Abstract:
A device and method for defining a signal transmission path having a selectable, continuous impedance. In one embodiment of the invention, a circuit board is provided with a signal conductor, and a conductive plane having an opening, wherein dimensions of the opening and proximity of the opening to the signal conductor are selected to affect an impedance of the signal conductor. The signal conductor and the conductive plane form a transmission path with the impedance of the transmission path being a function in part of the opening and the signal conductor. Such a circuit board provides a signal-transmission path having a selectable, continuous impedance return-signal path.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to the field of printed circuit boards (“PCBs”). More particularly, aspects of the present invention provide a selectable transmission-path impedance that is particularly suitable for high-frequency signals on a printed circuit board.  
         [0002]     A driver circuit is typically used to drive an electrical signal onto a signal conductor, such as a trace, which is connected to a receiver circuit. Once the signal reaches the receiver, it requires a return path from the receiver back to the driver and typically follows a path having the least impedance (if there are multiple return paths to choose from). The signal path or loop followed by the signal from driver to receiver, and from receiver back to driver, is referred to herein as a transmission path. A transmission path has a characteristic impedance that is a function of several variables as described below.  
         [0003]     Common types of PCBs are a double-sided PCB and a multi-layered PCB. A double-sided PCB includes conductive planes formed on both sides of an insulation layer. A multi-layered PCB includes a plurality of conductive planes and insulation layers. In a multi-layered PCB, an insulation layer is typically formed in between conductive planes. The multi-layered PCB can have three or more conductive planes. The term “conductive planes” herein refers to power planes, reference planes, and/or ground planes. A “transmission path” typically includes a signal conductor, such as a trace from a driver to a receiver, and a conductive plane acting as a return-signal path. A PCB structure provides a transmission path having a characteristic impedance, such as 50 ohms. It is often necessary to provide a higher-impedance transmission path than the characteristic impedance of a PCB structure to impedance match a driver and a receiver. Impedance mismatches produce several detrimental effects in high-frequency circuits, and are to be avoided. Detrimental effects include reflection of a signal between the driver and receiver, ringing on the signal, and electromagnetic interference (“EMI”).  
         [0004]     A signal conductor may be formed on a surface of a PCB or within a multi-layer PCB stack-up.  FIG. 1  is a perspective view of a multi-layered PCB  10  that includes a buried signal conductor, illustrated as signal trace  20 , insulation layers  14 ,  16 , and  18 , and conductive planes  30  and  40 . The signal trace  20  may be formed on an internal surface of the PCB  10  or, as illustrated, on a layer within the PCB stack-up. This type of PCB structure where the signal trace is buried in the PCB and adjacent to a first and second conductive plane is call a “strip line” construction. The term when the signal trace is on the surface and is adjacent to only a first conductive plane is called a “micro-strip” construction. The conductive planes  30  and  40  of  FIG. 1  can be used for any purpose. The insulation layers  14 ,  16 , and  18  may be any type of insulating material known in the art for forming a PCB. For purposes of clarity, a top layer  12  and a bottom layer  13  of the PCB are shown as having been omitted.  
         [0005]     The PCB stackup includes the conductive plane  30  formed on top of insulating layer  14 . The insulation layer  16  is formed on top of the conductive plane  30 . The buried signal trace  20  is formed on top of the insulation layer  16 , and has a trace width S. While the trace width S may be any value, trace widths of 0.005 inch are commonly used resulting in an impedance of substantially 50 ohms for typical insulation layer materials and typical spacings between conductive planes and signal traces. The insulating layer has a parameter called dielectric and for different dielectrics, different capacitance and thus Zo is achieved with the same spacing.  
         [0006]     Other traces (not shown) may also be formed on the insulation layer  16 . The insulation layer  18  is formed on top of the insulation layer  16  and the signal trace  20 . The conductive plane  40  is formed on top of the insulation  18 , which provides insulation between the conductive plane  40  and the buried signal trace  20 , and defines a separation distance D from the nearest surface of the trace  20 . The signal trace  20  can be used to conduct a signal from a driver circuit (not shown) to a receiver circuit (not shown), and a plane such as conductive plane  40  can be used to conduct the return signal, forming a transmission path.  
         [0007]     In recent years, double-sided and multi-layered PCBs have become increasingly thinner to meet the demand of consumers for smaller and more compact electronic products. One way to reduce PCBs thickness is reducing the thickness of the insulation layers between the conductive planes. However, reducing the thickness of an insulation layer between a signal trace and its conductive plane reduces the separation distance D, and thus the characteristic impedance, of the transmission path.  
         [0008]     The characteristic impedance of a signal conductor is primarily determined by inductance and capacitance as shown in equation (1):  
             Zo   =       L   C               (   1   )             
 
 where Zo is the characteristic impedance of the signal conductor, L is the inductance per unit length of the signal conductor, and C is the capacitance per unit length of the signal conductor. Furthermore, the capacitance per unit length C of the signal conductor is generally expressed as shown in equation (2): 
 
 C=KS/D   (2) 
 
 in which K is the dielectric constant of the insulation layer separating a conductor and its conductive plane, S is the electrode plate size (primarily width of the signal conductor), and D is the distance between two electrode plates, which in this case is the separation distance between the signal conductor and the nearest conductive plane. 
 
         [0009]     When these two equations are combined, the resulting equation is as shown in equation (3):  
             Zo   =       LD   KS               (   3   )             
 
 According to equation (3), if the inductance per unit length of the signal conductor (L), dielectric constant (K), and the width of the signal conductor (S) remain constant, the characteristic impedance of the signal conductor decreases by decreasing D, the separation distance. 
 
         [0010]     Reduction of the characteristic impedance in the thinner PCBs is typically beneficial because such reduction reduces cross-talk and lessens the effects of EMI on the signal conductors. However in certain applications, the reduction is not beneficial. Some signal conductors, such as video-signal conductors, require higher impedances to match properly with electronic components, such as video displays, that operate with higher impedances. Various techniques have been used to produce high-impedance transmission paths were necessary. These techniques include routing the signal conductor on the surface layer of the PCB. Disadvantages of this technique include a limited amount of board-surface-layer-space available, production difficulties in controlling impedance of a trace on a surface layer, and greater EMI generation by signal conductors on surface layers.  
         [0011]     Another technique for producing increased-impedance transmission paths includes routing the signal over signal conductors on internal PCB layers. According to Equation (3), the characteristic impedance of a signal conductor can be increased by keeping the factors L, K, and S constant and increasing D, which is the separation distance between the signal conductor and the conductive plane. This may be accomplished by increasing the thickness of an insulation layer, thereby causing the characteristic impedance of all other signal conductors on the insulation layer to be increased. However, this technique increases the thickness of the PCB rather than decreasing it.  
         [0012]     Another technique for increasing the separation distance D includes using a conductive plane positioned several layers away from the signal conductor and evacuating portions of intermediate planes between the signal conductor and the conductive plane in order to increase impedance. A disadvantage of this technique is that currents in the evacuated intermediate planes must flow around the evacuated areas. This may cause additional cross-talk and EMI, induce noise, and reduce signal integrity.  
         [0013]     Yet another way to increase the characteristic impedance of a signal conductor, according to Equation (3), is to decrease the width of the signal trace S. For example, the signal trace  20  can be reduced to a width such as 0.003 inch. However, decreasing the width increases the losses of the transmission path because the reduced width increases the trace resistance. In addition, decreasing the width of the signal trace may significantly increase the cost of fabricating a PCB and may violate manufacturing standards.  
       SUMMARY  
       [0014]     In one embodiment of the invention, a circuit board is provided with a signal conductor, and a conductive plane having an opening, wherein dimensions of the opening and proximity of the opening to the signal conductor are selected to affect an impedance of the signal conductor. The signal conductor and the conductive plane form a transmission path with the impedance of the transmission path being a function in part of the opening and the signal conductor. Such a circuit board provides a signal-transmission path having a selectable, continuous impedance return-signal path.  
         [0015]     These and various other features as well as advantages of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a perspective view of a conventional multi-layered PCB that includes a buried trace, a plurality of insulation layers, and two conductive planes;  
         [0017]      FIG. 2A  is a perspective view of a multi-layered PCB that includes a buried signal trace, a plurality of insulation layers, a conductive plane having a continuous opening, and another conductive plane, according to an embodiment of the invention;  
         [0018]      FIG. 2B  is a partial end view of the multi-layered PCB of  FIG. 2A ; and  
         [0019]      FIG. 3  is a perspective view of a multi-layered PCB that includes a buried-signal trace, a plurality of insulation layers, a conductive plane having two continuous openings, and another conductive plane, according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof. The detailed description and the drawings illustrate specific exemplary embodiments by which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is understood that other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the present invention. The following detailed description is therefore not to be taken in a limiting sense.  
         [0021]      FIG. 2A  is a perspective view of a multi-layered PCB  50  that includes a buried signal trace  20 , insulation layers  14 ,  16 , and  18 , a conductive plane  60  having a continuous opening  62 , and another conductive plane  30 , according to an embodiment of the invention. For purposes of clarity, any top and bottom layers of the PCB  50  have been omitted. In an alternative embodiment, the signal trace  20  can be on a surface of the PCB  50  and the conductive plane  60  can be buried within the PCB  50 . The PCB  50  is substantially similar to the PCB  10  of  FIG. 1  with an addition of the continuous opening  62  substantially aligned with the signal trace  20 . The conductive plane  60  may be a power, a dedicated reference, or a ground plane.  
         [0022]     After the conductive plane  60  is formed on the insulation layer  18 , the continuous opening  62  having a width  66  is formed by vacation of the conductive plane  60  in alignment with the route of the signal trace  20 . The width  66  may be less than, equal to, or greater than the width S of the signal trace  20 . Furthermore, a longitudinal centerline of the opening  62  may or may not coincide with a longitudinal centerline of the trace  20 . The signal trace  20  may be partially or fully beneath the conductive plane  60 . In an alternative embodiment, another conductive plane such as plane  30  can also have a continuous (not shown) opening aligned with the signal trace  20 .  
         [0023]     By creating the continuous opening  62 , the effective distance between the signal trace  20  and portions of the conductive plane  60  forming the return portion of the transmission path is greater than the distance D of circuit board  10  of  FIG. 1 .  FIG. 2B  is a partial end view of the multi-layered PCB  50  of  FIG. 2A . Part of the return signal will travel along an edge portion of the plane  60  that is separated from the trace  20  by D 1 , and the remainder of the return signal will travel along an edge portion of plane  60  that is separated from the trace  20  by distance D 2 . The effective distance between the trace  20  and the conductive plane  60  is a function of the distances D 1  and D 2 , and is greater than the distance D of  FIG. 1 . As the width  66  of the opening  62  increases relative to the width S of the trace  20 , the greater the distances D 1  and D 2  become relative to the trace  20 , and thus the greater the impedance Zo.  
         [0024]     According to equation (3) above, increasing the effective distance (a function of the distances D 1  and D 2 ) between the signal trace  20  and the conductive plane  60  increases the impedance Zo of the transmission path. Therefore, by selecting the width  66  and orientation of the continuous opening  62 , a PCB designer can advantageously select an impedance Zo that is relatively independent of the thickness of the insulation layer  18 . Specifically, the impedance Zo is selected for a given width  66  of the trace  20  by selecting the opening width  66 , and the orientation of a longitudinal centerline of the trace  20  relative to a longitudinal centerline of the continuous opening  62 . Per equation (3), making the trace  20  wider lowers the transmission-path impedance, and making the opening width  66  wider increases the transmission-path impedance.  
         [0025]     This allows the designer to change the resistance of the trace  20  independent of the transmission path impedance Zo. For example, the width S of the trace  20  could be increased over the typical trace width of the PCB to handle increased current, reduce resistive losses, reduce skin effects, or for some other reason. Normally, increasing the width S of the trace  20  decreases the transmission-path impedance. However, the width  66  of the continuous opening  62  can be selected relative the increased width S of the trace  20  to provide a transmission path having a desired impedance Zo that is not otherwise typical of the PCB  50 .  
         [0026]     The transmission path formed by the trace  20  and the plane  60  (with the opening  62 ) presents a transmission path having a relatively uniform and continuous impedance, and a small loop area. High-frequency signals are often adversely impacted by impedance discontinuities in the transmission path. The continuous opening  62  allow selection of the impedance Zo presented to signal with little or no degradation of the signal quality or generation of EMI, providing a significant advantage over the prior art.  
         [0027]     Another aspect of the invention allows the conductive plane  60  to carry other currents across the opening  62  without adding impedance to or creating a discontinuity in the transmission path formed by the trace  20  and the portions of the conductive plane  60  adjacent to the continuous opening  62 . At least one optional bridging conductor  64  may be formed when otherwise vacating the conductive plane  60  to form the continuous opening  62 . The bridging conductor  64  is generally formed perpendicular to a longitudinal centerline of the opening  62  and the return-signal path, and electrically couples the conductive plane  60  across the opening  62 . While only one bridging conductor  64  is shown across the opening  62 , it is generally anticipated that a plurality of bridging conductors  64  may be formed to conduct cross-currents.  
         [0028]     The width of the bridging conductor  64  is selected to minimize any tendency of the return signal to include the bridging conductor in the return-signal pathway. Because a signal seeks the lowest impedance path, as the width of the bridging conductor  64  increases, a return signal traveling along the edge portions of the opening  62  will begin to include portions of the bridging conductor  64  in its return path and a lower path impedance will result. Inclusion of significant portions of the bridging conductor  64  in the return-signal path will generate impedance discontinuities, resulting in EMI and signal degradation. To avoid impedance discontinuities or changes, the width of the bridging conductor  64  is selected to provide adequate cross-current pathway across the opening  62  while being too small to provide a significant return-signal pathway for the signal carried on the trace  20 . That is, by controlling the width of the bridging conductors  64 , the PCB designer can provide adequate paths for cross-currents with negligible effect on the value and continuity of Zo, and with negligible signal degradation and EMI generation.  
         [0029]     The dimensions of the elements of the PCB  50  may be varied by a designer to meet intended impedance requirements. For example, if the PCB  50  typically uses a 0.005-inch width S for the trace  20 , the width  66  of the opening  62  could be in a range based on the width S. For example, the width  66  could range between 80% and 300% of the width S, which for a 0.005-inch wide trace  20  ranges between 0.004″-0.015.″ If the optional bridging conductors  64  are used, the width of the bridging conductor  64  might be approximately equal to the width S, keeping in mind that whatever bridging-conductor width is selected should not provide a significant return-signal pathway that creates an impedance discontinuity. The number of bridging conductors  64  is typically selected by a designer based on the amount of cross-current anticipated. The bridging connectors  64  are typically equally spaced apart along a longitudinal length of the opening  62 , although the spacing may be unequal. For example, the distance between adjacent bridging conductors  64  could be a multiple of the width S or the opening width  66 . For example, if the opening width  66  is 0.01 inch, and the multiple is 10 times the opening width  66 , the resulting spacing is 0.100 inch between adjacent bridging conductors  64 .  
         [0030]      FIG. 3  is a perspective view of a multi-layered PCB  100  that includes a buried-signal trace  20 , insulation layers  14 ,  16 , and  18 , a conductive plane  110  having a return-signal conductor  120 , two continuous openings  122  and  124 , and another conductive plane  30 , according to an embodiment of the invention. PCB  100  is similar to PCB  50 , except that two continuous openings  122  and  124  having widths  126  and  128 , respectively, are vacated in the conductive plane  110  to form a return-signal conductor  120  having a width  127  in an alignment with the trace  20 . The PCB  50  of  FIG. 2  increases return-signal pathway impedance by vacation of a portion of the conductive plane to increase the distance D while leaving the conductive plane  60  essentially of infinite width with respect to the width S of the trace  20 . In contrast, the PCB  100  increases the transmission-path impedance by defining a finite width  127  of the return-signal conductor  120 . The characteristic impedance Zo of the signal trace expressed in equation (3) now also becomes a function of the width  127  of the return-signal conductor  120 . The width  127  may be larger, the same, or smaller than the width S of the trace  20 , as necessary to provide a selected impedance and current capacity.  
         [0031]     The conductive plane  110  may be used to form a plurality of transmission pathways by referencing a plurality of signal traces. For example, two transmission pathways may be defined in the PCB  100  using the conductive plane  110  by forming two return-signal conductors, each formed in an alignment with a trace in a manner similar to the conductor  120 . The two return-signal conductors may each be defined by vacating a respective pair of continuous openings, such as the openings  122  and  124 , in an alignment with a different trace formed on the insulation layer  16 . Each trace formed on the insulation layer  16  could have a different transmission loop impedance, as defined by a combination of the widths of the continuous openings and the width of its return-signal pathway defined thereby.  
         [0032]     As with the PCB  50  of  FIG. 2 , the width  127  may be less, equal to, or greater than the width S of the signal trace  20 . Further, a longitudinal centerline of the width  127  may or may not coincide with a longitudinal centerline of the trace width S. In an alternative embodiment, another conductive plane such as plane  30  can also have a continuous (not shown) opening aligned with the signal trace  20 . Also, as with the PCB  50 , optional bridging conductors may be formed across the openings  122  and  124  by leaving portions of the conductive plane  110  when otherwise vacating the continuous openings  122  and  124 .  
         [0033]     Moreover, if the openings  122  and  124  are narrow enough, then the return signal may also travel along one or both edges of the plane  110  in addition to returning along the conductor  120 . Therefore, the widths  126  and  128  can also be selected to set Zo at a desired value.  
         [0034]     Further, as with the PCB  50  of  FIG. 2 , the PCB  100  may include one or more bridging conductors across the openings  122  and  124  to conduct cross-currents.  
         [0035]     A printed circuit board utilizing aspects of the invention may be used in any electrical system to provide a transmission path having a selectable, continuous impedance, particularly systems involving high-frequency signals, such as computer systems.  
         [0036]     Although the present invention has been described in considerable detail with reference to certain preferred embodiments, other embodiments are possible. Therefore, the spirit or scope of the appended claims should not be limited to the description of the embodiments contained herein. It is intended that the invention resides in the claims.