Abstract:
The present invention involves connectors for reducing Far-End Crosstalk (FEXT) through the use of novel polarity swapping to negate the cumulative effect of FEXT. Skew adjustment is used to improve the FEXT cancellation from polarity swapping. The polarity reversal location or locations among FEXT sources are optimized to achieve maximum FEXT cancellation. The novelty polarity swapping technique can be applied to a wide variety of connectors, such as mezzanine connectors, backplane connectors, and any other connectors that can benefit from FEXT reduction.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This regular U.S. patent application is based on and claims the benefit of priority under 35 U.S.C. 119 from provisional U.S. patent application No. 61/146,614, filed on Jan. 22, 2009, the entire disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to systems for reducing Far-End Crosstalk (FEXT) and, more particularly, towards the reduction of FEXT in electrical connectors that are transmitting differential signals and may include both differential and single-ended signals. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    As the circuit speed increases, differential signaling has become a preferred method for data transmission in such applications as personal computers, servers, switches, and routers. 
         [0004]    For the differential victim pair being considered, unwanted electromagnetic coupling (i.e., crosstalk) from neighboring aggressor pairs occurs throughout the data transmission path when at least one of these neighboring pairs is active. When the aggressor&#39;s transmitter and victim&#39;s receiver are physically far away from each other (located at different chips, for example), the crosstalk induced is called far-end crosstalk (or, FEXT). In general, the chip package, connector, cable, printed circuit board (PCB) traces and vias are all sources of FEXT in a chip-to-chip communication system, due to the close proximity of signal lines. 
         [0005]    Several attempts at reducing FEXT have been made through the introduction of additional self or coupling inductance and/or capacitance in either a connector or a system. The idea was based on that FEXT is proportional to the difference between inductive and capacitive coupling coefficients, so therefore by balancing these two coupling coefficients, FEXT can be reduced. U.S. Pat. No. 7,317,318 B2 is an example of such an attempt as applied to a connector, and US Patent 2007/0275607 A1 is an example of such an attempt as applied to a system. However, such attempts have been relatively insufficient in reducing FEXT, particularly in higher frequency systems. Therefore, there is a need for better ways to reduce FEXT. 
         [0006]    There have also been several inventions that have implemented apparatus for reducing or cancelling crosstalk, such as U.S. Pat. Nos. 6,120,330 and 5,679,027. However, these apparatus are specifically for the reduction or cancellation of near-end crosstalk (NEXT) at frequencies of 100 MHz or below in specifically RJ45 connector systems. 
       SUMMARY OF THE INVENTION 
       [0007]    FEXT is a cumulative effect, and if there is more than one FEXT generator in the data transmission path (i.e., channel), the neighboring differential pair&#39;s polarity will be swapped at least once through a simple routing change. The swapping of neighboring pair&#39;s polarity results in the phase change of the following FEXT generators, so the cumulative FEXT from the following FEXT generators will cancel the cumulative FEXT from the preceding FEXT generators. Such polarity swapping, which is independent of inductive or capacitive coupling, can be applied to a chip package, a connector, a printed circuit board (PCB), or any differential system that experiences FEXT. Low-crosstalk chip packages and connectors can be designed with polarity swapping built-in. Systems with large individual FEXT components can also see big improvements in total FEXT by applying one or more embodiments of the invention. 
         [0008]    In one aspect of the present invention, the idea is implemented in a data transmission system that includes a transmission end, a receiving end, at least one differential pair connected to the transmission end and the receiving end, each differential pair comprising a positive signal line and a negative signal line, wherein at least one differential pair undergoes polarity reversal at one or more locations between the transmission end and the receiving end such that at least one signal transmitted to the receiving end experiences reduced FEXT due to the polarity reversal. 
         [0009]    Additional aspects of the present invention include a system involving a transmission end; a receiving end; at least one single ended signal line; at least one differential pair connected to the transmission end and the receiving end; each differential pair comprising a positive signal line and a negative signal line; and wherein one or more differential pairs undergo polarity reversal at one or more locations between the transmission end and the receiving end. 
         [0010]    Additional aspects of the present invention include a system that includes a transmission end, a receiving end, at least one differential pair connected to the transmission end and the receiving end, the differential pair comprising a positive signal line and a negative signal line, wherein the skew among differential pairs is minimized to improve FEXT cancellation. 
         [0011]    Additional aspects of the present invention include a system that includes a transmission end, a receiving end, at least one single ended signal line, at least one differential pair connected to the transmission end and the receiving end, the differential pair comprising a positive signal line and a negative signal line, wherein the skew among differential pairs and single-ended signals is minimized to improve FEXT cancellation. 
         [0012]    Additional aspects of the present invention include a system that includes a transmission end, a receiving end, at least one single ended signal line, at least one differential pair connected to the transmission end and the receiving end, the differential pair comprising a positive signal line and a negative signal line, wherein the polarity reversal location or locations among FEXT sources are optimized to achieve maximum FEXT cancellation. 
         [0013]    Additional aspects of the present invention include a connector that includes a transmission end; a receiving end; multiple differential pairs connected to the transmission end and the receiving end; each differential pair comprising a positive signal line and a negative signal line; wherein at least one differential pair undergoes a polarity reversal at a location between the transmission end and the receiving end. 
         [0014]    Additional aspects of the present invention include a connector that includes a transmission end; a receiving end; at least one single-ended signal; at least one differential pair connected to the transmission end and the receiving end; each differential pair comprising a positive signal line and a negative signal line; wherein at least one differential pair undergoes a polarity reversal at a location between the transmission end and the receiving end. 
         [0015]    Additional aspects related to the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Aspects of the invention may be realized and attained by means of the elements and combinations of various elements and aspects particularly pointed out in the following detailed description and the appended claims. 
         [0016]    It is to be understood that both the foregoing and the following descriptions are exemplary and explanatory only and are not intended to limit the claimed invention or application thereof in any manner whatsoever. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The accompanying drawings, which are incorporated in and constitute a part of this specification exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the inventive technique. Specifically: 
           [0018]      FIG. 1  shows an example chip-to-chip communication system where FEXT can occur. 
           [0019]      FIG. 2  shows an example connector where FEXT can occur. 
           [0020]      FIG. 3  illustrates two differential pairs of a system that contains two sources of FEXT. 
           [0021]      FIG. 4  illustrates an embodiment of the present invention implementing polarity swapping when applied to the differential system of  FIG. 3 . 
           [0022]      FIG. 5  illustrates an embodiment of the present invention implementing polarity swapping and skew minimization applied to the differential system of  FIG. 3 . 
           [0023]      FIG. 6  illustrates the FEXT of the differential pairs of  FIGS. 3-5 . 
           [0024]      FIG. 7  illustrates typical mezzanine connectors. 
           [0025]      FIG. 8  illustrates the pin assignment for the three piece BGA mezzanine connector of  FIG. 7   a  at both the entrance and exit. 
           [0026]      FIG. 9  illustrates the FEXT between differential pairs 8 and 9 on a typical mezzanine connector of  FIG. 7   a  and  FIG. 8 . 
           [0027]      FIGS. 10   a - 10   d  illustrate an example embodiment of the invention as applied to a mezzanine connector. 
           [0028]      FIGS. 11   a  and  11   b  illustrate the pin assignment of the mezzanine connector at the entrance and exit respectively, as indicated by an embodiment of the invention. 
           [0029]      FIG. 12  illustrates the improvement on the FEXT between differential pairs 8 and 9 on the mezzanine connector of  FIGS. 10 ,  11   a  and  11   b.    
           [0030]      FIG. 13  illustrates typical backplane connectors. 
           [0031]      FIG. 14  illustrates example embodiments of the invention as applied to backplane connectors. 
           [0032]      FIG. 15   a  illustrates a conventional card edge connector, and  FIG. 15   b  is an example embodiment of the invention as applied to a card edge connector. 
           [0033]      FIG. 16  illustrates a section of data transmission path in a chip-to-chip communication system with two connectors. 
           [0034]      FIG. 17  illustrates the application of an embodiment of the present invention to the system design of  FIG. 16 . 
           [0035]      FIG. 18  compares the FEXT of a typical system design versus the system design applying an embodiment of the invention. 
           [0036]      FIG. 19  illustrates how the FEXT between the single-ended signal 1− and differential pair signal 2 +and 2− is also significantly reduced with the application of an embodiment of the present invention. 
           [0037]      FIGS. 20   a - d  show examples of one or more embodiments of the invention with one or more polarity reversals among a plurality of FEXT sources. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    In the following detailed description, reference will be made to the accompanying drawing(s), in which identical functional elements are designated with like numerals. The aforementioned accompanying drawings show by way of illustration, and not by way of limitation, specific embodiments and implementations consistent with principles of the present invention. These implementations are described in sufficient detail to enable those skilled in the art to practice the invention and it is to be understood that other implementations may be utilized and that structural changes and/or substitutions of various elements may be made without departing from the scope and spirit of present invention. The following detailed description is, therefore, not to be construed in a limited sense. 
         [0039]      FIG. 1  shows an example of a chip-to-chip communication system, where the signal sent by the transmitter  101  can go through multiple chip packages  102 , connectors  103 , and traces  104  and vias  105  in printed circuit boards (PCB)  106  before arriving at the receiver  107 . Chip packages, connectors, PCBs, and cables are all components that can be utilized within such a chip-to-chip system. 
         [0040]      FIG. 2  shows an example of an electrical connector, where a signal being transmitted through the connector can go through mating portions  201 , retention portions  202 , solder tails  203 , conductors  204  or vias  205  before or after the connector. 
         [0041]      FIG. 3  illustrates two differential pairs  300 - 301  with two FEXT sources  302 - 303 . These FEXT sources can be chip packages, connectors, PCB vias or PCB traces as shown in  FIG. 1 . These FEXT sources can also be mating portions, retention portions, solder tails within a connector, traces, or vias before or after a connector as shown in  FIG. 2 . These FEXT sources can also be bond wires, traces, lead frames, or solder balls within a chip package. To illustrate how the polarity swapping idea works, we consider an example of two differential pairs, in which there are two FEXT sources, and the traces before, between, and after the sources contribute no crosstalk. 
         [0042]      FIG. 4  shows the modified differential pairs utilizing one or more embodiments of the inventive polarity swapping technique, where the relative positions of two differential pairs change from (1+, 1−, 2+, 2−) to (1+, 1−, 2−, 2+) at some point  401  between the two FEXT sources. Pair 2 has 5 ps more delay than pair 1 in this case because pair 2 requires additional trace length to swap positions. In this example, by swapping the relative positions of the positive and negative signal lines of the aggressor differential pair, a large amount of the accumulated FEXT becomes negated by the subsequent inverted FEXT. 
         [0043]      FIG. 5  illustrates the modified differential pairs utilizing one or more embodiments of the inventive polarity swapping technique  401  and a skew adjustment technique  501  at some point between the two FEXT sources. In this example, by swapping the relative positions of the positive and negative signal lines of the aggressor differential pair and matching the delay between the two differential pairs, the accumulated FEXT becomes almost completely negated by the subsequent inverted FEXT. 
         [0044]      FIG. 6  gives the FEXT of the differential pairs in  FIGS. 3-5  in frequency domain. From this plot, it can be seen that FEXT is significantly reduced by utilizing one or more embodiments of this polarity swapping technique. The reason for such reduction can be roughly explained by the following. The two sources of coupling between Pairs 1 and 2 are temporarily assumed to be of equal magnitude for simplicity. The polarity swapping results in these two FEXTs being opposite in phase, and therefore they cancel each other. When there is a 5 ps skew introduced by the additional length required to swap the relative positions of the aggressor pair, the two FEXTs arriving at the receiver are not exactly opposite in phase, resulting in more FEXT than when the skew is eliminated. By adding some delay to the victim pair as done in  FIG. 5  to remove the skew, the FEXT cancellation can be significantly improved as seen in  FIG. 6 . Rigorous mathematical derivation can be used to identify the propagating modes and explain the small left-over FEXT after the polarity change and skew elimination. Of especial interest in this simulation is the significant reduction of FEXT in high frequency systems where the differential pair is transmitting a signal of greater than 1 GHz. By utilizing the novel polarity reversal swapping technique, such higher frequency systems can be designed without significant worry in regards to FEXT. This would enable circuit speeds to go much faster than the current technology allows. However, lower frequency systems will also benefit from the use of the novel polarity swapping technique, which can be especially useful in systems that utilize both a differential pair and a single ended signal line. 
         [0045]    This optimal location for the polarity swap is where the FEXT accumulated from one side is close to 50% of the total FEXT. Although, some FEXT will still be cancelled even if the FEXT induced before the swap is unequal to the FEXT after the swap. 
         [0046]    In some cases, it may be optimal to swap multiple times among a plurality of FEXT sources. 
         [0047]    Moreover, the embodiments of the inventive polarity swapping technique can also be applied to the design of individual components.  FIG. 7  shows examples of a two piece mezzanine connector  705  and a three piece mezzanine connector  701  that the polarity swapping technique can be applied to. The two piece connector is comprised of a plug  706  and a receptacle  707 . The three piece connector is comprised of two receptacles  702  and one interposer  703 . The interposer, in turn, contains multiple wafers  704 , where each wafer has 10 signal traces and one ground plane. Solder balls are attached to the connector; their corresponding pin assignments for three wafers are shown in  FIG. 8 ; and differential FEXT between Pairs 8 and 9 is shown in  FIG. 9 .  FIG. 10  shows the improved three piece connector implementing an embodiment of the polarity swapping technique. The side view  1001  shows multiple wafers inserted into the interposer. The signal traces swap their relative positions on the wafers for every other differential pair. Two types of wafers are used. One type has the center pair swapped  1005 . The other type has the center pair non-swapped  1006 . These two wafer types are placed adjacent to each other within the interposer so that the swapped pairs are staggered from one wafer to the next. In this configuration the FEXT from the nearest neighbors are drastically reduced. Details of the swapping portion  1003  can be seen from the perspective view  1002 . To achieve optimal FEXT cancellation, the wafers were designed to have very little inter-pair skew. This was done by purposely adding delay to the non-swapped pairs. Details of the delay adjustment portion can be seen in  1004 . The corresponding pin assignments of solder balls and differential FEXT between Pairs (8+, 8−) and (9+, 9−) are shown in  FIGS. 11   a - b  and  12 , respectively. It is apparent that utilizing one or more embodiments of the invention results in FEXT of  FIG. 12  being significantly less than FEXT of  FIG. 9 . This polarity swapping technique is also applicable to the two piece mezzanine connector. 
         [0048]    The polarity swapping technique can also be applied to backplane connectors.  FIG. 13  shows examples of a three piece backplane connector  1301  and a two piece backplane connector  1305 . The three piece backplane connector consists of two receptacles  1302 - 1303  and an interposer  1304 . The two piece backplane connector is comprised of a plug  1306  and a receptacle  1307 . Examples of how the two piece backplane connector  1401  and the three piece backplane connector  1402  can utilize the polarity swapping technique are illustrated in  FIG. 14 . 
         [0049]      FIG. 15  illustrates an example of how one or more embodiments of the polarity swapping technique can be applied to a card edge connector. A conventional card edge connector  1501  has differential pairs that go straight through. Details of the straight pairs  1503  are shown. A card edge connector utilizing this polarity swapping technique  1502  has differential pairs that swap. Details of this implementation  1504  show how these pairs are swapped. 
         [0050]      FIG. 16   a  shows a portion of a conventional chip-to-chip communication system where two connectors  701  from  FIG. 7  are used in a “channel” with 3″ PCB trace  1602 , 6″ PCB trace  1603 , and 3″ PCB trace  1604  routed on the inner signal layers as an example. The top view layout of the PCB  1601  linking the two connectors is shown in  FIG. 16   b.  Details of how the PCB traces are routed out from the connectors are shown in the detail views  1605 - 1606 . In this conventional way of routing, the polarities of the two differential pairs, (1+, 1−) and (2+, 2−), are maintained from one connector to the next. The relative locations of the positive and negative signal lines remain the same at each connector. With such conventional routing, the FEXT generated by each connector will accumulate. 
         [0051]      FIG. 17   a  shows a portion of a chip-to-chip communication system utilizing one or more embodiments of the present invention. Similar to  FIG. 16   a,  it consists of two connectors  701  in a “channel” with 3″ PCB trace  1702 , 6″ PCB trace  1703 , and 3″ PCB trace  1704  routed on the inner signal layers. The top view layout of the PCB  1701  linking the two connectors is shown in  FIG. 17   b.  Details of how the PCB traces are routed out from the connectors are shown in detail views  1705 - 1706 . It can be seen that, unlike the system of  FIG. 16   a - b,  the system of  FIG. 17   a - b  has the polarity of differential pair (2+, 2−) swapped from one connector to the next. The differential pairs are configured 1+, 1−, 2+, and 2− at the connector to the left and configured 1+, 1−, 2−, and 2+ at the connector to the right. The relative locations of differential pair (2+, 2−) have changed from one connector to the next. Because of the polarity swapping, the FEXT generated from the two connectors will cancel. 
         [0052]      FIG. 18  shows the FEXT between differential pairs (1+, 1−) and (2+, 2−) for the systems of  FIGS. 16   a - b  and  FIGS. 17   a - b.  It can be seen that the use of one or more embodiments of the invention reduces FEXT dramatically in a system. 
         [0053]    So far, we have focused on FEXT between two differential pairs. However, the embodiments of the invention are applicable to FEXT between a differential pair and a single-ended signal as well. Let us consider 1+ and 1− in  FIGS. 16 and 17  as two independent single-ended signals.  FIG. 19  shows that FEXT between single-ended 1− and differential pair (2+, 2−) is also significantly reduced with the utilization of one or more embodiments of the invention. From this example, it is therefore possible to reduce FEXT even in a system involving a differential pair and a neighboring single-ended signal. 
         [0054]    The embodiments of the invention are not limited to only one polarity swap between two FEXT sources. The polarity can be swapped more than once among a plurality of FEXT sources.  FIGS. 20   a - d  show such examples, each consisting of four FEXT sources and at least one polarity reversal.  FIG. 20   a  shows a victim differential pair going through four FEXT sources. An aggressor differential pair, which does not swap polarity, is located along side the victim pair but is not shown here. At the first FEXT source  2001 , 10% of the aggressor signal couples over to the victim pair. At the second FEXT source  2002 , 20% couples over. Since the polarity is maintained from the first source to the second, the FEXT accumulates to 30%. The victim pair then swaps polarity before entering the third FEXT source  2003 , where 30% of the aggressor signal couples over. Because the polarity of the victim pair was swapped before entering the third source, the FEXT from  2003  is opposite in polarity to the FEXT accumulated from  2001 - 2002 , and they cancel each other out. This leaves 0% FEXT accumulated after three FEXT sources. The fourth FEXT source  2004  contributes 40% FEXT, so the total FEXT accumulated at the end of the system is 40%. This is a significant improvement over a system or component which would have seen 100% FEXT if routed in the conventional fashion. 
         [0055]    The FEXT in this example can be further reduced to 20% by swapping polarity at different locations as in  FIGS. 20   b - c.    FIG. 20   b  shows an example where the polarity is swapped three times: once after  2001 , once after  2002 , and once after  2003 . In this example 40% FEXT of one polarity, accumulated from  2001  and  2003 , cancels the 60% FEXT of opposite polarity, accumulated from  2002  and  2004 , leaving 20% FEXT in the victim pair.  FIG. 20   c  has the victim pair swap once between  2003  and  2004 . 40% FEXT from  2004  is canceled by 60% FEXT from  2001 - 2003 , resulting in 20% FEXT left over. 
         [0056]    With some optimization, it can be seen that FEXT can be cancelled even further using one or more embodiments of the present invention.  FIG. 20   d  shows an example where polarity is swapped between  2001  and  2002  and between  2003  and  2004 . 50% FEXT of one polarity is coupled from sources  2001  and  2004 , and 50% FEXT of the opposite polarity is couple from sources  2002  and  2003 . These FEXT sources cancel each other out and leave a minimal amount of FEXT in the victim pair. To achieve maximum FEXT cancellation, the location or locations of where to swap polarity can be optimized. The examples of  FIGS. 20   a - d  have shown four different examples of how utilizing one or more embodiments of the polarity swapping invention can be used to dramatically reduce FEXT. 
         [0057]    Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.