Patent Publication Number: US-8992237-B2

Title: Resonance modifying connector

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. Ser. No. 13/973,125, filed Aug. 22, 2013, now U.S. Patent No. TBD, which is a divisional application of U.S. Ser. No. 13/133,436, filed Aug. 26, 2011, now U.S. Pat. No. 8,540,525, which in turn is a national phase of PCT Application No. PCT/US09/67333, filed Dec. 9, 2009, which in turn claims priority to U.S. Provisional Appln. Ser. No. 61/122,216, filed Dec. 12, 2008, all of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF INVENTION 
     The present invention generally relates to connectors suitable for high data rate communications and, more particularly, to a connector with improved resonance characteristics. 
     BACKGROUND OF THE INVENTION 
     While a number of different configurations exist for high data rate connectors, one common configuration is to align a number of terminals in a row so that each terminal is parallel to an adjacent terminal. It is also common for such terminals to be closely spaced together, such as at a 0.8 mm pitch. Thus, high data rate connectors tend to include a number of tightly spaced and similarly aligned terminals. 
     High data rate communication channels tend to use one of two methods, differential signals or single-ended signals. In general, differential signals have a greater resistance to interference and therefore tend to be more useful at higher frequencies. Therefore, high data rate connectors (e.g., high-frequency capable connectors) such as small form factor pluggable (SFP) style connectors tend to use a differential signal configuration. An increasingly significant issue is that as the frequency of the signals increases (so as to increase the effective data rates), the size of the connector has a greater influence on the performance of the connector. In particular, the electrical length of the terminals in the connector may be such that a resonance condition can occur within the connector if the electrical length of the terminals and the wavelengths of the signals become comparable. Thus, even connector systems configured to use differential signal pairs may experience degradation of performance as operating frequencies increase. Potential resonance conditions in existing connectors tend to make them unsuitable for use in higher speed applications. Accordingly, improvements in the function, design and construction of a high data rate connector assembly is desirable. 
     SUMMARY OF THE INVENTION 
     A connector includes a housing that supports a plurality of ground and signal terminals. The terminals can have contact portions, tail portions and body portions extending between the contact and tail portions. The terminals can be positioned in wafers. The signal terminals can be provided as a pair of signal terminals in adjacent wafers that are used as a differential signal pair. A bridge is extends between two adjacent ground terminals while extending transversely and not in contact with signal terminals positioned between the ground terminals. If desired, multiple bridges may be used. In one embodiment, the bridge can be a pin that is inserted through multiple wafers and may extend transversely past a plurality of pairs of differential signal pairs. In another embodiment, the bridge can be a series of clips that are positioned in the wafers so as to allow each clip to engage a clip in an adjacent wafer. If the bridge is a pin, the pin can be inserted through a first side of the connector, pass through multiple wafers and extends to a second side of the connector. While a single bridge can couple three or more ground terminals, in an embodiment a first bridge can be used to couple a first pair of ground terminals and a second bridge can be used to couple a second pair of ground terminals, even if the first and second pair of ground terminals share a terminal. The ground terminals can include translatable arms that are deflected when the bridge engages the ground terminals. 
     The connector may include a light pipe structure that is supported by the housing. The connector may include a first opening having ground members and signal terminals adjacent thereto so at provide a first mating plane. The connector may include a second opening having ground members and signal terminals adjacent thereto so as to provide a second mating plane. The housing may be configured to be mounted on a circuit board with the upper surface of the circuit board forming a plane and the plane of the circuit board lying between the first and second mating plane. Alternatively, the connector may be configured so that both mating planes are on the same side of the supporting circuit board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various other objects, features and attendant advantages of the present invention will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, wherein: 
         FIG. 1  is a front perspective view of an embodiment of an electrical connector; 
         FIG. 2  is an exploded perspective of the connector of  FIG. 1  with certain components removed for clarity; 
         FIG. 3  is a front perspective view of the connector of  FIG. 1  with the front housing component removed for clarity; 
         FIG. 4  is a front perspective view similar to that of  FIG. 1  but with both of the front and rear housing components removed in order to show the subassembly of internal wafers; 
         FIG. 5  is a front perspective view similar to  FIG. 4  but with the insulation from around one of the ground wafers removed for clarity; 
         FIG. 6  is a front perspective view similar to that of  FIG. 4  but with the endmost ground wafer removed for clarity; 
         FIG. 7  is a perspective view similar to  FIG. 6  but taken from an orientation somewhat beneath the wafer subassembly; 
         FIG. 8  is a rear perspective view of the connector of  FIG. 1  with the rear housing component removed; 
         FIG. 9  is a perspective view of the wafer subassembly of  FIG. 4  but with all of the insulative components removed for clarity; 
         FIG. 10  is a view of the subassembly of  FIG. 9  but with some of the terminals removed for clarity; 
         FIG. 11  is a front elevational view of the subassembly of  FIG. 10 ; 
         FIG. 12  is a sectioned perspective view of  FIG. 1  taken generally along line  12 - 12  of  FIG. 1 ; 
         FIG. 13  is a side elevational view of a pair of ground terminals of  FIG. 12 ; 
         FIG. 14  is a side elevational view of an alternate embodiment of the ground terminals depicted in  FIG. 13 ; 
         FIG. 15  is a side-elevational view of still another alternate embodiment of the ground terminals depicted in  FIG. 14 ; 
         FIG. 16  is a perspective view of four pairs of signal terminals and one ground terminal associated with each row of signal terminals; 
         FIG. 17  is a side elevational view of the terminals of  FIG. 16  showing the relative widths of the body sections of the signal terminals compared to those of the ground terminals; 
         FIG. 18  is a perspective view similar to  FIG. 9  but showing only the ground terminals and the front bridging structure; 
         FIG. 18A  is an enlarged perspective view of a portion of  FIG. 18  showing the interaction between the ground terminals and the front bridging structure; 
         FIG. 19  is a top plan view of the front bridging structure; 
         FIG. 20  is a rear elevational view of the electrical connector of  FIG. 1  with the rear housing component removed and only two ground and two signal wafers inserted into the front housing component; 
         FIG. 21  is a rear perspective view of the electrical connector of  FIG. 1  but with the rear housing component and insulation around the wafers removed for clarity; 
         FIG. 21A  is an enlarged perspective view of a portion of  FIG. 21 ; 
         FIG. 22  is a rear perspective view similar to  FIG. 21  but with bridging pins inserted; 
         FIG. 22A  is an enlarged perspective view of a portion of  FIG. 22 ; 
         FIG. 23  is a front perspective view of another embodiment of an electrical connector; 
         FIG. 24  is a side elevational view of the electrical connector of  FIG. 23 ; 
         FIG. 25  is a perspective view of the electrical connector of  FIG. 23  incorporating a light pipe assembly; 
         FIG. 26  is a front perspective view of the electrical connector of  FIG. 23  but with the front and rear housing components removed in order to show the subassembly of internal wafers; 
         FIG. 27  is a front perspective view similar to  FIG. 26  but with the insulation removed from some of the wafers; 
         FIG. 28  is a side elevational view of  FIG. 27 ; 
         FIG. 29  is a perspective view of a subassembly of wafers utilizing an alternate form of grounding clips; 
         FIG. 30  is a sectioned perspective view of  FIG. 29  with the insulation above line  30 - 30  of  FIG. 29  removed for clarity; 
         FIG. 30A  is an enlarged perspective view of a portion of  FIG. 30 ; 
         FIG. 31  is a perspective view similar to that of  FIG. 29  but with the insulation removed from four of the wafers for clarity; 
         FIG. 32  is a perspective view similar to that of  FIG. 30A  but depicting only two ground and two signal wafers and with the insulation removed from the wafers for clarity; 
         FIG. 33  is a perspective view similar to  FIG. 32  but of an alternate embodiment of grounding clips; 
         FIG. 34  is a perspective view similar to  FIG. 32  but of another alternate embodiment of ground pins; 
         FIG. 35  is a front perspective view of an alternate embodiment of a ground terminal bridging structure with only a few ground terminals depicted for clarity; 
         FIG. 36  is a rear perspective view of the ground bridging structure and ground terminals of  FIG. 35 ; 
         FIG. 36A  is an enlarged perspective view of a portion of  FIG. 36 ; and 
         FIG. 37  is an enlarged perspective view similar to  FIG. 36A  but depicting an alternate embodiment of contact arms for the bridging structure. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     As required, detailed embodiments are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary and the depicted features may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the disclosed features in virtually any appropriate manner, including employing various features disclosed herein in combinations that might not be explicitly described. 
     Small form pluggable (SFP) style connectors are often used in systems where an input/output (I/O) data communication channel is desired. A number of variations in SFP-style connectors exist and different connectors are configured to meet different specifications, such as specifications commonly known as SFP, XFP, QSFP, SFP+ and the like. In general, the SFP-style connectors are configured to mate to modules or assemblies having circuit cards therein and include terminals that, at one end, removably mate with pads on the circuit card and, at an opposite end, extend to traces of a circuit board on which the SFP-style connector is mounted. The details discussed herein, which are based on embodiments of a connector suitable for use with such an SFP-style connector, are not so limited but instead are also broadly applicable to other connector types and configurations as well. For example, without limitation, features of the disclosure may be used for vertical and angled connectors as well as the depicted horizontal connector. In other words, other terminal and housing configurations, unless otherwise noted, may also be used. 
     In an electrical connector, adjacent terminals, when used to form a high data rate differential pair, electrically couple together to form what can be called a first, or intentional, mode. This mode is used to transmit signals along the terminals that make up the differential pair. However, if other signal terminals are also nearby this differential signal pair, it is possible that one (or both) of the terminals in the differential pair may also electrically couple to one or more of the other terminals (thus forming additional modes). These additional modes are typically undesirable as they can introduce cross-talk that acts as noise relative to the first mode. To prevent such cross-talk, therefore, it is known to shield the differential pair from other signals. 
     Due to the above-noted tendency to have the terminals located relatively close to each other, pairs of differential signal terminals are often separated from adjacent pairs of differential signal terminals by a ground terminal or a shield. For example, a repeating ground-signal-signal pattern may be used which results in a differential signal pair being surrounded by a ground on each side when the pattern is aligned in a row (e.g., G, S + , S − , G). A potential issue that arises due to the use of ground terminals as shields is that another mode is created by the coupling between each ground terminal and the pairs of signal terminals. In addition, the difference in voltage between two different grounds can also cause the grounds to couple together as transient signals pass through the connector. These various couplings create additional modes (and resultant electromagnetic fields) and introduce noise from which the first mode must be distinguished if the communication system is going to operate effectively. 
     The additional modes generally do not cause problems at low data rates as such additional modes tend to operate at higher frequencies and have less power compared to the first mode and thus do not cause a serious noise issue, assuming the connector is otherwise properly designed. However, as the frequency of the data transmission increases, the wavelength of the signal moves closer to the electrical length of the ground terminals. Therefore, at higher frequencies, it is possible that the transmission frequency will be high enough, and thus the wavelength short enough, to create undesirable resonance in the connector. Such resonance can amplify the secondary modes, which are typically noise, sufficiently to raise the amplitude of the noise as compared to the amplitude of the signal so that it becomes difficult to distinguish between signals and noise. Accordingly, it is desirable for the operating range of a connector to be sufficiently below the resonant frequency of the connector. 
     As used herein, the term resonant frequency refers to the lowest resonant frequency or fundamental frequency of the connector. Additional resonant frequencies, known as harmonics, exist above the lowest resonant frequency but may generally be ignored since a connector operating within a range below the lowest resonant frequency will also be operating below the harmonics and a connector operating within a range that includes the lowest resonant frequency will likely have issues with respect to noise (absent other steps taken to eliminate or reduce the noise) regardless of whether the operating range also overlaps with any of the harmonics. 
     The resonant frequency of a connector is a function of the longest effective electrical length between discontinuities or significant changes in impedance along the electrical path which includes the ground terminals. In other words, the resonant frequency depends on the effective electrical length between the points at which two adjacent ground paths are electrically connected. A non-limiting example of such a connection is a ground plane within a circuit board or card to which both of the adjacent ground terminals are connected. It should be noted that the effective electrical length is a function of numerous factors including the physical length of the terminal, the physical characteristics of the terminal (such as its geometry and surrounding dielectric material, both of which affect its impedance) and the physical length and characteristics beyond the terminal (such as within a circuit board) prior to reaching the discontinuity or intersection. 
     As an example, the physical distance between discontinuities of a pair of ground terminals having tails mounted in a circuit board and contact ends mated to conductive pads on a circuit card would be equal to the physical length of a ground terminal (defined as the distance from the point at which the terminals reach a common ground or reference plane within the circuit board on which they are mounted to the contact ends of the terminals at which they engage the conductive pads of the circuit card) plus the physical length from the conductive pads on the circuit card to a common ground plane within the circuit card. To determine the effective electrical length, which is measured in picoseconds, between discontinuities, one would also need to factor in characteristics that affect the impedance of the circuit path including the physical geometry of the conductors as well as the dielectric medium surrounding the paths. 
     A connector that can minimize resonance in the relevant frequency range of signaling can provide certain advantages. It has been determined that decreasing the effective electrical length of the ground terminals, which effectively decreases the length between discontinuities, can provide significant benefits in this regard. In particular, decreasing the electrical length of the terminal so that it is not more than one half the electrical length associated with a particular frequency (e.g., the electrical length between discontinuities is about one half the electrical length associated with a wavelength at the 3/2 Nyquist frequency) has been determined to significantly improve connector performance. It should be noted, however, that in certain embodiments the actual electrical length of the terminal is not the effective electrical length of the connector because there is an additional distance traveled outside the connector before a discontinuity is encountered. For example, the distance from the edge of the contact of the terminal along a contact pad and though a circuit board until reaching a common ground plane is part of the electrical length between discontinuities. Therefore, a connector with ground terminals that have an electrical length of about 40 picoseconds might, in operation, provide an effective electrical length of about 50 picoseconds between discontinuities once the circuit board and contact pad were taken into account. As can be appreciated, this difference can be significant at higher frequencies as a difference of 10 picoseconds in electrical length could result in a connector suitable for about 20 Gbps performance versus one suitable for about 30 Gbps performance. 
     As it is often not practicable to shorten or reduce the size of the entire connector, the resonance problem in a differential connector that provides rows of terminals has proven difficult to solve in an economical manner. To address this problem, however, it has been determined that one or a plurality of conductive bridges or commoning members can be used to connect multiple ground terminals so as to shorten the distance between discontinuities, thus reducing the electrical length and raising the resonant frequency. This reduced electrical length permits the establishment of a maximum effective electrical length below a desired level and allows higher frequencies to be transmitted over the connector without encountering resonance within the operating range of the connector. For example, placing a conductive bridge or commoning member so that it couples two ground terminals together at their physical mid-point can reduces the effective electrical length of the ground terminals in the connector approximately in half and therefore raises the resonant frequency by approximately doubling it. In practice, since a bridge has a physical length as it extends between the two ground terminals, placing a bridge at or near the physical midpoint may not reduce the electrical length exactly in half but the reduction can be relatively close to half of the original electrical length. 
     The features described below thus illustrate embodiment where certain features are used to provide a reduced electrical length. If desired, a connector may be provided having a dielectric housing, a first wafer positioned in the dielectric housing and supporting a first conductive ground terminal and a second wafer positioned in the dielectric housing and supporting a second conductive ground terminal. A pair of signal terminal may be positioned between the first and second ground terminals and at least one conductive bridge may extend between the first ground terminal and the second ground terminal with the conductive bridge electrically connecting the first and second ground terminals and configured so as to provide a reduced maximum effective electrical length of the first and second ground terminals. 
     If desired, the conductive bridge may be a conductive pin extending through the first and second wafers. Each of the first and second conductive ground terminals may include a contact section at one end for mating with a mating component, a tail at an opposite end for mounting to a circuit member and a generally plate-like body section therebetween. The conductive bridge may be positioned where appropriate and in an embodiment may be positioned so as to electrically connect the first and second ground terminals at a location generally towards a midpoint between the contact ends and the tails of the first and second ground terminals. In one configuration, the reduced maximum effective electrical length of the ground terminals may be less than about 38 picoseconds. In another configuration, the reduced maximum effective electrical length of the ground terminals may be less than about 33 picoseconds. In another configuration, the reduced maximum effective electrical length of the ground terminals may be less than about 26 picoseconds. The conductive bridge may extend transversely past a plurality of pairs of differentially coupled high data rate signal terminals. 
     If desired, a method of increasing a resonant frequency of an electrical connector above a desired operational frequency range of the connector may be utilized. Such method includes determining the desired operational frequency range of the connector, and providing first and second spaced apart ground members with the first ground member defining at least part of a first electrical path and the second ground member defining at least part of a second electrical path. A differential signal pair can be provided between the first and second ground members and the approximate maximum effective electrical length between discontinuities along the first and second electrical paths is determined. An initial resonant frequency is determined based on the approximate longest effective length between the discontinuities along the first and second electrical paths and a maximum desired effective electrical length between the discontinuities is determined in order to increase the resonant frequency of the electrical connector above the desired operational frequency range. At least one conductive bridge is connected between the first and second ground terminals to reduce the effective electrical length between discontinuities along the first and second ground members to a length that is less than the maximum desired effective electrical length. 
     If desired, determining the maximum effective electrical length between discontinuities along the first and second electrical paths may include simulating an electrical system. The simulating step may include analyzing physical characteristics of the ground members including their length, geometry and the dielectric medium surrounding the ground members. The simulating step may include analyzing additional circuit components that define at least part of the first and second electrical paths. Determining the maximum effective electrical length between discontinuities along the first and second electrical paths may include testing the electrical connector. 
     Referring now to the Figures,  FIGS. 1-13  illustrate an embodiment of a connector  500  that includes a first housing component  510  and a second housing component  520 . The first housing component  510  includes a first projection  530  and a second projection  532 , both of which have a card slot  534  configured to receive circuit cards (not shown) that are supported by a corresponding mating module (not shown). As depicted, each card slot  534  includes terminal receiving grooves  536  extending along the top and bottom inner surfaces thereof. 
     Pin receiving apertures  512  may be provided in a first side  514  of first housing component  510  and pin receiving apertures  516  aligned with pin receiving apertures  512  may be provided in a second side  518  of first housing component  510 . Similarly, pin receiving apertures  522  may be provided in a first side  524  of second housing component  520  and pin receiving apertures  526  aligned with pin receiving apertures  522  may be provided in a second side  528  of second housing component  520 . Depending upon the assembly process used, apertures may not be necessary on both sides of first housing component  510  nor on both sides of housing component  520 . In certain instances, apertures in the first and second housing components may not be necessary at all. 
     As depicted, the front housing component  510  includes a cavity  540  into which a plurality of insert-molded terminal wafers  550 ,  570 ,  580  may be inserted. As depicted, each wafer includes two pairs of conductive terminals with a plastic insulative body insert-molded around the terminals. Each terminal has a contact end for mating with a pad (not shown) on a mating circuit card, at least one tail for engaging a plated hole in a circuit board on which connector  500  is mounted, and a body connecting the contact end and the at least one tail. 
     More particularly, referring to  FIGS. 5 ,  9 ,  10 ,  12 , ground wafer  550  includes four ground terminals  552 ,  554 ,  556 ,  558 , each having a mating end  552   a ,  554   a ,  556   a ,  558   a  depicted as a deflectable contact beam or spring arm at one end for engaging a mating component (not shown) and tails  552   b ,  552   b ′,  554   b ,  556   b ,  556   b ′,  558   b  depicted as compliant pins for engaging a circuit member (not shown) on which connector  500  is mounted. Relatively large or wide body sections  552   c ,  554   c ,  556   c ,  558   c  extend between mating ends  552   a ,  554   a ,  556   a ,  558   a  and tails  552   b ,  554   b ,  556   b ,  558   b , respectively, of each terminal. In addition, each ground terminal  552 ,  554 ,  556 ,  558  includes a plurality of deflectable tabs or fingers  560  extending therefrom and a single, relatively wide tab  562  generally adjacent mating end  552   a ,  554   a ,  556   a ,  558   a . If desired, fingers  560  may be slightly angled towards one of the sides of housing components  510 ,  520 . A first joining member  564  may be provided between the longer two ground terminals  552 ,  554 , and second joining member  566  may be provided between the shorter two ground terminals  556 ,  558 . 
     Signal wafers  570 ,  580  can be configured in a substantially similar manner with respect to each other and can be somewhat similar to ground wafers  550 . As depicted in  FIGS. 16 ,  17 , each first signal wafer  570  has four signal terminals  572 ,  574 ,  576 ,  578  with a mating end  572   a ,  574   a ,  576   a ,  578   a  depicted as a deflectable contact beam or spring arm at one end for engaging a mating component (not shown) and a tail  572   b ,  574   b ,  576   b ,  578   b  depicted as a compliant pin for engaging a circuit member (not shown) on which connector  500  is mounted. Relatively small or narrow body sections  572   c ,  574   c ,  576   c ,  578   c  extend between mating ends  572   a ,  574   a ,  576   a ,  578   a  and tails  572   b ,  574   b ,  576   b ,  578   b , respectively, of each terminal. The difference in width between body sections  572   c ,  574   c ,  576   c ,  578   c  of ground terminals  552 ,  554 ,  556 ,  558  and body sections  572   c ,  574   c ,  576   c ,  578   c  of signal terminals  572 ,  574 ,  576 ,  578  is best seen in  FIG. 17 . Signal terminals  572 ,  574 ,  576 ,  578  further include transition sections  572   d ,  574   d ,  576   d ,  578   d  between body sections  572   c ,  574   c ,  576   c ,  578   c  and tails  572   b ,  574   b ,  576   b ,  578   b  in order to offset the tails from the body sections. 
     Second signal wafer  580  includes four signal terminals  582 ,  584 ,  586 ,  588  that, except as noted below, are substantially identical to the signal terminals  572 ,  574 ,  576 ,  578  of the first signal wafer  570  and the description of which is not repeated herein. However, as can be appreciated from  FIG. 11 , the tails  572   b ,  574   b ,  576   b ,  578   b  of first wafer  570  and the tails  582   b ,  584   b ,  586   b ,  588   b  of second wafer  580  are offset from the plane of their respective body sections in opposite directions towards the other wafer so that the tails of the signal terminals of both wafers are aligned in a single row. Upon insertion of the wafers  550 ,  570 ,  580  into the housing cavity  510   a , the contact sections of the terminals are positioned in and may be supported by the terminal receiving grooves  536  so as to form a row of contact ends. In operation, the row of contact sections facilitates mating between the connector and pads on circuit cards which may be inserted into card slots  534 . 
     As depicted, the wafers are positioned within cavity  510   a  in a repeating pattern with two signal wafers  570 ,  580  positioned next to each other to create pairs of horizontally aligned differential-coupled signal terminals. The depicted terminals are broadside-coupled, which has the benefit of provide a stronger coupling between the terminals that form the differential pair, but unless otherwise noted, broadside coupling is not required. Ground wafers  550  are positioned on both sides of each pair of signal wafers in order to achieve the desired electrical characteristics of the signal terminals and to create a repeating ground, signal, signal, pattern (e.g., G, S + , S − , G, S + , S − , G). If desired, other patterns of wafers could be utilized such as adding additional ground wafers (e.g., G, S + , S − , G, G, S + , S − , G) to further isolate the signal terminals and/or additional signal wafers could be added in which the addition signal terminals would typically be used for “lower” speed signals (e.g., G, S + , S − , G, S, S, S, G, S + , S − , G). In addition, if desired, rather than molding two separate signal wafers  570 ,  580  and then position them adjacent to each other during the assembly process, it is also possible that the two signal wafers could be combined so as to provide a single wafer molded around all of the terminals. In addition, if desired, the wafers need not be insert molded. For example, the wafer housing could be molded in a first operation and the terminals inserted into the wafer housing in a second, subsequent operation. Insert molded wafers, however, are beneficial to precisely control the orientation of terminals supported by the wafer. 
     In order to achieve the desired electrical characteristics, the depicted embodiment illustrates a connector with pins  600  (e.g., the pins providing the electrically conductive bridges) to be inserted once wafers  550 ,  570 ,  580  are loaded into the first and second housing components  510 ,  520 . The pins  600  engage and deflect fingers  560  of the ground terminals to couple together multiple ground terminals and thus form electrically conductive bridges. More particularly, as best seen in  FIG. 9 , a first pin  600   a  engages a first set of aligned fingers  560 ′ of ground terminals  552 , a second pin  600   b  engages a second set of aligned fingers  560 ″ of ground terminals  552 , and this can be repeated with additional pins so that ground terminals  552  are interconnected or commoned at multiple locations. It should be noted that the fingers  560  may be somewhat deflected out of the plane of the body section of each ground terminal but, for clarity, such deflection is not shown in the drawings. 
     The bridges (depicted as pins  600  in  FIGS. 1-28 ) couple fingers  560  that extend from the body portions  552   c ,  554   c ,  556   c ,  558   c  of the ground terminals  552 ,  554 ,  556 ,  558 . It has been determined that for a multi-row connector design, the height of the connector and the length of the ground terminals make the inclusion of a number of bridges desirable so as to ensure the effective electrical length is short enough. The pins  600  may be formed of a sufficiently conductive material such as a copper alloy with a desirable diameter, such as between 0.4 mm and 0.9 mm. It has been determined that such a construction allows for a pin  600  that has sufficient strength to allow for insertion while avoiding any significant increase in size of the connector. As can be appreciated, a shorter connector may be able to provide ground terminals with a desirable electrical length while only using one bridge. It is expected, however, that a plurality of bridges will be beneficial in many connector configurations. 
     For connector with multiple rows of contacts, such as those depicted, the terminals have different lengths, depending on the row in which they are positioned. Consequentially, a different number of bridges can be used with each row of ground terminals to ensure the corresponding row of ground terminals has the desired maximum electrical length. For example, in  FIG. 4 , the top row of ground terminals  552  in the first projection  530  is coupled to seven pins  600  while the opposing row of ground terminals  554  is coupled to five pins  600 . The top row of ground terminals  556  in the second projection  532  is coupled to three pins  600  while the opposing row of ground terminals  558  is coupled to one pin  600 . Thus, in the depicted embodiment, the number of pins in subsequent lower rows decreases by two as compared to the prior upper row. This helps ensure a desirable performance while minimizing complexity and cost. 
     The bridges extend transversely across the signal terminals, such as terminals  572 ,  582  that form the differential pair  540  ( FIG. 11 ). To minimize electrical interference and changes in impedance, each bridge may be positioned a distance  588  from the upper surface of the signal terminals  572 ,  582 . In an embodiment, the distance between the bridge and the terminals  572 ,  582  that form differential pair  540  is sufficient so that there is greater electrical separation between the bridge and the differential pair  540  than there is between the two terminals that form the differential pair. 
     As described above, the pairs of upper and lower ground terminals  552 ,  554  in the first projection  530  may be coupled by a first joining member  564  proximate to ground tails  552   b ,  554   b  and the pairs of upper and lower ground terminals  556 ,  558  in the second projection  532  may be coupled by second joining member  566  proximate to ground tails  555   b ,  558   b . These joining members can help further reduce potential differences between ground terminals and improve the overall performance of connector  500 . As can be appreciated from  FIGS. 13-15 , alternative embodiments of the ground terminals may be provided such as enclosing the space between the body sections  552   c ,  554   c  of ground terminals  552 ,  554  to create a single ground terminal body  552   c ′ to shield both of the signal terminals  572 ,  574  in the upper and lower rows of first projection  530 . Such a terminal could include fingers  560  extending from the upper and lower edges of the body or might include fingers  560 ′″ extending from only one side (such as depicted in  FIG. 14 ) or could include pins  600  extending through the middle of the ground terminals with an interference fit (as depicted in  FIG. 15 ). 
     Referring to  FIG. 19 , an embodiment of a bridge is illustrated. The bridge is provided by a clip  630  which is inserted into the first housing component  510  prior to insertion of wafers  550 ,  570 ,  580 . The clip  630  is conductive and may be once piece as shown. The clip  630  can include a plurality of spaced apart engagement notches  631  that engage projections on first housing component  510  so that the first housing component  510  retains the clip  630  therein with a press-fit type engagement. The clip  630  includes a plurality of spaced apart receiving channels  632 , which can be on an edge opposite notches  631 , with each channel having a pair of opposing spring arms  633  therein. As depicted, the distance between spring arms  633  is less than the thickness of wide tab  562  in order to establish a good electrical connection between the spring arms  633  and wide tab  562  upon insertion of wide tab  562  between spring arms  633 . If desired, a bump or projection  634  may be provided on each spring arm  633  in order to increase the reliability of the contact between the spring arms and the wide tab. 
     Clip  630  is preferably formed of an appropriate conductive material having sufficient spring and strength qualities so as to reliably retain clip  630  within front housing component  510  and maintain a reliable connection between spring arms  633  and wide tabs  562 . It may be desirable to use clip  630  in situations in which it is difficult to insert a pin  600  near the mating ends  552   a ,  554   a ,  556   a ,  558   a  of ground terminals  552 ,  554 ,  556 ,  558 . Depending on the available space within the connector  500 , channels  632  may be omitted from the outer lateral edges of clip  630  and replaced by a single spring arm  633  in which case the wide tabs of the outer ground wafers will only be engaged by a single spring arm  633 . Although clip  630  is depicted in  FIGS. 1-28  as a one-piece member, if desired, clip  630  could be formed of multiple components  890  ( FIGS. 29-34 ) that are secured within front housing component  510 . 
     During the assembly process, the wafers supporting the terminals may be inserted into the housing in a number of different manners. Some examples of the assembly process include: 1) individually loading or stitching the wafers into the housing in the sequence in which they are aligned in the housing (e.g., G S +  S +  G S −  S −  G); 2) inserting all of the wafers of a first type (e.g., all of the ground wafers  550 ) into cavity  540 , inserting all of the wafers of a second type (e.g., all of the first signal wafers  570 ) into cavity  540  and this process repeated until the cavity is fully populated; 3) configuring the wafers carrying the signal terminals so that the two signal wafers  570 ,  580  are coupled together first and then inserting the coupled wafer pair into the housing; or 4) coupling or positioning all of the wafers together in the desired pattern and then inserting the coupled subassembly of wafers into cavity  540  in a single loading operation. 
     For the first three assembly processes listed above, after the wafers  550 ,  570 ,  580  have been inserted into first housing  510 , pins  600  can be inserted into connector  500 . If the fingers  560  are all co-planar with body sections  552   c ,  554   c ,  556   c ,  558   c , pins  600  may be inserted from either side of the connector. More specifically, pins  600  could be inserted through the pin receiving apertures in either side of first housing component  510  and through the pin receiving apertures in either side of second housing component  520 . If desired, the pins  600  may extend essentially the entire width of connector  500  and through the pin receiving apertures on both sides of first housing component  510  and second housing component  520 . 
     As described above, fingers  560  may be slightly angled toward one of the sides of the respective first and second housing components  510 ,  520  and away from the direction of insertion of the pins  600  in order to ease insertion of the pins. As can be appreciated, in such case, it is preferable that the fingers  560  are all angled in the same direction (e.g., toward the same side) and the pins  600  could be inserted from the side opposite the side towards which the fingers are angled. In other words, fingers  560  may be bent out of the plane of the body section of their respective ground terminal and pins  600  can be inserted in the same direction as the fingers extend out of the plane of the body section. 
     If wafers  550 ,  570 ,  580  are coupled or positioned together in the desired pattern and then inserted as a subassembly of wafers into cavity  540  in a single loading operation as described above as the fourth assembly process, pins  600  could be inserted as described above once the wafer subassembly has been inserted into cavity  510   a  and second housing component  520  secured to first housing component  510 . In the alternative, shorter pins that only extend between the opposite sides of the wafer subassembly and not through the sidewalls of first or second housing components  510 ,  520  could be inserted into the wafer subassembly prior to insertion of the subassembly into first housing wafer  510 . In other words, the wafer subassembly may be joined by the pins and the entire subassembly inserted as a group into cavity  510   a . In such case, apertures in the first and second housing components  510 ,  520  would not be necessary. 
     Regardless of which assembly process is used, if first housing component  510  includes a clip  630 , during insertion of ground wafers  550 , the wide tab  562  of each ground terminal  552 ,  554 ,  556 ,  558  will slide into a receiving channel  632  and between spring arms  633  in order to establish a good electrical connection between clip  630  and one of the ground terminals  552 ,  554 ,  556 ,  558 . In other words, in an embodiment the clip can be first inserted into the housing component  510  and then the wafers can be inserted in the housing component  510  so that the ground terminals engage the clip  630 . 
     Referring to  FIGS. 23-28 , an embodiment of a connector  700  is depicted that is similar to that of  FIGS. 1-22A  except that the seating plane  702  (i.e., the plane of the circuit board on which the connector is mounted) has been moved upward so that the plane of one of the circuit card slots (lower slot  732  as depicted) is positioned below the plane of upper surface  52  of the circuit board  50 . Connector  700  includes a housing  710  with a first surface  712 , a first side  716  and second side  718 . Apertures  714  in the first side allow pins  740  to be inserted into the connector  700 . Projection  726 , which includes first surface  727  and second surface  728 , includes two vertically spaced apart card slots  730 ,  732  therebetween. The card slots  730 ,  732  may be chamfered and include terminal receiving grooves  734  for supporting terminals  750  inserted therein. 
     The sides of the connector  700  may include a curved wall  713  configured to retain a light pipe and may further include a shoulder  720  to help support the light pipe. If desired, a front face  729  of projection  726  may include apertures, such as aperture  736 , to support a light pipe assembly  738 . Slots  740  may be used to support shielding members (not shown). 
     The depicted housing  710  includes a block  722  that extends past an edge  54  of the circuit board  50  while the upper surface  52  of the circuit board  50  supports the connector. As can be appreciated, the depicted connector, while providing a press-fit (or thru-hole) mounting interface with respect to the circuit board, also allows the lower circuit card slot  732  to be positioned below the upper surface  52  of the circuit board. Thus, the depicted embodiment provides an advantageously compact and low profile package. 
     As with connector  500 , connector  700  includes an alternating array of wafers  745 ,  746 ,  747 . Wafers  745 ,  746 ,  747  are similar in construction to wafers  550 ,  570 ,  580  except that the seating plane  702  of connector  700  has been moved as compared to the seating plane of connector  500 . In addition, ground wafer  745  is different from ground wafer  550  in that it includes both ground terminals and signal terminals therein. More specifically, as best seen in  FIGS. 27 ,  28 , ground wafer  745  includes four terminals with the topmost and bottommost terminals  751 ,  752  being configured as ground terminals with wide body sections  751   c ,  752   c  and resilient tabs or fingers  756  extending therefrom. The middle two terminals  762 ,  764  are configured in a manner similar to the signal terminals  755  with the body sections  762   c ,  764   c  thereof being substantially narrower than the body sections  751   c ,  752   c  of the ground terminals. 
     As depicted, a first row  770  of terminals includes a plurality of pairs of differentially coupled high data rate signal terminals  771  with ground terminals  751  on opposite sides of each pair. Pins  780  engage fingers  756  of ground terminals  751  to common the ground terminals as described above in order to provide a desired maximum effective electrical length. A second row  772  of terminals  762  within the first card slot  730  has a similar configuration but does not include high data rate terminals and commoned ground terminals and thus the upper card slot  730  (which includes the first and second rows  770 ,  772 ) is configured for a high data rate version of the SFP-type connector (as SFP-style connectors include two high data rate channels in one of the two rows). The second card slot  732  is configured in a manner that is similar to the first card slot  730  as it has a third row  774  of terminals  764  not including commoned ground terminals while a fourth row  776  of terminals includes a pair of differentially coupled high data rate signal terminals  778  with commoned ground terminals  752  on opposite sides of each pair. Thus, both the first and second card slots  730 ,  732  are suitable for use in a high data rate variant of a SFP connector but the second card slot is rotated 180 degrees with respect to the orientation of the high data rate terminals surrounded by commoned ground terminals. Terminals  762 ,  764  of the middle two rows of terminals can be used as desired for lower-speed signals and/or power or the like. In an embodiment, the high data-rate terminals rows may be configured so that they are suitable for 17 Gbps performance or even 20 or 25 Gbps. As can be appreciated, flipping the orientation of the second card slot with respect to the first card slot is advantageous from a standpoint of signal separation in a dense package but is not required. 
       FIGS. 29-32  illustrate a subassembly of wafers similar to that of  FIGS. 1-22A  but which include an alternate embodiment of a structure for bridging the ground terminals in the wafers. Accordingly, like reference numbers are used with respect to like elements and the description of such elements is omitted. Wafers  850 ,  870 ,  880  include apertures  810  therethrough in which individual conductive, identically shaped, resilient ground clips  812 ,  814  are positioned. Ground clips  812 ,  814  may be inserted into apertures  810  either before or after molding of the plastic insulation around wafers  850 ,  870 ,  880 . The ground clips  812 ,  814  are configured to extend slightly beyond at least one side surface of its respective wafer so that each clip engages the clips on opposite sides thereof. In addition, the ground clips  812  associated with each ground wafer  850  also engage a tab  816  extending away from body section  552   c ,  554   c ,  556   c ,  558   c  of the ground terminals  552 ,  554 ,  556 ,  558 . Wafers  870 ,  880 , which include the high data rate signal terminals, are positioned between two ground wafers  850  so that grounding clips  814  of the signal wafers engage the grounding clips  812  of the ground wafers and form a continuous electrical bridge that extends between ground terminals and transversely to and spaced from an edge of the high data rate signal terminals. 
     As best seen in  FIG. 32  due to the removal of the plastic insulation of wafers  850 ,  870 ,  880 , the individual ground clips  812  secured within each ground wafer  850  conductively engage a tab  816  associated with each ground terminal  552 ,  554 ,  556 ,  558 . However, the individual ground clips  814  secured within each signal wafer  870 ,  880  are spaced from the edge of the closest signal terminal by a sufficient distance (similar to distance  588  of  FIG. 11 ) so as to avoid electrical interference and impedance affects on the signal terminals. The grounding clips may be formed of sheet metal or another resilient conductive material and, as depicted, are generally U-shaped or oval-shaped. 
     When the wafers  850 ,  870 ,  880  are assembled, the ground clips  812 ,  814  combine to serve the same purpose as pins  600 , namely, to interconnect the adjacent ground terminals along the length thereof in order to reduce the electrical length between discontinuities along the ground terminals. Thus, as with the embodiment of  FIGS. 29-32 , grounding clips  812 ,  814  permit the ground terminals  552 ,  554 ,  556 ,  558  to have a maximum effective electrical length that is substantially shorter than the effective electrical length of the terminals. 
     Referring to  FIG. 33 , another embodiment of individual ground clips is disclosed. As with ground clips  812 ,  814  discussed above, ground clips  820 ,  822  are identically shaped, resilient conductive members and may be formed of conductive sheet metal. Ground clips  820 ,  822  are similar in shape to ground clips  812 ,  814  except that they include an internal resilient, relatively small U-shaped section so that clips  820  may resiliently and conductively engage tabs  824  of the ground terminals. 
     In another embodiment, the resilient ground clips  812 ,  814  may be replaced by cylindrical posts  830  ( FIG. 34 ) that are retained within each wafer  850 ,  870 ,  880 . Upon assembling the wafers side-by-side, the posts  830  will combine to resemble pins  600 . In other words, if desired, pins  600  may be formed of multiple components rather than utilizing a one-piece construction. 
       FIGS. 35-36A  illustrate a subassembly of ground terminals that utilize an alternate embodiment of a structure for electrically bridging such terminals. The ground terminals are similar to those shown in  FIG. 10  and like reference numbers are used with respect to like elements and the description of such elements is omitted. Comparing  FIG. 35  to  FIG. 10 , it can be seen that all of signal terminals and all but a few of the ground terminals have been removed for clarity. More specifically, all of the terminals of  FIG. 10  have been removed except for those on the outer ends of the terminal array. A plate-like bridging structure is associated with each row of ground terminals. An upper row of ground terminals  552  has a first plate-like bridging structure  952  associated therewith, a second row of ground terminals  554  has a second plate-like bridging structure  954  associated therewith, a third row of ground terminals  556  has a third plate-like bridging structure  956  associated therewith and a lower row of ground terminals  558  has a fourth plate-like bridging structure  958  associated therewith. Each of the three upper bridging structures  952 ,  954 ,  956  are shaped as bent plates formed with multiple, interconnected, generally planar segments while the fourth bridging structure  958  is generally planar. 
     Each bridging structure includes a plurality of pairs of spaced apart, opposed resilient spring arms  970  positioned in a three-dimensional array and aligned with fingers  560  of each ground terminal. Each arm  970  is formed by stamping and forming the sheet metal so as to create the downwardly depending resilient arms and creating a window  972  in the sheet metal. While not shown, each signal contact is generally aligned with one of the edges  974  of window  972  opposite the edge  976  from which the spring arm depends. Each arm  970  is shaped so as to taper inward towards its opposing arm in order to create an enlarged inlet  978  to facilitate insertion of finger  560  into engagement with each pair of arms. Upon insertion of finger  560 , spring arms  970  deflect outward in a direction generally perpendicular to the plane of the body sections of the ground terminals. 
       FIG. 37  depicts an alternate embodiment of a plate-like bridging structure  980  in which each pair of spring arms  970  is replaced by a single spring arm  982  that is deflectable in a direction generally perpendicular to the plane of the segment of the bridging structure from which it depends. In other words, the single spring arms  982  are configured and positioned so as to be aligned with fingers  560  and deflect in the direction that each finger  560  extends away from its ground terminal. 
     As depicted, the bridging structures  952 ,  954 ,  956 ,  958 ,  980  are formed of sheet metal so as to have the desired electrical and mechanical characteristics. It should be noted that with respect to the embodiment depicted in  FIGS. 35-37 , fingers  560  were formed so as to be resilient and deflect to some extent upon engagement by pins  600 . Since the spring arms  970 ,  982  of the plate-like bridging structures are resilient, it is not necessary for fingers  560  be resilient when used with the plate like bridging structures depicted herein. 
     It should be noted that, in general, the longest section of the ground path between discontinuities will tend to control the resultant resonant frequency. Therefore, an electrical path that has a number of closely spaced bridges to create a series of short electrical lengths between discontinuities while also having a longer section between discontinuities will have an effective electrical length determined by the longer section between discontinuities. Consequently, it is beneficial to ensure that the maximum or longest effective electrical length between discontinuities is below or less than a predetermined length. 
     When designing a high data rate connector, a desired operational frequency range of the connector is typically known. Once the designer has designed a connector (or obtained a pre-existing connector), the connector can be analyzed to determine a maximum effective electrical length between discontinuities along adjacent ground paths in which the connector will be used. While this length is primarily the electrical length of the ground terminals, other factors contribute to the effective electrical length including any distance along the circuit path outside of the connector prior to reaching a discontinuity as well as other factors that affect the characteristics of the conductors. 
     Based upon the maximum effective electrical length between discontinuities, an initial or unmodified resonant frequency can be determined. If the initial or unmodified resonant frequency is too low (which means that the operational range of the connector will overlap with the resonant frequency), a maximum desired effective electrical length is determined such that the resonant frequency for such effective length will be sufficiently above the desired operational frequency range of the connector. At that point, one or more conductive bridges, such as those incorporating the structures disclosed herein, may be used to interconnect adjacent ground members and reduce the effective electrical length between discontinuities to a length less than the maximum desired effective length and thus increase the resonant frequency of the ground structure of the connector. In the alternative, the maximum desired effective length could be determined (based upon a desired resonant frequency) prior to determining the maximum effective electrical length between discontinuities. It should be noted that analyzing the connector to determine the longest effective electrical length between discontinuities and the desired maximum electrical length can be performed either by simulation of the circuitry or by actual measurement if physical samples of the connector exist. 
     It has been determined that a stacked SFP type connector with ground terminals that have an effective electrical length of about less than 38 picoseconds is suitable for use with signaling frequencies of about 8.5 GHz, which should provide about a 17 Gbps connector per differential signal pair when using a non-return to zero (NRZ) signaling method. 
     Careful placement of the bridges may allow the effective electrical length of the ground terminals to be reduced to about 33 picoseconds, which may be suitable for signaling frequencies of about 10 GHz (and thus may be suitable for about 20 Gbps performance). If the bridges are configured to be even closer together physically, the effective electrical length can be reduced to about 26 picoseconds, which may be suitable for transmitting signals at about 13 GHz or 25 Gbps performance (assuming NRZ signaling methodology). As can be appreciated, therefore, spacing the bridges closer together (and thus increasing the number of bridges) will have the tendency to reduce the effective electrical length of the ground terminals and consequentially help make the connector more suitable for higher frequencies and higher data rates. The desired maximum effective electrical length will vary depending on the application and the frequencies being transmitted. 
     In an embodiment, the connector can be configured so as to reduce the effective electrical length of a plurality of ground terminals so as to shift the resonant frequency sufficiently, thereby providing a substantially resonance free connector up to the Nyquist frequency, which is one half the sampling frequency of a discrete signal processing system. For example, in a 10 Gbps system using NRZ signaling, the Nyquist frequency is about 5 GHz. In another embodiment, the maximum electrical length of a plurality of ground connectors may be configured based on three halves (3/2) the Nyquist frequency which, for a 10 Gbps system is about 7.5 GHz, for a 17 Gbps system is about 13 GHz and for a 25 Gbps system is about 19 GHz. If the maximum electrical length is such that the resonance frequency is shifted out of the 3/2 Nyquist frequency range, a substantial portion of the power transmitted, potentially more than 90 percent, will be below the resonant frequency and thus most of the transmitted power will not cause a resonance condition that might otherwise increase noise within the system. 
     It should be noted that the actual frequency rate and effective electrical lengths vary depending upon the materials used in the connector, as well as the type of signaling method used. The examples given above are for the NRZ method, which is a commonly used high data rate signaling method. As can be appreciated, however, in other embodiments two or more ground terminals may be coupled together with a bridge at a predetermined maximum electrical length so that the connector is effective in shifting the resonance frequency for some other desired signaling method. In addition, as is known, electrical length is based on the inductance and capacitance of the transmission line in addition to the physical length and will vary depending on geometry of the terminals and materials used to form the connector. Thus, similar connectors with the same basic exterior dimensions may not have the same effective electrical length due to construction differences. 
     It will be understood that there are numerous modifications of the illustrated embodiments described above which will be readily apparent to one skilled in the art, such as many variations and modifications of the resonance modifying connector assembly and/or its components, including combinations of features disclosed herein that are individually disclosed or claimed herein, explicitly including additional combinations of such features, or alternatively other types of signal and ground contacts. For example, bridging structures can be used with arrays of signal and ground terminals regardless of whether the terminals are positioned in wafers that are inserted into a housing or the terminals are inserted directly into a housing. In addition, if the signal terminals are configured as differential pairs, they may be broad-side or edge coupled. Also, there are many possible variations in the materials and configurations. For example, components that are formed of metal may be formed of plated plastic provided that the necessary mechanical and electrical characteristics of the components are maintained. These modifications and/or combinations fall within the art to which this invention relates and are intended to be within the scope of the claims, which follow. It is noted, as is conventional, the use of a singular element in a claim is intended to cover one or more of such an element.