Abstract:
A waveguide assembly including a waveguide, a backshort member, and an adjustment member, where the adjustment member is capable of receiving or input and transforming it into an output that causes the backshort member to be displaced in response to said input.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a transition between a waveguide channel and a transmission line.  
           [0002]    It is well known in the prior art that electrical signals may be delivered through a variety of conductive media, such as solder traces, electrical wiring, coaxial or triaxial cables, waveguide channels, and microstrip lines, among numerous others. Usually, a given conductive medium will lend itself to a certain application, e.g. microcircuitry is better facilitated through the use of microstrip traces rather than triaxial cables.  
           [0003]    Often, a particular electrical application will require that an electrical signal transition between two or more types of conductive media. High-frequency testing of a silicon wafer serves as an effective illustration of this point. Such testing typically involves the interconnection of manufactured testing equipment with an electrical probe, the combination measuring voltages and/or currents at preselected nodes in the device-under-test (DUT) in response to a specific test signal.  
           [0004]    Wafer testing equipment is designed to be used repeatedly with a variety of test assemblies, and therefore includes input and output ports by which a particular probe system may be connected. Because coaxial adapters until recently have been unable to efficiently deliver signals above 65 GHz, frequently required for testing of today&#39;s high-speed semiconductor wafers, standard wafer testing equipment traditionally had been manufactured with ports that connect to waveguide channels, which are capable of delivering signals above 65 GHz.  
           [0005]    Probes, however, usually deliver the test signal to the DUT through either slender needles or contacts formed on a membrane that overlays the DUT. In addition, most wafer probe assemblies require shielding of the test signal to reduce undesired electrical coupling that may interfere with the test measurements.  
           [0006]    Accordingly, it is not uncommon for a probe assembly to allow a test signal to first transition from a waveguide to a coaxial line, then to a trace line that terminates at either a needle or a contact depending on the type of probe employed.  
           [0007]    Providing an efficient transition between a waveguide and a transmission line has proven problematic. For convenience, these types of transitions will be referred to as waveguide transitions. One widely used waveguide transition employs a waveguide channel into which the tip portion of a transmission line, such as the center pin of a coaxial cable, is inserted at a right angle to one of the interior surfaces of the waveguide. A backshort having a reflective face is also inserted into the waveguide. The backshort is typically made of brass and is oriented perpendicular to the waveguide channel so as to reflect the high-frequency signal towards the transmission line. The backshort is preferably located as close as possible to the transmission line. If properly positioned, the backshort will reflect the alternating signal within the waveguide into a standing wave pattern so that the signal will be induced in the transmission line with minimal degradation.  
           [0008]    The waveguide transition just described has a number of limitations. Because a waveguide channel cannot effectively transmit a DC signal, such a transition would be unable to deliver a high frequency signal together with a DC offset, required for example, to hold transistors in an active state during testing. Further, tuning of the waveguide transition is often difficult. Minimum signal transfer occurs when the backshort is spaced apart from the transmission line an integral multiple of one-half signal-wavelengths, while maximum signal transfer occurs at odd multiples of one-quarter signal-wavelengths. Thus at high frequencies, very small deviations from an optimal backshort position may lead to significant losses in signal transfer.  
           [0009]    An effective waveguide transition that may retain a DC offset is called a bias tee. Bias tees are used in a number of electrical configurations, including wafer probes. A bias tee typically includes a waveguide transition as previously described where the transmission line is a coaxial cable. A bias tee also includes a connection to a DC source that may provide a bias offset when desired. Any DC offset is combined with the alternating signal present within the waveguide channel by wiring the DC signal from the source to the center pin of the coaxial cable. Usually the DC signal is first passed through a choke so that any high-frequency signals induced in the coaxial cable by the waveguide are isolated from the DC source.  
           [0010]    Solutions to the difficulty encountered in tuning the waveguide transition are more problematical. With bias tees, current practice is to adjust the position of the backshort by hand. Traditionally, a backshort is constructed with a necked-down portion having low tensile strength that can be used as a handle. Conductive epoxy is applied around the perimeter of the backshort, which is then inserted into the waveguide channel. Adjustment of the backshort position within the waveguide channel is accomplished manually. Once the desired location of the backshort is obtained, the epoxy is cured by placing the bias tee in a heater. The handle is broken off and removed from the backshort.  
           [0011]    This accepted technique has a number of limitations. First, manual adjustment of the backshort does not permit effective fine-tuning, which becomes increasingly difficult at millimeter wavelengths where slight deviations in the backshort position can dramatically decrease performance. Second, if the backshort moves too far within the waveguide, bias circuit components can be damaged. Third, the backshort may shift during the curing process and the epoxy can seep into the waveguide channel which decreases performance. Fourth, once the backshort position is fixed, it is not suitable for a different test frequency range.  
           [0012]    In applications other than bias tees, a number of waveguide transitions have been developed that employ adjustable backshorts. Grote et al., U.S. Pat. No. 5,126,969, for example, disclose a W-Band waveguide variable oscillator having a brass backshort equipped with a locking screw. When the locking screw is released, the backshort may be moved manually, thereby adjusting the power output of the oscillator. Similarly, Simonutti, U.S. Pat. No. 4,835,495, discloses a sliding backshort that relies upon friction between the backshort and the surrounding waveguide to maintain the backshort in position unless the friction is overcome by hand pressure. Though these configurations allow the transition to be re-tuned to suit a variety of frequencies, in each of these mechanisms tuning of the backshort occurs by hand, with all of the attendant shortfalls discussed earlier.  
           [0013]    What is desired, therefore, is a waveguide transition having an adjustable backshort mechanism in which the backshort may be precisely positioned for maximum efficiency, without significant risk of overtravel and the attendant damage to circuit components. What is further desired is a waveguide transition with an adjustable backshort mechanism that, once adjusted, may be held in place without using conductive epoxy or a similar locking material within the waveguide channel. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 shows an exemplary embodiment of a bias tee that includes an adjustable backshort, a body portion, and a cap portion.  
         [0015]    [0015]FIG. 2 shows the adjustable backshort of the bias tee of FIG. 1 at an enlarged scale.  
         [0016]    [0016]FIG. 3 shows the body portion of the bias tee of FIG. 1 at an enlarged scale.  
         [0017]    [0017]FIG. 4 shows the cap portion of the bias tee of FIG. 1 at an enlarged scale. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Referring to the figures, wherein like numerals refer to like elements, FIG. 1 shows a bias tee  10  that is used to exemplify a preferred embodiment. It should be understood that other waveguide transitions exist apart from bias tees that may also benefit from the teachings herein. Some examples of alternate transitions are microstrip transitions, stripline transitions, and microwave antennas.  
         [0019]    The bias tee  10  allows an alternating electrical signal to transition from a waveguide  12  to a transmission line  14 , while also providing a DC offset voltage or current to be selectively added to the transmission line  14  from a connector  16 . In the preferred embodiment, the transmission line  14  is a coaxial cable, though a variety of other transmission lines, such as a triaxial cable, a single bare wire, etc. may be substituted for the coaxial cable depicted in FIG. 1. Preferably, the transmission line terminates in a connector. Alternatively, the transmission line may be terminated in probe contacts. Similarly, a number of connectors will appropriately provide the DC offset, but for illustrative purposes, the preferred embodiment depicts a right angle SSMC connector.  
         [0020]    As shown in FIG. 1, a portion of the coaxial cable  14 , including the center pin, protrudes into the waveguide  12 . A backshort member  18  with a reflecting face  22  is positioned at one end of the waveguide  12 . The backshort member  18  reflects an alternating signal present within the waveguide towards the center pin, thereby inducing within the coaxial cable  14  an alternating electrical signal desirably having approximately the same amplitude and frequency as that present within the waveguide  12 . A DC component may be selectively routed to the coaxial cable  14  from the connector  16 , thereby providing a DC offset to the induced alternating signal. Optionally, a choke  20  may electrically interconnect the connector  16  and the coaxial cable  14  to prevent the induced alternating signal from being transmitted through the connector  16 .  
         [0021]    Existing backshorts are designed to move in direct response to an input, such as hand pressure. The present inventor considered these existing backshorts, and determined that dramatic performance improvements may be achieved by operationally interposing an adjustment member  24  between the backshort  18  and any applied input. The adjustment member  24  receives an applied input, transforms it into an output that then controls the movement of the backshort  18 . Preferably, the output of the adjustment member  24  is less unwieldy than the input so that the reflecting face  22  may be moved to an appropriate position within the waveguide  12  with much more precision than that obtainable by previous design.  
         [0022]    In the preferred embodiment, a screw is used as the adjustment member  24 . As shown in FIG. 1, the screw  24  allows a rotational input applied at the screw head to be transformed into a transversal output applied on the backshort member  18 . This controllable adjustment of the position of the backshort  18  represents a dramatic improvement over existing designs in that the backshort  18  is capable of precise adjustment to obtain optimal tuning. Existing backshort mechanisms contained within waveguide transitions are either non-adjustable, or if adjustable, rely upon mere hand pressure to slide the backshort member  18  along the waveguide channel  12 . In the preferred embodiment, the adjustment member  24  allows the waveguide transition to be finely tuned, improving performance. Assuming, for example, that the adjustment member  24  is an 80 pitch screw and can be turned in 45 degree increments, a resolution of about 0.0016 inches may be achieved.  
         [0023]    Further, the preferred embodiment obviates any need to place conductive epoxy within the waveguide channel. If, for example, a screw is used as an adjustment member  24 , as described in the preferred embodiment, and it is desired that the backshort be permanently fixed in place, a thread-locking compound may be used on the screw  24 . The thread locking compound is preferably applied outside of the waveguide channel, eliminating any potential for epoxy to bleed into the waveguide channel. Alternately, the backshort need not be permanently positioned, but instead may be retuned.  
         [0024]    Because backshort movement within the waveguide channel may be positioned in much smaller increments in a controlled manner, there is a greatly reduced risk of damaging electrical components should the backshort be inadvertently pushed too far into the waveguide channel. The electrical components may include, for example, a crossover network and an out-of-band (waveguide band) signal termination for the bias tee. Again using a screw as an illustrative adjustment member  24 , should the backshort member  18  be moved further into the waveguide  12  than optimally desired, the direction of backshort travel may simply be reversed by turning the screw in the opposite direction. Preferably, a sprint  40  assists in reversing the path of the backshort.  
         [0025]    Though a screw is used to illustrate the manner in which the inclusion of an adjustment member  24  improves upon existing design, a variety of other devices or objects may be used as adjustment members. Examples might include a switch-activated electric positioner, a gear and pulley system operated by a handle, or a piezo-electric actuator. Similarly, the manner in which the input to the adjustment member is transformed may also vary. The adjustment member  24  may alter the nature of an applied input, the way the illustrative screw depicted in FIG. 1 converts a rotational input to a transversal output. Alternately, the adjustment member  24  may simply change the scale of an input, linearly or non-linearly, as would a gear and tooth assembly.  
         [0026]    Referring to FIG. 2, the backshort member  18  is preferably a unitary member, made from a casting or other process. In the preferred embodiment, the backshort member  18  includes a central elbow  25  having a supporting portion  26  and a cantilevered portion  27  oriented at substantially right angles to one another. The cantilevered portion  27  protrudes into the waveguide  12  and includes at its distal end a substantially planar reflecting face  22  oriented toward the coaxial cable  14 .  
         [0027]    The cantilevered portion  27  preferably has a width  29  and a depth  30  sized to fit securely within the waveguide  12  while retaining the ability to slide back and forth when the waveguide transition is being tuned. The cantilevered portion  27  has a length  31  measured from the supporting portion  26  preferably of sufficient length to permit the reflecting face  22  to closely approach the centerline of the coaxial cable  14 . The preferred embodiment has proven able to bring the reflecting face  22  to within 0.25 inches of the coaxial cable  14 , or closer. Other embodiments may have differing degrees of precision in this regard, though it should be noted that a waveguide transition performs better as these two elements are brought closer together. A stop (not shown) may be used to protect circuit components by limiting the movement of the backshort member  18  within the waveguide  12 .  
         [0028]    The backshort member  18  includes a base  32  from which the elbow  25  extends. The base  32  defines a hole  34  into which the screw  24  is engaged. The base  32  also includes two extensions  36  and  38  disposed laterally to either side of the hole  34 . As shown in FIG. 1, a plurality of spring members  40  are located within the body of the bias tee  10  on either side of the waveguide  12  to apply an outwardly directed force to extensions  36  and  38 , respectively. In the preferred embodiment, there are two such spring members  40 . Turning the screw  24  in one direction moves the reflecting face  22  inwardly into the waveguide channel  12 , compressing the spring members  40 . When compressed, the spring members  40  provide the requisite force to push the reflecting face  22  in an outwardly direction when the screw  24  is turned in the opposite direction.  
         [0029]    As shown in FIGS. 3 and 4, the bias tee  10  may be fashioned in two sections, namely, a bias tee body  42  and a bias tee cap  44 . The bias tee body  42  and the bias tee cap  44  are designed to be engaged through a selective number of fastening cavities  70   a  and  70   b  contained in the bias tee body  42  and the bias tee cap  44 , respectively.  
         [0030]    Referring to FIGS. 3 and 4, the bias tee body  42  forms a lower waveguide surface  50 A comprising three of the walls of the waveguide  12 . The bias tee cap  44  forms a waveguide ceiling  50 B that defines the fourth wall of the waveguide  12 . The lower waveguide surface  50 A and the waveguide ceiling  50 B are preferably composed of a conductive material suitable for the transmission of electromagnetic waves at frequencies up to and above 65 GHz.  
         [0031]    The bias tee body  42  also defines a coaxial cable port  54  within the lower wall of the lower waveguide channel surface  50 . A connector port  52  contained within a connector cavity  53  facilitates the attachment of a connector  16  that may route a signal from a DC power supply (not shown) to the coaxial cable  14  fitted within the coaxial cable port  54 . An opening  60  is defined by the side of the lower waveguide surface  50   a  to permit this connection. The connector cavity  53  preferably provides sufficient space so that, if desired, a choke  20  may be inserted between the connector  16  and the coaxial transmission line  14 .  
         [0032]    The bias tee body  42  includes a shelf portion  62 A, and the bias tee cap  44  includes a lip portion  62 B, both located at the side of the bias tee  10  with the backshort member  18 . As can be seen in FIGS. 3 and 4, the shelf portion  62 A of the bias tee body  42  and the lip portion  62 B of the bias tee cap  44  are sized so that when the bias tee body  42  and the bias tee cap  44  are engaged, a space is provided within which the backshort member  18  may be fitted.  
         [0033]    A threaded hole  56 A is defined by the shelf portion  62 A of the bias tee body  42  and an outer hole  56 B is defined by the lip portion  62 B of the bias tee cap  44 . As can be seen in FIG. 1, when assembled, the screw  24  may be inserted into the outer hole  56 B in the bias tee cap  44 , through the backshort member  18  and into the threaded hole  56 A in the bias tee body  42 . In this fashion, the adjustable backshort  18  may be readily tuned simply by turning the adjustment screw  24 . Bias tee body  42  defines two cylindrical cavities  58  and  59 , into which spring members  40  may be interested. Cylindrical cavities  58  and  59  are spaced symetrically about, and parallel to, the lower waveguide surface  58 A.  
         [0034]    It is to be understood that the adjustable backshort may likewise be used in other waveguide-to-transmission line structures apart from bias tees.  
         [0035]    The terms and expressions employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.