Abstract:
The invention relates to a tunable λ/4-filter subassembly whose frequency response can be adjusted. For this purpose, the λ/4-filter subassembly has a signal-conducting electrical conductor and an electrically conductive element that is at a reference potential. Furthermore, at least one short-circuit device is provided that electrically contacts the electrical conductor. An electrical coupling device is provided that couples the short-circuit device at a settable contact position to the electrically conductive element, wherein the electrical coupling device and the short-circuit device are movable relative to each other in order to set the length of the short-circuit device.

Description:
FIELD OF THE INVENTION 
     The invention relates to a tunable λ/4 filter subassembly with a signal conducting electrical conductor and an electrically conductive element which is at a reference potential and that may be in particular, a housing connected to ground. 
     BACKGROUND OF THE INVENTION 
     λ/4-filters are used, for example, in high frequency technology as bandpass filters. Such filters have, for example, a short-circuit line of fixed length between a signal-conducting conductor and a ground. The bandwidth of the filter depends on the length of the short-circuit conductor, which determines the center frequency of the filter according to the equation f 0 =c/λ. The bandpass character of such a filter is obtained from the special wave characteristic at high frequencies, since, at high frequencies, the short-circuit at the center frequency f 0  is blocked, so that signals with frequencies that are distributed over a certain bandwidth about the center frequency can pass through the conductor. 
     Such filters are also used for surge protection, for example, in mobile telephony. 
     Thus, for example in DE 9422 171 U1 describes a surge voltage protection plug connector for a coaxial cable having an inner and an outer conductor, where a spiral-shaped short-circuiting stub of fixed length short circuits the inner conductor with the outer conductor. 
     From U.S. Pat. No. 6,061,223, for example, a surge voltage protection device is known, in which an inner conductor is arranged in a housing, which together form a coaxial line. A spiral-shaped conductor of fixed length is used as a short-circuit element to transfer the energy during a surge voltage event to a ground connection. 
     One drawback of the mentioned filter is that, due to the fixed short-circuit length, the center frequency is determined in a fixed way. A different filter therefore must be used for each different frequency behavior. 
     SUMMARY OF THE INVENTION 
     The present invention is therefore based on the problem of providing a λ/4-filter subassembly, which is universally usable for a variable frequency range. 
     The main idea of the invention is to provide a tunable λ/4-filter, whose frequency response is modifiable by setting the length of a short-circuit device. 
     The technical problem is solved by a tunable λ/4-filter component, which presents a signal conducting electrical conductor and an electrically conductive element at a reference potential. In addition, at least one short-circuit device is provided that establishes electrical contact with the electrical conductor. Furthermore, an electrical coupling device is provided, that couples the short-circuit device at a settable contact position to the electrically conductive element, where the electrical coupling device and the short-circuit device are relatively movable in order thereby to set the length of the short-circuit device. 
     The length of the short-circuit device is preferably continuously adjustable over a predeterminable length range, so that the center frequency of the tunable λ/4-filter subassembly also continuously adjustable. In this way, a λ/4-filter with variable bandpass behavior can be realized. 
     A short-circuit device advantageously has a first contact section, which is connected to the electrical conductor, as well as a second contact section that is connected to the first contact section and with which the electrical coupling device engages electrically. 
     A compact construction with a wide setting range is obtained if the second section of the short-circuit device forms an essentially circular conductor element, which is arranged essentially concentrically to and at a distance r from the electrical conductor, and whose circumference is less than 2πr. 
     The electrical element is preferably an electrically conductive housing, which at least partially surrounds the conductor and the at least one short-circuit device. The housing is preferably cylindrical in design. 
     To be able to set the length of the short-circuit device, the coupling installation presents a first rotatable, electrically-conducting setting element with a contact element. The contact element engages at a settable contact position on the short-circuit device. For this purpose, a slot of predetermined length is provided in the housing, which allows the contact element to be shifted with the aid of the setting element along the short-circuit device. The setting element is applied against the housing and is thus at the potential of the housing. 
     To be able to increase the bandwidth of the λ/4-filter subassembly, at least one additional short-circuit device is provided, which is axially offset with respect to the longitudinal axis of the electrical conductor opposite the first short-circuit device. It presents a first contact section, which is connected to the electrical conductor, as well as a second contact section that is connected to the first contact section and with which the coupling device engages electrically. In this way, the λ/4-filter subassembly has two center frequencies, which increase its bandwidth. 
     To obtain a compact construction, the second contact section of the additional short-circuit device as well forms a circular conductor element, which is arranged essentially concentrically to and at a distance r from the electrical conductor, and whose circumference is again less than 2πr. The electrical element is again designed as a conductive housing that at least partially surrounds the conductor and the short-circuit devices. 
     In this case, the coupling device presents a second rotatable electrically-conductive setting element with a contact element. The contact element engages at a settable contact position on the second short-circuit device, where, in the housing, a slot is provided to shift the contact element. The setting element is again located outside of the housing, so that the setting element and the housing are at the same potential. If the housing has a circular cross section, the setting elements are annular in design. 
     According to an embodiment, the two setting elements can be connected to each other. However, to be able to adjust the length of the two short-circuit devices individually, the two setting elements are movable independently of each other. 
     To be able to change the bandwidth of the λ/4-filter, an additional short-circuit device located in the same plane as the first short-circuit device is provided according to an alternative embodiment. The additional short-circuit device presents a first contact section, which is connected to the electrical conductor, and a second contact section that is connected to the first contact section and that the electrical coupling device engages electrically. The second contact sections of the two short-circuit devices form in each case a conductor element in the shape of a segment of a circle, which in each case is arranged essentially at a distance r about the electrical conductor, where both second contact sections are arranged essentially diametrically opposite to each other. 
     The circumference of each second contact section is less than πr. 
     In this embodiment example, the electrical element can again be a conductive housing, which at least partially surrounds the conductor and the short-circuit device. 
     In the mentioned example, the coupling device can present a rotatable electrically conducting setting element with two contact elements, where one contact element engages at a settable contact position on the second contact section of the first short-circuit device, and the other contact element engages at a settable contact position on the second contact section of the other short-circuit device. A slot is advantageously provided in the housing for shifting the contact elements with the aid of the setting element, where the setting element is in electrical contact with the housing and thus at the housing potential. 
     A more flexible and better performing coupling device presents a first and a second rotatable electrically conductive setting element, each with a contact element, where the setting elements are movable independently of each other. The contact element of the first setting element engages at a settable contact position on the second contact section of the first short-circuit device, while the contact element of the second setting element engages at a settable contact position on the second contact section of the other short-circuit device. Again, at least one slot can be provided in the housing to move the contact elements by means of the setting element, where the setting elements are applied against the housing, so that the setting elements and the housing are at the same potential. 
     In a preferred embodiment, the contact elements are designed as sliding contacts. 
     Instead of using a movable design for the coupling device, and connecting the short-circuit device firmly to the conductor, it is conceivable to use a stationary design of the coupling device, and to attach the at least one short-circuit device in a movable way. In a possible embodiment variant, the short-circuit devices can be connected in a freely movable way to the electrical conductor, where an electrical connection must be guaranteed. Furthermore, the short-circuit device may be connected to a dielectric setting element, which is applied in a movable way to the housing. 
     In an additional embodiment, it is conceivable not to design the coupling device so that it is rotatable about the conductor. Instead, the coupling device could be designed so that it is movable axially with respect to the conductor, where, in this case, the at least one short-circuit device runs at least section-wise parallel to the conductor, and the coupling device engages with the short-circuit device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in greater detail below with reference to two embodiment examples in connection with the drawings in the appendix. 
       In the drawings: 
         FIG. 1  shows a tunable λ/4-filter subassembly according to the invention in longitudinal cross section, 
         FIG. 2  shows a cross section of the λ/4-filter subassembly represented in  FIG. 1 , 
         FIG. 3  shows a side view of the λ/4-filter subassembly shown in  FIG. 1 , 
         FIG. 4  shows an alternative embodiment of a tunable λ/4-filter subassembly according to the invention, 
         FIG. 5  shows a cross section of the λ/4-filter subassembly shown in  FIG. 4 , and 
         FIG. 6  shows the curve plots for the VSWR for a filter subassembly with one short-circuit device and with two short-circuit devices, respectively. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows, as an example, a λ/4-filter component  10 , whose center frequency and thus whose bandwidth is settable with the aid of two short-circuit devices  40  and  50  of variable length. The λ/4-filter subassembly  10 , which is represented schematically in longitudinal cross section, presents an inner signal-conducting conductor  20 , which has connections  25  at both of its ends. The electrical conductor  20  is held preferably in a central position in an electrically conductive housing  30 . In this example, the housing  30  has a circular cross section, where the diameter in the vicinity of the two line ends is less than in the middle area of the electrical conductor  20 . The two short-circuit devices  40  and  50 , for example, are soldered to the electrical conductor  20 , whose design is represented in greater detail in  FIG. 2 . In order to vary the center frequency, a coupling device  70  is provided that has two electrically conductive rings  73  and  75  as setting elements. The electrical housing can have two parallel grooves in which the rings are guided in a rotatable way. A sliding contact  72  is shaped on the ring  73 , which is in electrical contact with the short-circuit device  40 , while on the ring  75 , a sliding contact  77  is formed that is in electrical connection with the short-circuit device  50 . Slots are provided in the housing  30 , so that the sliding contacts can be moved along the given short-circuit device by means of the rings. 
       FIG. 2  shows a cross section of the filter subassembly  10  and particularly of electrically conductive housing  30 , which at least partially surrounds the electrical conductor  20  and the two short-circuit devices  40  and  50 . The short-circuit device  40  has a first wire-like contact section  40   a,  whose end is electrically connected at a predetermined location to the electrical conductor  20 . The wire-like contact section  40   a  extends perpendicularly away from the electrical conductor  20  a contact section  40   b  in the shape of a segment of a circle is formed and has a length r. on the outside end of the contact section  40   a.  The electrical conductor  20  forms the mid-point of the contact section  40   b  in the shape of a segment of a circle, where the radius of curvature of the contact section  40   b  essentially corresponds to the length r. The sliding contact  72  of the annular setting element  73  engages at a contact position  80  on the contact section  40   b  in the shape of a segment of a circle. In the case of one rotation of the annular setting element  73 , the sliding contact  72  moves along the contact section  40   b  in the shape of a segment of a circle, which results in the possibility of continuously changing the contact position  80  and thus the length of the short-circuit device  40 . The short-circuit connection extends from the conductor  20 , via the contact sections  40   a  and  40   b,  the contact position  80 , the sliding contact  72 , and the setting element  73 , to the electrically conductive housing  30 . 
     The second short-circuit device  50  represented in  FIG. 2  has a structure similar to the short-circuit device  40 . A wire-like contact section  50   a  is electrically connected to the electrical conductor  20  at a location, so that both short-circuit devices  40  and  50  lie in the same plane. The wire-like contact section  50   a  again extends perpendicularly away from the electrical conductor  20 . A contact section  50   b  in the shape of a segment of a circle is formed on its outside end. The wire-like contact section  50   a  may, if necessary, have the same length r as the wire-like contact section  40   a.  As shown in  FIG. 2 , both contact sections  40   b  and  50   b  in the shape of a segment of a circle have a circumference which is less [than] πr. In this way, a setting angle of less than 180° is obtained for the setting elements  73  and  75 . In the examples shown, the setting angle can be, for example, 150°. 
     The sliding contact  77  of the setting element  75  engages at variable changeable contact position  85  on the contact section  50   b  in the shape of a segment of a circle, so that the length of the short-circuit device  50  can be set. 
     It should be noted here that the contact sections  40   b  and  50   b  in the shape of a segment of a circle, represented in  FIG. 2 , may also be designed as linear contact sections that are arranged parallel to the conductor  20 . In this case, the setting elements  73  and  75  are not designed as rotatable rings, but as setting elements that can be shifted along the electrical conductor, where in this case, the sliding contacts  72  and  75  engage at the longitudinally shaped contact sections, that run parallel to the electrical conductor. 
       FIG. 3  shows a top view of the filter subassembly  10  shown in  FIG. 1 , with the two annular setting elements  73  and  75 , where markings or locating points can be provided on the housing  30  to allow the setting of predefined bandwidths for the λ/4-filter subassembly  10 . 
       FIG. 4  shows an alternative λ/4-filter subassembly  100 , in which two short-circuit devices  140  and  150  are arranged with axial offset with respect to an electrical conductor  120 , and thus form a parallel circuit. An electrical housing  130  is at least partially arranged about the electrical conductor  120  and the short-circuit devices  140  and  150 , and has a shape similar to that of the electrical housing  30  according to  FIG. 1 . A coupling device  170  is provided, which, with respect to the conductor  120 , comprises two mutually axially offset annular setting elements  173  and  125 . The annular setting element  173  has a sliding contact  172 , which engages with the short-circuit device  140  to change its length, while a sliding contact  177  of the annular setting element  175  engages with the short-circuit device  150 . 
       FIG. 5  is a schematic representation of the cross section of the filter subassembly  100  shown in  FIG. 4 . The electrical housing  130  with circular cross section surrounds the electrical conductor  120  as well as the short-circuit devices  140  and  150 , which is electrically connected to the conductor  130 . In  FIG. 5 , only the short-circuit device  140  is shown. The short-circuit device  150  has a shape similar to that of the short-circuit device  140  and is covered by the latter in the representation shown. The short-circuit device  140  has a wire-like contact section  140   a,  which is soldered, for example, at the contact position of the electrical conductor  120 . The wire-like contact section  140   a  extends perpendicularly away from the electrical conductor  120 . The length of the section  140   a  is r. A circular contact section  140   b  is connected to the outside end of the wire-like contact section  140   a  and runs essentially concentrically to and at a distance r from the electrical conductor  120 . The circumference of the circular contact section  140   b  is less than 2πr, so that the resulting setting angle for the setting element  173  is less than 360° but greater than the setting angle of the setting elements shown in  FIGS. 1 and 2 . 
     As shown in  FIG. 5 , the sliding contact  172  of the setting element  173  engages at a contact position  180  on the circular contact section  140   b,  resulting in the formation of an electrical connection between the electrical conductor  120  and the housing  130 , which is formed via the contact section  140   a,  the circular contact section  140   b,  the sliding contact  172  of the electrically conducting setting element  173 , and the housing  130 . At an axial separation with respect to the latter, the setting element  175  and the short-circuit device  150  are arranged. The setting element  173  and the setting element  175  can be moved independently of each other, so that the bandwidth of the λ/4-filter subassembly  10  can be varied and set. 
       FIG. 6  shows the curve plots of the VSWR (Voltage Standing Wave Ratio) vs frequency for a λ/4-filter with a single short-circuit device and for the λ/4-filter subassembly shown in  FIG. 4  with a parallel circuit of two short-circuit devices. It is evident that the bandwidth of a λ/4-filter subassembly, as shown in  FIG. 4 , presents a greater bandwidth than a λ/4-filter subassembly that uses only a single short-circuit device. The reason for this is that the two short-circuit devices  140  and  150  in each case establish a center frequency for the λ/4-filter, as can be seen in  FIG. 6  at the locations labeled  1  and  2 .