Patent Publication Number: US-7906730-B2

Title: Ground sleeve having improved impedance control and high frequency performance

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a ground sleeve. More particularly, the present invention is for a reference ground sleeve that controls impedance at the termination area of wires in a twinax cable assembly and provides a signal return path. 
     2. Background of the Related Art 
     Electrical cables are used to transmit signals between electrical components and are often terminated to electrical connectors. One type of cable, which is referred to as a twinax cable, provides a balanced pair of signal wires within a conforming shield. A differential signal is transmitted between the two signal wires, and the uniform cross-section provides for a transmission line of controlled impedance. The twinax cable is shielded and “balanced” (i.e., “symmetric”) to permit the differential signal to pass through. The twinax cable can also have a drain wire, which forms a ground reference in conjunction with the twinax foil or braid. The signal wires are each separately surrounded by an insulated protective coating. The insulated wire pairs and the non-insulated drain wire may be wrapped together in a conductive foil, such as an aluminized Mylar, which controls the impedance between the wires. A protective plastic jacket surrounds the conductive foil. 
     The twinax cable is shielded not only to influence the line characteristic impedance, but also to prevent crosstalk between discrete twinax cable pairs and form the cable ground reference. Impedance control is necessary to permit the differential signal to be transmitted efficiently and matched to the system characteristic impedance. The drain wire is used to connect the cable twinax ground shield reference to the ground reference conductors of a connector or electrical element. The signal wires are each separately surrounded by an insulating dielectric coating, while the drain wire usually is not. The conductive foil serves as the twinax ground reference. The spatial position of the wires in the cable, insulating material dielectric properties, and shape of the conductive foil control the characteristic impedance of the twinax cable transmission line. A protective plastic jacket surrounds the conductive foil. 
     However, in order to terminate the signal and ground wires of the cable to a connector or electrical element, the geometry of the transmission line must be disturbed in the termination region i.e., in the area where the cables terminate and connect to a connector or electrical element. That is, the conductive foil, which controls the cable impedance between the cable wires, has to be removed in order to connect the cable wires to the connector. In the region where the conductive foil is removed, which is generally referred to as the termination region, the impedance match is disturbed. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to control the impedance in the termination region of a cable. It is a further object of the invention to match the impedance in the termination region of differential signal wires. It is still another object of the invention to match the impedance in the termination region of a twinax cable. It is yet another object of the invention to control the impedance in the termination region of a twinax cable as it is connected to leads of an electrical connector. 
     In accordance with these and other objectives, the present invention is a connector that is terminated to one or more twinax cables. The connector includes a plastic insert molded lead frame, ground sleeve, twinax cable, and integrated plastic over molded strain relief. The lead frame is molded to retain both differential signal pins and ground pins. Mating sections are provided at the rear of the lead frame to connect each of the signal wires of the cables to respective signal leads. The ground sleeve has two general H-shape structures connected together by a center cross-support member. Each of the H-shaped structures have curved legs, each of which fits over the signal wires of one of the twinax cables. The wings of the ground sleeve are welded to the ground leads and the drain wire of the cable is welded to the ground sleeve to terminate the drain wire to a ground reference. The ground sleeve controls the impedance in the termination area of the cables, where the twinax foil is removed to connect with the leads. The ground sleeve also shields the cables to reduce crosstalk between multiple wafers when arranged in a connector housing. 
     These and other objects of the invention, as well as many of the intended advantages thereof, will become more readily apparent when reference is made to the following description, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective view of the connector having a ground sleeve in accordance with the preferred embodiment of the invention. 
         FIG. 2  is a perspective view of the connector of  FIG. 1  with the ground sleeve removed to show a twinax cable terminated to the lead frame. 
         FIG. 3(   a ) is a perspective view of the connector of  FIG. 1 , with the ground sleeve and cables removed to show the lead frame having pins and termination land regions. 
         FIG. 3(   b ) is a view of the connector having an overmold. 
         FIG. 4(   a ) is a perspective view of the ground sleeve. 
         FIGS. 4(   b )-( f ) illustrate the odd and even mode transmission improvement achieved by the present invention. 
         FIG. 5  is a perspective of a connection system having multiple wafer connectors of  FIG. 1 . 
         FIGS. 6-9  show an alternative embodiment of the invention in which the ground sleeve has a side pocket for connecting two single-wire coaxial cables. 
         FIGS. 10-11  show the ground sleeve in accordance with the alternative embodiment of  FIGS. 6-9 . 
         FIGS. 12-14  show a conductive slab utilized with the ground sleeve. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In describing a preferred embodiment of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents that operate in similar manner to accomplish a similar purpose. 
     Turning to the drawings,  FIG. 1  shows a connector wafer  10  of the present invention to form a termination assembly used with cables  20 . The connector  10  includes a plastic insert molded lead frame  100 , ground sleeve  200 , and pins  300 . The lead frame  100  retains the pins  300  and receives each of the cables  20  to connect the cables  20  with the respective termination land regions  130 ,  132 ,  134 ,  136  ( FIG. 3(   a )). The ground sleeve  200  fits over the cables  20  to control the impedance in the termination area of the cables  20 . The ground sleeve  200  also shields the cables  20  to reduce crosstalk between the wafers  10 . In addition, the ground sleeve terminates the drain wires  24  of the cables  20  to maintain a ground reference. 
     Referring to  FIG. 2 , the cables  20  are shown in greater detail. In the embodiment shown, two twin-axial cables, or twinax, are provided. Each of the cables  20  have two signal wires  22  which form a differential pair, and a drain wire  24  which maintains a ground reference with the cable conductive foil  28 . The signal wires  22  are each separately surrounded by an insulated protective coating  26 . The insulated wire pairs  22  and the non-insulated drain wire  24  are encased together in a conductive foil  28 , such as an aluminized Mylar, which shields the wires  22  from neighboring cables  20  and other external influences. The foil  28  also controls the impedance of the cables  20  by binding the cross sectional electromagnetic field configuration to a spatial region. Thus, the twinax cables  20  provide a shielded signal pair within a conformal shield. A plastic jacket  30  surrounds the conductive foil  28  to protect the wires  22 , which may be thin and fragile, from being damaged. 
     Referring to  FIG. 2 , the cables  20  are shown in greater detail. In the embodiment shown, two twin-axial cables, or twinax, are provided. Each of the cables  20  have two signal wires  22  which form a differential pair, and a drain wire  24  which maintains a ground reference with the cable conductive foil  28 . The signal wires  22  are each separately surrounded by an insulated protective coating  26 . The insulated wire pairs  22  and the non-insulated drain wire  24  are encased together in a conductive foil  28 , such as an aluminized Mylar, which shields the wires  22  from neighboring cables  20  and other external influences. The foil  28  also controls the impedance of the cables  20  by binding the cross sectional electromagnetic field configuration to a spatial region. Thus, the twinax cables  20  provide a shielded signal pair within a conformal shield. A plastic jacket  30  surrounds the conductive foil  28  to protect the wires  22 , which may be thin and fragile, from being damaged. 
     The air cavities provide for flexibility in controlling the transmission line characteristic impedance in the termination area. If smaller twinax wire gauges are used, the impedance will be increased. Additional plastic material may be added to fill the air cavities to lower the impedance. The H-shape is a feature used to accommodate the poorly controllable drain wire dimensional properties (e.g., mechanical properties including dimensional tolerances like drain wire bend radius, mylar jacket deformation and wrinkling, and electrical properties such as high frequency electromagnetic stub resonance and antenna effects, and the gaps can be used to tune the impedance if it is too low or high. Accordingly, this configuration provides for greater characteristic impedance control. The air cavities provide a mixed dielectric capability between the tightly-coupled transmission line conductors. 
     The termination region  110  also has two end members  122 ,  124 . The inside walls of the end members  122 ,  124  are straight so that the signal wires  22  are easily received in the receiving sections  131 ,  133  and guided to the bottom of the receiving sections  131 ,  133  to connect with the lands of the pins  300 . The outside surface of the end members  122 ,  124  are curved to generally conform with the shape of the insulated protective coating  26 . Thus, when the signal wires  22  are placed in the receiving sections  131 ,  133 , the termination regions  110  have a substantially similar shape as the portions of the cables  20  that have the insulated protective coating  26 . In this way, the ground sleeve  200  fits uniformly over the entire end length of the cable  20  from the ends of the signal wires  22  to the end of the plastic jacket  30 , as shown in  FIG. 1 . 
       FIG. 3(   a ) also shows the pins  300  in greater detail. In the preferred embodiment, there are seven pins  300 , including signal leads  304 ,  306 ,  310 ,  312 , and ground leads  302 ,  308 ,  314 . Each of the pins  300  have a mating portion  301  at one end and a termination region or attachment portions  103  at an opposite end. The mating portions  301  engage with the conductors or leads of another connector, as shown in  FIG. 5 . The termination regions  103  of the signal pins  304 ,  306 ,  310 ,  312 , engage the signal wires  22  of the cables  20 . The termination lands  103  of the ground pins  302 ,  308 ,  314  engage the ground sleeve  200 . The neighboring signal lands  130 ,  132 ,  134 ,  136  form respective differential pairs and connect with the wires  22  of the cables  20 . 
     The pins  300  are arranged in a linear fashion, so that the signal pins  304 ,  306 ,  310 ,  312  are co-planar with the ground leads  302 ,  308 ,  314 . Thus, the signal pins  304 ,  306 ,  310 ,  312  form a line with the ground pins  302 ,  308 ,  314 . In the preferred embodiment, the signal pins  304 ,  306 ,  310 ,  312  have an impedance determined by geometry and all of the pins  300  are made of copper alloy. 
     The pins  300  all extend through the lead frame  100 . The lead frame  100  can be molded around the pins  300  or the pins  300  can be passed through openings in the lead frame  100  after the lead frame  100  is molded. Thus, the mating portions  301  of the pins  300  extend outward from the front of the lead frame  100 , and the termination regions  103  extend outward from the rear surface of the lead frame  100 . The pins also have an intermediate portion which connects the mating portion  301  and the termination portion  103 . The intermediate portion is at least partially embedded in the lead frame  100 . 
     The ground pins  302 ,  308 ,  314  are longer than the signal pins  304 ,  306 ,  310 ,  312 , so that the ground pins  302 ,  308 ,  314  extend out from the front of the lead frame  100  further than the signal leads  304 ,  306 ,  310 ,  312 . This provides “hot-plugability” by assuring ground contact first during connector mating and facilitates and stabilizes sleeve termination. The ground pins  302 ,  308 ,  314  extend out from the rear a distance equal to the length of the ground sleeve  200 . Accordingly, the entire length of the wings of the ground sleeve  200  can be connected to the ground lands  144 ,  146 ,  148 . The wings can be attached by soldering, multiple weldings, conductive adhesive, or mechanical coupling. 
     As further shown in  FIG. 3(   a ), the center divider  112  and the end members  122 ,  124  define two receiving sections  131 ,  133 . The receiving sections  131 ,  133  are formed by one of the leg members  114 ,  116  of the center divider  112 , and an end member  122 ,  124 . A land end  130 ,  132 ,  134 ,  136  of each of the signal pins  312 ,  310 ,  306 ,  304 , respective, extends into each termination region to be situated between an end member  122 ,  124  and a respective leg member  114 ,  116 . The ends  130 ,  132 ,  134 ,  136  of the signal pins  312 ,  310 ,  306 ,  304  are flush with the rear surface of the end members  122 ,  124  and the rear surface of the leg members  114 ,  116 . The land ends  130 ,  132 ,  134 ,  136  are also positioned at the bottom of the termination region to form a termination platform within the receiving sections. 
     The lead frame  100  is insert molded and made of an insulative material, such as a Liquid Crystal Polymer (LCP) or plastic. The LCP provides good molding properties and high strength when glass reinforced. The glass filler has relatively high dielectric constant compared with polymers and provides a greater mixed dielectric impedance tuning capability. A channel  140  is formed at the top of the lead frame  100  to form a mechanical retention interlock with the overmold  18 , as best shown in  FIG. 3(   b ). 
     Stop members  142  are formed about the termination regions  110 . The openings (shown in  FIG. 1 ) are punched out during manufacturing to remove the bridging members used to prevent the pins  300  from moving during the process of molding the lead frame  100 . The projections or tabs  150  on the side of the frame  100  form keys that provide wafer retention in the connector housing or backshell  14  ( FIG. 5 ), and assures proper connector assembly. The latching of the backshell  14  is further described in U.S. Pat. No. 7,753,710, the contents of which are incorporated herein. The tabs  150  mate with organizer features in the connector housing  14  to help ensure proper alignment between the mating members of the board connector wafer and cable wafer halves. 
     Referring back to  FIG. 2 , the cable is prepared for termination with the lands  103  and the lead frame  100 . The plastic jacket  30  is removed from the cables  20  by use of a laser that trims away the jacket  30 . The laser also trims the foil  28  away to expose the insulated protective coating  26 . The foil  28  is removed from the termination section  32  of the cable  20  so that the cable  20  can be connected with the leads  300  at the lead frame  100 . The foil  28  is trimmed all the way back to expose the drain wire  24  and to prevent shorting between the foil and the signal wires. The insulation is then stripped away to expose the wire ends  34  of the cable  20 . The drain wire  24  is shortened to where the insulation  26  terminates. The drain wire  24  is shortened to prevent any possible shorting of the drain wire to the exposed signal wires  22 . 
     The cables  20  are then ready to be terminated with the lands  103  at the lead frame  100 . The cables  20  are brought into position with the lead frame  100 . The exposed bare signal ends  34  are placed within the respective receiving sections on top of the land ends  130 ,  132 ,  134 ,  136  of the signal pins  304 ,  306 ,  310 ,  312 . Thus, the termination regions of the frame  100  fully receive the length of the signal wire ends  34 . The bare wires  22  are welded or soldered to the lands  130 ,  132 ,  134 ,  136  of the signal leads  304 ,  306 ,  310 ,  312  to be electrically connected thereto. The drain wire  24  abuts up against the end of the center divider  116 , 118 . 
     The lead frame  100  and sleeve  200  are configured to maintain the spatial configuration of the wires  22  and drain wire  24 . The twinax cable  20  is geometrically configured so that the wires  22  are at a certain distance from each other. That distance along with the drain wire, conductive foil, and insulator dielectric maintains a characteristic and uniform impedance between the wires  22  along the length of the cable  20 . The divider separates the wires  22  by a distance that is approximately equal to the thickness of the wire insulation  26 . In this manner, the distance between the wires  22  stays the same when positioned in the receiving sections  131 ,  133  as when they are positioned in the cable  20 . Thus, the lead frame  100  and sleeve  200  cooperate to maintain the geometry between the wires  22 , which in turn maintains the impedance and balance of the wires  22 . In addition, the sleeve  200  provides for a smooth, controlled transition in the termination area between the shielded twinax cable and open differential coplanar waveguide or any other open waveguide connector. 
     Furthermore, the ground sleeve  200  serves to join or common the separate ground pins  302 ,  308 , and  314  ( FIG. 3(   a )) by conductive attachment in the regions  144 ,  146 , and  148 . This joining provides the benefit of preventing standing wave resonances between those ground pins in the region covered by the sleeve. Also, by reducing the longitudinal extent of the uncommoned portion of the ground pins, the sleeve  200  serves to increase the lowest resonant frequencies associated with that portion. A conductive element similar to the ground sleeve  200  may also be employed on the portion of the connector which attaches to a board, for the same purposes. 
     Turning to  FIG. 4(   a ), a detailed structure of the ground sleeve  200  is shown. The sleeve  200  is a single piece element, which is configured to receive the two twinax cables  20 . The sleeve  200  has two H-shaped receiving sections  210  joined together by a center support  224 . The sleeve  200 , the attachment portions  103  side of the ground leads  302 ,  308 ,  314 , and the twinax wires constitute geometries that result in an electromagnetic field configuration matched to 100 ohms, or any other impedance. The H-shaped geometry provides a smooth transition between two 100 ohm transmission lines of different geometries and therefore having different electromagnetic field configurations in the cross-section, i.e. shielded twinax to open differential coplanar waveguide. The H-shaped geometry of the sleeve  200  also makes an electrical connection between the drain/conductive foil ground reference of the twinax to the ground reference of the differential coplanar waveguide connector. The differential coplanar waveguide is the connector transmission line formed by the connector lands/pins. The sleeve could be adapted for other connector geometries. The H-shaped sleeve  200  provides a geometry that allows the characteristic impedance of this transmission line section (termination area) to be controlled more accurately than just bare wires by eliminating the effects of the drain wire. 
     Each of the receiving sections  210  receive a twinax cable  20  and include two legs or curved portions  212 ,  214  separated by a center support member formed as a trough  216 . The curved portions  212 ,  214  each have a cross-section that is approximately one-quarter of a circle (that is, 45 degrees) and have the same radius of curvature as the cable foil  28 . The trough  216  is curved inversely with respect to the curved portions  212 ,  214  for the purpose of drain wire guidance. A wing  222  is formed at each end of the ground sleeve  200 . The wings  222  and the center support member  224  are flat and aligned substantially linearly with one another. 
     The trough  216  does not extend the entire length of the curved portions  212 ,  214 , so that openings  218 ,  220  are formed on either side of the trough  216 . Referring back to  FIG. 1 , the rear opening  218  allows the drain wire  24  to be brought to the top surface of the sleeve  200  and rest within the trough  216 . The trough  216  is curved downward so as to facilitate the drain wire  24  being received in the trough  216 . In addition, the downward curve of the trough  216  is defined to maintain the geometry between the drain wire  24  and the signal wires  22 , which in turn maintains the impedance and symmetrical nature of the termination region. Though the opening  218  is shown as an elongated slot in the embodiment of  FIG. 4(   a ), the opening  218  is preferably a round hole through which the drain wire  24  can extend. Accordingly, the back end of the sleeve  200  is preferably closed, so as to eliminate electrical stubbing. 
     The lead opening  220  allows the ground sleeve  200  to fit about the top of the center divider  212 , so that the drain wire  24  can abut the center divider  112  (though it is not required that the drain wire  24  abut the divider  112 ). By having the drain wire  24  connect to the top of the sleeve  200 , the drain wire is electrically commoned to the system ground reference. The drain wire  24  is fixed to the trough  216  by being welded, though any other suitable connection can be utilized. The sleeve  200  also operates to shield the drain  24  from the signal wires  22  so that the signal wires  22  are not shorted. The drain wire  24  grounds the sleeve  200 , which in turn grounds the ground pins  302 ,  308 ,  314 . This defines a constant local ground reference, which helps to provide a matched characteristic impedance between twinax and differential coplanar waveguide, i.e. the attachment area. The controlled geometry of the sleeve  200  ensures that the characteristic impedance of the transmission lines with differing geometries can be matched. That is, the lead frame  100  and sleeve  200  cooperate to maintain the geometry between the wires  22 , which in turn maintains the impedance and balance of the wires  22 . 
     The electromagnetic field configuration will not be identical, and there will be a TEM (transverse-electric-magnetic) mode mismatch of minor consequence. The TEM (transverse-electric-magnetic) mode propagation is generally where the electric field and magnetic field vectors are perpendicular to the vector direction of propagation. The cable  20  and pins  300  are designed to carry a TEM propagating signal. The cross-sectional geometry of the cable  20  and the pins  300  are different, therefore the respective TEM field configurations of the cable  20  and the pins  300  are not the same. Thus, the electromagnetic field configurations are not precisely congruent and therefore there is a mismatch in the field configuration. However, if the cable  20  and the pins  300  have the same characteristic impedance, and since they are similar in scale, ground sleeve  200  provides an intermediate characteristic impedance step that is a smooth (geometrically graded) transition between the two dissimilar electromagnetic field configurations. This graded transition ensures a higher degree of match for both even and odd modes of propagation on each differential pair, over a wider range of frequencies when compared to sleeveless termination of just the ground wire. The connector  10  is generally designed to operate as a TEM, or more specifically quasi-TEM transmission line waveguide. TEM describes how the traveling wave in a transmission line has electric field vector, magnetic field vector, and direction of propagation vector orthogonal to each other in space. Thus, the electric and magnetic field vectors will be confined strictly to the cross-section of a uniform cross-section transmission line, orthogonal to the direction of propagation along the transmission line. This is for ideal transmission lines with a uniform cross-section down its length. The “quasi” arises from certain imperfections along the line that are there for ease of manufacturability, like shield holes and abrupt conductor width discontinuities. 
     The TEM transmission lines can have different geometries but the same characteristic impedance. When two dissimilar transmission lines are joined to form a transition, the field lines in the cross-section don&#39;t match identically. The field lines of the electromagnetic field configurations for particular transmission line geometries define a mode shape, or a “mode”. So when transmission occurs between dissimilar TEM modes, when the geometries are of similar shape or form and of the same physical scale or order (i.e., between the twinax cable  20  and the connector pins  300 ), there is some degree of transmission inefficiency. The energy that is not delivered to the second transmission line at a discontinuity may be radiated into space, reflected to the transmission line that it originated from, or be converted into crosstalk interference onto other neighbor transmission lines. This TEM mode mismatch results from the nature of all transmission line discontinuities, because some percentage of the incident propagating energy does not reach the destination transmission line even if they have an identical characteristic impedance. 
     The transition/termination area is designed so that the mismatch is of little consequence because a negligible amount of the incident signal energy is reflected, radiated, or takes the form of crosstalk interference. The efficiency is maximized by proper configuration of the transition between dissimilar transmission lines. The ground sleeve  200  provides a graded step in geometry between the cable  20  and the pins  300 . The configuration is self-defining by the geometrical dimensions of ground sleeve  200  that results in a sufficient (currently, about 110-85 ohms) impedance match between the cable and the pins. During the process of signal propagation along the transition area between two dissimilar transmission line geometries with the same characteristic impedance, most or all of the signal energy is transmitted to the second transmission line, i.e., from the cable  20  to the pins  300 , to have high efficiency. The high efficiency generally refers to a high signal transmission efficiency, which means low reflection (which is addressed by a sufficient impedance match). 
     Referring back to  FIG. 1 , the ground sleeve  200  is placed over the cables  20  after the cables  20  have been connected to the lead frame  100 . The sleeve  200  can abut up against the stop members  142  of the lead frame  100 . The wings  222  contact the lead frame  100 , and the wings  222  are welded to the outer ground leads  302 ,  314 . Likewise, the center support  224  is welded to the center ground lead  308 . The receiving sections  210  of the sleeve  200  surround the termination regions  110 , as well as the cables  20 . Though welding is used to connect the various leads and wires, any suitable connection can be utilized. 
     When the sleeve  200  is positioned over the cables  20 , each of the wings  222  are aligned with the lands  144 ,  148  to contact, and electrically connect with, the lands  144 ,  148 . In addition, the sleeve  200  center support  224  contacts, and is electrically connected to, the land  146  of the lead frame  100 . The ground pins  302 ,  308 ,  314  are grounded by virtue of their connection to the ground sleeve  200 , which is grounded by being connected to the drain wire  24 . 
     The ground sleeve  200  operates to control the impedance on the signal wires  20  in the termination region  32 . The sleeve  200  confines the electromagnetic field configuration in the termination region to some spatial region. That is, the proximity of the sleeve  200  allows the impedance match to be tuned to the desired impedance. Prior to applying the ground sleeve  200 , the bare signal wire ends  34  in this configuration and the entire termination region  32  have a unmatched impedance due to the absence of the conductive foil  28 . 
     In addition, the lead frame  100  and the ground sleeve  200  maintains a predetermined configuration of the signal wires  22  and the drain wire  24 . Namely, the lead frame  100  maintains the distance between the signal wires  22 , as well as the geometry between the signal wires  22  and the drain wire  24 . That geometry minimizes crosstalk and maximizes transmission efficiency and impedance match between the signal wires  22 . This is achieved by shielding between cables in the termination area and confining the electromagnetic field configuration to a region in space. The sleeve conductor provides a shield that reduces high frequency crosstalk in the termination area. 
     Turning to  FIG. 5 , the wafers  10  are shown in a connection system  5  having a first connector  7  and a second connector  9 . The first connector  7  is brought together with the second connector  9  so that the pins  300  of each of the wafers  10  in the first connector  7  mate with respective corresponding contacts in the second connector  9 . Each of the wafers  10  are contained within a wafer housing  14 , which surrounds the wafers  10  to protect them from being damaged and configures the wafers into a connector assembly. 
     Each of the wafers  10  are aligned side-by-side with one another within a connector backshell  14 . In this arrangement, the ground sleeve  200  operates as a shield. The sleeve  200  shields the signal wires  22  from crosstalk due to the signals on the neighboring cables. This is particularly important since the foil has been removed in the termination region. The sleeve  200  reduces crosstalk between signal lines in the termination region. Without a sleeve  200 , crosstalk in a particular application can be over about 10%, which is reduced to substantially less than 1% with the sleeve  200 . The sleeve  200  also permits the impedance match to be optimized by confining the electromagnetic field configuration to a region. 
     Only a bottom portion of the connector housing  14  is shown to illustrate the wafers  10  that are contained within the connector backshell  14 . The connector backshell  14  has a top half (not shown), that completely encloses the wafers  10 . Since there are multiple wafers  10  within the connector backshell  14 , many cables  20  enter the connector backshell  14  in the form of a shielding overbraid  16 . After the cables  20  enter the connector backshell  14 , each pair of cables  20  enters a wafer  10  and each twinax cable  20  of the pair terminates to the lead frame  100 . One specific arrangement of the wafer  10  is illustrated in U.S. Pat. No. 7,753,710, the contents of which are incorporated herein by reference. 
     The ground sleeve  200  is preferably made of copper alloy so that it is conductive and can shield the signal wires against crosstalk from neighboring wafers. The ground sleeve is approximately 0.004 inches thick, so that the sleeve does not show through the overmold  18 . As shown in  FIG. 3(   b ), the overmold  18  is injection-molded to cover all of the connector wafer  10  and part of the cable  20  features. The overmold interlocks with the channel  140  as a solid piece down through the twinax cables  20 . The overmold  18  prevents cable movement which can influence impedance in undesirable, uncontrolled ways. The channel  140  provides a rigid tether point for the overmold  18 . The overmold  18  is a thermoplastic, such as a low-temperature polypropylene, which is formed over the device, preferably from the channel  140  to past the ground sleeve  200 . The overmold  18  protects the cable  20  interface with the lead frame  100  and provides strain relief. The overmold  18  encloses the channel  140  from the top and bottom and enters the openings in the channel  140  to bind to itself. While the overmold  18  generally prevents movement, the channel  140  feature provides additional immunity to movement. 
     The approximate length and width of the sleeve are 0.23 inches and 0.27 inches, respectively, for a cable  20  having insulated signal wires with a diameter of about 1.34 mm. Ground sleeve  200  provides improved odd and even mode matching for cable termination. As an illustrative example not intended to limit the invention or the claims, the improvement in odd and even mode impedance matching can be observed in terms of increased odd and even mode transmission in  FIGS. 4(   b ) and  4 ( c ) respectively, or in terms of reduced odd and even mode reflection in  FIGS. 4(   d ) and  4 ( e ) respectively. It is readily apparent from  FIGS. 4(   b ) and  4 ( c ) that both the odd mode and even mode transmission efficiency is significantly improved when the ground sleeve  200  is employed. Similarly with odd and even mode reflection, in  FIGS. 4(   d ) and  4 ( e ) respectively, the use of ground sleeve  200  results in substantial reduction in magnitude of reflection due to the termination region. As shown in  FIG. 4(   f ), a further benefit of the geometrical symmetry inherent to ground sleeve  200  is the substantial reduction in transmitted signal energy which is converted from the preferred mode of operation (odd mode) to a less preferable mode of propagation (even mode) to which a portion of useful signal energy is lost. Of course, other ranges may be achieved depending on the specific application. 
     Though two twinax cables  20  are shown in the illustrative embodiments of the invention, each having two signal wires  22 , any suitable number of cables  20  and wires  22  can be utilized. For instance, a single cable  20  having a single wire  22  can be provided, which would be referred to as a signal ended configuration. A single-ended cable transmission line is a signal conductor with an associated ground conductor (more appropriately called a return path). Such a ground conductor may take the form of a wire, a coaxial braid, a conductive foil with drain wire, etc. The transmission line has its own ground or shares a ground with other single-ended signal wires. If a one-wire cable such as coaxial cable is used, the outer shield of this transmission line is captivated and an electrical connection is made between it and the single-ended connector&#39;s ground/return/reference conductor(s). A twisted pair transmission line inherently has a one-wire for the signal and is wrapped in a helix shape with a ground wire (i.e., they are both helixes and are intertwined to form a twisted pair). There are other one-wire or single-ended types of transmission lines than coax and twisted pairs, for example the Gore QUAD™ product line is an example of exotic high performance cabling. Or, there can be a single cable  20  having four wires  22  forming two differential pairs. 
     As shown in  FIGS. 1-5 , the preferred embodiment connects a cable  20  to leads  300  at the lead frame  100 . However, it should be apparent that the sleeve  200  can be adapted for use with a lead frame that is attached to a printed circuit board (PCB) instead of a cable  20 . In that embodiment, there is no cable  20 , but instead leads from the board are covered by the ground sleeve. Thus, the ground sleeve would common together the ground pins of the lead frame. The ground sleeve can provide a direct or indirect conductive path to the board through leads attached to the sleeve or integrated with the sleeve. 
     Another embodiment of the invention is shown in  FIGS. 6-11 . This embodiment is used for connecting two single-wire coaxial cables  410  to leads  430  at a lead frame  420 . Accordingly, the features of the connector  400  that are analogous to the same features of the earlier embodiment, are discussed above with respect to  FIGS. 1-5 . Turning to  FIGS. 6 and 7 , the connector wafer  400  is shown connecting the two single-cable coaxial wires  410  to the leads  430  at a lead frame  420 . A ground sleeve  440  covers the termination region of the cable  410 . As best shown in  FIG. 8 , the cables  410  each have a signal conductor and a ground or drain wire  412  wrapped by conductive foil and insulation. 
     Returning to  FIGS. 6-7 , the ground wire  412  extends up along the side of the ground sleeve  440  and rests in a side pocket  442  located on the curved portion of the ground sleeve  440 , which is along the side of the ground sleeve  440 . Referring to  FIG. 9 , the lead frame  420  is shown. Because each cable  410  has a single signal conductor, each mating portion only has a single receiving section  450  and does not have a center divider. 
     The ground sleeve  440  is shown in greater detail in  FIGS. 10 and 11 . The ground sleeve  440  has two curved portions  446 . Each of the curved portions  446  receive one of the cables  410  and substantially cover the top half of the received cable  410 . Instead of the trough  216  of  FIG. 4(   a ), the ground sleeve  440  has a side pocket  442  that is formed by being stamped out of and bent upward from one side of each curved portion  446 . The side pocket  442  receives the drain wire  412  and connects the drain wire  412  to the ground leads  430  via the wings and center support of the ground sleeve  440 . In addition, a side portion  444  of the curved portion  446  is cut out. The cutout  444  provides a window for the drain wire  412  to pass through the ground sleeve  440 . 
     Turning to  FIGS. 12-14 , an alternative feature of the present invention is shown. In the present embodiment, a conductive elastomer electrode slab  500  is provided. The slab  500  essentially comprises a relatively flat member that is formed over the surface of the sleeve  200  and cable  20 . The slab  500  has two rectangular leg portions  502  joined together at one end by a center support portion  504  to form a general elongated U-shape. The slab  500  can be a conductive elastomer, epoxy, or other polymer so that it can be conformed to the contour of the cable. Though the slab  500  is shown as being relatively flat in the embodiment of  FIGS. 12-14 , it is slightly curved to match the contour of the cable  20 . The elastomer, epoxy or polymer is impregnated with a high percentage of conductive particles. The slab  500  can also be a metal, such as a copper foil, though preferably should be able to conform to the contour of the cable  20  or is tightly wrapped about the cable  20 . The slab  500  is affixed to the top of the ground sleeve  200  and the cables  20 , such as by epoxy, conductive adhesive, soldering or welding. 
     The center support portion or connecting member  504  generally extends over the sleeve  200  and the legs  502  extend from the sleeve  200  over the cable  20 . The connecting member  504  allows for ease of handling since the slab  500  is one piece. The connection  504  ( FIG. 12 ) acts as a shield for small leakage fields at small holes and gaps between the openings  218  ( FIG. 4(   a )) and the drain wire  24  ( FIG. 2) . 
     The slab  500  contacts and electrically conducts with the ground wires  412  of the cable  20 . It preserves the continuity of the cable  20  ground return  412  through the insulative jacketing of the cable. The jacket insulator provides for a capacitor dielectric substrate between the slab  500  electrode and the cable conductor shield foil  28  surface. A capacitive coupling is formed between the slab leg  502 , which forms one electrode of a capacitor, and the cable shield conductor foil  28 , which forms the second electrode of the capacitor. The enhanced capacitive coupling at high frequencies (i.e., greater than 500 MHz) electrically “commons” the cable shield foil  28 , where physical electrical contact is essentially impossible or impractical. The protective insulator remains unaltered to preserve the mechanical integrity of the fragile cable shield conductor foil  28 . Exposing the very thin cable conductor foil  28  for conductive contact is impractical in that it requires much physical reinforcement, or may be impossible because the cable shield conductor foil  28  may be too thin and fragile to make contact with slab  502  if cable shield conductor foil  28  is a sputtered metal layer inside the protective insulator jacket  30 . 
     With reference to  FIG. 14 , it is desirable to have a low impedance to provide improved shielding because the slab  500  is more reflective. The low impedance can be obtained by increasing the capacitance and/or the dielectric constant. However, the capacitance is limited by the amount of surface area available on the cable  20  for a given application. The conductive properties of the slab should be as conductive as possible (conductivity of metal). For instance, the impedance of the series capacitive section between leg  502  and cable outer conductor  28  should be less than 0.50 ohms at frequencies greater than 500 MHz. The impedance can only get smaller as the operational frequency increases, assuming that capacitance remains constant. And, the dielectric constant is limited by the materials available for use, the capacitance can be enhanced by using high dielectric constant materials. 
     The size of the slab  500  or slab leg  502  can be varied to adjust the capacitor surface area and therefore adjust the capacitance. Generally the slab  500  and leg  502  should be as conductive as possible since they form one electrode of the enhanced capacitive area. The capacitance is dependent upon the dimensions of the application, the permittivity characteristics of the insulator material the cable protective jacket is made out of, and the operational frequency for the application. In general terms, the impedance of the ground return current at and above the desired operational frequency should be less than 1 ohm in magnitude. A simple parallel plate capacitor has a capacitance of: 
     
       
         
           
             C 
             = 
             
               
                 
                   ɛ 
                   r 
                 
                 ⁢ 
                 
                   ɛ 
                   0 
                 
                 ⁢ 
                 A 
               
               d 
             
           
         
       
     
     Where C represents the capacitance between the leg  502  and the foil  28 , ∈ 0  is the permittivity of vacuum, ∈ r  is the relative permittivity of the capacitor dielectric medium, A is the parallel plate capacitor surface area (i.e., leg  502 ), and d is the separation distance between the plate surfaces. 
     The impedance magnitude (|Z|) of a parallel plate capacitor (between the leg  502  and foil  28 ) is: 
                  Z        =     1     2   ⁢           ⁢     π   ·   f   ·   C               
Where f is the frequency in Hertz and C is the capacitance.
 
     For one example at 500 MHz, the length of slab leg  502  would be 0.2 inches and 0.1 inches in width, which forms a capacitor area of 0.02 square inches. The thickness d of a typical cable protective jacket is about 0.0025 inches thick and has a typical relative dielectric constant ∈ r , of 4. The capacitance of this specific element is approximately 730 pF. At 500 MHz, the impedance magnitude of this element is: 
     
       
         
           
             
                
               Z 
                
             
             = 
             
               
                 1 
                 
                   2 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     π 
                     · 
                     500 
                     · 
                     
                       10 
                       6 
                     
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     Hz 
                     · 
                     730 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   pF 
                 
               
               = 
               
                 0.43 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Ω 
               
             
           
         
       
     
     For frequencies above 500 MHz, this impedance will be reduced accordingly for this example. 
     An ideal capacitor provides a smaller path impedance as the operating frequency of the signal increases. So, increasing capacitance in alternating current signal (or in this case, the ground return) current paths provides an electrical short between conductor surfaces. Though the size and capacitance could vary greatly, it is noted for example that if the geometry in the cross section of ground sleeve  200  over the cable was kept constant and extruded by twice the length, the capacitance would be approximately doubled and the impedance of that element would be approximately half. Thus, because the capacitive coupling is enhanced to a great degree, it is not necessary for the shield  500  to make physical contact with the cable shield foil  28  while still being able to provide adequately low impedance return current path, i.e. the conductors may be separated by a thin insulating membrane. In fact, the thinner the insulating membrane, the larger the capacitance will be and therefore lower impedance path for the ground return current. 
     The slab  500  also improves crosstalk performance due to greater shielding around the termination area, where the enhanced capacitive coupling maintains high frequency signal continuity, and leakage currents are suppressed from propagating on the outside of the signal cable shield conductor. Since the enhanced capacitance provides a low impedance short-circuit impedance path, the return currents are less susceptible to become leakage currents on the cable shield foil  28  exterior, which can become spurious radiation and cause interference to electronic equipment in the vicinity. The shield  500  also eliminates resonant structures in the connector ground shield by commoning the metal together electrically. The slab  500  provides a short circuit to suppress resonance between geometrical structures on ground sleeve  200  that may otherwise be resonant at some frequencies. The end result of applying the slab  500  is the creation of an electrically uniform conductor consisting of several materials (conductive slab and ground sleeve  200 ). 
     As shown in  FIG. 13 , the slab  500  can be a flexible elastomer, which has the benefit of maintaining electrical conductivity while still allowing the cable  20  to have greater flexible mechanical mobility than a rigid conductive element provides. This flexibility is in terms of mechanical elasticity, so that the entire joint has some degree of play if the cable  20  needed to bend at the joint of ground sleeve  200  and the cable  20  for some reason or specific application, before the area is overmolded. Since the conductive elastomer/epoxy is applied in a plastic or liquid uncured state, it follows the contour of the cable protective insulator jacket to provide greater connection to sleeve  200  in ways that are difficult to achieve with a foil. Since the foil isn&#39;t able to conform to the surface contours of the ground sleeve  200  as well as with conductive elastomer/epoxy, and the foil realizes excess capacitance over the elastomer/epoxy. 
     Though the slab  500  has been described and shown as a relatively thin and flat U-shaped member that is formed of a single piece, it can have other suitable sizes and shapes depending on the application. For instance, the slab  500  can be one or more rectangular slab members (similar to the legs  502 , but without the connecting member  504 ), one of more of which are positioned over each signal conductor of the cable  20 . 
     The slab  500  is preferably used with the sleeve  200 . The sleeve  200  provides a rigid surface to which the slab  500  can be connected without becoming detached. In addition, the sleeve  200  is a rigid conductor that controls the transmission line characteristic impedance in the termination area. The ground sleeve  200  also provides an electrical conduction between the connector ground pins  144 ,  146 ,  148 , drain wire  24 , and eventually conductor foil  28 . In addition, the slab  500  and the sleeve  200  could be united as a single piece, though the surface conformity over the cables  20  would have to be very good. By having the slab  500  and the sleeve  200  separate, the slab  500  and the sleeve  200  can better conform to the surface of the cables  20 . However, the slab  500  can also be used without the sleeve  200 , as long as the area over which the slab  500  is used is sufficiently rigid, or the slab  500  sufficiently flexible, so that the slab  500  does not detract. 
     It is further noted that the sleeve  200  can be extended farther back along the cable  20  in order to enhance the capacitance. In other words, the sleeve  200  may have stamped metal legs as part of sleeve  200  that are similar to legs  502 . However, the capacitance would be inferior to the use of the slab  500  with legs  502  because the legs  502  are more flexible and therefore better conformed to the insulating jacket  30  surface area and are therefore as close as physically possible to the foil  28 . Thus, the series capacitance C is higher than would be the case with an extended sleeve  200   
     The legs  502  further enhances the electrical connection to the metalized mylar jacket of the cable  20 . The slab  500  is preferably utilized with the H-shaped configuration of the sleeve  200 . The slab  500  functions to short the two curved portions  212 ,  214  of the sleeve  200  to prevent electrical stubbing. The H-shaped configuration of the sleeve  200  is easier to manufacture and assemble as compared to the use of a round hole as an opening  218 . 
     The foregoing description and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of shapes and sizes and is not intended to be limited by the preferred embodiment. Numerous applications of the invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.