Abstract:
In one embodiment, an apparatus includes a line side of a network device. The line side is configured to connect to a device external to the network device. The apparatus also includes a physical side of the network device. The physical side is configured to communicate with an external entity. An isolation device is configured to isolate the physical side from the line side. An inductor is coupled between the line side and the physical side. The inductor has a value configured to control a matching of an impedance of the line side with an impedance of the physical side as seen through the isolation device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present disclosure claims priority to U.S. Provisional App. No. 61/494,601, filed on Jun. 8, 2011, which is incorporated herein by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     A network device is a device that processes data in a network. For example, common network devices include gateways, routers, bridges, switches, hubs, repeaters, and the like. In some applications, a communication cable may be coupled to a network device to permit the network device to communicate, via the communication cable, over the network. An interface between a communication cable and a network device is typically needed to couple the network device to the cable. However, the interface may be a source of an impedance mismatch between the network device and the combined impedance of the interface and communication cable. Impedance mismatches may lead to return loss, which corresponds to an unwanted loss of signal power at transitions between the network device, the interface, and the communication cable. 
       FIG. 1  shows a graph  10  showing an impedance of a network device in relation to frequency. The frequency shown in graph  10  corresponds to a frequency of operation of the network device. The impedance is shown on the y axis and the frequency is shown on the x axis of graph  10 . As shown, the impedance varies with respect to frequency. At higher frequencies, the impedance variation increases. The impedance of the network device may be acceptable between 1 MHz and 125 MHz because the return loss due to impedance mismatch is acceptable. However, in a network device that communicates at 10 gigabits, the bandwidth may extend to 500 MHz. In this case, the impedance variance causes an impedance mismatch that leads to a large return loss across the wide frequency band. 
     SUMMARY 
     In one embodiment, an apparatus includes a line side of a network device. The line side is configured to connect to a device external to the network device. The apparatus also includes a physical side of the network device. The physical side is configured to communicate with an external entity. An isolation device is configured to isolate the physical side from the line side. An inductor is coupled between the line side and the physical side. The inductor has a value configured to control a matching of an impedance of the line side with an impedance of the physical side as seen through the isolation device. 
     In one embodiment, a plurality of channels are provided between the line side and the physical side, wherein the impedance on the physical side of each channel is substantially matched to the impedance on the line side of each channel. 
     In one embodiment, the impedance of the line side includes a combined impedance of a connector of the network device, a connector of the device external to the network device, and the connection device. 
     In one embodiment, the isolation device includes a transformer. 
     In one embodiment, the connection device includes a cable. 
     In one embodiment, a method includes upon receiving a coupling of a cable to a connector of a network device configured to connect to the cable, isolating, by a transformer, a transmitter or receiver of the network device from the cable, wherein the transmitter or receiver sends or receives communications to and from the cable; and coupling a set of signal through a set of inductors between the transmitter or receiver and the cable, each inductor having a value configured to control a matching of an output impedance of the transformer with a combined impedance of the connector of the network device and the cable. 
     The following detailed description and accompanying drawings provide a more detailed understanding of the nature and advantages of particular embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a graph showing an impedance of a network device in relation to frequency. 
         FIG. 2  depicts a simplified system for impedance matching according to one embodiment. 
         FIG. 3  depicts a more detailed example of a system according to one embodiment. 
         FIG. 4  depicts a graph of the output impedance of a transformer versus frequency according to one embodiment. 
         FIG. 5  depicts a simplified flowchart of a method for matching impedance according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein are techniques for an impedance control system. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of particular embodiments. Particular embodiments as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein. 
       FIG. 2  depicts a simplified system  50  for impedance matching according to one embodiment. As shown, a physical side  52  and a line side  54  are shown. Physical side  52  and line side  54  may be part of a network device. Physical side  52  includes components that send and receive communications via line side  54 . Line side  54  may include an interface and be connected to a connector of a cable. One or more transformers  112  couple physical side  52  to line side  54 . Transformers  112  isolate physical side  52  from line side  54 . Also, transformers  112  are used to “step up” or “step down” alternating current (AC) voltage between line side  54  and physical side  52 . Although transformers  112  are described, other devices to isolate physical side  52  from line side  54  may be used. 
     Particular embodiments use inductors  120  to provide impedance matching between line side  54  and physical side  52 . Inductors  120 , as will be described in more detail below, improve the impedance matching over a wide bandwidth. For example, values of inductors  120  are selected to control the matching of impedances between line side  54  and physical side  52 . Thus, network devices operating at a wide range of frequency, such as 1 gigabit, may not suffer from return loss over a large bandwidth. 
       FIG. 3  depicts a more detailed example of system  150  according to one embodiment. A network device  104  connects to a communication cable  108 . Although this configuration is shown, a person of skill in the art will appreciate other configurations. 
     Network device  104  may be any type of computing device that communicates with cable  108 . For example, network device  104  includes a transmitter and receiver. In one example, network device  104  includes an electrical board having components mounted thereon. 
     Cable  108  may be a physical transmission medium. Cable  108  may include a category (CAT) 5, CAT 5E, CAT 6, or CAT 6A cable. Cable  108  includes a cable connector  130 . Cable connector  130  is configured to couple to a connector  106  of network device  104 . Connector  106  may be an interface that allows a connection to be made between network device  104  and cable  108 . Although the connection shown is physical, wireless connections may be used. 
     Electrical transfer of signals between network device  104  and cable  108  may occur via connector  106  and cable connector  130 . Cable  108  may include four channels: channel A, channel B, channel C, and channel D. Each channel may be a transmission medium, such as one or more wires. Connector  106  includes a corresponding channel A, channel B, channel C, and channel D. Also, cable connector  130  and network device connector  106  may be any type of connector, such as an RJ45-type connector. 
     Connector  106  couples to a set of lines L1-L8. Although 8 lines are shown, any other number of lines will be appreciated. Lines may be a set of wires. In one embodiment, each channel includes two lines, but channels may include a different number of lines. For example, channel A includes lines L1 and L2; channel B includes lines L3 and L4; channel C includes lines L5 and L6; and channel D includes lines L7 and L8. In one embodiment, the line pairs are twisted pair conductors. 
     A transformer  112  is provided to isolate a receiver and/or transmitter of network device  104  from cable  108 . Although one transformer  112  is shown, multiple transformers  112  may be provided. For example, a transformer  112  for each channel may be used. 
     Transformer  112  includes tap connectors  116 A- 116 D. Channel A including lines L1 and L2 connects to transformer tap connector  116 A, which includes a connection TRD1+ (positive connection) and TRD1− (negative connection) of transformer  112 . A center tap connector  134  may also be provided and be connected to one or more circuits (not shown) to obtain a desired circuit performance. Additionally, transformer  112  may have additional electrical connections that provide incoming signals to a receiver and outgoing signals to a transmitter to transformer  112 . 
     Inductors  120  are used to maintain consistent impedance across a wide bandwidth of frequency, such as of a frequency between 1 MHz to 500 MHz. For example, an impedance looking into transformer  112  (e.g., output impedance) may be kept at a consistent impedance of substantially 50 ohms. This matches an output impedance of transformer  112  to a combined impedance of connector  106 , cable connector  130 , and cable  108 , and minimizes return loss. 
     Inductors  120  may be included for each channel, such as for each line. For example, inductors  120 A are included on lines L1 and L2; inductors  1206  are included on lines L3 and L4; inductors  120 C are included on lies L5 and L6; and inductors  1206  are included on lines L7 and L8. The impedance of inductors  120  for different lines may be different or the same depending on the configuration of transformer  112 , connector  106 , circuits attached to center tap  134 , or cable  108 . In one embodiment, the inductor values for inductors  120 A and  120 D are between 2 and 3 nanohenrys and the inductor values for inductors  120 B and  120 C are between 3 and 4 nanohenrys. Other inductor values may also be appreciated. Additionally, all inductors  120  may be the same value. Or, the inductor values associated with a channel may also be different. For example, an inductor value for line L1 may be different from an inductor value for line L2. Inductors  120  may include discrete elements attached to network device  104 , or be built into a board that is implementing network device  104 , connector  106 , cable connector  130 , or transformer  112 . 
     Inductors  120  help match the output impedance from transformer  112  and a combined impedance of connector  106 , cable connector  130 , and cable  108 . In one embodiment, inductor values are selected to minimize return loss.  FIG. 4  depicts a graph  400  of the output impedance of transformer  112  versus frequency according to one embodiment. As shown, the impedance variance between 1 MHz and 500 MHz has been reduced when compared to the impedance variance as shown in  FIG. 1 . For example, the addition of inductors  112  draw the impedance up if the impedance is negative and also draw the impedance down if the impedance is positive. This reduces the variance of the impedance over the desired bandwidth. 
       FIG. 5  depicts a simplified flowchart  500  of a method for matching impedance according to one embodiment. At  502 , a coupling of cable  108  to network device  104  is received. For example, a cable connector  130  of cable  108  is coupled to connector  106 . At  504 , a connection is formed through inductors  120  that are coupled between transformer  112  and connector  106 . At  506 , electrical signals are sent between network device  104  and cable  108 . Inductors  120  maintain the output impedance of transformer  112  across a wide range of frequency. 
     As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
     The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents may be employed without departing from the scope of the invention as defined by the claims.