Abstract:
A pair manager for use in securing a twin-axial cable to a printed circuit board is described. The pair manager comprises a generally block-shaped portion containing a pair of channels. The channels extend from the front face to the rear face of the block-shaped portion. An integral flange and a pair of integral fingers extend perpendicularly from the front face of the block-shaped portion. The flange extends generally from the center of the front face and the fingers extend from opposite edges of the front face. The fingers and flange function as a partial shield cavity around each pair of conductors. This design helps to maintain better impedance matching when connecting twin-axial cables to a printed circuit board.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/487,778, filed Jun. 19, 2009, which claims priority to U.S. Provisional Patent Application No. 61/074,440, filed Jun. 20, 2008, and U.S. Provisional Patent Application No. 61/074,422, filed on Jun. 20, 2008, the subject matters of which are hereby incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to connectors, and more particularly, to an improved pluggable cable connector design. 
     BACKGROUND OF THE INVENTION 
     Network hardware vendors including Cisco, Extreme Networks, Arastra, and others offer families of 10 Gb/sec. switch products that unify Local Area Networks (LAN) and Storage Area Networks (SAN) using protocols for Unified Network Fabric Using Fiber Channel Over Ethernet (FCOE). Cisco, for example, has introduced the Nexus family of switches (Nexus 5000 and Nexus 7000) that seamlessly communicate with disparate communications protocols such as Fiber Channel (for SANs) and Ethernet/IP (LANs). 
     For relatively short digital links (&lt;20 meters), twin-ax cable is a preferred transmission medium due to the significantly lower cost per link compared to optical fiber. Twin-ax cable conductors are typically terminated on SFP+ (small form-factor pluggable) connectors, and in particular, on paddle boards or PCBs (Printed Circuit Boards) in the SFP+ pluggable connectors. At the cable termination interface, the reflections of the high-speed signals (e.g. 10 Gb/sec) are at their maximum. The SFP+ cable assemblies are used to interconnect from a Nexus 5000 (or similar) switch typically located at the top of a rack to other switches in the same or adjacent racks. Typical lengths of such connectivities are one, three, and five meters with no compensation on the connector&#39;s PCB for receive equalization and transmit pre-emphasis. Longer reaches of 10 to 20 meters are feasible and may require a pre-emphasis driver ASIC located on the connector&#39;s PCB. 
     However, terminating high-speed twin-ax cables to the paddle card in SFP+ cable assemblies used in Fiber Channel Over Ethernet (FCOE) deployment has been difficult. At the junction where the twin-ax conductors are soldered (or welded) to the paddle card pads, the reflection of high-speed signals (10 Gb/s) tends to be highest due to the fact that the shields are either stripped or folded back to accommodate attachment to the PCB. Improving the method of attachment (soldering, resistive welding, conductive epoxying, etc.) provides only marginal improvements in impedance matching. Further, there is a need to keep the spacing between the two pairs of twin-ax cable constant for manufacturability improvements. Protecting the soldered or welded cable-to-paddle card interface by means of strain relief is also desirable in the SFP+ cable assemblies. 
     In addition, the mechanism for latching the pluggable connector to the switch port and de-latching the pluggable connector from the switch port needs to be robust and reliable. 
     Needed is a quick and reliable method for attaching the twin-ax media to the host system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are perspective views of a pluggable cable connector; 
         FIG. 3  is an exploded view of a pluggable cable connector; 
         FIGS. 4 and 5  show a twin-ax cable being prepared for termination to a connector; 
         FIGS. 6-12  are perspective views of a pair manager, including views showing the provision of wires in a pair manager and the connection of the pair manager to a PCB; 
         FIGS. 13 and 14  show wires of a twin-ax cable terminated to a PCB; 
         FIGS. 15-23  are perspective views showing the termination of a twin-ax cable to a pluggable cable connector and further assembly of the connector; 
         FIGS. 24-27  are perspective views showing elements of a latch release mechanism and the operation of the latch mechanism; 
         FIGS. 28A-29B  are plan views of conductive traces of layers of a PCB; and 
         FIGS. 30 and 31  are perspective and exploded views of an alternative embodiment of a pair manager. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1-3  are perspective view illustrations (assembled and exploded) of a pluggable cable connector  100 , in accordance with an embodiment of the present invention. The connector  100  is preferably constructed to be part of a Small Form-factor Pluggable (SFP) cable assembly that complies with the physical requirements of SFF-8432 Specification for Improved Pluggable Form-Factor-Revision 5.0 dated Jul. 16, 2007. The connector  100  terminates a cable  102  and includes a shell  104  comprising a bottom shell  106  and a top shell  108  (See  FIG. 3 ). The bottom shell  106  and top shell  108  are preferably zinc die-cast housings assembled together by front inter-locks and formed integral rivets. 
     An EMI gasket  110  may be included for protection against EMI (Electro-Magnetic Interference) effects. A pull tab  112  acts on a latch release  114  to cause a latch  116  (loaded by springs  122 ) to release the connector  100  from a host receptacle (not shown) by recessing a latch tooth  172  while a pulling force is applied to the pull tab  112 . In an alternative embodiment, the pull tab  112  is integrally molded with the latch release  114 . 
     As shown in  FIG. 3 , a pair manager  118  preferably having at least metal walls is disposed inside the shell  104  to interface the cable  102  with a PCB (Printed Circuit Board)  120  in an organized manner that aids in reducing unwanted reflections and other potentially adverse effects. The pair-manager  118  facilitates pair-ground termination to the PCB  120 , shields exposed pairs, and helps position wire pairs during assembly to the PCB  120 . A crimp  124  assists in securing the cable  102  to the connector  100 . A bend radius control feature  160  (see  FIG. 22 ) may be included to assist in controlling bend radius where cable  102  enters the connector  100 . This external crimp/strain-relief mechanism eases assembly and crimp operation, and allows the connector shell  104  to be shorter. 
     Impedance matching at the cable termination interface is accomplished by using the metal walls of the pair manager  118  as a partial cavity that is designed to match the differential impedance of twin-ax pairs with the metal shield removed or folded back (see  FIGS. 4 and 5 ). The pair manager  118  also provides an electrical grounding system to which the drain wires of the twin-ax pairs are soldered (See  FIGS. 6-14 ). The pair manager  118  has metal flanges (see, e.g.,  FIG. 6 , reference numerals  136  and  148 ) that are designed to be soldered to the grounding pads on both surfaces of the PCB  120 , providing electrical grounding as well as a mechanically robust connection to the PCB  120 . Another useful design feature of the pair manager  118  is that it functions to position the twin-ax cable pairs  134  at a constant distance apart and enables at least a semi-automated termination process. 
       FIGS. 4 and 5  illustrate preparation of an end of the cable  102  for termination at the connector  100 , for an embodiment in which a standard twin-ax metal (e.g. copper) cable is being terminated. After removing the outer jacket  126 , the braid  128  is pulled back over the outer jacket  126 . The foil shield  130  is removed from the insulated wire pairs  134  and then the insulation  132  is removed from a length of the end of the wire pairs  134  suitable for attachment to pads on the PCB  120 . The crimp  124  is threaded onto the cable  102  and over the braid  128  near the end of the outer jacket  126 . 
       FIGS. 6-14  illustrate the pair manager  118  in further detail. The pair manager has been designed to provide good impedance matching with the PCB  120 . This is accomplished by sizing the depth, height, and spacing between the top flange  148  and fingers  149  such that the pair manager  118  functions as a partial shield cavity around each pair of conductors that are soldered to microstrip lines on the PCB  120 . According to some embodiments, the pair manager  118  may be plated with a metal layer whose conductivity is higher than that of the base metal. In one embodiment, if the pair manager is made of zinc as a base metal, the pair manager may be plated with copper, tin, or nickel. If aluminum is used as the base metal for the pair manager, it may be plated with another metal such as silver or nickel. The dimensions of the top flange  148  and fingers  149  are parameterized as a, b, and c, as shown in  FIG. 6 . According to one embodiment of the present invention for use with 30-AWG twin-ax cabling, the finger-to-flange spacing, a, is about 4.4 mm; the spacing between the fingers and the flange at the base of the fingers, b, is about 3.5 mm, and the finger height, c, is about 1.3 mm. 
       FIGS. 7-12  set forth two alternative techniques for interfacing the wire pairs  134  with the pair manager  118  and PCB  120 .  FIG. 7  illustrates the first technique, while  FIGS. 8-12  illustrate the second technique. The PCB  120  (sometimes referred to as a “paddle card” in the industry) in each technique includes a control side and a communication side, each having associated ground pads. The pair manager  118  can be the same for each technique, but need not be. The designs for the PCB  120  and the pair manager  118  are preferably customized for each wire gauge size used for wire pairs  134 . In a preferred embodiment, the pair manager  118  includes a bottom flange  136  and top flange  148  for receiving the PCB  120  between them. Ground slots  140  may be included on the bottom flange  136  to terminate ground wires  174  in accordance with the first technique. Alternatively and/or in addition, ground boss structure(s)  142  may be included on top of the pair manager  118  to terminate ground wires  174  in accordance with the second technique. The pair manager  118  is preferably constructed entirely or partially of a metal with good conductivity (such as copper, aluminum, zinc, etc.). To provide strain relief, an over-molded wire pair strain relief feature  152  (see  FIG. 16 ) may be included. The over-molded wire pair strain relief feature  152  overlies the wire pairs  134  between the point where the foil shield  130  and insulation  132  are removed from the pairs  134  to the point where the pairs  134  enter the pair manager  118 . 
     According to the first technique and as shown in  FIG. 7 , the twin-ax wire pairs  134  are positioned to have their associated ground wires  174  on the bottom (closer to the bottom flange  136 ) of the pair manager  118 . The wire pairs  134  are threaded through holes (preferably two separate holes) in the pair manager  118  until the insulation  132  on each wire pair  134  is flush with the front face  138  of the pair manager  118 . The ground wires  174  are then pulled through the ground slot  140  on the bottom flange  136 . The pair manager  118  is pressed onto the PCB  120 . The ground wires  174  are then soldered (or otherwise electrically connected) to a PCB ground pad  144  on the underside of the PCB  120  (see, e.g.,  FIGS. 12 and 13 ). 
     According to the second technique and as shown in  FIGS. 8-12 , the pair manager  118  is first assembled to the PCB  120 , such as by using reflow, crimp, or resistance welding. The twin-ax wire pairs  134  are positioned to have their associated ground wires  174  on the top (closer to the top flange  148 ) of the pair manager  118 . The wire pairs  134  are threaded through the pair manager  118  until the insulation  132  on each wire pair  134  is flush with the front face  138  of the pair manager  118 . The ground wires  174  are then positioned on the ground boss(es)  142  on the top of the pair manager  118 . Each ground boss  142  preferably includes a slot (as shown) or hole through which the ground wires  174  may pass. The ground wires  174  are then connected to the pair manager  118 , such as by soldering or crimping. The location on the pair manager  118  at which the ground wires  174  are connected provides one or more electrical connections to the PCB ground pad  144  on the communication side of the PCB  120 . 
     To provide electrical connectivity between the twin-ax wire pairs  134  and the PCB  120 , the wire pairs  134  are soldered to signal pairs on the PCB  120 , as shown in  FIGS. 13 and 14 . The signal pairs on the PCB  120  may be used to provide tuned impedance matching (e.g. by introducing distributed or lumped capacitance and/or inductance through conductive traces or discrete components on the PCB  120 ) and provide an electrical connection to the host receptacle, which may be part of a network switch, for example. 
     The high-speed signals are sent from the host system through the connector onto the PCB where they propagate along micro strip transmission lines to the PCB/twin-ax interface. The micro strip lines are designed to ensure the proper characteristic impedance by maintaining inductance and capacitance characteristics along the length of the transmission line. Controlling the conductor widths, spacing, height above a ground plane, and dielectric material between the traces and the ground plane accomplish this. Impedance-matching techniques are generally known and will likely be specific to the particular application, wire gauge, and configuration for which the connector  100  is used. 
     Next, if desired, the assembly can be tested to ensure that electrical performance requirements are met. Then, in accordance with a preferred embodiment, the various components of the connector  100  are assembled, as shown generally in  FIGS. 15-25 . First, the latch  116  is inserted into an opening in the bottom shell  106 . The assembly comprising the PCB  120 , the pair manager  118 , the cable  102 , and the crimp  124  is placed over support rails in the bottom shell  106 . To prevent upside-down assembly, locating pins  150   a - b  offset from one-another are aligned with correspondingly offset PCB slots  146   a - b  on the PCB  120 . The crimp  124  is placed over a bottom shell opening  154  and pressed into position. The springs  122  are loaded into latch spring pockets  156  located on the upper surface (away from the bottom shell  106 ) of the latch  116 . The front end of the top shell  108  is inserted under the front end of the bottom shell  106 . The top shell  108  is then rotated down over the bottom shell  106  so that sidewalls of the top shell  108  and bottom shell  106  align and the top shell  108  aligns over bottom shell bosses  158  located in the bottom shell  108 . The bottom shell bosses  158  may be flared out to permanently assemble the bottom shell  106  and top  108  to become shell  104 . Other techniques (such as ultrasonic welding, fastening, etc.) may be used to complete the assembly of shell  104 . 
     The cable  102  is then crimped using crimp  124  and the bend radius control feature  160  is molded over the crimp  124  and the cable  102 . The latch release  114  (with attached pull tab  112 ) is inserted into slots on the back face of the shell  104 . Finally, as shown in  FIGS. 28 and 29 , the EMI gasket  110  may be attached to the shell  104  using adhesive or snaps, for example. 
       FIGS. 23-27  illustrate the latch release  114  and its operation in further detail. Each side of the latch release  114  preferably includes a latch cam  162  and a latch release snap  164 . The latch cam  162  includes a latch cam face  170  (see  FIG. 24 ) and the latch release snap  164  includes a snap deflection slot  166  (see  FIG. 25 ). 
     The latch release snap  164  deflects downward (toward its snap deflection slot  166 ) as the latch release  114  is being inserted into the shell  104  and retracts back upward into a top shell pocket  168 . This limits subsequent travel of the latch release  114  and prevents the latch release  114  from pulling out. A top portion of the latch release snap  164  preferably contacts the upper surface (i.e. stop face) of the top shell pocket  168 . 
     When the pull tab  112  is pulled, the latch cam face  170  on the latch release  114  applies an upward force to the latch cam feature  176  on the latch  116  (i.e. the latch cam feature  176  rides up the ramped latch cam face  170  to cause the latch  116  to move upward (toward the top shell  108 ), thereby compressing the springs  122 . This, in turn, causes the latch tooth  172  to recede into the bottom shell  106 , which allows the connector  100  to be removed from the host receptacle. This transition is shown in  FIG. 26  (latch release position before pull) and  FIG. 27  (latch release position after pull). The resulting spring-loaded latch is (a) preferably housed entirely inside the connector cavity and (b) retracted in for de-latching. De-latching is done by a latch-release pull motion translated into an inward pull on the latch. 
     Pair managers according to some embodiments of the present invention maintain the differential impedance of twin-ax conductive pairs with the foil shields surrounding the twin-ax pairs removed or folded back. Preferably, transmission line impedance is maintained along a great extent of the signal pathway. Because the pair manager provides an efficient capacitive coupling between signal ground and the shield of the twin-ax cable, the common-mode return path is well balanced, thus assuring signal fidelity. According to some embodiments, grounding provided by a pair manager is isolated from the chassis ground path of the connector shells in the DC domain. 
     Connectors  100  and corresponding pair managers  118  can be designed for different gauges of twin-ax cable. 
     Ground pads  144  on PCB  120  may be soldered to tabs (fingers  149 ) of the pair manager. 
     The choice of soft metals such as zinc or aluminum for the pair manager makes the tabs (fingers  149 ) of the pair manager easier to crimp, eliminating the need for an overmolded strain relief in the region of termination of the twin-ax pairs to a PCB  120  and eliminating a process step in the manufacture of an SFP+ cable assembly. Because overmolding is not necessary in the region of termination, the likelihood of delamination of the PCB  120  due to mismatches in thermal expansion coefficients is minimal when compared to prior art connectors. In addition, there is a low likelihood of moisture absorption in the region of termination for the operating life of the cable assembly. 
     In various embodiments, the pair manager  118  may be only crimped to the PCB  120 , crimped and then soldered to the PCB  120 , or only soldered to the PCB  120 . 
     The following is a summary of the connections between a twin-ax cable and elements of an SFP connector according to one embodiment of the present invention:
         The outer shield  128  of the twin-ax cable is connected to the shell  104  of the SFP+ connector via the crimp  124 .   The foil pair shields  132  of the twin-ax conductive pairs and the drain wire  174  are connected to the pair manager  118  by soldering and/or crimping.   The pair manager  118  in turn is connected to the signal ground of the PCB  120  via ground pads  144  on the top and bottom of the PCB  120  by soldering and/or crimping.   Internal ground planes  80  of the PCB  120  are connected to the signal ground I/O of the connector through vias  64  as shown in  FIGS. 28A-29B .   In addition, the conductive signal pairs of the twin-ax cable are terminated via soldering to trances on the PCB  120 .       

     In addition to the conductive connections described above, all of the shields, including the drain wire, and the ground planes of the paddle card are coupled to each other by capacitive reactance in the AC domain. 
     The signal ground is isolated in the DC domain from the chassis ground (provided by the outer shield  128 , shell  104 , and crimp  124 ) of the connector. Signal ground is provided by the PCB and pair manager assembly which, after mating with an SFP host port, connect to the signal ground of a backplane PCB in a switch or host server. This DC isolation is important for the function of differential signaling, because in some embodiments, without this DC isolation, the host port cannot discern the logic states of the signals, resulting in communication failure. 
     Pair managers  118  according to some embodiments of the present invention may be provided in more than one piece. 
     According to one embodiment of the present invention, the PCB  120  is provided with four conductive layers. The layers of the PCB  120  are illustrated in  FIGS. 28A ,  28 B,  29 A, and  29 B.  FIGS. 28A and 28B  illustrate, respectively, the internal bottom side (control side) layer  50  and top (communication side) conductive layers  60  of the PCB  120 . The ground pad(s)  144  of the bottom layer  50  are visible in  FIG. 28A  and the ground pads  18  of the top layer  60  are shown in  FIG. 28B . 
       FIGS. 29A and 29B  illustrate, respectively, the internal ground plane  70  above the bottom layer  50  and the internal ground plane  80  below the top layer  60 . Resistors and capacitors are labeled, respectively, as R and C, and U 1  indicates a microcontroller. The ground pad  144  shown in  FIG. 28A  connects through vias (not visible) to the internal ground plane  70  shown in  FIG. 9A . The ground pads  144  shown in  FIG. 28B  also connect to the internal ground plane  70  shown in  FIG. 29A . 
     The vias  62  shown in  FIG. 28B  connect to the ground plane  80  of  FIG. 29B , which in turn connects (by three vias) to the signal ground I/O through vias  64 . 
       FIGS. 30 and 31  show an alternative embodiment of a pair manager  200  that comprises top and bottom halves  202  and  204 . The top half of the split pair manager  200  has top aperture halves  206  incorporating a rib  208  that serves to keep a twin-ax pair in place more firmly within the holes formed when the top and bottom halves  202  and  204  are assembled together and the top aperture halves  206  sit over the lower aperture halves  207  as shown in  FIG. 31 . As shown in  FIG. 31 , the top half  202  is provided with rivet holes  210  that accept rivets  212  provided in the bottom half  204 . 
     In situations where multiple gauges of wires are being terminated to PCBs  120 , different pair managers are used. When these pair managers are provided in halves, the rivets  212  and rivet holes  210  may be appropriately sized and/or spaced to provide a keying feature so that proper halves are mated. An additional keying hole  214  can be provided on PCBs  120  to mate with a keying feature  216  provided on the bottom half  204 , helping to make sure that the proper PCB is mated with the proper pair manager for a particular wire gauge being used. 
     While particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and compositions disclosed herein, and that various modifications, changes, and variations may be apparent from the foregoing descriptions without departing from the spirit and scope of the invention.