Abstract:
The present invention relates to a single-piece center conductor ( 15 ) for a LC-filter, and a LC-filter comprising at least one such center conductor. The center conductor comprises a first end section ( 20 ) with axially form confining fixing means and a second end section with means for direct connection to connecting means of a connector. A general object of the present invention is to provide an improved apparatus for conneting a LC-filter with a connector in terms of intermodulation interference, contact area with the LC-filter, accurate fastening to the LC-filter, assembly time, manufacturing, storage costs and logistics costs.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to a centre conductor for connecting an LC-filter with a connector, for example a coaxial connector fixed on a coaxial cable. The invention also relates to an LC-filter comprising at least one such centre conductor.  
         DESCRIPTION OF RELATED ART  
         [0002]    In LC-filter networks for filtering selected frequencies, for example when LC-filters are used in communication systems, for instance in radio base stations, it is a problem that mechanically joined electrical couplings between two components are likely to create a P-N-junction, thereby creating intermodulation interference problems Therefore, it is desirable to create a network with as few couplings as possible, especially friction couplings and brazed couplings, to reduce the intermodulation interference.  
           [0003]    Therefore, it is desirable to invent an apparatus with as few couplings and parts as possible for connecting, for example, a standard coaxial cable to an LC-filter. However, so far such an apparatus still involves brazed couplings and parts that create undesired intermodulation interference.  
           [0004]    U.S. Pat. No. 5,001,443-A discloses coaxial units for conducting microwaves to and from amplifying apparatus with splitter and combiner cavities. A coaxial coupling unit is fixed to a first plate member and has a tubular outer conductor and an inner conductor that are insulated from each other by a sleeve. The inner conductor has a lower pin part with a free end that has a frusto-conical exterior shape with slots, which extend upwards. The slots divide the pin into axially extending fingers, which are deflectable radially inwards. This enables the pin to be pressed into a hole in a second plate member. The inner conductor thus creates a friction coupling with the second plate member. The upper end of the inner conductor is shaped as a stub  
           [0005]    The embodiment of the inner conductor disclosed in U.S. Pat. No. 5,001,443-A has features that are undesirable if the coaxial unit for some reason would be used in an LC-filter. A friction coupling of the described type demands small limits of tolerance during manufacturing of both the coupling unit and the hole in a receiving member of the LC-filter, since it is a friction coupling exclusively by press fit It is also very difficult to achieve a predetermined desired coupling force between the inner conductor and the receiving member. Furthermore, if the inner conductor is fixed to the coaxial unit, the conductor and the hole in the sleeve would require very small limits of tolerance for a secure press fit, or an extra arrangement, for keeping the inner conductor fixed in the coaxial unit. Of course this increases the manufacturing costs of the coaxial unit. If the inner conductor is not a fixed part of the coaxial unit, a detachment of a to the coaxial unit before connected coaxial cable may withdraw the inner conductor from the hole or decrease the contact surface between the hole and the inner connector. This reduces the reliability of proper contact when a coaxial cable is to be attached again, or creates a renewed assembly time to reinstall the coaxial unit in a proper way. Also, since the upper part of the inner conductor is shaped as a stub, the inner conductor does not fit a standard connector that is fixed to a coaxial cable and has a pin member projecting out from the connector. This kind of connector is common for LC-filters, such as cavity filters.  
         SUMMARY  
         [0006]    It is therefore a general object of the present invention to solve or reduce the above mentioned problems and disadvantages by providing an improved apparatus for connecting an LC-filter with a connector, in terms of intermodulation interference, contact area with the LC-filter, accurate fastening to the LC-filter, assembly time, manufacturing, storage costs and logistics costs.  
           [0007]    It is also a general object of the present invention to provide an improved LC-filter in such a way that the LC-filter enables an improved connection with a connector, such as a coaxial connector fixed to a coaxial cable, by using the improved centre conductor and modifying the LC-filter for an improved fit with the centre conductor.  
           [0008]    The present invention therefore provides a single-piece centre conductor for an LC-filter. Thereby, the centre conductor does not involve two or more parts that have to be connected, e.g. by brazing. This reduces intermodulation interference problems and saves assembly time. Furthermore, it saves storage costs as well as logistics costs since fewer parts have to be manufactured, registered and stored.  
           [0009]    The centre conductor comprises a first end section with axially form confining fixing means, and a second end section with means for direct connection to connecting means of a connector, such as a coaxial connector fixed to a coaxial cable. With axially form confining fixing means are meant: fixing means that due to its form see to that the centre conductor substantially is not able to be axially removed in relation to the LC-filter when the centre conductor is mounted to the LC-filter and subjected to a tractive force. Thus when the first end section is mounted to the LC-filter, for example by engaging a partially threaded bore in the LC-filter, the fixing means substantially prevent axial movement of the centre conductor relative to the LC-filter when the centre conductor is subjected to a force substantially parallel with a longitudinal axis of the centre conductor. With that a problem with possible disconnection between the first end section and the LC-filter is avoided in case of disconnection of the coaxial cable from the LC-filter. The axially form confining fixing means also are allowed to have larger limits of tolerance than such a press fit as the one mentioned before in the discussion about U.S. Pat. No. 5,001,443-A. This makes the centre conductor easier to manufacture than if friction means were to be used.  
           [0010]    The means at the second end section preferably comprise a cavity for engaging the connecting means of the connector, where the connecting means is a pin member. This enables the centre conductor to be directly connected to a pin member of a standard coaxial connector.  
           [0011]    Suitably, the cavity is surrounded by at least two tongues, which are resiliently deflectable outwards from, and equiangularly distributed about, the longitudinal axis of the centre conductor. This provides easy and fast attachment and unattachment of the pin member to the centre conductor, as well as a press fit that secures a good contact between the centre conductor and the pin member.  
           [0012]    In order to improve the assembly time and the convenience for the installer, the centre conductor comprises at least one tool receiving means for engaging a tool during mounting of the centre conductor to the LC-filter. The tool receiving means may comprise a cross section with a regular polygonal shape, such as a hexagon shape, which enables engagement with an ordinary socket wrench. Alternatively or in addition to the above mentioned polygonal shape, the tool receiving means may comprise at least one aperture, for example a through-hole or an exterior slot, for engagement with pins, clutches or other kinds of assembly tools.  
           [0013]    Furthermore, the centre conductor preferably comprises a middle section with a larger cross sectional area than the first and second end sections. Suitably the middle section comprises a substantially flat ring-shaped surface, which is perpendicular to the longitudinal axis of the centre conductor and has a normal pointing outwards towards the first end section. This enables the ring-shaped surface to contact an opposite flat surface around the receiving bore of the LC-filter. Thus a larger contact area between the LC-filter and the centre conductor is created.  
           [0014]    The form closing fixing means are preferably threads. This enables the centre conductor to be fixed in the bore of the LC-filter with a predetermined force, which easily is determined since sufficient torque meters for screw joints are widely used in assembly workshops. The threads also enables a type of cold welding between the ring shaped surface and the opposite flat surface of the LC-filter as the ring shaped surface is screwed into contact with the opposite flat surface. The type of cold welding further improves the contact between the centre conductor and the LC-filter.  
           [0015]    The present invention further provides an LC-filter, which for instance is used in a radio base station, with at least one such a centre conductor as described above. Preferably the LC-filter is a cavity filter with an input resonator, comprising a bore having an outer mouth and wall, which comprises threads and engages the centre conductor, and a substantially flat surface that surrounds the outer mouth and is substantially perpendicular to a longitudinal axis of the bore. The LC-filter may also comprise an output resonator engaging a second centre conductor of the above described type. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The objects, advantages and effects as well as features of the present invention will be more readily understood from the following detailed description of a preferred embodiment, as well as other embodiments, when read together with the accompanying drawings in which:  
         [0017]    [0017]FIG. 1 shows a schematic block diagram of a radio base station;  
         [0018]    [0018]FIG. 2 shows a perspective view of an LC-filter in the form of a cavity filter, where one of the walls of the cavity filter is removed;  
         [0019]    [0019]FIG. 3 shows a partial cross sectional view of a part of the cavity filter according to a preferred embodiment of the invention;  
         [0020]    [0020]FIG. 4 shows a perspective view of a preferred centre conductor,  
         [0021]    [0021]FIG. 5 shows a cross sectional view of the preferred centre conductor;  
         [0022]    [0022]FIG. 6 shows an enlarged view of one of the ends of the preferred centre conductor, and  
         [0023]    [0023]FIG. 7 shows a perspective view of an embodiment of the centre conductor similar to the one in FIG. 4, but having another embodiment of a tool receiving means.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0024]    While the invention covers various modifications and alternative constructions, a preferred embodiment of the invention is shown in the drawings and will hereinafter be described in detail. It is to be understood, however, that the specific description and drawings are not intended to limit the invention to the specific forms disclosed. On the contrary, it is intended that the scope of the claimed invention include all modifications and alternative constructions thereof falling within the spirit and scope of the invention as expressed in the appended claims to the fall range of their equivalents.  
         [0025]    [0025]FIG. 1 shows a schematic block diagram of a radio base station used in a mobile telephone network. When an input signal S 1  is sent to the radio base station, a corresponding signal is transmitted out from the radio base station over a transmitter  1 , a filter apparatus  2  and an antenna  3 . When the antenna  3  receives a signal from the air, the radio base station provides an output signal S 2  over the filter apparatus  2  and a receiver  4 . The filter apparatus  2  typically includes LC-filters operating as a band-pass filter circuit.  
         [0026]    [0026]FIG. 2 shows an LC-filter in the form of a cavity filter  5 , for instance for the filter apparatus  2  in the radio base station, where an upper wall is removed to show the interior of the cavity filter  5 . The cavity filter has an output apparatus  6  according to the invention, here shown mounted to an output resonator  7 , for connecting the cavity filter  5  to a coaxial cable (not shown). An apparatus, similar to the output apparatus  6 , for connecting an input coaxial cable (schematically shown in FIG. 3) to the cavity filter  5  is generally designated as  8  and will be used to further explain the preferred embodiment. The cavity filter  5  comprises a grounded chassis  9  with a bore  10 , which preferably is threaded, but may be substantially smooth. In mounted position, the apparatus  8  is connected to the cavity filter  5  through the chassis  9  and an input resonator  11 .  
         [0027]    [0027]FIG. 3 is an enlarged partial cross sectional view of a part of the cavity filter  5  shown in FIG. 2, where the apparatus  8  for connecting a coaxial cable  12  to the cavity filter  5  is shown in more detail. The coaxial cable  12  with an associated connector is only schematically drawn since it is not a part of the invention. However, it is to be understood that the associated connector may be any suitable coaxial connector with an inner conductor and an outer conductor for engaging the apparatus  8 , wherein the inner conductor comprises connecting means, for example a pin member  13  projecting out from the connector, for engagement with the apparats  8 .  
         [0028]    The apparatus  8  generally comprises a connector unit  14  and a centre conductor  15  (see FIG. 2). The connector unit  14  comprises a tubular conductor shell  16  with outer fixing means in the form of screw threaded portions  17 , of which at least one is near each end of the connector unit  14 , for fixing the connector unit  14  to the chassis  9  of the cavity filter  5  and to ensure the connection to the coaxial cable  12 . The shell  16  surrounds a coaxial sleeve  18  of dielectric material. Preferably, the sleeve  18  is mainly positioned near the end of the connector unit  14  that is inserted into the cavity filter  5  (the left part of the connector unit  14  in FIG. 3) and extends towards the centre of the connector unit  14  along the longitudinal axis of the connector unit  14 . The sleeve  18  has a circular through-hole  19  concentric with the longitudinal axis of the connector unit  14  in order to allow insertion of the centre conductor  15  into the connector unit  14 .  
         [0029]    As especially seen in FIG. 5, the centre conductor  15  is fabricated from one piece of a conducting material that allows certain flexibility for a reason that will be explained later. All the parts of the centre conductor  15  are thus integrated with each other. Preferably the material is a tin-bronze alloy or beryllium-copper alloy. The centre conductor  15  comprises a cylindrical first end section  20  with external axially form confining fixing means in the form of screw threads  21 , a second end section in the form of a pin section  22 , and a middle section  23  with a larger cross sectional area than both of the end sections  20 ,  22 .  
         [0030]    As best seen in FIG. 6, the pin section  22  has means for engaging the connecting means of the inner conductor of the coaxial cable  12 , in the form of a cavity  24 , for instance a bore, which substantially opens out at the free end of the pin section  22  and is coaxial with the longitudinal axis of the centre conductor  15 . At least a part of the portion of the pin section  22  that surrounds the cavity  24  has a frusto-conical exterior shape and at least two, preferably four, slots  25 . These slots  25  are substantially equiangularly distributed about, and extend substantially parallel with, the longitudinal axis of the centre conductor  11  Furthermore, the slots  25  are substantially shaped as isosceles triangles with a base side  26  substantially perpendicular to the longitudinal axis of the centre conductor  11  and two sides  27  of equal length extending outwards to the free end of the pin section  22 , hereby creating equiangularly distributed tongues  28  between the slots  25 . The tongues  28  are resiliently deflectable outwards in order to ensure a manually detachable press fit around the pin member  13  of the connector belonging to the coaxial cable  12 .  
         [0031]    The middle section  23  is preferably cylindrical with a flat ring-shaped surface  29 , which is substantially perpendicular to the longitudinal axis of the centre conductor  15  and has a normal pointing out from the ring-shaped surface  29  in the direction towards the first end section  20 . The middle section  23  also comprises tool receiving means  30  for engagement with a tool during mounting of the centre conductor  15  to the input resonator  11  in a way described later. In FIG. 3 and  4 , the tool receiving means  30  comprises flat surfaces equiangularly distributed about the longitudinal axis of the centre conductor  15  so as to create a cross section with a substantially regular polygonal shape, e.g. a regular hexagonal shape, for engagement with a tool, such as a socket wrench. The tool receiving means  30  is preferably positioned at the end of the middle section  23  adjacent the pin section  22 , but may be positioned al some other place on the centre conductor  15 .  
         [0032]    The input resonator  11  has a bore  31  (see FIG. 3) with a wall, which preferably comprises threads  32 , corresponding to the first end section  20  of the centre conductor  15 . The input resonator  11  also comprises a substantially flat surface  33  surrounding an outer mouth of the bore  31 . The flat surface  33  is perpendicular to the longitudinal axis of the bore  31 .  
         [0033]    The mounting of the preferred embodiment of the apparatus  8  in the cavity filter will now be described. First, the centre conductor  15  is fastened to the input resonator  11  by screwing the first end section  20  with its screw threads  21  into the bore  31 . This is suitably done with the help from a socket wrench that mate with the regular polygonal shape of the tool receiving means  30  and together with the centre conductor  15  is inserted through the bore  10  of the cavity filter  5 . The threaded joint between the centre conductor  15  and the input resonator  11  easily allows the installer to tighten the joint to a predetermined desired torque level. The flat ring-shaped surface  29  of the centre conductor  15  contacts the flat surface  33  of the input resonator  11  to provide an even more fixed contact and a larger contact area between them. Also, when the flat ring-shaped surface  29  is in contact with the flat surface  33  of the input resonator  11  and is turned further towards the flat surface  33 , a type of cold welding between the two surfaces  29  and  33  is created, which here is advantageous and further improves the contact and stability between the centre conductor  15  and the input resonator  11 . In addition, the two surfaces  29  and  33  may be polished before the mounting in order to give an even better contact between them.  
         [0034]    When the centre conductor  15  is fixed to the cavity filter  5 , the connector unit  14  is easily pushed on to the pin section  22 , and introduced in the bore  10  of the cavity filter  5 . This is done by inserting the free end of the pin section  22  in the through-hole  19  of the sleeve  18 , using one of the screw threaded portions  17  on the shell  16  to mate with the corresponding threads in the bore  10 , and then screw a part of the connector unit  14  into the bore  10  of the cavity filter  5 . The connector of the coaxial cable  12  is fastened to the apparatus  8 , e.g. by screwing and/or pressing, so that the outer conductor of the coaxial cable  12  contacts the shell  16  and thus is connected to the grounded chassis  9 , and that the pin member  13  is inserted into the cavity  24  so as to connect to the input resonator  11 . The mouth of the cavity  24  has a smaller cross sectional area than the cross sectional area of the pin member  13 . When the pin member  13  is pushed into the cavity  24 , the resiliency of the material of the centre conductor  15  allows the pin member  13  to press the tongues  28  radially outwards. Hence a proper press fit between the centre conductor  15  and pin member  13  is obtained.  
         [0035]    Having described the preferred embodiment of the invention, alternative embodiments within the scope of the present invention will now be described as examples only.  
         [0036]    The first end section  20  of the centre conductor  15  may comprise other axially form confining fixing means than the screw threads  21 , as long as the fixing means in a mounted position in an LC-filter substantially prevent axial movement of the centre conductor  15  relative to the LC-filter when the centre conductor  15  is subjected to a force substantially parallel with the longitudinal axis of the centre conductor  15 . Such alternatives include bayonet couplings, which may require slots in the bore  31  of the input resonator  11  or in the first end section  20  for engaging corresponding locking pins. Other alternatives are clutches integrated at the first end section  20 . In a mounted position, these clutches may be L-shaped with a longitudinal portion, which is parallel with the longitudinal axis of the centre conductor  15  and is in contact with the walls of the bore  31 , and a transverse portion, which is substantially perpendicular to the longitudinal portion. These transverse portions of the clutches abut against a surface surrounding an inner mouth of the bore  31 . This prevents undesired withdrawal of the centre conductor  15  from the input resonator  11  and provides a larger contact surface between the centre conductor  15  and the input resonator  1 . Together with the flat ring-shaped surface  29 , these clutches axially fix the centre conductor  15  to the input resonator  11 .  
         [0037]    The cross sectional area of the middle section  23  may have substantially the same cross sectional area, as well as cross sectional shape, as the first end section  20  and the pin section  22 . This means that the centre conductor  15  is fabricated without the frusto-conical exterior shape at the free end of the pin section  22 . Although preferred, the ring-shaped surface  29  does not necessarily have to be perpendicular to the longitudinal axis of the centre conductor  15 , but may have any angle to the longitudinal axis of Me centre conductor  15 , as well as a curved shape.  
         [0038]    Example of an alternative embodiment of the tool receiving means  30  is at least one aperture in the centre conductor  15 , for example a through-hole or an exterior slot of any suitable shape for mating with corresponding means, such as pins or clutches, of an assembly tool. An aperture  34  is illustrated in FIG. 7, where the mouth of the aperture  34  is positioned at the envelope surface of the middle section, but the mouth may also be positioned at a second ring shaped surface  35 , which has a normal pointing out from the surface  35  in the direction of the pin section  22  (see FIG. 5).  
         [0039]    It is also within the scope of the invention to use a flange with a tubular bearing collar of any suitable material to be mounted around an outer bore, corresponding to the bore  10 , of an LC-filter to support the connector unit  14 . The bearing collar may be inserted in the bore  10  and/or may project outwards from the LC-filter. Any suitable type of fastener may fix the flange to the LC-filter.  
         [0040]    It is also to be understood that the connector unit  14  and the centre conductor  15  may be attached to each other in any suitable way before the installation in an LC-filter.