Impedance control in electrical connectors

[0001] This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/294,966, filed Nov. 14, 2002, which is a continuation-in-part of U.S. patent application Ser. No. 09/990,794, filed Nov. 14, 2001, now U.S. Pat. Nos. 6,692,272, and 10/155,786, filed May 24, 2002, now U.S. Pat. No. 6,652,318. The contents of each of the above-referenced U.S. patents and patent applications is herein incorporated by reference in its entirety.

[0002] Generally, the invention relates to the field of electrical connectors. More particularly, the invention relates to an impedance-controlled insert molded leadframe assembly (“IMLA”) in a “split” configuration.

[0003] Electrical connectors provide signal connections between electronic devices using signal contacts. Often, the signal contacts are so closely spaced that undesirable interference, or “cross talk,” occurs between adjacent signal contacts. As used herein, the term “adjacent” refers to contacts (or rows or columns) that are next to one another. Cross talk occurs when one signal contact induces electrical interference in an adjacent signal contact due to intermingling electrical fields, thereby compromising signal integrity. With electronic device miniaturization and high speed, high signal integrity electronic communications becoming more prevalent, the reduction of cross talk becomes a significant factor in connector design.

[0004] One commonly used technique for reducing cross talk is to position separate electrical shields, in the form of metallic plates, for example, between adjacent signal contacts. Another commonly used technique to block cross talk between signal contacts is to place ground contacts amongst the signal contacts of a connector. The shields and ground contacts act to block cross talk between the signal contacts by blocking the intermingling of the contacts' electric fields. FIGS. 1A and 1B depict exemplary contact arrangements for electrical connectors that use shields to block cross talk.

[0005] FIG. 1A depicts an arrangement in which signal contacts S and ground contacts G are arranged such that differential signal pairs S+, S− are positioned along columns 101–106. As can be seen in FIG. 1A, the signal pairs are edge coupled (i.e., where the edge of one contact is adjacent to the edge of an adjacent contact). Shields 112 can be positioned between contact columns 101–106. A column 101–106 can include any combination of signal contacts S+, S− and ground contacts G. The ground contacts G serve to block cross talk between differential signal pairs in the same column. The shields 112 serve to block cross talk between differential signal pairs in adjacent columns.

[0006] FIG. 1B depicts an arrangement in which signal contacts S and ground contacts G are arranged such that differential signal pairs S+, S− are positioned along rows 111–116. As can be seen in FIG. 1B, the signal pairs are broadside-coupled (i.e., where the broad side of one contact is adjacent to the broad side of an adjacent contact). Shields 122 can be positioned between rows 111–116. A row 111–116 can include any combination of signal contacts S+, S− and ground contacts G. The ground contacts G serve to block cross talk between differential signal pairs in the same row. The shields 122 serve to block cross talk between differential signal pairs in adjacent rows.

[0007] Because of the demand for smaller, lower weight communications equipment, it is desirable that connectors be made smaller and lower in weight, while providing the same performance characteristics. Shields and ground contacts take up valuable space within the connector that could otherwise be used to provide additional signal contacts, and thus limit contact density (and, therefore, connector size). Additionally, manufacturing and inserting such shields and ground contacts substantially increase the overall costs associated with manufacturing such connectors. For example, in some applications, shields are known to make up 40% or more of the cost of the connector. Another known disadvantage of shields is that they lower impedance. Thus, to make the impedance high enough in a high contact density connector, the contacts would need to be so small that they would not be robust enough for many applications. Furthermore, ground contacts can take up a large percentage of the available contacts in a connector, thus causing an increase in size and weight of the connector for a given number of differential signal pairs.

[0008] Therefore, a need exists for a lightweight, high-speed electrical connector that reduces the occurrence of cross talk without the need for separate shields or ground contacts, and provides for a variety of other benefits not found in prior art connectors. More particularly, what is needed is an impedance-controlled insert molded leadframe assembly (IMLA) that maintains a distance between broadside coupled signal pairs such that cross-talk between signal pairs may be limited without the use of shields or ground contacts.

[0009] The invention provides a high speed connector wherein differential signal pairs are arranged so as to limit the level of cross talk between adjacent differential signal pairs. The connector comprises a plurality of signal contact pairs, where the contacts of each pair are separated by a gap. The gap is formed over a distance such that insertion loss and cross talk between the plurality of signal contact pairs are limited. Thus, shields and/or ground contacts are not needed in an embodiment.

[0010] In one embodiment, the connector may be comprised of a header leadframe assembly and a receptacle leadframe assembly. Each leadframe assembly may include an overmolded housing and a set of contacts that extend through the housing. Each leadframe assembly may be adapted to maintain the width of the gap between contacts that form a pair along respective portions of the contacts that extend through the housing.

[0021] The subject matter of the present invention is described with specificity to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or elements similar to the ones described in this document, in conjunction with other present or future technologies. Moreover, certain terminology may be used in the following description for convenience only and should not be considered as limiting the invention in any way. For example, the terms “top,” “bottom,” “left,” “right,” “upper,” and “lower” designate directions in the figures to which reference is made. Likewise, the terms “inwardly” and “outwardly” designate directions toward and away from, respectively, the geometric center of the referenced object. The terminology includes the words above specifically mentioned, derivatives thereof, and words of similar import.

[0022] FIG. 2A is a schematic illustration of an electrical connector in which conductive and dielectric elements are arranged in a generally “I” shaped geometry. Such connectors are embodied in the assignee's “I-BEAM” technology, and are described and claimed in U.S. Pat. No. 5,741,144, entitled “Low Cross And Impedance Controlled Electric Connector,” the disclosure of which is hereby incorporated herein by reference in its entirety. Low cross talk and controlled impedance have been found to result from the use of this geometry.

[0023] The originally contemplated I-shaped transmission line geometry is shown in FIG. 2A. As shown, the conductive element can be perpendicularly interposed between two parallel dielectric and ground plane elements. The description of this transmission line geometry as I-shaped comes from the vertical arrangement of the signal contact shown generally at numeral 10 between the two horizontal dielectric layers 12 and 14 having a dielectric constant ε and ground planes 13 and 15 symmetrically placed at the top and bottom edges of the conductor. The sides 20 and 22 of the conductor are open to the air 24 having an air dielectric constant ε₀. In a connector application, the conductor could include two sections, 26 and 28, that abut end-to-end or face-to-face. The thickness, t₁ and t₂ of the dielectric layers 12 and 14, to first order, controls the characteristic impedance of the transmission line and the ratio of the overall height h to dielectric width w_d controls the electric and magnetic field penetration to an adjacent contact. Original experimentation led to the conclusion that the ratio h/w_d needed to minimize interference beyond A and B would be approximately unity (as illustrated in FIG. 2A).

[0024] The lines 30, 32, 34, 36 and 38 in FIG. 2A are equipotentials of voltage in the air-dielectric space. Taking an equipotential line close to one of the ground planes and following it out towards the boundaries A and B, it will be seen that both boundary A or boundary B are very close to the ground potential. This means that virtual ground surfaces exist at each of boundary A and boundary B. Therefore, if two or more I-shaped modules are placed side-by-side, a virtual ground surface exists between the modules and there will be little to no intermingling of the modules' fields. In general, the conductor width w_c and dielectric thicknesses t₁, t₂ should be small compared to the dielectric width W_d or module pitch (i.e., distance between adjacent modules).

[0025] Given the mechanical constraints on a practical connector design, it was found in actuality that the proportioning of the signal contact (blade/beam contact) width and dielectric thicknesses could deviate somewhat from the preferred ratios and some minimal interference might exist between adjacent signal contacts. However, designs using the above-described I-shaped geometry tend to have lower cross talk than other conventional designs.

[0026] In accordance with an embodiment of the invention, the basic principles described above were further analyzed and expanded upon and can be employed to determine how to even further limit cross talk between adjacent signal contacts. Such analysis first addresses the need to remove shields from between the contacts by determining an appropriate arrangement and geometry of the signal and ground contacts. FIG. 2B includes a contour plot of voltage in the neighborhood of an active column-based differential signal pair S+, S− in a contact arrangement of signal contacts S and ground contacts G according to the invention. As shown, contour lines 42 are closest to zero volts, contour lines 44 are closest to −1 volt, and contour lines 46 are closest to +1 volt. It has been observed that, although the voltage does not necessarily go to zero at the “quiet” differential signal pairs that are nearest to the active pair, the interference with the quiet pairs is near zero. That is, the voltage impinging on the positive-going quiet differential pair signal contact is about the same as the voltage impinging on the negative-going quiet differential pair signal contact. Consequently, the noise on the quiet pair, which is the difference in voltage between the positive- and negative-going signals, is close to zero.

[0027] Thus, as shown in FIG. 2B, the signal contacts S and ground contacts G can be scaled and positioned relative to one another such that a differential signal in a first differential signal pair produces a high field H in the gap between the contacts that form the signal pair and a low (i.e., close to ground potential) field L (close to ground potential) near an adjacent signal pair. Consequently, cross talk between adjacent signal contacts can be limited to acceptable levels for the particular application. In such connectors, the level of cross talk between adjacent signal contacts can be limited to the point that the need for (and cost of) shields between adjacent contacts is unnecessary, even in high speed, high signal integrity applications.

[0028] Through further analysis of the above-described I-shaped model, it has been found that the unity ratio of height to width is not as critical as it first seemed. It has also been found that a number of factors can affect the level of cross talk between adjacent signal contacts. For example, it has been found that one such factor is the distance between the broadside-coupled contacts that form a differential signal pair. In an embodiment, therefore, the careful control of the distance between the broadside-coupled contacts may be used to maintain an appropriate differential impedance Z₀ so as to reduce cross talk between signal pairs. Such a configuration is particularly suitable for mezzanine-style connectors, and such a connector will be discussed below in connection with FIGS. 5A–8. However, it will be appreciated that the invention is not limited to mezzanine connectors, and may be employed in a variety of connector applications.

[0029] FIG. 3 depicts a conductor arrangement in which signal pairs and ground contacts are arranged in rows. The conductor arrangement of FIG. 3 is shown for purposes of comparison, as the arrangement does not depict the “split IMLA” configuration to be discussed below in connection with FIGS. 4–8B. As shown in FIG. 3, each row 311–316 comprises a repeating sequence of two ground contacts and a differential signal pair. Row 311, for example, comprises, in order from left to right, two ground contacts G, a differential signal pair S1+, S1−, and two ground contacts G. Row 312, for example, comprises, in order from left to right, a differential signal pair S2+, S2−, two ground contacts G, and a differential signal pair S3+, S3−. In the embodiment shown in FIG. 3, it can be seen that the columns of contacts can be arranged as insert molded leadframe assemblies (“IMLAs”), such as IMLAs 1–3. The ground contacts may serve to block cross talk between adjacent signal pairs. However, the ground contacts take up valuable space within the connector. As can be seen, the embodiment shown in FIG. 3 is limited to only nine differential signal pairs for an arrangement of 36 contacts because of the presence of the ground contacts.

[0030] Regardless of whether the signal pairs are arranged into rows (broadside-coupled) or columns (edge coupled), each differential signal pair has a differential impedance Z₀ between the positive and negative conductors of the differential signal pair. Differential impedance is defined as the impedance existing between two signal contacts of the same differential signal pair, at a particular point along the length of the differential signal pair. As is well known, it is desirable to control the differential impedance Z₀ to match the impedance of the electrical device(s) to which the connector is connected. Matching the differential impedance Z₀ to the impedance of an electrical device minimizes signal reflection and/or system resonance that can limit overall system bandwidth. Furthermore, it is desirable to control the differential impedance Z₀ such that it is substantially constant along the length of the differential signal pair, i.e., such that each differential signal pair has a substantially consistent differential impedance profile. The distance d of an air dielectric between the contacts that form a differential signal pair (such as signal contacts S1+ and S1−, for example) can determine the impedance Z₀ between each of the contacts.

[0031] As noted above, the differential impedance profile can be controlled by the positioning of the signal and ground contacts. Specifically, differential impedance Z₀ can be determined by the proximity of an edge of a signal contact to an adjacent ground and by the gap distance d between edges of signal contacts within a differential signal pair. However, and significantly, if a proper geometry of broadside-coupled differential signal pairs is attained by precisely maintaining the distance between the contacts of the signal pair, the cross talk between multiple differential signal pairs can be reduced to the point that ground contacts are unnecessary. In other words, the signal quality that results from precisely maintaining an appropriate distance between broadside-coupled signal pairs is high enough to render any additional improvement in signal quality that may be gained by the presence of ground contacts either irrelevant for the connector's intended application, or not worth the attendant increase in size and/or weight of the connector.

[0032] To maintain acceptable differential impedance Z₀ control for high bandwidth systems, it is desirable to control the gap distance d between contacts to within a few thousandths of an inch. Gap variations beyond a few thousandths of an inch may cause unacceptable variation in the impedance profile; however, the acceptable variation is dependent on the speed desired, the error rate acceptable, and other design factors, any weighing or consideration of which is equally consistent with an embodiment of the present invention. When both contacts of a given signal pair are formed within the same IMLA, the distance d is difficulty to maintain at the levels of precision desired for establishing and maintaining a near-constant differential impedance Z₀.

[0033] According to an embodiment of the invention, a “split” IMLA configuration is provided where each IMLA has two lengthwise housing halves, each half corresponding to a respective contact column. It will be appreciated in the discussion that follows that the placing of one contact of a signal pair in a recess of each portion of the lead frame assembly (e.g., the header or receptacle portions of the IMLA) enables greater precision in maintaining the gap distance d between contacts. As a result, the differential impedance Z₀ can be controlled so as to minimize cross-talk between signal pairs to such an extent as necessary to enable removal of the ground contacts.

[0034] Referring now to FIG. 4, a mezzanine-style connector assembly in accordance with one embodiment of the invention is depicted. It will be appreciated that a mezzanine connector is a high-density stacking connector used for parallel connection of printed circuit boards and the like. Such a mezzanine connector can be used to relocate, for example, high pin count devices onto mezzanine or module cards to simplify board routing without compromising system performance. The mezzanine connector assembly 400 illustrated in FIG. 4 comprises a receptacle 410 having receptacle grounds 411 arranged around the outside of the receptacle 410, and a header 420 having header grounds 421 arranged around the outside of the header 420. The header 420 also contains header IMLAs (not individually labeled in FIG. 4 for clarity) and the receptacle 410 contains receptacle IMLAs (also not individually labeled in FIG. 4 for clarity). It will be appreciated that the receptacle 410 and header 420 can be mated to operatively connect the receptacle and header IMLAs. It will also be appreciated that, according to one embodiment of the invention, the grounds shown in FIG. 4, may be the only grounds in the connector.

[0035] As noted above, maintaining careful control of the distance between broadside-coupled contacts that form signal pairs can reduce cross talk between signal pairs. In an embodiment of the invention, such distance control is maintained by using each “split” half of an IMLA (e.g., receptacle and header IMLAs) to maintain precise spacing between contacts of a differential signal pair throughout a connector.

[0036] FIGS. 5A–C depict a receptacle IMLA pair in accordance with an embodiment of the invention. Referring first to FIG. 5A, a first receptacle IMLA 510 comprises an overmolded housing 511 and a series of receptacle contacts 530, and a second receptacle IMLA 520 comprises an overmolded housing 521 and a series of receptacle contacts 530. As can be seen in FIG. 5A, the receptacle contacts 530 are recessed into the housings of receptacle IMLAs 510 and B 520. It will be appreciated that fabrication techniques permit the recesses in each portion of the IMLA 510, 520 to be sized very precisely. As a result, the gap distance d between each signal contact can be maintained throughout a connector fabricated in accordance with an embodiment of the present invention.

[0037] Turning now to FIG. 5B, a detailed view of one such recessed receptacle contact 530 in receptacle IMLA 510 is shown. As can be seen in FIG. 5B, the housing 511 of receptacle IMLA 510 is recessed so the contact 530 sits within the housing such that the distance from the outside broad side of the contact 530 to the outside edge of the housing 511 is ½d. The total distance d extends from the outside broad side of the contact 530 to the outside broad side of a contact 530 of receptacle IMLA 520 (not shown in FIG. 5B for clarity), with which IMLA 510 will be operatively coupled. It will readily be appreciated that the distance provided by either IMLA 510 or IMLA 520 can be any fraction of d, so long as the total distance d is formed when IMLA 510 and IMLA 520 are operatively coupled.

[0038] FIG. 5C shows a detailed view of receptacle IMLA 510 operatively coupled to receptacle IMLA 520. It will be appreciated that in an embodiment any manner of operatively coupling receptacle IMLAs 510 and B 520 may be used. Thus, in an interference fit, fasteners and the like may be used alone or in any combination to affect such coupling.

[0039] In FIG. 5C, it can be seen that the housing 511 of receptacle IMLA 510 abuts the housing 521 of receptacle IMLA 520. Contacts 530 sit within respective recesses in the housings 511 and 521. It will be appreciated that operatively coupling the overmolded housings 511 and 521 as shown in FIG. 5C places a broad side of each contact 530 (i.e., the broad side that is facing the opposing contact 530) at a distance d from the opposing contact 530. In an embodiment, the distance d is able to be maintained at a high level of precision because of the low tolerances possible with overmolded housing fabrication, as well as contact fabrication. Because the distance d only depends on these two, highly-precise components, the distance d can be maintained within the very low acceptable variations that are needed to maintain an appropriate differential impedance Z₀.

[0040] It will be appreciated that, in an embodiment of the invention, the distance d may be bridged by an air dielectric as discussed above. Thus, the weight of the resulting connector, of which the receptacle IMLAs 510 and 520 are a part, may be minimized. It will also be appreciated that the ability to closely control the size of the recess within each overmolded housing 511, 521 enables the impedance Z₀ between the contacts that form signal pairs (and, consequently, cross-talk between signal pairs) to be closely controlled.

[0041] Because the above-mentioned differential impedance Z₀ (and therefore cross talk between signal pairs) is controlled by maintaining a precise distance d, it will be appreciated that a header ILMLA that is to be coupled to a receptacle IMLA should also carefully maintain a precise distance d between signal pairs. Therefore, and turning now to FIGS. 6A–C, a header IMLA pair in accordance with an embodiment of the present invention is depicted. Referring first to FIG. 6A, header IMLA 610 comprises an overmolded housing 611 and a series of header contacts 630, and header IMLA 620 comprises an overmolded housing 621 and a series of header contacts 630. As can be seen in FIG. 6A, the header contacts 630 are recessed into the housings of header IMLAs 610 and 620.

[0042] Turning now to FIG. 6B, a detailed view of one such recessed header contact 630 in header IMLA 610 is shown. As can be seen in FIG. 6B, the housing 611 of IMLA 610 is recessed so the contact 630 sits within the housing such that the distance from the inside broad side of the contact 630 to the inside edge of the housing 611 (i.e., the side of the housing 611 that will abut the housing 621 of header IMLA 620—not shown in FIG. 6B for clarity) is ½ the total distance d from the inside broad side of the contact 630 to the inside broad side of a contact 630 of IMLA 620. Again, it will readily be appreciated that the distance provided by either IMLA 610 or IMLA 620 can be any fraction of d, so long as the distance d is formed when IMLA 610 and IMLA 620 are operatively coupled.

[0043] FIG. 6C shows a detailed view of header IMLA 610 operatively coupled to header IMLA 620. It will be appreciated that in an embodiment any manner of operatively coupling header IMLAs 610 and 620 may be used. Thus, an interference fit, fasteners and the like may be used alone or in any combination to affect such coupling, and any such coupling may be accomplished by the same or a different method used to operatively couple the receptacle IMLAs discussed above in connection with FIGS. 5A–C.

[0044] In FIG. 6C, it can be seen that the housing 611 of header IMLA 610 abuts the housing 621 of header IMLA 620. Within respective recesses in both housings 611 and 621 are contacts 630. It will be appreciated that operatively coupling the housings 611 and 621 as shown in FIG. 6C places a respective broad side of each contact 630 (i.e., the broad side that is facing the opposing contact 630) at a distance d from the opposing contact 630. Thus, the differential impedance Z₀ as discussed above in connection with FIG. 3 may be established because of the distance d maintained between the contacts 630 of header IMLAs 610 and 620. It will also be appreciated that the aforementioned ability to closely control the size of the recess within each housing 611, 621, as well as the contact size, enables differential impedance Z₀ and cross-talk to be closely controlled.

[0045] Turning now to FIG. 7, a header and receptacle IMLA pair in operative communications in accordance with an embodiment of the present invention is depicted. In FIG. 7, it can be seen that header IMLAs 610 and B 620 are operatively coupled to form a single and complete header IMLA. Likewise, receptacle IMLAs 510 and B 520 are operatively coupled to form a single and complete receptacle IMLA. While FIG. 7 illustrates an interference fit between the contacts 630 of the receptacle IMLA and the contacts of the header IMLA, it will be appreciated that any method of causing electrical contact, and/or for operatively coupling the header IMLA to the receptacle IMLA, is equally consistent with an embodiment of the present invention.

[0046] As can be seen in FIG. 7, the contacts of the receptacle IMLA may be flared to accept the contacts of the header IMLA. As a result, the precise maintenance of the distance d between contacts within both the receptacle IMLA and the header IMLA enables the differential impedance Z₀ to be carefully controlled through the connector. This, in turn, minimizes cross talk between signal pairs, even in the absence of ground contacts.

[0047] Turning now to FIG. 8A, a conductor arrangement is depicted in which signal pairs are arranged in rows. As can be seen in FIG. 8A, each row 811–816 comprises a plurality of differential signal pairs. First row 811 comprises, in order from left to right, three differential signal pairs: S1+ and S1−, S2+ and S2−, and S3+ and S3−. Each additional row in the exemplary arrangement of FIG. 8A contains three differential signal pairs. In the embodiment shown in FIG. 8A, and as was the case with FIG. 3, it can be seen that the columns of contacts can be arranged as IMLAs, such as IMLAs 1–3. In addition, each IMLA has two lengthwise halves in a split configuration, A and B, that correspond to each column. Unlike the arrangement discussed above in connection with FIG. 3, no ground contacts are needed because the cross talk between adjacent signal pairs may be minimized by the proper selection of the differential impedance Z₀ that is possible by maintaining a precise distance d between signal contacts. Thus, in an embodiment of the invention, and as shown in FIG. 8A, the connector may be devoid of ground contacts.

[0048] As can be seen, therefore, the embodiment shown in FIG. 8A provides 18 differential signal pairs for an arrangement of 36 contacts, which is a significant improvement over the nine differential signal pairs in the arrangement depicted above in FIG. 3. Thus, a connector according to the invention may be lighter and smaller for a given number of differential signal pairs, or have a greater concentration of differential signal pairs for a given weight and/or size of the connectors.

[0049] It will be appreciated that an embodiment of the present invention encompasses any number of conductor arrangements. For example, the conductor arrangement depicted in FIG. 8B shows that adjacent columns of broadside-coupled pairs may be offset from each other. The conductor arrangement, like the arrangement of FIG. 8A, above, has 36 contacts in 18 signal pairs that are equally divided between IMLAs 1–3 in rows 811–816. It can be seen that IMLAs 1–3 are in the aforementioned split configuration, where each IMLA has a lengthwise half denoted as A and B. In addition, and as noted above, each contact in a given signal pair is separated by a precisely-maintained distance d, which enables the differential impedance Z₀ to be carefully controlled through the connector.

[0050] Unlike the connector of FIG. 8A, however, the pairs disposed along IMLA 2 are offset from the pairs disposed along IMLAs 1 and 3 by an offset distance o. For comparison, it can be seen that in FIG. 8A, the IMLAs 1–3 are arranged such that the conductor pairs that comprise each row 811–816 are in alignment. It will be appreciated that the magnitude of the offset distance o in FIG. 8B may be determined by any number and type of considerations, such as for example the intended application of the connector or the like. In addition, it will be appreciated that any or all of the IMLAs present in a given connector may be offset from any other IMLA within the connector by any offset distance o. In such embodiments, the offset distance o between any two IMLAs may be the same as or different from the offset distance o between any other IMLAs within the connector.

[0051] It will be further appreciated that the offset distance o and the distance d may be set so as to achieve a desired differential impedance Z₀. Therefore, while some embodiments may achieve a desired differential impedance Z₀ by precisely maintaining the distance d alone, other embodiments may achieve a desired differential impedance Z₀ by maintaining the distance d in combination with setting one or more offset distances o.

[0052] Thus, a method and system for split IMLA impedance control has been disclosed. It is to be understood that the foregoing illustrative embodiments have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the invention. Words which have been used herein are words of description and illustration, rather than words of limitation. Further, although the invention has been described herein with reference to particular structure, materials and/or embodiments, the invention is not intended to be limited to the particulars disclosed herein. Rather, the invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. Those skilled in the art, having the benefit of the teachings of this specification, may affect numerous modifications thereto and changes may be made without departing from the scope and spirit of the invention in its aspects.