Patent Publication Number: US-7221245-B2

Title: Balanced microwave cable adaptor having a connector interface secured by a slidable nut

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This patent application is a continuation-in-part of U.S. patent application Ser. No. 10/309,543, entitled BALANCED MICROWAVE CONNECTOR AND TRANSITION, filed Dec. 4, 2002 by Hassan Tanbakuchi, Paul E. Cassanego, and Kenneth H. Wong, which issued on Aug. 30, 2005 as U.S. Pat. No. 6,937,109 B2. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO MICROFICHE APPENDIX 
   Not applicable. 
   FIELD OF THE INVENTION 
   The present invention relates generally to high-frequency components and more particularly to a cable having a connector interface with two coaxial microwave structures. 
   BACKGROUND OF THE INVENTION 
   High-frequency connectors are used in cable ends, package feedthroughs, adaptors, probes, and similar applications. Connector interfaces typically provide a single coaxial structure that maintains the characteristic impedance of the transmission line through the connector. Balanced techniques, which use two high-frequency transmission lines, are desirable in some applications because they can provide a larger signal and superior noise immunity compared to unbalanced techniques, but generally involve making twice as many connections to a device or circuit. 
   Balanced cables are presently available with two coaxial cables that are joined within a single cable housing for most of the length of the cable, but these balanced cables are basically two coaxial cables with regular coaxial cable ends. Joining the cables together for most of their length avoids some inter-cable movement and keeps the cables reasonably balanced, but connecting the cables to a device requires connecting each of the cable ends causing relative movement between the cable ends that can introduce measurement error or uncertainty. Other presently available types of balanced cables extend center conductors of two coaxial transmission lines through a single connector without maintaining the coaxial structures of the transmission lines through the connector. While these types of balanced cables are typically used at low frequencies (e.g. below 200 MHz), they are not well suited for use in high-frequency applications. 
   BRIEF SUMMARY OF THE INVENTION 
   An adaptor includes a connector interface having a first coaxial structure with a first center pin configured to be coupled to a first center conductor of a first coaxial transmission line and a second coaxial structure with a second center pin configured to be coupled to a second center conductor of a second coaxial transmission line. A nut surrounds the first coaxial structure and the second coaxial structure. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a simplified perspective view of a connector interface according to an embodiment of the present invention incorporated in a package launch. 
       FIG. 1B  is a simplified perspective view of a connector interface according to another embodiment of the present invention incorporated in the end of a balanced cable. 
       FIG. 1C  shows a cross section of the connector interface of  FIG. 1A  connected to the connector interface of  FIG. 1B . 
       FIG. 1D  is a simplified perspective view of a connector interface according to another embodiment of the present invention incorporated in a package launch. 
       FIG. 2A  shows an electronic device with connector interfaces according to the present invention coupled to a vector network analyzer with balanced cables. 
       FIG. 2B  is a simplified perspective view of a connector interface incorporated in the end of a balanced cable according to an alternative embodiment of the present invention. 
       FIG. 3A  shows a connector interface according to an embodiment of the present invention incorporated into an adaptor assembly connected to a package launch. 
       FIG. 3B  shows the adaptor assembly of  FIG. 3A  with the slidable nut retracted. 
       FIG. 3C  shows the adaptor assembly of  FIG. 3A  with the slidable nut extended. 
       FIG. 3D  is a cross section of a portion of the adaptor assembly of  FIG. 3A . 
       FIG. 4A  is an isometric view of an adaptor connected to a connector body according to an embodiment of the invention. 
       FIG. 4B  shows a cross section of the adaptor of  FIG. 4A . 
       FIG. 5A  is an isometric view of an adaptor according to another embodiment of the invention. 
       FIG. 5B  is a simplified cross section of the adaptor of  FIG. 5A . 
       FIG. 6A  is a front view of a connector body according to an embodiment of the invention. 
       FIG. 6B  is a cross section taken along A—A of  FIG. 6A . 
       FIG. 7  is a front view of a connector body according to another embodiment of the invention. 
       FIG. 8  shows adaptor assemblies illustrated in  FIG. 3A  connecting an electronic device having conventional package feedthroughs to a balanced vector network analyzer. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   I. Introduction 
   A connector interface constructed according to the embodiments of the present invention includes two coaxial structures within a single connector provides superior balanced high-frequency performance and allows closer pin spacing compared to conventional coaxial connectors. Balanced high-frequency techniques are used in a variety of applications, such as digital communication analysis, digital oscilloscopes, wafer testing, differential vector network analysis, or to run separate signals side-by-side, such as a test signal with a clock signal or a test signal with a reference signal. Conventional balanced measurement techniques use a pair of connectors. If conventional connectors are used to connect coaxial transmission lines to an electronic circuit, such as a printed wiring board (“PWB”), differential probe, integrated circuit, or thick-or thin-film hybrid microcircuit, the connectors are spaced far apart, to allow for connecting and disconnecting each connector. It is difficult to achieve high-frequency balanced circuits with the spacing resulting from paired conventional connectors. 
   II. Exemplary Connectors 
     FIG. 1A  is a simplified perspective view of a connector interface  9  according to an embodiment of the present invention incorporated in a package launch  10 . The package launch includes mounting flanges  12 ,  14  with through holes  16 ,  18  for attaching the package launch to a package of an electronic device. Two coaxial structures  20 ,  22  are incorporated into the connector interface. The coaxial structures typically correspond to a connector standard, such as 1.0 mm, 1.85 mm, 2.4 mm, SMA, or other connector standard. Alternatively, the coaxial structures are not in accordance with any connector standard. It is not necessary that each coaxial structure within a connector interface have the same dimensions. In one example, each coaxial structure conforms to a 1.85 mm connector standard, with center pins  24 ,  26  supported within the conductive outer walls  28 ,  30  of the coaxial structures. The center pins are male-female type, but alternatively are overlapping or butt-contact center pins, which are known as sexless connectors. 
   The 1.85 mm connector standard provides high-frequency performance up to 70 GHz. The center pins have compliant fingers to accept a mating center conductor (see  FIG. 1B , ref. nums.  46 ,  48 ). Connectors with center pins that accept center conductors, such as the differential package launch interface illustrated in  FIG. 1A , are typically referred to as “female” connectors, and the corresponding connectors with protruding center conductors or pins are referred to as “male” connectors. 
   A barrel  32  includes threads  34  for securing a nut captivated on the mating part (see  FIG. 1C , ref. num.  76 ) configured to screw onto the threads. In one alternative, the nut is on the barrel and the mating connector part is threaded. In another alternative, a bayonet-type, snap-on, or other mechanical coupling technique is used. An alignment feature  36  polarizes the connector interface and aligns the center conductors of the mating parts, as well as prevents twisting of one part relative to the other when the nut is tightened. The alignment feature is a countersunk hole that is configured to accept an alignment pin (see  FIG. 1B , ref. num.  54 ), which is typically rounded or chamfered to facilitate insertion into the hole. In a particular embodiment, each half of a connector interface pair includes a pin and an alignment hole corresponding to the alignment hole and pin on the mating part. In another embodiment, one half of a connector interface pair has two pins, and the mating part has two alignment holes. The pins and holes may be offset or of different diameter to further prevent misalignment. Polarization of the connector interface insures that the correct coaxial structures are coupled to their respective transmission lines on the mating part. Other alignment features, such as a key and slot outside the barrel of the connector interface are alternatively used. 
   It is generally desirable that the alignment pin contacts the alignment feature before the center pins contact the center conductors. The mating part also has a rim that contacts the inner diameter  38  of the connector interface. The rim works in conjunction with the alignment pin to guide the center conductors into the center pins without twisting the center conductors with respect to the center pins. Twisting might deform the center conductors and/or center pins, and might even break fingers off of the center pins. Even if the center conductors and center pins are not permanently bent, misalignment or twisting of the connectors can degrade measurement accuracy. The center pins and center conductors of conventional connectors having radial symmetry are typically not deformed or broken by mere twisting between the mating connector parts. To ensure that the outer conductors of the connectors make electrical contact around the 1.85 mm bores, the surface around the bores of the male connector may be raised slightly to minimize the impact of surface flatness. 
     FIG. 1B  is a simplified perspective view of a connector interface  9 , according to another embodiment of the present invention incorporated in the end of a balanced cable  41 . This connector interface  9 , is configured to mate with the connector interface  9  illustrated in  FIG. 1A . The barrel  42  of the connector interface includes a rim  44  that is partially inserted into the inner diameter (see  FIG. 1A , ref. num.  38 ) before the center conductors  46 ,  48  of the coaxial transmission lines  50 ,  52  contact the center pins of the connector interface on the package launch. A pin  54  is also partially inserted into the alignment feature (see  FIG. 1A , ref. num.  36 ) before the center conductors contact the center pins. A nut (not shown in  FIG. 1B  for clarity of illustration) is retained by ridges  56  on the connector end, allowing the nut to spin as it is tightened onto the threads of the package launch to secure the face  58  of the connector interface on the balanced cable against the opposing face of the connector interface on the package launch. To facilitate the proper orientation of the alignment pin to the alignment feature, the coupling nut or mechanism may be configured to be retractable so that the alignment pin is visible and can be oriented to align with the alignment features. 
     FIG. 1C  shows a cross section of the connector interface of  FIG. 1A  connected to the connector interface of  FIG. 1B . The package launch  10  is shown mounted on a circuit package  60 . The screws that would typically be inserted through the mounting holes  16 ,  18  of the package launch and screwed into the screw holes  62 ,  64  of the circuit package are omitted for clarity of illustration. 
   The center pins  24 ,  26  of the connector interface of the package launch  10  are supported with dielectric stand-offs  66 ,  68  inside the coaxial structures and accept the center conductors  46 ,  48  of the two coaxial cables  70 ,  72  in the balanced cable  41 . A cable end  74  is machined from metal and securely holds the ends of the coaxial cables. The coaxial cables may be semi-rigid coaxial cables that include center conductors separated from outer conductors by dielectric spacers. The balanced cable is filled with compliant polymer  75  to support the coaxial cables and generally maintain their relationship to each other as the balanced cable is bent. A nut  76  on the cable end  74  engages the threads on the package launch  10  to securely connect the mating connector interfaces. Alternatively, the nut is provided on the package launch and the cable end is threaded. Similarly, the package launch is alternatively a male connector, and the cable end is a female connector. Alternatively, the cable end may be connected to a twin coaxial structure such that the other end of the coaxial structure are made with the connector features of  FIG. 1B . 
   In a particular embodiment, the nut  76  is a slidable nut that may be slid backwards (retracted) to expose the center conductors  46 ,  48  of the two coaxial cables  70 ,  72  in the balanced cable  41  and an alignment pin (not shown, see  FIG. 1B , ref. num.  54 ). Providing a slidable nut is particularly desirable with connector interfaces having two coaxial structures because it allows accurate, concurrent alignment of the alignment pin and of the two coaxial structures. Viewing conventional connector interfaces having a single coaxial structure as they are brought together is not critical because there is not a pin or other structure to align with a mating feature. Generally, conventional single-coaxial connectors may be rotated about the center axis. 
   Feedthrough pins  78 ,  80  extend from the opposite (distal) end of the package launch through glass feedthroughs  82 ,  84  into the interior of the circuit package  60 . The feedthrough pins may then be electrically connected to an electronic circuit  86 . The feedthrough pins include a glass-to-metal seal, which seals the circuit package. Alternatively, the feedthrough pins extend into the package without a glass-to-metal seal. 
     FIG. 1D  is a simplified perspective view of a connector interface  9  according to another embodiment of the present invention incorporated in a package launch. A first coaxial structure  20 ′ includes a male center conductor  24 ′ and a second coaxial structure  22 ′ includes a second male center conductor  26 . The connector interface  9  also includes the mounting flange  12 , barrel  32  and alignment feature  36 , as described above in reference to  FIG. 1A . 
   III. Balanced VNA Measurements and Adaptors 
     FIG. 2A  shows an electronic device  102 , commonly referred to as a device under test (“DUT”), with connector interfaces  104 ,  106  according to the present invention coupled to a vector network analyzer (“VNA”)  100  with balanced cables  41 ,  41 ′. Each balanced cable contains two coaxial transmission lines and has a cable end with a connector interface according to the present invention that is connected to the corresponding connector interface of the electronic device. 
     FIG. 2B  is a simplified perspective view of a connector interface  110  incorporated in the end of a balanced cable according to an alternative embodiment of the present invention. The balanced cable is similar to the balanced cable illustrated in  FIG. 11B ; however, the connector interface is a female connector interface, similar to the female connector interface illustrated in  FIG. 1A , rather than the male connector interface illustrated in  FIG. 11B . The connector interface has two coaxial structures  112 ,  114  with center pins  116 ,  118  that accept center conductors of the mating connector part. An alignment feature  36  keeps the connector interface from twisting when connecting or disconnecting the mating part. 
     FIG. 3A  shows an adaptor assembly  130  with a connector interface  136  according to an embodiment of the present invention connected to a package launch  10 . The adaptor assembly joins two coaxial cables  132 ,  134 , such as semi-rigid coaxial cable, into the connector interface  136 . A slidable nut  137  on the package launch engages threads on the connector interface  136  of the adapter assembly  130 . The opposite ends of the coaxial cables have conventional connector ends  138 ,  140 , such as 1.85 mm or 2.4 mm cable ends. 
   The package launch provides differential feedthrough pins  78 ,  80  that are about 3 mm apart. Providing differential feedthrough pins in such close proximity facilitates electrical connection to PCBs, microcircuits, or integrated circuits (“ICs”) and enables measurement of common-mode and differential-mode signals. The connector interfaces on the adaptor and the mating connector interface on the package launch are referred to as “differential connectors” for purposes of discussion. In a particular embodiment, a differential connector is used with a wafer probe to provide accurate, high-frequency measurements of unpackaged ICs. It is desirable that the feedthrough pins are not more than 5 mm apart (center-to-center) to facilitate the transition from the connector interface to a balanced device or circuit. In particular, it is desirable to avoid having to change the spacing between balanced transmission lines on a circuit to accommodate pin spacing. Balanced transmission lines are usually parallel, and introducing an angle between the balanced transmission lines can cause unwanted radiation patterns. Balanced transmission lines on circuits packaged using conventional side-by-side coaxial connectors usually diverge near the package wall to accommodate the wider pin spacing (typically about 11 mm), which alters the characteristics of the balanced transmission lines. 
   Package launches according to embodiments of the present invention can provide pins 2 mm apart, and in another embodiment, 3 mm apart. A pin spacing of about 3 mm (±10%) is particularly desirable for connecting to balanced high-frequency circuits and devices because it allows connecting the pins to parallel, balanced transmission lines, thus maintaining superior transmission characteristics at high frequencies. Alternatively, a 5 mm spacing or a 7 mm pin spacing is provided by other embodiments of the present invention. 
   The adaptor assembly  130  can be used to connect a balanced test cable to an electronic device with conventional differential package launches, to connect an electronic device having a package launch with a connector interface according to an embodiment of the present invention to a conventional VNA, or to use a balanced test cable to perform a two-port measurement (or a four-port measurement with two balanced test cables and two adaptors), for example. The part of a connector pair with the nut is typically the male part; however, adaptor assemblies are alternatively male-male, male-female, female-male, or female-female, and the differential connector interface  136  of the adaptor assembly  130  is alternatively threaded. 
     FIG. 3B  shows the adaptor assembly  130  of  FIG. 3A  with the slidable nut  137  retracted. Retracting the slidable nut  137  exposes the pin  54  and the face  139  of the connector interface. This allows an operator to align the pin  54  to a mating hole or other alignment feature as the face  139  of the connector interface is aligned to a mating connector interface. The slidable nut  137  is then slid forward (extended) to engage threads on the mating connector interface. This avoids the nut from obscuring the operator&#39;s view when aligning the pin to its mating hole. 
     FIG. 3C  shows the adaptor assembly  130  of  FIG. 3A  with the slidable nut  137  extended. Once the connector interface is aligned to its mating interface, the nut is slid forward (extended) to engage mating threads and secure the connector interfaces to each other. 
     FIG. 3D  is a cross section of a portion of the adaptor assembly of  FIG. 3A . The slidable nut  137  is captivated on a connector body  141  with a C-ring  143 . The C-ring  143  forms a back stop and a ridge  145  of the connector body  141  forms a forward stop that a foot  147  of the slidable nut  137  slides between. Female-female center pins  149 ,  151  adapt the center conductors  153 ,  155  of the coaxial cables  132 ,  134  to a female-type connector interface. The center pins  149 ,  151  are held in the connector body  141  with dielectric standoffs  157 ,  159 . 
   In some embodiments, the dimensions of the coaxial cable center conductors are suitable for directly connecting them to a mating connector interface (see, e.g.,  FIG. 1B ). In other embodiments, it is desirable to provide a transition from the dimensions of the coaxial cable to a connector interface having more suitable dimensions for a particular connector interface standard. Similarly, the center conductors of coaxial cables are often relatively soft copper or silver-plated copper. This allows convenient bending of the cable, but the copper center conductors might not withstand the repeated connecting and disconnecting that arises in some applications, such a microwave component testing. 
     FIG. 4A  is an isometric view of an adaptor  160  connected to a connector body  161  according to an embodiment of the invention. The adaptor adapts two coaxial cables  132 ,  134  to a connector interface  162 . Alternatively, the adaptor adapts a balanced cable having to coaxial cables to a connector interface (see  FIG. 1C ). The first slidable nut  137  slides relative to the connector body  161 , and a second slidable nut  170  slides relative to an adaptor barrel  172 . 
   The connector interface  162  includes two male-type coaxial structures  164 ,  166  and a pin  54 . A raised ground plane  167  surrounds the coaxial structures  164 ,  166 . The raised ground plane  167  is essentially a mesa-type feature that extends a selected height above the field  168  of the connector interface  162 . The selected height is typically about 0.08 mm to about 0.5 mm. The raised ground plane contacts the face of a mating connector, either on a flat face on at another raised ground plane area so that the ground-to-ground electrical coupling occurs close to the coaxial structures, which in turn provides superior transmission characteristics. 
     FIG. 4B  shows a cross section of the adaptor  160  of  FIG. 4A . The adaptor  160  includes two female-to-male center pins  174 ,  176  disposed in the adaptor barrel  172  with dielectric standoffs  178 ,  180 ,  182 ,  184 . In a particular embodiment, the center pins  174 ,  176  are made of metal that is harder than the center conductor material (typically copper or silver-plated copper) of the coaxial cables. This provides a more rugged connector interface capable of being connected and disconnected more times without significant degradation of transmission characteristics. In a particular embodiment, the center pins are made from a beryllium-copper alloy and are gold plated. Alternatively, the center pins are made from an iron alloy, such as stainless steel, and are plated or unplated. 
   In a further embodiment, the adaptor transitions from the dimensions of the coaxial cable to the dimensions of a connector standard. For example, semi-rigid coaxial cable is often manufactured so that the diameter of the center conductor is close to the diameter of a center pin of a connector standard. A small change in diameter from the center conductor to the center pin might be acceptable in some applications, but unacceptable in others. Using an adaptor that provides a transition from coaxial cable dimensions to connector interface dimensions improves transmission characteristics from the end of the cable to the device that the cable is attached to. Similarly, use of an adaptor that provides a transition from coaxial cable dimensions to connector interface dimensions allows greater design freedom in selecting what type of coaxial cable to use in a particular application (i.e., with a particular connector interface standard). 
     FIG. 5A  is an isometric view of an adaptor  200  according to another embodiment of the invention. The adaptor  200  adapts two coaxial cables  132 ,  134  to a connector interface  202 . Alternatively, the adaptor adapts a balanced cable having two coaxial cables to a connector interface (see  FIG. 1C ). The adaptor  200  includes a base  204  and a shell  206  that provide a larger grasping surface for manipulating the adaptor  200 . The shell  206  also protects where the coaxial cables are connected to the base  204  (see  FIG. 5B ). The connector interface  202  includes a raised ground plane  167 . 
     FIG. 5B  is a simplified cross section of the adaptor  200  of  FIG. 5A . The shell  206  surrounds a connector body  161  and first slidable nut  137 . The shell  206  and base  204  of the adaptor provide a more rugged assembly by providing a large-diameter exterior for an operator to grasp when tightening or loosening the second sliding nut  208 . 
     FIG. 6A  is a front view of a connector body  210  according to an embodiment of the invention. A raised ground plane portion  212  of the face of the connector body  210  extends a selected height above the field  214  of the connector body  210 . The raised ground plane portion is in the shape of a figure-8 or hourglass, which facilitates machining the raised ground plane portion because it is not separated between the coaxial outer conductors  216 ,  218 . The raised ground plane portion  212  increases the pressure between mating connectors (at a given force between the mating connectors) around the coaxial outer conductors  216 ,  218 , improving the ground continuity and hence the transmission characteristics. 
     FIG. 6B  is a cross section taken along A—A of  FIG. 6A . The raised ground plane portion  212  is between about 0.08 mm and about 0.5 mm above the field  214  of the connector face  218 . A chamfer  220  is formed on the rim of the connector body  210  to facilitate alignment and reduce burring during use. The pin  54  is fitted to a hole drilled in the connector body  210 . 
     FIG. 7  is a front view of a connector body  230  according to another embodiment of the invention. Separated raised ground plane portions  232 ,  234  surround coaxial outer conductors  236 ,  238 . Raised ground plane portions are formed using a variety of techniques, such as milling, etching, abrasive blasting, and electronic discharge machining. 
     FIG. 8  shows adaptor assemblies  130 ,  130 ′ illustrated in  FIG. 3A  connecting an electronic device  150  having conventional package feedthroughs  152 ,  154 ,  156 ,  158  to a balanced VNA  100 . The adaptor assembly  130  separates the two coaxial transmission paths from a balanced cable  41  into two coaxial transmission lines  132 ,  134 . These separated coaxial transmission lines are connected to conventional coaxial package feedthroughs  152 ,  154  with conventional coaxial cable ends  138 ,  140  of the adaptor assembly  130 . Another adaptor assembly  130 ′ similarly connects conventional coaxial package feedthroughs  156 ,  158  with conventional coaxial cable ends  138 ′,  140 ′ to a second balanced cable  41 ′. This configuration may be used to perform balanced two-port measurements on a conventional differential two-port electronic device, or to perform four-port measurements on a four-port electronic device, using a balanced VNA and balanced cables. 
   A balanced cable with a cable end incorporating a connector interface constructed according to an embodiment of the present invention provides desirable advantages over conventional cables used with VNA systems because of the stability of the balanced cable. Most of the transmission line length between the VNA  100  and the electronic device  150  is a balanced test cable  41 , which maintains balance through the connector interface and is less likely to introduce measurement error due to movement of the test cables, compared to conventional four-cable systems or balanced cables with conventional cable ends. 
   While the preferred embodiments of the present invention have been illustrated in detail, it should be apparent that modifications and adaptations to these embodiments may occur to one skilled in the art without departing from the scope of the present invention as set forth in the following claims.