Patent Publication Number: US-2018048336-A1

Title: Diversity switch circuit, radio-frequency module, and communication device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority to Japanese Patent Application No. 2017-103471 filed on May 25, 2017 and Japanese Patent Application No. 2016-158048 filed on Aug. 10, 2016. The entire contents of this application are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a diversity switch circuit, a radio-frequency module, and a communication device. 
     2. Description of the Related Art 
     Hitherto, a sub antenna, which is called a diversity antenna, provided separately from a main antenna, has been used with the main antenna to improve the quality and the reliability of communication for receiving signals. Lately, there has been an increasing demand for multiband- and multimode-support communication devices, such as cellular phones, in which a single terminal handles multiple frequency bands and multiple wireless systems. To satisfy such a demand, a diversity switch circuit is used for separating signals received by a diversity antenna according to the frequency band. More specifically, Japanese Unexamined Patent Application Publication No. 2015-517753 discloses a diversity switch circuit such as that shown in  FIG. 7 .  FIG. 7  is a schematic diagram illustrating an example of a known diversity switch circuit  400 . 
     As shown in  FIG. 7 , in the diversity switch circuit  400 , a switch  410  connected to a diversity antenna ANT and a multiband-support switch circuit  420  are connected in series with each other. The switch circuit  420  includes switches  423   a,    423   b,  . . . , and  423   n  as n switches connected to signal paths through which multiple signals of different frequency bands are transmitted. Circuits, such as filters that filters received signals or low-noise amplifiers (LNAs) for amplifying received signals, are connected to the switches  423   a,    423   b,  . . . , and  423   n,  though they are not shown in  FIG. 7 . The diversity switch circuit  400  may receive a high power signal for some reasons, for example, due to a short distance between a base station and a communication device having the diversity switch circuit  400  mounted thereon. In this case, the switch  410  and the switch circuit  420  may be broken or the performance may be decreased. To avoid such a situation, when the diversity antenna ANT receives a high power signal, the switch  410  is turned OFF so that the switch  410  and the switch circuit  420  will not receive such a signal. 
     Although a diversity antenna is typically used as a receive antenna, it may be expected to be used also as a transmit antenna in accordance with an increase in the multiband-support terminals and the communication traffic. However, the diversity switch circuit disclosed in the above-described publication is a receive-only circuit for receiving multiband signals, and is unable to handle sending signals. A transmit diversity antenna is thus required to be separate from the receive diversity antenna. This increases the number of antennas required in a communication device, such as a cellular phone. A larger antenna space is accordingly required in a housing of the communication device, thus making it difficult to reduce the size of the communication device. 
     SUMMARY OF THE INVENTION 
     Accordingly, preferred embodiments of the present invention provide diversity switch circuits, radio-frequency modules, and communication devices that are able to use a diversity antenna as a transmit-and-receive antenna. 
     According to a preferred embodiment of the present invention, a diversity switch circuit includes first and second switches. The first switch includes a first common terminal connected to a diversity antenna, a first selection terminal connected to a first signal path, and a second selection terminal connected to a second signal path. The second signal path is a path different from the first signal path. The second switch is disposed in the first signal path and includes a second common terminal connected to the first selection terminal and includes at least two selection terminals. A received signal received by the diversity antenna is transmitted to the first signal path when the first common terminal and the first selection terminal are connected to each other. A sending signal to be sent by the diversity antenna is transmitted to the second signal path when the first common terminal and the second selection terminal are connected to each other. 
     When the first common terminal and the first selection terminal are connected to each other, the single diversity antenna is able to be used as a receive antenna. By connecting the second common terminal and a specific one of the at least two selection terminals to each other in the second switch, the diversity antenna receives a desired signal. The specific selection terminal is a selection terminal to which a filter (elastic wave filter, for example) having the pass band corresponding to the frequency band of the desired signal is connected. When the first common terminal and the second selection terminal are connected to each other, a sending signal transmitted to the second signal path is sent to the single diversity antenna. Thus, the single diversity antenna is also able to be used as a transmit antenna. By using the diversity switch circuit that is able to receive signals of different frequency bands, the diversity antenna may be used as a transmit-and-receive antenna. It is thus possible to reduce the size of a communication device, such as a cellular phone. 
     A terminating resistor may be connected to one of the at least two selection terminals of the second switch. 
     The diversity antenna is preferably used as a transmit-and-receive antenna. For this reason, even when the first common terminal and the first selection terminal are not connected to each other, a high-power sending signal transmitted to the second signal path may leak to the second switch. Additionally, even when the first common terminal and the first selection terminal are not connected to each other, a high power signal received by the diversity antenna may leak to the second switch. To address this issue, a terminating resistor is connected to one of the at least two selection terminals of the second switch. When the diversity antenna is used as a transmit antenna or when the diversity antenna receives a high power signal, by connecting the second common terminal and the selection terminal to which the terminating resistor is connected, the terminating resistor is able to output energy of a high power signal to a ground. It is thus less likely that a circuit, such as a filter or an LNA, connected to the second switch will be broken or the performance will be decreased by a high power signal. 
     A matching circuit may be connected between the first selection terminal and the second common terminal. 
     The matching circuit performs impedance matching between the first and second switches, thus decreasing the level of loss (return loss) which may occur in a signal transmitted to the first signal path. 
     The second switch may include two or more second switches. A first multiplexer may be connected between the first selection terminal and the second common terminal of each of the two or more second switches. 
     It is thus possible to simultaneously receive multiple signals of different frequency bands, such as a low band (LB), a middle band (MB), and a high band (HB), received by the diversity antenna, thus achieving CA communication. 
     The first switch may also include a third selection terminal connected to the first signal path. A third switch may be connected between the first multiplexer and the second common terminal. A received signal received by the diversity antenna may be transmitted to the first signal path without passing through the first multiplexer when the first common terminal and the third selection terminal are connected to each other and when the third switch is OFF. 
     By connecting the first common terminal and the third selection terminal to each other and by turning OFF the third switch, a received signal received by the diversity antenna is able to be transmitted to the first signal path without passing through the first multiplexer. When CA communication is performed, the first common terminal and the first selection terminal are connected to each other and the third switch is turned ON. When CA communication is not performed, the first common terminal and the third selection terminal are connected to each other and the third switch is turned OFF. It is thus possible to reduce loss (insertion loss) which may occur in a received signal by the first multiplexer when CA communication is not performed. 
     The diversity switch circuit may further include a fourth switch. The fourth switch is disposed in the second signal path and includes a third common terminal connected to the second selection terminal and includes at least two selection terminals. 
     The fourth switch enables the diversity switch circuit to simultaneously send multiple sending signals of different frequency bands. 
     A matching circuit may be connected between the second selection terminal and the third common terminal. 
     The matching circuit performs impedance matching between the first and fourth switches, thus decreasing the level of loss (return loss) which may occur in a signal transmitted to the second signal path. 
     The fourth switch may include two or more fourth switches. A second multiplexer may be connected between the second selection terminal and the third common terminal of each of the two or more fourth switches. 
     It is thus possible to simultaneously send multiple signals of different frequency bands, such as the LB, MB, and HB, thus achieving CA communication. 
     The first switch may also include a fourth selection terminal connected to the second signal path. A fifth switch may be connected between the second multiplexer and the third common terminal. A sending signal to be sent by the diversity antenna may be transmitted to the second signal path without passing through the second multiplexer when the first common terminal and the fourth selection terminal are connected to each other and when the fifth switch is OFF. 
     By connecting the first common terminal and the fourth selection terminal to each other and by turning OFF the fifth switch, a signal transmitted to the second signal path is able to be sent to the diversity antenna without passing through the second multiplexer. When CA communication is performed, the first common terminal and the second selection terminal are connected to each other and the fifth switch is turned ON. When CA communication is not performed, the first common terminal and the fourth selection terminal are connected to each other and the fifth switch is turned OFF. It is thus possible to reduce loss (insertion loss) which may occur in a sending signal by the second multiplexer when CA communication is not performed. 
     According to a preferred embodiment of the present invention, a radio-frequency module includes the above-described diversity switch circuit, a filter, and an amplifier circuit. The filter is connected to the at least two selection terminals of the second switch or to the at least two selection terminals of the fourth switch. The amplifier circuit is connected to the filter. 
     It is thus possible to provide a radio-frequency module that is able to use a diversity antenna as a transmit-and-receive antenna. 
     According to a preferred embodiment of the present invention, a communication device includes a radio-frequency signal processing circuit that processes the above-described received signal and sending signal and the above-described radio-frequency module. The radio-frequency module sends the sending signal and receives the received signal between the diversity antenna and the radio-frequency signal processing circuit. 
     It is thus possible to provide a communication that is able to use a diversity antenna as a transmit-and-receive antenna. 
     According to preferred embodiments of the present invention, it is possible to provide diversity switch circuits, radio-frequency modules, and communication devices that are able to use a diversity antenna as a transmit-and-receive antenna. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an example of a diversity switch circuit according to a first preferred embodiment of the present invention. 
         FIG. 2  is a schematic diagram illustrating an example of a diversity switch circuit according to a second preferred embodiment of the present invention. 
         FIG. 3  is a schematic diagram illustrating an example of a diversity switch circuit according to a third preferred embodiment of the present invention. 
         FIG. 4  is a schematic diagram illustrating an example of a diversity switch circuit according to a fourth preferred embodiment of the present invention. 
         FIG. 5  is a schematic diagram illustrating an example of a diversity switch circuit according to a fifth preferred embodiment of the present invention. 
         FIG. 6  is a schematic diagram illustrating an example of a communication device according to a sixth preferred embodiment of the present invention. 
         FIG. 7  is a schematic diagram illustrating an example of a known diversity switch circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. All of the preferred embodiments described below illustrate general or specific examples. Numerical values, components, and positions and connection states of the components illustrated in the following preferred embodiments are only examples, and do not limit the present invention. Among the components illustrated in the following preferred embodiments, the components that are not recited in the independent claims which embody the broadest concept of the present invention will be described as optional components. The drawings are only schematic drawings and do not necessarily illustrate the components precisely. In the drawings, the same components are designated by like reference numeral. 
     First Preferred Embodiment 
     A diversity switch circuit  1  according to a first preferred embodiment of the present invention will be described below with reference to  FIG. 1 . 
       FIG. 1  is a schematic diagram illustrating an example of the diversity switch circuit  1  according to the first preferred embodiment. 
     The diversity switch circuit  1  is a circuit that is able to receive signals of different frequency bands received by a diversity antenna ANT. In the first preferred embodiment, the diversity switch circuit  1  is also able to simultaneously send multiple sending signals of different frequency bands. The diversity antenna ANT is a sub antenna provided separately from a main antenna and is used to improve the quality and the reliability of communication. The diversity antenna is typically used as a receive antenna. In the preferred embodiments of the present invention, however, the diversity antenna is able to be used as a transmit-and-receive antenna. 
     The diversity switch circuit  1  includes a first switch  10 , a second switch  20 , and a fourth switch  30 . 
     The first switch  10  includes a first common terminal  11  connected to the diversity antenna ANT. The first switch  10  also includes a first selection terminal  12  connected to a first signal path  200  and a second selection terminal  14  connected to a second signal path  300 . The second selection terminal  14  is a path different from the first signal path  200 . In the first preferred embodiment, the first switch  10  includes first common terminals  11   a  and  11   b,  which are collectively called the first common terminal  11 . The first switch  10  includes switches  13  and  15 . Turning ON the switch  13  connects the first common terminal  11   a  and the first selection terminal  12 . Turning ON the switch  15  connects the first common terminal  11   b  and the second selection terminal  14 . When the switch  13  is ON, the switch  15  is OFF. When the switch  15  is ON, the switch  13  is OFF. However, both of the switches  13  and  15  may be ON during a certain period. 
     The switches  13  and  15  are semiconductor switches such as diode switches or field-effect transistor (FET) switches, and are turned ON or OFF in accordance with a control signal from an external source (radio-frequency (RF) signal processing circuit (radio frequency integrated circuit (RFIC))  120 , which will be discussed later, for example) outside the first switch  10 . The first common terminals  11   a  and  11   b  may be one integrated terminal. In this case, instead of the switches  13  and  15 , the first switch includes a switch that changes the connection between the integrated first common terminal  11  and one of the first and second selection terminals  12  and  14 . However, the first common terminal may still be able to simultaneously connect to the first selection terminal  12  and to the second selection terminal  14 . 
     The second switch  20  includes a second common terminal  21  connected to the first selection terminal  12  and also includes at least two selection terminals  22 . The second common terminal  21  may be directly connected to the first selection terminal  12  or be indirectly connected to the first selection terminal  12  via another circuit (a matching circuit  50  or a multiplexer  60 , which will be discussed later, for example). The second switch  20  includes a switch  23 . Turning ON the switch  23  connects the second common terminal  21  and the selection terminal  22 . In the first preferred embodiment, the second switch  20  includes n second common terminals, that is, second common terminals  21   a,    21   b,  . . . , and  21   n,  which are collectively called the second common terminal  21 . The second switch  20  also includes n selection terminals, that is, selection terminals  22   a,    22   b,  . . . , and  22   n,  which are collectively called the selection terminal  22 . The second switch  20  includes n switches, that is, switches  23   a,    23   b,  . . . , and  23   n,  which are collectively called the switch  23 . Turning ON the switch  23   n,  for example, connects the second common terminal  21   n  and the selection terminal  22   n.    
     The switch  23  preferably is a semiconductor switch such as a diode switch or an FET switch, and is turned ON or OFF in accordance with a control signal from an external source (RF signal processing circuit  120 , which will be discussed later, for example) outside the second switch  20 . The second common terminals  21   a,    21   b,  . . . , and  21   n  may be one integrated terminal. In this case, instead of the switches  23   a,    23   b,  . . . , and  23   n,  the second switch includes a switch that changes the connection between the integrated second common terminal  21  and one of the selection terminals  22   a,    22   b,  . . . , and  22   n.  However, the second common terminal  21  may still be able to simultaneously connect to two or more of the selection terminals  22   a,    22   b,  . . . , and  22   n.    
     The second switch  20  is disposed on the first signal path  200 . This means that the second switch  20  is disposed on a path connected to the first selection terminal  12  of the first switch  10 . For example, when the second common terminal  21   n  and the selection terminal  22   n  are connected to each other, a signal passes through a path connecting the first selection terminal  12 , the second common terminal  21   n,  and the selection terminal  22   n  as the first signal path  200 . 
     The fourth switch  30  includes a third common terminal  31  connected to the second selection terminal  14  and also includes at least two selection terminals  32 . The third common terminal  31  may be directly connected to the second selection terminal  14  or be indirectly connected to the second selection terminal  14  via another circuit. The fourth switch  30  includes a switch  33 . Turning ON the switch  33  connects the third common terminal  31  and the selection terminal  32 . In the first preferred embodiment, the fourth switch  30  includes n third common terminals, that is, third common terminals  31   a,    31   b,  . . . , and  31   n,  which are collectively called the third common terminal  31 . The fourth switch  30  also includes n selection terminals, that is, selection terminals  32   a,    32   b,  . . . , and  32   n,  which are collectively called the selection terminal  32 . The fourth switch  30  includes n switches, that is, switches  33   a,    33   b,  . . . , and  33   n,  which are collectively called the switch  33 . Turning ON the switch  33   n,  for example, connects the third common terminal  31   n  and the selection terminal  32   n.  The switch  33  is a semiconductor switch such as a diode switch or an FET switch, and is turned ON or OFF in accordance with a control signal from an external source (RF signal processing circuit  120 , which will be discussed later, for example) outside the fourth switch  30 . The third common terminals  31   a,    31   b,  . . . , and  31   n  may be one integrated terminal. In this case, instead of the switches  33   a,    33   b,  . . . , and  33   n,  the fourth switch  30  includes a switch that changes the connection between the integrated third common terminal  31  and one of the selection terminals  32   a,    32   b,  . . . , and  32   n.  However, the third common terminal  31  may still be able to simultaneously connect to two or more of the selection terminals  32   a,    32   b,  . . . , and  32   n.    
     The fourth switch  30  is disposed on the second signal path  300 . This means that the fourth switch  30  is disposed on a path connected to the second selection terminal  14  of the first switch  10 . For example, when the third common terminal  31   n  and the selection terminal  32   n  are connected to each other, a signal passes through a path connecting the second selection terminal  14 , the third common terminal  31   n,  and the selection terminal  32   n  as the second signal path  300 . 
     A signal received by the diversity antenna ANT passes through the first signal path  200  when the first common terminal  11   a  and the first selection terminal  12  are connected to each other. That is, the first signal path  200  is a path that transmits a signal received by the diversity antenna ANT. A receiving filter (filter constituted by an elastic wave resonator or an LC circuit) or an amplifier circuit, such as an LNA, is thus connected to the selection terminal  22  of the second switch  20 . Filters having different pass bands from each other are connected to the selection terminals  22   a,    22   b,  . . . , and  22   n,  so that the diversity switch circuit  1  is able to receive signals of different frequency bands by using the second switch  20 . 
     A signal to be sent from the diversity antenna ANT is transmitted to the second signal path  300  when the first common terminal  11   b  and the second selection terminal  14  are connected to each other. That is, the second signal path  300  is a path that transmits a sending signal to be sent from the diversity antenna ANT. A sending filter (filter constituted by an elastic wave resonator or an LC circuit) or an amplifier circuit, such as a power amplifier (PA), is thus connected to the selection terminal  32  of the fourth switch  30 . Filters having different pass bands from each other are connected to the selection terminals  32   a,    32   b,  . . . , and  32   n,  so that multiple sending signals of different frequency bands are able to be simultaneously sent via the fourth switch  30 . That is, among multiple sending signals of different frequency bands, one or more sending signals selected by the fourth switch  30  can be sent from the diversity antenna ANT. As described above, the first switch  10  includes the second selection terminal  14 . When the first common terminal  11   b  and the second selection terminal  14  are connected to each other, a sending signal transmitted to the second signal path  300  can be sent to the diversity antenna ANT. That is, the diversity antenna ANT is able to be used as a transmit antenna as well as a receive antenna. 
     As shown in  FIG. 1 , the first signal path  200  and the second signal path  300  are different paths. The path connecting the first signal path  200  and the diversity antenna ANT and the path connecting the second signal path  300  and the diversity antenna ANT branch off from the diversity antenna ANT at a position closer to the diversity antenna ANT than to the switches  13 , and  15 . Accordingly, the switch  13  is not disposed on the path connecting the second signal path  300  and the diversity antenna ANT. It is thus less likely that loss caused by the switch  13  will occur in a sending signal transmitted to the second signal path  300 . As stated above, the first signal path  200  and the second signal path  300  are different paths. This means that a path connected to the selection terminal  22  (the path of the second switch  20  opposite the path close to the diversity antenna ANT as viewed from the diversity antenna ANT, that is, the path connected to the right side of the second switch  20  in  FIG. 1 ) and a path connected to the selection terminal  32  (the path of the fourth switch  30  opposite the path close to the diversity antenna ANT as viewed from the diversity antenna ANT, that is, the path connected to the right side of the fourth switch  30  in  FIG. 1 ) are not integrated together by a circuit, such as a switch circuit in a subsequent stage. To put it another way, if the path connected to the selection terminal  22  and the path connected to the selection terminal  32  are integrated together by a switch that selects one of multiple inputs and outputting the selected input in a subsequent stage, the first signal path  200  and the second signal path  300  are not different paths. If the path connected to the selection terminal  22  and the path connected to the selection terminal  32  are each connected to a common ground or connected to different terminals of one RF signal processing circuit, the first signal path  200  and the second signal path  300  are different paths. 
     By using the diversity switch circuit  1  that is able to receive multiple signals of different frequency bands and simultaneously send multiple sending signals of different frequency bands, the diversity antenna ANT is able to be used as a transmit-and-receive antenna. 
     Second Preferred Embodiment 
     A diversity switch circuit  1   a  according to a second preferred embodiment of the present invention will be described below with reference to  FIG. 2 . 
       FIG. 2  is a schematic diagram illustrating an example of the diversity switch circuit  1   a  according to the second preferred embodiment. 
     The diversity switch circuit  1   a  according to the second preferred embodiment is different from the diversity switch circuit  1  according to the first preferred embodiment in that it includes a second switch  40  instead of the second switch  20 . The configurations of the other components are similar to those of the first preferred embodiment, and an explanation thereof will thus be omitted. The second switch  40  is different from the second switch  20  in that it includes a terminating resistor  41 . The configurations of the other components of the second switch  40  are similar to those of the first preferred embodiment, and an explanation thereof will thus be omitted. 
     The terminating resistor  41  is connected to one of at least two selection terminals  22  of the second switch  40 . One end of the terminating resistor  41  is connected to the second selection terminal  22 , while the other end thereof is grounded. The terminating resistor  41  outputs energy of a signal input into the second switch  40  to a ground, and the resistance of the terminating resistor  41  is about 50Ω, for example. The terminating resistor  41  may be built in the second switch  40 . In the second preferred embodiment, the terminating resistor  41  is connected to the selection terminal  22   n,  as shown in  FIG. 2 . 
     The diversity antenna ANT is used as a transmit-and-receive antenna. For this reason, even when the first common terminal  11  and the first selection terminal  12  are not connected to each other, a high-power sending signal transmitted to the second signal path  300  may leak to the second switch  40 . Additionally, even when the first common terminal  11  and the first selection terminal  12  are not connected to each other, a high power signal received by the diversity antenna ANT may leak to the second switch  40 . A circuit, such as a filter that filters a received signal or an LNA that amplifies a received signal, is connected to the second switch  40 . When receiving a high power signal, such a circuit may be broken or the performance may be decreased. To address this issue, the terminating resistor  41  is connected to one of the at least two selection terminals  22  of the second switch  40 . When the diversity antenna ANT is used as a transmit antenna or when the diversity antenna ANT receives a high power signal, by connecting the second common terminal  21  and the selection terminal to which the terminating resistor  41  is connected, the terminating resistor  41  is able to output energy of a high power signal to a ground. It is thus less likely that a circuit, such as a filter or an LNA, connected to the second switch  40  will be broken or the performance will be decreased. 
     Third Preferred Embodiment 
     A diversity switch circuit  1   b  according to a third preferred embodiment of the present invention will be described below with reference to  FIG. 3 . 
       FIG. 3  is a schematic diagram illustrating an example of the diversity switch circuit  1   b  according to the third preferred embodiment. 
     The diversity switch circuit  1   b  according to the third preferred embodiment is different from the diversity switch circuit  1   a  according to the second preferred embodiment in that it includes a matching circuit  50 . The configurations of the other components are similar to those of the second preferred embodiment, and an explanation thereof will thus be omitted. In the third preferred embodiment, the diversity switch circuit  1   b  includes matching circuits  50   a  and  50   b,  which are collectively called the matching circuit  50 . 
     The matching circuit  50   a  is connected between the first selection terminal  12  and the second common terminal  21 . The matching circuit  50   b  is connected between the second selection terminal  14  and the third common terminal  31 . The matching circuit  50  is an impedance matching circuit including elements, such as capacitors or inductors. The matching circuit  50  is able to perform impedance matching between the first switch  10  and the second switch  40  and between the first switch  10  and the fourth switch  30 , thus decreasing the level of loss (return loss) which may occur in a signal transmitted to the first and second signal paths  200  and  300 . 
     As described above, filters having different pass bands from each other are connected to the at least two selection terminals  22  of the second switch  40 . That is, the frequency band for which the matching circuit  50   a  performs impedance matching differs depending on which switch  23  will be turned ON. The matching circuit  50   a  predicts which switch  23  in the second switch  40  will be turned ON and adjusts itself in advance to the frequency band of a signal to be subjected to impedance matching. If the parameter used to perform impedance matching in the matching circuit  50   a  is variable, a control signal for turning ON or OFF the switch  23  received by the second switch  40  is also received by the matching circuit  50   a.  That is, the matching circuit  50   a  is able to identify the switch  23  to be turned ON in the second switch  40 , that is, the matching circuit  50   a  is able to identify the frequency band to be subjected to impedance matching. The matching circuit  50   a  includes plural circuits having different parameters, such as the value of an inductor or a capacitor, or having different connection modes. In each of these circuits, the parameter, such as the value of an inductor or a capacitor, or the connection mode has been adjusted so that impedance matching is able to be performed according to the associated frequency band. The matching circuit  50   a  includes a switch that is able to select one of these circuits to be connected between the first switch  10  and the second switch  40  in accordance with a control signal received by the matching circuit  50   a.  This configuration enables the matching circuit  50   a  to perform impedance matching for the frequency band associated with the switch  23  to be turned ON in the second switch  40 . 
     Similarly, filters having different pass bands from each other are connected to the at least two selection terminals  32  of the fourth switch  30 . That is, the frequency band for which the matching circuit  50   b  performs impedance matching differs depending on which switch  33  will be turned ON. The matching circuit  50   b  predicts which switch  33  in the fourth switch  30  will be turned ON and adjusts itself in advance to the frequency band of a signal to be subjected to impedance matching. If the parameter used to perform impedance matching in the matching circuit  50   b  is variable, a control signal to turn ON or OFF the switch  33  received by the fourth switch  30  is also received by the matching circuit  50   b.  That is, the matching circuit  50   b  is able to identify the switch  33  to be turned ON in the fourth switch  30 , that is, the matching circuit  50   b  is able to identify the frequency band to be subjected to impedance matching. The matching circuit  50   b  includes plural circuits having different parameters, such as the value of an inductor or a capacitor, or having different connection modes. In each of these circuits, the parameter, such as the value of an inductor or a capacitor, or the connection mode has been adjusted so that impedance matching is able to be performed according to the associated frequency band. The matching circuit  50   b  includes a switch that is able to select one of these circuits to be connected between the first switch  10  and the fourth switch  30  in accordance with a control signal received by the matching circuit  50   b.  This configuration enables the matching circuit  50   b  to perform impedance matching for the frequency band associated with the switch  33  to be turned ON in the fourth switch  30 . 
     As described above, the impedance matching parameter or the connection mode of the matching circuit  50  is adjusted in advance to the frequency band to be subjected to impedance matching, or is changed in accordance with a control signal for turning ON or OFF the switch  23  received by the second switch  40  or the switch received by the fourth switch  30 . It is thus possible to achieve significantly improved or optimal impedance matching according to the frequency band. 
     Fourth Preferred Embodiment 
     A diversity switch circuit  1   c  according to a fourth preferred embodiment of the present invention will be described below with reference to  FIG. 4 . 
       FIG. 4  is a schematic diagram illustrating an example of the diversity switch circuit  1   c  according to the fourth preferred embodiment. 
     The diversity switch circuit  1   c  according to the fourth preferred embodiment is different from the diversity switch circuit  1   a  according to the second preferred embodiment in that the diversity switch circuit  1   c  includes a multiplexer  60 , the second switch  40  includes two or more second switches  40 , and the fourth switch  30  includes two or more fourth switches  30 . The configurations of the other components are similar to those of the second preferred embodiment, and an explanation thereof will thus be omitted. In the fourth preferred embodiment, the diversity switch circuit  1   c  includes first and second multiplexers  60   a  and  60   b,  which are collectively called the multiplexer  60 . The diversity switch circuit  1   c  includes second switches  40   a  and  40   b,  which are collectively called the second switch  40 . The diversity switch circuit  1   c  includes fourth switches  30   a  and  30   b,  which are collectively called the fourth switch  30 . 
     The first multiplexer  60   a  is connected between the first selection terminal  12  and the second common terminal  21  of each of the two or more second switches  40 . The second multiplexer  60   b  is connected between the second selection terminal  14  and the third common terminal  31  of each of the two or more fourth switches  30 . The multiplexer  60  is a filter which is used in frequency division duplexing (FDD) communication, for example, and separates one signal into multiple signals of different frequency bands or integrates multiple signals of different frequency bands into one signal. Although the multiplexer  60  is a duplexer in the fourth preferred embodiment, it may be a triplexer or a quadplexer, for example. 
     The first multiplexer  60   a  enables the diversity switch circuit  1   c  to simultaneously receive multiple signals of different frequency bands, such as a low band (LB), a middle band (MB), and a high band (HB), received by the diversity antenna ANT. The diversity switch circuit  1   c  can thus perform CA communication. The LB is about 700 MHz to about 900 MHz, for example. The MB is about 1800 MHz to about 2200 MHz, for example. The HB is about 2300 MHz to about 2700 MHz, for example. The second multiplexer  60   b  enables the diversity switch circuit  1   c  to simultaneously send multiple signals of different frequency bands, such as the LB, MB, and HB. The diversity switch circuit  1   c  is thus able to perform CA communication. 
     The multiplexer  60  may include filters constituted by elastic wave resonators, LC circuits, or both of them. The elastic wave resonator may be a surface acoustic wave (SAW) resonator or a bulk acoustic wave (BAW) resonator. The SAW resonator includes a substrate and interdigital transducer (IDT) electrodes. The substrate is a substrate having piezoelectricity at least on its surface. For example, the substrate may include a piezoelectric thin film on its surface, and may be a multilayer body including the piezoelectric thin film and another film having different acoustic velocities, and a support substrate. Alternatively, the entirety of the substrate may have piezoelectricity. In this case, the substrate is a piezoelectric substrate defined by a single piezoelectric layer. 
     The filters of the multiplexer  60  may be band pass filters. Other examples of the filters are a high pass filter, a low pass filter, and a band elimination filter. 
     Fifth Preferred Embodiment 
     A diversity switch circuit  1   d  according to a fifth preferred embodiment of the present invention will be described below with reference to  FIG. 5 . 
       FIG. 5  is a schematic diagram illustrating an example of the diversity switch circuit  1   d  according to the fifth preferred embodiment. 
     The diversity switch circuit  1   d  according to the fifth preferred embodiment is different from the diversity switch circuit  1   c  according to the fourth preferred embodiment in that it includes a first switch  100  instead of the first switch  10  and also includes a third switch  65  between the first multiplexer  60   a  and the second switch  40  and a fifth switch  66  between the second multiplexer  60   b  and the fourth switch  30 . The first switch  100  and the first signal path  200  are connected to each other via multiple paths. The first switch  100  and the second signal path  300  are also connected to each other via multiple paths. The configurations of the other components are similar to those of the fourth preferred embodiment, and an explanation thereof will thus be omitted. 
     The first switch  100  includes a first common terminal  11  connected to the diversity antenna ANT, a first selection terminal  12  and a third selection terminal  16  connected to the first signal path  200 , and a second selection terminal  14  and a fourth selection terminal  18  connected to the second signal path  300 . A path connecting the third selection terminal  16  and the first signal path  200  is called a first bypass path. A path connecting the fourth selection terminal  18  and the second signal path  300  is called a second bypass path. The first switch  100  includes switches  13 ,  15 ,  17 , and  19 . In the fifth preferred embodiment, the first switch  100  includes first common terminals  11   a  through  11   f,  which are collectively called the first common terminal  11 . The first switch  100  also includes third selection terminals  16   a  and  16   b,  which are collectively called the third selection terminal  16 , and fourth selection terminals  18   a  and  18   b,  which are collectively called the fourth selection terminal  18 . The first switch  100  also includes switches  17   a  and  17   b,  which are collectively called the switch  17 , and switches  19   a  and  19   b,  which are collectively called the switch  19 . 
     A third switch  65   a  is connected between the first multiplexer  60   a  and each second common terminal  21  of the second switch  40   a.  More specifically, as shown in  FIG. 5 , the diversity switch circuit  1   d  includes the third switch  65   a  at a position closer to the first multiplexer  60   a  than to a node between a path connected to the third selection terminal  16   a  and a path connecting the first multiplexer  60   a  and the second switch  40   a,  as viewed from this node. Similarly, the diversity switch circuit  1   d  includes a third switch  65   b  between the first multiplexer  60   a  and the second switch  40   b.    
     A fifth switch  66   a  is connected between the second multiplexer  60   b  and each third common terminal  31  of the fourth switch  30   a.  More specifically, as shown in  FIG. 5 , the diversity switch circuit  1   d  includes the fifth switch  66   a  at a position closer to the second multiplexer  60   b  than to a node between a path connected to the fourth selection terminal  18   a  and a path connecting the second multiplexer  60   b  and the fourth switch  30   a,  as viewed from this node. Similarly, the diversity switch circuit  1   d  includes a fifth switch  66   b  between the second multiplexer  60   b  and the fourth switch  30   b.  In the fifth preferred embodiment, the third switches  65   a  and  65   b  are collectively called the third switch  65 , and the fifth switches  66   a  and  66   b  are collectively called the fifth switch  66 . 
     By turning ON the switch  13  and the third switches  65   a  and  65   b,  a signal received by the diversity antenna ANT is transmitted to the first signal path  200  via the first multiplexer  60   a.  By turning ON the switch  15  and the fifth switches  66   a  and  66   b,  a signal transmitted to the second signal path  300  is sent to the diversity antenna ANT via the second multiplexer  60   b.  When the first common terminal  11   c  and the third selection terminal  16   a  are connected to each other by turning ON the switch  17   a  and when the third switch  65   a  is OFF, a signal received by the diversity antenna ANT is transmitted to the first signal path  200  without passing through the first multiplexer  60   a.  When the first common terminal  11   d  and the third selection terminal  16   b  are connected to each other by turning ON the switch  17   b  and when the third switch  65   b  is OFF, a signal received by the diversity antenna ANT is transmitted to the first signal path  200  without passing through the first multiplexer  60   a.  When the first common terminal  11   e  and the fourth selection terminal  18   a  are connected to each other by turning ON the switch  19   a  and when the fifth switch  66   a  is OFF, a signal transmitted to the second signal path  300  is sent to the diversity antenna ANT without passing through the second multiplexer  60   b.  When the first common terminal  11   f  and the fourth selection terminal  18   b  are connected to each other by turning ON the switch  19   b  and when the fifth switch  66   b  is OFF, a signal transmitted to the second signal path  300  is sent to the diversity antenna ANT without passing through the second multiplexer  60   b.    
     When the switch  13  is ON, the switches  15 ,  17   a,    17   b,    19   a,  and  19   b  are OFF. When at least one of the switches  17   a  and  17   b  is ON, the switches  13 ,  15 ,  19   a,  and  19   b  are OFF. When the switch  15  is ON, the switches  13 ,  17   a,    17   b,    19   a,  and  19   b  are OFF. When at least one of the switches  19   a  and  19   b  is ON, the switches  13 ,  15 ,  17   a,  and  17   b  are OFF. When one of the switches  13 ,  17   a,  and  17   b  is ON, one of the switches  15 ,  19   a,  and  19   b  may be ON. Similarly, when one of the switches  15 ,  19   a,  and  19   b  is ON, one of the switches  13 ,  17   a,  and  17   b  may be ON. 
     The switches  17  and  19  are semiconductor switches such as diode switches or FET switches, and are turned ON or OFF in accordance with a control signal from an external source (RF signal processing circuit  120 , which will be discussed later, for example) outside the first switch  100 . Similarly, the third and fifth switches  65  and  66  are semiconductor switches such as diode switches or FET switches, and are turned ON or OFF in accordance with a control signal from an external source (RF signal processing circuit  120 , which will be discussed later, for example) outside the diversity switch circuit  1   d.  The first common terminals  11   a  through  11   f  may be one integrated terminal. In this case, instead of the switches  13 ,  15 ,  17 , and  19 , the first switch  100  includes a switch that changes the connection between the integrated first common terminal  11  and one of the first selection terminal  12 , the second selection terminal  14 , the third selection terminal  16 , and the fourth selection terminal  18 . However, the first common terminal  11  may still be able to simultaneously connect to one of the first and third selection terminals  12  and  16  and to one of the second and fourth selection terminals  14  and  18 . 
     By using the multiplexer  60 , CA communication is able to be performed. However, when a signal passes through the multiplexer  60 , loss occurs because of the multiplexer  60 . For example, in the diversity switch circuit  1   c  shown in  FIG. 4 , all signals sent and received by the diversity antenna ANT pass through the multiplexer  60 . However, CA communication may not be necessary, depending on the purpose or the situation of use of a communication device on which the diversity antenna circuit  1   c  is mounted. Even in this case, a signal passes through the multiplexer  60 , which causes loss in the signal. 
     To address this issue, in the fifth preferred embodiment, when the first common terminal  11  and the third selection terminal  16  are connected to each other and when the third switch  65  is OFF, a signal received by the diversity antenna ANT passes through the first bypass path without passing through the first multiplexer  60   a  and is transmitted to the first signal path  200 . As a result, when CA communication is not performed, loss which may occur in a received signal is able to be reduced. When the first common terminal  11  and the fourth selection terminal  18  are connected to each other and when the fifth switch  66  is OFF, a signal transmitted to the second signal path  300  passes through the second bypass path without passing through the second multiplexer  60   b  and is sent to the diversity antenna ANT. As a result, when CA communication is not performed, loss which may occur in a sending signal is able to be reduced. 
     Sixth Preferred Embodiment 
     A diversity switch circuit according to a preferred embodiment of the present invention is applicable to a radio-frequency module and a communication device. In a sixth preferred embodiment of the present invention, a radio-frequency module  110  including the diversity switch circuit  1  and a communication device  130  including the radio-frequency module  110  will be described. 
       FIG. 6  is a schematic diagram illustrating an example of the communication device  130  according to the sixth preferred embodiment. 
     The communication device  130  includes the radio-frequency module  110  and an RF signal processing circuit (RFIC)  120 . 
     The radio-frequency module  110  sends signals to be sent from the diversity antenna ANT and receives signals received by the diversity antenna ANT between the diversity antenna ANT and the RF signal processing circuit  120 . The radio-frequency module  110  includes the diversity switch circuit  1 , plural filters  70  connected to the at least two selection terminals  22  of the second switch  20  and to the at least two selection terminals  32  of the fourth switch  30 , and an amplifier circuit  80  connected to the plural filters  70 . The radio-frequency module  110  also includes a sixth switch  90 . The amplifier circuit  80  is connected to the filters  70  via the sixth switch  90 . The radio-frequency module  110  also includes a switch  91  connected to a terminal of the sixth switch  90  on the side closer to the RF signal processing circuit  120 . The diversity switch circuit  1  is similar to that of the first preferred embodiment, and an explanation thereof will thus be omitted. 
     In the sixth preferred embodiment, the radio-frequency module  110  includes an LNA  80   a  and a PA  80   b,  which are collectively called the amplifier circuit  80 . The radio-frequency module  110  includes six switches  90   a  and  90   b,  which are collectively called the sixth switch  90 . As discussed above, the radio-frequency module  110  includes the switch  91 . 
     The filters  70  are filters including elastic wave resonators, LC circuits, or both of them. The elastic wave resonators may be SAW resonators or BAW resonators. The SAW resonator includes a substrate and IDT electrodes. The substrate is a substrate having piezoelectricity at least on its surface. For example, the substrate may include a piezoelectric thin film on its surface, and may be a multilayer body including the piezoelectric thin film and another film having different acoustic velocities, and a support substrate. Alternatively, the entirety of the substrate may have piezoelectricity. In this case, the substrate is a piezoelectric substrate defined by a single piezoelectric layer. 
     The filters  70  may be band pass filters. Other examples of the filters  70  are a high pass filter, a low pass filter, and a band elimination filter. The filters  70  have different pass bands from each other. The filters  70  are connected to the selection terminal  22 , so that the diversity switch circuit  1  is able to receive signals of different frequency bands. The pass bands of the filters  70  are in a range of about 700 MHz to about 3.5 GHz, for example. The filters  70  are connected to the selection terminal  32 , so that one or more sending signals selected by the fourth switch  30  among multiple sending signals of different frequency bands can be sent from the diversity antenna ANT. 
     The sixth switch  90  have plural selection terminals connected to the filters  70 , a common terminal connected to the amplifier circuit  80 , and a common terminal connected to the switch  91 . The sixth switch  90  selects one of the common terminals and one of the selection terminals in accordance with a control signal output from the RF signal processing circuit  120 , for example. For example, the sixth switch  90  selects the selection terminal connected to the filter  70  associated with a desired frequency band and the common terminal connected to the amplifier circuit  80  or the switch  91 . 
     The LNA  80   a  is a receiving amplifier circuit which amplifies a received signal and outputs it to the RF signal processing circuit  120 . The PA  80   b  is a sending amplifier circuit which amplifies a sending signal and outputs it to the sixth switch  90   b.    
     The switch  91  is turned ON when amplifying of a received signal is not necessary. When the switch  91  is ON, the selection terminal connected to the filter  70  is connected to the common terminal connected to the switch  91  instead of that connected to the LNA  80   a.    
     The RF signal processing circuit  120  is a circuit that processes a sending signal to be sent from the diversity antenna ANT and a signal received by the diversity antenna ANT. The RF signal processing circuit  120  performs signal processing, such as down-conversion, on a received signal input from the diversity antenna ANT via the first signal path  200  and outputs the resulting received signal to a baseband signal processing circuit (not shown). The RF signal processing circuit  120  performs signal processing, such as up-conversion, on a sending signal input from the baseband signal processing circuit (not shown) and outputs the resulting sending signal to the diversity antenna ANT via the second signal path  300 . 
     As described above, the diversity switch circuit according to a preferred embodiment of the present invention may be applied to the radio-frequency module  110  and the communication device  130 . 
     Other Preferred Embodiments 
     The diversity switch circuits, the radio-frequency module, and the communication device according to preferred embodiments of the present invention have been discussed. However, the present invention is not restricted to the above-described preferred embodiments. 
     Although in the above-described preferred embodiments the diversity switch circuit includes the fourth switch  30 , it may not necessarily include the fourth switch  30 . 
     In the second through fifth preferred embodiments, the terminating resistor  41  is built in the second switch  40  and is connected to the selection terminal  22 . However, the terminating resistor  41  may be disposed outside the second switch  40  and be connected to the selection terminal  22 . 
     In the second through fifth preferred embodiments, the diversity switch circuit includes the second switch  40  including the terminating resistor  41 . However, the diversity switch circuit may include the second switch  20  without a terminating resistor  41 , such as that of the first preferred embodiment. 
     In the third preferred embodiment, the matching circuit  50  is connected between the first selection terminal  12  and the second common terminal  21  and between the second selection terminal  14  and the third common terminal  31 . Alternatively, the matching circuit  50  may be connected only one of between the first selection terminal  12  and the second common terminal  21  and between the second selection terminal  14  and the third common terminal  31 . 
     In the fourth and fifth preferred embodiments, a matching circuit  50  is connected neither between the first selection terminal  12  and the second common terminal  21  nor between the second selection terminal  14  and the third common terminal  31 . However, a matching circuit  50  may be connected at least one of between the first selection terminal  12  and the second common terminal  21  and between the second selection terminal  14  and the third common terminal  31 . The multiplexer  60  may have the function of the matching circuit  50 . In other words, the multiplexer  60  and the matching circuit  50  may be provided integrally. 
     In the fifth preferred embodiment, a matching circuit  50  is connected neither between the third selection terminal  16  and the second common terminal  21  nor between the fourth selection terminal  18  and the third common terminal  31 . However, a matching circuit  50  may be connected at least one of between the third selection terminal  16  and the second common terminal  21  and between the fourth selection terminal  18  and the third common terminal  31 . 
     Although in the fourth and fifth preferred embodiments the diversity switch circuits  1   c  and  1   d  each include two second switches  40  and two fourth switches  30 , they may include three or more second switches and three or more fourth switches. 
     Although in the sixth preferred embodiment the radio-frequency module  110  includes the diversity switch circuit  1 , it may alternatively include one of the diversity switch circuits  1   a  through  1   d.    
     Preferred embodiments obtained by making various modifications to the above-described preferred embodiments by those skilled in the art and preferred embodiments achieved by combining the elements and functions of the above-described preferred embodiments without departing from the scope and spirit of the invention are also encompassed in the present invention. 
     Preferred embodiments of the present invention are widely applicable to communication devices, such as cellular phones, as diversity switch circuits, radio-frequency modules, and communication devices that are able to use a diversity antenna as a transmit-and-receive antenna. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.