Abstract:
Tunable duplexers and related methods are disclosed for use in communications networks. A tunable radiating duplexer can include a first antenna comprising a first variable capacitor and a second antenna comprising a second variable capacitor.

Description:
RELATED APPLICATIONS 
     The presently disclosed subject matter claims the benefit of U.S. Provisional Patent Application Ser. No. 61/125,747, filed Apr. 28, 2008, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The subject matter disclosed herein relates generally to duplexers for use in communications networks. More particularly, the subject matter disclosed herein relates to duplexers used in conjunction with communications antennas. 
     BACKGROUND 
     In communications systems, duplexers provide the ability to receive and transmit signals while using the same antenna. In a typical transmission operation, only signals of a designated transmission frequency are passed to an antenna, which transmits the signal as a radio signal into the air. In a typical receiving operation, a signal received by an antenna is transmitted to the duplexer to select only a signal of the designated frequency. A duplexer uses resonant circuits to isolate a transmitter from a receiver for allowing the transmitter and the receiver to operate on the same antenna at the same time without the transmitter adversely affecting the receiver. Duplexers use filters, such as various pass band filters and notches to accomplish isolation and continuity in signal transfer. In duplexer operation, filters must pass the desired signal while rejecting as much as possible of the undesired signals. 
     The increased diversity in the functionality of mobile phones has resulted in an increase in the complexity of duplexer design. For example, increased mobile phone functions such as dual mode (e.g., a combination of an analog mode and a digital mode, or a combination of digital modes, such as TDMA or CDMA), and a dual band (e.g., a combination of an 800 MHz band and a 1.9 GHz band, or a combination of a 900 MHz band and a 1.8 GHz band or a 1.5 GHz band) have been increasing the complexity of mobile phone architecture and circuitry. Increased implementation of frequency related functions affect antenna bandwidth. Antenna bandwidth is generally the range of frequencies over which the antenna can operate while some other characteristic remains in a given range. Therefore, increased frequency ranges increase demand for performance over a number of frequency channels, or a wide bandwidth antenna. Moreover, to support these multiple, diverse functions while maintaining proper isolation and reliable signal transfer between transmitter and receiver operations, present communication devices use fixed, redundant circuitry, such as an increased quantity of switches and filters to compensate and broaden duplexer capabilities. Accordingly, such increased use and quantity of filters creates the need for optimizing filter performance. 
     There is a continuing demand for component reduction and high performance communications devices. Elimination of redundant components, functions, or circuitry is highly desired in communication electronics. Increased performance in communication devices without increasing device size or weight is similarly desirable. Further, there is a continuing need for reliable and quality signal transfer, improved transmitter-receiver isolation, and very high Q value circuitry with respect to duplexers. In addition, further considerations include polarization, tradeoffs between isolation and size, tuning precision, and transmit/receive frequency spacing for a given band versus wholesale tuning between bands. 
     Micro-electro-mechanical system (MEMS) technology is currently implemented for various filtering circuitry. Exemplary MEMS components that have been used for filtering include MEMS capacitors and acoustic resonators. Although there have been improvements in the development of MEMS components for filtering, there is a continuing need for improved performance and stability of these components as well as tunability for optimal performance and multi-band applications. 
     Therefore, room for improvement exists for improved duplexer circuitry and related components and improved MEMS components for use in duplexer circuitry. 
     SUMMARY 
     In accordance with this disclosure, duplexers and related methods for use in communications networks are provided. Specifically, in one embodiment, a tunable radiating duplexer is provided and can include a first antenna comprising a first variable capacitor and a second antenna comprising a second variable capacitor. 
     It is an object of the presently disclosed subject matter to provide a tunable duplexing antenna that can combine the functions of an antenna and a duplexer (and potentially also a matching network and/or filters) and that can lower insertion losses, provide a smaller overall solutions and that may provide operation over multiple communication bands. 
     Some of the objects of the subject matter disclosed herein having been stated hereinabove, and which are achieved in whole or in part by the presently disclosed subject matter, other objects will become evident as the description proceeds when taken in connection with the accompanying drawings as best described hereinbelow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the present subject matter will be more readily understood from the following detailed description which should be read in conjunction with the accompanying drawings that are given merely by way of explanatory and non-limiting example, and in which: 
         FIG. 1  is a schematic drawing of a tunable radiating duplexer according to an embodiment of the presently disclosed subject matter; 
         FIGS. 2A and 2B  are plan views of arrangements for tunable antennas in a tunable radiating duplexer according to two different embodiments of the presently disclosed subject matter; 
         FIG. 3  is a side view of a tunable radiating duplexer according to an in-line embodiment of the presently disclosed subject matter; 
         FIG. 4  is a plan view of a tunable radiating duplexer according to a side-by-side embodiment of the presently disclosed subject matter; 
         FIG. 5  is a perspective view of a tunable radiating duplexer according to a side-by-side embodiment of the presently disclosed subject matter; 
         FIGS. 6A through 6C  are return loss graphs for different configurations of the tunable radiating duplexer illustrated in  FIG. 5 ; 
         FIG. 7  is a plan view of a tunable radiating duplexer according to a side-by-side embodiment of the presently disclosed subject matter in which the antennas are positioned opposing each other; 
         FIG. 8  is a plan view of a tunable radiating duplexer according to a side-by-side embodiment of the presently disclosed subject matter in which an isolation fence separates the antennas; and 
         FIG. 9  is a schematic drawing of a tunable radiating duplexer according to an embodiment of the presently disclosed subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     The present subject matter provides designs and methods for tunable duplexing antennas. In one aspect, the present subject matter provides a tunable radiating duplexer, generally designated  100  in  FIG. 1 , which can include two or more tunable narrow band antennas in or on a single substrate. For instance, tunable radiating duplexer can include a first antenna  101  in communication with a receive terminal RX and a second antenna  102  in communication with a transmit terminal TX. First antenna  101  can further include a first tunable filter F 1  (e.g., a TX filter and/or matching network, if required), and second antenna  102  can include a second tunable filter F 2  (e.g., a harmonic filter and/or matching network, if required). First and second antennas  101  and  102  can be tuned to address multiple frequency bands, and can be tuned to adjust frequency spacing between the elements. For example, a distance between the first and second antennas can be less than about ¼ of a free-space wavelength, and/or first and second antennas  101  and  102  can be sized less than about ¼ of a free-space wavelength. Further, the first and second antennas can be co-polarized. 
     As a result, a tunable antenna of the type in  FIG. 1  can be smaller than a full-band antenna and much smaller than multi-band antenna. In particular, more than two tunable antennas can be configured to fit into a volume of an existing fixed antenna. In addition, if two antennas with individual feeds are closely spaced but tuned to slightly different frequencies, there will be substantial isolation between the ports. As a result, tunable duplexing antennas can provide a number of advantages over current duplexer designs. Specifically, tunable duplexing antennas can combine the functions of an antenna, a duplexer, a matching network, and/or filters. Further, tunable duplexing antennas can produce lower insertion losses by optimizing matches (e.g. about 1 dB), improving antenna efficiency (e.g., about 0.5 dB), and eliminating duplexer losses (e.g., about 3 dB). 
     As is shown in  FIGS. 2A and 2B , tunable radiating duplexer  100  can be provided in a variety of different configurations. For instance, referring to  FIG. 2A , tunable radiating duplexer  100  can include first antenna  101  and second antenna  102  arranged in a back-to-back configuration. In this arrangement, first antenna  101  and second antenna  102  can be positioned such that they share a common short (S 1 , S 2 ) and have tunable end-loads  111  and  112 , respectively. In particular, end loads  111  and  112  can, respectively, be a first and second variable capacitor, such as a MEMS capacitor. The configuration with first and second antennas  101  and  102  being arranged in a back-to-back arrangement can provide desirable isolation for antennas  101  and  102 . 
     Alternatively and as shown in  FIG. 2B , first antenna  101  and second antenna  102  can be arranged in a side-by-side configuration. Such a side-by-side configuration can involve first and second antennas  101  and  102  aligned in the same direction as shown, or first and second antennas  101  and  102  can be aligned in opposing directions. In this kind of configuration, each of first and second antennas  101  and  102  can be connected to an independent short S 1  and S 2 , respectively. 
     Each different configuration provides different characteristics to tunable radiating duplexer  100 . For instance, with an in-line configuration (i.e., back-to-back configuration shown in  FIG. 2A ), coupling can be dominated by mutual inductance, whereas with a side-by-side configuration, coupling is a combination of mutual capacitance and mutual inductance, and ground return currents can yield a magnetic quadrupole. Regardless of the specific arrangement, however, similar elements can be incorporated into the system. Specifically, tunable radiating duplexer  100  can be configured such that each of first and second antennas  101  and  102  includes a tunable end load (e.g., a variable capacitor) including first and second terminals and an antenna comprising first and second transmission lines each including first and second ends. The first ends of the transmission lines can be connected to the circuit terminal and the second ends of the transmission lines can be connected to the first terminal of the variable capacitor and to a ground terminal, respectively. In addition, the second terminal of the variable capacitor can be connected to a ground terminal. Again as described with reference to  FIG. 1 , first antenna  101  can be in communication with receive terminal RX and second antenna  102  can be in communication with transmit terminal TX in both of the duplexer or antenna embodiments shown in  FIGS. 2A and 2B . 
       FIG. 3  provides a more detailed view of a specific embodiment tunable radiating duplexer  100  in a back-to-back configuration. Tunable radiating duplexer  100  can include a substrate  200  on which both first antenna  101  and second antenna  102  can be formed (e.g., a Roger R4350 PCB). First antenna  101  can have first and second transmission lines generally designated T 1 - 1  and T 1 - 2 , respectively, with first transmission line T 1 - 1  connecting receive terminal RX to a first terminal of end load  111  (e.g., a tunable capacitor) and second transmission line T 1 - 2  connected to a ground Gnd. Similarly, second antenna  102  can have first and second transmission lines T 2 - 1  and T 2 - 2 , respectively, with first transmission line T 2 - 1  connecting transmit terminal TX to a first terminal of end load  112  (e.g., a tunable capacitor) and second transmission line T 2 - 2  connected to ground Gnd. The first and second transmission lines of at least one of first and second antennas  101  and  102  can be attached to opposing sides of substrate  200 . Also, as noted above, tunable radiating duplexer  100  can include at least one short (S 1 , S 2 ), wherein the second ends of the first transmission lines can be connected to the at least one short. 
     The first transmission lines of first and second antennas  101  and  102  can be substantially parallel to the second transmission lines of first and second antennas  101  and  102 , respectively. Further, in the back-to-back configuration shown in  FIG. 3 , the first transmission lines of first and second antennas  101  and  102  can be at least substantially aligned so as to be collinear with each other. In addition, the first transmission lines of first and second antennas  101  and  102  can be at least substantially parallel to each another and can be between 1/50 and ¼ of a free space wavelength in length. Similarly, one or both of second transmission lines T 1 - 2  and T 2 - 2  of first and second antennas  101  and  102 , respectively, can be sized between about 1/50 and ¼ of a free space wavelength. 
     The first transmission lines of first and second antennas  101  and  102  have different lengths d 1  and d 2 , respectively, with the ratio approximately equal to a default transmit/receive frequency ratio. For instance, the length d 1  of first transmission line T 1 - 1  of first antenna  101  can be about 10 mm, whereas the length d 2  of first transmission line T 2 - 1  of second antenna  102  can be greater than 10 mm. Alternatively, the first transmission lines of first and second antennas  101  and  102  have substantially the same length. 
       FIGS. 4 and 5  illustrate a different configuration for tunable radiating duplexer  100 . In this alternative configuration, first and second antennas  101  and  102  can be arranged in a side-by-side configuration (e.g., as part of a Planar Inverted F Antenna). First and second antennas  101  and  102  can be arranged such that they are facing the same direction (i.e., end loads  111  and  112  are on a same end of first and second antennas  101  and  102 , respectively, relative to each other). First antenna  101  can have a first width w 1 , second antenna  102  can have a second width w 2 , and first and second antennas  101  and  102  can be separated by a distance d 3 . For instance, the first transmission lines of first and second antennas  101  and  102  can be spaced from one another between about 1/50 and ¼ of a free space wavelength. Widths w 1  and w 2  of the antennas can influence efficiency and coupling, while distance d 3  between them can influence individual radiating frequencies and isolation. 
     First and second antennas  101  and  102  can be connected to first and second end loads  111  and  112 , respectively, which can be tuning capacitors or single components with multiple terminals. As with the previous configuration, the first transmission lines of first and second antennas  101  and  102  have different lengths d 1  and d 2 , respectively, with the ratio approximately equal to a default transmit/receive frequency ratio. For example,  FIG. 5  illustrates an example of a tuned PCB PIFA Duplexer in which the lengths of first and second antennas  101  and  102  can be offset so that the frequency at transmit terminal TX is lower than the frequency at receive terminal RX under the same end loading conditions. Alternatively, the lengths can be designed to be the same, but the capacitance on second antenna  102  (i.e. connected to transmit terminal TX) is higher. Again as described previously, first antenna  101  can be in communication with receive terminal RX and second antenna  102  can be in communication with transmit terminal TX in both of the duplexer or antenna embodiments shown in  FIGS. 2A and 2B . 
     Referring to  FIGS. 6A through 6C , return loss graphs for a tuned PCB PIFA duplexer are provided. Referring to  FIG. 6A , it can be seen that if both end loads  111  and  112  (e.g., MEMS capacitor banks) are tuned together to sweep transmit and receive signals (i.e., TX/RX) as a pair, the isolation can be between about 12-14 dB, the return loss can be about 14-15 dB, and the spacing can be about 169-187 MHz. Referring to  FIG. 6B , however, it can be seen that if one of end loads  111  or  112  (end load  111  in  FIG. 6B ) is tuned to adjust the spacing between the transmit/receive frequencies, the isolation can be about 8-15 dB, the return loss can be about 14-16 dB, and the tuned spacing can be between 111 and 263 MHz with a 0.2 pF change. Further, in the third example configuration shown in  FIG. 6C , it can be seen that if the lateral spacing between antenna elements is adjusted (i.e., the gap varied from about 1 to 6 mm), and the end loads  111  and  112  held at a constant 1 pF, the isolation at low spacing (e.g., about 1 mm) is only about 5 dB. Good isolation can be achieved, however, where the spacing is on the order of one-half the antenna length (e.g., about 6 mm for a 10 mm antenna). It is also noted that inter-coupling also affects frequency spacing, thereby causing larger transmit/receive separation. 
     Still further alternative configurations are shown in  FIGS. 7 and 8 . In the configuration shown in  FIG. 7 , first and second antennas  101  and  102  can be arranged in a side-by-side configuration, but in opposing directions (i.e., end loads  111  and  112  are on different ends of first and second antennas  101  and  102 , respectively, relative to each other), which can improve isolation. Tunable radiating duplexer  100  can further optionally include a grounded isolation fence IF positioned between first and second antennas  101  and  102 , which can also serve to improve isolation. In the configuration shown in  FIG. 8 , first and second antennas  101  and  102  are arranged to be facing in the same direction in a side-by-side configuration with a grounded isolation fence IF positioned between them. Of course, tunable radiating duplexer  100  can be used for a variety of applications beyond PIFA designs, including loops, a directly tuned cellular antenna, a DVB-H antenna, cellular diversity antenna, or any other loaded antenna concept, whether they be PCB based or case-mounted. 
     Regardless of the specific configuration of first and second antennas  101  and  102 , tunable radiating duplexer  100  can include a tunable matching network on a circuit terminal of each of first and second antennas  101  and  102 . Specifically, tunable radiating duplexer  100  can include RF circuitry coupled to either or both of the antenna circuit ports to provide amplification and filtering for transmit and receive signals. For instance, referring to  FIG. 9 , amplifiers, filters, and/or matching circuits can be included in the design of tunable radiating duplexer  100 . Specifically, elements such as a low noise amplifier LNA and/or a first filter F 1  (e.g., a matching network, TX filter or other blockers) can be provided in communication with receive terminal RX, and other elements such as a power amplifier PA or a second filter F 2  (e.g., a harmonics or operating point matching filter) can be provided in communication with transmit terminal TX. 
     The present subject matter can be embodied in other forms without departure from the spirit and essential characteristics thereof. The embodiments described therefore are to be considered in all respects as illustrative and not restrictive. Although the present subject matter has been described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art are also within the scope of the present subject matter.