DELAY EQUALIZATION IN WIRELESS COMMUNICATION SYSTEMS

Abstract: A receive front-end useful for receiving a communication signal includes a bandpass filter having a high selectivity for selecting a passband of the communication signal, an equalizer to compensate for group delay variance introduced by the high selectivity of the bandpass filter, and a low noise amplifier protected by the bandpass filter from signals outside the passband and coupled to the equalizer to establish noise figure for the receive front-end. The filter may be self-equalizing to provide all or a portion of the requisit delay compensation, and both the filter and the equalizer may include high-temperature superconducting components.

Description: DELAY EQUALIZATION IN WIRELESS COMMUNICATION SYSTEMS FIELD OF THE INVENTION The present invention relates generally to wireless communication systems and, more particularly, to such systems that address delay equalization in connection with a highly selective filter. BACKGROUND OF THE INVENTION Radio frequency (RF) receivers for wireless communication stations must provide high degrees of both selectivity (the ability to distinguish between signals separated by small frequency differences) and sensitivity (the ability to receive weak signals) in an increasingly hostile frequency spectrum. In a typical base station, an incoming RF signal is first passed through a low loss, RF bandpass filter to remove signal components outside of the frequency range of the desired signal. Because the resulting filtered signal is usually very weak, the signal is coupled to an amplifier that does not introduce a significant amount of noise ( . e. , a low-noise amplifier or LNA). The relatively recent advancements in superconducting technology have given rise to a new type of RF filter, namely, the high-temperature, superconducting (HTS) filter. HTS filters contain components that act as superconductors at or above the liquid nitrogen temperature of 77 K. Such filters provide greatly enhanced performance in terms of both sensitivity and selectivity over conventional (i.e. , non-superconducting) filters. Specifically, HTS filters have been shown to provide excellent rejection with signal losses much lower than that possible using conventional filters. These improvements in selectivity without higher insertion losses have generally allowed wireless telephone carriers to increase the range of each base station. As long as the system remains within its capacity, the increased range will result in an increase in the minutes of use (MOUs). However, these performance enhancements have been gained at the cost of a more complicated system of components in each RF receiver. More particularly, HTS filters must be accompanied by a cooling system to ensure the filters are maintained at relatively low temperatures (e.g. , approximately 90 K or lower). In order to maintain the devices at such temperatures, the cooling system includes some type of cryorefrigerator. The cryorefrigerator typically includes a compressor for maintaining a supply of pressurized coolant and a heat exchanger or cold head to remove heat from the devices being cooled. In addition, an HTS filter must minimize the amount of heat transfer from the environment by enclosing the cooled devices in a cryostat. The cryostat is then often evacuated of any gaseous material to reduce convection heating. Complications have also arisen in connection with systems utilizing such highly selective filters with regard to group or envelope delay. In general, an ideal RF filter has a constant group delay for all frequencies in the passband. In this manner, signals in the passband arrive at the filter output without any phase distortion between differing frequencies. Filters utilized in the context of satellite communications (i.e. , C- and X-band signals), however, have presented the need for delay equalization or, generally speaking, compensation for the phase non-linearity introduced into the passband. In contrast, delay equalization in conjunction with amplitude correction has been addressed in wireless communication systems at the baseband frequency after demodulation of the carrier frequency. One manner in which such phase distortion may be remedied is through use of a delay equalizer (hereinafter "equalizer"), which typically addresses the distortion by providing an all-pass transfer function (i.e. , having an amplitude of one for all frequencies in the passband) that compensates for the non-linear effects of the filter in order to equalize the delay for all frequencies in the passband. The non-linearities may, for example, be addressed by designing the equalizer to have a transfer function with appropriately placed complex zeros to counter the effects of the poles of the filter. Conventional equalizers include a circulator having an input port, a second port coupled to a one-port network, and an output port. The circulator presents a passive waveguide junction for passing signals in a single direction from the input port to the second port, and from there to the output port. Conventional equalizers, however, are not suited for use with the HTS filters that have been recently incorporated into wireless communication stations for several reasons. Because a filter introduces delay variation at RF frequencies in proportion to the rejection, use of a highly selective HTS filter may result in equalization demands more extreme than previously encountered, which, among other matters, raises issues of design complexity and tuning capabilities. Furthermore, conventional equalizers may be generally incompatible with the cryogenic environment imposed by the HTS filter and/or, more specifically, the stringent tuning and other demands imposed thereby. SUMMARY OF THE INVENTION In accordance with one aspect of the present invention, a receive front-end for receiving a communication signal includes a bandpass filter having a high selectivity for selecting a passband of the communication signal, an equalizer to compensate for group delay variance introduced by the high selectivity of the bandpass filter, and a low-noise amplifier protected by the bandpass filter from signals outside the passband and coupled to the equalizer to establish a noise figure for the receive front-end. In a preferred embodiment, the bandpass filter includes a high- temperature superconducting component, and the equalizer includes a further high-temperature superconducting component. The bandpass filter, the equalizer, and the low-noise amplifier may be disposed in a cryostat for operation in a cryogenic environment. The receive filter may be useful in connection with different types of wireless communication systems, such that the communication signal may include either a PCS signal or a cellular signal. Preferably, the equalizer includes a plurality of resonators. The equalizer may further include a housing defining a plurality of cavities in which the plurality of resonators are respectively disposed. The plurality of resonators may include a pair of non-sequential resonators that are coupled, and which may be adjacent. The equalizer preferably includes a high- performance, external equalizer. In another preferred embodiment, the bandpass filter includes a self- equalized filter. The receive front-end preferably includes a cryostat in which the self-equalized filter is disposed. The self-equalized filter may then include a high-temperature superconducting component. The equalizer may then also include a further high-temperature superconducting component. In accordance with another aspect of the present invention, a receive front-end for receiving a communication signal includes a highly selective, bandpass filter that selects a passband of the communication signal and includes a plurality of coupled stages to establish complex zeros for delay compensation within the passband of the communication signal, and an equalizer coupled to the highly selective, bandpass filter to provide further delay compensation within the passband of the communication signal. In a preferred embodiment, the highly selective, bandpass filter and the equalizer include high-temperature superconducting components. The receive front-end may further include a low-noise amplifier protected by the highly selective, bandpass filter from signals outside the passband and coupled to the equalizer to establish a noise figure for the receive front-end. The highly selective, bandpass filter, the equalizer, and the low-noise amplifier may then be disposed in a cryostat for operation in a cryogenic environment. The equalizer preferably includes a plurality of resonators, which may be disposed in a plurality of cavities, respectively, the cavities being defined by a housing of the equalizer. The plurality of resonators may include a pair of non-sequential resonators that are coupled. The pair of non-sequential resonators may be adjacent. In accordance with yet another aspect of the present invention, a bandpass filter having a passband includes a plurality of resonators, each resonator comprising high-temperature superconducting material. The plurality of resonators includes a pair of non-sequential resonators, and the pair of non-sequential resonators are coupled to establish complex zeros for delay compensation within the passband. The plurality of resonators may be arranged such that the pair of non-sequential resonators are adjacent. The bandpass filter may also include a housing defining a plurality of cavities such that the plurality of resonators are disposed in the plurality of cavities, respectively. The housing may include a wall separating the pair of non-sequential resonators, and the wall may include an aperture for cross-coupling the pair of non-sequential resonators. In one embodiment, the bandpass filter is combined with an equalizer coupled to the bandpass filter to provide further delay compensation. The equalizer may include a high-temperature superconducting component, and the combination may also include a low-noise amplifier coupling the bandpass filter to the equalizer to establish a noise figure for the combination. The equalizer may be a high-performance equalizer and/or a multiple-resonator device. The multiple-resonator device may include a housing defining a plurality of cavities, and further include at least two, nonsequential resonators that are coupled. The two, non-sequential resonators may be adjacent. Other features and advantages are inherent in the receive front-ends and filters claimed and disclosed or will become apparent to those skilled in the art from the following detailed description in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a receive front-end in accordance with one aspect of the present invention; FIG. 2 is a schematic representation of an equalizer of the receive front-end of FIG. 1 in accordance with one embodiment of the present invention; FIGS. 3A and 3B are simplified, perspective views of a portion of the equalizer of FIG. 2; FIG. 4 is a schematic representation of a pre-selection filter of the receive front-end of FIG. 1 in accordance with one aspect of the present invention; FIG. 5 is a schematic representation of a self-equalized, pre-selection filter of the receive front-end of FIG. 1 in accordance with another aspect of the present invention; FIGS. 6A and 6B are simplified, perspective views of the self- equalized, pre-selection filter of FIG. 5 in accordance with one embodiment of the present invention; FIG. 7 is a perspective view of a portion of the self-equalized, preselection filter of FIGS. 5, 6A, and 6B; and FIG. 8 is a sectional view of the self-equalized, pre-selection filter of FIG. 5 taken along the plane 8-8 of FIG. 7. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention is generally directed to compensation for group delay variances effected by one or more highly selective filters in a receive front-end of a wireless communication system. Delay compensation in accordance with the present invention incorporates either internal equalization, external equalization, or some combination thereof. More particularly, delay compensation in receive front-ends having a highly selective filter may include an improved self-equalized filter, an external equalizer, or a combination thereof. With reference to FIG. 1, a base station or any other communication station for receiving and transmitting communication signals includes a receiver, which, in turn, includes a front-end (hereinafter "receive front- end") generally indicated at 10. An antenna 12 is coupled to the receive front-end 10 via coaxial or other cabling 14 of a proper impedance, or any other type of connection known to those skilled in the art. The receive front-end 10 may be housed in a cabinet 16 having an output port 18 for connection to the remaining components of the receiver. The cabinet 16 may include components other than those dedicated to the receive front-end 10, such as a transmit filter, and may further include redundant systems for establishing multiple signal paths. The receive front-end 10 may also be incorporated into a duplexer configuration in which components in the transmit path are packaged and/or shared with those components in the receive path. It should also be noted that one or more antennae (that may or may not be omni-directional as is known to those skilled in the art) may be associated with the receive front-end 10 for coverage of a multiple-sector cell. Thus, for simplicity in description only, the present invention will be set forth in a receive-only, single cell or sector environment with the understanding that certain components, aspects, or elements described hereinbelow may need to be replicated, combined or otherwise modified for operation in a transceiver, multiple-sector environment. In accordance with certain embodiments of the present invention, the cabinet 16 includes a cryostat 20 for maintaining a cryogenic environment for components disposed therein. The cryostat 20 may be a thin-walled cryostat, such as the device disclosed in co-pending and commonly assigned U.S. patent application Serial No. 08/831,175, the disclosure of which is hereby incorporated by reference, or any other cryostat suitable for establishing a constant temperature and pressure environment. To this end, the cryostat 20 is coupled to a cryorefrigerator (not shown). In accordance with one embodiment of the present invention, the cryostat 20 preferably houses several cryogenic components, including a pre-selection, bandpass filter (hereinafter "filter") 22, a low-noise amplifier 24, and an equalizer 26. The filter 22 is coupled to an input port 28 to receive the communication signal collected by the antenna 12 and delivered by the coaxial cable 14 to the receive front-end 10. The filter 22 is generally designed to protect the low-noise amplifier 24 from signals collected by the antenna 12 outside of a passband of interest to the wireless communication system. The low-noise amplifier 24, in turn, is designed to strengthen signals at frequencies within the passband of the filter 22 without adding any significant noise, thereby essentially capturing or establishing the noise figure for the receive front-end 10. The filtered and amplified communication signal developed by the filter 22 and the low-noise amplifier 24 is then supplied to the equalizer 26, which has been designed to compensate for variances in group delay introduced by the filter 22 within the passband. The filter 22 may be a filter system, in the sense that multiple filters are cascaded to provide higher degrees of reflection. The filters may be separated by an isolator or other buffer, or an amplifier as described in co- pending, commonly assigned U.S. patent application Serial No. 09/130,274, the disclosure of which is hereby incorporated by reference. Regardless of the particular configuration, the filter or filters 22 are preferably HTS-based filters, in the sense that the filter 22 includes resonator or other components having an HTS material. The filter 22 further preferably includes multiple resonant cavities that utilize HTS materials to provide excellent rejection while maintaining an extremely low insertion loss. More particularly, such HTS filters may include one or more components having a thick film coating of an HTS material. Suitable HTS filters are available from Illinois Superconductor Corporation (Mt. Prospect, Illinois) for a number of cellular and PCS bands and may be obtained in combination with a low-noise amplifier in an integrated device marketed under the trademarks SpectrumMaster ® and RangeMaster ® . Other HTS and non-HTS filters suitable for practice of the present invention are further described in the above-identified co-pending application and/or will be described in greater detail hereinbelow. It should be noted that the filter 22 need not include thick-film resonators, or cavity-based resonators, but may be based on a thin-film superconducting material deposited on a suitable substrate that may be disposed in a resonant cavity. The filter 22 may also constitute a dual-mode, all-temperature filter as disclosed in co-pending and commonly assigned U.S. patent application Serial No. 09/158,631, the disclosure of which is also hereby incorporated by reference. Such dual-mode filters provide for continued operation during power failures that result in operating temperatures exceeding the critical temperature of the superconducting material. Other types of highly selective filters may be utilized in lieu of an HTS filter to provide high rejection with low insertion loss. For example, those skilled in the art are readily aware of non-superconducting filters that provide improved responses when operating in a cryogenic environment, as well as filters containing components made of certain non-superconducting ceramics that have low-noise floors. Regardless of the component materials of the filter, a highly selective filter (or alternatively a filter having a high selectivity), as used herein, constitutes any ten or more pole filter for a bandwidth of about 1 % or more (typically for full or whole band systems), or any filter having five or more poles for a bandwidth of about 0.3% or less (typically for channel-specific systems). More preferably, the filter 22 has twelve or more poles for a bandwidth of about 1 % or more, while the filter 22 has six or more poles for a bandwidth of about 0.3 % or less. Most preferably, the filter 22 has 16 or more poles for a bandwidth of about 1 % or more, while the filter 22 has eight or more poles for a bandwidth of about 0.3% or less. The HTS or other highly selective filter 22 also preferably has an additive noise contribution of about 1 dB or less, more preferably about 0.7 dB or less, and most preferably about 0.5 dB or less. It shall be noted that the foregoing noise figures are, of course, temperature-dependent and, therefore, may need to be adjusted therefor. The filter 22 is coupled to the low-noise amplifier 24 via a respective 50 Ohm coaxial cable or other suitable transmission line known to those skilled in the art. To avoid reflection, and, therefore, signal loss, the transmission line should be impedance-matched to the filter 22 and low-noise amplifier 24. The low-noise amplifier 24 outputs a filtered, amplified RF signal having a fixed amount of gain over a frequency range set to correspond with the passband of the filter 22. For example, the low-noise amplifier 24 may provide about 25 dB of gain over the frequency range 1850 to 1910 MHZ with a maximum noise figure of about 1.2 dB (at room temperature). The low-noise amplifier 24 may, but need not, be a GaAs- based amplifier to allow for operation at cryogenic temperatures. Such a low-noise amplifier is available from JCA Technology (Camarillo, California) as product number JC12-2342D. Alternatively, the low-noise amplifier 24 provides similar gain levels over a lower frequency range (824 to 849 MHZ). Such a low-noise amplifier is available from JCA Technology as product number JCA01-3149. The equalizer 26 is shown in greater detail in FIG. 2. In accordance with one embodiment of the present invention, the equalizer 24 includes a circulator 30 having an input port 32, an intermediate port 34, and an output port 36. The circulator 30 may be of a conventional design, but preferably is operable at cryogenic temperatures for compatibility with the other components of the receive front-end 10. Such a circulator is available from UTE Microwave, Inc.(Hasbury Park, New Jersey) as model number CT- 2409-N. In operation, the input port 32 is coupled to the low-noise amplifier 24 to receive the filtered and amplified communication signal having group delay variation, which is then passed to the intermediate port 34 for application to a one-port device 38. The one-port device 38 is preferably an all-pass network having a transfer function that compensates for the delay variation effected by the filter 22. The signal presented to the intermediate port 34 enters the one-port device 38 for processing by a plurality of resonators E-l through E-6, reflection after the resonator E-6, and further processing from the resonator E-6 back through the resonator E- 1. The resonators E-l through E-6 may include either HTS or non-HTS components and need not be operated in a cryogenic environment. However, in accordance with one aspect of the present invention, it is advantageous to have similar materials and/or components in both the filter 22 and the equalizer 26, inasmuch as the similar materials or components have similar temperature response characteristics. That is, even if both the filter 22 and the equalizer 26 are operated at cryogenic temperatures, any temperature deviation from the exact temperature at which the filter 22 and the equalizer 26 are tuned would otherwise affect the filter 22 and the equalizer 26 to differing degrees. As a result, any efforts to tune the components of the receive front-end 10 to achieve delay compensation may be thwarted by subsequent, uncontrolled events affecting the operating temperature of the receive front-end 10. The resonators E-l through E-6 of the one-port device 38 of the equalizer 26 may be realized as cavity resonators or in thin-film HTS technology. Thus, to the extent that HTS components are utilized, it should be understood that the resonators E-l through E-6 may incorporate either thick or thin film components. In the event that the resonators E-l through E-6 are cavity resonators, the respective cavities may be coupled via apertures disposed in interior walls of a device housing, which will be described in greater detail hereinbelow in connection with FIGS. 3 A and 3B. In accordance with one embodiment of the present invention, the manner in which the resonators E- 1 through E-6 are coupled is schematically shown in FIG. 2 by openings or gaps 40 in walls 42 separating adjacent resonators. The openings 40 are positioned to correspond with apertures shown and described in connection with FIGS. 3 A and 3B. The circulator 30 may be replaced by a 3-dB hybrid network, as is well known to those skilled in the art. Referring now to FIGS. 3A and 3B, the one-port device 38 of the equalizer 26 is shown in greater detail from two different perspectives. In accordance with one embodiment of the present invention, the resonators E- 1 through E-6 are realized in respective cavities defined by a housing 50 having a pair of end walls 52, an upper wall 54, and a lower wall 56. The housing 24 also includes a pair of plates (not shown) that are secured via screws or the like to the end walls 52, the upper wall 54, and the lower wall 56. The housing 50 also includes multiple inner walls 58 for separating adjacent resonant cavities E-l through E-6. Any inner walls 58 obscured from view or otherwise not shown in the two perspective views of FIGS. 3 A and 3B, such as those defining the cavities associated with the resonators E- 1, E-4, and E-5, are preferably similar to those shown in connection with the cavities more clearly shown. The housing 50 has an input/output cavity corresponding with the resonator E-l that includes an aperture 60 (FIG. 3 A) for insertion of an input/output coupling mechanism indicated generally at 62 (FIG. 3B). The coupling mechanism 62 includes an antenna (not shown) for propagating (or collecting) electromagnetic waves within the cavity associated with the resonator E-l . 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. An example of such a coupling mechanism is described in U.S. Patent No. 5,731,269, the disclosure of which is hereby incorporated by reference. The coupling mechanism 62 further includes a plate 64 for securing the antenna and a cable connector 66 to the side wall 52 as shown in FIG. 3B. Each cavity includes a resonant element (not shown), which preferably, in turn, includes a split-ring, toroidal resonator (see, for example, FIGS. 7 and 8). The toroidal resonator may be oriented within the cavity to achieve a certain degree and type of coupling, as shown in FIGS. 7 and 8 or otherwise as is known to those skilled in the art. Each toroidal resonator may be secured to the lower wall 56 by a dielectric mounting mechanism. The mounting mechanism may be secured to the lower wall 56 via screws (not shown) or the like that extend through apertures 68. The mounting mechanism may be positioned and constructed as shown in FIGS. 7 and 8. Further details on exemplary mounting mechanisms may be found in U.S. Patent No. 5,843,871, the disclosure of which is hereby incorporated by reference. Another suitable dielectric mounting mechanism is described and shown in U.S. Patent Application Serial No. 08/869,399, the disclosure of which is also hereby incorporated by reference. The tuning of the resonators E-l through E-6 is primarily adjusted by a tuning disk (not shown) that projects into the respective cavity near a gap (not shown) in the toroidal resonator. Each tuning disk is coupled to a screw assembly 70 that extends through an aperture (not shown) in the upper wall 54. 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 No. 5,843,871. With continued reference to Figs. 3 A and 3B, the resonators E-l and E-2 are coupled via a coupling aperture 72 in the inner wall 58. The positioning, size, and shape of the coupling aperture 72 may vary greatly, as will be appreciated by those skilled in the art. The resonator E-2 is coupled to the resonator E-3 by a coupling aperture 74, while the resonator E-3 is coupled to the resonator E-4 by a coupling aperture 76, which may be T- shaped. The resonators E-4 and E-5 may be coupled by an aperture (not shown) similar in size and shape to an aperture 78 that cross-couples the non-sequential resonators E-3 and E-6. Lastly, the resonators E-5 and E-6 are coupled by an aperture 80. Each of the above-identified apertures establish positive coupling between respective resonator pairs to establish complex zeros for delay compensation within the passband for the communication signal. It should be noted that the resonators E-l through E-6 are arranged in a zigzag configuration to allow for easy cross-coupling between the resonators E-3 and E-6 and to otherwise optimize performance. For example, establishing cross-coupling between the resonators E-3 and E-6 has been found to improve equalization performance to the extent that the six-resonator design shown in FIGS. 3 A and 3B performs on the order of an eight-resonator design. The zig-zag, six-stage configuration also allowed the coupling strengths to be minimized, which, in turn, decreased the extent of any undesired, non-adjacent coupling. In general, the configuration or layout of the resonators within the housing 50 allows for non-sequential resonators (e.g. , resonators E-3 and E- 6) that are to be cross-coupled to be adjacent. It should further be noted that the resonators E-l through E-6 and the coupling apertures depicted in, and described in connection with, FIGS. 3A and 3B may be designed to minimize the coupling between sequential resonators (e.g. , resonators E-l and E-2), as well as between non-sequential resonators, to avoid any undesired coupling between non-adjacent resonators. Undesired coupling may become particularly problematic due to the strong coupling possible when coupling resonators utilizing HTS components. Adjustment of the coupling between resonators E-l through E-6 to further tune the one -port device 38 and establish the complex zeros necessary for delay compensation is accomplished via coupling screws 82 disposed in apertures 84 in the upper wall 54. The apertures 84 are preferably positioned such that each coupling screw 82 projects into a respective aperture in the inner walls 58. The housing 50 of the one-port network 38 is preferably made of silver-coated aluminum, but may be made of a variety of materials having a low resistivity. Similarly, the split-ring resonators may be made of a low resistance metal and, in one embodiment of the present invention, be coated with an HTS material. Further details on the chemical composition and method for manufacturing such HTS materials may be found in U.S. Patent No. 5,789,347, the disclosure of which is hereby incorporated by reference. While the one-port device 38 shown in FIGS. 3A and 3B includes six resonators or cavities defining six stages, the one-port device 38 may have any number of resonators or stages as needed to achieve a certain degree of equalization. Generally, if about 90% of the passband requires some degree of equalization (e.g. , for a non-trivial amount of delay variance), the number of resonators needed in the one-port network 38 has been found to be approximately half the number of resonators in the filter 22 from which the delay variance arises. If additional portions of the passband also require equalization, the number of resonators may increase. It should be noted that the above approximations assume that the filter is a quasi-elliptic filter, as is each filter disclosed herein. In the event that Chebyshev or other filter types are utilized, less equalization may be needed, in which case a lesser number of resonators may be appropriate. Generally speaking, however, the HTS and other highly selective filters discussed hereinabove may generate a significant amount of delay variance, particularly in wide bandwidth systems such as W-CDMA. Such instances may require significant amounts of delay equalization, thereby requiring a high-performance equalizer. As used herein, a high-performance equalizer has a one-port device having six or more resonators for a wireless communication system having a bandwidth of about 1 % or more, and four or more resonators for a wireless communication system having a bandwidth of about 0.3% or less. More preferably, the one-port device 38 has eight or more resonators for a bandwidth of about 1 % or more, while the one-port device 38 has six or more resonators for a bandwidth of about 0.3% or less. Most preferably, the one-port device 38 has ten or more resonators for a bandwidth of about 1 % or more, while the device 38 has eight or more resonators for a bandwidth of about 0.3% or less. The equalizer 26 may be utilized with an HTS filter having a configuration similar to the one-port network 38 shown in FIGS. 3A and 3B. With reference to FIG. 4, the filter 22 may include sixteen resonators (F-l through F-l 6) that may be realized utilizing either thick or thin film technology. In eithe ' r case, the resonators F-l through F-l 6 may be arranged in two rows as shown to facilitate sequential and non-sequential coupling. The communication signal enters the filter 22 via an input port 100 and the resonator F-l, after which the signal is processed by the resonators F-2 through F-l 6 in a sequential manner through couplers 102, as shown schematically in FIG. 4. In the event that the filter 22 utilizes thick film technology, each coupler 102 may correspond with an aperture disposed in an interior wall (in much the same fashion as that described hereinabove in connection with the equalizer 26). Further details regarding the size, positioning, and shape of the apertures may be found in the above-reference application Serial Number 09/130,274. In general, however, the couplers 102 (and any coupling adjustment mechanism such as the screw described hereinabove) are designed to establish positive coupling for each sequential coupling pair of resonators. The filter 22 also includes couplers 104 for non-sequential coupling between the resonators F-6 and F-11 and between the resonators F-7 and F- 10. Such non-sequential coupling is designed to establish zeros of transmission for the filter 22 (rather than complex zeros for delay compensation) and may include coupling adjustment in a manner known to those skilled in the art (via screws and the like) to establish positive coupling between the resonators F-6 and F-11 , and negative coupling between the resonators F-7 and F-10. The strength or magnitude of such coupling is a matter of degree to be established on a case-by-case basis for each application of the present invention. After the communication signal has propagated around (sequentially) and about (non-sequentially) the resonators of the filter 22, the signal is coupled to an output port 106. Both the input port 100 and the output port 106 may be similar in design to the input/output coupling mechanism 62 (FIG. 3B) or any other suitable coupling port known to those skilled in the art. For certain applications of the present invention, the amount of delay compensation provided by the equalizer 26 may be insufficient. For example, the equalizer 26 may be limited in practice to a certain order by difficulties in tuning. In that event, it may be desirable to provide a fraction of the necessary delay compensation via the equalizer, with the remainder supplied internally via a redesigned filter. That is, in one embodiment of the present invention, the filter 22 shown in FIG. 4 is replaced by a filter indicated generally at 108 in FIG. 5. The filter 108 is said to be a self- equalizing filter, inasmuch as the resonators thereof are coupled in such a fashion as to provide the complex zeros necessary for delay equalization and compensation. With reference now to FIG. 5, the filter 108 is similar in many respects to the filter 22 of FIG. 4. For example, the filter 108 may be realized in either thin or thick film technology with input/output coupling arrangements similar to the input and output ports 100 and 106. Furthermore, the filter 108 is also preferably a highly selective filter, and may, as shown, have sixteen resonators SE-1 through SE-16, it being understood that different numbers of resonators may be appropriate for different applications. The resonators SE-1 through SE-16 are sequentially coupled via the couplers 102 in a manner similar to that described hereinabove in connection with FIG. 4, although the coupling strength or magnitude may need to be adjusted as described hereinbelow. The filter 108 differs from the filter 22 of FIG. 4 in several cross-coupling arrangements shown schematically in FIG. 5 between the following non-sequential resonators: SE-5 and SE-12; SE-6 and SE-11; and, SE-7 and SE-10. Through the complex zeros established by these cross-coupling arrangements, either all or at least a portion of the equalization that would otherwise be provided by the equalizer 26 is achieved internally rather than externally. In the embodiment shown schematically in FIG. 5 and to be further shown and described in connection with FIGS. 6A and 6B, the non- sequential or cross-coupling between the resonators SE-5 and SE-12 is positive, while the coupling between the resonators SE-6 and SE-11 and the coupling between the resonators SE-7 and SE-10 are both negative. With regard to quasi-elliptic filters with n= 16 (such as that shown in FIG. 5), while the coupling between the (n/2-l)/2 resonator (i.e. , the resonator SE-7) and the (n/2 + l)/2 resonator (i.e. , the resonator SE-10) is typically opposite in sign to, but similar to the coupling between the n 2-3 resonator (i.e. , the resonator SE-5) and the n/2+2 resonator (i.e. , the resonator SE-12), the coupling in accordance with this embodiment of the present invention between the resonators SE-7 and SE-10 is rather negative and similar to the coupling between the resonators SE-6 and SE-11 (which is also negative). With reference now to FIGS. 6 A and 6B, one embodiment of the self-equalized filter 108 is shown in greater detail, particularly in connection with the above-described coupling arrangements. The filter 108 includes many of the same structural components as the cavity resonator-based approach to the embodiment of the one-port network 38 shown in FIGS. 3 A and 3B, with the exception of two input/output ports rather than merely one. In fact, the similarities in design and structure are beneficial in proper implementation of the filter 108 in the receive front-end 10, inasmuch as the similar mechanisms and structures will all react similarly to temperature changes and, therefore, minimize the likelihood of a de-tuning of the receive front-end 10. To that end, the self-equalized filter includes a housing 150, end walls 152, an upper wall 154, a lower wall 156, inner walls 158, a pair of plates (not shown) for completely defining the cavities associated with the resonators SE-1 through SE-16, an input coupling mechanism 160 (FIG. 6B) for the resonator SE-1, an output coupling mechanism 162 for the resonator SE-16 (both of which are similar to the coupling mechanism 62), respective split-ring toroidal resonators disposed and mounted in each cavity (as shown in FIGS. 7 and 8), respective tuning disks (not shown) for each cavity, and respective coupling screws (not shown) for adjusting each sequential coupling, all of which are similar to that referenced hereinabove and, accordingly, will not be further described. Sequential coupling is also similarly established via coupling apertures 164 disposed in the inner walls 158. Those coupling apertures associated with sequential coupling in the row of resonators not shown in the perspective views of FIGS. 6A and 6B preferably correspond in size, shape, and position with the coupling apertures 164 associated with the row of resonant cavities actually shown. It should be noted, however, that the sequential or adjacent coupling between the resonators SE-8 and SE-9 was found to be too strong in the sense that the coupling between the resonators SE-8 and SE-9 was affecting the coupling of the resonators SE-7 and SE-10. As a result, a coupling aperture 166 was designed to decrease the coupling between the resonators SE-8 and SE-9. With regard to the non-sequential coupling utilized in part to establish the complex zeros necessary for delay equalization, a coupling aperture 168 effects the requisite positive coupling between the resonators SE-5 and SE-12, while coupling apertures 170 and 172 effect the requisite negative coupling between the resonators SE-6 and SE-11, and between the resonators SE-7 and SE-10, respectively. The coupling aperture 172 was also designed to be smaller in size to assist in efforts to reduce any spurious, non-sequential coupling via the resonators SE-8 and SE-9. By reducing the strength of the coupling between the resonators SE-7 and SE-10 (as well as between the resonators SE-8 and SE-9), the tuning of the self-equalized filter 108 was found, in practice, to be a much more manageable and practicable effort. Referring now to FIGS. 7 and 8, the negative coupling between the resonators SE-6 and SE-11 and the resonators SE-7 and SE-10 is facilitated by a coupling assembly indicated generally at 180. The resonators SE-7 and SE-10 are shown as implemented with a split-ring, toroidal resonator 181 and a mounting mechanism 182. The coupling assembly 180 is preferably disposed near the resonators 181 and in the coupling apertures 170 and 172, and includes a screw 182 having a screwhead 184. The screw 182 is movably disposed in an aperture (not shown) in the upper wall 154 for positioning a metallic bar 186 that acts as an antenna for establishing the cross-coupling. To this end, a washer 188 is disposed in a seat 190 formed in the upper wall 154 for maintaining screw position between adjustments. Further, the screw 182 is coupled to an elliptically shaped, dielectric insert 192 having a threaded interior for receiving the screw 182. The insert 192 is fitted for the coupling apertures 170 and 172 such that rotational movement of the screwhead 184 is prevented and thereby translated into vertical displacement of the metallic bar 186. In this manner, the metallic bar 186 may be positioned to establish the proper magnitude of coupling between the resonators to be cross-coupled. The cross-coupling, in turn, is designed to establish the requisite complex zeros for delay compensation. Utilizing the self-equalized filter 108 in conjunction with the equalizer 26 of FIG. 1 (i.e. , substituting the filter 22 with the filter 108) may significantly decrease the complexity of the one-port device 38 of the equalizer 26. For example, it has been found that the order of the one-port device may decrease from six to four in such an embodiment. Generally speaking, the filter 108 will provide a reasonable amount of internal equalization, which may be sufficient for completely compensating for any group delay variance in certain applications of the present invention. However, in the interest of reducing complexity in system design as well as minimizing problems associated with tuning the system, it may be desirable to add an external equalizer to provide further delay compensation regardless of whether internal equalization could provide, in theory, sufficient equalization. In practice, such an embodiment would allow a system designer or technician to tune the filter 108 and the equalizer 26 separately before the receive front-end 10 is connected to the communication system. The equalizer 26 may then be fine-tuned to ensure that system specifications for group delay equalization are met. It shall be further understood that the present invention is not limited to any particular physical configuration of the components in the receive front-end 10, inasmuch as the highly selective filters and other components described hereinabove may or may not be integrated into a single, standalone unit. Furthermore, the present invention is not limited to a receive front-end having either a simplex or duplex configuration, nor is it limited to use in a base station having a certain number of sectors. When the receive front-end 10 includes one or more HTS filters, the receive front-end 10 may include a bypass mechanism to determine when the HTS filters should or should not process an incoming signal. The election to bypass an HTS filter may be addressed by a controller integrated into the stand-alone unit, as described in commonly assigned and co-pending application Serial number 09/255,896, the disclosure of which is hereby incorporated by reference. In alternative embodiments, any one or more components of the receive front-end 10 need not include HTS-based components. Furthermore, such components may or may not be operated at non-cryogenic temperatures. It may, however, for purposes of tuning and other calibration, be less onerous to prepare and operate a system having similar components or components designed for operation at similar temperatures. The communication signal processed by the receive front-end 10 is not limited to any particular type of RF communication signal, nor is it limited to any one type of wireless communication signal. Accordingly, practice of the present invention is well-suited for, but limited to, PCS, cellular, and other wireless systems, and may be particularly well-suited for third generation (i.e. , "3G") and other systems having a wide bandwidth (and therefore more potential for delay variances), such as W-CDMA. Although the receive front-end 10 is particularly well suited for use with such wireless communication systems and is 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, receivers constructed pursuant to the teachings of the invention may be employed in any application which would benefit from the high performance filtering and/or delay equalization that it provides without departing from the scope or spirit of the invention. The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications would be obvious to those skilled in the art.

Claims: Claims: 1. A receive front-end for receiving a communication signal, comprising: a bandpass filter having a high selectivity for selecting a passband of the communication signal; an equalizer to compensate for group delay variance introduced by the high selectivity of the bandpass filter; and a low-noise amplifier protected by the bandpass filter from signals outside the passband and coupled to the equalizer to establish a noise figure for the receive front-end. 2. The receive front-end of claim 1, wherein the bandpass filter comprises a high-temperature superconducting component. 3. The receive front-end of claim 2, wherein the equalizer comprises a further high-temperature superconducting component. 4. The receive front-end of claim 3, wherein the bandpass filter, the equalizer, and the low-noise amplifier are disposed in a cryostat for operation in a cryogenic environment. 5. The receive front-end of claim 1, wherein the communication signal is selected from the group consisting of a PCS signal and a cellular signal. 6. The receive front-end of claim 1, wherein the equalizer comprises a plurality of resonators. 7. The receive front-end of claim 6, wherein: the equalizer further comprises a housing defining a plurality of cavities; and the plurality of resonators are disposed in the plurality of cavities, respectively. 8. The receive front-end of claim 6, wherein: the plurality of resonators comprises a pair of non-sequential resonators; and the pair of non-sequential resonators are coupled. 9. The receive front-end of claim 8, wherein the pair of nonsequential resonators are adjacent. 10. The receive front-end of claim 1 , wherein the equalizer comprises a high-performance, external equalizer. 11. The receive front-end of claim 1 , wherein the bandpass filter comprises a self-equalized filter. 12. The receive front-end of claim 11 , further comprising a cryostat in which the self-equalized filter is disposed. 13. The receive front-end of claim 11, wherein the self-equalized filter comprises a high-temperature superconducting component. 14. The receive front-end of claim 13, wherein the equalizer comprises a further high-temperature superconducting component. 15. A receive front-end for receiving a communication signal, comprising: a highly selective, bandpass filter that selects a passband of the communication signal and includes a plurality of coupled stages to establish complex zeros for delay compensation within the passband of the communication signal; and an equalizer coupled to the highly selective, bandpass filter to provide further delay compensation within the passband of the communication signal. 16. The receive front-end of claim 15, wherein the highly selective, bandpass filter comprises a high-temperature superconducting component. 17. The receive front end of claim 16, wherein the equalizer comprises a further high-temperature superconducting component. 18. The receive front-end of claim 15, further comprising a low- noise amplifier protected by the highly selective, bandpass filter from signals outside the passband and coupled to the equalizer to establish a noise figure for the receive front-end. 19. The receive front-end of claim 18, wherein the highly selective, bandpass filter, the equalizer, and the low-noise amplifier are disposed in a cryostat for operation in a cryogenic environment. 20. The receive front-end of claim 15, wherein the communication signal is selected from the group consisting of a PCS signal and a cellular signal. 21. The receive front-end of claim 15, wherein the equalizer comprises a plurality of resonators. 22. The receive front-end of claim 21, wherein: the equalizer further comprises a housing defining a plurality of cavities; and the plurality of resonators are disposed in the plurality of cavities, respectively. 23. The receive front-end of claim 22, wherein: the plurality of resonators comprises a pair of non-sequential resonators; and the pair of non-sequential resonators are coupled. 24. The receive front-end of claim 23, wherein the pair of nonsequential resonators are adjacent. 25. The receive front-end of claim 23, wherein the equalizer comprises a high-performance, external equalizer. 26. A bandpass filter having a passband, comprising: a plurality of resonators, each resonator comprising high-temperature superconducting material; wherein — the plurality of resonators includes a pair of non-sequential resonators; and the pair of non-sequential resonators are coupled to establish complex zeros for delay compensation within the passband. 27. The bandpass filter of claim 26, wherein the plurality of resonators are arranged such that the pair of non-sequential resonators are adjacent. 28. The bandpass filter of claim 26, further comprising a housing defining a plurality of cavities wherein the plurality of resonators are disposed in the plurality of cavities, respectively. 29. The bandpass filter of claim 28, wherein: the housing includes a wall separating the pair of non-sequential resonators; and the wall includes an aperture for cross-coupling the pair of nonsequential resonators. 30. The bandpass filter of claim 26 in combination with an equalizer coupled to the bandpass filter to provide further delay compensation. 31. The combination of claim 30, wherein the equalizer comprises a high-temperature superconducting component. 32. The combination of claim 30, further comprising a low-noise amplifier coupling the bandpass filter to the equalizer to establish a noise figure for the combination. 33. The combination of claim 32, wherein the equalizer is a high- performance equalizer. 34. The combination of claim 32, wherein the equalizer comprises a multiple-resonator device. 35. The combination of claim 34, wherein the multiple-resonator device comprises a housing defining a plurality of cavities. 36. The combination of claim 35, wherein the multiple-resonator device further comprises at least two, non-sequential resonators that are coupled. 37. The combination of claim 36, wherein the two, non-sequential resonators are adjacent.

Inventors: ABDELMONEM AMR, REMILLARD STEPHEN K, ABDELMONEM, AMR, REMILLARD, STEPHEN, K.
Applicants: ILLINOIS SUPERCONDUCTOR CORP, ILLINOIS SUPERCONDUCTOR CORPORATION
Application year: 2001
Main classification: H01P1
IPC codes: H01P1, H04B1, H04B3