Abstract:
A filter network designed for providing high frequency selectivity with a high degree of reliability and availability. The filter network comprises a superconducting filter and a non-superconducting filter, or a combination thereof to form multiplexers. A receive side of the non-superconducting filter pre-filters received RF signals before inputting them to the superconducting filter. The non-superconducting filter is constructed and arranged to pass RF signals having a frequency within a first pass band to the superconducting filter. The superconducting device is constructed and arranged to exhibit a high-degree of frequency selectivity in further narrowing the received RF signals. Other aspects are directed to the arrangement, construction, and uses of the same structures to accomplish different but similar goals. In a multiplexed configuration, various combinations of transmit filters are used to enable the use of a common antenna with the receive side electronics, which may be located at the top of the antenna tower or in the base station.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of co-pending U.S. application Ser. No. 09/818,100, filed Mar. 26, 2001, allowed, which is fully and expressly incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to structures and techniques for filtering radio waves and more particularly to the implementation of a filter network using a combination of superconducting filters and non-superconducting filters.  
         BACKGROUND OF THE INVENTION  
         [0003]    Radio frequency (RF) equipment have used a variety of approaches and structures for receiving and transmitting radio waves in selected frequency bands. The type of filtering structure used often depends upon the intended use and the specifications for the radio equipment. For example, dielectric filters may be used for filtering electromagnetic energy in the ultra-high frequency (HF) band, such as those used for cellular communications in the 800+ MHz frequency range. Typically, such filter structures are implemented by coupling a number of dielectric resonator structures together. One can also use metal coaxial resonators in such filters are coupled together via capacitors, inductors, or by apertures in walls separating the resonator structures. The number of resonator structures used for any particular application also depends upon the system specifications and, typically, added performance is realized by increasing the number of intercoupled resonator structures.  
           [0004]    However, because of an increase in the number of users utilizing a limited bandwidth, demand has increased for greater frequency selectivity than can be provided by normal or non-superconducting resonator filters, especially for RF signals in the ultra-high frequency bands used for cellular communications. High frequency selectivity has previously been accomplished using High Temperature Superconducting (HTS) filters, usually as front-end filters for cellular base station receivers. However, HTS front-end filters may be susceptible to failure, or degradation in performance, induced by lightning surges or other high power signals. In addition, the non-linearity of HTS filters produces in-band intermodulation spurious signals from out-of-band interferers.  
           [0005]    For cellular or similar base stations, typical lightning protectors have only one resonator and do not provide sufficient protection from high power co-located radio frequency signals originating from the transmit side of the base stations. These co-located transmission signals are especially troublesome because they are relatively closely spaced to the operating frequency of the base station receivers. Accordingly, there is a need for a filter that overcomes the above-mentioned and other disadvantages associated with the prior art.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed toward a filter network that provides high frequency selectivity to a receiver. The filter network of the present invention comprises a non-superconducting filter and a superconducting filter. The output of the non-superconducting filter is coupled to the input of a superconducting filter. The non-superconducting filter pre-filters received RF signals by passing RF signals having a frequency within a first pass band to the superconducting filter. The superconducting filter further filters the RF signals to provide a high degree of frequency selectivity at its output.  
           [0007]    The filter network of the present invention is able to provide high frequency selectivity while overcoming many of the disadvantages associated with superconducting filters. This is achieved by pre-filtering the RF signals with the non-superconducting filter before inputting them to the superconducting filter. The non-superconducting filter protects the superconducting filter from lightning surges or other high power signals. In addition, the non-superconducting filter filters out interferers that produce in-band intermodulation spurious signals at the superconducting filter output. In a multiplexed configuration, the non-superconducting filter protects the superconducting filter directly from transmit signal energy.  
           [0008]    According to one embodiment of the present invention, the non-superconducting resonator filter comprises a housing enclosing three resonators. The resonators are coupled to each other through apertures in the housing. The effect of using this coupling with the three resonators is to produce a filter response with a pass band and a finite frequency transmission zero located outside the pass band. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Other aspects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0010]    [0010]FIG. 1 shows a communications system incorporating a filter network according to one embodiment of the invention.  
         [0011]    [0011]FIG. 2A shows a top view of a non-superconducting filter according to one embodiment of the present invention.  
         [0012]    [0012]FIG. 2B shows a cross-sectional side view of the non-superconducting filter according to one embodiment of the present invention.  
         [0013]    [0013]FIG. 3 shows a plot of the filter response of the non-superconducting filter according to one embodiment of the present invention.  
         [0014]    [0014]FIG. 4 shows a multiplexer according to one embodiment of the present invention.  
         [0015]    [0015]FIG. 5 shows a double-duplexer according to one embodiment of the present invention.  
         [0016]    [0016]FIG. 6 shows a double-duplexer according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The present invention is believed to be applicable to a variety of radio frequency (RF) applications in which achieving low insertion loss in the pass band with high attenuation in the stop band, and an extremely high degree of selectivity in the pass band are necessary. The present invention is particularly applicable and beneficial for cellular-communication base stations, and other communication applications. While the present invention is not so limited, an appreciation of the present invention is best presented by way of a particular example application, in this instance, in the context of such a communication system.  
         [0018]    Now turning to the drawings, FIG. 1 shows a front-end receiver system  10  of a base station, according to a particular application and embodiment of the present invention. The front-end receiver system  10  includes an antenna  12  for receiving RF signals  15 , a filter network  100  for filtering the received RF signals, and a receiver  16 . The filter network  100  is used to selectively pass received RF signals within a designated pass band to the receiver  16 , while filtering out interferers. The interferers are interfering signals located outside the operating frequency of the receiver  16 , and include RF signals transmitted by other cellular service providers. The interferers also include co-located transmission signals transmitted by the transmitter side of the same base station.  
         [0019]    The filter network  100  comprises a non-superconducting filter  20  and a superconducting filter  30 , preferable a High Temperature Superconducting (HTS) filter. The input of the non-superconducting filter  20  receives RF signals  15  from the antenna  12 . The output of the non-superconducting filter  20  is coupled to the input of the superconducting filter  30 , and the output of the superconducting filter is coupled to the receiver  16 . The non-superconducting filter  20  pre-filters the received RF signals  15  before they are filtered by the superconducting filter  30 .  
         [0020]    The non-superconducting filter  20  is a bandpass filter tuned to pass the received RF signals having a frequency within a first pass band to the superconducting filter  30 . Preferably, the first pass band encompasses a receiving frequency range of the base station. For base stations using the Advanced Mobile Phone Service (AMPS) standard, for example, the total receiving frequency range is approximately 824 MHz to 849 MHz. The superconducting filter  30  is a bandpass filter tuned to pass the pre-filtered RF signals having a frequency within a second pass band to the receiver  16 . The second pass band is a narrow pass band located inside the first pass band for providing high frequency selectivity to the receiver  16 .  
         [0021]    The non-superconducting filter  20  protects the superconducting filter  30  from high power out-of-band signals that can cause catastrophic failure of the superconducting filter  30 . The high power signals include electrical surges caused by lightning strikes. In addition, the non-superconducting  20  filter filters out interferers located outside the first pass band before they are inputted to the superconducting filter  30 . This is done because these interferers produce in-band intermodulation spurious signals in the superconducting filter  30 . By filtering out these interferers before they are inputted to the superconducting filter  30 , the non-superconducting filter  20  dramatically reduces the in-band intermodulation spurious signals.  
         [0022]    The superconducting filter  30  provides high frequency selectivity to the receiver  16  for rejecting undesirable signals that are closely spaced in frequency to desirable signals. The advantage of using a superconducting filter is its ability to provide a precise narrow pass band around the desired signals with low insertion loss due to its low resistance. This allows the superconducting filter  30  to provide high frequency selectivity without adversely affecting the signal sensitivity of the receiver  16 .  
         [0023]    Therefore, the filter network  100  according to the present invention exhibits high frequency selectively and low insertion loss without many of the disadvantages associated with a superconducting filter. This is achieved by pre-filtering the RF signals with the non-superconducting filter  20  before inputting the RF signals to the superconducting filter  30 . That way, catastrophic failure due to high power out-of-band signals and performance degradation due to in-band intermodulation spurious signals are dramatically reduced.  
         [0024]    [0024]FIG. 2A of a non-superconducting filter  200  according to one embodiment of the present invention. The non-superconducting filter  200  comprises a housing  210  enclosing three round-rod resonators  215 ,  220  and  225 . Alternatively, the resonators  215 ,  220  and  225  can be waveguide resonators, cavity resonators, dielectric resonators, stripline resonators, or other resonators known in the art. The housing  210  and resonators  215 ,  220  and  225  may be machined from aluminum and silver plated to minimize insertion loss. The resonators  215 ,  220  and  225  are placed in three cavities  230 ,  235 , and  240 , respectively, formed inside the housing  210 , creating coaxially resonant structures. The input  275  and the output  285  of the non-superconducting  200  filter are directly coupled  290  and  295  to resonators  215  and  225 , respectively. Alternatively, the input  275  and the output  285  may be coupled to the resonators  215  and  225 , respectively, using capacitors, inductors or any other coupling technique used by those skilled in the art.  
         [0025]    [0025]FIG. 2B shows a cross-sectional view of the non-superconducting filter  200  taken along line  2 B in FIG. 2A. FIG. 2B shows a top plate  310  placed over the housing  210  of the non-superconducting filter  200 . In addition, tuning screws  320  are inserted into each resonator  215 ,  220  and  225  though the top plate  310 . The tuning screws  320  are secured to the top plate  310  by nuts  330 . The functionality of the tuning screws  320  will be discussed later.  
         [0026]    Each resonator  215 ,  220  and  225  is electro-magnetically coupled to each one of the other two resonators  215 ,  220  and  225  through apertures in the housing  210 . The aperture coupling resonators  215  and  220  is shown in FIG. 2A as the opening between cavities  230  and  235 . The aperture coupling resonators  220  and  225  is shown in FIG. 2A as the opening between cavities  235  and  240 . The aperture coupling resonators  215  and  225  is best shown in FIG. 2B as an opening  270  in a housing wall  275  positioned between resonators  215  and  225 . Alternatively, the resonators can be coupled to each other using transformers or capacitors.  
         [0027]    The turning screws  320  are used to adjust the capacitance of the resonators  215 ,  220  and  225 . Turning the tuning screws  320  inwardly increases the capacitance of the resonators  215 ,  220  and  225 , which lowers the resonance frequency of the resonators  215 ,  220  and  225 . Turning the tuning screws  320  outwardly decreases the capacitance of the resonators, which increases the resonance frequency of the resonators  215 ,  220  and  225 .  
         [0028]    The non-superconducting filter  200  of FIGS. 2A and 2B produces a first pass band and a finite frequency transmission zero positioned at a frequency outside the first pass band. The finite frequency transmission zero provides enhanced rejection of signals located in its vicinity. The position of the finite frequency transmission zero can be controlled by adjusting the dimensions of the aperture coupling resonators  215  and  225 . Preferably, the finite frequency transmission zero is positioned at a frequency within a frequency range containing powerful interferers to provide enhanced rejection of these interferers. For example, the co-located transmission signals transmitted by the transmitter side of the base station can be powerful due to the proximity between the transmitter and receiver side of the base station. In this example, the finite frequency transmission zero can be positioned at a frequency inside the transmitting frequency range of the base station to enhance rejection of the co-located transmission signals. For base stations using the AMPS standard, for example, the transmitting frequency range is approximately 869 MHz to 894 MHz, which is located near the receiving frequency range of 824 MHz to 849 MHz. The finite frequency transmission zero can be positioned at a frequency either above or below the first pass band, depending on the location of powerful interferers.  
         [0029]    In one specific example of the non-superconducting filter  200  in FIGS. 2A and 2B, the non-superconducting filter  200  structure has the dimensions given below. The housing  210  has a height H1 of 2.30 inches. Chamber  235  has a width W1 of 3.50 inches and a length L1 of 2.75 inches, and chambers  230  and  240  each have a width W2 of 2.55 inches and a length L2 of 2.55 inches. Each one of the resonators  215 ,  220  and  225  has a diameter d of 0.75 inches and a height H2 of 2.15 inches. The center of resonator  220  is positioned in chamber  235  a length L5 of 1.275 inches from one side of the housing  210  and width W4 of 1.75 from another side of the housing  210 . The center of resonator  225  is position in chamber  240  a length L6 of 1.275 inches from one side of the housing  210  and a width W5 of 1.275 from another side of the housing  210 . The center of resonator  215  is in the same relative position in chamber  230  as the center of resonator  235  is in chamber  240 . The housing wall  275  separating resonators  215  and  225  has a width W3 of 0.20 inches and a length L3 of 2.75 inches. Finally, the aperture  270  coupling resonators  215  and  225  has a height H3 of 0.70 inches and a length L4 of 1.70 inches.  
         [0030]    [0030]FIG. 3 shows a plot  345  of the frequency response of a non-superconducting filter  200  made from silver-plated aluminum and having the above dimensions. Specifically, the plot  345  shows an insertion loss  350  measured in decibels (dB) between the input  275  and the output  285  of the non-superconducting filter  200  versus frequency in the range of 750 MHz to 950 MHz. The filter  200  passes frequencies at which the insertion loss  350  is low and rejects frequencies at which the insertion loss  350  is high. In FIG. 3, the insertion loss  350  is low within a receiving frequency range of about 824 MHz to 849 MHz, which is bounded by lines  355  and  360 . In contrast, the insertion loss is high within a transmitting frequency range of 869 MHz to 894 MHz, which is bounded by lines  365  and  370 . Thus, the non-superconducting filter  200  measured in plot  345  passes signals within the receiving frequency range of 824 MHz to 849 MHz, while rejecting signals within the transmitting frequency range of 869 MHz to 894 MHz. These frequency ranges correspond to the receiving and transmitting frequency ranges used by cellular base stations in the AMPS standard.  
         [0031]    In this specific example, the effect of the cross coupling between the resonators  215 ,  220  and  225  produces a finite frequency transmission zero, which can been seen as a deep spike  375  in the insertion loss  350  in the plot  345 . This transmission zero is located inside the base station transmitting frequency range of 869 MHz to 894 MHz and provides enhanced rejection of frequencies within this frequency range.  
         [0032]    [0032]FIG. 4 shows a multiplexer  410  according to one embodiment of the present invention. The multiplexer  410  comprises at least one transmit filter  420 - n  and at least one receive filter network  425 - n . The receive filter network  425 - n  further comprises a non-superconducting filter  430 - n , and a superconducting filter and receive electronics  440 - n . The output of the transmit filter  420 - n  and the input of the receive filter network  425 - n  are coupled to a common antenna port  450 - n . The transmit filter  420 - n  and the receive filter network  425 - n  may be coupled to the common antenna port  450 - n  by an interconnecting phasing network (not shown), the construction of which is well known in the art. The common antenna port  450 - n  is coupled to an antenna  460 , for example, through a cable. The multiplexer  410  may be located in close proximity to the antenna  460 . For example, the multiplexer  410  and the antenna  460  may be mounted to the same antenna tower. Alternatively, the multiplexer  410  may be located away from the antenna  460 , such as in a base station.  
         [0033]    The transmit filter  420 - n  filters incoming transmit signals  422 - n  from the transmitter side of a base station (not shown). The transmit filter  420 - n  is a bandpass filter constructed to pass signals within a transmitting frequency range of the base station, for example, approximately 869 MHz to 894 MHz for the AMPS standard. The transmit filter  420 - n  may include one or more finite frequency transmission zeros for providing enhanced rejection of signals located outside of the transmitting frequency range, such as the receive signals on the common antenna port  450 - n . The non-superconducting filter  430 - n  of the receive filter network  425 - n  pre-filters receive signals from the antenna  460 . The non-superconducting filter  430 - n  is a bandpass filter constructed to pass signals within a receiving frequency range of the base station, for example, 824 MHz to 849 MHz for the AMPS standard. The non-superconducting filter  430   n  may include one or more finite frequency transmission zeros for providing enhanced rejection of signals located outside of the receiving frequency range, such as the transmit signals on the common antenna port  450 - n . The superconducting filter  440 - n  is a sharp bandpass filter for providing high frequency selectivity of the receive signals. The receive electronics  440 - n  further processes the receive signals. The receive electronics  440 - n  may include a Low Noise Amplifier (LNA), which may or may not be cryogenically cooled, for amplifying the receive signals. The receive electronics  440 - n  may also include protection circuits for protecting the superconducting filter  440 - n  and/or base station (not shown) from electrical surges. The protection circuits may include gas discharge tube voltage arrestors, quarter wavelength stubs, and any other protection circuits that are well known in the art. The receive signals are outputted  445 - n  by the receive filter network  425 - n  to the receiver side of a base station (not shown).  
         [0034]    The multiplexer  410  according to the present invention enables the same antenna  460  to both transmit and receive signals, thereby reducing costs. This is achieved by coupling the transmit filter  420 - n  and the receive filter network  425 - n  to the common antenna port  450 - n  of the multiplexer  410 , and coupling the common antenna port  450 - n  to the antenna  460 .  
         [0035]    [0035]FIG. 5 shows a double duplexer  510  according to another embodiment of the present invention. The double duplexer  510  includes a transmit filter  515  and a receive filter network  520 . The receive filter network  520  further includes a first non-superconducting filter  530 , a second non-superconducting filter  550 , and a superconducting filter and receive electronics  540  coupled between the first and second non-superconducting filter  530 ,  550 . The output of the transmit filter  515  and the input of the receive filter network  520  are coupled to a common antenna port  560 . The common antenna port  560  is coupled to an antenna  565 , for example, through a cable. The input of the transmit filter  515  and the output of the receive filter network  520  are coupled to a common port  570 . The common port  570  is coupled to a base station (not shown) through a cable  575 .  
         [0036]    The transmit filter  515  filters incoming transmit signals from the base station (not shown) in a manner similar to the transmit filter  420 - n  of the multiplexer  410 . The first non-superconducting filter  530  pre-filters receive signals from the antenna  565  in a manner similar to the non-superconducting filter  430  of the multiplexer  410 . The superconducting filter  540  is a sharp bandpass filter for providing high frequency selectivity of the receive signals. The receive electronics  540  further processes the receive signal in a manner similar to the receive electronics  440 - n  of the multiplexer  410 . The second non-superconducting filter  550  is a bandpass filter that passes the receive signals to the common port  570  while blocking the transmit signals on the common port  570  from the entering the receive electronics  540 . The second non-superconducting filter  550  may be the identical to the first non-superconducting filter  530 .  
         [0037]    The double-duplexer  510  according to the present invention enables the same antenna  565  to both transmit and receive signals, thereby reducing costs. In addition, the double-duplexer  510  enables the transmit signals and the receive signals to flow between the double-duplexer  510  and the base station (not shown) through the common port  570 . As a result, a single cable  575  can be used to coupled the double-duplexer  510  to the base station. Because the base station uses a single cable  575  to both transmit signals to and receive signals from the double-duplexer  510 , additional filters may be needed to split the transmit and receive signals at the base station. This may be accomplished by providing a transmit filter  580  between the transmitter side of the base station (not shown) and the cable  575 , and a receive filter  585  between the receiver side of the base station (not shown) and the cable  575 .  
         [0038]    Although, the double-duplexer  510  was described as including one transmit filter  515  and one receive filter network  520 , those skilled in the art will appreciate that any number of transmit filters and receive filter network may be added to the double-duplexer to realize a double-multiplexer.  
         [0039]    [0039]FIG. 6 shows a double-duplexer  610  according to another embodiment of the present invention. In this embodiment, the receive filter network  620  includes a first superconducting filter  630 , a second superconducting filter  650 , and receive electronics  640  coupled between the first and second superconducting filter  630 ,  650 . The first superconducting filter  630  is a sharp bandpass filter for providing high frequency selectivity of the receive signals from the antenna  565 . The receive electronics  630  further processes the receive signals and may include an LNA and protection circuits. The second superconducting filter  650  is a bandpass filter that passes the receive signals to the common port  570  while blocking transmit signals on the common port  570  from entering the receive electronics  640 . Alternatively, the second superconducting filter  650  may be replaced by a non-superconducting filter.  
         [0040]    Additionally, to alleviate catastrophic failure of the receive side of the systems shown in FIGS. 4 and 5, a switched bypass (not shown) may be used. In the event of an electrical surge in a receive path of the systems, the switched bypass directs the receive signals around the superconducting filters shown in the receive electronics  440 - n  and  540 . Also included in this bypass function may be one or more low noise amplifiers, which may or may not be cooled, along with any other circuitry in the path of the receive signals that may be considered prone to failure.  
         [0041]    Other aspects and embodiments of the present invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. For example, the non-superconducting filter illustrated in FIGS. 2A and 2B may be implemented in a variety of ways to achieve similar results according to the design and specifications. In addition, those skilled in the art will appreciate that the invention is not restricted to frequency bands used in the AMPS standard, and may, in principle, operate in other frequency bands used in other mobile phone standards. It is intended that the specification and illustrated embodiments be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.