the required capacitance, the thickness (t) will also need to vary depending on the implementation as well as variables such as the length (l) of ground plane  206  and/or the width (d) of conductor  202 . Because the selection of these other parameters, e.g., the length (l) and width (d), can be effected by other implementation concerns, the thickness (t) of the dielectric is preferably used to control the capacitance formed between conductor  202  and ground plane  206 . For example, the width (d) of conductor  202  is dependent on the wavelength of the signals that it will transmit and receive, as explained above. Further, the length (l) of ground plane  206  is dependent on the length of the cylinder and/or the length of the DUT&#39;s antenna. Therefore, once (l) and (d) are selected as required by these other concerns, the thickness (t) of cylinder  204  can be selected to provide the requisite capacitance. 
     It should also be noted that the dielectric material used for cylinder  204  will impact the capacitance formed between conductor  202  and ground plane  206 . Dielectric materials are rated based upon their ability to support electrostatic forces in terms of a number called a dielectric constant. The higher the dielectric constant the greater the ability of the dielectric to support electrostatic forces, which means that as the dielectric constant increases, capacitance increases. But the selection of the material used to construct cylinder  204  can also be constrained by other parameters, such as price or availability of the material. As a result, the type of dielectric material may need to be selected for reasons unrelated to the capacitance required between conductor  202  and ground plane  206 . 
     Therefore, the thickness (t) is preferably the primary manner to control the capacitance between conductor  202  and ground plane  206 , but clearly there are numerous design choices that will impact the capacitance. The designer should keep such impact in mind when making such choices for each application. 
     FIG. 2B illustrates a cross sectional view of cylinder  200 , with a DUT antenna  208  positioned within cylinder  200  for testing. Thus, test signals will travel between conductor  202  and DUT antenna  208  in order to test the over the air performance of the DUT. Because DUT antenna  208  is shielded both from external interference and from interference generates by its own transmissions, the tolerance on the position (r) of DUT antenna  208  is relatively loose. Therefore, DUT antenna  208  just needs to be approximately in the center of cylinder  200 . The tolerance on the position of conductor  202  relative to DUT antenna  208  is also relatively loose. Thus, DUT antenna  208  needs to only be positioned within cylinder  200  such that conductor  202  will be at approximately the midpoint of DUT antenna  208 . 
     Because of the wide tolerances, and immunity to interference that inheres from the use of closed field antenna  200 , the reliability and repeatability of the test results achieved are greatly increased. Moreover, a high degree of correlation between fixtures is achievable. Thus, the use of a closed field antenna  200  allows for the construction of an inexpensive fixture that not only improves reliability and repeatability, but also provides a high degree of correlation between fixtures. 
     The cost of a test fixture that incorporates a closed field antenna, such as antenna  200 , can be reduced even further by an appropriate selection of material for use in constructing the closed field antenna. The attributes and functionality of such a closed field antenna allow for the use of common, inexpensive material in its design. For example, a PVC pipe and copper tape can be used for the cylinder and ground plane, respectively. The conductor can also be formed as a microstrip antenna using copper tape or equivalent. A microstrip transmission line is a type of printed circuit construction, consisting of a track of copper or equivalent conductor on an insulating substrate. Additionally, there is a backplane on the other side of the insulating substrate, which is formed from a similar material as the conductor. 
     Referring again to the view of cylinder  200  illustrated in FIG. 2B, it is easy to see that conductor  202 , cylinder  204 , and ground plane  206  form a microstrip transmission line. Thus, conductor  204  is preferably formed from copper tape or the equivalent, which is common and widely available it is also common to use plastic for the insulating substrate in microstrip transmission line construction, so as mentioned above simple PVC will suffice for the cylinder. The result is an extremely efficient and low cost test fixture in which alignment tolerances are much less critical than conventional fixtures. Testing will of course produce some more precise alignment tolerances, but the results from test fixture to test fixture should be highly correlative. 
     To complete the construction of a closed field antenna in accordance with the systems and method for using a closed field antenna for air interface testing, a means for interfacing the closed field antenna with the test equipment is required. Example closed field antenna  300  in FIG. 3 illustrates one embodiment of such an interface. The interface comprises a RF connector  306  that can be connected to the test equipment. RF connector  306  is configured to pass RF signals  312  received from the DUT to the test equipment, where they can be evaluated, and to receive RF signals from the test equipment, which are then transmitted to the DUT via conductor  304 . 
     The interface from RF connector  306  to conductor  304  preferably consists of some form of RF transmission line  318 . Thus, for example the portion  316  of transmission line  318  that is inside cylinder  302  can preferably comprise a microstrip transmission line. In one embodiment, RF connector  306  is then coupled to RF transmission line  318  right at the edge of cylinder  302 . Alternatively, RF transmission line  318  can also comprise a portion  308  that extends beyond cylinder  302 , which preferably consists of a cabled RF transmission line complete with shielding to prevent both external interference and interference from the signals  312  and  314  generated during testing. 
     The RF impedance of RF transmission line  318  must match that of conductor  304  otherwise RF energy arriving at the interface between RF transmission line  318  and conductor  304  will be reflected back in the direction from which it came. Thus, an impedance matching network  310  is preferably included that is configured to match the impedance of transmission line  318  with that of conductor  304 . In this manner reflections are reduced and the efficiency of conductor  304  is increased. 
     FIG. 4 illustrates a logical block diagram of an example air interface tester  400  that utilizes a closed field antenna in accordance with the systems and methods described herein. Air interface tester  400  comprises computer  404  and test equipment  402 . Test equipment  402  can, for example, comprise a RF signal generator and an RF signal receiver. Computer  404  runs a software program that controls the testing. Test equipment  402  is interfaced with a closed field antenna  408 , and a DUT  410  positioned so that its antenna is inserted within closed field antenna  408 . Computer  404  then controls, based on the software program, the generation of signals to be transmitted to DUT  410  and the evaluation of signals received from DUT  408 . Preferably tester  400  also includes an output device  406 , which can, for example, be a printer to print out the test results and/or a display to display the test results both during and after testing. 
     Another advantage of the immunity to interference and wide tolerance of a closed field antenna designed in accordance with the systems and methods described herein, is that setup and calibration for an air interface tester, such as tester  400 , is quick an reliable. This not saves time and money and allows for easy and quick reconfiguration for different DUTs. Thus, use of a closed field antenna  408  in an air interface tester, such as tester  400 , provides repeatable, reliable test data that is highly correlative with other similar fixtures and that can be reconfigured quickly and reliably. Moreover, such a tester can be designed using inexpensive common material, which provides a tremendous cost savings. 
     While embodiments and implementations of the invention have been shown and described, it should be apparent that many more embodiments and implementations are within the scope of the invention. Accordingly, the invention is not to be restricted, except in light of the claims and their equivalents.