Abstract:
A high-frequency filter arrangement comprising at least one filter consisting of a plurality of high-frequency inter-coupled cavities in which a locally fixed respective dielectric resonator element is disposed and in which a respective dielectric body can be modified, in order to tune the frequency of the filter, in the position thereof in relation to the dielectric resonator element. The structure of the inventive filter arrangement is simple, compact and economical and excellent filter and tuning properties are obtained by virtue of the fact that the dielectric body is arranged in an eccentric recess of the dielectric resonator element and that the dielectric body is rotatably arranged in the eccentric recess.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to the field of radio-frequency engineering. It relates in particular to a tunable radio-frequency filter arrangement as claimed in the precharacterizing clause of claim  1 , and to a method for its production.  
         [0002]     A radio-frequency filter arrangement such as this is known, for example, from U.S. Pat. No.  6 , 147 , 577 .  
         [0003]     A single tunable dielectric resonator, in which the moving dielectric body can move linearly in the vertical or horizontal direction in a cutout in the dielectric resonator element is known, by way of example, from EP-A1-0 601 369.  
       PRIOR ART  
       [0004]     Transportable radio link connections (LOS=Line of Sight) have been proven for rapid and flexible construction of wire-free communication networks, in particular in rugged terrain without a suitable infrastructure, and these operate in the frequency range of two or more GHz (for example 4.4 to 5 GHz; or 14.62 to 15.23 GHz). Appropriate filters, in particular bandpass filters, are required for signal processing within the transmission and reception appliances for such directional radio links, which filters are designed not only for individual frequencies but are automatically tunable and are distinguished by constant high Q-factors over the tuning range.  
         [0005]     In addition to the essential electrical and radio-frequency characteristics, filters such as these must, however, also be producible at low cost, must have a robust design, and must be designed to be reliable in use and to be space-saving and weight-saving. Space (volume) and weight, in particular, are major factors for the mobility of the overall communication system.  
         [0006]     In the past, in order to reduce the size of the cavities for filters such as these, solutions have increasingly been proposed which have a dielectric resonator element arranged in a cavity as the tunable basic element, whose resonant configuration can be varied in order to tune the filter. One such solution is described, by way of example, in U.S. Pat. No. 6,147,577, which was cited initially. In this known solution, a first round dielectric disk (ceramic puck) is arranged in a fixed position as a resonator in each of the cavities of the filter. An identical second round dielectric disk is located parallel above the first, and can be raised vertically, and lowered again, relative to the first disk by means of an electronically controlled motor drive. The linear movement that is required for this purpose is produced by a digital stepping motor, whose rotary movement is converted to a linear movement by a complex threaded rod mechanism.  
         [0007]     This known filter arrangement has various disadvantages: firstly, it is comparatively difficult to achieve the comparatively high accuracy and reproducibility of the disk position during a linear movement of the moveable disk, as is required for good tunability of the filter. Secondly, the adjustment mechanism that is required for the linear movement requires a very large amount of space. As can easily be seen from FIG. 4 in U.S. Pat. No. 6,147,577, the motorized adjustment mechanism that is arranged above the cavities occupies about ⅔ of the entire physical volume of the filter. Furthermore, owing to the capability of the upper disk to move in the vertical direction, the cavity must be designed to be comparatively large, from the start.  
         [0008]     EP-A1-0 601 369, which was likewise cited initially, proposes a single tunable dielectric resonator in which an eccentric cutout is provided in the dielectric disk that is arranged in a fixed position in a cavity, which cutout can be entered to a greater or lesser extent by a dielectric body that is shaped to match the cutout. The resonator is tuned by adjustment of the insertion depth. For this purpose, the dielectric body can be moved linearly via a holder in the form of a rod in the vertical direction ( FIG. 1  in EP-A1-0 601 369) or in the horizontal direction ( FIG. 2  in EP-A1-0 601 369). No further details are stated about the tuning response that can be achieved by this solution. Furthermore, no mechanically worked-out adjustment mechanism is specified either, so that this proposal should in fact be regarded just as paper prior art, and its feasibility is more than questionable. In particular, this solution proposal is also subject to the same disadvantages resulting from the linear movement as those which have already been discussed further above.  
       DESCRIPTION OF THE INVENTION  
       [0009]     One object of the invention is thus to provide a tunable radio-frequency filter arrangement which can be produced cost-effectively, is distinguished by a particularly compact and robust design with good radio-frequency characteristics, and has an advantageous tuning response, and to specify a cost-effective and simple method for its production.  
         [0010]     The object is achieved by the totality of features in claims  1  and  27 . The essence of the invention is to provide, as a tunable filter module, a cavity with a dielectric resonator element which is arranged in a fixed position and has an eccentric cutout in which a dielectric body is arranged such that it can rotate. The arrangement of the body such that it can rotate in the cutout allows the dielectric resonator element to be designed to be extremely compact. The rotary movement can be designed with high precision, thus allowing high tuning accuracy and reproducibility to be achieved.  
         [0011]     One preferred refinement of the filter arrangement according to the invention is distinguished in that the dielectric resonator element is in the form of a planar, round circular disk, and in that the dielectric body can rotate about a rotation axis which is at right angles to the disk plane of the dielectric resonator element, in that the dielectric resonator element has a predetermined thickness, and in that the dielectric body has a height in the direction of the rotation axis which is essentially equal to the thickness of the dielectric resonator element.  
         [0012]     A development of this refinement has been found to have a particularly advantageous tuning characteristic, in which the cutout in the dielectric resonator element is a circular cylindrical through-hole which is concentric with respect to the rotation axis, in which the external dimensions of the dielectric body are matched to the cutout in the dielectric resonator element in such a way that the two are separated from one another by only narrow air gaps, and the dielectric body is bounded by two parallel planar surfaces in a first direction at right angles to the rotation axis ( 60 ), and is bounded by two cylindrical envelope surfaces, which are concentric with respect to the rotation axis, in a second direction, which is at right angles to the rotation axis and to the first direction.  
         [0013]     Undesirable interference fields in the dielectric resonator element and in the metallic cavity are preferably suppressed by the dielectric resonator element having a central through-hole.  
         [0014]     It is also expedient for the dielectric resonator element and the dielectric body each to be composed of the same material.  
         [0015]     The filter arrangement has a particularly simple and compact design, overall, if, according to another development, the at least one filter is accommodated in a preferably rectangular filter housing, in that the filter housing is formed from a base plate and wall plates, which are at right angles to the base plate for the side walls, and is covered on the top face by a motor mounting plate, which is parallel to the base plate, and in that the cavities in the filter are formed by separating plates which are incorporated in the filter housing and are at right angles to the base plate, and mounting slots are provided in the base plate, in the wall plates and in the separating plates, by means of which the plates are plugged into one another and are connected to one another, in particular by being soldered. The electromagnetic interaction of the cavities is in this case achieved in a particularly simple manner in that coupling openings, in particular coupling slots, are provided at predetermined points in individual separating plates.  
         [0016]     Another development of the invention is distinguished in that a preferably circular opening is provided in the motor mounting plate above each of the cavities, through which the respective dielectric resonator element and the respective dielectric body are held in the cavity, in that the dielectric resonator element and the dielectric body are part of a tuning element which is associated with the cavity and is mounted on the motor mounting plate, and in that the tuning element in each case has a fixed holder, which passes through the opening in the motor mounting plate, for the dielectric resonator element, a holder which passes through the opening in the motor mounting plate and is mounted such that it can rotate, for the dielectric body, a motor, in particular a stepping motor, and a gearbox unit, which transmits the rotational movement of the motor to the holder, which is mounted such that it can rotate.  
         [0017]     The arrangement is particularly space-saving if, according to one preferred development, the gearbox unit is accommodated in a housing, in that the housing is mounted on a motor mounting plate, in that the motor is flange-connected to the housing, and in that the holder for the dielectric resonator element is attached to the housing.  
         [0018]     Particularly precise tuning is achieved in that the gearbox unit has a rotating element which is known in the form of a shaft, is mounted in a prestressed precision bearing and is firmly connected to the holder for the dielectric body, and in that the rotating element is driven by a drive shaft within the gearbox unit via a gearwheel which is firmly seated on the rotating element, with the drive shaft being connected to the motor and engaging with the gearwheel via a worm gear, and in that the rotating element is prestressed in the rotation direction in order to overcome play, preferably by means of a spiral spring.  
         [0019]     Furthermore, space can be saved by the gearwheel being in the form of a circle segment, rather than a complete wheel. A configuration such as this in the form of a segment with a segment angle of about 100° is completely sufficient to cover the entire worthwhile adjustment range of about 90° of the dielectric body in the cutout in the dielectric resonator element.  
         [0020]     Particularly reliable tuning with high reproducibility is achieved in that, a controller, which has a control block, a memory and an input unit, is provided in the eccentric cutouts in the dielectric resonator bodies in order to control the rotation of the dielectric bodies, in that position sensors, in particular in the form of light barriers which are connected to the control block, are provided in order to determine the initial position of the dielectric bodies in the radio-frequency filter arrangement, and in that value tables are stored in the memory and associate an appropriate angle position of the dielectric bodies with a small number of selected frequencies of the radio-frequency filter arrangement.  
         [0021]     One preferred refinement of the method according to the invention is distinguished in that the sheet-metal parts are silver-plated, and are soldered to one another by means of a silver solder, the sheet-metal parts have mounting aids, in particular in the form of crossing slots mounting slots and mounting lugs which are matched to one another, in that the sheet-metal parts are initially loosely plugged together by means of the mounting aids and the crossing slots, mounting slots and mounting lugs in order to form the filter housing, and the plugged-together filter housing is made mechanically robust by pushing the mounting lugs into the mounting slots, in that silver solder, preferably in paste form, is applied to the junction points between the plugged-together sheet-metal parts, and in that the plugged-together sheet-metal parts are heated, preferably in an oven, until the silver solder melts and flows into the junction points.  
         [0022]     The production process is particularly simple and cost-effective if all of the sheet-metal parts of a filter housing are cut from a common metal sheet, which has not been silver-plated, by means of a cutting method, preferably by means of laser cutting, in such a way that the cut-out sheet-metal parts are connected to the remaining area of the metal sheet only by a small number of narrow webs, in that the metal sheet together with the cut-out sheet-metal parts is then silver-plated, in that the sheet-metal parts are detached from the metal sheet after being silver-plated, and are then used to construct the filter housing, in particular with the majority of the webs remaining at those points on the sheet-metal parts which are located outside the cavities when the filter housing is complete.  
         [0023]     Further embodiments are specified in the dependent claims. 
     
    
     BRIEF EXPLANATION OF THE FIGURES  
       [0024]     The invention will be explained in more detail in the following text using exemplary embodiments and in conjunction with the drawing, in which:  
         [0025]      FIG. 1  shows a perspective overall view of the filter housing (the filter box) of a radio-frequency filter arrangement according to one preferred exemplary embodiment of the invention for a total of three filters which are arranged alongside one another and each have four cavities which are arranged in a square and are coupled to one another (the tuning units with the dielectric resonator elements and adjustable dielectric bodies have been omitted, for clarity reasons);  
         [0026]      FIG. 2  shows the filter housing from  FIG. 1 , in the form of a side view of the longitudinal face with the inputs and outputs of the three filters;  
         [0027]      FIG. 3  shows the filter housing from  FIG. 1 , in the form of a side view of the transverse face;  
         [0028]      FIG. 4  shows a perspective view of a metal sheet, which is used as a wall plate for the transverse faces of the filter housing as shown in  FIG. 1 , and has a transverse separating plate between the three filters;  
         [0029]      FIG. 5  shows the perspective view of a metal sheet which is used in the filter housing as shown in  FIG. 1  as a transverse separating plate with a coupling opening between the four cavities within each of the three filters;  
         [0030]      FIG. 6  shows the perspective view of a metal sheet which is used in the filter housing as shown in  FIG. 1  as a separating plate running in the longitudinal direction with coupling openings between the front and rear cavities of all three filters;  
         [0031]      FIG. 7  shows the perspective view of the base plate of the filter housing as shown in  FIG. 1  with a large number of mounting slots, into which the lugs of the separating plates and wall plates as shown in FIGS.  2  to  5  can be inserted, and can be soldered.  
         [0032]      FIG. 8  shows the perspective view of a tuning unit with a motor, a gearbox unit, a dielectric resonator element and a dielectric body which can rotate;  
         [0033]      FIG. 9  shows the tuning element from  FIG. 8 , in a view from underneath;  
         [0034]      FIG. 10  shows a longitudinal section through the gearbox unit of the tuning unit from  FIG. 8 ;  
         [0035]      FIG. 11  shows the perspective view of the gearwheel, which is in the form of a circle segment, from the gearbox unit shown in  FIG. 10 ;  
         [0036]      FIG. 12  shows the perspective view of the dielectric resonator element of the tuning element shown in  FIG. 8 ;  
         [0037]      FIG. 13  shows the perspective view of the dielectric body, which can rotate, of the tuning unit shown in  FIG. 8 ;  
         [0038]      FIG. 14  shows the fundamental arrangement of the cavities of a filter in a square according to the exemplary embodiment shown in  FIG. 1 , and the orientation of the associated dielectric resonator elements and bodies within the cavities, with respect to the coupling slots;  
         [0039]      FIG. 15  shows an alternative arrangement to that in  FIG. 14  of the cavities of a filter, in a row;  
         [0040]      FIG. 16  shows the outline circuit diagram of a control system for the radio-frequency filter arrangement according to the invention;  
         [0041]      FIG. 17  shows the arrangement and configuration of the sheet-metal parts for a filter housing as shown in  FIG. 1  on a common metal sheet;  
         [0042]      FIG. 18  shows the relationship between the filter frequency of the filter according to the exemplary embodiment and the rotation angle of the dielectric body  45 ;  
         [0043]      FIG. 19  shows the measured frequency profile of the S parameters S 11  (reflection coefficient at the input; curve B) and S 21  (transmission coefficient in the forward direction; curve A) of the filter according to the exemplary embodiment for the tuned frequency of 4.7 GHz over a frequency range of ±15 MHz about the respective mid-frequency; and  
         [0044]      FIG. 20  shows the measured frequency profile of the S parameter S 21  of the filter according to the exemplary embodiment, for the tuned frequency of 4.7 GHz over a wider frequency range of ±60 MHz about the respective mid-frequency. 
     
    
     APPROACHES TO IMPLEMENTATION OF THE INVENTION  
       [0045]     The tunable radio-frequency filter arrangement which is described in the following text has a filter housing ( 10   FIG. 1 ) in which a number of tuning units ( 40  in  FIG. 8 ) are inserted and are screwed to the motor mounting plate ( 13  in  FIG. 1 ). The filter housing and the tuning units will be explained separately. The completely assembled filter arrangement is not illustrated, for reasons of clarity.  
         [0046]     The rectangular filter housing (filter box)  10  illustrated in  FIG. 1  is composed of a thicker (at the top) motor mounting plate  13  and of a number of sheet-metal parts, which form the base, side walls and (inner) separating walls of the filter housing  10 . The sheet-metal parts include the baseplate  11 , which is illustrated individually in  FIG. 7 , the wall plates  12  and  20  (see also  FIG. 4 ) which run in the transverse direction, the wall plates  14  and  32  ( FIGS. 1, 2 ) which run in the longitudinal direction, the transverse (inner) separating plates  15 , . . . , 19  which are illustrated individually in  FIGS. 4 and 5 , and the (inner) separating plate  33 , which is located in the longitudinal direction and is illustrated individually in  FIG. 6 . The sheet-metal parts are composed, for example, of 1 mm thick silver-plate sheet steel (material No. 1.4301). The motor mounting plate  13  is composed of the same material and is likewise silver-plated, but has a thickness of, for example, 4 mm.  
         [0047]     As can be seen from  FIG. 17 , the sheet-metal parts can be produced particularly easily and cost-effectively by cutting all of the sheet-metal parts of the filter housing  10  out of a common metal sheet  69  of suitable size, in the manner illustrated in  FIG. 17 . First of all, the metal sheet  69  is not silver-plated. Initially, the contours of the required sheet-metal parts  11 ,  12 ,  14 , . . . , 20 ,  32  and  33  are cut out in the metal sheet  69  by laser cutting and by a comparable cutting technique, with the sheet-metal parts that have been cut out still being connected to the rest of the metal sheet  69  at various points by narrow webs. The majority of the webs are arranged at points on the sheet-metal parts which are located outside the cavities  21 , . . . , 24  in the subsequent filter housing  10 . The lack of any silver layer at these points means that there will be no affects on the radio-frequency characteristics of the cavities. Once the cut metal sheet  69  is in the form shown in  FIG. 17 , a silver layer is provided over its entire surface. This results in the sheet-metal parts being virtually completely silver-plated. Such silver plating is missing only in the areas of the webs which will later be cut through. However, since these are largely located outside the cavities, this is not disadvantageous.  
         [0048]     The filter housing  10  is formed from the individual sheet-metal parts  11 ,  12 ,  14 , . . . , 20 ;  32 ,  33  and the motor mounting plate  13  by soldering and pinning. The soldering is carried out by means of a suitable silver solder in an oven. The sheet-metal parts  11 ,  12 ,  14 , . . . , 20 ;  32 ,  33  are for this purpose first of all provisionally connected by plugging mounting lugs and mounting slots that are provided for this purpose into one another, and the sheet-metal housing that is formed is made mechanically robust by pushing the mounting lugs into the mounting slots. Only the wall plates  14 ,  32  on the longitudinal face of the filter housing  10  are pinned at the upper edge to the end faces of the motor mounting plate  11 . A suitable amount of solder in the form of solder paste is applied to the junction points between the sheet-metal parts and is distributed such that the gaps at the junction points are reliably closed during the soldering process. The housing that has been prepared in this way is then heated in an oven to the temperature required for soldering, and is cooled down again once the solder has melted and has run in the junction points.  
         [0049]     In order to plug the sheet-metal parts  11 ,  12 ,  14 , . . . , 20 ;  32 ,  33  into one another, the baseplate  11  and the wall plates  14 ,  32  which are arranged on the longitudinal faces of the housing are provided with a number of mounting slots  39  (some of which cross). The wall plates  12 ,  14 ,  20  and  32  and the separating plates  15 , . . . , 19  and  33  are equipped on their lower edges with mounting lugs L 1  appropriate for this purpose, by means of which they can be plugged through the mounting slots  39  in the baseplate  11 , and can be soldered. The transverse wall plates  12  and  20  and separating plates  15 , . . . , 19  additionally have mounting lugs L 2  on their side edges, which can be plugged through corresponding mounting slots in the longitudinal wall plates  14 ,  32 , and can be soldered. In order to allow unimpeded crossing of the transverse wall and separating plates  12 ,  14 , . . . , 20 ;  32  with the longitudinally running separating plate  33 , special crossing slots  34 ,  36 ,  37  and  38  ( FIGS. 4-6 ) are provided in these sheet-metal parts. In this case, the crossings are alternate on the upper face and lower face (alternating crossing slots  37 ,  38  in  FIG. 6 ).  
         [0050]     The longitudinally running separating plate  33  and the transverse separating plates  15 , . . . , 19  result in a total of 3×4=12 identical cavities, each with a square base area (A 1 , . . . ,A 4  in  FIG. 7 ) being formed in the filter housing  10 , four associated cavities of which are annotated, by way of example, with the reference symbols  21 , . . . , 24  in  FIG. 1 . The four associated cavities  21 , . . . , 24  which are arranged in a square form a filter F 3 . In addition to the filter F 3 , the filter housing  10  shown in  FIG. 1  has two further identical filters F 2  and F 1  which likewise each comprise four cavities arranged in a square. Each of the filters F 1 , F 2  and F 3  as shown in  FIG. 2  has an associated input  26 ,  28 ,  30 , and an output  27 ,  29 ,  31 , respectively.  
         [0051]     The four cavities of each of the filters F 1 , F 2  and F 3  are coupled to one another for radio-frequency purposes. This is achieved by means of suitably arranged, elongated coupling slots  35  in the transverse separating plates  15 ,  17 , and  19  ( FIG. 5 ) and in the longitudinally running separating plate  33  ( FIG. 6 ). The coupling slots  35  are positioned in the present example such that they are located in the center of the wall of the adjacent cavity and on the vertical center plane of the cavities to be coupled. The importance of this position for the coupling characteristics will be described in more detail later. The transverse separating plates  16  and  18  which separate the filters F 1 , F 2  and F 3  from one another are, of course, not equipped with coupling openings.  
         [0052]     A circular dielectric resonator element  44  ( FIG. 12 ) in the form of a disk is arranged in the center of each of the cavities  21 , . . . , 24  formed in the filter housing  10  and governs the overall radio-frequency and transmission characteristic of the individual cavity and of the respective filter. The dielectric resonator element  44  is part of a compact tuning unit  40  that is associated with each cavity ( FIGS. 8-10 ). The tuning unit  40  is screwed onto the robust motor mounting plate  13  from above and has a fixed holder  46  ( FIG. 10 ), to whose end the dielectric resonator element  44  is attached, which projects through a (circular) opening  25  ( FIG. 1 ), which is associated with the cavity, into the cavity located underneath.  
         [0053]     The dielectric resonator element  44  has a central circular through-hole  58  and an eccentrically arranged circular cutout  59  ( FIG. 12 ). A dielectric body  45  ( FIG. 13 ) of the same thickness is mounted in the eccentric cutout  59  such that it can rotate about a rotation axis  60  that is at right angles to the disk plane of the dielectric resonator element  44 . The cutout  59  is in the form of a circular-cylindrical through-hole that is concentric about the rotation axis  60 . The external dimensions of the dielectric body  45  are matched to the cutout  59  in such a way that the two are separated from one another only by narrow air gaps. For this purpose, the dielectric body  45  is bounded in a first direction (which is at right angles to the rotation axis  60 ) by two parallel, planar surfaces  61 ,  62 , and is bounded in a second direction (which is at right angles to the rotation axis  60  and to the first direction) by two cylindrical envelope surfaces  63 ,  64 , which are concentric about the rotation axis  60  (see  FIG. 13 ; the body  45  inserted into the cutout can be seen in  FIG. 9 ).  
         [0054]     The dielectric body  45  is preferably formed from the same dielectric material as the dielectric resonator element  44 . It is attached to the end of a holder  47  which is mounted such that it can rotate, and can be rotated by means of the mechanism that is accommodated in the tuning unit  40  relative to the dielectric resonator element  44 , about the rotation axis  60 . The rotation allows the resonant frequency of the resonator element, and thus the mid-frequency of the filter, to be varied.  
         [0055]     The tuning unit  40  ( FIGS. 8-10 ) essentially comprises a gearbox unit  42  and a motor  41 , which is flange-connected to the gearbox unit  42  at the side and drives the holder  47  (which can rotate) via the gearbox unit  42 . The motor  41  is preferably a stepping motor. As can be seen from  FIG. 10 , the gearbox unit  42  has a housing  43  on whose lower face the holder  46  for the stationary dielectric resonator element  44  is mounted. A rotating element  49  in the form of a shaft is mounted by means of a precision bearing  48  such that it can rotate in a through-hole which passes through the base of the housing  43  at right angles, and this rotating element  49  is firmly connected to the holder  47  that can rotate. By way of example, a special bearing which can be prestressed, is provided with two ball bearings and is used in hard disk memories of PCs is used as the precision bearing  48 . Bearings such as these can be obtained, for example, using the name “RO bearing” (after the inventor Rikuro Obara from the Japanese Company Minebea Co, Ltd. Their principle is described inter alia, in U.S. Pat. No. 5,556,209. The precision bearing  48  contributes to the achievement of a positioning accuracy of the dielectric body  45  in the region of a few micrometers, as is required for accurate tuning of the filters F 1 , F 2  and F 3 .  
         [0056]     A gearwheel  51  in the form of a circle sector is mounted on the rotating element  49 , as shown in  FIG. 11 . Since the full tuning range of the configuration shown in  FIG. 9  and comprises the dielectric resonator element  44  and the dielectric body  45  can be covered by rotation of the body through 90° from the position shown in  FIG. 9 , a sector angle of 100° is more than adequate for the gearwheel  51 . Designing the gearwheel  51  to be in the form of a circle sector means that the gearbox unit  42  and thus the tuning unit  40  can be designed to be extremely compact.  
         [0057]     The worm gear on a driveshaft  55 , which is at right angles to the rotation axis  60  and is connected directly to the motor  41 , engages with the gearwheel  51 . In order to ensure that there is no play in the engagement between the worm gear and the gearwheel  51 , the rotating element is prestressed in the rotation direction by means of a spiral spring  50 , which is mounted on the housing  43 . Two light barriers  52  and  53  are provided in the gearbox unit  42 , in order to control the drive unit  40 . The first light barrier  52  scans a marking element (not shown in  FIG. 10 ) which is in the form of a rod, is seated in an appropriate mounting hole  56 ,  57  in the gearwheel  51  ( FIG. 11 ) and marks the end points of the pivoting range. The second light barrier  53  scans a position sensor disk  54 , which is seated on the driveshaft  55  and is provided with a radial slot. The interaction of the two light barriers allows the initial or zero position of the gearwheel  51 , and thus the initial position of the dielectric body  45 , to be determined precisely.  
         [0058]     As already mentioned further above, the four cavities  21 , . . . , 24  with the dielectric resonator elements  44  and bodies  45  placed centrally in them are arranged in a square in each of the filters F 1 , . . . ,F 3 . This is illustrated once again in  FIG. 14  on the basis of the example of the filter F 3 . The RF energy is injected into the first cavity  21 , propagates by means of the coupling slots  35  via the adjacent cavities  22 ,  23  and  24 , and is emitted again from the last cavity  24 . The coupling slots  35  are located on the vertical center planes or in the center of separating walls of the cavities  21 , . . . , 24 . The dielectric resonator elements  44  are rotated together with their eccentric cutouts  59  from the vertical center plane of the coupling slot  35  that is located closest to the cutout through a predetermined angle which, in the example, is about 57°. This particular configuration of the cutout and coupling slot results in the filter having a radio-frequency response in which the coupling factor decreases as the frequency increases, when the dielectric body  45  is rotated toward the next coupling slot. An additional degree of freedom is provided by the capability for additional coupling between the first cavity  21  and the last cavity  24 , as is indicated by the S-shaped coupling element in  FIG. 14 .  
         [0059]     Another configuration of a filter F′ by means of which—apart from the transverse coupling—the same effect can be achieved is for the cavities  21 , . . . , 24  to be arranged in a few, as shown in  FIG. 15 . In this case as well, the coupling slots  35  are arranged centrally, and the dielectric resonator elements  44  are rotated, together with their cutouts, through about 60° from the center plane.  
         [0060]     A control system is provided for tuning of the filter arrangement by means of the tuning elements  40 , and a highly simplified block diagram of this control system is illustrated in  FIG. 16 . The controller  65  has a control block  66  which, for example, has a suitable microprocessor and a number of power outputs corresponding to the number of motors  41 . The control block  66  controls the stepping motors  45  via the power outputs, and is activated from the outside via an input unit  68 . The control block  66  interacts with a memory (EPROM)  67 , in which value tables are stored, which associate a specific step number of the stepping motors  41  with a number of selected frequency values of the filter. Intermediate values are produced by interpolation. Furthermore, the control block  66  receives signals from the two light barriers  52 ,  53  for each tuning unit  40 . If a specific frequency for the filter or filters is intended to be set (during start-up), the dielectric bodies  45  are first of all moved back to their initial position. The reaching of the initial position is signaled by appropriate signals from the two light barriers  52 ,  53 . The stepping motors  41  are then switched forward from the initial position by the number of steps corresponding to the table value taken from the memory  67 , or to a value determined by interpolation for the desired frequency. The motors  41  for a filter may in this case all be switched largely at the same time, or may be switched following a specific algorithm.  
         [0061]     If the radio-frequency filter arrangement with the filter housing  10  according to the exemplary embodiment ( FIG. 1 ) is intended to be designed for band  4 , that is to say for a tunable frequency range from about 4.4 GHz to 5 GHz, the housing (without the tuning units) has a base area of approximately 66 mm×186 mm, and a height of approximately 30 mm. Each of the cavities has a base area (A 1 , . . . ,A 4  in  FIG. 7 ) of 28 mm×28 mm, and a height of 20 mm. The dielectric resonator element  44  has a thickness of approximately 6 mm, an external diameter of approximately 15 mm, and an internal diameter of approximately 6.5 mm. The diameter of the eccentric cutout  59  is approximately 6 mm, the width of the dielectric body  45  between the parallel vertical boundary surfaces is approximately 3 mm. The tuning unit  40  projects only approximately 24 mm beyond the surface of the motor mounting plate  13 .  
         [0062]     Characteristic curves as are shown in FIGS.  18  to  20  are obtained for a filter arrangement designed in this way:  
         [0063]      FIG. 18  shows the relationship between the tunable filter frequency and the rotation angle of the dielectric body  45  in the eccentric cutout  59  in the dielectric resonator element  44 . The rotation angle range is from 0° to 90°. At 0° the straight sides of the dielectric body  45  are tangential with respect to the dielectric resonator elements  44 .  
         [0064]      FIG. 19  shows the measured curves for a number of S parameters of the filters according to the exemplary embodiment, specifically the reflection coefficient at the input S 11  (curve B), the transmission coefficient in the forward direction, S 21  (curve A), as a function of the frequency for a selected mid-frequency of 4.7 GHz. The frequency range is in this case ±15 MHz about the respective mid-frequency. The graph is logarithmic. The scale in the vertical direction is 0.5 dB per division for S 21 , and 5 dB per division for S 11 .  
         [0065]      FIG. 20  shows the measured curve for S 21  for 4.7 GHz over an extended frequency range of ±60 MHz about the respective mid-frequency. The graph is logarithmic. In this case, the scale in the vertical direction is 10 dB per division.  
         [0066]     Overall, the invention provides a tunable radio-frequency filter arrangement which can be designed such that it is simple and costs little, can be tuned very accurately and reproducibly over a wide frequency range, is extremely space-saving, and is distinguished by very good radio-frequency characteristics. In particular, a number of identical filters can be accommodated in a common filter housing, with little additional complexity.  
       LIST OF REFERENCE SYMBOLS  
       [0000]    
       
           10  Filter housing (Filter box)  
           11  Base plate  
           12 ,  20  Wall plate (transverse)  
           13  Motor mounting plate  
           14 ,  32  Wall plate (longitudinal)  
           15 , . . . , 19  Separating plate (transverse)  
           21 , . . . , 24  Cavity  
           25  Opening (Circuit)  
           26 , 28 , 30  Input (Filters F 1 , F 2 , F 3 )  
           27 , 29 , 31  Output (Filters F 1 , F 2 , F 3 )  
           33  Separating plate (longitudinal)  
           34 ,  36 ,  37 ,  38  Crossing slot  
           39  Mounting slot  
           35  Coupling slot  
           40  Tuning unit  
           41  Motor (stepping motor)  
           42  Gearbox unit  
           43  Housing (Gearbox unit)  
           44  Dielectric resonator element (stationary)  
           45  Dielectric body (moving)  
           46  Holder (in the form of a half shell)  
           47  Holder (which can rotate)  
           48  Precision bearing  
           49  Rotating element  
           50  Spiral spring  
           51  Gearwheel (in the form of a circle segment)  
           52 ,  53  Light barrier  
           54  Position sensor disk  
           55  Drive shaft (with worm gear)  
           56 ,  57  Attachment hole (position sensor pin)  
           58  Central through-hole  
           59  Eccentric cutout  
           60  Rotation axis  
           61 , . . . , 64  Boundary surface  
           65  Controller  
           66  Control block  
           67  Memory (EPROM)  
           68  Input unit  
           69  Metal sheet  
          A 1 , . . . A 4  Surface  
          F,F 1 ,F 2 ,F 3  Filter (Bandpass filter)  
          K 1 , K 2  Curve  
          L 1 , L 2  Mounting lug