Patent Publication Number: US-8968035-B2

Title: Communications plugs and patch cords with mode conversion control circuitry

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §120 as a divisional application of U.S. patent application Ser. No. 13/827,421, filed Mar. 14, 2013, the entire content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to communications connectors and, more particularly, to communications plugs such as RJ-45 plugs that may exhibit improved crosstalk performance when mated with a communications jack to form a mated plug jack connection, 
     BACKGROUND 
     Many hardwired communications systems use plug and jack connectors to connect a communications cable to another communications cable or to computer equipment. By way of example, high speed communications systems routinely use such plug and jack connectors to connect computers, printers and other devices to local area networks and/or to external networks such as the Internet.  FIG. 1  depicts a highly simplified example of such a hardwired high speed communications system that illustrates how plug and jack connectors may be used to interconnect a computer  11  to, for example, a network server  20 . 
     As shown in  FIG. 1 , the computer  11  is connected by a cable  12  to a communications jack  15  that is mounted in a wall plate  19 . The cable  12  is a patch cord that includes a communications plug  13 ,  14  at each end thereof. Typically, the cable  12  includes eight insulated conductors. As shown in  FIG. 1 , plug  14  is inserted into a cavity or “plug aperture”  16  in the front side of the communications jack  15  so that the contacts or “plug blades” of communications plug  14  mate with respective contacts of the communications jack  15 . If the cable  12  includes eight conductors, the communications plug  14  and the communications jack  15  will typically each have eight contacts. The communications jack  15  includes a wire connection assembly  17  at the back end thereof that receives a plurality of conductors (e.g., eight) from a second cable  18  that are individually pressed into slots in the wire connection assembly  17  to establish mechanical and electrical connections between each conductor of the second cable  18  and a respective one of a plurality of conductive paths through the communications jack  15 . The other end of the second cable  18  is connected to a network server  20  which may be located, for example, in a telecommunications closet. Communications plug  13  similarly is inserted into the plug aperture of a second communications jack (not pictured in  FIG. 1 ) that is provided in the back of the computer  11 . Thus, the patch cord  12 , the cable  18  and the communications jack  15  provide a plurality of electrical paths between the computer  11  and the network server  20 . These electrical paths may be used to communicate information signals between the computer  11  and the network server  20 . 
     When a signal is transmitted over a conductor (e.g., an insulated copper wire) in a communications cable, electrical noise from external sources may be picked up by the conductor, degrading the quality of the signal. In order to counteract such noise sources, the information signals in the above-described communications systems are typically transmitted between devices over a pair of conductors (hereinafter a “differential pair” or simply a “pair”) rather than over a single conductor. The two conductors of each differential pair are twisted tightly together in the communications cables and patch cords so that the eight conductors are arranged as four twisted differential pairs of conductors. The signals transmitted on each conductor of a differential pair have equal magnitudes, but opposite phases, and the information signal is embedded as the voltage difference between the signals carried on the two conductors of the pair. When the signal is transmitted over a twisted differential pair of conductors, each conductor in the differential pair often picks up approximately the same amount of noise from these external sources. Because the information signal is extracted by taking the difference of the signals carried on the two conductors of the differential pair, the subtraction process may mostly cancel out the noise signal, and hence the information signal is typically not disturbed. 
     Referring again to  FIG. 1 , it can be seen that a series of plugs, jacks and cable segments connect the computer  11  to the server  20 . Each plug, jack and cable segment includes four differential pairs, and thus a total of four differential transmission lines are provided between the computer  11  and the server  20  that may be used to carry two way communications therebetween (e.g., two of the differential pairs may be used to carry signals from the computer  11  to the server  20 , while the other two may be used to carry signals from the server  20  to the computer  11 ). The cascaded plugs, jacks and cabling segments shown in  FIG. 1  that provide connectivity between two end devices (e.g., computer  11  and server  20 ) is referred to herein as a “channel.” Thus, in most high speed communications systems, a “channel” includes four differential pairs. Unfortunately, the proximities of the conductors and contacting structures within each plug-jack connection (e.g., where plug  14  mates with jack  15 ) can produce capacitive and/or inductive couplings. These capacitive and inductive couplings in the connectors (and similar couplings that may arise in the cabling) give rise to another type of noise that is known as “crosstalk.” 
     In particular, “crosstalk” refers to unwanted signal energy that is capacitively and/or inductively coupled onto the conductors of a first “victim” differential pair from a signal that is transmitted over a second “disturbing” differential pair. The induced crosstalk may include both near-end crosstalk (NEXT), which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose induced voltage signal travels in an opposite direction to that of an originating, disturbing signal in a different path), and far-end crosstalk (FEXT), which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose signal travels in the same direction as the disturbing signal in the different path). Both types of crosstalk comprise an undesirable noise signal that interferes with the information signal that is transmitted over the victim differential pair. 
     While methods are available that can significantly reduce the effects of crosstalk within communications cable segments, the communications connector configurations that were adopted years ago—and which still are in effect in order to maintain backwards compatibility—generally did not arrange the contact structures so as to minimize crosstalk between the differential pairs in the connector hardware. For example, pursuant to the ANSI/TIA-568-C.2 standard approved Aug. 11, 2009 by the Telecommunications Industry Association, in the connection region where the contacts of a modular plug mate with the contacts of the modular jack (referred to herein as the “plug-jack mating region”), the eight contacts  1 - 8  of the jack must be aligned in a row, with the eight contacts  1 - 8  arranged as four differential pairs specified as depicted in  FIG. 2 . As known to those of skill in the art, under the TIA/EIA 568 type B configuration, contacts  4  and  5  in  FIG. 2  comprise pair  1 , contacts  1  and  2  comprise pair  2 , contacts  3  and  6  comprise pair  3 , and contacts  7  and  8  comprise pair  4 . Contacts  1 ,  3 ,  5  and  7  are the so-called “tip” contacts, while contacts  2 ,  4 ,  6  and  8  are the “ring” contacts. As is apparent from  FIG. 2 , this arrangement of the eight contacts  1 - 8  will result in unequal coupling between the differential pairs, and hence both NEXT and FEXT is introduced in each connector in industry standardized communications systems. The unequal coupling that occurs as a result of the industry standardized RJ-45 plug-jack interface is typically referred to as “offending” crosstalk. 
     As hardwired communications systems have moved to higher frequencies in order to support increased data rate communications, crosstalk in the plug and jack connectors has became a more significant problem. To address this problem, communications jacks now routinely include crosstalk compensation circuits that introduce “compensating” crosstalk that is used to cancel much of the “offending” crosstalk that is introduced in the plug-jack mating region as a result of the industry-standardized connector configurations. In order to ensure that plugs and jacks manufactured by different vendors will work well together, the industry standards specify amounts of offending crosstalk that must be generated between the various differential pair combinations in an RJ-45 plug for that plug to be industry-standards compliant. Thus, while it is now possible to manufacture RJ-45 plugs that exhibit much lower levels of offending crosstalk, it is still necessary to ensure that RJ-45 plugs inject the industry-standardized amounts of offending crosstalk between the differential pairs so that backwards compatibility will be maintained with the installed base of RJ-45 plugs and jacks. Typically, so-called “multi-stage” crosstalk compensation circuits are used. Such crosstalk circuits are described in U.S. Pat. No. 5,997,358 to Adriaenssens et al., the entire content of which is hereby incorporated herein by reference as if set forth fully herein. 
     Crosstalk can be classified as either differential crosstalk or as common mode crosstalk. Differential crosstalk refers to a crosstalk signal that appears as a difference in voltage between two conductors of a victim differential pair. This type of crosstalk degrades any information signal carried on the victim differential pair as the difference in voltage does not subtract out when the information signal carried on the victim differential pair is extracted by taking the difference of the voltages carried by the conductors on the victim differential pair. Common mode crosstalk refers to a crosstalk signal that appears on both conductors of a differential pair. Common mode crosstalk typically does not disturb the information signal on the victim differential pair, as the disturbing common mode signal is cancelled by the subtraction process used to recover the information signal on the victim differential pair. 
     Common mode crosstalk, however, can generate another type of crosstalk called “alien” crosstalk. Alien crosstalk refers to crosstalk that occurs between two communication channels. Alien crosstalk can arise, for example, in closely spaced connectors (e.g., patch panels) or in communications cables that are bundled together. For example, a differential pair in a first communications cable can crosstalk with a differential pair in a second, immediately adjacent communications cable. Common mode signals that may be carried on a differential pair are particularly likely to generate alien crosstalk, as common mode signals are generally not self-cancelling in the way that differential signals are. Obviously, physical separation between connectors and cables may be used to reduce alien crosstalk. However, this is typically impractical because bundling of cables and patch cords and locating communications connectors in close proximity on patch panels is common practice due to “real estate” constraints and/or ease of wire management. 
     SUMMARY 
     Pursuant to embodiments of the present invention, patch cords are provided that include a communications cable that has first through eighth conductors. The fourth and fifth conductors are twisted together to form a first twisted pair, the first and second conductors are twisted together to form a second twisted pair, the third and sixth conductors are twisted together to form a third twisted pair, and the seventh and eighth conductors are twisted together to form a fourth twisted pair. A plug is attached to the communications cable. This plug includes a housing that receives the communications cable and first through eighth plug contacts that include plug contact regions that are substantially aligned in a row in numerical order. The plug further includes a printed circuit board that has first through eighth conductive paths that connect the first through eighth conductors to the respective first through eighth plug contacts. A first portion of the first conductive path and a first portion of the second conductive path are routed as a transmission line, and a first portion of the sixth conductive path is routed therebetween. 
     In some embodiments, the first portion of the first conductive path, the first portion of the second conductive path and the first portion of the sixth conductive path are all on the same side of the printed circuit board. In other embodiments, the first portion of the first conductive path and the first portion of the second conductive path are on a first layer of the printed circuit board, and the first portion of the sixth conductive path is on a second layer of the printed circuit board that is different than the first layer. A first portion of the seventh conductive path and a first portion of the eighth conductive path may also be routed in side-by-side fashion as a transmission line, and a first portion of the third conductive path may be routed therebetween. The third and sixth conductive paths may cross over each other at least twice and/or may form an expanded loop on the printed circuit board. 
     In some embodiments, the first portion of the sixth conductive path may be configured to couple substantially equal amounts of energy onto the first portions of the first and second conductive paths when a signal is incident on the sixth conductive path. The first portion of the sixth conductive path that is routed between the first portions of the first and second conductive paths may comprise a differential-to-common mode crosstalk cancellation circuit that at least partially cancels the common mode crosstalk that is injected from the third plug contact onto the first and second plug contacts. At least a portion of the differential-to-common mode crosstalk cancellation circuit may be located on a front half of the printed circuit board that receives the first through eighth plug blades. 
     Pursuant to further embodiments of the present invention, patch cords are provided that include a communications cable that has first through eighth conductors. The fourth and fifth conductors are twisted together to form a first twisted pair, the first and second conductors are twisted together to form a second twisted pair, the third and sixth conductors are twisted together to form a third twisted pair, and the seventh and eighth conductors are twisted together to form a fourth twisted pair. A plug is attached to the communications cable. This plug includes a housing that receives the communications cable and first through eighth plug contacts that include plug contact regions that are substantially aligned in a row in numerical order. The plug further includes a printed circuit board that has first through eighth conductive paths that connect the first through eighth conductors to the respective first through eighth plug contacts. On the printed circuit board, a first portion of the second conductive path is closer to the seventh and eight conductive paths than is a first portion of the sixth conductive path, and a first portion of the seventh conductive path is closer to the first and second conductive paths than is a first portion of the third conductive path. 
     In some embodiments, the first portion of the sixth conductive path is routed between substantially parallel first portions of the first and second conductive paths. The first portion of the sixth conductive path may be substantially equidistant from the first portions of the first and second conductive paths. The first portion of the sixth conductive path may be configured to couple substantially equal amounts of energy onto the first portions of the first and second conductive paths. The first portions of the first and second conductive paths may be routed generally side-by-side as a differential transmission line, and the first portion of the sixth conductive path may be routed between the first portions of the first and second conductive paths. 
     Pursuant to still further embodiments of the present invention, patch cords are provided that include a communications cable that has first through fourth conductors. The first and second conductors form a first differential pair, and the third and fourth conductors form a second differential pair. A plug is attached to the communications cable. This plug includes a housing that receives the communications cable and first through fourth plug contacts. The plug further includes a printed circuit board that has first through fourth conductive paths that connect the first through fourth conductors to the respective first through fourth plug contacts. The third plug contact injects common mode crosstalk onto the first and second plug contacts, and the fourth conductive path includes a section that couples with the first and second conductive paths to at least partially cancel this common mode crosstalk. 
     In some embodiments, the first through fourth plug contacts include plug contact regions that are substantially aligned in a row in numerical order, and/or the third and fourth conductive paths form an expanded loop on the printed circuit board. First portions of the first and second conductive paths may be routed in side-by-side fashion as a transmission line, and a first portion of the fourth conductive path may be routed therebetween. The first portions of the first, second and fourth conductive paths may all be on the same side of the printed circuit board. The third and fourth conductive paths may cross over each other at least twice. The first portion of the fourth conductive path may be configured to couple substantially equal amounts of energy onto the first portions of the first and second conductive paths. 
     Pursuant to still further embodiments of the present invention, patch cords are provided that include a communications cable that has first through eighth conductors, where the fourth and fifth conductors are twisted together to form a first twisted pair, the first and second conductors are twisted together to form a second twisted pair, the third and sixth conductors are twisted together to form a third twisted pair, and the seventh and eighth conductors are twisted together to form a fourth twisted pair. A plug is attached to the communications cable. The plug has a housing that receives the communications cable, first through eighth plug contacts that include plug contact regions that are substantially aligned in a row in numerical order, and a printed circuit board that is at least partly within the housing. The printed circuit board includes first through eighth conductive paths that connect the first through eighth conductors to the respective first through eighth plug contacts. A first portion of the sixth conductive path is routed so that, when excited by a signal, it will couple substantially equal amounts of signal energy onto a first portion of the first conductive path and a first portion of the second conductive path. 
     In some embodiments, the first portion of the sixth conductive path includes a first current carrying path that is positioned adjacent the first portion of the first conductive path and a second current carrying path that is positioned adjacent the first portion of the second conductive path. In such embodiments, the first portion of the first conductive path, the first portion of the second conductive path and the first portion of the sixth conductive path may all be on the same layer of the printed circuit board. In some embodiments, the first portion of the first conductive path and the first portion of the second conductive path may be between the first current carrying path of the first portion of the sixth conductive path and the second current carrying path of the first portion of the sixth conductive path. In other embodiments, the first current carrying path of the first portion of the sixth conductive path may be vertically stacked with the first portion of the first conductive path and the second current carrying path of the first portion of the sixth conductive path may be vertically stacked with the first portion of the second conductive path 
     In some embodiments, the first portion of the first conductive path and the first portion of the second conductive path may be on a first layer of the printed circuit board, and the first portion of the sixth conductive path may be a widened trace that is on a second layer of the printed circuit board that is different than the first layer. In some embodiments, the first portion of the sixth conductive path may overlap the first portion of the first conductive path and the first portion of the second conductive path. The printed circuit board may be a flexible printed circuit board. The first portion of the sixth conductive path may be routed between the first portion of the first conductive path and the first portion of the second conductive path. 
     Pursuant to additional embodiments of the present invention, patch cords are provided that include a communications cable that has first through eighth conductors. The fourth and fifth conductors are twisted together to form a first twisted pair, the first and second conductors are twisted together to form a second twisted pair, the third and sixth conductors are twisted together to form a third twisted pair, and the seventh and eighth conductors are twisted together to form a fourth twisted pair. A plug is attached to the communications cable. This plug includes a housing that receives the communications cable and first through eighth plug contacts. The plug further includes a printed circuit board that has first through eighth conductive paths that connect the first through eighth conductors to the respective first through eighth plug contacts. A first crosstalk injection circuit is provided between the second conductive path and the sixth conductive path, and a second crosstalk injection circuit is provided between the first conductive path and the sixth conductive path. 
     In some embodiments, the first and second crosstalk injection circuits substantially cancel the common mode crosstalk injected from the third twisted pair onto the second twisted pair when the third twisted pair is excited differentially. The plug may also include a third crosstalk injection circuit between the second conductive path and the third conductive path. In such embodiments, the first, second and third crosstalk injection circuits may substantially cancel the common mode crosstalk injected from the third twisted pair onto the second twisted pair when the third twisted pair is excited differentially. In other embodiments, the plug includes a third crosstalk injection circuit that is provided between the first conductive path and the third conductive path. 
     In some embodiments, the first crosstalk injection circuit comprises a first capacitor on the printed circuit board between the second conductive path and the sixth conductive path. Likewise, the second crosstalk injection circuit may be a second capacitor on the printed circuit board between the first conductive path and the sixth conductive path. The first capacitor may connect to the second conductive path directly adjacent the second plug contact and may connect to the sixth conductive path directly adjacent the sixth plug contact. 
     Pursuant to yet additional embodiments of the present invention, RJ-45 communications plugs are provided that have first through eighth conductive paths where the fourth and fifth conductive paths are part of a first differential transmission line, the first and second conductive paths are part of a second differential transmission line, the third and sixth conductive paths are part of a third differential transmission line, and the seventh and eighth conductive paths are part of a fourth differential transmission line. The plugs further have first through eighth plug blades that are electrically connected to the respective first through eighth conductive paths, where the first through eighth plug blades aligned in a row in numerical order. A differential-to-common mode crosstalk cancellation circuit is provided that substantially cancels common mode crosstalk that is injected within the plug from the third differential transmission line onto the second differential transmission line when the third differential transmission line is excited differentially. Additionally, the third differential transmission line is configured to inject differential crosstalk onto the second differential transmission line when the third differential transmission line is excited differentially. 
     In some embodiments, the amount of differential crosstalk injected from the third transmission line onto the second differential transmission line when the third differential transmission line is excited by a differential signal may be an industry standards specified amount of offending crosstalk. The differential-to-common mode crosstalk cancellation circuit may comprise a first reactive circuit between the second conductive path and the sixth conductive path, and a second reactive circuit between the first conductive path and the sixth conductive path. The first reactive circuit may be a first capacitor on a printed circuit board and the second reactive circuit may be a second capacitor on the printed circuit board. In other embodiments, the first reactive circuit may be a first inductive coupling section on a printed circuit board between the second conductive path and the sixth conductive path, and the second reactive circuit may be a second inductive coupling section on the printed circuit board between the first conductive path and the sixth conductive path. 
     In some embodiments, the differential-to-common mode crosstalk cancellation circuit includes a third reactive circuit between the second conductive path and the third conductive path. The differential crosstalk injected onto the second transmission line by the third differential transmission line when the third differential transmission line is excited differentially is greater than twice amount of coupling between the second plug blade and the third plug blade minus twice the amount of coupling between the first plug blade and the third plug blade. In other embodiments, the differential-to-common mode crosstalk cancellation circuit includes a third reactive circuit between the first conductive path and the third conductive path. In these embodiments, the differential crosstalk injected onto the second transmission line by the third differential transmission line when the third differential transmission line is excited differentially may be less than twice amount of coupling between the second plug blade and the third plug blade minus twice the amount of coupling between the first plug blade and the third plug blade. The differential-to-common mode crosstalk cancellation circuit may substantially cancel the common mode crosstalk that is injected within the plug from the second differential transmission line onto the third differential transmission line when the second differential transmission line is excited differentially. 
     Pursuant to even further embodiments of the present invention, RJ-45 communications plugs are provided that have first through eighth conductive paths where the fourth and fifth conductive paths are part of a first differential transmission line, the first and second conductive paths are part of a second differential transmission line, the third and sixth conductive paths are part of a third differential transmission line, and the seventh and eighth conductive paths are part of a fourth differential transmission line. The first, third, fifth and seventh conductive paths are tip conductive paths and the second, fourth, sixth and eighth conductive paths are ring conductive paths. These plugs further have first through eighth plug blades that are electrically connected to the respective first through eighth conductive paths, the first through eighth plug blades aligned in a row in numerical order. An offending crosstalk circuit that is separate from the plug blades is provided that injects crosstalk between the second and third differential transmission lines, where the offending crosstalk circuit is between a ring conductive path and a tip conductive path. Additionally, a differential-to-common mode crosstalk cancellation circuit is provided that is electrically connected between the second differential transmission line and the third differential transmission line. 
     In some embodiments, the differential-to-common mode crosstalk cancellation circuit substantially cancels common mode crosstalk that is injected within the plug from the third differential transmission line to the second differential transmission line when the third differential transmission line is excited differentially. The differential-to-common mode crosstalk cancellation circuit may include a first reactive circuit between the second conductive path and the sixth conductive path, a second reactive circuit between the first conductive path and the sixth conductive path and a third reactive circuit between the second conductive path and the third conductive path. The first through third reactive circuits may comprise first through third capacitors on a printed circuit board. 
     Pursuant to further embodiments of the present invention, RJ-45 communications plugs provided that include first through eighth conductive paths that are arranged as first through fourth differential transmission lines. A capacitor and a resistor are electrically coupled in series between two of the first through eighth conductive paths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic diagram illustrating the use of conventional communications plugs and jacks to interconnect a computer with network equipment. 
         FIG. 2  is a schematic diagram illustrating the TIA/EIA 568 type B modular jack contact wiring assignments for a conventional 8-position communications jack as viewed from the front opening of the jack. 
         FIG. 3  is a stylized partial perspective view of blades and conductors of a prior art communications plug. 
         FIG. 4  is a perspective view of a patch cord according to certain embodiments of the present invention. 
         FIG. 5  is a top, rear perspective view of a plug that is included on the patch cord of  FIG. 4 . 
         FIG. 6  is a bottom, rear perspective view of the plug of  FIG. 5 . 
         FIG. 7  is a side view of the plug of  FIG. 5 . 
         FIG. 8  is a perspective view of the blades and printed circuit board of the plug of  FIG. 5 . 
         FIG. 9  is a perspective view of an alternative printed circuit board that may be used in the plug of  FIG. 5 . 
         FIGS. 9A-9D  are schematic illustrations of portions of additional alternative printed circuit boards that may be used in the plug of  FIG. 5 . 
         FIG. 10  is a schematic circuit diagram of a front portion of the printed circuit board of  FIG. 8  that illustrates four printed circuit board capacitors that may be provided that inject offending crosstalk between various of the plug blades that are mounted on the printed circuit board. 
         FIG. 10A  is a schematic circuit diagram of a front portion of a revised version of the printed circuit board of  FIG. 8  according to embodiments of the present invention. 
         FIG. 11  is a schematic diagram of a known crosstalk compensation scheme that compensates for differential-to-differential crosstalk. 
         FIG. 12  is a schematic diagram of a known crosstalk compensation scheme that compensates for differential-to-common mode crosstalk. 
         FIGS. 13A-13C  are schematic diagrams of crosstalk compensation schemes for communications plugs according to embodiments of the present invention. 
         FIG. 14  is a perspective view of the blades and printed circuit board of a communications plug according to further embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to communications plugs such as RJ-45 plugs. As used herein, the terms “forward” and “front” and derivatives thereof refer to the direction defined by a vector extending from the center of the plug toward the portion of the plug that is first received within a plug aperture of a jack when the plug is mated with a jack. Conversely, the terms “rearward” and “back” and derivatives thereof refer to the direction directly opposite the forward direction. The forward and rearward directions define the longitudinal dimension of the plug. The vectors extending from the center of the plug toward the respective sidewalls of the plug housing defines the transverse (or lateral) dimension of the plug. The transverse dimension is normal to the longitudinal dimension. The vectors extending from the center of the plug toward the respective top and bottom walls of the plug housing (where the top wall of the plug housing is the wall that includes slots that expose the plug blades) defines the vertical dimension of the plug. The vertical dimension of the plug is normal to both the longitudinal and transverse dimensions. 
     Pursuant to embodiments of the present invention, communications plugs, as well as patch cords that include such communications plugs, are provided that may exhibit reduced levels of differential-to-common mode crosstalk (which is also referred to as “mode conversion”). By reducing the amount of mode conversion that occurs in a communications plug, the need to compensate for such mode conversion in a mating communications jack may be reduced. Moreover, all other factors equal, it may be more efficient to reduce such common mode crosstalk in the plug rather than having to cancel it in the mating jack, since typically most offending crosstalk is generated in the plug and attempting to cancel it in mating jack is subject to the limitations imposed by the transmission delay between the offending and compensating crosstalk. The plugs according to some embodiments of the present invention may substantially cancel the differential-to-common mode crosstalk that arises between selected of the differential transmission lines in the communications plug, while still providing any industry standardized amounts of differential-to-differential crosstalk between these differential transmission lines. 
     In some embodiments, the communications plug may comprise an RJ-45 plug. The RJ-45 plug may have a printed circuit board that includes first through eighth conductive paths and first through eighth plug blades that are mounted on the printed circuit board and connected to the respective first through eighth conductive paths. The eight conductive paths and plug blades may be arranged as the four differential transmission lines with the conductive paths numbered pursuant to the TIA/EIA 568 type B configuration. The third and sixth conductive paths (i.e., the third differential transmission line) may form an expanded loop on the printed circuit board in order to cancel differential-to-common mode crosstalk that arises between (1) the plug blades of the second and third differential transmission lines and/or (2) the plug blades of the third and fourth differential transmission lines. This expanded loop may substantially cancel the common mode crosstalk injected by the third plug blade onto the first and second plug blades and by the fourth plug blade onto the seventh and eighth plug blades. 
     In some embodiments, a first portion of the first conductive path and a first portion of the second conductive path may be routed as a transmission line, and a first portion of the sixth conductive path may be routed between the first portion of the first conductive path and the first portion of the second conductive path. The first portion of the sixth conductive path may be configured to couple substantially equal amounts of energy onto the first portion of the first conductive path and the first portion of the second conductive path when a signal is incident on the sixth conductive path. A first portion of the seventh conductive path and a first portion of the eighth conductive path may similarly be routed as a transmission line, and a first portion of the third conductive path may be routed between the first portion of the seventh conductive path and the first portion of the eighth conductive path. 
     Pursuant to further embodiments of the present invention, communications plugs are provided that include eight plug blades and a printed circuit board that has eight conductive paths that are electrically connected to respective ones of the eight plug blades. The plug blades and the conductive paths may be arranged and numbered pursuant to the TIA/EIA 568 type B configuration. The plug may further include a first crosstalk injection circuit between the second conductive path and the sixth conductive path and a second crosstalk injection circuit between the first conductive path and the sixth conductive path. In some embodiments, the plug may further include a third crosstalk injection circuit between the third conductive path and either the first conductive path or the second conductive path. 
     The first and second crosstalk injection circuits (and the third crosstalk injection circuit, if provided) may substantially cancel the differential-to-common mode crosstalk injected from the third differential pair onto the second differential pair. The crosstalk injection circuits may comprise, for example, capacitors that are implemented on the printed circuit board. 
     Pursuant to still further embodiments of the present invention, communications plugs are provided that include first through fourth differential transmission lines. These plugs further include a differential-to-common mode crosstalk cancellation circuit that substantially cancels differential-to-common mode crosstalk that is injected within the plug from the third differential transmission line onto the second differential transmission line. Moreover, the third differential transmission line in these plugs is configured to inject differential-to-differential crosstalk onto the second transmission line. 
     Patch cords are also provided that include the above-described communications plugs. 
     Embodiments of the present invention will now be discussed in greater detail with reference to the drawings. 
     As discussed above, differential-to-common mode crosstalk may be injected from a first differential transmission line to a second differential transmission line in a communications connector such as a modular plug or jack (e.g., from pair  3  to pair  2  and/or to pair  4  in an RJ-45 jack). This differential-to-common mode crosstalk may give rise to alien crosstalk that may degrade the performance of other channels in the communications system in which the connectors are used. The prior art has suggested at least two solutions to the above-described problem of differential-to-common mode crosstalk. In the first solution, the differential-to-common mode crosstalk that is generated in the plug of a mated plug-jack connection and in the plug-jack mating area of the jack is compensated for in the jack. This approach is illustrated in U.S. Pat. No. 5,967,853, which is discussed in greater detail herein, and in U.S. Pat. No. 7,204,722 (“the &#39;722 patent”), which discloses including a crossover in the contact wires of pair  3  in order to cancel such differential-to-common mode crosstalk. In the second solution, an expanded loop on the conductors of pair  3  is provided in an otherwise conventional RJ-45 plug. This approach is illustrated in U.S. Pat. No. 7,220,149 (“the &#39;149 patent”). As explained in the &#39;149 patent, both the plug blades and conductors of pair  3  in most conventional plugs are spatially unbalanced relative to the outside pairs  2  and  4 , particularly in the plug blades and the region approaching the blades. The &#39;149 patent discloses providing an expanded loop in the conductors of pair  3  that corrects for the spatial imbalance between (a) pairs  2  and  3  and (b) pairs  3  and  4  caused by the positions of the blades and conductors in a conventional plug. 
       FIG. 3  is a stylized partial perspective view of blades and conductors of the prior art plug  30  disclosed in the &#39;149 patent as a solution to the problem of mode conversion. 
     As shown in  FIG. 3 , the plug  30  includes eight blades  32   a ,  32   b ,  34   a ,  34   b ,  36   a ,  36   b ,  38   a ,  38   b  and eight conductors  40   a ,  40   b ,  42   a ,  42   b ,  44   a ,  44   b ,  46   a ,  46   b  that are twisted into pairs and attached to the blades in the TIA/EIA 568 B configuration pairings. The conductors  44   a ,  44   b  of pair  3  are arranged such that, after a first crossover point  45  adjacent the blade region, the conductors  44   a ,  44   b  form an expanded loop  48  that terminates at a second crossover point  52 . The expanded loop  48  includes segments  50   a ,  50   b  that are positioned adjacent to conductors  42   a ,  42   b  of pair  2  and conductors  46   a ,  46   b  of pair  4 , respectively, and that are spaced apart from conductors  40   a ,  40   b  of pair  1 . The expanded loop reduces mode conversion that would otherwise occur between (a) pairs  2  and  3  and (b) pairs  3  and  4 . 
       FIGS. 4-10  illustrate a patch cord  100  and various components thereof according to certain embodiments of the present invention. In particular,  FIG. 4  is a perspective view of the patch cord  100 .  FIG. 5  is a top-rear perspective view of a plug  116  that is included on the patch cord  100  of  FIG. 4 .  FIG. 6  is a bottom-rear perspective view of the plug  116 .  FIG. 7  is a side view of the plug  116 .  FIG. 8  is a perspective view of the plug contacts  141 - 148  and a printed circuit board  150  of plug the  116  of  FIGS. 5-7 .  FIG. 9  is a perspective view of an alternative printed circuit board  150 ′ that may be used in the plug  FIG. 5 . Finally,  FIG. 10  is a schematic circuit diagram of a front portion of the printed circuit board  150  that illustrates four printed circuit board capacitors that may be provided that inject offending crosstalk between various of the plug blades. 
     As shown in  FIG. 4 , the patch cord  100  includes a cable  109  that has eight insulated conductors  101 - 108  enclosed in a jacket  110  (the conductors  101 - 108  are not individually numbered in  FIG. 4 , and conductors  104  and  105  are not visible in  FIG. 4 ). The insulated conductors  101 - 108  may be arranged as four twisted pairs of conductors, with conductors  104  and  105  twisted together to form twisted pair  111  (pair  111  is not visible in  FIG. 4 ), conductors  101  and  102  twisted together to form twisted pair  112 , conductors  103  and  106  twisted together to form twisted pair  113 , and conductors  107  and  108  twisted together to form twisted pair  114 . Each twisted pair  111 - 114  may carry a differential signal. A separator  115  such as a tape separator or a cruciform separator may be provided that separates one or more of the twisted pairs  111 - 114  from one or more of the other twisted pairs  111 - 114 . A first plug  116  is attached to a first end of the cable  109  and a second plug  118  is attached to the second end of the cable  109  to form the patch cord  100 . 
       FIGS. 5-7  are enlarged views that illustrate the first plug  116  of the patch cord  100 . A rear cap of the plug housing and various wire grooming and wire retention mechanisms are omitted to simplify these drawings. As shown in  FIGS. 5-7 , the communications plug  116  includes a housing  120  that has a bi-level top face  122 , a bottom face  124 , a front face  126 , and a rear opening  128  that receives a rear cap (not shown). A plug latch  129  extends from the bottom face  124 . The top and front faces  122 ,  126  of the housing  120  include a plurality of longitudinally extending slots. The communications cable  109  (see  FIG. 4 ) is received through the rear opening  128 . The rear cap (not shown) locks into place over the rear opening  128  of housing  120  and includes an aperture that receives the communications cable  109 . 
     As is also shown in  FIGS. 5-7 , the communications plug  116  further includes a printed circuit board  150  which is disposed within the housing  120 , and a plurality of plug contacts  141 - 148  in the form of low profile plug blades that are mounted at the forward edge of the printed circuit board  150 . The top and front surfaces of the plug blades  141 - 148  are exposed through the slots in the top face  122  and front face  126  of the housing  120 . The housing  120  may be made of an insulative plastic material that has suitable electrical breakdown resistance and flammability properties such as, for example, polycarbonate, ABS, ABS/polycarbonate blend or other dielectric molded materials. 
     The conductors  101 - 108  may be maintained in pairs within the plug  116 . A cruciform separator  130  may be included in the rear portion of the housing  120  that separates each pair  111 - 114  from the other pairs  111 - 114  in the cable  109  to reduce crosstalk in the plug  116 . The conductors  101 - 108  of each pair  111 - 114  may be maintained as a twisted pair all of the way from the rear opening  128  of plug  116  up to the back edge of the printed circuit board  150 . 
       FIG. 8  is a perspective top view of the printed circuit board  150  and the plug blades  141 - 148  that illustrate these structures in greater detail.  FIG. 8  also shows how the conductors  101 - 108  of communications cable  109  may be electrically connected to the respective plug blades  141 - 148  through the printed circuit board  150 . The printed circuit board  150  may comprise, for example, a conventional printed circuit board, a specialized printed circuit board (e.g., a flexible printed circuit board) or any other appropriate type of wiring board. In the depicted embodiment, the printed circuit board  150  comprises a conventional multi-layer printed circuit board. 
     As shown in  FIG. 8 , the printed circuit board  150  includes four plated pads  151 ,  152 ,  154 ,  155  on a top surface thereof and four plated pads  153 ,  156 - 158  on a bottom surface thereof. The insulation is removed from an end portion of each of the conductors  101 - 108  (see  FIGS. 4-6 ) and the metal (e.g., copper) core of each conductor  101 - 108  may be soldered, welded or otherwise attached to a respective one of the plated pads  151 - 158 . It will be appreciated that other techniques may be used for terminating the conductors  101 - 108  to the printed circuit board  150 . It will also be appreciated that in other embodiments different numbers of the conductors  101 - 108  may be mounted on the top and bottom surfaces of the printed circuit board  150  (e.g., all eight on one surface, six on one surface and two on another surface, etc.). 
     The plug blades  141 - 148  are configured to make mechanical and electrical contact with respective contacts, such as, for example, spring jackwire contacts, of a mating communications jack. Each of the eight plug blades  141 - 148  is mounted at the front portion of the printed circuit board  150 . The plug blades  141 - 148  may be substantially aligned in a side-by-side relationship along the transverse dimension. Each of the plug blades  141 - 148  includes a first section that extends forwardly (longitudinally) along a top surface of the printed circuit board  150 , a transition section that curves through an angle of approximately ninety degrees and a second section that extends downwardly from the first section along a portion of the front edge of the printed circuit board  150 . The portion of each plug blade  141 - 148  that is in physical contact with a contact structure (e.g., a jackwire contact) of a mating jack during normal operation is referred to herein as the “plug-jack mating point” of the plug contact  141 - 148 . The plug contacts  141 - 148  are also referred to herein as “plug blades.” 
     In some embodiments, each of the plug blades  141 - 148  may comprise, for example, an elongated metal strip having a length of approximately 140 mils, a width of approximately 20 mils and a height (i.e., a thickness) of approximately 20 mils. Each plug blade  141 - 148  may include a projection that extends downwardly from the bottom surface of the first section of the plug blade. The printed circuit board  150  includes eight metal-plated vias  131 - 138  that are arranged in two rows along the front edge thereof. The downwardly-extending projections of each plug blade  141 - 148  is received within a respective one of the metal-plated vias  131 - 138  where it may be press-fit, welded or soldered into place to mount the plug blades  141 - 148  on the printed circuit board  150 . In some embodiments, the projections may be omitted and the plug blades  141 - 148  may be soldered or welded directly onto conductive structures (e.g., pads) that are deposited on top of the respective vias  131 - 138 . 
     Turning again to  FIG. 8  it can be seen that a plurality of conductive paths  161 - 168  are provided on the top and bottom surfaces of the printed circuit board  150 . Each of these conductive paths  161 - 168  electrically connects one of the plated pads  151 - 158  to a respective one of the metal-plated vias  131 - 138  so as to provide a conductive path between each of the conductors  101 - 108  that are terminated onto the plated pads  151 - 158  and a respective one of the plug blades  141 - 148  that are mounted in the metal-plated vias  131 - 138 . Each conductive path  161 - 168  may comprise, for example, one or more conductive traces that are provided on one or more layers of the printed circuit board  150 . When a conductive path  161 - 168  includes conductive traces that are on multiple layers of the printed circuit board  150 , metal-filled through holes (or other layer-transferring structures known to those skilled in this art) may be provided that provide an electrical connection between the conductive traces on different layers of the printed circuit board  150 . 
     A total of four differential transmission lines  171 - 174  are provided through the plug  116 . The first differential transmission line  171  includes the end portions of conductors  104  and  105 , the plated pads  154  and  155 , the conductive paths  164  and  165 , and the plug blades  144  and  145 . The second differential transmission line  172  includes the end portions of conductors  101  and  102 , the plated pads  151  and  152 , the conductive paths  161  and  162 , and the plug blades  141  and  142 . The third differential transmission line  173  includes the end portions of conductors  103  and  106 , the plated pads  153  and  156 , the conductive paths  163  and  166 , and the plug blades  143  and  146 . The fourth differential transmission line  174  includes the end portions of conductors  107  and  108 , the plated pads  157  and  158 , the conductive paths  167  and  168 , and the plug blades  147  and  148 . As shown in  FIG. 8 , the conductive paths that form each the differential transmission lines  171 ,  172  and  174  are generally run together, side-by-side, on the printed circuit board  150 , which may provide improved impedance matching. 
     In contrast, the conductive paths  163  and  166  that form the third differential transmission line  173  do not run in a side-by-side fashion across the printed circuit board  150 . Instead, adjacent the conductive pad  156 , conductive path  166  transitions from the bottom surface of printed circuit board  150  to the top surface at a first conductive via  191 . The top of a first conductive via  191  is positioned between conductive paths  161  and  162 , and conductive path  166  runs between conductive paths  161  and  162  from the top of the first conductive via  191  to the top of a second conductive via  192 . Conductive path  166  then transitions at the second conductive via  192  to the bottom surface of printed circuit board  150 , where it is routed to connect to the conductive via  136  that is used to mount plug blade  146  onto the printed circuit board  150 . 
     In a similar fashion, conductive path  163  is routed from conductive pad  153  to the other side of printed circuit board  150 , where it transitions from the bottom surface of the printed circuit board  150  to the top surface at a third conductive via  193 . Conductive path  163  then travels a short distance on the top surface of printed circuit board  150  to a fourth conductive via  194  that transitions conductive path  163  back to the bottom surface of the printed circuit board  150 . The bottom of the fourth conductive via  194  is positioned between conductive paths  167  and  168 , and conductive path  163  runs between conductive paths  167  and  168  from the bottom of the fourth conductive via  194  to the bottom of a fifth conductive via  195 . Conductive path  163  then transitions at the fifth conductive via  195  to the top surface of printed circuit board  150 , where is routed to connect to a sixth conductive via  196 . Conductive path  163  then transition at the sixth conductive via  196  back to the bottom surface of the printed circuit board  150  where it is routed to the conductive via  133  that is used to mount plug blade  143  onto the printed circuit board  150 . Conductive vias  193 - 196  are merely used to transition conductive path  163  between the top and bottom surfaces of the printed circuit board  150  so that conductive path  163  may cross other of the conductive paths  161 - 168  without short-circuiting. 
     As shown in  FIG. 8 , the above-described routing of conductive paths  163  and  166  forms a loop  190  on the printed circuit board  150 . In particular, conductive paths  163  and  166  cross over each other at two crossover points  197 ,  198 , thereby, in effect, carrying over the twist that is present in conductors  103  and  106  onto the printed circuit board  150 . Moreover, instead of maintaining a tight twist as is the case in the conductors  103 ,  106 , conductive paths  163  and  166  spread far apart on the printed circuit board  150  so that the loop  190  is an “expanded loop”  190 . 
     As is apparent from  FIG. 8 , plug blade  146  will couple more heavily with plug blades  147  and  148  than will plug blade  143 , forming a first unbalanced coupling region  201 . As a result of the unbalanced coupling in region  201 , when a signal is transmitted over differential transmission line  173 , unequal amounts of signal energy will flow from conductors  103  and  106  of differential transmission line  173  onto differential transmission line  174  in the plug blade region of plug  116 , thereby injecting differential-to-common mode crosstalk from differential transmission line  173  onto differential transmission line  174 . In a similar fashion, plug blade  143  will couple more heavily with plug blades  141  and  142  than will plug blade  146 , forming a second unbalanced coupling region  202 . As a result of the unbalanced coupling in region  202 , when a signal is transmitted over differential transmission line  173 , unequal amounts of signal energy will flow from conductors  103  and  106  of differential transmission line  173  onto differential transmission line  172  in the plug blade region of plug  116 , thereby injecting differential-to-common mode crosstalk from differential transmission line  173  onto differential transmission line  172 . As noted above, this differential-to-common mode crosstalk may generate alien crosstalk in other channels of the communications system that includes plug  116 , degrading the performance of those other communications channels. 
     The expanded loop  190  is provided in differential transmission line  173  to reduce or cancel the differential-to-common mode crosstalk that is injected from differential transmission line  173  onto differential transmission lines  172  and  174 . In particular, by routing a segment  166 ′ of conductive path  166  so that it runs between segments  161 ′,  162 ′ of differential transmission line  172 , while corresponding segment  163 ′ of conductive path  163  is maintained far away from segments  161 ′,  162 ′ of differential transmission line  172 , a third unbalanced coupling region  203  is formed in plug  116 . This third unbalanced coupling region  203  injects differential-to-common mode crosstalk from differential transmission line  173  onto differential transmission line  172  that has the opposite polarity of the differential-to-common mode crosstalk that is injected in region  202 , and which hence acts to cancel the differential-to-common mode crosstalk that is injected in region  202 . Similarly, by routing a segment  163 ′ of conductive path  163  so that it runs between segments  167 ′,  168 ′ of differential transmission line  174 , while corresponding segment  166 ′ of conductive path  166  is maintained far away from segments  167 ′,  168 ′ of differential transmission line  174 , a fourth unbalanced coupling region  204  is formed in plug  116 . This fourth unbalanced coupling region  204  injects differential-to-common mode crosstalk from differential transmission line  173  onto differential transmission line  174  that has the opposite polarity of the differential-to-common mode crosstalk that is injected in region  201 , and which hence acts to cancel the differential-to-common mode crosstalk that is injected in region  201 . 
     As shown in  FIG. 8 , the segments  161 ′,  162 ′ of differential transmission line  172  that couple with segment  166 ′ of conductive path  166  are not twisted (as is the case with the plug design disclosed in the aforementioned &#39;149 patent), but instead comprise a pair of generally parallel trace segments  161 ′,  162 ′ on the printed circuit board  150 . In order to have generally equal coupling between segment  166 ′ of conductive path  166  and the segments  161 ′,  162 ′ of differential transmission line  172 , segment  166 ′ is routed between and parallel to the segments  161 ′,  162 ′ and is generally equidistant from each of segments  161 ′ and  162 ′ in the region  203  where the three conductive trace segments  161 ′,  162 ′  166 ′ are routed generally in parallel to each other on the printed circuit board  150 . In like fashion, the segments  167 ′,  168 ′ of differential transmission line  174  that couple with segment  163 ′ of conductive path  163  are not twisted, but instead comprise a pair of generally parallel trace segments  167 ′,  168 ′ on the printed circuit board  150 . In order to have generally equal coupling between segment  163 ′ of conductive path  163  and the segments  167 ′,  168 ′ of differential transmission line  174 , segment  163 ′ is routed between and parallel to the segments  167 ′,  168 ′ and is generally equidistant from each of segments  167 ′ and  168 ′ in the region  204  where the three conductive trace segments  166 ′  167 ′,  168 ′ are routed generally in parallel to each other on the printed circuit board  150 . 
     As is also shown in  FIG. 8 , in the region  203  where conductive trace segments  161 ′,  162 ′ and  166 ′ run generally in parallel, conductive path  162  is closer to differential transmission line  174  than is conductive path  166 . This feature results because conductive path  166  is routed between conductive paths  161  and  162 . Similarly, in the region  204  where conductive trace segments  167 ′,  168 ′ and  163 ′ run generally in parallel, conductive path  167  is closer to differential transmission line  172  than is conductive path  163 . This feature results because conductive path  163  is routed between conductive paths  167  and  168 . Once again, this is in contrast to the plug design of the &#39;149 patent where the expanded loop on pair  3  stays between the outside twisted pairs. 
     In the particular embodiment depicted in  FIG. 8 , segments  161 ′,  162 ′ are sufficiently close together that there is not sufficient room for the first and second conductive vias  191 ,  192  therebetween without the danger of a short circuit and/or undesired effects on the impedance of differential transmission line  172  from the vias  191 ,  192 . Accordingly, conductive paths  161  and  162  include bends/arcuations  199  where the conductive paths  161 ,  162  split farther apart to accommodate the conductive vias  191 ,  192 . Similar bends/arcuations  199  are provided in conductive paths  167 ,  168  to accommodate the conductive vias  194 ,  195 . 
     While in the embodiment of  FIG. 8 , the segment  166 ′ of conductive path  166  is routed between segments  161 ′,  162 ′ of differential transmission line  172  on the same side of the printed circuit board  150  (and segment  163 ′ of conductive path  163  is likewise routed between segments  167 ′,  168 ′ of differential transmission line  174  on the same side of the printed circuit board  150 ), it will be appreciated that embodiments of the present invention are not limited to this configuration. For example,  FIG. 9  illustrates an alternative printed circuit board  150 ′ in which segment  166 ′ of conductive path  166  is routed between segments  161 ′,  162 ′ of differential transmission line  172  on a different layer of the printed circuit board  150 ′ (i.e., segments  161 ′,  162 ′ are on a first layer of printed circuit board  150 ′, while segment  166 ′ is on a second different layer of printed circuit board  150 ′). In some embodiments, the first layer could be, for example, a top layer and the second layer could be a bottom layer, while in other embodiments the second layer could be an intermediate layer. Similarly, segment  163 ′ of conductive path  163  is routed between segments  167 ′,  168 ′ of differential transmission line  174  but on a different layer of the printed circuit board  150 ′ (i.e., segments  167 ′,  168 ′ are on the bottom layer of printed circuit board  150 ′, while segment  166 ′ is on an intermediate layer or on the top layer of printed circuit board  150 ′). Such a design may be used where the dielectric layer(s) of printed circuit board  150 ′ are sufficiently thin such that sufficient coupling may be achieved between traces that run in overlapping or near overlapping fashion on the top and bottom sides of the printed circuit board  150 ′. Segment  166 ′ of conductive path  166  may be equidistant from segments  161 ′,  162 ′ of differential transmission line  172 , and segment  163 ′ of conductive path  163  may be equidistant from segments  167 ′,  168 ′ of differential transmission line  174 . The printed circuit board  150 ′ may be a conventional printed circuit board or a flexible printed circuit board. 
     As noted above, in some embodiments, segments  163 ′ and/or  166 ′ may be routed on an intermediate layer of the printed circuit board  150 ′. In order to ensure that intermediate printed circuit board layers can manage the current flow without, excessive heating, the segments  163 ′ and/or  166 ′ may be widened to reduce the current density per unit volume in these conductive traces. Notably, the widened trace segments  166 ′ and  163 ′ may exhibit increased capacitive coupling with the segments  161 ′,  162 ′ of differential transmission line  172  and the  167 ′,  168 ′ of differential transmission line  174 , respectively. Such increased capacitive coupling may be disadvantageous in some cases, as it may be more effective to locate as much of the capacitive coupling as possible very near the plug jack mating point. Routing segments  166 ′ and  163 ′ between the segments  161 ′,  162 ′ of differential transmission line  172  and the  167 ′,  168 ′ of differential transmission line  174 , respectively, may also negatively impact the return loss on differential transmission lines  172  and  174 . 
       FIGS. 9A-9D  schematically illustrate additional configurations for the coupling region  203 . These configurations may exhibit reduced capacitive coupling and/or reduced impact on the return loss on differential transmission line  172 . In the plan views of  FIGS. 9B-9D , the solid traces are conductive traces that are on a top layer of the printed circuit board and the cross-hatched traces are traces that are on an intermediate or bottom layer of the printed circuit board. It will be appreciated that these designs may also be implemented on conductive segments  163 ′,  167 ′ and  168 ′ to implement coupling region  204 . 
     Turning first to  FIG. 9A , another implementation of the coupling region  203  is illustrated under reference numeral  203 - 1 .  FIG. 9A  is a schematic cross-sectional view of a portion of the printed circuit board  150 - 1 . In the embodiment of  FIG. 9A , segments  161 ′ and  162 ′ are routed in a vertically stacked arrangement on the top and bottom sides of the printed circuit board  150 - 1  (which typically would be implemented as a flexible printed circuit board). Segment  166 ′ is routed on an intermediate layer of the printed circuit board  150 - 1 ′ between segments  161 ′ and  162 ′. Segment  166 ′ may be equidistant from segments  161 ′ and  162 ′, and may be generally vertically stacked with segments  161 ′ and  162 ′. Segment  166 ′ may be wider than segments  161 ′ and  162 ′ in order to reduce the current density in segment  166 ′ since segment  166 ′ is implemented in an intermediate layer of the printed circuit board  150 - 1 . 
     Turning next to  FIG. 9B , another implementation  203 - 2  of the coupling region  203  is illustrated that is implemented on a flexible printed circuit board  150 - 2 .  FIG. 9B  is a schematic plan view of a portion of the printed circuit board  150 - 2 . In the embodiment of  FIG. 9B , segments  161 ′ and  162 ′ are routed in a side-by-side fashion on a first layer of the printed circuit board  150 - 2  (e.g., on the top layer). Segment  166 ′ is routed on a different layer of the printed circuit board  150 - 2  such as an intermediate layer of a bottom layer. Segment  166 ′ is routed to overlap segments  161 ′ and  162 ′, as shown. The centerline of segment  166 ′ may be generally equidistant from the centerlines of segments  161 ′ and  162 ′. Segment  166 ′ may be wider than segments  161 ′ and  162 ′ in order to reduce the current density in segment  166 ′ if it is implemented in an intermediate layer of the printed circuit board  150 - 2 . 
     Turning next to  FIG. 9C , another implementation  203 - 3  of the coupling region  203  is illustrated that is implemented on a flexible printed circuit board  150 - 3 .  FIG. 9C  is a schematic plan view of a portion of the printed circuit board  150 - 3 . In the embodiment of  FIG. 9C , segments  161 ′ and  162 ′ are again routed in a side-by-side fashion on a first layer of the printed circuit board  150 - 3  (e.g., on the top layer). Segment  166 ′ is routed on a different layer of the printed circuit board  150 - 3  such as an intermediate layer of a bottom layer. In the embodiment of  FIG. 9C , segment  166 ′ splits into two separate current paths at a first junction  206 - 1 . Segment  166 ′ then recombines into a single current path at a second junction  206 - 2 . The two current paths for segment  166 ′ that are provided between the junctions  206 - 1 ,  206 - 2  are routed to overlap the respective segments  161 ′,  162 ′, as shown. Thus, segment  161 ′ may be generally vertically stacked with one of the two current paths of segment  166 ′ and segment  162 ′ may be generally vertically stacked with the other of the two current paths of segment  166 ′. The two current paths of segment  166 ′ may be equidistant from segments  161 ′ and  162 ′. By splitting the current on segment  166 ′ along two current paths it may be possible to use thinner trace segments, as shown. 
     Turning next to  FIG. 9D , another implementation  203 - 4  of the coupling region  203  is illustrated that is implemented on a flexible printed circuit board  150 - 4 .  FIG. 9D  is a schematic plan view of a portion of the printed circuit board  150 - 4 . In the embodiment of  FIG. 9D , segments  161 ′ and  162 ′ are again routed in a side-by-side fashion on a first layer of the printed circuit board  150 - 4  (e.g., on the top layer). Segment  166 ′ is again split into two separate current paths. In particular, starting at the left hand side of  FIG. 9D , it can be seen that segment  166 ′ initially is on a lower layer (e.g., an intermediate layer or bottom layer) of the printed circuit board  150 - 4  while segments  161 ′ and  162 ′ are on the top layer. A conductive via  207 - 1  acts as a first junction that splits segment  166 ′ into the two current paths  166 - 1 ′ and  166 - 2 ′. Segment  166 - 1 ′ extends on the lower layer of printed circuit board  150 - 4  from the first via  207 - 1  to a second via  207 - 2  where it transitions to the top layer of. Section  166 - 1 ′ then extends on the top layer parallel and immediately adjacent to segment  161 ′ to a third conductive via  207 - 3 . At the base of via  207 - 3 , segment  166 - 1 ′ then connects to a fourth conductive via  207 - 4  where segment  166 - 1 ′ recombines with segment  166 - 2 ′. Segment  166 - 2 ′ extends from the first via  207 - 1  to the fourth via  207 - 4  and extends on the top layer parallel and immediately adjacent to segment  162 ′. Current path  166 - 1 ′ may be the same distance from segment  161 ′ as is current path  166 - 2 ′ from segment  162 ′. 
     The embodiments of FIGS.  9  and  9 B- 9 D do not interpose any conductive segments between the segments  161 ′ and  162 ′. This may improve the performance of the transmission line  172 . Moreover, the embodiments of  FIGS. 9C and 9D  may exhibit reduced capacitive coupling between segment  166 ′ and segments  161 ′ and  162 ′, which may also improve performance. 
     The third and fourth unbalanced coupling regions  203  and  204  may be designed to inject differential-to-common mode crosstalk between differential transmission line  173  and differential transmission lines  172  and  174 , respectively, that is sufficient to substantially cancel the differential-to-common mode crosstalk that is injected by differential transmission line  173  onto differential transmission lines  172  and  174  in the plug blade region of plug  116 . If it is anticipated that additional differential-to-common mode crosstalk may be injected by differential transmission line  173  onto differential transmission lines  172  and  174  in the leadframe of a mating jack, the amount of differential-to-common mode crosstalk injected by differential transmission line  173  onto differential transmission lines  172  and  174  may be increased so that this additional differential-to-common mode crosstalk is also substantially cancelled by the differential-to-common mode crosstalk that is injected in the third and fourth unbalanced coupling regions  203  and  204 . The amount of differential-to-common mode crosstalk that is introduced in the third and fourth unbalanced coupling regions  203  and  204  may be adjusted in a variety of ways including, for example, adjusting the lengths of the coupling segments  161 ′/ 162 ′/ 166 ′ and  166 ′/ 167 ′/ 168 ′, adjusting the thickness of these segments, adjusting the separation of these segments, etc. 
     As noted above, the plug blades  141 - 148  may comprise “low profile” plug blades that have much smaller facing surface areas. This may significantly reduce the amount of offending crosstalk that is generated between the various differential pair combinations in the plug  116 . The terminations of the conductors  101 - 108  onto the printed circuit board  150  and the routings of the conductive paths  161 - 168  may also be designed to reduce or minimize the amount of offending crosstalk that is generated between the differential pairs  171 - 174 . As a result, the amount of offending crosstalk that is generated in the plug  116  may be significantly less than the offending crosstalk levels specified in the relevant industry-standards documents. A plurality of offending crosstalk circuits thus may be provided in plug  116 , if necessary, that inject additional offending crosstalk between the pairs in order to bring the plug  116  into compliance with these industry standards documents. 
     The use of low profile plug blades and offending crosstalk circuits may be beneficial, for example, because if everything else is held equal, more effective crosstalk cancellation may generally be achieved if the offending crosstalk and the compensating crosstalk are injected very close to each other in time (as this minimizes the phase shift that occurs between the point(s) where the offending crosstalk is injected and the point(s) where the compensating crosstalk is injected). The plug  116  may be designed to generate low levels of offending crosstalk in the back portion of the plug (i.e., in portions of the plug  116  that are at longer electrical delays from the plug jack mating regions of the plug blades  141 - 148 ), and the offending crosstalk circuits are provided to inject the bulk of the offending crosstalk at very short delays from the plug-jack mating regions of the plug blades  141 - 148 . This may allow for more effective cancellation of the offending crosstalk in a mating jack. 
     As shown in the circuit diagram of  FIG. 10 , four offending crosstalk capacitors  181 - 184  may be provided adjacent the plug blades  141 - 148  (different numbers of capacitors may be provided in other embodiments). Capacitor  181  injects offending crosstalk between plug blades  142  and  143  (i.e., between differential transmission lines  172  and  173 ), capacitor  182  injects additional offending crosstalk between plug blades  143  and  144  (i.e., between differential transmission lines  171  and  173 ), capacitor  183  injects offending crosstalk between plug blades  145  and  146  (i.e., between differential transmission lines  171  and  173 ), and capacitor  184  injects offending crosstalk between plug blades  146  and  147  (i.e., between differential transmission lines  173  and  174 ). Each of the four offending crosstalk capacitors  181 - 184  are configured to inject the offending crosstalk at a location that is very near to the plug-jack mating region of each plug blade  142 - 147 . In particular, the electrodes for each crosstalk capacitor  181 - 184  connect to the top edges of the conductive vias  132 - 137  (note that only vias  131  and  138  are numbered in  FIG. 10 , but each via is clearly pictured in  FIG. 10 ). Thus, the offending crosstalk that is generated by each offending crosstalk capacitor  181 - 184  is injected at the underside of the plug blades  142 - 147 , directly opposite the plug-jack mating region of the respective plug blades. 
       FIG. 10A  is a schematic circuit diagram of a portion of a revised version  150 ′ of the printed circuit board  150  of  FIG. 8 . As shown in  FIG. 10A , the printed circuit board  150 ′ includes the offending crosstalk capacitors  182  and  183  that are provided in the embodiment of  FIG. 10 , but replaces offending crosstalk capacitors  181  and  184  with offending crosstalk capacitors  181 ′ and  184 ′. Capacitor  181 ′ injects offending crosstalk between plug blades  141  and  146  (i.e., between differential transmission lines  172  and  173 ), and capacitor  184 ′ injects additional offending crosstalk between plug blades  143  and  148  (i.e., between differential transmission lines  173  and  174 ). Thus, capacitor  181 ′ injects crosstalk between the same two transmission lines as capacitor  181  of  FIG. 10  that has the exact same polarity as the crosstalk injected by capacitor  181  of  FIG. 10 , and capacitor  184 ′ injects crosstalk between the same two transmission lines as capacitor  184  of  FIG. 10  that has the exact same polarity as the crosstalk injected by capacitor  184  of  FIG. 10 . However, by providing capacitors  181 ′ and  184 ′ that couple between plug blades  141  and  146  and between plug blades  143  and  148 , respectively, it may be possible to further reduce mode conversion. As with the embodiment of  FIG. 10 , offending crosstalk capacitors  181 ′ and  184 ′ are configured to inject the offending crosstalk at locations that are very close to the plug-jack mating region. 
     Pursuant to further embodiments of the present invention, communications plugs (and related patch cords) are provided that may substantially cancel the differential-to-common mode crosstalk that is injected between various of the differential transmission lines though the plug while maintaining predetermined amounts of differential-to-differential crosstalk between these differential transmission lines. These plugs may be industry standards compliant plugs that exhibit the required amounts of differential-to-differential crosstalk while generating significantly lower levels of differential-to-common mode crosstalk, thereby reducing any need to cancel substantial amounts of differential-to-common mode crosstalk in a mating jack. Before describing these communications plugs, it is helpful to briefly discuss various known schemes for cancelling differential-to-differential and differential-to-common mode crosstalk. 
     In particular,  FIG. 11  is a schematic diagram of a known crosstalk compensation scheme that compensates for differential-to-differential crosstalk between pairs  2  and  3  in a four-pair modular mated plug/jack combination that conforms to the TIA/EIA T568-B wiring convention. Referring to  FIG. 11 , if pair  3  is driven differentially, the differential signal energy that is coupled onto pair  2  may be substantially canceled out (ignoring the effects of delay) by virtue of the crossover in pair  2 . Unfortunately, however, coupled common-mode signals on pair  2  are not addressed by the compensation scheme of  FIG. 11 , as conductor T 3  (tip of pair  3 ) will couple more signal energy onto pair  2  than will conductor R 3  (ring of pair  3 ). 
       FIG. 12  is a schematic diagram of a known crosstalk compensation scheme that compensates for differential-to-common mode crosstalk. As shown in  FIG. 12 , a crossover is added to the conductors of pair  3  (T 3 , R 3 ), so that the differential-to-common mode crosstalk that is injected onto pair  2  in the “crosstalking region” may be substantially cancelled by the opposite polarity differential-to-common mode crosstalk that is injected from pair  3  onto pair  2  in the “compensation region.” While the compensation scheme of  FIG. 12  may effectively cancel out any coupled common-mode signals, unfortunately it does not address differential-to-differential crosstalk. 
       FIGS. 13A-13C  are schematic diagrams of crosstalk compensation schemes for communications plugs according to embodiments of the present invention. In  FIGS. 13A-13C , only the conductive paths and plug blades of pairs  2  and  3  are illustrated (namely, conductive paths  261 - 263  and  266  and plug blades  241 - 243  and  246 ) in order to simplify the drawings. The plug blades  241 - 243  and  246  may, for example, be identical to the plug blades  141 - 143  and  146  included in the plug  116  that is discussed above. 
     As noted above, it may be advantageous to reduce the amount of differential-to-common mode crosstalk that arises in a communications plug in order to reduce or eliminate any need to compensate for this crosstalk in a mating jack. However, unlike a mated plug-jack combination, many communications plugs such as plugs that comply with the ANSI/TIA-568-C.2 standard are required to exhibit specified levels of offending differential-to-differential crosstalk between the various transmission lines through the plug. Pursuant to embodiments of the present invention, communications plugs are provided that may exhibit little or no differential-to-common mode crosstalk between various pair combinations while providing the requisite levels of differential-to-differential crosstalk between each pair combination. The plugs according to embodiments of the present invention include a plurality of crosstalk injection circuits that inject crosstalk between various of the conductive paths through the plug where the magnitudes of the crosstalk injected by these circuits are selected to cancel the differential-to-common-mode crosstalk while providing the requisite levels of offending differential-to-differential crosstalk. 
     The crosstalk injection circuits that are provided and methods for selecting the values for these crosstalk injection circuits will now be discussed with reference to  FIG. 13A .  FIG. 13A  illustrates a crosstalk compensation scheme for pairs  2  and  3  in a four pair communications plug that may be used, for example, in plugs having plug blades that inject more differential-to-differential crosstalk than is required by the relevant industry standards document. 
     As is apparent from  FIG. 13A , crosstalk will inherently arise between the plug blades  241 - 243  and  246 . This crosstalk will typically include both capacitive coupling and inductive coupling (with more capacitive coupling than inductive coupling). This inherent crosstalk is represented in  FIGS. 13A-13C  as four crosstalk couplings Cs 1 , Cs 2 , Cs 3 , and Cs 4 . Coupling Cs 1  represents the crosstalk coupled between plug blade  241  and plug blade  243 , coupling Cs 2  represents the crosstalk coupled between plug blade  242  and plug blade  246 , coupling Cs 3  represents the crosstalk coupled between plug blade  241  and plug blade  246 , and coupling Cs 4  represents the crosstalk coupled between plug blade  242  and plug blade  243 . While these couplings Cs 1 , Cs 2 , Cs 3  and Cs 4  are shown as being capacitive in nature, it will be appreciated that they will also typically include an inductive component. The values of couplings Cs 1 , Cs 2 , Cs 3  and Cs 4  are determined by the geometries of the plug blades and the electrical properties of the medium material (as well as the geometries of the conductive traces that are electrically connected to the plug blades), and can be measured directly or inferred from measurements of actual crosstalk levels. 
     As shown in  FIG. 13A , a plurality of crosstalk injection circuits are also coupled between the conductors of pairs  2  and  3 . In the embodiments of  FIG. 13A , these crosstalk injection circuits include a first crosstalk injection circuit Cc 1  that is connected between conductive path  261  (the tip line of pair  2 ) and conductive path  263  (the tip line of pair  3 ), a second crosstalk injection circuit Cc 2  that is connected between conductive path  262  (the ring line of pair  2 ) and conductive path  266  (the ring line of pair  3 ), and a third crosstalk injection circuit Cc 3  that is connected between conductive path  261  and conductive path  266 . As shown in  FIG. 13A , in one example implementation the crosstalk injection circuits Cc 1 , Cc 2 , and Cc 3  may be implemented as capacitors that are connected at or directly adjacent to the plug blades  241 - 243  and  246  in order to inject the “compensating” crosstalk provided by circuits Cc 1 , Cc 2 , and Cc 3  at or very near the injection point of the “offending” crosstalk Cs 1 , Cs 2 , Cs 3  and Cs 4 . If the magnitudes of the crosstalk injected by crosstalk injection circuits Cc 1 , Cc 2 , and Cc 3  are chosen correctly, the differential-to-common-mode couplings between pairs  2  and  3  may be substantially canceled while still providing the requisite level of offending differential-to-differential crosstalk between pairs  2  and  3 , regardless which of the two pairs is driven and which is idle. 
     The following analysis shows how to calculate the amount of crosstalk to inject between pairs  2  and  3  using the crosstalk injection circuits Cc 1 , Cc 2  and Cc 3  in order to substantially cancel the differential-to-common-mode crosstalk while achieving the requisite amount of differential-to-differential crosstalk between pairs  2  and  3 . The differential-to-differential and differential-to-common-mode crosstalk coupling effects in the crosstalking region can be represented by Equations (1)-(3) as follows:
 
 Csu=Cs 3 +Cs 4 −Cs 1 −Cs 2  (1)
 
 Csb 23 =Cs 2 +Cs 4 −Cs 1 −Cs 3  (2)
 
 Csb 32 =Cs 1 +Cs 4 −Cs 2 −Cs 3  (3)
 
where:
 
     Csu is the unbalanced coupling (both capacitive and inductive) in the crosstalking region, responsible for differential-to-differential crosstalk between pairs  2  and  3 ; 
     Csb 23  is the balanced coupling (both capacitive and inductive) in the crosstalking region, responsible for differential-to-common-mode crosstalk when pair  2  is driven and pair  3  is idle; and 
     Csb 32  is the balanced coupling (both capacitive and inductive) in the crosstalking region, responsible for differential-to-common-mode crosstalk when pair  3  is driven and pair  2  is idle. 
     The term “unbalanced coupling” describes the total coupling between two pairs that contributes to differential-to-differential crosstalk, and the term “balanced coupling” describes the total coupling between two pairs contributing to differential-to-common-mode crosstalk. For total differential-to-common mode crosstalk cancellation while providing the industry-standardized amount of differential-to-differential crosstalk between pairs  2  and  3 , the three crosstalk injection circuits Cc 1 , Cc 2 , and Cc 3  should be chosen to produce balanced couplings that are equal to and opposite in polarity to those in the crosstalking region while producing unbalanced couplings that are equal to and opposite in polarity to in the crosstalking region minus the industry-standardized amount of offending crosstalk. Thus, the three crosstalk injection circuits Cc 1 , Cc 2 , and Cc 3  should inject crosstalk having the magnitudes expressed in Equations (4)-(6) as follows:
 
− Csu=Cc 3 −Cc 1 −Cc 2− K   (4)
 
− Csb 23 =Cc 2 −Cc 1 −Cc 3  (5)
 
− Csb 32 =Cc 1 −Cc 2 −Cc 3  (6)
 
where:
 
     K is the magnitude of the offending differential-to-differential crosstalk that should be injected between pairs  2  and  3  according to the industry standards. 
     Solving Equations (4)-(6) for Cc 1 , Cc 2 , and Cc 3  yields Equations (7)-(9) as follows:
 
 Cc 1=( Csu+Csb 23 −K )/2  (7)
 
 Cc 2=( Csu+Csb 32 −K )/2  (8)
 
 Cc 3=( Csb 23 +Csb 32)/2  (9)
 
     Substituting for Csu, Csb 23 , and Csb 32  from Equations (1)-(3) into Equations (7)-(9) yields Equations (10)-(12) as follows:
 
 Cc 1 =Cs 4 −Cs 1 −K/ 2  (10)
 
 Cc 2 =Cs 4 −Cs 2 −K/ 2  (11)
 
 Cc 3 =Cs 4− Cs 3  (12)
 
     As indicated by Equations (10)-(12), knowing Cs 1 , Cs 2 , Cs 3 , and Cs 4 , the values of Cc 1 , Cc 2 , and Cc 3  can be calculated. The same can be achieved by inferring Csu, Csb 23 , and Csb 32  from differential-to-differential and differential-to-common-mode crosstalk measurements performed for the crosstalking region. 
     While the above analysis uses three crosstalk injection circuits Cc 1 , Cc 2  and Cc 3  to inject crosstalk that will substantially cancel the differential-to-common mode crosstalk while leaving the industry standardized amount of differential-to-differential offending crosstalk between pairs  2  and  3 , it will be appreciated that a fourth crosstalk injection circuit Cc 4  could be added between R 2  and T 3 . The addition of this fourth crosstalk injection circuit Cc 4  provides an additional degree of freedom. 
     Subtracting above equation (11) from above equation (10) yields,
 
 Cc 1 −Cc 2 =Cs 2 −Cs 1  (13)
 
     Typically Cs 1  is greater that Cs 2 , since plug blade  241  is physically closer to plug blade  243  than is plug blade  242  to plug  246  for the pairs  2  and  3 . As a consequence of this and above equation (10), Cc 2  has to be greater than Cc 1  for positive values of Cc 1  and Cc 2 . This implies that the compensation scheme of  FIG. 13A  cannot have an offending crosstalk greater than the amount that would result from having Cc 1 =0. This maximum differential-to-differential offending crosstalk achievable using the compensation scheme of  FIG. 13A  can be derived by substituting Cc 1 =0 into above equation (10), which yields,
 
 K= 2( Cs 4 −Cs 1)  (14)
 
     Thus the crosstalk compensation scheme of  FIG. 13A  is applicable when the required offending differential-to-differential crosstalk between pairs  2  and  3  is less than twice amount of coupling between plug blade  242  and plug blade  243  minus twice the amount of coupling between plug blade  241  and plug blade  243 . 
     Next, reference is made to  FIG. 13B , which illustrates the crosstalk compensation scheme for pairs  2  and  3  in a four pair communications plug that may be used, for example, if the amount of differential-to-differential crosstalk that is specified by the relevant industry standards document is equal to twice amount of coupling between plug blade  242  and plug blade  243  minus twice the amount of coupling between plug blade  241  and plug blade  243 . 
     As shown in  FIG. 13B , in this embodiment, only two crosstalk injection circuits are used: namely, the second crosstalk injection circuit Cc 2 ′ that is connected between conductive paths  262  and  266 ; and the third crosstalk injection circuit Cc 3 ′ that is connected between conductive paths  261  and  266 . The crosstalk injection circuits Cc 2 ′ and Cc 3 ′ may again be implemented as capacitors that are connected at or directly adjacent to the plug blades  241 - 242  and  246  in order to inject the “compensating” crosstalk provided by circuits Cc 2 ′ and Cc 3 ′ at or very near the injection point of the “offending” crosstalk Cs 1 , Cs 2 , Cs 3 , Cs 4 . If the magnitudes of the crosstalk injected by crosstalk injection circuits Cc 2 ′ and Cc 3 ′ are chosen correctly, the differential-to-common-mode couplings between pairs  2  and  3  may be substantially canceled while still providing the requisite level of offending differential-to-differential crosstalk between pairs  2  and  3 , regardless which of the two pairs is driven and which is idle. In particular, to achieve this result Cc 2 ′ and Cc 3 ′ should have the following values based on Equations (11) and (12) above
 
 Cc 2 ′=Cs 4 −Cs 2 −K 12  (15)
 
 Cc 3 ′=Cs 4 −Cs 3  (16)
 
     Finally, reference is made to  FIG. 13C , which illustrates a crosstalk compensation scheme for pairs  2  and  3  in a four pair communications plug that may be used, for example, if the amount of differential-to-differential crosstalk that is specified by the relevant industry standards document is greater than twice amount of coupling between plug blade  242  and plug blade  243  minus twice the amount of coupling between plug blade  241  and plug blade  243 . 
     As shown in  FIG. 13C , in this embodiment, three crosstalk injection circuits may be used, namely the second crosstalk injection circuit Cc 2 ″ that is connected between conductive paths  262  and  266 , the third crosstalk injection circuit Cc 3 ″ that is connected between conductive paths  261  and  266 , and a fourth crosstalk injection circuit Cc 4 ″ that is connected between conductive paths  262  and  263 . The crosstalk injection circuits Cc 2 ″, Cc 3 ″ and Cc 4 ″ may again be implemented as capacitors that are connected at or directly adjacent to the plug blades  241 - 243  and  246 . If the magnitudes of the crosstalk injected by crosstalk injection circuits Cc 2 ″, Cc 3 ″, and Cc 4 ″ are chosen correctly, the differential-to-common-mode couplings between pairs  2  and  3  may be substantially canceled while still providing the requisite level of offending differential-to-differential crosstalk between pairs  2  and  3 , regardless which of the two pairs is driven and which is idle. In particular, to achieve this result Cc 2 ″, Cc 3 ″ and Cc 4 ″ should be chosen such that:
 
− Csu=Cc 3 ″+Cc 4 ″−Cc 2 ″−K   (17)
 
− Csb 23 =Cc 2 ″+Cc 4 ″−Cc 3″  (18)
 
− Csb 32 =Cc 4 ″−Cc 2 ″−Cc 3″  (19)
 
     Solving Equations (17) through (19) for Cc 2 ″, Cc 3 ″, and Cc 4 ″ yields Equations (20) through (22) as follows:
 
 Cc 2″=( Csb 32 −Csb 23)/2  (20)
 
 Cc 3″=( Csb 32 −Csu+K )/2  (21)
 
 Cc 4″=(− Csb 23 −Csu+K )/2  (22)
 
     Substituting for Csu, Csb 23 , and Csb 32  from Equations (1) through (3) into Equations (20) through (22) yields Equations (23) through (25) as follows:
 
 Cc 2 ″=Cs 1 −Cs 2  (23)
 
 Cc 3 ″=Cs 1 −Cs 3 +K/ 2  (24)
 
 Cc 4 ″=Cs 1 −Cs 4 +K/ 2  (25)
 
     As shown in the analysis above, the solution presented with respect to  FIG. 13A  may be used if the requisite differential-to-differential coupling between pairs  2  and  3  is less than 2 (Cs 4 -Cs 1 ). The solution presented with respect to  FIG. 13B  may be used if the requisite differential-to-differential coupling between pairs  2  and  3  is substantially equal to 2 (Cs 4 -Cs 1 ). The solution presented with respect to  FIG. 13C  may be used if the requisite differential-to-differential coupling between pairs  2  and  3  is greater than 2 (Cs 4 -Cs 1 ). It will also be appreciated that while the scenarios of  FIGS. 13B and 13C  have been solved assuming that two or three crosstalk injection circuits are used, as with the scenario of  FIG. 13A , all four crosstalk injection circuits may also be used in these scenarios, providing at least one additional degree of freedom with respect to solutions that substantially cancel the differential-to-common-mode couplings between pairs  2  and  3  while providing the requisite level of offending differential-to-differential crosstalk between pairs  2  and  3 . 
     It will also be appreciated that the above calculations derive values for the four crosstalk injection circuits that provide solutions for pairs  2  and  3  of a four pair connector. Those skilled in the art will understand that the above analysis is equally applicable to pairs  3  and  4  and that the same principles can be extended to derive values for crosstalk injection circuits that will compensate for crosstalk between other pair combinations in a four pair connector or for pair combinations in other types of mated plug jack connectors. 
     It will be appreciated that the printed circuit board  150  that is illustrated in  FIGS. 4-8  and  10  above could be modified to include the first, second, third and/or fourth various crosstalk injection circuits that are illustrated in  FIGS. 13A-13C  in order to implement these crosstalk compensation schemes in plug  116 . 
     As noted above, in some embodiments, the first, second, third and/or fourth crosstalk injection circuits may be implemented as capacitors that inject the crosstalk close in time to the offending crosstalk Cs 1 , Cs 2 , Cs 3  and Cs 4 . This may reduce and/or minimize the delay, which may more effectively cancel the differential-to-common mode crosstalk. However, differential-to-common mode crosstalk may appear as both NEXT and FEXT, and hence it may be desirable in some embodiments to include inductive components in at least some of the first, second, third and/or fourth crosstalk injection circuits in order to better cancel both differential-to-common mode NEXT and FEXT. However, at least some of the inductive components may have greater associated delays which may degrade the cancellation, and hence there may be inherent tradeoffs with respect to whether or not to include inductive components in the first, second, third and/or fourth crosstalk injection circuits, at least in some embodiments. 
     Thus, pursuant to some embodiments of the present invention, RJ-45 communications plugs (and related patch cords) are provided that include at least a first crosstalk injection circuit that is connected between a first conductive path of a first differential pair and a first conductive path of a second differential pair, and a second crosstalk injection circuit that is connected between the second conductive path of the first differential pair and the first conductive path of the second differential pair. The first and second crosstalk injection circuits may be designed to substantially cancel the differential-to-common mode crosstalk injected between the first and second differential pairs. In some embodiments, the plug may further include a third crosstalk injection circuit that is connected either (1) between the first conductive path of the first differential pair and the second conductive path of the second differential pair or (2) between the second conductive path of the first differential pair and the second conductive path of the second differential pair. This third crosstalk injection circuit may act in conjunction with the first and second crosstalk injection circuits to substantially cancel the differential-to-common mode crosstalk injected between the first and second differential pairs. 
     In some embodiments, the first, second and/or third crosstalk injections circuits may be implemented as capacitors on a printed circuit board of the plug. These capacitors may, for example, inject crosstalk onto the signal carrying paths directly adjacent to the connection of each path to its respective plug blade. 
     As is also made clear above, the plugs (specifically including RJ-45 plugs) according to embodiments of the present invention may include a differential-to-common mode crosstalk cancellation circuit that substantially cancels differential-to-common mode crosstalk that is injected within the plug from a first differential transmission line onto a second differential transmission line while still ensuring that differential-to-differential crosstalk is injected from the first differential transmission line onto the second differential transmission line when the first differential transmission line is excited differentially. The amount of differential-to-differential crosstalk that is injected may, for example, be the amount specified in a relevant industry standards document. 
     The plugs according to embodiments of the present invention thus may include an offending crosstalk circuit that is separate from the plug blades that injects crosstalk between first and second differential transmission lines, where the offending crosstalk circuit is between a ring conductive path and a tip conductive path, as well as a differential-to-common mode crosstalk cancellation circuit that is electrically connected between the first and second differential transmission lines. The differential-to-common mode crosstalk cancellation circuit may substantially cancel the differential-to-common mode crosstalk that is injected within the plug between the first and second differential transmission lines. 
     Thus, using the above-described techniques, mode conversion between for example, pairs  2  and  3  may be managed (i.e., cancelled) in the communications plug. This may reduce any need to compensate for mode conversion in a mating communications jack. As known to those of skill in the art, one technique for compensating for mode conversion in a four-pair T-568B type communications jack is to include a crossover on pair  3  as is described, for example, in the above-referenced U.S. Pat. No. 7,204,722. However, as communications plugs and jacks are designed to operate at higher data rates, it may be difficult to physically implement such crossovers in a reliable fashion so that they will inject the compensating crosstalk at sufficiently short delays. Thus, by compensating for the differential-to-common mode crosstalk in the communications plug it may be possible to omit such crossovers in some jack designs. 
     It will be appreciated that in shielded communications systems, the impact of differential-to-common mode crosstalk may be reduced as the shielding may reduce the amount of alien crosstalk in the communications system. However, the plugs according to embodiments of the present invention may still be useful in shielded communications systems for various reasons including further reducing the amount of alien crosstalk and improving insertion loss performance. 
     Pursuant to further embodiments of the present invention, resistors may be placed in series with one or more of the first, second, third and/or fourth crosstalk injection circuits. These series resistors may further reduce mode conversion and/or facilitate managing the return loss along one or more of the differential transmission lines.  FIG. 14  is a perspective view of some of the plug blades and a portion of a printed circuit board  250  of a communications plug according to further embodiments of the present invention that includes such a series resistor. 
     As shown in  FIG. 14 , the printed circuit board  250  includes a plurality of conductive vias  231 - 238  and a plurality of conductive paths  261 - 268 . A plurality of plug blades  241 - 248  are mounted in the respective conductive vias  231 - 238 . Plug blades  245  and  246  are shown using dotted lines in  FIG. 14  in order to better illustrate capacitors that are included underneath those plug blades. The conductive vias  231 - 238 , the conductive paths  261 - 268 , and the plug blades  241 - 248  may be identical to the conductive vias  131 - 138 , the conductive paths  161 - 168  and the plug blades  141 - 148  that are described above with respect to  FIGS. 5-8  except as described below. 
     As is further shown in  FIG. 14 , a plurality of capacitors  281 - 285  are provided in the printed circuit board  250 . Each of these capacitors is implemented as a plate capacitor that has a first plate on a first layer of the printed circuit board  250  that electrically connects to a first of the conductive paths  261 - 268  and a second plate on a second layer of the printed circuit board  250  that electrically connects to a second of the conductive paths  261 - 268 . In particular, capacitor  281  is interposed between conductive paths  262  and  263 , capacitor  282  is interposed between conductive paths  263  and  264 , capacitor  283  is interposed between conductive paths  265  and  266 , capacitor  284  is interposed between conductive paths  261  and  266 , and capacitor  285  is interposed between conductive paths  266  and  267 . The capacitors  281 - 285  may be configured to ensure that the plug exhibits the industry standards required amounts of offending crosstalk between the different pair combinations, and may also be used to at least partially cancel differential-to-common mode crosstalk that arises between pairs in the plug. 
     As is further shown in  FIG. 14 , a resistor  286  is provided in series with the capacitor  284  between conductive path  261  and conductive path  266 . In an example embodiment, the resistor  286  may be a 1000 ohm resistor and the capacitor  284  may be a 0.1 pF capacitor. The resistor  286  may improve return loss on conductive path  261  by increasing the impedance of the connection to capacitor  284  and so reducing its effect as an electrical stub; and it may also limit the crosstalk between conductive path  261  and conductive path  266  at high frequencies. Additional resistors may be provided in series with any other of the capacitors  281 - 283  and  285 . 
     While in the description above with reference to  FIGS. 13A-13C  and  14  the analysis is made with reference to pairs  3  and  2  according the TIA 568 type B pair assignment counting the plug blades as shown in  FIG. 8  from left to right, it will be appreciated that if the conductors are counted from the opposite direction the description would apply equally well to pairs  3  and  4  of the TIA 568 type B pair assignment. Thus, the above description and the pending claims cover both cases. 
     The present invention is not limited to the illustrated embodiments discussed above; rather, these embodiments are intended to fully and completely disclose the invention to those skilled in this art. In the drawings, like numbers refer to like elements throughout. Thicknesses and dimensions of some components may be exaggerated for clarity. 
     Spatially relative terms, such as “top,” “bottom,” “side,” “upper,” “lower” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. Thus, the exemplary term “under” can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Herein, the term “signal current carrying path” is used to refer to a current carrying path on which an information signal will travel on its way from the input to the output of a communications plug. Signal current carrying paths may be formed by cascading one or more conductive traces on a wiring board, metal-filled apertures that physically and electrically connect conductive traces on different layers of a printed circuit board, portions of plug blades, conductive pads, and/or various other electrically conductive components over which an information signal may be transmitted. Branches that extend from a signal current carrying path and then dead end such as, for example, a branch from the signal current carrying path that forms one of the electrodes of an inter-digitated finger or plate capacitor, are not considered part of the signal current carrying path, even though these branches are electrically connected to the signal current carrying path. While a small amount of current will flow into such dead end branches, the current that flows into these dead end branches generally does not flow to the output of the plug that corresponds to the input of the plug that receives the input information signal. 
     Well-known functions or constructions may not be described in detail for brevity and/or clarity. As used herein the expression “and/or” includes any and all combinations of one or more of the associated listed items. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes” and/or “including” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     All of the above-described embodiments may be combined in any way to provide a plurality of additional embodiments. 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.