Patent Publication Number: US-2003227350-A1

Title: Dual operation mode all temperature filter using superconducting resonators

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates generally to filters, and, more particularly, to a dual operation mode all temperature filter using superconducting resonators.  
       BACKGROUND OF THE INVENTION  
       [0002] Radio Frequency (RF) filters have been used with cellular base stations and other telecommunications equipment for some time. Such filters are conventionally used to filter out noise and other unwanted signals. For example, bandpass filters are conventionally used to filter out or block radio frequency signals in all but one or more predefined band(s). By way of another example, notch filters are conventionally used to block signals in a predefined radio frequency band.  
       [0003] The relatively recent advancements in superconducting technology have given rise to a new type of RF filter, namely, the high temperature superconducting (HTSC) filter. HTSC filters contain components which are superconductors at or above the liquid nitrogen temperature of 77K. Such filters provide greatly enhanced performance in terms of both sensitivity (the ability to select signals) and selectability (the ability to distinguish desired signals from undesirable noise and other traffic) as compared to conventional filters. However, since known high temperature superconducting (HTSC) materials are only superconductive at relatively low temperatures (e.g., approximately 90K or lower), and are relatively poor conductors at ambient temperatures, such superconducting filters require accompanying cooling systems to ensure the filters are maintained at the proper temperature during use. As a result, the reliability of traditional superconducting filters has been tied to the reliability of the power source. Specifically, if the power source (e.g., a commercial power distribution system) fails (e.g., a black out, a brown out, etc.) for any substantial length of time, the cooling system would likewise fail and, when the corresponding superconducting filters warm sufficiently to prevent superconducting, so too would the filters.  
       [0004] To prevent systems serviced by such filters from failing during these power outages, additional circuitry in the form of RF bypass circuitry was often needed to switch out the failed filter until a suitably cooled environment was returned. Such bypass circuitry added expense and complexity to known systems.  
       SUMMARY OF THE INVENTION  
       [0005] In accordance with an aspect of the invention, a filter is provided. The filter includes a housing defining at least two cavities, an input port, and an output port. It also includes a first non-superconducting resonator disposed in a first one of the cavities; and a first superconducting, resonator disposed in a second one of the cavities.  
       [0006] Preferably, the superconducting resonator comprises a superconducting material including 8-15% silver bu weight.  
       [0007] In some embodiments, the filter is further provided with a second superconducting resonator disposed in a third cavity and a second non-superconducting resonator disposed in a fourth cavity. In such embodiments, the first cavity may optionally define an input cavity and the fourth cavity may optionally define an output cavity.  
       [0008] In accordance with another aspect of the invention, a combination comprising a dual operation mode filter and a conventional filter cascaded with the dual operation mode filter is provided. The dual operation mode filter provides a first level of filtering at temperatures below a threshold temperature and a second level of filtering at temperatures above the threshold temperature. The first level is higher than the second level.  
       [0009] In some embodiments, a low noise amplifier is coupled between the dual operation mode filter and the conventional filter. In other embodiments, an isolator is coupled between the dual operation mode filter and the conventional filter.  
       [0010] In some embodiments, the dual operation mode filter comprises a bandpass filter.  
       [0011] Other features and advantages are inherent in the apparatus claimed and disclosed or will become apparent to those skilled in the art from the following detailed description and its accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012]FIG. 1 is a schematic illustration of a dual operation mode all temperature filter constructed in accordance with the teachings of the instant invention.  
     [0013]FIG. 2 is a cross-sectional view of the filter of FIG. 1.  
     [0014]FIG. 3 is a schematic illustration of a second dual operation mode all temperature filter constructed in accordance with the teachings of the invention.  
     [0015]FIG. 4 is a schematic illustration of a circuit employing the dual operation mode filter. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0016] A dual operation mode all temperature filter  10  constructed in accordance with the teachings of the invention is shown in FIG. 1. As discussed below, the filter  10  provides a first level of filtering when its temperature is maintained at a temperature below a threshold temperature, and a second level of filtering which is less than the first level when its temperature exceeds the threshold value. More specifically, when maintained in a cooled environment, the filter  10  produces the enhanced level (high rejection and low insertion loss) of filtering expected of HTSC filters, but when exposed to a non-cooled environment (e.g., due to a failure in the cooling system), the filter  10  delivers filtering at a level (high rejection with some insertion loss) expected of conventional (non-HTSC) RF filters. Thus, the disclosed filter  10  provides enhanced performance as compared to conventional filters and enhanced reliability as compared to prior art HTSC filters. Specifically, it provides enhanced filtering levels in most instances and ensures acceptable levels of filtering are maintained in adverse circumstances such as during power interruptions.  
     [0017] Although the disclosed filter  10  is particularly well suited for use with wireless telecommunication systems and will be discussed in that context herein, persons of ordinary skill in the art will readily appreciate that the teachings of the invention are in no way limited to such an environment of use. On the contrary, filters constructed pursuant to the teachings of the invention can be employed in any application which would benefit from the high performance filtering and enhanced reliability it provides without departing from the scope or spirit of the invention.  
     [0018] For the purpose of defining a chamber to contain, direct and filter electromagnetic signals, the filter  10  is provided with a housing  12 . As shown in FIG. 1, the housing  12  includes a pair of end walls  14 , an upper wall  16 , a lower wall  18 , and a pair of side plates (not shown) secured via conventional fasteners such as screws or the like to the end wall  14 , the upper wall  16 , and/or the lower wall  18 .  
     [0019] To divide the housing chamber into a plurality of resonant cavities  20 , the housing  12  is further provided with an inner partition wall  22  and a plurality of inner walls  24 . As shown in FIG. 1, the inner partition wall  22  and the inner walls  24  together define two parallel rows of resonant cavities  20 . To couple the rows of cavities  20 , the inner partition wall  22  defines a coupling aperture  28 .  
     [0020] In order to input electromagnetic signals into the housing  12  and to retrieve filtered signals from the housing  12 , an end wall  14  of the housing  12  respectively defines an input aperture  30  and an output aperture  32 . As shown in FIG. 1, the input and output apertures  30 ,  32  are defined at an end of the housing  12  opposite the coupling aperture  28 . Thus, an electromagnetic signal delivered to the filter  10  via the input aperture  30  will travel down the first row of resonant cavities  20 , pass through the coupling aperture  28 , and return up the second row of resonant cavities  20  and out the output port  32 .  
     [0021] The thickness of the inner partition wall  22  is preferably selected to accommodate the requirements of the coupling mechanism employed to deliver electromagnetic signals to the filter  10 . The two resonant cavities  20  located adjacent the end wall defining the input and output apertures  30 ,  32  form an input cavity  36  and an output cavity  38  which respectively receive at least a portion of a conventional input coupling mechanism and a conventional output coupling mechanism (not shown). In the disclosed embodiment, the input and output cavities  36 ,  38  are separated by a thickened section  42  of the inner partition wall  22 . This thickened section  42  has approximately twice the thickness of the remainder of the inner partition wall  22 . As will be appreciated by persons of ordinary skill in the art, the precise dimensions of the thickened section  42  of the inner partition wall  22  are selected based upon the frequency and loading conditions the filter  10  is expected to accommodate.  
     [0022] As is conventional, the input and output coupling mechanisms are connected to respective RF transmission lines (not shown) that carry RF signals to and from the filter  10 . In general, each coupling mechanism includes an antenna (not shown) for propagating (or collecting) electromagnetic waves within the input and output cavities  36  and  38 . The antenna may include a simple conductive loop or a more complex structure that provides for mechanical adjustment of the position of a conductive element within the cavity  36 ,  38 . An example of such a coupling mechanism is described in U.S. Pat. No. 5,731,269, the disclosure of which is hereby incorporated in its entirety by reference.  
     [0023] For the purpose of tuning each cavity  20  to remove an undesirable frequency or range of frequencies from the RF signal being processed, each resonant cavity  20  is provided with a resonator  46 . (For simplicity of illustration, only two resonators  46  are shown in FIG. 1.) Although persons of ordinary skill in the art will readily appreciate that resonators of various types can be employed in this role without departing from the scope or the spirit of the invention, in the preferred embodiment, the resonators  46  are each preferably implemented as a split-ring, toroidal resonator  46 . The resonators  46  are each located within their respective resonant cavity  20  as shown in FIGS. 1 and 2. Each resonator is individually adjustable within its respective cavity. By selecting its orientation, the degree and type of coupling between each resonator  46  and the electromagnetic signals in its cavity can be adjusted as is known to those skilled in the art. Each resonator  46  is secured to the lower wall  18  by a dielectric mounting mechanism generally indicated at  48  in FIG. 2. The mounting mechanism  48  is secured to the lower wall  18  via conventional fasteners (not shown) such as screws or the like that extend through apertures (not shown) defined in the wall  18 . Further details on exemplary mounting mechanisms may be found in U.S. patent application Ser. No. 08/556,371, the disclosure of which is hereby incorporated in its entirety by reference. Another suitable dielectric mounting mechanism is described and shown in U.S. patent application Ser. No. 08/869,399, the disclosure of which is also hereby incorporated in its entirety by reference.  
     [0024] For the purpose of individually tuning the cavities, each cavity is provided with a tuning disk  52  (FIG. 2). The tuning disks  52  are the primary mechanism for tuning the resonant cavities  20 . As most easily seen in FIG. 2, each tuning disk  52  projects into its associated resonant cavity  20  near a gap  54  (best seen in FIG. 2) in the resonator  46 . Preferably, each tuning disk  52  is coupled to a screw assembly  56  (FIG. 2) that extends through an aperture  58  (FIG. 1) defined in the upper wall  16 . Such a mechanism for tuning split-ring resonators is well known to those skilled in the art and will not be further described herein. Further details, however, may be found in the disclosure of U.S. patent application Ser. No. 08/556,371, which is hereby incorporated in its entirety by reference.  
     [0025] For the purpose of facilitating transmission of electromagnetic signals between respective pairs of the resonant cavities  20 , the inner walls  32  disposed between adjacent coupled resonant cavities  22  of the RF filter  20  define coupling apertures  60 . The size and shape of the individual coupling apertures  60  may vary greatly, as will be appreciated by those skilled in the art. For instance, as shown in FIG. 2, the coupling apertures  60  are generally rectangular. In contrast, other adjacent resonant cavities  22  are coupled together by larger and/or differently shaped apertures (e.g., T-shaped apertures).  
     [0026] In order to further tune the RF filter  20  and to thereby establish a particular response curve for the device, adjustment of the coupling between adjacent resonant cavities  22  can be further effected via coupling screws (not shown) disposed in bores (also not shown) in the upper wall  28 , as is conventional. The bores are preferably positioned such that each coupling screw projects into a respective coupling aperture  60 .  
     [0027] The housing  24  of the RF filter  20  is preferably made of silver-coated aluminum, but may be made of a variety of materials having a low resistivity.  
     [0028] In accordance with an aspect of the invention, at least one, but not all, of the resonators  46  is made from a high temperature superconducting (HTSC) material which is doped with 8-15% silver. This high level of silver doping (conventional levels are on the order of 1-2%) enables the HTSC material to maintain a reasonable level of conductivity at temperatures above the superconducting threshold (i.e., to have a reasonably high Q factor at normal ambient temperatures).  
     [0029] At least one of the resonators  46  in the filter  10  is not made from an HTSC material. Instead, these resonators are made of a conventional conductive material such as copper. The copper resonator(s), therefore, exhibit conventional levels of conductivity at higher environmental temperatures such as room temperature.  
     [0030] More specifically, in a preferred embodiment shown in FIG. 3, a four pole filter  100  comprising four resonant cavities  20 , and four resonators  46  (see FIG. 1) is provided. In the disclosed embodiment, the resonators  46  in the input and output cavities  36 ,  38  are implemented as copper toroids with no high temperature superconducting properties. The remaining two resonators  46  are also toroids. However, these last two resonators  46  are made out of an HTSC material doped with approximately 10% silver. As a result, when the filter  100  is cooled below a superconducting threshold temperature (typically to approximately 77K), the superconducting toroids  46  will exhibit their superconducting properties and the filter  100  will enjoy the enhanced filtering associated with HTSC filters. In the event of a failure in the cooling system (e.g., a power failure), the filter  100  will continue operating at the enhanced filtering level for some dwell time (typically on the order of several hours) until the filter  100  warms above the superconducting threshold. Once such warming has occurred, the high silver doping of the HTSC resonators  46  ensures that the HTSC resonators  46  will still conduct at conventional levels (i.e., not at superconducting levels). As a result of this property of the HTSC resonators  46  and as a result of the presence of the conventional (non-HTSC) resonators  46 , the filter  100  automatically switches to a conventional filtering mode of operation wherein the filter  100  filters signals as if it were a conventional (i.e., non-superconducting) filter. Upon returning to the super cooled state (e.g., upon resumption of power to the cooling system), the filter  100  automatically switches into its ultra-high performance mode where it performs filtering at the enhanced level typical of HTSC filters. Filters constructed in accordance with the teachings of the invention exhibit very low insertion loss. For example, the four pole filter  100  shown in FIG. 3 exhibited an insertion loss of 2-5 dB at room temperature and an insertion loss of 0.2 dB at 77K.  
     [0031] As will be appreciated by persons of ordinary skill in the art, the ability of the dual operation mode filter  10 ,  100  to automatically switch between operating modes renders the filter  100  operational at all temperatures, thereby removing the need for the RF bypass circuitry and/or temperature control circuitry associated with prior art HTSC filters. The elimination of this circuitry reduces the size and cost of the filter  100 . The filter  100  is, thus, less expensive, more reliable and smaller than conventional HTSC filters.  
     [0032] A process for manufacturing HTSC resonators  46  is disclosed in U.S. Pat. No. 5,789,347, which issued on Aug. 4, 1998 and which is hereby incorporated in its entirety by reference. The &#39;347 Patent, however, discloses the use of 2% by weight of silver powder in the HTSC material. The HTSC resonators  46  used in filters constructed in accordance with the present invention can be manufactured pursuant to the process disclosed in the &#39;347 Patent with silver doping levels increased to 8-15% by weight. Although silver doping in the range of 8-15% is presently believed to be acceptable, at the present time doping at approximately a 10% level by weight is preferred. In addition, although the HTSC resonators described above can be made of heavily silver doped HTSC material, persons of ordinary skill in the art will appreciate that other approaches can be taken without departing from the scope or spirit of the invention. For example, the HTSC resonators  46  can be made of stainless steel toroids coated with HTSC material which is heavily silver doped in accordance with the ranges specified above without departing from the teachings of the invention.  
     [0033] Persons of ordinary skill in the art will readily appreciate that, although the preferred embodiment uses high silver doping to increase the ambient temperature conductivity of its HTSC resonators  46 , other conductive doping materials can be used in this role without departing from the scope or spirit of the invention. Persons of ordinary skill in the art will further appreciate that although the filters disclosed herein are low order filters having six or fewer poles, filters with other numbers of poles can be constructed in accordance with the teachings of the invention. However, filters with four to six poles are presently preferred.  
     [0034] The filters  10 ,  100  shown in FIGS. 1 and 3 are bandpass filters (i.e., filters designed to pass frequencies in a predetermined range and to block signals in frequencies higher and lower than that range). However, persons of ordinary skill in the art will appreciate that the teachings of the invention are not limited to such filters. For example, a notch filter (i.e., a filter designed to block frequencies in a predetermined range) can be constructed pursuant to the teachings of the invention. Unlike the bandpass filters  10 ,  100  described above, such notch filters employ HTSC resonators  46  whose HTSC material is not doped (in order to completely decouple at room temperature). Also like the bandpass filters  10 ,  100  described above, the notch filter filters at an enhanced level typical of HTSC filters when maintained at a temperature at or below the superconducting threshold. However, when the notch filter is warmed above the threshold level, it acts as a pass through filter within the predetermined range (i.e., it stops blocking signals in the predetermined range). As a result, if the cooling system associated with the notch filter fails, the notch filter will permit signals having frequencies in the predetermined range to pass through without impediment, and, thus, will not prevent the serviced telecommunication device (e.g., a base station) from operating. The notch filter achieves this result because, at ambient temperatures, the notch range will shift to a different range. Accordingly, at ambient temperatures a different range of frequencies will be blocked than at superconducting temperatures. The filter designer should consider this shift to ensure that desirable signals are not blocked at ambient temperatures.  
     [0035] An exemplary HTSC notch filter is disclosed in co-pending U.S. application Ser. No. 08/556,371, which is hereby incorporated in its entirety by reference. The notch filter described in this document is constructed like the notch filter described in the &#39;371 application, but with the resonator modifications described above (and preferably limited to 6 or fewer poles). Accordingly, the interested reader is referred to the &#39;371 application for a detailed discussion of the implementation details of HTSC notch filters.  
     [0036] In order to enhance the filtering performance of the dual operation mode filter  10 ,  100 , the dual operation mode filters (bandpass or notch)  10 ,  100 , may be cascaded with one or more conventional filters  50  as shown in FIG. 4. By using cascaded filters  50 , it is possible to achieve high performance filtering typically associated with high order filters while using only low order pole filters. A detailed discussion of the virtues of cascading filters is provided in co-pending U.S. patent application Ser. No. 09/130,274, filed Aug. 6, 1998, which is hereby incorporated in its entirety by reference.  
     [0037] As shown in FIG. 4, the conventional filter  50  is preferably connected to the dual operation mode filter  10 ,  100 , via either a low noise amplifier  52  or an isolator  54 . A low noise amplifier  52  would be used in applications where it is desirable to amplify the filtered signal output by the dual operation mode filter  10 ,  100 , prior to filtering by the conventional filter  50 . The isolator  54  would be used in applications where low loss transmission between the filter  10 ,  100 , and  50  is desired, but where it is undesirable to permit operation of the conventional filter  50  to effect the operation of the dual operation mode filter  10 ,  100 . A cascaded filter implemented with a dual operation mode, 4 pole bandpass filter  100 , an isolator  54 , and a conventional, high rejection filter  50 , experienced increased insertion loss as compared to the statistics quoted above, but was tuned while achieving more than 20 dB/1 MHz rejection.  
     [0038] Persons of ordinary skill in the art will appreciate that the RF spectrum is divided into A, B, A′ and B′ bands. The B band separates the A and A′ bands. The A′ band separates the B and B′ bands. Such persons will further appreciate that it is often desirable to broadcast in the A and A′ bands without broadcasting in the B band and/or to broadcast in the B and B′ bands without broadcasting in the A′ band. Prior art systems solved this problem by using two bandpass filters in parallel and multiplexing the outputs of the parallel filters.  
     [0039] By using a bandpass filter (either conventional or dual operation mode) cascaded with a notch filter (either conventional or dual operation mode), the same result can be achieved without requiring multiplexing. For example, if the bandpass filter is designed to pass signals in the A, B and A′ bands and the notch filter blocks signals in the B band, an A, A′ band filter is achieved. Alternatively, if the bandpass filter is designed to pass signals in the B, A′ and B′ bands and the notch filter is designed to block signals in the A′ band, a B, B′ band filter is achieved.  
     [0040] Although certain instantiations of the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all instantiations of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.