Patent Publication Number: US-6664873-B2

Title: Tunable resonator

Description:
TECHNICAL FIELD 
     The present invention relates generally to the field of filters and, in particular, to a tunable resonator for a filter. 
     BACKGROUND 
     Wireless telecommunications systems transmit signals to and from wireless terminals using radio frequency (RF) signals. A typical wireless system includes a plurality of base stations that are connected to the public switched telephone network (PSTN) via a mobile switching center (MSC). Each base station includes a number of radio transceivers that are typically associated with a transmission tower. Each base station is located so as to cover a geographic region known colloquially as a “cell.” Each base station communicates with wireless terminals, e.g. cellular telephones, pagers, and other wireless units, located in its geographic region or cell. 
     A wireless base station includes a number of modules that work together to process RF signals. These modules typically include, by way of example, mixers, amplifiers, filters, transmission lines, antennas and other appropriate circuits. One type of filter that finds increased use in wireless base stations is known as a microwave cavity filter. These cavity filters include a number of resonators formed in a plurality of cavities so as to provide a selected frequency response when signals are applied to an input of the filter. 
     Each resonator in a filter is tuned to have a selected resonant frequency. Many techniques are conventionally available for remotely tuning the resonant frequency of these filters. These techniques include electromagnetic actuators and stepper motors. Unfortunately, these techniques each have limitations and drawbacks. For example, many of the remote tuning techniques have a limited tuning range or require large movement amplitudes to gain the required tuning range. Further, many of the remote tuning techniques are not reliable. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved tunable resonator. 
     SUMMARY 
     The above-mentioned problems with tunable resonators and other problems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. Embodiments of the present invention provide a tunable resonator that is tuned by varying the size of a gap between a resonator body and a ground plane, or a portion of a ground plane, of the resonator. 
     More particularly, in one embodiment a tunable resonator is provided. The resonator includes a housing having a cavity. A resonator body is disposed adjacent to a first surface within the cavity. A gap is formed between the resonator body and the first surface. The resonator is tuned by controlling the size of the gap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a first embodiment of a tunable resonator constructed according to the teachings of the present invention. 
     FIG. 2 is a partial cross-sectional view illustrating tuning of the first embodiment. 
     FIG. 3 is a cross-sectional two of a second embodiment of a tunable resonator constructed according to the teachings of the present invention. 
     FIG. 4 is a cross-sectional view of an embodiment of a filter having tunable resonators according to the teachings of the present invention. 
     FIG. 5 is an exploded view of another embodiment of a tunable filter including a tunable x-resonator constructed according to teachings of the present invention. 
     FIG. 6 is a block diagram of an embodiment of a tunable resonator with a control loop according to the teachings of the present invention. 
     FIG. 7 is a block diagram of an embodiment of a tunable resonator according to the teachings of the present invention. 
     FIG. 8 is an exploded view of another embodiment of a tunable filter including a tunable multi-mode resonator constructed according to teachings of the present invention. 
     FIG. 9 is an exploded view of another embodiment of a tunable filter including a tunable multi-mode resonator constructed according to teachings of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. 
     Embodiments of the present invention provide improvements in tunable resonators for cavity filters. Embodiments of the present invention include a resonator body that is disposed either directly on, or very close to, a grounding structure of the resonator cavity. The resonator is tuned by varying the distance between the resonator body and the grounding structure or a part of the grounding structure. Advantageously, when the ground plane is very close to the resonator, only small variations in the distance between the resonator body and the ground plane, or part of the ground plane, are required to achieve a wide tuning range. This tuning technique is used, for example, with dielectric filters in which a dielectric block is located close to the ground plane. Examples of this kind of resonator include a Transverse Magnetic (TM) mode dielectric rod, a half cut Transverse Electric (TE) mode dielectric body, a quarter cut TE mode dielectric body, a TE mode x-resonator, any appropriate multi-mode dielectric body and a conductor loaded Hybrid mode (HE-mode) resonator body. Other resonator structures can also be used. Each of these resonator structures can be used in the embodiments shown in FIGS. 1-7 described in detail below. 
     With a resonator body mounted directly to, or in close proximity with, the conducting cavity surface, very small changes of the distance between the surface and the resonator cause significant change in the resonant frequency of the resonator. For example, it has been discovered that changing the distance from the 0 mm to 0.2 mm changes the resonant frequency over 200 MHz in some embodiments of the present invention. 
     FIG. 1 is a cross-sectional view of a first embodiment of a tunable resonator, indicated generally at  100 , constructed according to the teachings of the present invention. Tunable resonator  100  includes housing  102 . In one embodiment, housing  102  comprises a conductive, e.g., metal, shell having a cavity  103 . The resonator body  104  is disposed within housing  102  in close proximity to surface  105 . Surface  105  comprises a ground plane of resonator  100 . 
     Tunable resonator  100  includes a mechanism for adjusting the resonant frequency of tunable resonator  100 . This mechanism includes opening  106  in housing  102 . Member or shaft  108  extends through opening  106  and is coupled to resonator body  104 , e.g., a dielectric resonator body. In one embodiment, shaft  108  also extends through support  110  fastened to an exterior surface of housing  102 . The position of shaft  108  in opening  106  is controlled by any appropriate mechanical actuator, e.g., a piezoelectric actuator, piezoelectric stack, piezoelectric multilayer, piezoelectric bimorph actuator, a stepper motor, a linear motor, a solenoid, and a magnetostrictive GMM material. 
     In operation, the resonant frequency of resonator  100  is adjusted by adjusting the size of a gap between resonator body  104  and the ground surface, e.g., surface  105 . In this embodiment, this is accomplished by moving the relative position of resonator body  104  with respect to surface  105  as indicated by arrows  114 . To accomplish this, shaft  108  moves in opening  106  as indicated by arrows  112 . For example, as illustrated in FIG. 2, the resonant frequency of tunable resonator  100  is adjusted by moving resonator block  104  away from surface  105  to adjust the size of gap  116 . When gap  116  increases, the resonant frequency also increases. Conversely, when gap  116  decreases the resonant frequency also decreases. 
     FIG. 3 is a cross-sectional view of a second embodiment of a tunable resonator, indicated generally at  200 , constructed according to the teachings of the present invention. Tunable resonator  200  includes housing  202  with cavity  203 . Tunable resonator  200  further includes resonator body  204  that is disclosed on, or in close proximity to, surface  205  of housing  202 . 
     Tunable resonator  200  further includes a mechanism for adjusting the resonant frequency of tunable resonator  200 . This mechanism includes movable tuning plate  220  that moves within opening  221  of housing  202 . In one embodiment, this mechanism includes an optional flexible membrane  222  that couples movable plate  220  to housing  202  within opening  221 . In other embodiments, flexible membrane  222  is omitted and movable plate  220  is fitted to move within opening  221 . In one embodiment, movable plate  220  and flexible membrane  222  comprise conductive material that are electrically connected to housing  202 . In one embodiment, movable plate  220  and flexible membrane  222  are formed from the material of housing  202  using an appropriate machining process. In other embodiments, movable plate  220  and flexible membrane  222  are formed by forging, impact extrusion or from separate pieces that are joined together. 
     Movable plate  220  is separated from dielectric body  204  by gap  223 . Movement of movable plate  220  adjusts the size of gap  223  and thereby adjusts the resonant frequency of tunable resonator  200 . 
     Movement of movable plate  220  is controlled by actuation device  224 . Actuation device  224  comprises one of a number of mechanical/electrical mechanisms for moving plate  220  within opening  221 . For example, actuation device  224  comprises one of a piezoelectric actuator, piezoelectric stack, piezoelectric multilayer, piezoelectric bimorph actuator, a stepper motor, a linear motor, a solenoid, and a giant magnetostrictive material (GMM). In other embodiments, other appropriate to mechanical/electrical devices are used to control the position of movable plate  220 . It is noted that when a piezoelectric actuator is used, in some embodiments, the actuator itself acts as the movable plate. 
     FIG. 4 is a cross-sectional view of an embodiment of a filter, indicated generally at  300 , having tunable resonators  330  and  340  according to the teachings of the present invention. For sake of clarity, only the tuning mechanism for filter  300  is shown. Mechanisms for coupling signals between resonators to implement the filter have been omitted from the figure, but would be included in an implementation. 
     Filter  300  includes first and second tunable resonators  330  and  340 , respectively. In this embodiment, tunable resonators  330  and  340  are disposed back to back to allow the two tunable resonators to share actuator  324  for simultaneously tuning resonators  330  and  340 . 
     Resonator  330  includes conductive, e.g., metal, housing  302 . Housing  302  forms cavity  303 . Dielectric body  304  is disposed on, or in close proximity to, surface  305  of housing  302 . Resonator  330  also includes a mechanism for tuning resonator  330 . This mechanism includes movable plate  320  that is disposed within opening  321  of housing  302 . In one embodiment, this mechanism further includes flexible membrane  322  that is coupled to housing  302  in opening  321  to allow movement of movable plate  320  and to provide contact with housing  302 . In one embodiment, membrane  322  and movable plate  320  are formed from material of housing  302  by an appropriate machining process. 
     Similarly, resonator  340  includes conductive, e.g., metal, housing  402 . Housing  402  forms cavity  403 . Dielectric body  404  is disposed on, or in close proximity to, surface  405  of housing  402 . Resonator  340  also includes a mechanism for tuning resonator  340 . This mechanism includes movable plate  420  that is disposed within opening  421  of housing  402 . In one embodiment, this mechanism further includes flexible membrane  422  that is coupled to housing  402  in opening  421  to allow movement of movable plate  420  and to provide contact with housing  402 . In one embodiment, membrane  422  and movable plate  420  are formed from material of housing  402  by an appropriate machining process. 
     Resonators  330  and  340  share actuation device  324 . Actuation device  324  is provided in contact with movable plates  322  and  422 . Actuation device  324  controls the size of gap  323  of resonator  330  and gap  423  of resonator  340 . Thus, actuation device  324  controls the resonant frequency of both resonators. In one embodiment, actuation device  324  provides similar displacement to both movable plates at the same time. For example, actuation device  324  simultaneously provides a force on movable plates  320  and  420  to move movable plates  320  and  420  toward their respective resonator bodies, e.g., bodies  304  and  404 , or a force that moves plates  320  and  420  away from their respective resonator bodies. Advantageously, this reduces the number of parts necessary to control the frequency of filter  300 . 
     FIG. 5 is an exploded view of another embodiment of a tunable filter, indicated at  500 , including an x-resonator constructed according to teachings of the present invention. In this embodiment, filter  500  includes conductive, e.g., metal, housing  502  that forms cavity  503 . Resonator body  504 , e.g., a cross shaped dielectric body, is disposed on, or in close proximity to, surface  505  of housing  502  as indicated by outline  511 . 
     Filter  500  includes a mechanism for tuning of the resonant frequency and the coupling between modes for filter  500 . In this embodiment, this mechanism includes a plurality of openings  521  in surface  505  of housing  502 . In one embodiment, these openings are positioned under members  530 ,  531 ,  532  and  533  of resonator body  504  as shown in FIG.  5 . In other embodiments, openings  521  are provided in other orientations to allow an appropriate level of tuning for a given application. In one embodiment, an additional opening  523  is provided below mode coupling member  507 . This allows for tuning of the mode coupling in a multimode resonator. In the embodiment of FIG. 5, only a single mode coupling member  507  is shown. It is understood that in other embodiments any appropriate number of mode coupling members  507  are incorporated with resonator body  504 . 
     The tuning mechanism further includes a plurality of movable plates  522  with one movable plate provided for each opening in surface  505  of housing  502 . In one embodiment, the movable plates each include a flexible membrane. In one embodiment, the movable plates  522  are formed from the material of housing  502 . It is noted that the distance or gap between the movable plates  522  and resonator body  504  and mode coupling member  507  controls resonant frequencies and mode coupling, respectively. 
     Finally, the tuning mechanism includes actuation device  524 . In one embodiment, actuation device  524  comprises a single actuation device for a plurality of movable plates  522  as shown in FIG.  5 . In other embodiments, separate control for one or more of the movable plates is achieved by providing more than one, independent actuation device. 
     In operation, filter  500  provides an adjustable filter function. The filter function is adjusted by controlling the resonant frequencies provided by the resonator body. In this embodiment, the resonator body is a multimode resonator body with first and second modes that are coupled through mode coupling member  507 . The resonant frequency of each of the modes and the mode coupling is controlled by adjusting the relative position of movable plates  522  within openings  521  of housing  502 . As with the embodiments described above, movable plates  522  below resonator body  504  affect the resonant frequency of resonator  500  proportionate with the change in a gap between the respective plate and resonator body  504 . For example, when the gap increases, the resonant frequency increases and when the gap decreases the resonant frequency also decreases. With respect movement of plates  522  relative to coupling member  507 , the affect Varies based on the placement and number of coupling members. For example, when two coupling members  507  are located on adjacent corners of dielectric body  504 , movement of plate  522  toward a first coupling member increases coupling and movement of plate  522  toward the second coupling member decreases the coupling. 
     FIG. 6 is a block diagram of an embodiment of a tunable resonator, indicated generally at  600 , with a control loop according to the teachings of the present invention. Resonator  600  includes cavity resonator  602  that has a resonant frequency that is adjusted by controlling the distance between a resonator body and an interior surface of the cavity. For example, resonator  602 , in one embodiment, comprises one of resonators or filters shown and described above with respect to FIGS. 1-5. 
     Resonator  600  further includes a control loop with monitor  604  and actuator  606 . Monitor  604  is coupled to an output of cavity resonator  602 . Monitor  604  is further coupled to control actuator  606 . Actuator  606  is coupled to control the resonant frequency of resonator  602 . 
     In operation, resonator  600  uses automatic feedback control to control the resonant frequency of resonator  602 . Resonator  602  processes signals received at its input. At the output of resonator  602 , monitor  604  monitors the output power and determines whether adjustments need to be made to the resonant frequency. If adjustments are required, monitor  604  provides control signals to actuator  606  to move the position of the resonator body of resonator  602 . 
     FIG. 7 is a block diagram of an embodiment of a tunable resonator, indicated generally at  700 , according to the teachings of the present invention. Resonator  700  includes cavity resonator  702 . Cavity resonator  702  has a resonant frequency that is adjusted by controlling the distance between a resonator body and an interior surface of the cavity of cavity resonator  702 . For example, cavity resonator  702 , in one embodiment, comprises one of the resonators or filters shown and described above with respect to FIGS. 1-5. 
     Resonator  700  includes a mechanism to select the resonant frequency of the resonator. This mechanism includes controller  704 , e.g., a processor, logic circuit or other circuit that is capable of providing a control signal to adjust the resonant frequency of resonator  700 . Controller  704  is coupled to input  708  and memory  710 . Memory  710  comprises a circuit such as a memory device or other circuit that stores control values for setting the resonant frequency of resonator  700 . Controller  704  is further coupled to actuator  706 . Actuator  706  is coupled to selectively adjust a gap between a resonator body and a ground plane of cavity resonator  702  that sets the resonant frequency of resonator  700 . 
     In operation, the resonant frequency of resonator  700  is established based on an input received at input  708 . Based on the input, controller  704  selects an appropriate control signal from memory  710 . This control signal is applied to actuator  706 . Actuator  706  uses the control signal to establish the size of a gap in cavity resonator  702  to control the resonant frequency of resonator  700 . 
     Advantageously, resonator  700  can be preset with values stored in memory  710  for resonant frequencies for a plurality of service bands. Based on the pre-set values, an end user can configure the resonator as a filter for a specific service operating in one of the bands, e.g., analog AMPS, digital, PCS, GSM, or other appropriate cellular or PCS service. 
     FIG. 8 is an exploded view of another embodiment of a tunable filter, indicated at  800 , including a multi-mode resonator constructed according to teachings of the present invention. In this embodiment, filter  800  includes conductive, e.g., metal, housing  802  that forms cavity  803 . Resonator body  804 , e.g., a dielectric body, is disposed on, or in close proximity to, surface  805  of housing  802  as indicated by outline  811 . Resonator body  804  is shown as a round body. However, in other embodiments, resonator body  804  comprises any other appropriate multimode resonator body. 
     Filter  800  includes a mechanism for tuning of the resonant frequency of the various modes of filter  800 . In this embodiment, this mechanism includes a plurality of openings  821  in surface  805  of housing  802 . In one embodiment, these openings are positioned under selected portions of resonator body  804  as shown in FIG.  8 . In other embodiments, openings  821  are provided in other orientations to allow an appropriate level of tuning for a given application. 
     The tuning mechanism further includes a plurality of movable plates  822  with one movable plate provided for each opening in surface  805  of housing  802 . In one embodiment, the movable plates each include a flexible membrane. In one embodiment, the movable plates  822  are formed from the material of housing  802 . It is noted that the distance or gap between the movable plates  822  and resonator body  804  controls the resonant frequencies of the various modes. 
     Finally, the tuning mechanism includes actuation device  824 . In one embodiment, actuation device  824  comprises a single actuation device for a plurality of movable plates  822  as shown in FIG.  8 . In other embodiments, separate control for one or more of the movable plates is achieved by providing more than one, independent actuation device. 
     In operation, filter  800  provides an adjustable filter function. The filter function is adjusted by controlling the resonant frequencies provided by the resonator body. In this embodiment, the resonator body is a multimode resonator body. The resonant frequency of each of the modes is controlled by adjusting the relative position of movable plates  822  within openings  821  of housing  802 . As with the embodiments described above, movable plates  822  below resonator body  804  affect the resonant frequency of resonator  800  proportionate with the change in a gap between the respective plate and resonator body  804 . For example, when the gap increases, the resonant frequency increases and when the gap decreases the resonant frequency also decreases. 
     FIG. 9 is an exploded view of another embodiment of a tunable filter, indicated at  900 , including an x-resonator constructed according to teachings of the present invention. In this embodiment, filter  900  includes conductive, e.g., metal, housing  902  that forms cavity  903 . Resonator body  904 , e.g., a cross shaped dielectric body with rounded top surface  950 , is disposed on, or in close proximity to, surface  905  of housing  902  as indicated by outline  911 : 
     Filter  900  includes a mechanism for tuning of the resonant frequency of the various modes for filter  900 . It is noted that in other embodiments, mode coupling mechanisms are also included, such as those shown in FIG. 5 above. In this embodiment, the frequency tuning mechanism includes a plurality of openings  921  in surface  905  of housing  902 . In one embodiment, these openings are positioned under members  930 ,  931 ,  932  and  933  of resonator body  904  as shown in FIG.  9 . In other embodiments, openings  921  are provided in other orientations to allow an appropriate level of tuning for a given application. 
     The tuning mechanism further includes a plurality of movable plates  922  with one movable plate provided for each opening in surface  905  of housing  902 . In one embodiment, the movable plates each include a flexible membrane. In one embodiment, the movable plates  922  are formed from the material of housing  902 . It is noted that the distance or gap between the movable plates  922  and resonator body  904  controls the resonant frequencies. 
     Finally, the tuning mechanism includes actuation device  924 . In one embodiment, actuation device  924  comprises a single actuation device for a plurality of movable plates  922  as shown in FIG.  9 . In other embodiments, separate control for one or more of the movable plates is achieved by providing more than one, independent actuation device. 
     In operation, filter  900  provides an adjustable filter function. The filter function is adjusted by controlling the resonant frequencies provided by the resonator body. In this embodiment, the resonator body is a multimode resonator body. The resonant frequency of each of the modes is controlled by adjusting the relative position of movable plates  922  within openings  921  of housing  902 . As with the embodiments described above, movable plates  922  below resonator body  904  affect the resonant frequency of resonator  900  proportionate with the change in a gap between the respective plate and resonator body  904 . For example, when the gap increases, the resonant frequency increases and when the gap decreases the resonant frequency also decreases. 
     Although specific embodiments have been illustrated and described in this specification, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention.