Abstract:
A filter circuit includes: a filter element having a first terminal connected to an antenna, a second terminal connected to a receiving circuit, and a third terminal connected to a transmission circuit; a first inductor, a second inductor, and a third inductor connected in series between the first terminal and the third terminal of the filter element; a fourth inductor that has one end connected to a connecting node connecting the first inductor and the second inductor and that has the other end grounded; and a fifth inductor that has one end connected to a connecting node connecting the second inductor and the third inductor and that has the other end grounded.

Description:
This application claims the benefit of Japanese Application No. 2012-013737, filed in Japan on Jan. 26, 2012, which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a filtering technology in a front end module of a wireless communication device. 
     2. Description of Related Art 
     In wireless communication devices, reduction in size of a front end module including a duplexer is sought after. In order to achieve the size reduction, some of the components provided in the module are formed inside a substrate. However, it is difficult to form a capacitor or an inductor inside the substrate when it has a large element value. 
     A front end module as shown in  FIG. 1 , for example, has been traditionally used. In the front end module shown in  FIG. 1 , one end of an inductor L 101  (2.2 nH, for example) is connected to a terminal  1  of an SAW (Surface Acoustic Wave) duplexer  1001 , which is connected to an antenna (ANT), and the other end of the inductor L 101  is grounded. One end of an inductor L 100  (4.9 nH, for example) is connected to a terminal  2  of the SAW duplexer  1001 , which is connected to a transmission circuit, and the other end of the inductor L 100  is grounded. Between a terminal  3  and a terminal  4  of the SAW duplexer  1001 , which are connected to a receiving circuit, an inductor L 102  (8.2 nH, for example) and the primary coil of a transformer T 101  are connected in parallel. One end of the secondary coil of the transformer T 101  is grounded, and the other end thereof is connected to another circuit. Terminals  5  and  6  of the SAW duplexer  1001  are grounded. The inductors L 100  and L 101  are matching elements. 
       FIG. 2  shows frequency characteristics of this front end module. In  FIG. 2 , the frequency characteristics at the transmitting terminal (TX) and the frequency characteristics at the receiving terminal (RX) are shown. The horizontal axis represents the frequency, and the vertical axis represents the gain. If a wireless communication device is designed to receive through an antenna a signal in the band around 1.5 GHz for GPS (Global Positioning System) and a signal in the band around 2.11 GHz to 2.17 GHz for receiving wireless communication, for example, because the transmission frequency characteristics show excessive gain and the insufficient attenuation amount in the areas indicated with the circles in  FIG. 2 , a noise that is generated by the transmitting terminal is received by the receiving terminal, resulting in a problem. 
     Although there exist various conventional technologies with regard to a front end module, a technology that can solve such a problem and that can also achieve the size reduction has not yet been provided. 
     RELATED ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2005-124139 
     Patent Document 2: WO2010/116776 
     SUMMARY OF THE INVENTION 
     Thus, an object of the present invention, in one aspect, is to provide a technology that makes it possible to reduce the size of a frond end module that can achieve desired frequency characteristics. 
     Additional or separate features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect, a filter circuit according to the present invention includes: (A) a filter element having a first terminal connected to an antenna, a second terminal connected to a receiving circuit, and a third terminal connected to a transmission circuit; (B) a first inductor, a second inductor, and a third inductor that are connected in series between the first terminal and the third terminal of the filter element; (C) a fourth inductor having one end connected to a connecting node connecting the first inductor and the second inductor, the fourth inductor having the other end grounded; and (D) a fifth inductor having one end connected to a connecting node connecting the second inductor and the third inductor, the fifth inductor having the other end grounded. 
     By employing such an inductor configuration, even if the inductance values of the second, fourth, and fifth inductors are small, the sufficient attenuation amount can be ensured in desired frequency bands. 
     In the above-mentioned filter circuit, the second inductor, the fourth inductor, and the fifth inductor may be formed inside a ceramic substrate where the filter element is provided. This makes it possible to further reduce the size of a front end module. 
     The above-mentioned filter circuit may further include a capacitor that is connected in parallel with the third inductor or in parallel with the first inductor. By adjusting the capacitance value of this capacitor, it becomes possible to make a further adjustment in the desired frequency bands. 
     The above-mentioned filter element may be a surface acoustic wave filter. In another aspect, a substrate structure for a filter circuit according to the present invention includes: a substrate; a first terminal on the substrate to be connected to the filter and an antenna; a second terminal on the substrate to be connected to the filter and a receiving circuit; a third terminal on the substrate to be connected to the filter and a transmission circuit; a first inductor, a second inductor, and a third inductor, the first, second, and third inductors being connected in series between the first terminal and the third terminal, the second inductor being provided in the substrate, the first and second inductors being provided on the substrate; a fourth inductor in the substrate, the fourth inductor having one end connected to a connecting node connecting the first inductor to the second inductor, and having the other end grounded; and a fifth inductor in the substrate, the fifth inductor having one end connected to a connecting node connecting the second inductor to the third inductor, and having the other end grounded. In the above-mentioned substrate structure, the substrate may be a ceramic substrate. In the above-mentioned substrate structure, the substrate may include a ceramic substrate and a printed circuit board under the ceramic substrate, and the second, fourth, and fifth inductors may be formed in the ceramic substrate. 
     Circuits described in the following embodiments are examples of possible circuits, and various modifications can be made thereto. 
     According to one aspect of the present invention, it becomes possible to reduce the size of a front end module that can achieve desired frequency characteristics. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit example of a conventional front end module. 
         FIG. 2  shows the frequency characteristics of the circuit example of the conventional front end module. 
         FIG. 3  shows a circuit example of a front end module. 
         FIG. 4  shows frequency characteristics of the circuit example of the front end module. 
         FIG. 5  shows a circuit example according to Embodiment 1. 
         FIG. 6A  is an explanatory diagram for Y-Δ conversion. 
         FIG. 6B  is an explanatory diagram for Y-Δ conversion. 
         FIG. 6C  is an explanatory diagram for Δ-Y conversion. 
         FIG. 6D  is an explanatory diagram for Δ-Y conversion. 
         FIG. 7  shows the frequency characteristics of the circuit example of Embodiment 1. 
         FIG. 8  is a perspective view showing a mounting example. 
         FIG. 9  is a cross-sectional view of a mounting substrate. 
         FIG. 10  shows the first layer of the substrate. 
         FIG. 11  shows the second layer of the substrate. 
         FIG. 12  shows the third layer of the substrate. 
         FIG. 13  shows the fourth layer of the substrate. 
         FIG. 14  shows a circuit example of Embodiment 2. 
         FIG. 15  shows the frequency characteristics of the circuit example of Embodiment 2. 
         FIG. 16  shows a modification example of Embodiment 2. 
         FIG. 17  shows the frequency characteristics of the modification example of Embodiment 2. 
         FIG. 18  shows the second modification example of Embodiment 2. 
         FIG. 19  shows the frequency characteristics of the second modification example of Embodiment 2. 
         FIG. 20  shows the third modification example of Embodiment 2. 
         FIG. 21  shows frequency characteristics of the third modification example of Embodiment 2. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Desired frequency characteristics cannot be obtained with the circuit shown in  FIG. 1 . To solve this problem, a configuration shown in  FIG. 3  in which the terminal  1  and the terminal  2  of the SAW duplexer  1001  are connected to an inductor L 103  (150 nH, for example) can be employed. With this circuit, the frequency characteristics shown in  FIG. 4  are obtained. In a manner similar to  FIG. 2 , in the example of  FIG. 4 , the frequency characteristics at the transmitting terminal (TX) and the frequency characteristics at the receiving terminal (RX) are shown. The horizontal axis represents the frequency, and the vertical axis represents the gain. As indicated with the circles in  FIG. 4 , in the transmission frequency characteristics, the gain in the 1.5 GHz band and the gain in the 2.11 GHz to 2.17 GHz band are sufficiently low. That is, by adjusting the inductance value of the inductor L 103  and the like, the gain in the desired frequency bands can be lowered so as not to adversely affect the signal reception. However, in this configuration, if the inductance value of the inductor L 103  is reduced to a level that is low enough to allow the inductor to be formed inside the substrate, it would become impossible to reduce the transmission frequency characteristics in the desired frequency bands. This means that the inductor L 103  must be mounted as a separate element, which results in an increase in the size of the front end module. 
     In order to make possible the reduction in size of the front end module that can achieve desired frequency characteristics, the following embodiments are provided. 
     Embodiment 1 
       FIG. 5  shows an example of a circuit of a front end module according to Embodiment 1. One end of an inductor L 2  (2.2 nH, for example) is connected to a terminal  1  of an SAW duplexer  101 , which is connected to an antenna (ANT), and the other end of the inductor L 2  is connected to one end of an inductor L 5  (1.7 nH, for example) and one end of an inductor L 4  (0.27 nH, for example). The other end of the inductor L 4  is grounded. 
     One end of an inductor L 1  (4.9 nH, for example) is connected to a terminal  2  of the SAW duplexer  101 , which is connected to a transmission circuit, and the other end of the inductor L 1  is connected to the other end of the inductor L 5  and one end of an inductor L 3  (0.68 nH, for example). The other end of the inductor L 3  is grounded. 
     Between a terminal  3  and a terminal  4  of the SAW duplexer  101 , which are connected to a receiving circuit, an inductor L 6  (8.2 nH, for example) and the primary coil of a transformer T 1  are connected in parallel. One end of the secondary coil of the transformer T 1  is grounded, and the other end thereof is connected to another circuit. The terminals  5  and  6  of the SAW duplexer  101  are grounded. 
     In the above configuration, while the inductors L 1  and L 2  are not changed from those in the conventional example, the inductors L 3 , L 4 , and L 5  are provided in place of the inductor of 150 nH. However, the inductance value of the inductor L 5  is 1.7 nH, the inductance value of the inductor L 3  is 0.68 nH, and the inductance value of the inductor L 4  is 0.27 nH, and because of such small inductance values, these inductors can be formed inside a ceramic substrate (LTCC: Low Temperature C 0 -fired Ceramics, for example), which makes possible the reduction in size. 
     Techniques called the Y-Δ conversion and the Δ-Y conversion, which is the reverse thereof, are known. Using there conversions, the newly developed configuration of  FIG. 5  can be shown to be substantially equivalent to the circuit shown in  FIG. 3  in terms of circuit operations as follows. A circuit shown in  FIG. 6A  in which inductors are connected in the letter Y shape is an equivalent circuit of a circuit shown in  FIG. 6B  in which inductors are connected in a reverse Δ shape. Conversely, a circuit shown in  FIG. 6C  in which inductors are connected in a reverse Δ shape is an equivalent circuit of a circuit shown in  FIG. 6D  in which inductors are connected in the letter Y shape. The inductors in the circuit shown in  FIG. 3  are connected in the same manner as those in  FIG. 6B , and can thus be converted to the circuit shown in  FIG. 6A . When the top two inductors in  FIG. 6A  are respectively divided into two inductors, the circuit shown in  FIG. 6D  can be obtained, and therefore, the circuit shown in  FIG. 6A  can be converted to the circuit shown in  FIG. 6C , which is an equivalent circuit of the circuit shown in  FIG. 6D . The circuit shown in  FIG. 6C  has the same configuration as that of the circuit in  FIG. 5 . It should be noted that the respective inductance values need to be calculated so as to obtain precise equivalent circuits. There are, of course, numerous ways to construct equivalent circuits of a particular circuit. Among such numerous possibilities, the present inventor has devised space-efficient and effective circuit configurations that make possible implementation into a small package. 
       FIG. 7  shows frequency characteristics of the circuit shown in  FIG. 5 . In  FIG. 7 , the frequency characteristics at the transmitting terminal (TX) and the frequency characteristics at the receiving terminal (RX) are shown. The horizontal axis represents the frequency, and the vertical axis represents the gain. Although these characteristics do not completely coincide with those of  FIG. 4 , the transmission frequency characteristics in the 1.5 GHz band and in the 2.11 GHz to 2.17 GHz band are sufficiently low as indicated with the circles in  FIG. 7 . That is, desired frequency characteristics can be obtained in the bands that are used for GPS and for wireless communication, respectively. 
     Next, an example of forming the inductors L 3  to L 5  inside the actual LTCC will be described.  FIG. 8  is a perspective view of the substrate. The substrate includes an LTCC portion A and a printed circuit board portion B, and on a surface of the LTCC portion A, electrodes for the inductor L 1 , electrodes for the inductor L 2 , electrodes for the inductor L 6 , and eight electrodes for the SAW duplexer  101  are formed. Although the SAW duplexer  101  in  FIG. 5  was shown to have six terminals, because additional two terminals are for grounding, the SAW duplexer  101  here is substantially the same as that in  FIG. 5 . As shown in the perspective view, inside the LTCC portion A, the inductor L 3  is formed below the electrode for the inductor  1 , the inductor L 4  is formed below the electrode for the inductor L 2 , and the inductor L 5  is formed by a line connecting the inductor L 3  to the inductor L 4 . 
       FIG. 9  is a cross-sectional view along the line XX′ in  FIG. 8 . As shown in the figure, the LTCC portion A has four electrode layers. The vertical black portions are holes that connect different electrodes to each other. These electrode layers are represented as Layer  1 , Layer  2 , Layer  3 , and Layer  4  from the top. 
       FIG. 10  shows Layer  1  that is the surface of the LTCC portion A. As also shown in  FIG. 8 , the eight electrodes for the SAW duplexer  101  and the respective electrodes for the inductor L 1 , the inductor L 2 , and the inductor L 6  are formed in Layer  1 . 
       FIG. 11  shows Layer  2  inside the LTCC portion A. In Layer  2 , a part of the inductor L 4 , a part of the inductor L 3 , and the entire inductor L 5  that connects the inductor L 3  to the inductor L 4  are formed. 
       FIG. 12  shows Layer  3  inside the LTCC portion A. In Layer  3 , ground sections, a part of the inductor L 4 , and a part of the inductor L 3  are formed. 
       FIG. 13  shows Layer  4  inside the LTCC portion A. The entire Layer  4  is a ground layer. 
     As described above, because the inductor L 3 , the inductor L 4 , and the inductor L 5  are formed inside the LTCC, the reduction in size of the front end module can be made possible. 
     Embodiment 2 
     The inductance values of the inductors used in the circuit shown in  FIG. 5  are set in accordance with the characteristics of the SAW duplexer  101 . Therefore, when another SAW duplexer  111  is used, different inductors need to be used.  FIG. 14  shows a circuit example of a front end module of the present embodiment. The basic circuit configuration is similar to that of  FIG. 5 . In  FIG. 14 , an inductor L 11  (4.7 nH, for example) replacing the inductor L 1 , an inductor L 12  (2.8 nH, for example) replacing the inductor L 2 , an inductor L 13  (0.5 nH, for example) replacing the inductor L 3 , an inductor L 14  (0.3 nH, for example) replacing the inductor L 4 , an inductor L 15  (0.9 nH, for example) replacing the inductor L 5 , and an inductor L 16  (8 nH, for example) replacing the inductor L 6  are used. 
       FIG. 15  shows frequency characteristics of the circuit shown in  FIG. 14 . In  FIG. 15 , the frequency characteristics at the transmitting terminal (TX) and the frequency characteristics at the receiving terminal (RX) are shown. The horizontal axis represents the frequency, and the vertical axis represents the gain. In the frequency characteristics shown in  FIG. 15 , the sufficiently large attenuation amount is obtained around the 2.11 GHz to 2.17 GHz band, thus causing no adverse effect to the receiving circuit in wireless communication. On the other hand, around the 1.5 GHz band for GPS, although the attenuation amount is large in the vicinity thereof, the frequency at which the attenuation amount reaches its peak is slightly off from 1.5 GHz. One may consider adjusting the inductance values of the inductors in a manner similar to Embodiment 1 in order to change the peak frequency, but instead, the following modifications are made in the present embodiment. 
       FIG. 16  shows a circuit after modification. One end of an inductor L 22  (2.8 nH, for example) is connected to a terminal  1  of the SAW duplexer  111 , which is connected to an antenna (ANT), and the other end of the inductor L 22  is connected to one end of an inductor L 25  (0.9 nH, for example) and one end of an inductor L 24  (0.3 nH, for example). The other end of the inductor L 24  is grounded. 
     Also, one end of an inductor L 21  (4.7 nH, for example) and one end of a capacitor C 1  (0.3 pF, for example) are connected to a terminal  2  of the SAW duplexer  111 , which is connected to the transmission circuit. The other end of the inductor L 21  and the other end of the capacitor C 1  are connected to the other end of the inductor L 25  and one end of an inductor L 23  (0.5 nH, for example). The other end of the inductor L 23  is grounded. That is, the inductor L 21  and the capacitor C 1  are connected in parallel. 
     Between a terminal  3  and a terminal  4  of the SAW duplexer  111 , which are connected to the receiving circuit, an inductor L 26  (8 nH, for example) and the primary coil of a transformer T 3  are connected in parallel. One end of the secondary coil of the transformer T 3  is grounded, and the other end thereof is connected to another circuit. 
     When the capacitor C 1  is provided as described above, frequency characteristics shown in  FIG. 17  are obtained. In  FIG. 17 , the frequency characteristics at the transmitting terminal (TX) and the frequency characteristics at the receiving terminal (RX) are shown. The horizontal axis represents the frequency, and the vertical axis represents the gain. As indicated with E and F in  FIG. 17 , the sufficient attenuation amount is ensured around the 1.5 GHz band for GPS and around the 2.11 GHz to 2.17 GHz band for receiving the wireless communication. This modification is particularly effective for the 1.5 GHz band, but because the characteristics around the 2.11 GHz to 2.17 GHz band are also changed as compared with those in  FIG. 15 , the capacitance value of the capacitor C 1  need to be adjusted so as to obtain the desired frequency characteristics in both bands. 
     Because the inductors are connected in the letter H shape, and are symmetrical, it is also possible to connect a capacitor C 2  in parallel with the inductor L 22  as shown in  FIG. 18 , instead of connecting the capacitor C 1  in parallel with the inductor L 21 . In this case, the frequency characteristics shown in  FIG. 19  are obtained. As indicated with the circles G and H in  FIG. 19 , the frequencies at which the attenuation amount reaches its peak are different from those in the frequency characteristics shown in  FIG. 17 . By changing the capacitance of the capacitor C 2 , the frequencies at which the attenuation amount reaches its peak can be adjusted. 
     Further, as shown in  FIG. 20 , it is also possible to connect the capacitor C 1  in parallel with the inductor L 21  and connect the capacitor C 2  in parallel with the inductor L 22 . In this case, the frequency characteristics shown in  FIG. 21  are obtained. In a manner similar to  FIG. 17 , in  FIG. 21 , the sufficient attenuation amount can be ensured in the desired frequency bands (as indicated with the circles in  FIG. 21 ). As described above, by changing the capacitance of the capacitors C 1  and C 2 , the frequencies at which the attenuation amount reaches its peak can be adjusted. The capacitors C 1  and C 2  can be integrally formed within the LTCC portion or provided externally using conventional technologies. 
     The embodiments of the present invention were described above, but the present invention is not limited to such, and it is possible to modify the circuits in such a manner to provide for effects similar to those of the embodiments above. 
     It will be apparent to those skilled in the art that various modification and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents. In particular, it is explicitly contemplated that any part or whole of any two or more of the embodiments and their modifications described above can be combined and regarded within the scope of the present invention.