Patent Publication Number: US-8970445-B2

Title: Radio device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2011-058342, filed on Mar. 16, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments of the present invention relate to a radio device, for example, to a radio transceiver with a differential balanced antenna. 
     BACKGROUND 
     Conventionally, there has been proposed an antenna device in which two antennas are connected to a switching device, respectively, and one of two signals of the two antennas can be input to a receiver through selection by the switching device. 
     This antenna device can change a radiation pattern by selecting the antennas, providing a diversity effect. However, providing the diversity effect involves a plurality of antennas and receivers, resulting in an increase in a mounting area and a higher mounting cost. 
     Also, according to another conventional art, a known antenna device has a loop antenna both ends of which are short-circuited each other so as to change a radiation pattern. In this case, a single antenna can allow the pattern to be changed. However, since each terminal of the antenna is short-circuited, there arises a problem that a signal cannot be input or output when a transceiver to process a differential signal is connected to the antenna device. It is noted that because a differential signal, generally, can drastically reduce performance degradation caused from external noises and hard-wiring connected to a radio device, it is said that the differential signal can present a considerably workable improvement in performance of a radio device configured to process a high frequency. 
     As described above, there have conventionally been problems, such as an increase in the mounting area, a higher cost or a lack of connection capability to a differential transceiver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a radio device according to a first embodiment; 
         FIG. 2  shows an example of a first configuration of the radio device; 
         FIG. 3  shows an example of another configuration of a balun; 
         FIG. 4  shows a manner in which a radiation pattern is changed; 
         FIG. 5  shows an example of a second configuration of the radio device; 
         FIG. 6  shows an example of a third configuration of the radio device; 
         FIG. 7  shows an example of a fourth configuration of the radio device; 
         FIG. 8  shows a radio device according to a second embodiment; 
         FIG. 9  shows an example of a configuration of the radio device according to the second embodiment; 
         FIG. 10  shows a radio device according to a third embodiment; 
         FIG. 11  shows an example of a configuration of the radio device according to the third embodiment; 
         FIG. 12  shows a radio device according to a fourth embodiment; 
         FIG. 13  shows a radio device according to a fifth embodiment; 
         FIG. 14  shows an example of a configuration of the radio device according to the fifth embodiment; and 
         FIG. 15  shows an example of a configuration of a balun formed of a coupled line. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment, there is provided a radio device including an antenna, a first impedance converting circuit, a second impedance converting circuit and a differential output unit. 
     The antenna has a first terminal and a second terminal to receive a signal. 
     The first impedance converting circuit and the second impedance converting circuit have a first impedance and a second impedance, respectively. The first impedance and the second impedance each are controllable. One end of the first impedance converting circuit and one end of the second impedance converting circuit are connected to the first terminal and the second terminal of the antenna, respectively. 
     The differential output unit is connected to the other end of the first impedance converting circuit and the other end of the second impedance converting circuit through which the signal received by the antenna is input to the differential output unit, and transform the signal into a differential signal. 
     Hereinafter, embodiments according to the present invention will be described below with reference to the drawings. 
     (First Embodiment) 
       FIG. 1  shows a configuration of a radio device according to a first embodiment. 
     This radio device is constructed as a receiver including an antenna  11 , an impedance converting unit  21 , and a differential output unit  31 . 
     The antenna  11  is a differential input and output antenna including two terminals  12 ,  13  as a first and second terminal, respectively. The antenna including two antennas described above is, for example, a loop antenna or a dipole antenna. At these two terminals  12 ,  13 , a differential signal is input and output. The antenna, in receiving operation, outputs an analog signal at these terminals  12 ,  13 , the analog signal originating from a radio signal incoming from the air. 
     The impedance converting unit  21  includes two impedance converting circuits  22 ,  23  (a first and second impedance converting circuit). Each one end of the impedance converting circuits  22 ,  23  is connected to the terminals  12 ,  13 , respectively. 
     The impedance converting circuits  22 ,  23  have a first impedance and a second impedance, each being controllable. Each of the impedance converting circuits can switch, for example, between a low impedance state and a high impedance state (for example, infinite). Accordingly, an impedance with respect to each of the antenna terminals  12 ,  13  can take a different value, and a combination of the impedances with respect to the antenna terminals  12 ,  13  can change a radiation pattern of the antenna, providing the antenna diversity effect. 
     When impedances of the impedance converting circuits  22 ,  23  are equal to one another (in the case that the impedances are both low), then, signals at the terminals  12 ,  13  of the antenna  11  pass through the impedance converting circuits  22 ,  23  under the equal condition, and a differential signal, accordingly, is output at terminals  24 ,  25  corresponding to the other end of the impedance converting circuit  22  and the other end of the impedance converting circuit  23 , respectively. 
     On the other hand, when the impedances of the impedance converting circuits  22 ,  23  are not equal to one another, an unbalanced signal (single ended signal) is output at the terminals  24 ,  25 . For example, when the impedance of the impedance converting circuit  22  is low and the impedance of the impedance converting circuit  23  is infinite, the output signal at the terminal  12  is output at the terminal  24 , and the terminal  25  is grounded. 
     The differential output unit  31  receives the signal at the terminals  24 ,  25 . 
     The differential output unit  31  provides the signal (differential signal) received at the terminals  24 ,  25  to output terminals  32 ,  33  when the impedances of the impedance converting circuits  22 ,  23  are equal to one another (in the case that the impedances are low). In doing so, the differential output unit  31  may amplify the differential signal or convert its frequency before outputting. The output terminals  32 ,  33  output the provided signal to a circuit in a subsequent stage (not shown). 
     On the one hand, when the impedances of the impedance converting circuits  22 ,  23  are not equal to one another, the differential output unit  31  transforms the signal (unbalanced signal) received at the terminals  24 ,  25  into a differential signal and provides the transformed signal to the output terminals  32 ,  33 . In doing so, the differential output unit  31  may amplify the differential signal or convert its frequency. 
     In this manner, the output signal from the differential output unit  31  is consistently in the form of differential signal whether the input signal to the differential output unit  31  is in the form of differential signal or in the form of unbalanced signal, furthermore both cases are possible. 
     The configuration described above can provide a radio device with the diversity function using a single antenna, and allow a signal received by an antenna to be output in the form of differential signal. 
     An example of a specific configuration of the radio device shown in  FIG. 1  will be described below. 
       FIG. 2  shows an example of a first configuration of the radio device. 
     A loop antenna  51  is used as a differential input and output antenna. 
     An impedance converting unit  60  includes a first impedance converting circuit  61  and a second impedance converting circuit  71 . 
     The first impedance converting circuit  61  includes a quarter-wave line (λ/4 line)  62  and a first switch  63 . The quarter-wave line is a transmission line having the equivalent electrical length of a quarter wavelength of a signal frequency. An input portion of the quarter-wave line  62  is connected to a terminal  52  of the antenna  51 . An output portion of the quarter-wave line  62  is connected to one end of the first switch  63 . The other end of the first switch  63  is connected to a high-frequency grounding point (ground). 
     The second impedance converting circuit  71  includes a quarter-wave line (λ/4 line)  72  and a second switch  73 . The quarter-wave line is a transmission line having the equivalent electrical length of a quarter wavelength of a signal frequency. An input portion of the quarter-wave line  72  is connected to a terminal  53  of the antenna  51 . An output portion of the quarter-wave line  72  is connected to one end of the second switch  73 . The other end of the second switch  73  is connected to a high-frequency grounding point (ground). 
     A differential output unit  81  includes a balun (transformer)  82  and a differential amplifier  83 . The balun can be constructed by using a coupling coil or a coupled line. The illustrated example shows an example in which a coupling coil is used. The balun  82  includes a coil  74  with two terminals on the input side and a coil  75  with two terminals on the output side. The coils  74 ,  75  are disposed so that they can be coupled to one another. The two terminals on the input side are connected to one end of the first switch  63  and one end of the second switch  73 , respectively. A signal received at the two terminals on the input side is coupled to the two terminals on the output side through coil coupling. In doing so, when the signal received at the two terminals on the input side is a differential signal, the balun  82  couples the signal directly in the form of differential signal to the output terminals. When the signal is an unbalanced signal (in this example, a single ended signal), the balun  82  operates so that the signal is transformed into a differential signal and coupled to the output terminals. 
     A configuration shown in  FIG. 3  may be used for an example of another configuration of the balun with a coupling coil. The balun  78  is disposed in a differential output unit  84 . Coils  76 ,  77  are disposed so that they can be coupled to one another. One terminal of the coil  76  and one terminal of the coil  77 , as terminals on the input side, respectively, are connected to one end of the switch  63  and one end of the switch  73 , respectively. Further, the other terminal of the coil  76  and the other terminal of the coil  77 , as terminals on the output side, respectively, are connected to differential input terminals of the amplifier  83 , respectively. Also, a configuration shown in  FIG. 15  may be used for an example of a configuration of the balun with a coupled line. In a differential output unit  80 , coupled lines  80   b ,  80   c  are disposed so that they can be coupled to a coupled line  80   d , resulting in a balun  80   a.    
     The differential amplifier  83  amplifies a signal provided by the balun  82 , i.e. a differential signal, and outputs it at output terminals  32 ,  33 . 
     An example of operation of the radio device shown in  FIG. 2  will be described. 
     The antenna  51  outputs an incoming signal to the impedance converting circuits  61 ,  71  in the impedance converting unit  60  via the two terminals. The signal to be output is a differential signal. 
     Suppose that the first switch  63  and the second switch  73  in the impedance converting unit  60  are both open. Then, an input differential signal is directly sent through the balun  82  to the differential amplifier  83 , and amplified to be output. 
     On the one hand, suppose that one of the first switch  63  and the second switch  73  in the impedance converting unit  60  is short-circuited. Then, an impedance of the switch in a short-circuited state as seen from the antenna  51  corresponds to an impedance of a circuit in which a quarter-wave line is short-circuited, and accordingly the impedance becomes very high. In this manner, a terminal condition under which one of the two terminals of the antenna is differs from a terminal condition of the other of the two terminals, and accordingly a radiation pattern can be changed, allowing the diversity effect to be provided. That is, by turning one of the two switches on, or turning both the switches off, a radiation pattern can be changed, as shown in  FIG. 4 . 
     When one of the two switches is turned on, a signal output by the impedance converting unit  60  is an unbalanced signal in which one output of the two impedance converting circuits  61 ,  71  is at a high-frequency grounding point potential (ground potential). 
     When the unbalanced signal as described above is input, the balun  82  in the differential output unit  81  transforms this unbalanced signal into a differential signal. Accordingly, the differential input and output amplifier  83  is provided with the differential signal similarly to the case where the switches are both open, and the differential input and output amplifier  83  can provide an amplified differential signal. 
     As described above, according to the example of the first configuration, a single antenna element can provide the diversity function, and also a differential signal can be output from the differential output unit. 
       FIG. 5  shows an example of a second configuration of the radio device. 
     This example differs from the example of the first configuration shown in  FIG. 2  in a differential output unit  91 . Parts similar to those of the example of the first configuration have like reference numbers and description about them will be omitted. The differential output unit  91  will be described below with a focus on it. 
     The differential output unit  91  includes an amplifier of differential-pair type having single ended input and output amplifiers  92 ,  93  and a current source  94 . Output portions of impedance converting circuits  61 ,  71  are connected to input portions of the two single ended input and output amplifiers  92 ,  93 . Common mode terminals of the amplifiers  92 ,  93  are both connected to the single current source  94 . It is noted that this current source  94  may include an active element such as a transistor, or include a resistor and/or an inductor. 
     When a first and second switch  63 ,  73  are both open, then, similarly to the example of the first configuration, a differential signal is input to the differential output unit  91  by an impedance converting unit  60 , and this differential signal is amplified by the amplifier of differential-pair type ( 92 ,  93 ,  94 ) to be output. 
     On the one hand, when one of the first switch  63  and the second switch  73  is short-circuited, then, similarly to the example of the first configuration, the impedance converting unit  60  provides an unbalanced signal in which one output of the two impedance converting unit  60  is at a high-frequency grounding point potential. Accordingly, the impedance converting circuit whose switch is open outputs a signal having a large amplitude, and the impedance converting circuit whose switch is short-circuited outputs a signal having a very small amplitude (for example, the amplitude is substantially equal to zero). 
     In the amplifier of differential-pair type ( 92 ,  93 ,  94 ) in the differential output unit  91 , from the common mode terminal of one of the single ended amplifiers, a current signal is sent to the other single ended amplifier. Accordingly, the single ended amplifier receiving the signal of a large amplitude provides the single ended amplifier receiving the signal of a very small amplitude with a difference signal via the common mode terminal, thus allowing a differential signal having the same amplitude (amplified differential signal) to be output at output terminals of the differential output unit  91 . 
     As described above, also according to the example of the second configuration, a single antenna element can provide the diversity function, and also a differential signal can be output from the differential output unit. 
       FIG. 6  shows an example of a third configuration of the radio device. 
     This example differs from the examples of the first and second configuration shown in  FIG. 2  in a differential output unit  101 . Parts similar to those of the examples of the first and second configuration have like reference numbers, and description about them will be omitted. The differential output unit  101  will be described below with a focus on it. 
     The differential output unit  101  includes amplifiers  103 ,  104  and a frequency converter of balance type  102 . 
     Output portions of impedance converting circuits  61 ,  71  are connected to input portions of two single ended input and output amplifiers  103 ,  104 , respectively. Output portions of the amplifiers  103 ,  104  are connected to an input portion of the balance type frequency converter  102  which uses a differential local signal (differential LO signal). 
     When a first and second switch  63 ,  73  are both open, then, a differential signal is input to the amplifiers  103 ,  104  by an impedance converting unit  60 . The input differential signal is amplified by the amplifiers  103 ,  104  and is subsequently frequency-converted to be output by the balance type frequency converter  102 . 
     On the one hand, when one of the first switch  63  and the second switch  73  is short-circuited, then, similarly to the example of the first configuration, in the impedance converting unit  60 , one output of the two impedance converting circuits  61 ,  71  is an unbalanced signal at a high-frequency grounding point potential. Accordingly, output signals of the two single ended amplifiers  103 ,  104  are also unbalanced signals, but a frequency-converted differential signal can be output by the balance type frequency-converter  102  using the differential LO signal. 
     As described above, also according to the example of the third configuration, a single antenna element can provide the diversity function, and also a differential signal can be output from the differential output unit. 
       FIG. 7  shows an example of a fourth configuration of the radio device. 
     This example differs from the example of the first configuration in that the loop antenna is replaced by a dipole antenna  54 . A further point is similar to the example of the first configuration, and description thereof will be omitted. 
     As described above, also according to the example of the fourth configuration, a single antenna element can provide the diversity function, and also a differential signal can be output from the differential output unit. 
     (Second Embodiment) 
     The first embodiment has described the configurations of a receiver as a radio device. In this embodiment, configurations of a transmitter will be described. 
       FIG. 8  shows a configuration of a radio device according to a second embodiment. 
     This radio device is a transmitter including an antenna  111 , an impedance converting unit  121  and a differential input unit  131 . 
     The antenna  111  includes two terminals  112 ,  113 . The antenna  111 , in transmission operation, radiates an analog transmission signal received at the two terminals  112 ,  113  in the air as a radio signal. 
     The impedance converting unit  121  includes two impedance converting circuits  122 ,  123  (a first and second impedance converting circuit). Output portions of the impedance converting circuits  122 ,  123  are connected to the terminals  112 ,  113  of the antenna  111 , respectively. 
     The impedance converting circuits  122 ,  123  include a first impedance and a second impedance, each being controllable. A configuration of the impedance converting circuits  122 ,  123  is similar to that of the first embodiment. That is, each of the impedance converting circuits  122 ,  123  can be switched into, for example, a low impedance state or a high impedance state (for example, an infinite impedance state), and take a variable value of their impedance with respect to the terminals  112 ,  123 . Thus, an antenna radiation pattern can be changed, providing the antenna diversity. 
     The differential input unit  131  receives a differential signal at the terminals  124 ,  125  as a transmission signal. The differential input unit  131  transforms the transmission signal into an unbalanced signal corresponding to the difference between the impedances of the impedance converting circuits  122 ,  123 . When both impedances are equal to one another, the transformed signal is also a differential signal. 
     The transformed signal is provided to input portions of the impedance converting circuits  122 ,  123 . In particular, when the impedances of the impedance converting circuits  122 ,  123  are equal to one another (in the case where the impedances are low), the input differential signal is provided to the input portions of the impedance converting circuits  122 ,  123 . In doing so, the differential input unit  131  may output after processing amplification of the differential signal or frequency conversion. The differential signal input to the impedance converting circuits  122 ,  123  is input to the antenna  111  and radiated in the air as radio waves. 
     On the one hand, in the differential input unit  131 , when the impedances of the impedance converting circuits  122 ,  123  are not equal to one another, then, one of the output portions of the differential input unit  131  is in a low impedance state and the other output portion is in a high impedance state, and the differential input unit  131 , accordingly, outputs an unbalanced signal. 
     For example, when the impedance converting circuit  122  is in a low impedance state and the impedance converting circuit  123  is in a high impedance state, then, a signal of a large amplitude passes through the impedance converting circuit  122 , and is input to the terminal  112  of the antenna, and a signal of a small amplitude (for example, substantially equal to zero) is input to the terminal  113  of the antenna through the impedance converting circuit  123 . That is, an unbalanced signal is input to the terminals  112 ,  113  of the antenna  111 . The input unbalanced signal is radiated in the air as radio waves. The direction of radiation is different from the direction when the impedance converting circuits  122 ,  123  are both in a low impedance state. Thus, also in the case of transmission, a single antenna element can provide the diversity function even though the input signal is a differential signal. 
     It is noted that when the differential input unit  131  outputs the unbalanced signal, the unit  131  may output after processing amplification of the differential signal or frequency conversion. 
       FIG. 9  shows an example of a configuration of the radio device according to the second embodiment. 
     The configuration of  FIG. 9  corresponds to a configuration created by modifying, for a transmitter, the example of the configuration according to the first embodiment shown in  FIG. 2 . 
     For an antenna, a loop antenna  141  is used. Also, a dipole antenna may be used. 
     A first impedance converting circuit  151  includes a quarter-wave line (λ/4 line)  152  and a first switch  153 . The quarter-wave line  152  is a transmission line having the equivalent electrical length of a quarter wavelength of a signal frequency. One end of the quarter-wave line  152  is connected to a terminal  142  of the antenna  141 . The other end of the quarter-wave line  152  is connected to one end of the first switch  153 . The other end of the first switch  153  is connected to a high-frequency grounding point (ground). 
     A second impedance converting circuit  161  includes a quarter-wave line (λ/4 line)  162  and a second switch  163 . The quarter-wave line  162  is a transmission line having the equivalent electrical length of a quarter wavelength of a signal frequency. One end of the quarter-wave line  162  is connected to a terminal  143  of the antenna  141 . The other end of the quarter-wave line  162  is connected to one end of the second switch  163 . The other end of the second switch  163  is connected to a high-frequency grounding point (ground). 
     A differential input unit  171  includes a balun (transformer)  172  and a differential amplifier  173 . The balun can be constructed by using a coupling coil or a coupled line. The illustrated example shows the balun  172  including a coil  175  having two terminals on the output side and a coil  174  having two terminals on the input side. The coils are disposed so that they can be coupled to one another. The two terminals on the output side are connected to one end of the first switch  153  and one end of the second switch  163 , respectively. The two terminals on the input side are connected to an output portion of the differential amplifier  173 . The differential amplifier  173  receives and amplifies an input differential signal and provides the amplified differential signal to input terminals of the balun  172 . It is noted that, similarly to the first embodiment, the balun may be constructed according to the configuration of  FIG. 3 . 
     The balun  172  couples the differential signal received at the terminals of the coil  174  to the coil  175 . In doing so, the balun  172  directly couples the differential signal received at the input terminals to the coil  175  when the two switches are both open. The differential signal coupled to the coil  175  is output at output terminals of the coil  175 , and radiated by the antenna  141  through the impedance converting circuits  151 ,  161 . 
     On the one hand, when one of the two switches is off, the differential signal input to the balun  172  is transformed into an unbalanced signal. That is, to an output terminal connected to the impedance converting circuit in a high impedance state, a signal having a very small amplitude is provided, and to an output terminal connected to the impedance converting circuit in a low impedance state, a signal having a large amplitude is provided. As the result, to the two terminals  142 ,  143  of the antenna  141 , an unbalanced signal is input through these two impedance converting circuits  152 ,  162 . The input unbalanced signal is radiated in the air as radio waves. 
     As described above, according to this configuration, a single antenna element can realize the diversity function even though the input signal (transmission signal) is a differential signal. 
     (Third Embodiment) 
     A third embodiment is a combination of the first embodiment and the second embodiment. 
       FIG. 10  shows a configuration of a radio device (transceiver) according to the third embodiment. 
     This radio device includes an antenna  181 , an impedance converting unit  191 , a differential output unit  201  and a differential input unit  211 . The antenna  181  includes terminals  182 ,  183 . The antenna  181 , the differential output unit  201  and the differential input unit  211  are similar to elements of the first and second embodiment described above having like reference numbers, and description about them will be omitted. 
     The impedance converting unit  191  includes a first impedance converting circuit  192 , a second impedance converting circuit  193 , a third impedance converting circuit  194  and a fourth impedance converting circuit  196 . Each of the circuits can control its impedance (a first impedance to a fourth impedance). 
     The first impedance converting circuit  192  is disposed between a terminal  182  of the antenna  181  and an input terminal  202  of the differential output unit  201 . The second impedance converting circuit  193  is disposed between a terminal  183  of the antenna  181  and an input terminal  203  of the differential output unit  201 . 
     The third impedance converting circuit  194  is disposed between the terminal  182  of the antenna  181  and an output terminal  204  of the differential input unit  211 . The fourth impedance converting circuit  195  is disposed between the terminal  183  of the antenna  181  and an output terminal  205  of the differential input unit  211 . 
     In receiving operation, the third and fourth impedance converting circuit  194 ,  195  are always set to be in a high impedance state, which accordingly prevents a signal received by the antenna  181  from being input to the differential input unit  211 . On the one hand, the first and second impedance converting circuit  192 ,  193  are both set to be in a low impedance state, or one of them is set to be in a high impedance state, and, accordingly, the diversity is provided and a differential signal or an unbalanced signal is input to the differential output unit  201 . 
     In transmission operation, the first and second impedance converting circuit  192 ,  193  are set to be in a high impedance state, which accordingly prevents a transmission signal from being input to the differential output unit  201 . On the one hand, the third and fourth impedance converting circuit  194 ,  195  are both set to be in a low impedance state, or one of them is set to be in a high impedance state, and, accordingly, the diversity is provided, and a differential signal or an unbalanced signal is input to the terminals  182 ,  183  of the antenna  181 . 
     As described above, according to this configuration, a single antenna element can provide the diversity function on receiving, and a differential signal can be output by the differential output unit. Also, on transmission, a single antenna element can realize the diversity function even though the input signal (transmission signal) is a differential signal. 
       FIG. 11  shows an example of a configuration of the radio device according to the third embodiment. 
     This configuration is basically based on a combination of the configuration of the first embodiment shown in  FIG. 2  and the configuration of the second embodiment shown in  FIG. 11 , but a configuration of an impedance converting unit  231  largely differs from the configurations described above (the quarter-wave lines, the switches and the high-frequency grounding point). 
     An antenna  211  is a loop antenna. A differential output unit  232  is similar to the differential output unit  81  shown in  FIG. 2 . A differential input unit  234  is similar to the differential input unit  171  shown in  FIG. 9 . Accordingly, description about them will be omitted. 
     The impedance converting unit  231  includes a first to fourth impedance converting circuit  232 ,  233 ,  234 ,  235 . 
     Each of the impedance converting circuits includes switches. The antenna is controlled to be connected or disconnected to and from the differential output unit  232  and the differential input unit  234  by short-circuiting or opening the switches. That is, a low impedance state or a high impedance state is set. Accordingly, each of the impedance converting circuits  232  to  235  is in a low impedance state when its switch is short-circuited, and it is in a high impedance state when open. 
     As described above, according to this configuration, a single antenna element can provide the diversity function on receiving, and a differential signal can be output by the differential output unit. Also, on transmission, a single antenna element can realize the diversity function even though the input signal (transmission signal) is a differential signal. 
     (Fourth Embodiment) 
       FIG. 12  shows a configuration of a radio device (transceiver) according to a fourth embodiment. 
     This radio device includes an antenna  251 , an impedance converting unit  261 , a differential output unit  271  and a differential input unit  273 . The impedance converting unit  261  includes impedance converting circuits  262 ,  263 . 
     The antenna  251 , the impedance converting unit  261 , the differential output unit  271  and the differential input unit  273  are basically similar to those of the first and second embodiment described above. A different point lies in that, in receiving operation, the differential output unit  271  is powered on, and the differential input unit  273  is powered off, and, on transmission, conversely, the differential output unit  271  is powered off, and the differential input unit  273  is powered on. 
     Accordingly, in receiving operation, a signal received by the antenna  251  (differential signal) is provided directly in the form of differential signal or in the form of unbalanced signal to the differential output unit  271  through the impedance converting unit  261 . In transmission operation, a transmission signal (differential signal) is provided directly in the form of differential signal or in the form of unbalanced signal to the antenna  251  through the differential input unit  273  and the impedance converting unit  261 . 
     An example of a particular configuration of the impedance converting circuits  262 ,  263  may be the configuration shown in  FIG. 8  (the quarter-wave lines, the switches, the high-frequency grounding point), or the configuration may include only the switches as shown in  FIG. 11 . 
     As described above, according to this configuration, a single antenna element can provide the diversity function, and a differential signal can be output by the differential output unit. Also, a single antenna element can realize the diversity function even though the input signal (transmission signal) is a differential signal. 
     (Fifth Embodiment) 
     In the Embodiments described above, the impedance of each of the impedance converting circuits is controlled two ways, that is, to be high or low, but in order to improve the diversity function, the impedance can be controlled to take any one of three or more values. 
       FIG. 13  shows an example of a configuration of a radio device including an impedance converting circuit whose impedance can be controlled to take any one of three or more values. 
     An impedance converting unit  65  includes impedance converting circuits  67 ,  77 . An antenna  51  and a differential output unit  81  are similar to those of the first embodiment, as shown in  FIG. 2 , and description about them will be omitted. 
     The impedance converting circuit  67  is a circuit in which the switch  63  of the impedance converting circuit  61  shown in  FIG. 2  is replaced by a variable impedance element  64  (a variable resistor in the example shown). Similarly, the impedance converting circuit  77  is a circuit in which the switch  73  of the impedance converting circuit  62  shown in  FIG. 2  is replaced by a variable impedance element  74  (a variable resistor in an example shown). 
     In this manner, use of a variable impedance element(s) allows the impedance to be controlled more accurately, and the diversity effect, accordingly, can be enhanced. 
     A particular configuration of the variable impedance elements  64 ,  74  may include an optional element, such as a MOSFET or a variable capacitor. 
       FIG. 14  shows an example of a configuration in which a MOSFET is used. 
     An impedance converting unit  66  includes impedance converting circuits  69 ,  79 . The impedance converting circuit  69  uses a MOSFET  68  as a variable impedance element. The impedance converting circuit  79  uses a MOSFET  78  as a variable impedance element. In order to set a variable impedance, a phenomenon of a MOSFET is used in which a resistance corresponding to a control voltage is generated between both ends of the MOSFET (between a drain and a source), and by switching this control voltage from 0 (ground) to 1 (supply voltage) in a step-by-step manner, a broad range of impedance can be set. 
     The present invention is not limited to the exact embodiments described above and can be embodied with its components modified in an implementation phase without departing from the scope of the invention. Also, arbitrary combinations of the components disclosed in the above-described embodiments can form various inventions. For example, some of the all components shown in the embodiments may be omitted. Furthermore, components from different embodiments may be combined as appropriate.