Abstract:
A tunable filter and a tunable duplexer are provided, both of which comprise an input terminal, an output terminal, four fixed capacitors, three variable capacitors and three fixed coils; wherein the variable capacitors are grounded at one end and, at the other end, connected to the fixed coils to form three sets of series-connected LC circuit; wherein the connecting points of the three sets of series-connected LC circuit, the input terminal and the output terminal are connected with the fixed capacitors, respectively.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a tunable (frequency-variable) filter and a tunable duplexer, both for mobile communication terminals, and a mobile communication terminal using them. 
         [0002]    New mobile communications technologies currently being considered for use on mobile phones include WCDMA, which is already in service, and LTE. WCDMA and LTE allow simultaneous transmission and reception and therefore use different frequency bands for transmission and reception. These communication systems use a duplexer that separates the transmission band and the reception band. 
         [0003]    The WCDMA and the LTE systems each have a plurality of frequency bands and, to produce desired high-frequency characteristics, use a duplexer for each frequency band in a mobile communication terminal front end. Further the LTE system requires the same number of reception circuits as the antennas since it employs a MIMO (Multiple Input Multiple Output) technology to realize high-speed communication. So, as the communication grows in speed in future, the scale of the reception circuit is expected to become large, calling for a new technology to render the duplexers tunable. Patent Literature 1 (JP-A-2011-120120) describes a tunable filter technology to switch the duplexer into a tunable state and a canceler technology to cancel leakage components of a transmitted signal found in a received signal output from the tunable filter and thermal noise leakage components in the reception band. So, although the amount by which out-of-band signals are suppressed by the tunable filter is about 20 dB smaller than that suppressed by a conventional untunable duplexer or a frequency-fixed duplexer, the tunable filter, when used in combination with the canceler technology, can be put into practical use. 
         [0004]    With the conventional tunable filters, a high-frequency filter has been formed, as described in Patent Literature 2 (JP-A-2010-45478), by connecting three meander line inductors formed on a dielectric substrate, five transmission lines approximately λ/4 long and three varactors with their one end grounded to make the capacitance of the varactors variable. 
       SUMMARY OF THE INVENTION 
       [0005]    The technology described in the Patent Literature 2 has a drawback that the filter becomes large in size because a number of meander line inductors and λ/4-long transmission lines are formed on a dielectric substrate. This problem becomes conspicuous especially when a tunable filter is constructed in low frequency ranges because the meander line inductors and the λ/4-long transmission lines become long. 
         [0006]    In the WCDMA and LTE systems, specifications on Band 1 -Band 17  have already been defined and the number of bands tends to further increase in future. For mobile communication terminals capable of handling these multiple bands, an effective solution involves making the duplexer tunable to reduce the size of their front end portion and also minimizing the size of the tunable filter. 
         [0007]    It is an object of this invention to provide a mobile communication terminal that performs transmission and reception operations simultaneously by using different frequency bands for transmission and reception and which is small in size and highly reliable and can handle a plurality of frequency bands. 
         [0008]    To make improvements on the aforementioned problem, a tunable filter is used which comprises: an input terminal; an output terminal; a first series-connected LC circuit composed of a first variable capacitor and a first fixed coil; a second series-connected LC circuit composed of a second variable capacitor and a second fixed coil; a third series-connected LC circuit composed of a third variable capacitor and a third fixed coil; a first fixed capacitor with one of its ends connected to the input terminal and the other end connected to a connecting point of the first variable capacitor and the first fixed coil; a second fixed capacitor with one of its ends connected to the output terminal and the other end connected to a connecting point of the second variable capacitor and the second fixed coil; a third fixed capacitor with one of its ends connected to the connecting point of the first variable capacitor and the first fixed coil and the other end connected to a connecting point of the third variable capacitor and the third fixed coil; and a fourth fixed capacitor with one of its ends connected to the connecting point of the second variable capacitor and the second fixed coil and the other end connected to the connecting point of the third variable capacitor and the third fixed coil; wherein the frequencies of a passband and a stopband of the tunable filter are made variable by changing capacitance values of the variable capacitors. 
         [0009]    With this invention, a mobile communication terminal can be provided which is small and highly reliable and can handle a plurality of frequency bands. 
         [0010]    Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a circuitry showing an example configuration of a tunable filter tuned to relatively high frequencies in a first embodiment of this invention. 
           [0012]      FIG. 2  shows a frequency characteristic of a receiving filter tuned to a highest frequency band in Band 1  in the tunable filter of the first embodiment. 
           [0013]      FIG. 3  shows frequency characteristic of a receiving filter tuned to a lowest frequency band in Band 3  in the tunable filter of the first embodiment. 
           [0014]      FIG. 4  is a circuitry showing an example configuration of a tunable filter for relatively low frequencies in a second embodiment of this invention. 
           [0015]      FIG. 5  shows a frequency characteristic of a receiving filter tuned to a highest frequency band in Band 3  in the tunable filter of the second embodiment. 
           [0016]      FIG. 6  shows a frequency characteristic of a receiving filter tuned to a lowest frequency band in Band 17  in the tunable filter of the second embodiment. 
           [0017]      FIG. 7  shows a circuitry of a tunable filter module as a third embodiment of this invention. 
           [0018]      FIG. 8  shows a circuitry of a tunable duplexer tuned to relatively high frequencies as a fourth embodiment of this invention. 
           [0019]      FIG. 9  shows a frequency characteristic of the duplexer of the fourth embodiment tuned to a highest frequency band in Band 1 . 
           [0020]      FIG. 10  shows a frequency characteristic of the duplexer of the fourth embodiment tuned to a lowest frequency band in Band 3 . 
           [0021]      FIG. 11  shows a circuitry of a tunable duplexer tuned to relatively low frequencies as a fifth embodiment of this invention. 
           [0022]      FIG. 12  shows a frequency characteristic of the duplexer of the fifth embodiment tuned to a highest frequency band in Band 8 . 
           [0023]      FIG. 13  shows a frequency characteristic of the duplexer of the fifth embodiment tuned to a lowest frequency band in Band 17 . 
           [0024]      FIG. 14  shows a circuitry of a tunable duplexer module as a sixth embodiment of this invention. 
           [0025]      FIG. 15A  is a circuit block diagram showing as a seventh embodiment a multiband-enabled mobile communication terminal that applies the tunable filter module and the tunable duplexer module; and  FIG. 15B  shows an example of frequency bands available to the tunable duplexer. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    Embodiments of this invention will be described as follows. 
       Embodiment 1 
       [0027]      FIG. 1  is a circuitry showing an example configuration of a tunable filter tuned to relatively high frequencies in the first embodiment. This tunable filter is obtained as a pass-through characteristic in a direction from an input terminal  1 H to an output terminal  2 H, or in a reverse direction. 
         [0028]    The tunable filter has a first fixed capacitor  3 H connected between an input terminal  1 H and a connecting point of a first pair of a variable capacitor  10 H and a fixed coil  7 H; a second fixed capacitor  4 H connected between the connecting point of the first pair of the variable capacitor  10 H and the fixed coil  7 H and a connecting point of a second pair of a variable capacitor  11 H and a fixed coil  8 H and; a third fixed capacitor  5 H connected between the connecting point of the second pair of the variable capacitor  11 H and the fixed coil  8 H and a connecting point of a third pair of a variable capacitor  12 H and a fixed coil  9 H; and a fourth fixed capacitor  6 H connected between the connecting point of the third pair of the variable capacitor  12 H and the fixed coil  9 H and an output terminal  2 H, with one end of the variable capacitors  10 H,  11 H,  12 H, whose opposite end is connected to the fixed coils  7 H,  8 H,  9 H, grounded and with one end of the fixed coils  7 H,  8 H,  9 H, whose opposite end is connected to the variable capacitors  10 H,  11 H,  12 H, connected together. 
         [0029]    This tunable filter is characterized in that the frequency of a passband that is formed by a resonant circuit composed of the fixed capacitors  3 H,  6 H, the fixed coils  7 H,  9 H and the variable capacitors  10 H,  12 H and the frequency of a stopband (notch) that is formed by a resonant circuit composed of the fixed coil  8 H and the variable capacitor  11 H are made variable by changing the capacitance values of the variable capacitors  10 H,  11 H,  12 H. 
         [0030]    Here, the fixed coils  7 H,  8 H,  9 H have constants of about 4.3 nH, 4.1 nH and 4.3 nH, respectively, and use solenoid coils with Q values of about 90 at the operation frequencies. The fixed capacitors  3 H,  4 H,  5 H,  6 H have constants of about 0.41 pF, 0.01 pF, 0.01 pF and 0.41 pF, respectively. Since the fixed capacitors  4 H and  5 H have very small values of capacitance, they can be provided, respectively, by a stray capacitance formed between lands mounting the fixed coil  7 H and the fixed coil  8 H and by a stray capacitance formed between lands mounting the fixed coil  8 H and the fixed coil  9 H. In practical use, this allows for a size reduction of the device by not actually mounting these fixed capacitors. The fixed capacitors  3 H,  6 H are constructed of a chip capacitor. As for constants of the variable capacitors, the variable capacitors  10 H,  12 H are in a range of between about 0.8 pF and 1.35 pF and the variable capacitor  11 H in a range of between about 1.53 pF and 2.07 pF, allowing the filter characteristic to be variable from the highest frequency channel in the Band 1  reception band to the lowest frequency channel in the Band 3  reception band. That is, the constant of this tunable filter is distributed symmetric between the input terminal  1 H side and the output terminal  2 H side with respect to the second pair of the variable capacitor  11 H and the fixed coil  8 H located at the center. The variable capacitors  10 H,  11 H,  12 H are constructed of MEMS variable capacitors. 
         [0031]      FIG. 2  shows a frequency characteristic of a receiving filter of the tunable filter of the first embodiment tuned to the highest frequency band of Band 1 , with the variable capacitors  10 H and  12 H at 0.8 pF and the variable capacitor  11 H at 1.53 pF. As shown in  FIG. 2 , the Band 1  receiving filter has a passband at the highest frequency band (2.17 GHz) in Band 1  and a stopband at the highest frequency band (1.98 GHz) in the transmission band of Band 1 . 
         [0032]      FIG. 3  shows a frequency characteristic of a receiving filter tuned to the lowest frequency band in Band 3 , with the variable capacitors  10 H and  12 H at 1.35 pF and the variable capacitor  11 H at 2.07 pF. As shown in  FIG. 3 , the Band 3  receiving filter has a passband at the lowest frequency band (1.805 GHz) in Band 3  and a stopband at the lowest frequency band (1.71 GHz) in the transmission band of Band 3 . 
         [0033]    From  FIG. 2  and  FIG. 3 , making the variable capacitors  10 H,  12 H variable in a range from about 0.8 pF to 1.35 pF and the variable capacitor  11 H variable in a range from about 1.53 pF to 2.07 pF allows the tunable filter to deal also with those bands included in the frequency range of Band 1  and Band 3 , such as Band 2 , Band 4  and Band 9 . 
       Embodiment 2 
       [0034]      FIG. 4  is a circuitry showing a configuration of a tunable filter tuned to relatively low frequencies in a second embodiment. This tunable filter is obtained as a pass-through characteristics in a direction from an input terminal  1 L to an output terminal  2 L, or in a reverse direction. 
         [0035]    A first fixed capacitor  3 L is connected between the input terminal  1 L and a connecting point of a first pair of a variable capacitor  10 L and a fixed coil  7 L; a second fixed capacitor  4 L is connected between the connecting point of the first pair of the variable capacitor  10 L and the fixed coil  7 L and a connecting point of a second pair of a variable capacitor  11 L and a fixed coil  8 L; a third fixed capacitor  5 L is connected between the connecting point of the second pair of the variable capacitor  11 L and the fixed coil  8 L and a connecting point of a third pair of a variable capacitor  12 L and a fixed coil  9 L; and a fourth fixed capacitor  6 L is connected between the connecting point of the third pair of the variable capacitor  12 L and the fixed coil  9 L, with one end of the variable capacitors  10 L,  11 L,  12 L, the opposite end of which is connected to the fixed coils  7 L,  8 L,  9 L, grounded and with one end of the fixed coils  7 L,  8 L,  9 L, opposite end of which is connected to the variable capacitors  10 L,  11 L,  12 L, connected together. 
         [0036]    Similar to the operating principle explained in the first embodiment, the tunable filter is characterized in that its applied band frequency can be varied by changing the capacitance values of the variable capacitors  10 L,  11 L,  12 L. 
         [0037]    Here, the fixed coils  7 L,  8 L,  9 L have constants of about 11.5 nH, 7.9 nH and 11.5 nH, respectively, and use solenoid coils with Q values of about 90 for the operation frequencies. The fixed capacitors  3 L,  4 L,  5 L,  6 L have constants of about 0.83 pF, 0.15 pF, 0.15 pF and 0.83 pF, respectively. Since the fixed capacitors  4 L and  5 L have very small values of capacitance, they can be provided, respectively, by a stray capacitance formed between lands mounting the fixed coil  7 L and the fixed coil  8 L and by a stray capacitance formed between lands mounting the fixed coil  8 L and the fixed coil  9 L. In practical use, this allows for a size reduction of the device by not actually mounting these fixed capacitors. The fixed capacitors  3 L,  6 L are constructed of a chip capacitor. As for constants of the variable capacitors, the variable capacitors  10 L,  12 L are in a range of between about 1.20 pF and 3.02 pF and the variable capacitor  11 L in a range of between about 3.12 pF and 5.77 pF, allowing the filter characteristic to be variable from the highest frequency channel in the Band 8  reception band to the lowest frequency channel in the Band 17  reception band. That is, the constant of this tunable filter is symmetrically distributed between the input terminal  1 L side and the output terminal  2 L side with respect to the second pair of the variable capacitor  11 L and the fixed coil  8 L located at the center. The variable capacitors  10 L,  11 L,  12 L are constructed of MEMS variable capacitors. 
         [0038]      FIG. 5  shows a frequency characteristic of a receiving filter of the tunable filter of the second embodiment tuned to the highest frequency band of Band 8 , with the variable capacitors  10 L and  12 L at 1.2 pF and the variable capacitor  11 L at 3.12 pF. As shown in  FIG. 5 , the Band 8  receiving filter has a passband at the highest frequency band (0.96 GHz) in Band 8  and a stopband at the highest frequency band (0.915 GHz) in the transmission band of Band 8 . 
         [0039]      FIG. 6  shows a frequency characteristic of a receiving filter tuned to the lowest frequency band in Band  17 , with the variable capacitors  10 L and  12 L at 3.02 pF and the variable capacitor  11 L at 5.77 pF. As shown in  FIG. 6 , the Band 17  receiving filter has a passband at the lowest frequency band (0.734 GHz) in Band 17  and a stopband at the lowest frequency band (0.704 GHz) in the transmission band of Band 17 . 
         [0040]    From  FIG. 5  and  FIG. 6 , making the variable capacitors  10 L,  12 L variable in a range from about 1.2 pF to 3.02 pF and the variable capacitor  11 L in a range from about 3.12 pF to 5.77 pF allows the tunable filter to also handle those bands included in the frequency range of Band 8  and Band 17 , such as Band 5  and Band 6 . 
       Embodiment 3 
       [0041]      FIG. 7  shows a circuitry of a tunable filter module of the third embodiment. In this embodiment, a high-band tunable filter  27  uses the tunable filter of the first embodiment and a low-band tunable filter  28  the tunable filter of the second embodiment. An input terminal  1 H of the high-band tunable filter  27  and an input terminal  1 L of the low-band tunable filter  28  are connected to an antenna  21  through a SPDT (Single Pole Dual Throw) switch  20 . That is, the SPDT switch  20  selects between the high-band tunable filter  27  and the low-band tunable filter  28  for connection to the antenna  21 . The SPDT switch is formed of CMOS, SOS (Silicon on Sapphire) or GaAs switch. The tunable filter module of this configuration can be used as a diversity receiver circuit that covers almost all bands used in communication systems, such as WCDMA and LTE. 
       Embodiment 4 
       [0042]      FIG. 8  shows a circuitry of a tunable duplexer tuned to relatively high frequencies in the fourth embodiment. As shown in  FIG. 8 , input terminals of a receiving tunable filter  31  and a transmitting tunable filter  32  are connected to an antenna  22 H to form a tunable duplexer that splits the received signals and transmission signals. The configuration of the tunable filter is the same as that of the tunable filter of the first embodiment. The operating principle that makes the passband and the stopband tunable is the same as that explained in the first embodiment. The configuration of the receiving tunable filter  31  will be described as follows. 
         [0043]    A first fixed capacitor  3 HR is connected between the antenna  22 H and a connecting point of a first pair of a variable capacitor  10 HR and a fixed coil  7 HR; a second fixed capacitor  4 HR is connected between the connecting point of the first pair of the variable capacitor  10 HR and the fixed coil  7 HR and a connecting point of a second pair of a variable capacitor  11 HR and a fixed coil  8 HR; a third fixed capacitor  5 HR is connected between the connecting point of the second pair of the variable capacitor  11 HR and the fixed coil  8 HR and a connecting point of a third pair of a variable capacitor  12 HR and a fixed coil  9 HR; and a fourth fixed capacitor  6 HR is connected between the connecting point of the third pair of the variable capacitor  12 HR and the fixed coil  9 HR and a receiving terminal  2 HR, with one end of the variable capacitors  10 HR,  11 HR,  12 HR, whose opposite end is connected to the fixed coils  7 HR,  8 HR,  9 HR, grounded and with one end of the fixed coils  7 HR,  8 HR,  9 HR, whose opposite end is connected to the variable capacitors  10 HR,  11 HR,  12 HR, connected together. The receiving tunable filter  31  is characterized in that its applied band frequency can be varied by changing the capacitance values of the variable capacitors  10 HR,  11 HR,  12 HR. 
         [0044]    Here, the fixed coils  7 HR,  8 HR,  9 HR have constants of about 4.3 nH, 4.1 nH and 4.3 nH, respectively, and use solenoid coils with Q values of about 90 at the operation frequencies. The fixed capacitors  3 HR,  4 HR,  5 HR,  6 HR have constants of about 0.41 pF, 0.01 pF, 0.01 pF and 0.41 pF, respectively. Since the fixed capacitors  4 HR and  5 HR have very small values of capacitance, they can be provided, respectively, by a stray capacitance formed between lands mounting the fixed coil  7 HR and the fixed coil  8 HR and by a stray capacitance between lands mounting the fixed coil  8 HR and the fixed coil  9 HR. In practical use, this allows for a size reduction of the device by not actually mounting these fixed capacitors. The fixed capacitors  3 HR,  6 HR are composed of a chip capacitor. As for constants of the variable capacitors, the variable capacitors  10 HR,  12 HR are in a range of between about 0.8 pF and 1.35 pF and the variable capacitor  11 HR in a range of between about 1.53 pF and 2.07 pF, allowing the filter characteristic to be varied from the highest frequency channel in the Band 1  reception band to the lowest frequency channel in the Band 3  reception band. That is, this tunable filter has its constant distributed symmetrically between the antenna  22 H side and the receiving terminal  2 HR side with respect to the second pair of the variable capacitor  11 HR and the fixed coil  8 HR located at the center. The variable capacitors  10 HR,  11 HR,  12 HR are constructed of MEMS variable capacitors. 
         [0045]    Next, the configuration of the transmitting tunable filter  32  will be explained. 
         [0046]    A first fixed capacitor  3 HT is connected between the antenna  22 H and a connecting point of a first pair of a variable capacitor  10 HT and a fixed coil  7 HT; a second fixed capacitor  4 HT is connected between the connecting point of the first pair of the variable capacitor  10 HT and the fixed coil  7 HT and a connecting point of a second pair of a variable capacitor  11 HT and a fixed coil  8 HT; a third fixed capacitor  5 HT is connected between the connecting point of the second pair of the variable capacitor  11 HT and the fixed coil  8 HT and a connecting point of a third pair of a variable capacitor  12 HT and a fixed coil  9 HT; and a fourth fixed capacitor  6 HT is connected between the connecting point of the third pair of the variable capacitor  12 HT and the fixed coil  9 HT and a transmitting terminal  2 HT, with one end of the variable capacitors  10 Ht,  11 HT,  12 HT, whose opposite end is connected to the fixed coils  7 HT,  8 HT,  9 HT, grounded and with one end of the fixed coils  7 HT,  8 HT,  9 HT, whose opposite end is connected to the variable capacitors  10 HT,  11 HT,  12 HT, connected together. 
         [0047]    The transmitting tunable filter  32  is characterized in that its applied band frequency can be varied by changing the capacitance values of the variable capacitors  10 HT,  11 HT,  12 HT. 
         [0048]    Here, the fixed coils  7 HT,  8 HT,  9 HT have constants of about 4.4 nH, 4.5 nH and 4.4 nH, respectively, and use solenoid coils with Q values of about 90 at the operation frequencies. The fixed capacitors  3 HT,  4 HT,  5 HT,  6 HT have constants of about 0.67 pF, 0.01 pF, 0.01 pF and 0.67 pF, respectively. Since the fixed capacitors  4 HT and  5 HT have very small capacitances, they can be provided, respectively, by a stray capacitance formed between lands mounting the fixed coil  7 HT and the fixed coil  8 HT and by a stray capacitance formed between lands mounting the fixed coil  8 HT and the fixed coil  9 HT. In practical use, this allows the device to be reduced in size by not actually mounting these fixed capacitors. The fixed capacitors  3 HT,  6 HT are constructed of a chip capacitor. As for constants of the variable capacitors, the variable capacitors  10 HT,  12 HT are in a range of between about 0.95 pF and 1.50 pF and the variable capacitor  11 HT in a range of between about 1.16 pF and 1.7 pF, allowing the filter characteristic to be varied from the highest frequency channel in the Band 1  transmission band to the lowest frequency channel in the Band 3  transmission band. That is, this tunable filter has its constant distributed symmetrically between the antenna  22 H side and the transmitting terminal  2 HT side with respect to the second pair of the variable capacitor  11 HT and the fixed coil  8 HT located at the center. The variable capacitors  10 HT,  11 HT,  12 HT are constructed of MEMS variable capacitors. 
         [0049]      FIG. 9  shows a frequency characteristic of a tunable duplexer of the fourth embodiment tuned to the highest frequency band in Band 1 , with the variable capacitors  10 HR,  12 HR at 0.8 pF, the variable capacitor  11 HR at 1.53 pF, the variable capacitors  10 HT,  12 HT at 0.95 pF and the variable capacitor  11 HT at 1.16 pF. In  FIG. 9 , a thick line represents a pass-through characteristic in a direction from the antenna  22 H to the receiving terminal  2 HR and a thin line represents a pass-through characteristic in a direction from the transmitting terminal  2 HT to the antenna  22 H, and a medium thin line an isolation characteristic in a direction from the transmitting terminal  2 HT to the receiving terminal  2 HR. 
         [0050]    As shown in  FIG. 9 , the resonant frequency of the second pair of the variable capacitor  11 HR and the fixed coil  8 HR in the receiving tunable filter  31  matches the passband of the transmitting tunable filter  32 , thus forming a notch. Further, the resonant frequency of the second pair of the variable capacitor  11 HT and the fixed coil  8 HT in the transmitting tunable filter  32  matches the passband of the receiving tunable filter  31 , thus forming a notch. This results in a satisfactory isolation characteristic in a direction from the transmitting terminal  2 HT to the receiving terminal  2 HR. 
         [0051]      FIG. 10  shows a frequency characteristic of the tunable duplexer of the fourth embodiment tuned to the lowest frequency band in Band 3 , with the variable capacitors  10 HR,  12 HR at 1.35 pF, the variable capacitor  11 HR at 2.05 pF, the variable capacitors  10 HT,  12 HT at 1.50 pF and the variable capacitor  11 HT at 1.7 pF. In  FIG. 10 , a thick line represents a pass-through characteristic in a direction from the antenna  22 H to the receiving terminal  2 HR, a thin line represents a pass-through characteristic in a direction from the transmitting terminal  2 HT to the antenna  22 H and a medium thin line an isolation characteristic in a direction from the transmitting terminal  2 HT to the receiving terminal  2 HR. As shown in  FIG. 10 , the resonant frequency of the second pair of the variable capacitor  11 HR and the fixed coil  8 HR in the receiving tunable filter  31  matches the passband of the transmitting tunable filter  32 , thus forming a notch. Further, the resonant frequency of the second pair of the variable capacitor  11 HT and the fixed coil  8 HT in the transmitting tunable filter  32  matches the passband of the receiving tunable filter  31 , thus forming a notch. This results in a satisfactory isolation characteristic in a direction from the transmitting terminal  2 HT to the receiving terminal  2 HR. 
       Embodiment 5 
       [0052]      FIG. 11  shows a circuitry of a tunable duplexer tuned to relatively low frequencies in the fifth embodiment. As shown in  FIG. 11 , input terminals of a receiving tunable filter  33  and a transmitting tunable filter  34  are connected to an antenna  22 L to form a tunable duplexer that splits the received signals and sending signals. The configuration of the tunable filter is the same as that of the tunable filter of the first embodiment. The operating principle that makes the passband and the stopband tunable is the same as that explained in the first embodiment. The configuration of the receiving tunable filter  33  will be explained in the following. 
         [0053]    A first fixed capacitor  3 LR is connected between the antenna  22 L and a connecting point of a first pair of a variable capacitor  10 LR and a fixed coil  7 LR; a second fixed capacitor  4 LR is connected between the connecting point of the first pair of the variable capacitor  10 LR and the fixed coil  7 LR and a connecting point of a second pair of a variable capacitor  11 LR and a fixed coil  8 LR; a third fixed capacitor SLR is connected between the connecting point of the second pair of the variable capacitor  11 LR and the fixed coil  8 LR and a connecting point of a third pair of a variable capacitor  12 LR and a fixed coil  9 LR; and a fourth fixed capacitor  6 LR is connected between the connecting point of the third pair of the variable capacitor  12 LR and the fixed coil  9 LR and a receiving terminal  2 LR, with one end of the variable capacitors  10 LR,  11 LR,  12 LR, whose opposite end is connected to the fixed coils  7 LR,  8 LR,  9 LR, grounded and with one end of the fixed coils  7 LR,  8 LR,  9 LR, whose opposite end is connected to the variable capacitors  10 LR,  11 LR,  12 LR, connected together. The receiving tunable filter  33  is characterized in that its applied band frequency can be varied by changing the capacitance values of the variable capacitors  10 LR,  11 LR,  12 LR. 
         [0054]    Here, the fixed coils  7 LR,  8 LR,  9 LR have constants of about 11.5 nH, 7.92 nH and 11.5 nH, respectively, and use solenoid coils with Q values of about 90 at the operation frequencies. The fixed capacitors  3 LR,  4 LR,  6 LR,  6 LR have constants of about 0.83 pF, 0.15 pF, 0.15 pF and 0.83 pF, respectively. Since the fixed capacitors  4 LR and  5 LR have very small capacitances, they can be provided, respectively, by a stray capacitance formed between lands mounting the fixed coil  7 LR and the fixed coil  8 LR and by a stray capacitance formed between lands mounting the fixed coil  8 LR and the fixed coil  9 LR. In practical use, this allows the device to be reduced in size by not actually mounting these fixed capacitors. The fixed capacitors  3 LR,  6 LR are constructed of a chip capacitor. As for constants of the variable capacitors, the variable capacitors  10 LR,  12 LR are in a range of between about 1.2 pF and 3.02 pF and the variable capacitor  11 LR in a range of between about 3.15 pF and 5.8 pF, allowing the filter characteristic to be varied from the highest frequency channel in the Band 8  reception band to the lowest frequency channel in the Band 17  reception band. That is, this tunable filter has its constant distributed symmetrically between the antenna  22 L side and the receiving terminal  2 LR side with respect to the second pair of the variable capacitor  11 LR and the fixed coil  8 LR located at the center. The variable capacitors  10 LR,  11 LR,  12 LR are constructed of MEMS variable capacitors. 
         [0055]    Next, the configuration of the transmitting tunable filter  34  will be explained. 
         [0056]    A first fixed capacitor  3 LT is connected between the antenna  22 L and a connecting point of a first pair of a variable capacitor  10 LT and a fixed coil  7 LT; a second fixed capacitor  4 LT is connected between the connecting point of the first pair of the variable capacitor  10 LT and the fixed coil  7 LT and a connecting point of a second pair of a variable capacitor  11 LT and a fixed coil  8 LT; a third fixed capacitor  5 LT is connected between the connecting point of the second pair of the variable capacitor  11 LT and the fixed coil  8 LT and a connecting point of a third pair of a variable capacitor  12 LT and fixed coil  9 LT; and a fourth fixed capacitor  6 LT is connected between the connecting point of the third pair of the variable capacitor  12 LT and the fixed coil  9 LT and a transmitting terminal  2 LT, with one end of the variable capacitors  10 LT,  11 LT,  12 LT, whose opposite end is connected to the fixed coils  7 LT,  8 LT,  9 LT, grounded and with one end of the fixed coils  7 LT,  8 LT,  9 LT, whose opposite end is connected to the variable capacitors  10 LT,  11 LT,  12 LT, connected together. 
         [0057]    The transmitting tunable filter  34  is characterized in that its applied band frequency can be varied by changing the capacitance values of the variable capacitors  10 LT,  11 LT,  12 LT. 
         [0058]    Here, fixed coils  7 LT,  8 LT,  9 LT have constants of about 14.5 nH, 7.5 nH and 14.5 nH, respectively, and use solenoid coils with Q values of about 90 at the operation frequencies. The fixed capacitors  3 LT,  4 LT,  5 LT,  6 LT have constants of about 1.32 pF, 0.3 pF, 0.3 pF and 1.32 pF, respectively. Since the fixed capacitors  4 LT and  5 LT have very small capacitances, they can be provided, respectively, by a stray capacitance formed between lands mounting the fixed coil  7 LT and the fixed coil  8 LT and by a stray capacitance formed between lands mounting the fixed coil  8 LT and the fixed coil  9 LT. In practical use, this allows the device to be reduced in size by not actually mounting these fixed capacitors. The fixed capacitors  3 LT,  6 LT are constructed of a chip capacitor. As for constants of the variable capacitors, the variable capacitors  10 LT,  12 LT are in a range of between about 0.88 pF and 2.30 pF and the variable capacitor  11 LT in a range of between about 2.11 pF and 4.55 pF, allowing the filter characteristic to be varied from the highest frequency channel in the Band 8  transmission band to the lowest frequency channel in the Band 17  transmission band. That is, this tunable filter has its constant distributed symmetrically between the antenna  22 L side and the transmitting terminal  2 LT side with respect to the second pair of the variable capacitor  11 LT and the fixed coil  8 LT located at the center. The variable capacitors  10 LT,  11 LT,  12 LT are constructed of MEMS variable capacitors. 
         [0059]      FIG. 12  shows a frequency characteristic of the tunable duplexer of the fifth embodiment tuned to the highest frequency band in Band 8 , with variable capacitors  10 LR,  12 LR at 1.2 pF, the variable capacitor  11 LR at 3.15 pF, the variable capacitors  10 LT,  12 LT at 0.88 pF and the variable capacitor  11 LT at 2.11 pF. In  FIG. 12 , a thick line represents a pass-through characteristic in a direction from the antenna  22 L to the receiving terminal  2 LR, a thin line represents a pass-through characteristic in a direction from the transmitting terminal  2 LT to the antenna  22 L and a medium thin line an isolation characteristic in a direction from the transmitting terminal  2 LT to the receiving terminal  2 LR. As shown in  FIG. 12 , the resonant frequency of the second pair of the variable capacitor  11 LR and the fixed coil  8 LR in the receiving tunable filter  33  matches the passband of the transmitting tunable filter  34 , thus forming a notch. Further, the resonant frequency of the second pair of the variable capacitor  11 LT and the fixed coil  8 LT in the transmitting tunable filter  34  matches the passband of the receiving tunable filter  33 , thus forming a notch. This results in a satisfactory isolation characteristic in a direction from the transmitting terminal  2 LT to the receiving terminal  2 LR. 
         [0060]      FIG. 13  shows a frequency characteristic of the tunable duplexer of the fifth embodiment tuned to the lowest frequency band in Band 17 , with variable capacitors  10 LR,  12 LR at 3.02 pF, the variable capacitor  11 LR at 5.80 pF, the variable capacitors  10 LT,  12 LT at 2.30 pF and the variable capacitor  11 LT at 4.55 pF. In  FIG. 13 , a thick line represents a pass-through characteristic in a direction from the antenna  22 L to the receiving terminal  2 LR, a thin line represents a pass-through characteristic in a direction from the transmitting terminal  2 LT to the antenna  22 L and a medium thin line an isolation characteristic in a direction from the transmitting terminal  2 LT to the receiving terminal  2 LR. As shown in  FIG. 13 , the resonant frequency of the second pair of the variable capacitor  11 LR and the fixed coil  8 LR in the receiving tunable filter  33  matches the passband of the transmitting tunable filter  34 , thus forming a notch. Further, the resonant frequency of the second pair of the variable capacitor  11 LT and the fixed coil  8 LT in the transmitting tunable filter  34  matches the passband of the receiving tunable filter  33 , thus forming a notch. This results in a satisfactory isolation characteristic in a direction from the transmitting terminal  2 LT to the receiving terminal  2 LR. 
       Embodiment 6 
       [0061]      FIG. 14  shows a circuitry of a tunable duplexer module of the sixth embodiment. In this embodiment, the high-band tunable duplexer  25  uses the tunable duplexer of the fourth embodiment and the low-band tunable duplexer  26  uses the tunable duplexer of the fifth embodiment. An antenna side terminal  22 HP of the high-band tunable duplexer  25  and an antenna side terminal  22 LP of the low-band tunable duplexer  26  are connected through a SPDT switch  24  to an antenna  23 . That is, the SPDT switch  24  selects between the high-band tunable duplexer  25  and the low-band tunable duplexer  26  for connection to the antenna  23 . The SPDT switch is formed of GaAs (gallium arsenide) material. The tunable duplexer module of this configuration can be used as a mobile communication module that covers almost all bands of the communication systems such as WCDMA and LTE. 
         [0062]    While, in all of the foregoing embodiments, solenoid coils are used as stationary coils, if their Q value is about 60 or higher at the operation frequency, other means may be used, such as IPD (Integrated Passive Device) coils in which solenoid coils are formed on a silicon substrate, or chip-laminated coils. Further, although in this embodiment chip capacitors are used as the fixed capacitors, it is also possible to use other means, such as IPD capacitors and MEMS capacitors, or coils formed as inner layer patterns in laminated substrates. Furthermore, although this embodiment uses MEMS variable capacitors as the variable capacitors, other means such as varicap may also be used. The constants shown in this embodiment are just one example and it is noted that desired tunable filters can be formed by using other constants than those described above, as needed, to be able to deal with other bands than Band 1  and Band  11 . Further, while bands used for WCDMA and LTE have been taken for example, adjusting the applied frequencies by changing the constants appropriately can make the device applicable to 4G (fourth generation mobile communication system). 
       Embodiment 7 
       [0063]      FIG. 15  is a block diagram of the seventh embodiment showing a tunable filter module and a tunable duplexer module of this invention applied to a multiband-enabled mobile communication terminal. As shown in this block diagram, the mobile terminal has a tunable duplexer module and a tunable filter module, the tunable duplexer module comprising the high-band tunable duplexer  25 , the low-band tunable duplexer  26 , the SPDT switch  24  and the antenna  23 , the tunable filter module comprising the high-band tunable filter  27 , the low-band tunable filter  28 , the SPDT switch  20  and the antenna  21 . 
         [0064]    Main communication is done by transferring signals through the tunable duplexer module and, for improved reception quality, uses tunable filter module as the diversity receiver circuit. The high-band receiving terminal  2 HR and transmitting terminal  2 HT are connected to a high-band jamming wave and distortion canceler block  35  and the low-band receiving terminal  2 LR and transmitting terminal  2 LT are connected to a low-band jamming wave and distortion canceler block  36 . The high-band and low-band received signals and transmitting signals are each connected through LNA and PA to RF-IC and BB (Base Band) blocks that perform subsequent steps of signal processing. The high-band receiving terminal  2 H of the tunable filter module is connected to a high-band jamming wave and distortion canceler block  37  and the low-band receiving terminal  2 L is connected to a low-band jamming wave and distortion canceler block  38 . The high-band and low-band received signals in the tunable filter module are connected through LNA to RF-IC and BB blocks that perform subsequent steps of signal processing. 
         [0065]    The mobile communication terminal of this configuration can handle multiple bands, such as shown in “Example of frequency bands available to tunable duplexer” of  FIG. 15B , without having to use a large number of duplexers and diversity filters, and can be made small in size and simplified. While the configuration of this embodiment has been described to use the tunable filter module for a diversity reception circuit to obtain a very high reception sensitivity, it is also possible to provide a simplified configuration that uses only a tunable duplexer module that performs main signal processing.