High frequency RJ45 plug with non-continuous ground planes for cross talk control

There is provided a communication connector including a housing and a plurality of electrical contacts received by the housing. A printed circuit board (PCB) is provided and includes at least first and second pairs of electrical conductive traces. The first pair of traces is connected to a first pair of contacts. The second pair of traces is connected to a second pair of contacts. The PCB has first and second regions. The first pair of contacts is located in the first region and the second pair of contacts is located in the second region. The PCB further includes at least a first ground plane. The first ground plane has first and second sections. The first and second sections are electrically isolated from one another. At least a portion of the first section being adjacent to at least a portion of the first pair of traces. At least a portion of the second section is adjacent to at least a portion of the second pair of traces whereby signal transmission is enhanced. A layer of dielectric material which has a relatively low dielectric constant is located between the first and second traces and the first ground plane.

BACKGROUND OF THE INVENTION

This invention relates to reducing electrical signal interference and return loss which arises due to parallel contacts in electrical connectors. More particularly, it relates to reducing return loss and pair to pair as well as common mode cross talk interference in FCC type plugs. The Federal Communications Commission has adopted certain architectural standards with respect to electrical connectors utilized in the telecommunications industry so as to provide intermatability. The connectors most commonly utilized are FCC type modular plugs and jacks, also referred to as RJ45 plugs and jacks. The plug is commonly terminated to a plurality of conductors which may be connected to a communication device. Normally, each plug is terminated to eight conductors which form four pairs. The corresponding jack is commonly mounted to a panel or a printed circuit board or a wall outlet, which in turn is connected to the telecommunication network. To complete the circuit, the plug and jack are intermated.

While most of the cross talk and return loss problems arise in the plug, normally the reduction of cross talk and return loss is dealt with in the jack. For example, U.S. Pat. No. 5,299,956 assigned to Optical Cable Corporation, assignee of this application, shows a jack connected to a circuit board in which traces on the board are used to cancel cross talk which primarily arises in a corresponding plug. U.S. Pat. No. 5,310,363, also assigned to Optical Cable Corporation, shows a similar jack having both cross talk cancellation and return loss reduction features.

Industry standards for maximum cross talk, both near end cross talk (NEXT) and far end cross talk (FEXT), common mode coupling, and maximum return loss are governed by the Telecommunications Industry Association (TIA). The TIA is currently addressing performance standards at the so-called Category 8 level, which is in a bandwidth of up to 2,000 MHz and designed to enable data transmission rates up to 40 Gbits per second. The TIA standards are pair and frequency dependent. The TIA standards as currently drafted for NEXT, FEXT and return loss for various frequencies and pairs are shown in the graphs illustrated inFIGS. 6 through 14.

As previously stated and as taught in Optical Cable Corporation's U.S. Pat. No. 5,299,956, cross talk compensation is normally primarily carried out on a circuit board which is attached to the jack. A properly designed plug, however, is essential so as not to degrade the cross talk cancellation features of the jack and, in addition, to ensure minimal return loss.

SUMMARY OF THE INVENTION

In accordance with one form of this invention, there is provided a communication connector comprising a housing, a plurality of pairs of electrical contacts received by the house, and a printed circuit board (PCB). The PCB includes at least first and second pairs of electrical conductive signal traces. The first pair of traces is connected to a first pair of contacts. The second pair of traces is connected to a second pair of contacts. The PCB has first and second regions. The first pair of traces is located in the first region and the second pair of traces is located in the second region. The PCB further includes at least a first ground plane. The first ground plane has first and second sections. The first and second sections are substantially electrically isolated from one another. At least a portion of the first section is adjacent to at least a portion of the first pair of traces. At least a portion of the second section is adjacent to at least a portion of the second pair of traces whereby signal transmission performance is improved.

In accordance with another form of this invention, there is provided a communication connector comprising a housing, a plurality of pairs of electrical contacts received by the housing, and a printed circuit board (PCB). The PCB includes at least first and second pairs of electrically conductive signal traces. The first pair of traces is connected to a first pair of contacts. The second pair of traces is connected to a second pair of contacts. The PCB has first and second regions. At least one trace of the first pair of traces is located in the first region and at least one trace of the second pair of traces is located in the second region. The PCB further includes at least a first ground plane. The first ground plane has a gap thereby forming first and second sections which are not electrically connected to each other. At least a portion of the first section is adjacent to at least a portion of at least one trace of the first pair. At least a portion of the second section is adjacent to at least a portion of at least one trace of the second pair.

In accordance with yet another form of this invention, there is a provided a communication connector including a housing, a plurality of pairs of electrical contacts received by the housing, and a printed circuit board (PCB). The PCB includes at least first and second pairs of electrical conductive signal traces. The first pair of signal traces is connected to a first pair of contacts and the second pair of signal traces is connected to a second pair of contacts. The PCB further including a ground plane, a first dielectric layer having a first dielectric constant and a second dielectric layer having a dielectric constant lower than the dielectric constant of the first dielectric layer. The first dielectric layer is adjacent to one side of the ground plane and the second dielectric layer is adjacent to the other side of the ground plane and the second dielectric layer is also adjacent to the first and second pairs of signal traces. Preferably, ground plane coupling is primarily to the adjacent traces above it.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now more particularly toFIG. 1, there is provided plug10having plastic body12. Preferably, plug10has the connection architecture of a standard FCC RJ45 plug for connecting four signal pairs of conductors to a corresponding RJ45 jack, which except for jack contacts14, is not shown. Plug10includes eight contacts16which are ordered in pairs in accordance with FCC standards. According to the FCC standards, pair1consists of the fourth and fifth contacts, pair2consists of the first and second contacts, pair3consists of the third and sixth contacts, and pair4consists of the seventh and eighth contacts.

Plug10also includes circuit board18. Circuit board18preferably consists of seven layers of conductors and dielectric materials which is better illustrated inFIG. 3which shows an inverted cross-section of circuit board18. The top layer20of circuit board18includes circuit board traces corresponding to two of the four conductor pairs of traces, such as trace22. The second layer24of the board18includes a high performance, high frequency dielectric material such as FR408 which is commercially available from Isola Laminate Systems Corp. The third layer26of circuit board18consists of a first split ground plane26, which is also shown inFIG. 5. The fourth layer28of circuit board18consists of standard dielectric material such as FR4 also commercially available from Isola Laminate Systems Corp. The fifth layer30of circuit board18consists of a second split ground plane30. The sixth layer32of circuit board18preferably consists of the same high performance, high frequency dielectric as second layer24. The seventh or bottom layer34includes another layer of circuit board traces corresponding to the other two of the four conductor pairs of traces, such as trace35.

As can be seen best inFIG. 5, first ground plane26is split forming a gap36extending lengthwise along the ground plane in the direction of the pairs of traces so as to effectively divide ground plane26into two electrically isolated elements50and52. The position of the split relative to the traces affects electrical performance. Second ground plane30is also split and is preferably substantially identical to first ground plane26. The traces on the top side and bottom side of circuit board18are terminated to circuit board contacts38which extend from one side of circuit board18. For reference, the side of circuit board18from which the circuit board contacts38extend is referred to as the top side. A plurality of via holes, such as via hole40, extend through the circuit board18so that a trace on the bottom side of the board can be connected to a corresponding circuit board contact. For example, trace35is connected to conductor46which extends through a via hole (not shown so conductor46and its connections are clear) which in turn connects to circuit board contact48. The eight circuit board contacts38are connected to eight corresponding plug contacts16.

Referring again toFIG. 5, while item26is referred to as a ground plane, it includes at least two, and preferably two, substantially electrically isolated sections, namely, sections50and52which are separated by gap36. By referring to sections50and52as being “substantially electrically isolated” it is meant that the two elements do not contact one another and the gap36is wide enough so that electrical energy is not substantially transferred across the gap36even at 2,000 MHz. That is, it is important that gap36not be so small that substantial current or voltage from section50could be transferred to section52and vice verse either by capacitive or by inductive coupling. In addition, while it is preferred that sections50and52of ground plane26are on the same plane as illustrated inFIG. 2, it is not intended to exclude sections which are on different planes, however, it is important that sections50and52not be in electrical contact with one another. It is preferred that the gap36be at least 0.08 millimeters wide. It is also preferred that gap36not be so wide that the return loss reduction characteristics of ground plane26be degraded. It is preferred that gap36be no wider than 6.35 millimeters.

As can be seen inFIG. 7, pair37is above and adjacent to section50of split ground plane26and pair39is above and adjacent to section52of split ground plane26. Any energy transferred from pair37to section50will not be transferred to section52or pair39because gap36electrically isolates return current of the signal on each trace to sections50and52. The same electrical isolation occurs between the two split sections of ground plane30and pair41and43on the other side of board18as shown inFIG. 8. It is also possible that only one trace of pair37be adjacent to section50and only one trace of pair39be adjacent to section52.

It has been found that the use of a split ground plane increases common mode isolation between the traces which enhanced differential pair transmission characteristics of plug10. This is best illustrated in reference toFIGS. 18, 19 and 20. For example, as shown inFIG. 18, a plug having a split ground plane was compared to a plug having a non-split ground plane, with all of the other elements of the plug being the same. As can be seen inFIG. 18, common mode to common mode NEXT loss for pairs1-2for the split ground plane achieved greater performance for a bandwidth up to 2,000 megahertz.

As shown inFIG. 19, common mode to common mode NEXT loss for pairs1-4for a plug having a split ground plane was superior to a plug having a non-split ground place as was pairs2-4shown inFIG. 20.

In addition, forward NEXT for the plug of the subject invention having split ground planes26and30with the gap formed by the split being between approximately 0.45 millimeters and 1.65 millimeters has been measured.FIGS. 9 through 12show plug forward NEXT magnitude for various pairs measured against the current Category 8 standards.FIG. 9shows NEXT measurements for pairs1-3.FIG. 10shows NEXT measurements for pairs2-3and pairs3-4.FIG. 11shows NEXT measurements for pairs1-2and pairs1-4.FIG. 12shows NEXT measurements for pairs2-4. All of the measurements shown inFIGS. 9-12exceed the current Category 8 NEXT standard. At 2,000 MHz, the NEXT for pairs1-3was approximately −15.1 dB; for pairs2-3, it was approximately −23.0 dB; for pairs3-4, it was approximately −23.5 dB; for pairs1-2, it was approximately −34.3 dB; for pairs1-4, it was approximately −41.5 dB; and for pairs2-4, it was approximately −41.4 dB.

Plug forward FEXT for the same plug of the subject invention has also been measured.FIGS. 13 through 16show plug forward FEXT for various pairs.FIG. 13shows FEXT measurements for pairs1-3.FIG. 14shows FEXT measurements for pairs2-3and pairs3-4.FIG. 15shows FEXT measurements for pairs1-2and pairs1-4.FIG. 16shows FEXT measurements for pairs2-4. All of these readings show that the plug of the subject invention exceeds the current Category 8 FEXT standard. At 2,000 MHz, the FEXT for pair1-3was approximately −29.3 dB; for pairs2-3, it was approximately −36.1 dB; for pairs3-4, it was approximately −32.3 dB; for pairs1-2, it was approximately −36.2 dB; for pairs1-4, it was approximately −36.7 dB; and for pairs2-4, it was approximately −44.5 dB.

FIG. 17shows the return loss for all four pairs for the same plug of the subject invention which also exceed the draft Category 8 standard for return loss as of Dec. 1, 2014. At 2,000 MHz, the return loss for pair1was approximately −15.5 dB; for pair2, it was approximately −17.3 dB; for pair3, it was approximately −12.5 dB; and for pair4, it was approximately −17.3 dB.

Referring again toFIG. 3, fourth or center layer28of the circuit board18is made from an inexpensive standard dielectric material such as FR4 as the central core and is used primarily as a substrate for the circuit board. FR4 is made from woven fiberglass clothe with an epoxy resin binder and has a fairly high dielectric constant, dependent on the frequency of the signal, such as, for example, a dielectric constant of 4.5. However, the material which more directly affects the transmission qualities of the plug is the dielectric used for the second layer24and the sixth layer32of the circuit board, because those layers are in close proximity to the traces in layers20and34. The dielectric second and sixth layers have a dielectric constant less than 4.5 and preferably no more than 3.7. This lower dielectric constant material is well suited to high frequency content and supports the higher frequency transmission of Category 8, i.e., up to 2,000 megahertz. It is preferred that the material in the second layer and in the sixth layer be FR408. FR408 is also made from woven fiberglass cloth with an epoxy resin binder but is designed for faster signal speeds. It has been found that the use of the FR408 as the second and sixth layers helps enable the improved differential return loss referred to above. The second and sixth FR408 layers should be thinner than the fourth FR4 layer. Preferably, the thickness of the second and sixth layers are 0.127±0.025 mm. Since FR408 is more expensive than FR4, substantial cost benefits are achieved by using only thin layers of FR408 without degrading signal quality. The thickness of the FR408 could be any standard available size. The PCB referred to herein uses 0.005 inch layers. The thickness is driven by the desired trace geometry.

The use of a ground plane in an RJ45 plug improves return loss. However, Applicant has found that without the use of split or gap36in ground plane26, cross talk with respect to the conductor pairs becomes a problem. The split in the ground plane forces return currents of the transmission lines to remain in proximity to the transmission lines, resulting in increased common mode isolation between traces of plug and thus reduced cross talk. Preferably, the thickness of ground plane26is in the range of 0.0178 millimeters to 0.0771 millimeters. In addition, it is preferred that ground plane26be made of copper. It is also preferred that the distance between traces on one side of circuit board18and its adjacent ground plane26be between 0.051 millimeters and 0.61 millimeters. The use of the term split ground plane as used herein can also mean two or more separate ground planes. The separate ground planes or separate sections of a split ground plane may or may not be at the same potential. It has been found that placement of the split ground plane below pairs of traces as defined in the connector PCB allows tuning of coupling parameters between other pairs and individual conductors in the overall plug.

Referring to the foregoing description of the preferred embodiments of the invention, it would be apparent that many modifications may be made therein. It is to be understood that all such modifications are embodied in the accompanying claims which come within the true spirit and scope of the invention.