Abstract:
A high density capacitor filter bank for use in connection with printed circuit boards is provided. Capacitive elements are disposed within a conductive shield such that the capacitive elements are substantially orthogonal to the plane of the printed circuit board, and such that they can interconnect with corresponding contacts on the printed circuit board while occupying a minimal amount of surface area on the printed circuit board. The conductive shield may comprise a Faraday shield.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to the reduction of noise in electrical circuits. In particular, the present invention is directed to the provision of an electrically shielded high density capacity filter bank for use in connection with printed circuit boards. 
     BACKGROUND OF THE INVENTION 
     One important consideration in the design of electrical circuits is electromagnetic compatibility (EMC). In particular, electromagnetic fields resulting from noise signals within electrical circuits must be held to within acceptable limits, in order to prevent interference with neighboring circuits. As the density with which electrical circuitry is packaged, and the sensitivity of such circuitry increases, the standards for EMC compliance have become increasingly stringent. 
     A particular problem within the field of electromagnetic compatibility is to provide effective filtering for a large number of closely spaced conductors, such as conductors associated with high density connectors. A high density connector that is interconnected to a printed circuit board (PCB) must have its pins electrically connected to the conductors of the PCB. Usually, one PCB conductor is mapped to each connector pin. Since PCB conductors propagate high frequency noise currents, it is highly desirable to provide effective filtering against these noise currents. Typically, such filtering is provided by using appropriately valued surface-mountable capacitors to shunt undesirable noise currents to ground, so that the noise currents can return to their sources. Such capacitors have been either embedded onto the high density connector, or they have been soldered onto the PCB, one for each conductor. In either approach, the capacitors are coplanar with the PCB conductors. 
     Because the capacitors used to shunt noise currents have been coplanar with the PCB conductors, some of the shunted noise currents can be recoupled back into the conductors, due to the magnetic fields from these noise currents being closely located to the conductors. This effect is caused by the phenomena of magnetic field coupling. In addition, where discrete capacitors are placed alongside PCB conductors, a significant amount of PCB real estate (or area) is required. In addition, the conductive traces necessary to connect the capacitors between the circuit and ground can be difficult to route. 
     Another problem encountered with such approaches is that the shunted high frequency noise currents propagate on one of the PCB surfaces for a short distance before reaching a ground plane, in which the noise currents can return to their sources. This can result in the free space propagation of the electromagnetic fields associated with the noise currents. Such free space propagation can contribute to EMC noncompliance. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to solving these and other problems and disadvantages of the prior art. 
     According to an embodiment of the present invention, a high density capacitor filter bank is provided. The high density capacitor filter bank includes a number of capacitive elements substantially enclosed within a volume defined by a conductive shield. The capacitive elements are oriented such that, when the filter bank is interconnected to a printed circuit board (PCB), the recoupling of noise currents back onto the PCB conductors is reduced. In addition, the capacitive elements include a first terminal located substantially within a first plane for interconnection to a PCB conductor, and a second terminal, substantially located within a second plane and interconnected to the conductive shield, which is in turn connected to ground. This configuration limits the area of the PCB required by the filter bank. In addition, the provision of a shield limits the free space propagation of electromagnetic radiation. 
     In accordance with an embodiment of the present invention, the conductive shield comprises a Faraday shield formed from a substantially continuous sheet or sheets of conductive material. In accordance with another embodiment of the present invention, the conductive shield has no aperture with a maximum linear dimension greater than about 1/30 of the wavelength of the highest frequency signal of concern. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a filter bank in accordance with an embodiment of the present invention, interconnected to a printed circuit board; 
         FIG. 2  is a cross-section of the filter bank and printed circuit board of  FIG. 1 ; 
         FIG. 3  is a plan view of a filter bank in accordance with an embodiment of the present invention; 
         FIG. 4A  is a perspective view of a capacitive element in accordance with an embodiment of the present invention; 
         FIG. 4B  is a perspective view of a capacitive element in accordance with another embodiment of the present invention; 
         FIG. 5A  is an exploded view of the capacitive element of  FIG. 4A ; 
         FIG. 5B  is an exploded view of the capacitive element of  FIG. 4B ; and 
         FIG. 6  is a flow diagram of a method for providing a high density capacitor filter bank in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference now to  FIG. 1 , a high density capacitor filter bank  100  in accordance with an embodiment of the present invention is shown interconnected to a printed circuit board (PCB)  104 . As shown in  FIG. 1 , the printed circuit board  104  includes a number of conductive traces  108 . As can be appreciated by one of skill in the art, the conductive traces  108  generally comprise electrical signal lines. As can also be appreciated by one of skill in the art, the conductive signal lines  108  shown on the visible surface of the printed circuit board  104  comprise a first conductive layer of the printed circuit board. In general, the printed circuit board  104  may comprise a conductive ground plane  112 , separated from the conductive traces  108  by a dielectric substrate  116 . Of course, various other configurations are possible. For example, a printed circuit board  104  may have more than two conductive layers (i.e., more than a single layer of conductive traces combined with a ground plane). Although the present invention is readily adaptable to any multiple layer PCB, for clarity of the present description, a PCB  104  having two conductive layers is shown. 
     The high density capacitor filter bank  100  generally includes a conductive shield  120  and a plurality of capacitive elements  124 . In general, the conductive shield  120  is shaped such that it forms an enclosure defining a volume that substantially contains the capacitive elements  124 . For instance, as illustrated in  FIG. 1 , the conductive shield  120  may be shaped like a rectangular box having four side surfaces  128   a–d  and a capacitive element attachment surface  132 . The side of the box or enclosure opposite the capacitive element attachment surface  132  of the conductive shield  120  is defined by or adjacent to the surface of the PCB  104  when the high density capacitor filter bank  100  is interconnected to the PCB  104 . Cutouts  136  may be formed in the side surfaces  128  of the conductive shield  120  so that direct electrical contact between the conductive traces  108  and the conductive shield  120  is prevented. Alternatively or in addition, mounting tabs  140  may be provided on the side surfaces  128  of the conductive shield  120  to form a gap between the side surfaces  128  of the conductive shield  120  and the conductive traces  108 . 
     As will be described in greater detail herein, the conductive shield  120  provides an electrically conductive path for noise currents shunted from the conductive traces  108  by the capacitive elements  124 , to the ground plane  112  of the PCB  104 . In addition, the conductive shield  120  may be substantially continuous. For example, as illustrated in  FIG. 1 , the conductive shield  120  may have no apertures, apart from the side corresponding to the plane of the PCB  104  being open and any provided gaps  136  or space between mounting tabs  140 . By providing a continuous or substantially continuous conductive surface, the conductive shield  120  prevents or reduces the generation of radiation due to currents conducted by the conductive shield  120 . A continuous or substantially continuous conductive shield  120  as illustrated in  FIG. 1  may be formed from any electrically conductive material. For example, the conductive shield  120  may be formed from a folded plate of sheet metal. Suitable metals include copper, aluminum, silver or gold. 
     With reference now to  FIG. 2 , a cross-section of the filter bank  100  and PCB  104  of  FIG. 1  is shown. In general, the cross-section illustrated in  FIG. 2  is taken along section line A—A of  FIG. 1 . As seen in  FIG. 2 , each capacitive element  124  is interconnected to a corresponding conductive trace  108  of the PCB  104  at a first end, and to the circuit element attachment surface  132  of the conductive shield  120  at a second end. To provide a sufficient area for attaching a corresponding capacitive element  124 , each conductive trace  108  may be widened at the attachment point of a corresponding capacitive element  124 , to form an attachment pad  144  (see also  FIG. 1 ). The conductive shield  120  is interconnected to the ground plane  112  of the PCB  104  by electrical vias  204  passing through the substrate  116  of the PCB  104 . In order to facilitate the electrical interconnection of the conductive shield  120  to the vias  204 , connection tabs  148  (see also  FIG. 1 ) may be provided. In order to facilitate the manufacture of the high density capacitor filter bank  100  in accordance with the present invention, and to facilitate the attachment of such a filter bank  100  to a PCB  104 , it is generally preferable to use like-sized capacitive elements  124 . In accordance with such an embodiment, the areas of the circuit element attachment surface  132  corresponding to attachment points of the second ends of the capacitive elements  124  should be a uniform distance from the pads  144  of the conductive traces  108 . In particular, that distance should be substantially equal to the length of the capacitive elements  124 . Therefore, the capacitive element attachment surface  132  may comprise a planar surface that is substantially parallel to the surface of the PCB  104 . In addition, the distance between the capacitive element attachment surface  132  and the surface of the PCB  104  may be such that a high density capacitor filter bank  100  in accordance with an embodiment of the present invention comprises a low profile component. 
     In order to reduce the area of the PCB  104  taken up by the high density capacitor filter bank  100 , the capacitive elements  124  are mounted such that they are substantially orthogonal to the plane of the PCB  104 . This configuration also prevents the recoupling of noise currents back into the conductive traces  108 , and facilitates the routing of conductive traces  108  on the PCB  104 . 
       FIG. 3  is a plan view of the high density capacitor filter bank  100  shown in  FIGS. 1 and 2 . In particular,  FIG. 3  shows a high density capacitor filter bank  100  in accordance with an embodiment of the present invention, as it would appear from the surface of the PCB  104  to which the high density capacitor filter bank  100  would be mounted. As best seen in  FIG. 3 , the capacitive elements  124  may be arranged in staggered rows, to facilitate the routing of conductive traces  108 . Although such an arrangement may be convenient, it should be appreciated that the filter elements  124  may be arranged in any configuration deemed desirable or necessary given the configuration of the PCB  104  to which the high density capacitor filter bank  100  is to be attached. 
     As shown in  FIGS. 2 ,  4 A,  4 B,  5 A and  5 B, the capacitive elements  124  generally include first  208  and second  212  terminals or plates, separated by a dielectric  216 . As shown in  FIGS. 4A and 5A , first  404  and second  408  end caps may be provided to facilitate interconnection of the capacitive elements  124  having a single capacitor  512  to the conductive shield  120  and attachment pads  144  formed on or as part of the PCB  104 . The capacitive values are selected such that a relatively low impedance is presented to noise within an interconnected conductive trace  108 . In addition, the capacitive values of the capacitive elements  124  are selected such that a relatively high impedance is presented to desired signals in the conductive traces  108 . Alternatively, as shown in  FIGS. 4B and 5B , capacitive elements  124  having multiple capacitors  512  held between end caps  412  and  416  may be provided. The use of multiple capacitors  512  can facilitate the filter over a larger frequency range. 
     With reference now to  FIG. 5A , the capacitive element  124  of  FIG. 4A  is shown in an exploded view. As shown in  FIG. 5A , each end cap  404 ,  408  may include a planar mounting surface  504  and a receptacle  508 . In general, the planar surface  504  provides a relatively large surface area for interconnecting (e.g., soldering) the capacitive element  124  to the PCB  104  at a first end and to the capacitive element attachment surface  132  of the conductive shield  120  at a second end. The receptacle  508  provides a mechanical structure in which the terminals  208 ,  212  can be held. In accordance with an embodiment of the present invention, the terminals  208 ,  212  and dielectric  216  of each capacitive element  124  are provided as a conventional surface mount capacitor  512 . For example, the capacitor  512  may comprise an  0603  or an  0402  capacitor. Such devices are easily obtainable, and are available in a wide variety of capacitive values. Where capacitors  512  having different sizes are used in connection with the same high density capacitor filter bank  100 , certain or all of the end caps  404 ,  408  may perform a spacing function so that the capacitive elements  124  all have the same length. In  FIG. 5B , the capacitive element  124  of  FIG. 4B  is shown in an exploded view. As shown in  FIG. 5B , each end cap  412 ,  416  has a planar mounting surface  516 , and a receptacle  520  sized to accommodate the included capacitors (e.g., first capacitor  512   a  and second capacitor  512   b ). Although  FIGS. 4B and 5B  show a capacitive element  124  with two capacitors  512 , different numbers of capacitors  512  can be provided. 
     With reference now to  FIG. 6 , a method for providing a high capacity filter bank in accordance with an embodiment of the present invention is illustrated. Initially, at step  600 , the conductive shield  120  is formed. In accordance with an embodiment of the present invention, the conductive shield is formed by folding an appropriately patterned piece of electrically conductive sheet metal. At step  604 , end caps  404 ,  408  are interconnected to opposite terminals  208 ,  212  of capacitors  512 , to form capacitive elements  124 . In accordance with an embodiment of the present invention, the end caps  404 ,  408  are interconnected to the respective terminals or plates  208 ,  212  of each capacitor  412  using a conductive adhesive or by soldering. As noted above, the capacitive value of each capacitor  412  is selected to present a relatively low impedance to noise, while presenting a relatively high impedance to desired signals carried by a conductive trace  108  to which the capacitive element  124  is to be interconnected. This allows the capacitive element  124  incorporating the capacitor  412  to shunt noise to ground, while allowing non-noise signals to pass through the conductive trace  108  relatively unimpeded. 
     At step  608 , an end of each capacitive element  124  is interconnected to the capacitive element attachment surface  132  of the conductive shield. Each capacitive element is positioned so that it will mate with a corresponding attachment pad  144  on the PCB  104 . For example, as shown in  FIGS. 1 ,  2  and  3 , the second end cap  408  of each capacitive element  124  may be soldered to the capacitive element attachment surface  132  of the conductive shield  120 . The interconnection of the capacitive elements  124  to the conductive shield  120  completes assembly of the high density capacitor filter bank  100 . 
     At step  612 , the high density capacitor filter bank  100  is positioned such that the end caps  404  of each capacitive element  124  opposite the capacitive element attachment surface  132  of the capacitive shield  120  is over a corresponding attachment pad  144  on the surface of the PCB  104  and each connection tab  148  provided by the conductive shield is over a corresponding via  204  of the PCB  104 . Next, each capacitive element  124  is interconnected to a corresponding mounting pad  144 , and each connecting tab  148  is interconnected to a corresponding via  204  (step  616 ), for example by soldering. 
     When a circuit or circuits associated with the PCB  104  is in operation, signals are conducted by the conductive traces  108 . Noise, such as may be created by integrated circuits or other componentry interconnected to the conductive traces  108  is shunted to ground by the high density capacitor filter bank  100 . In particular, the capacitive value of each capacitive element  124  is selected such that a low impedance path to ground is presented to noise within a corresponding trace  108 , while a relatively high impedance is presented to desired signals within the corresponding conductive trace  108 . Accordingly, desired signals are allowed to pass along the conductive trace  108 , while noise signals are filtered out. As the noise signals pass through the capacitive elements  124 , they travel away from the conductive traces  108  in a direction that is substantially orthogonal to the plane of the PCB  104 . Accordingly, recoupling of the noise back into the conductive traces  108  is substantially prevented. After passing through the capacitive elements  124 , the noise signals are conducted by the conductive shield  120  to the vias  204  provided in the PCB  104 , and then to the ground plane  112 , from which the noise signals may return to their source. Because the conductive shield  120  is substantially continuous, conduction of the noise signals by the conductive surface  120  does not result in the free space propagation of those signals. 
     In accordance with other embodiments of the present invention, the conductive shield  120  may be provided with apertures or holes, for example to provide air flow for the cooling of componentry within or beneath the conductive shield  120 . The maximum linearly dimension of any apertures provided in the conductive shield  120  should be small enough that the free space propagation of electromagnetic radiation is substantially prevented. The maximum linear dimension of apertures in the conductive shield  120  can be determined from the wavelength of signals at frequencies of concern (i.e., at frequencies comprising noise). For example, in accordance with an embodiment of the present invention, the maximum linear dimension of an aperture within a conductive shield  120  is given by the wavelength of the highest noise frequency of concern divided by thirty. Thus, if the highest noise frequency of concern was 2 gigahertz, which has a wavelength of about 1.5×10 −1  m, the largest linear dimension of any aperture in the conductor shield  120  itself should be no larger than about 0.5 cm. Thus, if cooling is of particular concern, and air flow through the conductive shield  120  is desirable, a large number of relatively small apertures is preferable to a smaller number of larger apertures. 
     In accordance with an embodiment of the present invention, the capacitive elements  124  are arranged such that a first end surface of each capacitive element  124  lies substantially within a first plane proximate the surface of the PCB  104  when the high density capacitor filter bank  100  is interconnected to the PCB  104 . A second end of each capacitive element  124  is proximate to a second plane defined by the capacitive element attachment surface  132  of the conductive shield. 
     In accordance with another embodiment of the present invention, the attachment surface  132  is not planar, but is shaped to provide an appropriate spacing between the attachment surface  132  and the PCB  104  at points where capacitive elements are attached. For example, the attachment surface  132  may be stepped or corrugated. In addition, a non-planar attachment surface may be provided to facilitate the use of capacitive elements  124  of different lengths. In accordance with still other embodiments of the present invention, a high density capacitor filter bank  100  need not be rectangular in plan view. Instead, any shape considered desirable to facilitate attachment of a high density capacitor filter bank  100  to a PCB  104 , and/or to facilitate manufacture of the high density capacitor filter bank  100  may be used. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with various modifications required by their particular application or use of the invention. It is intended that the appended claims be construed to include the alternative embodiments to the extent permitted by the prior art.