Abstract:
A dielectric body is provided with at least two coupling structures for coupling in and coupling out electronic signals, and one or more conductors on surface portions of the body. An outer surface of the body includes one or more indentations. The coupling structure includes a coupling conductor for signal input and/or output, and in relation to an end portion of the conductor a coupling hole extending from the first surface and/or from a second surface opposite the first surface into the body.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to European Application No. 08104660 filed on Jul. 7, 2008, the contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    Described below is a filter for electronic signals and to a method for manufacturing it. The filters under consideration may be filters having a ceramic body with appropriate metallizations thereon such as ceramic monoblock filters. 
         [0003]    Ceramic single mode monoblock filters are used in small and medium power base transmitting site (BTS) products for the reason of size and cost. Also the electrical performance is satisfactory, especially if the dimensions of the filter body are increased. This causes that the rectangular filter body starts passing through electromagnetic energy at higher resonance modes and begins to leak through power beyond a certain cut off frequency. The smaller the body is the higher the cut off frequency is. It is roughly relative to equation: 1/((εr)½×W×L). This leakage is caused by non-desired higher propagation modes in the ceramic material. That is similar to the propagation in a waveguide that is bigger than required to allow just the lowest order mode. The leaking energy is at the harmonics of the desired operating frequency. This would be tolerable if the second and third harmonic at least could be reduced sufficiently. 
         [0004]      FIGS. 1A to 1E  illustrate known ceramic monoblock filters  1  which has a ceramic body  10  with a carefully designed shape and metallizations thereon and with a selected dielectric constant. The filter  1  may have coupling structures  19  for signal input and output, respectively. These coupling structures include conductors on a first surface  11  of the filter body. These conductors  19  may be coupled to external wiring, such as wiring on a printed circuit board (PCB). From the first surface  11  of the body  10  holes may be extending into the body and often through the body down to the opposing surface  12 . 
         [0005]    Surfaces of the body  10  including the inner surfaces of holes  17  may have a more or less continuous metallization thereon. It is to be noted that on one or more particular surfaces this more or less continuous metallization may not or only partially be provided, such as the first surface  11 . Reference numeral  18  indicates conductors which may be metallizations and which are shown in  FIG. 1A  as continuous conductive layers covering all surfaces of the body except the first surface  11 . The conductor  18  also covers the inner walls of the hole  17  and may be grounded in use. 
         [0006]    The holes  17  act as resonators. They have circular or elliptic cross-section.  101  and  102  define symmetry axes of the filter  1 .  101  is a longitudinal axis defined by the mid-points of two holes  17  provided in the ceramic monoblock filter  1 . It coincides with the symmetry axis of the body  10  itself in that it is the centerline between surfaces  13  and  14 , so that the holes  1   8  are arranged symmetrically in block  10 . The holes  17  extend perpendicularly with respect to the first surface  11 .  102  is another symmetry axis. The two holes  17  are symmetrically positioned with respect to this axis  102  which is also the centerline between surfaces  15  and  16 . 
         [0007]    The body  10  and the overall filter  1  is a cuboid with three pairs of opposing surfaces ( 11  and  12 ,  13  and  14 ,  15  and  16 ), the surfaces being substantially flat/plane and rectangular to each other. The body  10  is formed of a ceramic material with a certain relative dielectric constant, which is again selected in view of electronic properties. Holes  17  together with their cladding  18  and the surrounding body  10  and its outside conductors  18  of the filter  1  serve as resonators, the resonating frequencies being adjusted by defining the geometrical dimensions of the body, by forming the coupling conductors  19  and appropriately selecting the dielectric constant ε r . The thickness is the predominant parameter for defining the resonating frequency. The filter has a length L, a width W, and a height H. The holes extend in height direction. 
         [0008]      FIG. 1B  shows a vertical cross-section of the filter  1 . It shows the through-holes  17  extending from the first surface  11  to the second surface  12 , and it shows all recognizable body surfaces except the first surface  11  coated by a conductor  18 . In use, the conductor  18  may be grounded. The first surface  11  may carry coupling conductors  19  of appropriate shape, size and arrangement. It may also carry conductors connected to the conductors of the other surfaces. The conductors of the other surfaces  12  to  16  are continuously connected to each other along the respective common edges. 
         [0009]      FIG. 1C  shows a ceramic monoblock filter  1  on a printed circuit board (PCB)  103 . The holes  17  extend in a direction more or less parallel to the surface of the PCB  103 . The surface conductors  18  are connected to conductors on PCB  103 . Particularly, they may be grounded. The first surface  11  pointing upwards in  FIG. 1A  points now towards the viewer of FIG.  1 C. The coupling connectors  19  are connected to wiring  105  on the PCB and may, through this wiring, be connected to other electronic components  104  and/or to a connector  106 . 
         [0010]    The field of use of such ceramic monoblock filters is wireless communication. They are used both in mobile stations and in base stations. In mobile stations, size and cost are very relevant criteria. In base stations, quality and costs are relevant criteria. The holes  17  together with the coupling conductors  19  and the metallizations/conductors  18  serve as resonators, and through the resonating effect they provide filtering as desired. The design is such that desired propagation modes of electric fields and magnetic fields are supported as far as possible, whereas undesired propagation modes and frequencies and harmonics are suppressed as far as possible. 
         [0011]    The manufacturing method is that first a cuboid as desired is pressed from powder of the material that is to form the body  10 . After pressing, the body has a consistency similar to sugar cubes, i.e. it withstands some mechanical impact, but is destroyable. After pressing, the body is fired under a certain temperature profile over time. It may be exposed to temperatures higher than 1000 or 1500° C. for several hours. Through this firing, the powder particles do not melt, but are sintered together. After firing, the body is machined to the desired final external shape and the desired holes are drilled into the body  10 , and thereafter the body is immersed into a bath of silver paint, what may be repeated several times. After drying, the paint-covered body is again fired for increasing conductivity of the conductor cladding on the walls. Finally, the conductors on the top surface (first surface  11 ) may be structured as required for the tuning of the filtering performance. 
         [0012]    The disadvantage of the present ceramic monoblock filters is that, at a given dimensioning and external circuitry requirement, they show insufficient suppression of certain modes and frequencies, particularly, harmonics are insufficiently suppressed. For the reason that touching the filter dimensions leads to reduced performance, alternative methods, like additional filtering on the PCB  103  must be used. This is sufficiently good but causes additional loss and consumes PCB space. A receiver is protected by using higher power low noise amplifiers (LNA) and an additional small filter behind it. This consumes space and adds to costs. This is not so critical in big units but is getting increasingly important when the units get smaller like in medium range and active antenna products. 
         [0013]    Another disadvantage of prior art circuitry is insufficient or undesired coupling of incoming and outgoing signals. The present coupling is made in voltage made or in current mode using one of the resonator rods as a coupling element. In voltage mode, the coupling conductor approaches a resonator hole  17 , but is not in electrical contact with the conductor on the inner wall of the hole, whereas in current mode coupling the coupling conductor is in electrical contact with the conductor on the inner wall of the resonator hole. 
         [0014]      FIGS. 1D and 1E  show such structures schematically.  FIG. 1D  shows voltage mode coupling. A coupling conductor  19   a  runs from the rim of the first surface  11  towards the hole  17  and may surround it fully or partially, without, however, contacting the conductor  18  on the inner wall of hole  17 . The rim end  191  of coupling conductor  190  is for rendering connections to external circuitry, whereas the resonator end portion  192  provides coupling with resonator hole  17 . In voltage mode coupling as shown in  FIG. 1  D, an electromagnetic coupling is provided between coupling conductor part  192  and conductor  18  on the inside of the resonator hole  17 . The coupling mechanism goes predominantly via the electric field having effect on the conductor  18  on the inside of the hole. 
         [0015]      FIG. 1E  shows current mode coupling in which the rim end  191  of the coupling conductor  190  runs towards the resonator hole  17  and, there, contacts the conductor  18  on the inside of hole  17 . A conductive rim  192  may surround the hole  17 , where it contacts the conductor  18  on the inside wall of resonator hole  17 . For DC, this may constitute a short circuit to the potentially grounded metallization  18 , but for HF it is a suitable direct coupling. 
         [0016]    The disadvantage of known couplings is that they are not optimized either in the sense of matching or in the sense of coupling efficiency or mode/frequency selectivity. The current mode coupling ( FIG. 1   e ) aligns the exciting current better with the resonating current of the desired first order resonance mode, but may be insufficient in the sense of controlling the matching and the coupling. Voltage mode coupling creates roughly a suitable electric field for coupling the sufficient amount of energy in the basic mode into the filter, but it also easily excites other modes, because the direction and distribution of the exciting electric field cannot sufficiently be controlled. 
       SUMMARY 
       [0017]    An aspect is to provide a filter for electronic signals and a manufacturing method therefor which are cheap and result in improved harmonics suppression. 
         [0018]    Another aspect is to provide a filter for electronic signals having an improved coupling structure. 
         [0019]    A filter for electronic signals has a dielectric body, at least two coupling structures for coupling in and coupling out electronic signals, and one or more conductors on surface portions of the body. An outer surface of the body has one or more indentations. The indentations may have rounded surface portions and their contours may follow a hole in the body of the filter. They may be provided in a pairwise manner and may be symmetrical. They may have an internal symmetry, and two or more of them may be symmetrical with respect to each other. Particularly, two opposing surfaces may have indentations, preferably symmetrical to each other, whereas at least another pair of opposing surfaces does not have indentations (except the holes/throughholes/resonator holes) and are substantially flat. 
         [0020]    The indentations maintain the effective diameter of the resonators above a certain value and, thus, have little effect on the Q-value and the performance, but help to suppress harmonics, because the cut-off frequency for non-desired modes is roughly doubled so that particularly second and third harmonics are better suppressed. 
         [0021]    According to the filters described below, cavities of resonators formed by resonator holes are separated by reducing the width in between the cavities so that one nevertheless can maintain their diameter and the effective filter width can be reduced roughly to half or even less. “Cavity” in this sense is the space between the resonator hole conductor and the outer wall conductor. It is filled by the material of the filter body. This reducing roughly doubles the cut off frequency for the non-desired modes to around the fourth harmonic, so that the difficult second and third harmonics would be covered. 
         [0022]    Further, a filter for electronic signals, which may optionally be formed as mentioned above, includes a dielectric body, one or more conductors on surface portions of the body, at least one resonator hole extending from a first surface of the body into the body, and a coupling structure for coupling in and/or out an electromagnetic signal. On the first surface of the body the coupling structure includes a conductor for signal input and/or output from/towards external, and in relation to an end portion of the conductor a coupling hole extending from the first surface and/or from an opposing second surface into the filter body. 
         [0023]    The coupling hole may have or—together with other components—provide no or one or more predetermined resonance frequencies. It may have or provide insignificant resonance frequencies (i.e., sufficiently remote in frequency space from the desired frequency band, or non-existent). The coupling hole may be a through-hole or may be a blind hole extending from the first surface where the coupling conductor is provided or extending from the second surface opposing the first surface. 
         [0024]    The location and dimension can be selected to optimize coupling, incoming impedance and excited resonance mode. The coupling hole may be provided asymmetrically in the cavity, whereas the resonator hole/s is/are in the centre of the cavity and of the filter body. The structure may be such that the coupling element is shorted at the same end as the resonator is and it is fed with a stripline at the other end of the resonator. i.e. it can be manufactured with the same steps as the actual resonator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
           [0026]      FIGS. 1A and 1C  to  1 E are perspective views of known filters; 
           [0027]      FIG. 1B  is a vertical cross-section of the filter illustrated in  FIG. 1A ; 
           [0028]      FIGS. 2A and 2B  are perspective views of a first set of embodiments; 
           [0029]      FIG. 2C  is a cross-section of  FIG. 2B ; 
           [0030]      FIG. 3  is a plan view of a second embodiment; 
           [0031]      FIG. 4  is a plan view of a third embodiment; and 
           [0032]      FIGS. 5A to 5C  is a plan view of a fourth embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0033]    Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0034]    Generally speaking, features described in this specification shall be deemed combinable with each other, even if this is not said explicitly, to the extent that a combination is technically possible. Same reference numerals shall denote same components. 
         [0035]      FIGS. 2A to 2C  illustrate first embodiments showing an improved shape of a filter. The filter may be what is addressed as a ceramic monoblock filter. As far as not otherwise stated in the following, the filters may have features as described with reference to  FIGS. 1A to 1E . 
         [0036]    The body  10  of the filter  1  has a first surface  11  from which two holes  17 , which may be resonator holes, extend into the body, preferably perpendicularly to the first surface  11 , and preferably through the entire body down to the second (not visible) surface  12 . A surface of the body  10 , from which the holes do not extend, namely in  FIG. 2A  the fourth surface  14 , has an indentation  20  reaching into the body, i.e., towards the third surface  13 . Edges  20   a,    20   b  and  20   c  defining or defined by this indentation may be in parallel to the axis of at least one of the holes  17 . 
         [0037]    The indentation  20  has an innermost point  21 , i.e., a surface point most remote from surface  14  from which the indentation starts. The indentation may in itself be symmetrical in a sense that left and right surface portions thereof, i.e. portions left and right of the innermost point  21 , are symmetrical to each other with respect to a plane symbolized by dashed line  22 . The symmetry plane may be rectangular to the plane defined by the axes of the holes  17 . The innermost portion may, in cross section, be the vertex of an angle, the angle preferably being smaller than 120° or smaller than 90°. 
         [0038]    The indentation  20  may be positioned to indent at a space between two resonator holes  17  for reducing the width between resonators. More particularly, the indentation may have the same symmetry plane as two holes  17  left and right thereof in lengthwise direction. Generally speaking, the most indented portion  21  may project on a mid portion of a connecting line between adjacent holes  17 . The mid portion may be the middle of the connecting line plus/minus 30% of the line length. 
         [0039]    Indentations may be provided symmetrically with respect to a plane defined by axes through adjacent holes  17 . This symmetry is shown in  FIG. 2C  where indentation  20   a  is symmetrical to indentation  20   b  with respect to a plane indicated by axis  101  in  FIG. 1 . Not only the indentations may have this symmetry with respect to plane  101 , but also the remaining surface portions  13   a,    13   b,    14   a,    14   b.    
         [0040]    Turning back to  FIGS. 2A and 2B , the indentation restricts the body width between adjacent holes  17  and resonators built thereby while maintaining material thickness (“cavity” width) in a radial direction, seen from hole  17 , above a given value. The material of the filter body  10  confined between the conductor on the inner wall of hole  17  and the conductor provided on the outer wall of the body may be considered like a cavity in a coax conductor. The indentations serve to maintain a certain minimum distance between inner conductor and outer conductor separated by the “cavity”, but restrict the material volume at the coupling of adjacent resonators. This is effective for suppressing certain modes and, particularly, for suppressing harmonics (second, third harmonics), without compromising other quality factors of the respective resonators and the overall filter. 
         [0041]    The dimensioning of the indentations  20  may optionally be such that the remaining body material between opposing indentations ( 20   a  and  20   b  in  FIG. 3 ) or between an innermost portion  21  of an indentation  20  and the opposing wall is at least a certain percentage of the minimum material thickness between hole conductor and wall conductor, the percentage being 10% or 20% or 50% or 100%. Vice versa, an indentation or two opposing indentations together may take away at least 10% or at least 30% or at least 50% of the width of the body. 
         [0042]      FIGS. 2A to 2C  illustrate an embodiment where four of the six surfaces of the cuboid, namely surfaces  11 ,  12 ,  15  and  16  are more or less conventionally built, whereas the at least one surface  14 , and possibly also the opposing surface  13 , are provided with indentations.  FIG. 2B  shows an embodiment where the indentations follow a rounded contour which also effects surface  16  which remained conventional in the embodiment of  FIG. 2A . As shown, the contour (rim) of the first surface  11  is rounded and has a discontinuity only where it meets the rounded contour around another hole  17 . 
         [0043]    The rounded contour may follow a circle or an ellipse. Likewise, the cross-section of the hole  17  may follow a circle or an ellipse. The mentioned circles or ellipses may be concentric. The rounded outer wall (corresponding to walls  13 ,  14 ,  15  and  16  in  FIGS. 1A to 1E ) may be completely covered by a conductor. The same applies to the (not shown/visible) bottom surface  12 . Again conductors on the side walls, the bottom wall and the inner wall of holes  17  may be connected to each other and may, in use, be grounded. 
         [0044]    In this specification, various shapes of indentations and holes are described. In a preferred embodiment, these shapes may be constant along the height direction of a filter. However, they may also be variable, and then the indications may apply to only a portion along the height of the filter or only to a cross-section at a particular height position. 
         [0045]      FIG. 2C  shows an embodiment that is a geometrical mix of the embodiments in  FIGS. 2A and 2B . The indentations themselves have a cross-sectional contour following a circle of a radius R or an oval shape. However, once these rounded wall portions run into the respective straight walls, they continue as flat walls  13   a,    13   b,    14   a,    14   b  until they reach the respective terminating wall  15 ,  16 . The dashed line in the top part of  FIG. 2C  indicates the case that an indentation is provided only at wall  14 , wall  13  remaining conventional, as indicated by the dashed lines. 
         [0046]    The dimension G indicates the remaining width between an indentation  20   a  and the opposing surface which may again be an indentation or, if no indentation is provided, the opposing wall as indicated by the dashed lines in  FIG. 2C . It may be sized as mentioned above. 
         [0047]      FIG. 3  shows an embodiment with five holes  17  in a row, aligned along a straight axis/plane  101 . The axes of holes  17  may be parallel to each other. Holes  17  may all be coated by conductors  18 . The outer wall may also be completely coated, just like the bottom wall opposing wall  11 . Generally speaking, a filter may have a succession of two, three, four, five, six, seven or more resonators formed by a hole with conducting walls and a surrounding surface on the body  10  of the filter  1 . The respective rounded portions (hole, wall portions) may follow circles which may be concentric. The involved radii of holes and outer wall portions may be same or different. Similarly, the distance between adjacent resonator holes  17  may be constant or different from each other. 
         [0048]    The overall size of the filter may be set in relation to the desired operating frequency range of the filter. The operating frequency range may be a frequency or frequency range between 200 MHz and 10 GHz. It may be for mobile communication applications, particularly for base stations and stationary equipment thereof, and may be suitable for one or more of the frequency ranges required there. 
         [0049]    The manufacturing method may be as follows: 
         [0050]    First, a powder of the desired material is prepared. The powder may have an average grain size as desired. The material is selected also in view of its dielectric constant. 
         [0051]    Thereafter, the powder is pressed into the desired shape of the filter body  10 . This pressing may include the provision of holes  17  by having respective rods in the mold for pressing the powder. The mold for pressing the powder may already have the indentations  20 . Likewise, however, at this stage, the walls may still be conventional, i.e. as shown in  FIG. 1 . 
         [0052]    Next, the pressed body is fired, i.e. heated up to a certain temperature and kept at a certain temperature profile over time. Time may be several hours (two or more hours), temperatures may exceed 1000° C. or 1200° C. Through this, the powder particles bake together as in sintering and become a solid body of high mechanical strength. 
         [0053]    If not already provided in the pressed form, the so fired body would now be provided with the indentations  20 , preferably after having cooled down. This may be done by mechanical treatment like grinding or using a diamond tool. A profiled wheel for grinding may be used. The mechanical treatment, however, may also be made when the fired body has already indentations. The mechanical treatment may then be for refining surfaces or bringing the wall geometry down to a finally defined shape. 
         [0054]    Once the final geometry of the body, including the desired indentations  20  and holes  17 , is reached, it will be covered with conductors, which may be metallizations in certain embodiments. This may be done by immersing the body into a silver paint bath and drying it. This may be done repeatedly. 
         [0055]    For improving the conductivity the so obtained silver coating may again be fired for achieving the desired better conductivity of the conductors covering substantial parts of most of the surfaces. The surfaces (except one) may be covered by at least 50% or at least 70% thereof by conductors, these conductors are preferably interconnected and may be grounded in use. Metallization coverage may also be 100%. This also applies to the inner walls of the holes  17 . 
         [0056]    This covering process may also include the first (top) surface which, thereafter, would be structured as desired. Particularly, metallization must be removed by an appropriate process (e.g. etching), so that only the desired pattern remains. Particularly, the coupling conductors  19  must be formed. Further, as far as present, the connection to the wall metallizations must be interrupted. Bringing the desired structures onto the first surface may also be done by an appropriate printing technique, such as the silk screen method. 
         [0057]      FIGS. 4 and 5  show improved couplings for filters. A coupling structure  40  is shown in the lower right corner of the filter body  10  in  FIG. 4 . A conductor  18  covering the walls as described above is only partially shown there, although it is present as shown in, and described with respect to  FIGS. 1 to 3 . The coupling  40  has a coupling hole  41  and a coupling conductor  42 . The coupling conductor  42  is provided on the first surface  11 . It runs from a rim of the first surface towards the coupling hole  41 . Coupling hole  41  may be provided asymmetrically with respect to symmetry lines given by the shape of the filter body  10 . Particularly, it may be away from horizontal or vertical symmetry lines  101 ,  102 . 
         [0058]    The coupling hole  41  may practically be non-resonant, or it may have a resonance frequency remote from the working frequency range of the filter, e.g. more than 2% or more than 5% of the nominal frequency away therefrom. The coupling impedance is tuned to the desired value at the operating frequency. The coupling conductor  42  serves to make electrical contact between outside circuitry to which it is connectable via its rim side end  42   e.  From there, it runs towards the coupling hole  41  and, there, may make voltage mode coupling as schematically indicated in  FIG. 4  and also  FIG. 1B , or may make current mode coupling as shown in  FIGS. 5A and 5B  and also  FIG. 1E . 
         [0059]    Coupling hole  41  may be a through-hole as shown in  FIG. 5A  or may be a blind hole as shown in  FIGS. 5B and 5C . In the one case, it may extend from the first surface into the depth of the body  10 , particularly with rounded or circular cross-section, preferably constant over at least a part of or all of the depth ( FIG. 5B ). In the other case, the blind hole  41   c  may extend from another surface, e.g., the second surface  12  opposing the first surface  11  into the body, again preferably with rounded or circular contour which is preferably constant over at least a part or all of the depth of the hole. Coupling hole  41  is also provided with a conducting layer, such as a metallization. The conductor may be connected to conductor  18  covering the other walls of the filter body  10  ( FIGS. 5A and 5   c ). 
         [0060]    In  FIG. 5B , the conductor covering the coupling hole surface may be isolated from conductor  18 . The positioning, shaping and contacting of the coupling hole  41  may be done in view of coupling performance. Particularly, coupling efficiency and impedance matching aspects may be considered for selecting the make ( FIG. 5A  or  5 B or  5 C) and the positioning of the coupling hole  41  and the design of the coupling conductor  42 .  FIG. 5A  is a current mode coupling, because the coupling conductor  42   a  is in contact with the conductor  18  of coupling hole  41   a.    FIG. 5B  is a mixed mode variant of  FIG. 5A . Here, the coupling conductor  42   a  contacts the conductor/metallization of coupling hole  41   b  which, however, is isolated from the covering conductor  18 .  FIG. 5C  is called “loaded electrical coupling”. It is similar to voltage mode coupling, because the coupling conductor  42   b  is not in contact with the conductor covering the walls of coupling hole  41   c.    
         [0061]    Generally speaking, either voltage mode coupling or current mode coupling is combinable with any of the makes of coupling holes  41   a  or  41   b  as shown in  FIGS. 5   a  and  5   b.    
         [0062]    The coupling holes  41  can substantially be manufactured just as the resonator holes  17 . Also providing their respective conductor on their surfaces can be made in the same way. Some extra steps need to be taken for  FIG. 5B  when immersion into silver paint is used, because this may not cover the first surface  11  and, thus, also not the surface of coupling hole  41   b.    
         [0063]    In the described filters, the coupling structure described with reference to  FIGS. 4 and 5A  to  5 C is combinable with the indentation technology described with reference to  FIGS. 2A to 2C  and  3 . However, these aspects may also be separate from each other. 
         [0064]    A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).