1. Field of the Invention
The present invention relates to a system for reducing noise in a backplane system. More particularly, the present invention reverses pin connections between backplanes to reduce crosstalk generated between pin connections in the backplanes.
2. Background of the Related Art
Electronic systems are often assembled from multiple printed circuit boards (PCBs). PCBs that contain electronic components are sometimes called “daughter cards.” Often, electrical signals are routed from one daughter card to another through a PCB called a “backplane.” A backplane includes conducting paths called “signal traces,” or just “traces,” etched onto the PCB using a conductive material, such as copper. Connectors are mounted on the backplane to make connections to the traces. The portion of a printed circuit board that allows a trace to be connected to a contact element in a connector or other electronic element is called a “signal launch,” or just “launch.” When connectors are used to mate daughter cards with the launches on a backplane, signal paths are established between the daughter cards via the signal traces on the backplane.
As electronic systems have become smaller, faster, and more complex, backplanes generally have been required to provide more signal traces without increasing in size, or while actually decreasing in size. As a result of the increased density of traces, electrical noise between signals has become more problematic. Electrical noise is usually considered any undesirable electrical energy in an electronic system, including but not limited to reflections, electromagnetic interference, mode conversions, and unwanted coupling, such as crosstalk.
The trend toward smaller, faster, and more complex electronic systems has also required daughter cards, backplanes, and their connectors to carry more and faster data signals in a smaller space without degrading the characteristics of those signals. Daughter cards, backplanes, and their connectors can be made to carry more signals in less space by placing signal conductors closer together. A major difficulty with placing signal conductors closer together is that electrical noise between the signal conductors increases as the distance between signal conductors decreases and as the speed of the signals increases. In addition, as frequency content increases, there is a greater possibility of energy loss. Energy loss may be attributed to impedance discontinuities, mode conversion, leakage from imperfect shielding, or undesired coupling to other signal conductors, such as crosstalk.
Differential signals are signals represented by a pair of conducting paths, called a “differential signal pair.” The voltage difference between the conductive paths represents the signal. In general, the two conducing paths of a differential signal pair are arranged to run near each other. If any other source of electrical noise is electromagnetically coupled to the differential signal pair, the effect on each conducting path of the pair should be similar. Because the signal on the differential signal pair is treated as the difference between the voltages on the two conducting paths of the pair, a common noise voltage that is coupled to both conducting paths in the differential signal pair does not affect the signal. That renders a differential signal pair less sensitive to crosstalk noise, as compared with a single-ended signal path. At high speeds, the available conducting paths may encounter crosstalk, impedance, and attenuation mismatch characteristics, and the signal transmission characteristics may degrade.
Crosstalk generally refers to electromagnetic interference that comes from an adjacent wire. Current that flows down one of the traces, referred to as the “aggressor line,” couples into the adjacent trace, called the “victim line,” and creates two different noise signals in the victim line. One of the noise signals, called “forward crosstalk” or “far end crosstalk,” flows in the victim line in the same direction as the current flowing in the aggressor line. The other noise signal, called “backward crosstalk,” flows in the victim line in the opposite or backward direction as the current flowing in the aggressor line.
Referring to FIG. 1(a), a standard backplane connection system 100 is shown having a first connector 110, a second connector 120, and various trace pairs 130-140. The first connector 110 is connected to input trace pairs 130 and 132 on a first daughter card. Trace pairs 134 and 136 are provided on a backplane to connect the first connector 110 to the second connector 120. And, the second connector 120 is connected to output trace pairs 138 and 140 on a second daughter card. In a backplane environment, the input trace pairs 130 and 132 coming into the first connector 110 can be, for example, from a line card of a router or a processor card on the first daughter card, and the output trace pairs 138 and 140 going out from the second connector 120 can be, for example, to a fabric card or a switch card on the second daughter card.
The impact of crosstalk on the system 100 is exemplified in FIGS. 1(b)-(i). In the system 100 of FIG. 1(a), the bottom trace pairs 130, 134, and 138 are the aggressor trace pairs and the top trace pairs 132, 136, and 140 are the victim trace pairs. After the signal on the aggressor trace pair 130 reaches the first connector 110, as shown in FIG. 1(b), noise in the form of crosstalk is created at the first connector 110, as shown in FIG. 1(c). That noise is generated inside the first connector 110 by coupling between the differential signal pairs.
Noise is also generated in the portion of the backplane that the first connector 110 is pressed into, which is called the “footprint” of the first connector 110. The footprints of the first and second connectors 110 and 120 each have mechanisms for mating the contact elements of the first and second connectors 110 and 120 to their respective launches in the backplane, such as surface-mount or compliant pin attachments. The vias created in the substrate of the backplane to make those attachments are comprised of cylinders extending through the thickness of the board. Those via cylinders can also couple to each other and generate noise. That effect is directly related to the thickness of the board, such that the thicker the board is, the more crosstalk that is generated. For a thick board, the crosstalk in the backplane can be greater than the crosstalk from the connector. Noise may be generated in a similar manner in the daughter card backplanes.
There is no noise on the second connector 120 at this point, as shown in FIG. 1(d), so that the noise at the first connector 110 is the total noise for the system 100, as shown in FIG. 1(e). The noise from the first connector 110 continues at the output of the first connector 110 on the victim trace pair 136.
When the signal reaches the second connector 120, as shown in FIG. 1(f), there is noise on the victim trace 136 from the first connector 110, as shown in FIG. 1(g), in addition to the noise that is generated at the second connector 120, as shown in FIG. 1(h). The noise generated in the second connector 120 is substantially the same as the noise generated in the first connector 110, i.e., noise from the coupling between pairs, the footprint attachments, and the via cylinder coupling. The noise on the victim trace pair 136 and the noise on the second connector 120 get combined inside the second connector 120. Thus, the total noise in the system 100, as shown in FIG. 1(i), is approximately double that at each connector 110 and 120, as shown in FIGS. 1(g) and (h), respectively.
While it is possible to minimize those deleterious coupling effects by adding space between the victim and aggressor lines, that comes at the disadvantage of losing density in the backplane and the connector. Thus, as more and more signals are populated onto backplanes and more and more signal conductors are provided in connectors, the problem with coupling becomes more probable and more problematic. Accordingly, there is a need for a system that reduces noise generated at the connector footprints and connectors of a connection system. Moreover, there is a need for a connection system that allows connectors that increase speed and density without degrading the electrical characteristics of the signals they route.