Abstract:
There is disclosed a duplexer comprising: a first external terminal; a second external terminal; an antenna terminal; a first frequency variable filter electrically connected between the first external terminal and the antenna terminal, and composed of at least one resonator and a reactance element electrically connected to the resonator and capable of being voltage-controlled; a second frequency variable filter electrically connected between the second external terminal and the antenna terminal, and composed of at least one resonator and a reactance element electrically connected to the resonator and capable of being voltage-controlled; the predetermined reactance element of the first frequency variable filter being in the on state when the reactance element of the second frequency variable filter is in the on state.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a duplexer for use in a microwave band, for example, and a communication apparatus. 
     2. Description of the Related Art 
     A transmission frequency band required for the transmission side circuit of a duplexer for use in PCS is 1850-1910 MHz, and a reception frequency band for a reception side circuit is 1930-1990 MHz. It is necessary for both of the transmission side circuit and reception side circuit to have a wide pass-band of 60 MHz. On the other hand, the separation assured to separate the transmission frequency band from the reception frequency band is 20 MHz. That is, the separation between the both bands is very narrow. 
     Further, the duplexer composes the phase of the transmission side circuit and that of the reception side circuit. In the case of PCS, the phase of the transmission side circuit and that of the reception side circuit are ideally composed by setting the transmission side circuit to have a high impedance (open) in the reception frequency band of 1930-1990 MHz, and setting the reception side circuit to have a high impedance (open) in the transmission frequency band of 1850-1910 MHz. 
     FIG. 8 shows an example of the circuit configuration of a prior art duplexer  1 . In the case of a PCS system, the separation between the transmission frequency band and reception frequency band is narrow, namely, 20 MHz. Accordingly, the transmission frequency band is divided into two ranges of 1850-1880 MHz and 1880-1910 MHz, and also, the reception frequency band is divided into two ranges of 1930-1960 MHz and 1960-1990 MHz. That is, the frequency bands become narrow, and the separations are wide. In particular, reactance elements (PIN diode) are connected to resonators, respectively, and control the voltages of the resonators, so that the two types of pass-bands of each of the transmission side circuit  25  and the reception side circuit  26  can be changed over, resulting in reduction of the number of the filter stages. Like this, it is attempted to downsize the duplexer and give high qualities thereto. In FIG. 8, a transmission terminal is designated by Tx, a reception terminal by Rx, an antenna terminal by ANT, resonators in the transmission side circuit  25  by 2 and 3, resonators in the reception side circuit  26  by 4 to 6, coupling coils by L 1  and L 11 , coupling capacitors for determining a rejection-band attenuation by C 1  and C 2 , capacitors by C 5 , C 6 , and C 24 , frequency band variable capacitors by C 3 , C 4 , and C 7  to  9 , PIN diodes by D 2  to D 6 , choke coils by L 2 , L 3 , and L 6  to  8 , control voltage supply resistances and capacitors by R 1  and R 2 , and C 22  and C 23 , respectively, coils and capacitors constituting phase circuits by L 20  and L 21 , and C 15 , respectively, and coupling capacitors by C 11  to C 14 . 
     CONT 1  designates a voltage control terminal for controlling the voltages of the PIN diodes D 2  and D 3  of the transmission circuit  25 , and CONT 2  a voltage control terminal for controlling the voltages of the PIN diodes D 4  to D 6 . When positive voltages are applied to the voltage control terminals CONT 1  and CONT 2 , the PIN diodes D 2  to D 6  are in the on state, and the duplexer  1  operates through the LOW channel. That is, as shown in FIG. 9, the pass-band of the transmission side circuit  25  becomes 1850-1880 MHz, and that of the reception side circuit  26  becomes 1930-1960 MHz. To the contrary, when the control voltages are zero with no voltages being applied to the voltage control terminals CONT 1  and CONT 2 , the PIN diodes D 2  to D 6  turn off, and the duplexer  1  operates through the HIGH channel. That is, as shown in FIG. 9, the pass-band of the transmission side circuit  25  becomes 1880-1910 MHz, and that of the reception side circuit  26  becomes 1960-1990 MHz. 
     A portable telephone is put on standby for a reception wave except the time when speech is carried out. In case the frequency during the reception wave standby is 1930 MHz and the reception wave standby is carried out with positive voltages being applied to the voltage control terminals CONT 1  and CONT 2 , the battery of the portable telephone is quickly exhausted, which causes the problem that the reception wave standby time becomes short. 
     It may be supposed that as countermeasures against the problem, the control voltage of the voltage control terminal CONT 1  is set at 0V and a positive voltage is applied to the voltage control terminal CONT 2  only. Since a consumption current flows through only the reception side circuit  26  during the reception wave standby, the exhaustion of the battery can be suppressed. However, as to a system such as PCS in which the frequency of the transmission frequency band is lower than that of the reception frequency band, the separation between the pass-band (1880-1910 MHz) of the transmission side circuit  25  and that (1930-1960 MHz) of the reception side circuit  26  is very narrow, as shown in FIG. 10, when the PIN diodes D 2  and D 3  in the transmission side circuit  25  is turned off (in the off state) and the PIN diodes D 4  to D 6  in the reception side circuit  26  is turned on (in the on state). Therefore, it is difficult to set the transmission side circuit  25  to have a high impedance (open) in the reception frequency band of 1930-1960 MHz. Thus, there arises the in problem that the insertion loss of the reception side circuit  26  is large. 
     FIG. 11 is a graph showing the measurement results of the band-pass characteristic S 32  and reflection characteristic S 22  (see FIG. 8) of the reception side circuit  26  obtained when positive voltages are applied to the voltage control terminals CONT 1  and CONT  2 . In this case, the insertion loss of the reception side circuit  26  was 3.3 dB. On the other hand, FIG. 12 is a graph showing the measurement results of the band-pass characteristic S 32  and reflection characteristic S 22  of the reception side circuit  26  obtained when a positive voltage is applied to the voltage control terminal CONT  2  only. In FIG. 12, the waveform is distorted in the part thereof shown by a circle A. In this case, the insertion loss of the reception side circuit  26  was deteriorated to be 5.0 dB. 
     SUMMARY OF THE INVENTION 
     To overcome the above described problems, preferred embodiments of the present invention provide a duplexer of which the consumption current is small and the insertion loss is low, and a communication apparatus. 
     One preferred embodiment of the present invention provides A duplexer comprising: a first external terminal; a second external terminal; an antenna terminal; a first frequency variable filter electrically connected between the first external terminal and the antenna terminal, and composed of at least one resonator and a reactance element electrically connected to the resonator and capable of being voltage-controlled; a second frequency variable filter electrically connected between the second external terminal and the antenna terminal, and composed of at least one resonator and a reactance element electrically connected to the resonator and capable of being voltage-controlled; the predetermined reactance element of the first frequency variable filter being in the on state when the reactance element of the second frequency variable filter is in the on state. 
     Hereupon, the first frequency variable filter is a transmission filter, for example, and the second frequency variable filter is a reception filter, for example. As the reactance elements, for example, PIN diodes and variable capacitance diodes are used. 
     When the reactance element of the second frequency variable filter is in the on state, the predetermined reactance element of the first frequency variable filter is in the on state. Thereby, the impedance of the first frequency variable filter is enhanced in the resonant frequency band of the second frequency variable filter. Accordingly, the insertion loss of the second frequency variable filter is suppressed. In addition, since only the predetermined reactance element of the first frequency variable filter is in the on state, the current consumption is reduced as compared with the case where all the reactance elements of the first frequency variable filter are in the on state. Thus, the power consumption during reception wave standby is decreased. 
     Another preferred embodiment of the present invention provides a communication apparatus including any one of the duplexers described above. Accordingly, the power consumption during reception wave standby is suppressed, and the loss of the reception side circuit is reduced. 
    
    
     Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an electric circuit diagram according to a first embodiment of the duplexer of the present invention. 
     FIG. 2 is a perspective view showing the mounting structure of the duplexer of FIG.  1 . 
     FIG. 3 is a cross sectional view an example of the resonators used in the duplexer of FIG.  1 . 
     FIG. 4 is a graph showing the pass and reflection characteristics of the reception side circuit of the duplexer of FIG.  1 . 
     FIG. 5 is an electric circuit diagram of a duplexer according to a second embodiment of the present invention. 
     FIG. 6 is an electric circuit diagram of a duplexer of the present invention. 
     FIG. 7 is a block diagram of a communication apparatus according to an embodiment of the present invention. 
     FIG. 8 is an electric circuit diagram showing the constitution of a prior art antenna device. 
     FIG. 9 is an illustration of the filter characteristic of a duplexer. 
     FIG. 10 is an illustration of the filter characteristic of a prior art duplexer. 
     FIG. 11 is a graph showing the pass and reflection characteristics of the reception side circuit of a prior art duplexer when positive voltages are applied to voltage control terminals CONT 1  and CONT 2 . 
     FIG. 12 is a graph showing the pass and reflection characteristics of a reception side circuit of a prior art duplexer when a positive voltage is applied to a voltage control terminal CONT 2  only. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment, FIGS.  1  to  4   
     FIG. 1 shows the circuit configuration of a duplexer  31  in a communication apparatus. FIG. 2 is a perspective view of the duplexer  31  in which the respective components are mounted onto a circuit substrate  40 . In the duplexer  31 , a transmission side circuit  25  is electrically connected between a transmission terminal TX and an antenna terminal ANT, and a reception side circuit  26  between a reception terminal Rx and the antenna terminal ANT. 
     The transmission side circuit  25  includes a frequency variable band elimination filter circuit  27  and a phase circuit  29 . The frequency variable band elimination filter circuit  27  comprises resonant circuits in two stages coupled to each other, that is, it comprises a resonator  2  electrically connected to the transmission side terminal Tx via a resonance capacitor C 1 , and a resonator  3  electrically connected to the phase circuit  29  via a resonance capacitor C 2 . The resonance capacitors C 1  and C 2  are capacitors for determining the rejection-band attenuation. The series resonant circuit comprising the resonator  2  and the resonance capacitor C 1  is electrically connected to the series resonant circuit comprising the resonator  3  and the resonance capacitor C 2  via a coupling coil L 1 . Further, capacitors C 5  and C 6  are electrically connected in parallel to these two series resonant circuits, respectively. 
     To the intermediate node between the resonator  2  and the resonance capacitor C 1 , the PIN diode D 2  as a reactance element is electrically connected in parallel with the resonator  2  with the cathode being grounded. On the other hand, to the intermediate node between the resonator  3  and the resonance capacitor C 2 , the PIN diode D 3  is electrically connected via a band variable capacitor C 4 , in parallel with the resonator  3 . The band variable capacitors C 3  and C 4  are capacitors for changing two attenuation extreme frequencies of the attenuation characteristic of the frequency variable band elimination filter circuit  27 , respectively. Further, a capacitor  24  is connected in parallel to the band variable capacitor C 4 . 
     The phase circuit  29  is a T-shaped type circuit comprising a coil L 20  electrically connected between the frequency variable band elimination filter circuit  27  and the antenna terminal ANT, a capacitor  15  electrically connected between the ground and the antenna terminal ANT, and a coil L 21  electrically connected between the band-pass filter circuit  28  (described later) of the reception side circuit  26  and the antenna terminal ANT. 
     On the other hand, the reception side circuit  26  contains the frequency variable band-pass filter circuit  28  and the phase circuit  29 . The reception side circuit  26  of the first embodiment shares the phase circuit  29  with the transmission side circuit  25 . However, needless to say, the transmission side circuit  25  and the reception side circuit  26  include independent phase circuits, respectively. 
     The frequency variable band-pass filter circuit  28  comprises a resonant-circuit in three stages coupled to each other, that is, it comprises a resonator  4  electrically connected to the phase circuit  29  via a resonance inductance L 9 , a resonator  6  electrically connected to the reception terminal Rx via a resonance inductance L 10 , and a resonator  5  electrically connected between the resonators  4  and  6  via coupling capacitors C 11 , C 12 , C 13 , and C 14 . 
     To the intermediate node between the resonator  4  and the resonance inductance L 9 , a series circuit comprising a band variable capacitor C 7  and a PIN diode D 4  is electrically connected in parallel with the resonator  4 . To the intermediate node among a resonator  5  and the coupling capacitors C 12  and C 13 , a series circuit comprising a band variable capacitor C 8  and a PIN diode D 5  is electrically connected in parallel with the resonator  5 . To the intermediate node between the resonator  6  and the resonance inductance L 10 , a series circuit comprising a band variable capacitor C 9  and a PIN diode D 6  is electrically connected in parallel with the resonator  6 . 
     A voltage control terminal CONT 1  is electrically connected to the intermediate node between the anode of the PIN diode D 2  and the band-variable capacitor C 3 , via a control voltage supply resistance R 1 , a capacitor C 22 , and a choke coil L 2 . On the other hand, a voltage control terminal CONT 2  is electrically connected to the intermediate node between the anode of the PIN diode D 3  and the band variable capacitor  4 , via a control voltage supply resistor R 2 , a capacitor C 23 , and a choke coil L 3 , is electrically connected to the intermediate node between the anode of the PIN diode D 4  and the band variable capacitor C 7 , via the control voltage supply resistance R 2 , the capacitor C 23 , and a choke coil L 6 , is electrically connected to the intermediate node between the anode of the PIN diode D 5  and the band variable capacitor C 8 , via the control voltage supply resistor R 2 , the capacitor C 23 , and a choke coil L 7  and further is electrically connected to the intermediate node between the anode of the PIN diode D 6  and the band variable capacitor C 9 , via the control voltage supply resistor R 2 , the capacitor C 23 , and a choke coil L 8 . The capacitors C 22  and C 23  function as noise-cut bypass capacitors, and are electrically connected among the voltage control terminals CONT 1  and CONT 2  and the ground, respectively. 
     Further, for example, dielectric resonators are used as the resonators  2  to  6 , as shown in FIG.  3 . FIG. 3 shows the resonator  2  as a typical example. The dielectric resonators  2  to  6  each comprise a cylindrical dielectric  21  made of a material with a high dielectric constant such as a TiO 2  type ceramic or the like, an outer conductor  22  formed on the outer peripheral surface of the cylindrical dielectric  21 , and an inner conductor  23  formed on the inner wall of the cylindrical dielectric  21 . The outer conductor  22  is electrically opened (separated) from the inner conductor  23  at one open-end  21   a  (hereinafter, referred to as an open-end face  21   a ) of the dielectric  21 , and electrically short-circuited (conducting) to the inner conductor  23  at the other open-end face  21   b  (hereinafter, referred to as a short-circuited end face  21   b ). Regarding the dielectric resonator  2 , the series circuit comprising the band variable capacitor C 3  and the PIN diode D 2  is electrically connected in such a manner that one end of the band variable capacitor C 3  is connected to the inner conductor  23  at the open-end face  21   a , and the cathode of the PIN diode D 2  is connected to the ground. The outer conductor  22  is connected to the ground. 
     Hereinafter, the operation and effects of the duplexer  31  having the above-described configuration will be described. In this duplexer  31 , a transmission signal, input to the transmission terminal Tx from a transmission circuit system is output from the antenna terminal ANT via the transmission side circuit  25 , while a reception signal input through the antenna terminal ANT is output to a reception circuit system via the reception side circuit  26 . 
     The trap frequency of the frequency variable band elimination filter circuit  27  in the transmission side circuit  25  is determined by the resonance frequency of a resonance system comprising the band variable capacitor C 3 , the resonance capacitor C 1 , and the resonator  2 , and the resonance frequency of a resonance system comprising the band variable capacitor C 4 , the resonance capacitor C 2 , and the resonator  3 . When positive voltages as control voltages are applied to the voltage control terminals CONT 1  and CONT 2 , the PIN diodes D 2  and D 3  are in the on state, respectively. Accordingly, the band variable capacitors C 3  and C 4  are grounded via the PIN diodes D 2  and D 3 , so that both of the attenuation extreme frequencies are decreased, and the pass-band of the transmission side circuit  25  becomes a LOW channel (1850-1880 MHz). 
     To the contrary, when the control voltages are 0V with no voltages being applied to the voltage control terminals CONT 1  and CONT 2 , the PIN diodes D 2  and D 3  are in the off state, respectively. Negative voltages may be applied to the voltage control terminals CONT 1  and CONT 2 , instead of applying the control voltages of 0 V, so that the PIN diodes D 2  and D 3  are in the off state. Thereby, the band variable capacitors C 3  and C 4  become open, respectively, so that both of the two attenuation frequencies are increased, and the pass-band of the transmission side circuit  25  becomes a HIGH channel (1880-1910 MHz). Like this, to the transmission side circuit  25 , the two different pass-band characteristics can be rendered by carrying out the voltage control to ground or open the band-variable capacitors C 3  and C 4 . 
     On the other hand, the pass frequency of the frequency variable band-pass filter circuit  28  in the reception side circuit  26  is determined by the resonance frequency of a resonance system comprising the band variable capacitor C 7 , the resonance inductance L 9 , and the resonator  4 , the resonance frequency of a resonance system comprising the band variable capacitor C 8  and the resonator  5 , and the resonance frequency of a resonance system comprising the band variable capacitor C 9 , the resonance inductance L 10 , and the resonator  6 . When positive voltages as control voltages are applied to the voltage control terminal CONT 2 , the PIN diodes D 4 , D 5 , and D 6  are in the on state. Accordingly, the band variable capacitors C 7 , C 8 , and C 9  are grounded via the PIN diodes D 4 , D 5 , and D 6 , respectively, and the pass frequency is decreased, whereby the pass band of the reception side circuit  26  becomes a LOW channel (1930-1960 MHz). 
     To the contrary, when the control voltage is made 0 V with no voltages being applied to the voltage control terminal CONT 2 , the PIN diodes D 4 , D 5 , and D 6  are in the off state. Thereby, the band variable capacitors C 7 , C 8 , and C 9  become open and the pass frequency is increased, whereby the pass band of the reception side circuit  26  becomes a HIGH channel (1960-1990 MHz). Like this, to the reception side circuit  26 , two different pass band characteristics can be rendered by carrying out the voltage control to ground or open the band variable capacitors C 7  to C 9 . 
     This duplexer  31  is voltage-controlled in such a manner that the two pass bands, namely, the high and low pass bands, are changed over. That is, when the low frequency pass band is selected as a transmission band, the pass frequency of the reception side circuit  26  is decreased, and when the high frequency pass band is selected as the transmission band, the pass frequency of the reception side circuit  26  is increased. Thereby, the phase of the transmission side circuit  25  and that of the reception side circuit  26  can be ideally composed. 
     If the frequency of a reception wave on standby is 1930 MHz, the duplexer  31  is put into the reception wave standby state, by making the control voltage of the voltage control terminal CONT 1  0 V and applying a positive voltage to the voltage control terminal CONT 2  only. That is, the PIN diodes D 4  to D 6  of the reception side circuit  26  and the PIN diode D 3  electrically connected to the antenna terminal ANT in the position nearer to the antenna terminal ANT than the PIN diode D 2  in the transmission side circuit  25  during the reception wave standby are in the on state. Accordingly, the transmission side circuit  25  is allowed to have a high impedance in the reception frequency band of 1930-1960 MHz, and the insertion loss of the reception side circuit  26  can be suppressed. FIG. 4 is a graph showing the measurement results of the band-pass characteristic S 32  and the reflection characteristic S 22  (see FIG. 1) of the reception side circuit  26  obtained when a positive voltage is applied to the voltage control terminal CONT 2  only. In this case, the insertion loss of the reception side circuit  26  was 3.5 dB. Further, satisfactorily, only the PIN diode D 3  is in the on state during the reception wave standby. Both of the PIN diodes D 2  and D 3  in the transmission side circuit  25  are not in the on state. Thus, the power consumption during the reception wave standby can be suppressed. 
     Second Embodiment, FIG.  5   
     FIG. 5 illustrates another embodiment of the duplexer of the present invention. In a duplexer  40 , a transmission side circuit  47  is electrically connected between a transmission terminal Tx and an antenna terminal ANT, and a reception side circuit  48  is electrically connected between a reception terminal Rx and the antenna terminal ANT. 
     The transmission side circuit  47  is a frequency variable band elimination filter having resonant circuits in stages coupled together. A resonator  41  is electrically connected to a transmission terminal Tx via a resonance capacitor C 41 . A series resonant circuit comprising the resonator  41  and the resonance capacitor C 41 , a series resonant circuit comprising a resonator  42  and a resonance capacitor C 42 , and a series resonant circuit comprising a resonator  43  and a resonance capacitor C 43  are electrically connected together via coupling coils L 41  and L 42 . Capacitors C 47 , C 48 , and C 49  are electrically connected in parallel to these three series resonant circuits, respectively. The antenna terminal ANT is electrically connected to the series resonant circuit comprising the resonator  43  and the resonance capacitor C 43  via an L-LC circuit comprising a coupling coil L 43  and a capacitor  50 . The resonance capacitors C 41  to C 43  are capacitors for determining a rejection-band attenuation. 
     To the intermediate node between the resonator  41  and the resonance capacitor C 41 , a PIN diode D 41  as a reactance element is electrically connected in parallel with the resonator  41  via band variable capacitor C 44 , with the cathode being grounded. To the intermediate node between the resonator  42  and the resonance capacitor C 42 , a PIN diode D 42  is electrically connected in parallel with the resonator  43  via a band variable capacitor C 45 . Further, to the intermediate node between the resonator  43  and the resonance capacitor C 43 , a PIN diode D 43  is electrically connected in parallel with the resonator  43  via a band variable capacitor C 46 . The band variable capacitors C 44  to C 46  are capacitors for changing the attenuation extreme frequencies of the transmission side circuit  47 . Further, a capacitor  64  is connected in parallel to the band variable capacitor C 46 . 
     The reception side circuit  48  is a frequency variable band-pass filter comprising resonant circuits in three stages connected together. A series resonant circuit comprising a resonator  44  and a resonance capacitor C 55 , a resonator  45 , and a series resonant circuit comprising a resonator  46  and a resonance capacitor C 56  are electrically connected via coupling capacitors C 52  and C 53 . Further, the series resonant circuit comprising the resonator  44  and the resonance capacitor C 55  is electrically connected to the antenna terminal ANT via a coupling capacitor C 51 . The series resonant circuit comprising the resonator  46  and the resonance capacitor C 56  is electrically connected to the reception terminal Rx via a coupling capacitor C 54 . 
     To the intermediate node between the resonator  44  and the resonance capacitor C 55 , a series circuit comprising a band variable capacitor C 57  and a PIN diode D 44  is electrically connected in parallel with the resonator  44 . To the intermediate node among the resonator  45  and the coupling capacitors C 52  and C 53 , a series circuit comprising a band variable capacitors C 58  and C 59 , and the PIN diode D 45  is electrically connected in parallel with the resonator  45 . To the intermediate node between the resonator  46  and the resonance capacitor C 56 , a series circuit comprising a band variable capacitor C 60  and a PIN diode D 46  is electrically connected in parallel with the resonator  46 . 
     A voltage control terminal CONT 1  is electrically connected to the intermediate node between the anode of the PIN diode D 41  and the band variable capacitor C 44  via a control voltage supply resistor R 41 , a capacitor C 62 , and a choke coil L 44 , and is electrically connected to the intermediate node between the anode of the PIN diode D 42  and the band variable capacitor C 45  via the control voltage supply resistor R 41 , the capacitor C 62 , and a choke coil L 45 . 
     On the other hand, a voltage control terminal CONT 2  is electrically connected to the intermediate node between the anode of the PIN diode D 43  and the band variable capacitor C 46  via a control voltage supply resistor R 42 , a capacitor C 63 , and a choke coil L 46 , electrically connected to the intermediate node between the anode of the PIN diode D 44  and the band variable capacitor C 57  via the control voltage supply resistor R 42 , the capacitor C 63 , and a choke coil  47 , and moreover, electrically connected to the intermediate node between the anode of the PIN diode D 46  and the band variable capacitor C 60  via the control voltage supply resistor R 42 , the capacitor C 63 , and a choke coil L 49 . 
     The duplexer  40  having the above-described configuration has the same operation and effects as those of the duplexer  31  of the first embodiment. 
     Third Embodiment, FIG.  6   
     FIG. 6 shows a duplexer according to a third embodiment of the present invention. A duplexer  70  is the same as the duplexer  40  of the second embodiment except that a voltage control terminal CONT 3  for independently voltage-controlling the PIN diode D 43 , connected to the resonator  43  which is electrically connected to the antenna terminal ANT in the position nearest thereto in the transmission side circuit  47  is newly provided. The voltage control terminal CONT 3  is connected to the intermediate node between the anode of the PIN diode D 43  and the band variable capacitor C 46  via a control voltage supply resistorR 73 , a capacitor C 74 , and a choke coil L 46 . 
     Hereinafter, the operation and effects of the duplexer  70  having the above-described configuration will be described. 
     As described previously, if consumption current is caused to flow through the reception side circuit  48  only of the duplexer  70  during reception wave standby, the insertion loss of the respectively side circuit  48  is increased. However, as seen in the pass characteristic S 32  of FIG. 12, the insertion loss of the reception side circuit  47  is deteriorated at about 1930 MHz only, which is near to the transmission frequency band (1850-1910 MHz), while substantially no deterioration of the insertion loss occurs near to 1960 MHz. 
     Accordingly, in the case where frequencies near to 1960 MHz are used as the frequency of a reception wave on standby, the control voltages of the voltage control terminals CONT 1  and CONT 3  are made 0V and a positive voltage is applied to the voltage control terminal CONT 2  only. That is, a consumption current is made to flow through the reception side circuit  48  only during reception wave standby. 
     On the other hand, in the case where about 1930 MHz is used as the frequency of a reception wave on standby, the control voltage of the voltage control terminal CONT 1  is made 0V, and positive voltages are applied to the voltage control terminals CONT 2  and CONT 3 . That is, during the reception wave standby, the PIN diodes D 44  to D 46 , and the PIN diode D 43  electrically connected in the position nearest to the antenna terminal ANT among the PIN diodes D 41  to D 43  in the transmission side circuit  47  are in the on state. 
     Like this, appropriate control voltages are applied to the voltage control terminals CONT 1  to CONT 3 , depending on whether the frequency of a reception wave on standby is near to 1960 MHz or 1930 MHz, so that the power consumption can be further decreased. 
     Fourth Embodiment, FIG.  7   
     The fourth embodiment will be described with reference to a portable telephone as an example of the communication apparatus of the present invention. 
     FIG. 7 is an electric circuit block diagram of the RF part of a portable telephone  120 . In FIG. 7, an antenna element is designated by  122 , a duplexer by  123 , a transmission side isolator by  131 , a transmission side amplifier by  135 , a reception side interstage band-pass filter by  136 , a reception side mixer by  137 , a voltage control oscillator (VCO) by  138 , and a local band-pass filter by  139 . 
     As the duplexer  123 , the duplexers  31 ,  40 , and  70  of the first to third embodiments may be used. A portable telephone with a low power consumption and a low loss of the reception side circuit during reception wave standby can be realized by mounting the duplexer  31 ,  40 , or  70 . 
     The duplexer and the communication apparatus of the present invention are limited onto the above-described embodiments, and variations may be made without departing from the sprit and the scope of the present invention. Especially, as the reactance element, variable capacitance diodes, transistors or the like are available in addition to the PIN diode. Further, as the resonators, strip line resonators or the like may be employed, in addition to the dielectric resonators. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the forgoing and other changes in form and details may be made therein without departing from the spirit of the invention.