Abstract:
A tunable filter has a plurality of variable capacitors and a plurality of inductor elements, each being formed on a common substrate, a filter circuit formed by using at least a portion of the plurality of variable capacitors and a portion of the plurality of inductor elements, a monitor circuit formed by using at least a portion of the plurality of variable capacitors and a portion of the plurality of inductor elements, a detecting circuit which detects a prescribed circuit constant of the monitor circuit, a storage which stores information relating to a reference circuit constant of the monitor circuit, and a capacitance control circuit which controls capacitance of the variable capacitors in the monitor circuit and capacitance of the variable capacitors in the filter circuit, based on a result detected by the detecting circuit and information stored in the storage.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of priority under 35USC§119 to Japanese Patent Application No. 2004-23012, filed on Jan. 30, 2004, the entire contents of which are incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a tunable filter having a variable capacitor of which capacitance is variably controlled by a thin-film piezoelectric actuator and an inductor element. 
   2. Related Art 
   In the radio communication field, there is a strong demand for realizing a tunable filter that can continuously and freely change a pass band and a block band. Characteristics required for the tunable filter are, for example, a large band-change width, a continuous band change, small insertion loss and precipitous shielding characteristic as a filter (i.e., high Q), compactness and lightness, and high reliability and excellent reproducibility. However, filters having only remarkably limited performance can be achieved at present. 
   A filter is a circuit basically having a combination of an inductor element and a capacitor. A tunable filter can be realized when either inductance of the inductor element or capacitance of the capacitor can be changed. 
   Inductance of an inductor element is determined by a length and shape of a transmission path, and permeability in space. A variable inductor which changes permeability by placing and taking out a ferromagnetic core in the center of the coil using an inductor element having low Q is in practical use. However, an attempt to obtain large variable inductance is not known, using an inductor element of high Q used in a high-frequency band of a few hundred MHz or above for portable telephones or the like. 
   On the other hand, capacitance of a capacitor is determined by area and an interval of a pair of opposite electrodes, and permeability in space. A ferroelectric substance such as barium titanate and lead zirconate titanate has a characteristic that its permittivity changes by a few times at a maximum, when a direct current bias is applied to this ferroelectric substance. Therefore, a variable capacitor can be formed using these ferroelectric substances. However, a ferroelectric substance generally has large dielectric loss and has low Q. 
   A variable capacitor can be configured when a distance between electrodes is variable, and therefore, an electromagnetic driving mechanism such as a motor can be utilized. However, this has a slow response and has a large size. Therefore, the electromagnetic driving mechanism is not suitable for a mobile radio terminal such as a cellular phone. 
   A variable capacitor using an electrostatic driving type MEMS (Micro-Electro-Mechanical System) recently calls attention. For example, an experiment example of a tunable filter that has a troidal coil and many electrostatic driving type variable capacitors connected together is introduced (see 2003 IEEE Microwave Theory and Technique Symposium Digest p. 1781). 
   However, the electrostatic driving type variable capacitor has a phenomenon called pull-in in operation. While capacitance changes in the order of about two digits in on-off operations, capacitance disadvantageously changes continuously within a range of 1.5 times. Therefore, when the electrostatic driving type variable capacitor is applied to the tunable filter, it is difficult to realize the most demanded characteristic of continuously changing a band on a large scale, although a band can be digitally switched on a large scale. 
   SUMMARY OF THE INVENTION 
   The present invention provides a tunable filter which can change a pass band at wide frequency range, can continuously change the pass band, has small insertion loss, has precipitous shielding characteristic, can downsize, and has excellent reliability and reproducibility. 
   According to one embodiment of the present invention, a tunable filter, comprising: 
   a plurality of variable capacitors and a plurality of inductor elements, each being formed on a common substrate; 
   a filter circuit formed by using at least a portion of said plurality of variable capacitors and a portion of said plurality of inductor elements; 
   a monitor circuit formed by using at least a portion of said plurality of variable capacitors and a portion of said plurality of inductor elements; 
   a detecting circuit which detects a prescribed circuit constant of said monitor circuit; 
   a storage which stores information relating to a reference circuit constant of said monitor circuit; and 
   a capacitance control circuit which controls capacitance of said variable capacitors in said monitor circuit and capacitance of said variable capacitors in said filter circuit, based on a result detected by said detecting circuit and information stored in said storage. 
   According to one embodiment of the present invention, a portable telephone, comprising: 
   an antenna which sends and receives a wireless signal modulated by phase; 
   a receiver which receives a reception signal received by said antenna; and 
   a transmitter which sends a transmission signal sent by said antenna, 
   wherein said receiver includes: 
   a high frequency amplifier which amplifies the reception signal modulated by phase; and 
   a tunable filter which is provided at former stage or subsequent stage of said high frequency amplifier and extracts the reception signal in a prescribed frequency component, 
   said tunable filter having: 
   a plurality of variable capacitors and a plurality of inductor elements, each being formed on the same substrate; 
   a filter circuit formed by using at least a portion of said plurality of variable capacitors and a portion of said plurality of inductor elements; 
   a monitor circuit formed by using at least a portion of said plurality of variable capacitors and a portion of said plurality of inductor elements; 
   a detecting circuit which detects a prescribed circuit constant of said monitor circuit; 
   a storage which stores information relating to a reference circuit constant of said monitor circuit; and 
   a capacitance control circuit which controls capacitance of said variable capacitors in said monitor circuit and capacitance of said variable capacitors in said filter circuit, based on a result detected by said detecting circuit and information stored in said storage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an equivalent circuit diagram of a tunable filter according to a first embodiment of the present invention. 
       FIG. 2  is a top plan diagram of the variable capacitors  7  and  9  that are used in the tunable filter shown in  FIG. 1 . 
       FIG. 3  is a cross-sectional diagram of the variable capacitors cut along a line A–A′ in  FIG. 2 . 
       FIG. 4  is a diagram showing a relation between driving voltages applied to the thin-film piezoelectric actuators  35  and  36  and capacitances of the variable capacitors  7  and  9 . 
       FIG. 5  is a cross-sectional configuration diagram of the film bulk acoustic resonator  8 . 
       FIG. 6  is a diagram showing impedance characteristic of the film bulk acoustic resonator  8 . 
       FIG. 7  is a diagram showing a phase characteristic of the film bulk acoustic resonator  8 . 
       FIG. 8  is a diagram for explaining the principle of the operation of the tunable filter shown in  FIG. 1 . 
       FIG. 9  is a diagram showing passage characteristics of the tunable filter shown in  FIG. 1 . 
       FIG. 10  is a circuit diagram of the filter main body  11  according to the second embodiment. 
       FIG. 11  is a diagram showing passage characteristics of the tunable filter using the filter main body  11  shown in  FIG. 10 . 
       FIG. 12  is a circuit diagram of the filter main body  11  according to the third embodiment. 
       FIG. 13  is a diagram showing passage characteristics of the tunable filter using the filter main body  11  shown in  FIG. 12 . 
       FIG. 14  is an equivalent circuit diagram of a tunable filter according to the fourth embodiment of the present invention. 
       FIGS. 15A and 15B  are diagrams for explaining the principle of the operation of the tunable filter shown in  FIG. 14 . 
       FIG. 16  is an equivalent circuit diagram of a tunable filter according to the fifth embodiment of the present invention. 
       FIGS. 17A and 17B  are diagrams for explaining the principle of the operation of the tunable filter shown in  FIG. 16 . 
       FIG. 18  is an equivalent circuit diagram of a tunable filter according to the sixth embodiment of the present invention. 
       FIG. 19  is an equivalent circuit diagram of a tunable filter according to the seventh embodiment of the present invention. 
       FIG. 20  is an equivalent circuit diagram of a tunable filter according to the eighth embodiment of the present invention. 
       FIG. 21  is a diagram showing an example of a state that one actuator  121  is used to drive variable capacitor within plural resonators. 
       FIG. 22  is a top plan diagram showing one example of a surface acoustic wave element. 
       FIG. 23  is a cross-sectional diagram of the surface acoustic wave element shown in  FIG. 22  cut along a line A—A. 
       FIG. 24  is a diagram showing an example of a resonator that is configured by a variable capacitor and a film bulk acoustic resonator that are connected in series, and a variable capacitor that is connected in parallel with them. 
       FIG. 25  is a block diagram showing one example of a schematic configuration of a portable telephone that incorporates the tunable filter according to the above embodiments. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereafter, an embodiment of the present invention will be described more specifically with reference to the drawings. 
   (First Embodiment) 
     FIG. 1  is an equivalent circuit diagram of a tunable filter according to a first embodiment of the present invention. The tunable filter shown in  FIG. 1  includes a filter main body  11 , and a control circuit  12  that controls the filter main body  11 . 
   The filter main body  11  is a ladder filter including a series resonance unit  3  having two resonance units  1  and  2  connected in series, and parallel resonance units  4  and  5  that are connected to between one end of the resonance units  1  and  2 , respectively and an input/output common terminal. Each of the resonance units  1 ,  2 ,  4 , and  5  has a variable capacitor  7  and a thin-film piezoelectric resonator, i.e., a film bulk acoustic resonator (FBAR)  8  that are connected in parallel, and a variable capacitor  9  that is connected in series with them. An upper electrode of the film bulk acoustic resonator  8  within the series resonance unit  3  and an upper electrode of the film bulk acoustic resonator  8  within the parallel resonance unit  6  have mutually different thicknesses. Based on this, a resonance frequency of the series resonance unit  3  and a resonance frequency of the parallel resonance unit  6  are slightly different from each other. Configurations of the variable capacitors  7  and  9 , and the film bulk acoustic resonator  8  are described later. 
   The control circuit  12  includes a first voltage controlled oscillator (VCO 1 )  13  that oscillates in a first oscillation frequency, a second voltage controlled oscillator (VCO 2 )  14  that oscillates in a second oscillation frequency, a temperature compensated crystal oscillator (TCXO)  15  that generates a reference frequency signal, a PLL (phase-locked loop) circuit (PLL 1 )  16  that controls the oscillation frequency of the first voltage controlled oscillator  13 , a voltage applying circuit  17  that controls capacitance of a part of the variable capacitors within the tunable filter, a PLL circuit (PLL 2 )  18  that controls the oscillation frequency of the second voltage controlled oscillator  14 , a voltage applying circuit  19  that controls capacitance of other part of the variable capacitors within the tunable filter, a base band circuit  20 , and a storage circuit  21  that stores reference frequencies of the first and the second voltage controlled oscillators  13  and  14 . The first voltage controlled oscillator  13  and the second voltage controlled oscillator  14  constitute a monitoring circuit. 
     FIG. 2  is a top plan diagram of the variable capacitors  7  and  9  that are used in the tunable filter shown in  FIG. 1 .  FIG. 3  is a cross-sectional diagram of the variable capacitors cut along a line A–A′ in  FIG. 2 . As shown in these diagrams, each of the variable capacitors  7  and  9  have fixed electrode  32  formed on a silicon substrate  31 , a dielectric film  33  formed on the upper surface of the fixed electrode  32 , and a variable electrode  34  disposed opposed above the dielectric film  33 . 
   Bimorph type thin-film piezoelectric actuators  35  and  36  are formed at the left and right sides of the variable electrode  34 . Each of the thin-film piezoelectric actuators  35  and  36  has a first electrode  38  formed above the silicon substrate  31  via an anchor  37 , a piezoelectric film  39  formed on the upper surface of the first electrode  38 , a second electrode  40  formed on the piezoelectric film  39 , and a support beam  41  formed on the upper surface of the second electrode  40 . 
   When a voltage is applied to between the first electrode  38  and the second electrode  40 , bimorph operation occurs to displace the actuators  35  and  36 . Maximum capacitance is obtained when the variable electrode  34  and the dielectric film  33  are brought into contact with each other. Minimum capacitance is obtained when the variable electrode  34  is furthest from the dielectric film  33 . The dielectric film  33  formed on the upper surface of the fixed electrode  32  prevents occurrence of short-circuit between the fixed electrode  32  and the variable electrode  34 . 
     FIG. 4  is a diagram showing a relation between driving voltages applied to the thin-film piezoelectric actuators  35  and  36  and capacitances of the variable capacitors  7  and  9 . A distance between electrodes changes in proportion to an application voltage. Capacitance changes in inverse proportion to a distance between electrodes. Capacitance can change continuously in the order of about two digits. When electrodes have a large film thickness to have a low direct-current resistance, Q becomes very large too. 
   The first electrode  38  and the second electrode  40  of the thin-film piezoelectric actuators  35  and  36 , and the variable electrode  34  and the fixed electrode  32  of the variable capacitors  7  and  9  can have a thickness within a range of 10 nm to 1 μm, by taking a resistance into account, respectively. According to the present embodiment, these electrodes are assumed to have a thickness of 50 nm, respectively. The piezoelectric film  39  can have a thickness within a range of 10 nm to 1 μm, by taking displacement into account. According to the present embodiment, the piezoelectric film  39  is assumed to have a thickness of 500 nm. The dielectric film  33  is assumed to have a thickness of 50 nm, and equivalent area of the variable capacitors  7  and  9  is assumed to be 6400 μm. 
   Capacitances of the variable capacitors  7  and  9  are measured by changing control voltages Vtune applied to the thin-film piezoelectric actuators  35  and  36  within a range of 0 to 3 volts. As a result, minimum capacitance is 0.34 pF and maximum capacitance is 2.86 pF, which shows a large change of 8.4 times. 
     FIG. 5  is a cross-sectional configuration diagram of the film bulk acoustic resonator  8 . The film bulk acoustic resonator  8  shown in  FIG. 5  includes a lower electrode  53  formed on a silicon substrate  51  via an anchor  52 , a piezoelectric unit  54  that covers the surrounding of the lower electrode  53 , and an upper electrode  55  formed on the upper surface of the piezoelectric unit  54 . An aluminum nitride film that grows in orientation to a direction of axis c is used for the piezoelectric unit  54 . Aluminum is used for the upper electrode  55  and the lower electrode  53 , respectively. A resonator  56  including the lower electrode  53 , the piezoelectric unit  54 , and the upper electrode  55  is fixed to the substrate via the anchor  52 . 
   When an alternate current is applied to between the upper electrode  55  and the lower electrode  53 , an alternate stress occurs due to a piezoelectric adverse effect, thereby exciting a resonance of an elastic wave in a thickness vertical mode. A film thickness of the piezoelectric unit  54  substantially corresponds to a half wave length of the resonance frequency. 
     FIG. 6  is a diagram showing impedance characteristic of the film bulk acoustic resonator  8 .  FIG. 7  is a diagram showing a phase characteristic of the film bulk acoustic resonator  8 . Impedance becomes minimum at a resonance point Fr, and impedance becomes maximum at an antiresonance point Fa. The inductor can have very high Q between Fr and Fa. 
   When an oriented thin film of aluminum nitride or zinc oxide is used for the piezoelectric unit  54 , a distance between Fr and Fa can be taken by 5 to 6 percent cent. Therefore, a filter having a relatively wide band can be configured. 
   As is clear from a comparison between  FIG. 3  and  FIG. 5 , the variable capacitors  7  and  9  and the film bulk acoustic resonator  8  that are driven with the thin-film piezoelectric actuators  35  and  36  have very similar configurations. Therefore, these units can be manufactured in a common manufacturing process. When they are hollow sealed, a larger advantage can be obtained. Particularly, when plural elements are prepared on the same substrate, a variance between the elements can be reduced, which contributes to improve performance of the filter. 
   According to the present embodiment, in order to obtain 2 GHz of resonance frequency, the piezoelectric unit  54  has a film thickness of 1100 nm, the lower electrode  53  has a film thickness of 100 nm, and the upper electrode  55  has a film thickness of 150 nm. 
   The first voltage controlled oscillator  13  shown in  FIG. 1  has a tank circuit  61  and an amplifier  62  connected in parallel. The second voltage controlled oscillator  14  has a tank circuit  63  and an amplifier  64  that are connected in parallel. The tank circuit  61  has a film bulk acoustic resonance unit  65  and a variable capacitor  66  that are connected in parallel. The tank circuit  63  also has a film bulk acoustic resonance unit  67  and a variable capacitor  68  that are connected in series. The film bulk acoustic resonators  65  and  67  within the tank circuits  61  and  63  have configurations similar to those shown in  FIG. 5 . The variable capacitors  66  and  68  have configurations similar to those shown in  FIG. 3 . 
   The voltage applying circuit  17  controls capacitance of the variable capacitor  66  within the first voltage controlled oscillator  13 , and controls capacitance of the variable capacitor  9  within the series resonance unit  3  and capacitance of the variable capacitor  9  within the parallel resonance unit  6 . 
   The voltage applying circuit  19  controls capacitance of the variable capacitor  68  within the second voltage controlled oscillator  14 , and controls capacitance of the variable capacitor  7  within the series resonance unit  3  and capacitance of the variable capacitor  7  within the parallel resonance unit  6 . 
     FIG. 8  is a diagram for explaining the principle of the operation of the tunable filter shown in  FIG. 1 . The oscillation frequency of the first voltage controlled oscillator  13  is determined by a control voltage V 1  that is output from the voltage applying circuit  17 . The oscillation frequency of the second voltage controlled oscillator  14  is determined by a control voltage V 2  that is output from the voltage applying circuit  19 . 
   The storage circuit  21  stores information concerning the oscillation frequencies of the first voltage controlled oscillator  13  and the second voltage controlled oscillator  14  so that band passage characteristics that are optimum for selecting a channel individual to the communication system are obtained at the time of manufacturing the tunable filter. The base band circuit  20  reads this information, and controls the PLL circuits  16  and  18 , thereby accurately controlling the oscillation frequencies of the first voltage controlled oscillator  13  and the second voltage controlled oscillator  14 . 
   The center frequency and the bandwidth in the passage characteristics of the ladder filter (i.e., filter main body)  11  are determined by the control voltages V 1  and V 2  that are output from the voltage applying circuits  17  and  19 , respectively. More specifically, as shown in  FIG. 8 , the center frequency of the filter is determined by the control voltage V 2  that is output from the voltage applying circuit  19 , and the bandwidth of the filter is determined by the control voltage V 1  output from the voltage applying circuit  17 . 
     FIG. 9  is a diagram showing passage characteristics of the tunable filter shown in  FIG. 1 . As shown in  FIG. 9 , when the voltages applied by the voltage applying circuits  17  and  19  are changed within the range of 0 to 3 volts, the center frequency changes within a range of 2.95 MHz to 3.08 MHz, thereby obtaining a large range of a frequency change of 43 percent. At the same time, very precipitous shielding characteristic can be obtained. 
   As explained above, according to the first embodiment, a feedback control, in which capacitances of the variable capacitors  7  and  9  within the filter main body  11  are controlled in accordance with the oscillation frequencies within the first and the second voltage controlled oscillators  13  and  14  as a monitoring circuit, is performed continuously during communication. With this arrangement, stable filter characteristics can be obtained without being affected by frequency drift due to rise in the temperature of the device. 
   While the monitoring circuit including the first and the second voltage controlled oscillators  13  and  14  is used in  FIG. 1 , the configuration of the monitoring circuit is not particularly limited. By using this type of monitoring circuit, capacitances of variable capacitors during operation are accurately measured. Further, a resonance frequency of a resonance circuit combined with an inductor element is accurately monitored. Capacitances are calculated based on a result of monitoring the resonance frequency, and are fed back to the voltage applying circuits  17  and  19  that drive the variable capacitors. As a result, characteristics of the filtering circuit consisting of the variable capacitors  7  and  9  and the film bulk acoustic resonator  8  can be controlled accurately. 
   (Second Embodiment) 
   A tunable filter according to a second embodiment is the same as that according to the first embodiment, except the circuit configuration of the filter main body  11  is different. Therefore, the difference is mainly explained hereinafter. 
     FIG. 10  is a circuit diagram of the filter main body  11  according to the second embodiment. The filter main body  11  shown in  FIG. 10  includes two capacitors  71  and  72  that are connected in series, and a parallel resonance unit  75  having two resonance units  73  and  74  connected to between one end of the capacitors  71  and  72 , respectively and an input/output common terminal. Each of the resonance units  73  and  74  has the film bulk acoustic resonator  8  and the variable capacitor  7  that are connected in parallel, and the variable capacitor  9  that is connected in series with them, like the resonator shown in  FIG. 1 . The number of resonators that constitute the parallel resonance unit  6  is not particularly limited to two. 
     FIG. 11  is a diagram showing passage characteristics of the tunable filter using the filter main body  11  shown in  FIG. 10 .  FIG. 11  shows a change in the passage characteristics when the application voltages output from the voltage applying circuits  17  and  19  shown in  FIG. 1  are changed. 
   As is clear from a comparison between  FIG. 1  and  FIG. 10 , number of elements of the filter main body  11  shown in  FIG. 10  is smaller than that of the filter main body shown in  FIG. 1 . Therefore, the area in which the elements are formed can be reduced, and the passage bandwidth becomes half of that in  FIG. 9 . Further, when the capacitances of the variable capacitors  7  and  9  are changed, a total change in the impedance of the filter is small. On the other hand, attenuation characteristic becomes milder than that in  FIG. 9 . Shielding characteristics in areas other than the passage band are different between at the low-frequency side and at the high-frequency side. 
   As explained above, according to the second embodiment, the filter main body  11  can be made smaller. 
   (Third Embodiment) 
   A tunable filter according to a third embodiment is the same as that according to the first embodiment, except the circuit configuration of the filter main body  11  is different. Therefore, the difference is mainly explained. 
     FIG. 12  is a circuit diagram of the filter main body  11  according to the third embodiment. The filter main body  11  shown in  FIG. 12  has a lattice filter configured by four resonators  76  connected in a bridge. Each resonance unit  76  has the film bulk acoustic resonator  8  and the variable capacitor  7  that are connected in parallel, and the variable capacitor  9  that is connected in series with them, like the resonator shown in  FIG. 1 . 
   Among the four resonators  76  shown in  FIG. 12 , the film bulk acoustic resonators  8  included in the two resonators on one diagonal line and the film bulk acoustic resonators  8  included in the two resonators on the other diagonal line have mutually different thicknesses in their upper electrodes  55 . Therefore, resonance frequencies of the resonators on one diagonal line and resonance frequencies of the resonators on the other diagonal line are different from each other by a predetermined level. 
     FIG. 13  is a diagram showing passage characteristics of the tunable filter using the filter main body  11  shown in  FIG. 12 . This diagram shows passage characteristics when the voltage applying circuit controls capacitances of the variable capacitors  7  and  9  within a range of control voltage 0 to 3 volts. The center frequency changes within a range of 2.98 MHz to 3.12 MHz, thereby obtaining a large range of a frequency change of 5.2 percent. At the same time, very large out-of-band attenuation characteristics are obtained. 
   As explained above, when a lattice filter is configured by plural resonators, a large variable-frequency range can be obtained, in a similar manner to that according to the first embodiment. 
   (Fourth Embodiment) 
   According to a fourth embodiment, a circuit configuration of the control circuit  12  is different from that according to the first embodiment. 
     FIG. 14  is an equivalent circuit diagram of a tunable filter according to the fourth embodiment of the present invention. The tunable filter shown in  FIG. 14  has the control circuit  12  having a circuit configuration different from that shown in  FIG. 1 . The control circuit  12  shown in  FIG. 14  has a monitoring circuit  81  that constitutes a voltage controlled oscillator, the temperature compensated crystal oscillator  15 , the voltage applying circuits  17  and  19 , the base band circuit  20 , the storage circuit  21 , and an operating circuit  82 . 
   The monitoring circuit  81  has the amplifier  62  and a resonance unit  83  that are connected in parallel. The resonance unit  83  has the film bulk acoustic resonator  8  and the variable capacitor  7  that are connected in parallel, and the variable capacitor  9  that is connected in series with them, like the resonator shown in  FIG. 1 . 
   The voltage applying circuit  17  controls capacitance of the variable capacitor  9  within the monitoring circuit  81 , capacitance of the variable capacitor  9  within the series resonance unit  3  of the filter main body  11 , and capacitance of the variable capacitor  9  within the parallel resonance unit  6 . The voltage applying circuit  19  controls capacitance of the variable capacitor  7  within the monitoring circuit  81 , capacitance of the variable capacitor  7  within the series resonance unit  3  of the filter main body  11 , and capacitance of the variable capacitor  7  within the parallel resonance unit  6 . 
     FIGS. 15A and 15B  are diagrams for explaining the principle of the operation of the tunable filter shown in  FIG. 14 . As shown in  FIG. 15A , when the voltage applying circuit  17  controls the control voltage V 1  supplied to the monitoring circuit  81 , the oscillation frequency of the monitoring circuit  81  changes within a range of Fr to Fa. As shown in  FIG. 15B , a center frequency is determined by the control voltage V 2  output from the voltage applying circuit  19 , and a passage bandwidth is determined by the control voltage V 1  output from the voltage applying circuit  17 . 
   The storage circuit  21  stores in advance at a manufacturing time, the oscillation frequency of the monitoring circuit  81  corresponding to the band passage characteristics optimum for selecting a channel individual to a communication system. With this arrangement, the operating circuit  82  can accurately control the oscillation frequency of the monitoring circuit  81  corresponding to the passage characteristics desirable during communications. This feedback control of the oscillation frequency is carried out continuously during communications. 
   As explained above, according to the fourth embodiment, stable filter characteristics can be obtained without being affected by frequency drift due to rise in the temperature of the device, in a similar manner to that according to the first embodiment. 
   (Fifth Embodiment) 
   According to a fifth embodiment, a circuit configuration of the monitoring circuit is different from that according to the fourth embodiment. 
     FIG. 16  is an equivalent circuit diagram of a tunable filter according to the fifth embodiment of the present invention. The tunable filter shown in  FIG. 16  has a monitoring circuit  91  having a circuit configuration different from that of the monitoring circuit  81  shown in  FIG. 14 . The tunable filter shown in  FIG. 16  is input with an oscillation signal of a predetermined frequency from a voltage controlled oscillator  92  provided at the outside. 
   The monitoring circuit  91  shown in  FIG. 16  includes resonators similar to those shown in  FIG. 1 . Each resonator has the film bulk acoustic resonator  8  and the variable capacitor  7  that are connected in parallel, and the variable capacitor  9  that is connected in series with the film bulk acoustic resonator  8  and the variable capacitor  7 . 
   The voltage applying circuit  17  controls capacitance of the variable capacitor  9  within the monitoring circuit  91 , capacitance of the variable capacitor  9  within the series resonance unit  3  of the filter main body  11 , and capacitance of the variable capacitor  9  within the parallel resonance unit  6 . The voltage applying circuit  19  controls capacitance of the variable capacitor  7  within the monitoring circuit  91 , capacitance of the variable capacitor  7  within the series resonance unit  3  of the filter main body  11 , and capacitance of the variable capacitor  7  within the parallel resonance unit  6 . 
     FIGS. 17A and 17B  are diagrams for explaining the principle of the operation of the tunable filter shown in  FIG. 16 . As shown in the diagram, capacitances of the variable capacitors  7  and  9  within the monitoring circuit  91  are controlled based on the control voltage V 1  output from the voltage applying circuit  17  and the control voltage V 2  output from the voltage applying circuit  19 , respectively. As a result, oscillation frequencies change, and the passage bandwidth and the center frequency of the filter main body  11  are controlled. Mainly, as shown in  FIG. 17B , the center frequency of the filter main body  11  is controlled based on the control voltage V 1 , and the bandwidth of the filter main body  11  is controlled based on the control voltage V 2 . 
   The voltage applying circuits intermittently control the control voltages V 1  and V 2  during communications. 
   As explained above, according to the fifth embodiment, stable filter characteristics can be obtained without being affected by frequency drift due to rise in the temperature of the device, in a similar manner to that according to the first embodiment. 
   (Sixth Embodiment) 
   According to a sixth embodiment, the variable capacitors  7  and  9  and the film bulk acoustic resonator  8  of the filter main body  11  are used as a part of the control circuit  12 . 
     FIG. 18  is an equivalent circuit diagram of a tunable filter according to the sixth embodiment of the present invention. The tunable filter shown in  FIG. 18  has the control circuit  12  of which circuit configuration is different from that shown in  FIG. 1 . 
   The control circuit  12  shown in  FIG. 18  has the temperature compensated crystal oscillator  15 , a voltage applying circuit  101 , the base band circuit  20 , the storage circuit  21 , the operating circuit  82 , switching circuits  102 ,  103 , and  104 , a detecting circuit  106  that detects the amplitude of a signal output from the filtering circuit  11 , and a temperature detector  107  that detects the ambient temperature. The filter main body  11  has a circuit similar to that shown in  FIG. 1 . At the time of adjusting filter characteristics, the outside voltage controlled oscillator  92  inputs an oscillation signal having a predetermined frequency to the filter main body  11  via the switching circuit  104 . 
   The switching circuit  103  uses the variable capacitors  7  and  9  of any one of the resonators of the filter main body  11 , as a part of the monitoring circuit  81 , and is used to control capacitances of these variable capacitors  7  and  9 . The variable capacitors  7  and  9  that are not selected by the switching circuit  102  hold charges held when these variable capacitors are connected to the switching circuit  102  before. 
   The switching circuit  103  is switched at the time of monitoring the variable capacitors  7  and  9  of any one of the resonators of the filter main body  11 . 
   According to the present embodiment, the output from the voltage controlled oscillator is intermittently sweep input to the filter main body  11  via the switching circuit  104  when the power is turned on or during communications. The detecting circuit  106  detects the amplitude of the output signal from the filter main body  11 . The operating circuit  82  controls capacitances of the variable capacitors  7  and  9  based on a result of detecting the amplitude by the detecting circuit  106  and a result of detecting the temperature by the temperature detector  107 . More specifically, the operating circuit  82  controls capacitances of the variable capacitors  7  and  9  so that the amplitude of the output signal from the filter main body  11  becomes maximum. With this arrangement, stable filter characteristics can be obtained without being affected by frequency drift due to rise in the temperature of the device. 
   As explained above, according to the sixth embodiment, the filter main body  11  can be used as a monitoring circuit by switching the switching circuits  102  to  104 . As a result, an exclusive monitoring circuit is not necessary, thereby simplifying a circuit configuration. 
   (Seventh Embodiment) 
   According to a seventh embodiment, a circuit configuration of a monitoring circuit is different from those according to the preceding embodiments. 
     FIG. 19  is an equivalent circuit diagram of a tunable filter according to the seventh embodiment of the present invention. The tunable filter shown in  FIG. 19  includes the voltage applying circuits  17  and  19 , the base band circuit  20 , the storage circuit  21 , monitoring circuits  111  and  112 , and a temperature detecting circuit  113 . 
   Each of the monitoring circuits  111  and  112  has a variable capacitor  114  and a capacitance detecting circuit  115  that are connected in parallel. The capacitance detecting circuit  115  measures capacitances of the variable capacitors  7  and  9  that are connected in parallel, and transmits a result of the measuring to the operating circuit  82 . 
   The voltage applying circuit  17  controls capacitance of the variable capacitor  114  within the monitoring circuit  111 , capacitance of the variable capacitor  9  within the series resonance unit  3  of the filter main body  11 , and capacitance of the variable capacitor  9  within the parallel resonance unit  6 . The voltage applying circuit  19  controls capacitance of the variable capacitor  114  within the monitoring circuit  112 , capacitance of the variable capacitor  7  within the series resonance unit  3  of the filter main body  11 , and capacitance of the variable capacitor  7  within the parallel resonance unit  6 . 
   The operation of the principle of the tunable filter shown in  FIG. 19  is explained hereinafter. A resonance frequency Fr′ and an antiresonance frequency Fa′ of the resonator in the filter main body  11  can be calculated based on the following expressions (1) and (2), using the resonance frequency Fr and the antiresonance frequency Fa of the film bulk acoustic resonator  8 , the capacitance VC 1  of the variable capacitor  7  connected in parallel, and the capacitance VC 2  of the variable capacitor  9  connected in series. 
   
     
       
         
           
             
               
                 
                   f 
                   R 
                   ′ 
                 
                 = 
                 
                   
                     f 
                     R 
                   
                   ⁢ 
                   
                     
                       1 
                       + 
                       
                         
                           C 
                           1 
                         
                         
                           
                             C 
                             0 
                           
                           + 
                           
                             V 
                             C1 
                           
                           + 
                           
                             V 
                             C2 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
           
             
               
                 
                   f 
                   A 
                   ′ 
                 
                 = 
                 
                   
                     f 
                     R 
                   
                   ⁢ 
                   
                     
                       1 
                       + 
                       
                         
                           C 
                           1 
                         
                         
                           
                             C 
                             0 
                           
                           + 
                           
                             V 
                             C1 
                           
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   Capacitors C 0  and C 1  correspond to an equivalent capacitance and a parallel equivalent capacitance, respectively when the film bulk acoustic resonator  8  is expressed by a BVD model equivalent circuit. 
   Therefore, when the resonance frequency and the antiresonance frequency of each resonator in the filter main body  11 , and the capacitances of the variable capacitor  7  connected in parallel and the variable capacitor  9  connected in series within each resonator are controlled based on the measured capacitances of the variable capacitors  114  within the monitoring circuits  111  and  112 , band passage characteristics of the filtering circuit can be set to a value that is optimum for selecting a channel individual to a communication system. 
   As explained above, according to the seventh embodiment, configurations of the monitoring circuits  111  and  112  can be simplified. Using a simpler circuit than that according to the first embodiment, stable filter characteristics can be obtained without being affected by frequency drift due to rise in the temperature of the device. 
   (Eighth Embodiment) 
   An eighth embodiment is a modified example of the seventh embodiment, and differences from the seventh embodiment will be mainly described hereinafter. 
     FIG. 20  is an equvalent circuit diagram of a tunable filter according to the eighth embodiment of the present invention. The tunable filter of  FIG. 20  has voltage applying circuits  116  and  117 , and monitor circuits  118  and  119 , in addition to the constituents of  FIG. 19 . 
   Each of the monitor circuits  118  and  119  has a variable capacitor  114  and a capacitance detecting circuit  115  connected in parallel, similarly to the monitor circuit  111 . The capacitance detecting circuit  115  measures the capacitance of the variable capacitor  114  connected in parallel, and transmits the measured result to the operating circuit  82 . 
   The voltage applying circuit  17  controls capacitance of the variable capacitor  114  in the monitor circuit  111  and capacitance of the variable capacitor  9  in the series resonance unit  3  in the filter main body  11 . The voltage applying circuit  19  controls capacitance of the variable capacitor  114  in the monitor circuit  112  and capacitance of the variable capacitor  7  in the parallel resonance unit  6  in the filter main body  11 . The voltage applying circuit  116  controls capacitance of the variable capacitor  114  in the monitor circuit  118  and capacitance of the variable capacitor  9  in the series resonance unit  3  in the filter main body  11 . The voltage applying circuit  117  controls capacitance of the variable capacitor  114  in the monitor circuit  119  and capacitance of the variable capacitor  9  in the parallel resonance unit  6  in the filter main body  11 . 
   According to the eighth embodiment, resonance frequency and antiresonance frequency of the series resonance unit  3  in the filter main body  11 , and capacitances of the variable capacitor  7  connected in parallel and the variable capacitor  9  connected in series in the series resonance unit  3  can be controlled based on the measured capacitances of the variable capacitors  114  in the monitor circuits  111  and  112 . Resonance frequency and antiresonance frequency of the parallel resonance unit  6 , and capacitances of the variable capacitor  7  connected in parallel and the variable capacitor  9  connected in series in the parallel resonance unit  6  can be controlled based on the measured capacitances of the variable capacitors in the monitor circuits  118  and  119 . 
   Therefore, it is possible to control band-pass property of the filter circuit, especially, central pass frequency and band-pass over a range of broad frequency band, and to set the band-pass property to an optimum value for channel selection inherent to the communication system. 
   As described above, according to the eighth embodiment, it is possible to simplify the configurations of the monitor circuits  111  and  112 , and to obtain stable filter property corresponding to the central frequency and the band-pass width at a range broader than that of the first embodiment. 
   (Nineth Embodiment) 
   According to the fourth, the fifth, and the seventh embodiments, when the variable capacitors  114  within the monitoring circuits  111  and  112  and the variable capacitors  7  and  9  within the filter main body  11  apply the same voltage to the respective piezoelectric driving actuators, the same capacitance needs to be obtained. Further, the variable capacitors  7  connected in parallel or the variable capacitors  9  connected in series in the resonators within the filter main body  11  need to exhibit the same characteristics and the same responses. 
   The MEMS elements formed on the same substrate according to the semiconductor process usually obtain the same characteristics within a narrow area of at least the same wafer even when there is a large variance among lots or among wafers. Therefore, the control systems according to the fourth, the fifth, and the seventh embodiments can be employed. 
   In order to enable the resonators to have the same capacitance by receiving control voltages from the voltage applying circuits, one actuator can be shared as shown in  FIG. 21  in place of individually providing actuators to the variable capacitors. 
     FIG. 21  shows an example of a state that one actuator  121  is used to drive variable capacitor within plural resonators. With this arrangement, a variance of characteristics of individual variable capacitors can be reduced. 
   Actuators of all variable capacitors to which one voltage applying circuit supplies a control voltage can be set together into one. Actuators of all variable capacitors within the resonators  3  connected in series can be set together into one. Actuators of all variable capacitors within the resonators  6  connected in parallel can be set together into one. 
   As explained above, according to the ninth embodiment, one actuator is used to control capacitances of plural variable capacitors. Therefore, characteristics of the variable capacitors  7  and  9  can be arranged. 
   (Other Embodiments) 
   In the above embodiments, a film bulk acoustic resonator is used for the inductor element. Alternatively, a surface acoustic wave element (i.e., a SAW device) can be used. An inductor including a general waveguide and a coil can be also used. 
     FIG. 22  is a top plan diagram showing one example of a surface acoustic wave element.  FIG. 23  is a cross-sectional diagram of the surface acoustic wave element shown in  FIG. 22  cut along a line A—A. As shown in these diagrams, the surface acoustic wave element has a comb electrode  132  and an input/output electrode  133  formed on a piezoelectric monochristalline substrate  131 . 
   A monitoring circuit of the above variable capacitors can have various forms. For example, a voltage controlled oscillator using a film bulk acoustic resonator and a variable capacitor can be used, or a filter module having a film bulk acoustic resonator and a variable capacitor connected in series or in parallel can be used. Alternatively, a tunable filter itself can be used to carry out monitoring during the operation. 
   A filter main body configured by a variable capacitor and an inductor element has various types such as a ladder type and a lattice type. Many circuit systems can be also applied to the monitoring circuit. 
   In the above embodiments, a resonator uses the variable capacitor  7  and the film bulk acoustic resonator  8  that are connected in parallel, and the variable capacitor  9  that is connected in series with them. However, the circuit configuration of the resonator is not limited to this. For example,  FIG. 24  shows an example of a resonator that is configured by a variable capacitor and a film bulk acoustic resonator that are connected in series, and a variable capacitor that is connected in parallel with them. The resonator shown in  FIG. 24  can be used in the filter main body  11 , shown in  FIG. 1  etc., and in the monitoring circuit  83  shown in  FIG. 14 . 
   The tunable filters explained in the above embodiments are used in various electric appliances. Because the tunable filter according to the present invention is formed on the semiconductor substrate, the device can be made small. Therefore, the tunable filter can be applied to various portable devices such as a portable telephone. 
     FIG. 25  is a block diagram showing one example of a schematic configuration of a portable telephone that incorporates the tunable filter according to the above embodiments. This portable telephone is a direct conversion type. The portable telephone shown in  FIG. 25  includes an antenna  141 , a directional coupler  142  that switches between a transmission and a reception, a transmitter  143 , a receiver  144 , and a base band processor  145 . 
   The receiver  144  includes a tunable filter  146  explained above, a low noise amplifier (LNA)  147 , a phase demodulator  148  that demodulates the phase of an output signal from the LNA  147 , and an A/D converter  149  that A/D converts the phase-modulated signal. The transmitter  143  includes a D/A converter  151  that D/A converts a transmission signal generated by the base band processor, a low-pass filter  152  that extracts only a predetermined frequency component of a signal output from the D/A converter  151 , a phase modulator  153  that modulates the phase of an output from the low-pass filter  152 , and an amplitude modulator  154  that modulates the amplitude of a phase-modulated signal. 
   The tunable filter can be connected to a latter stage of the LNA.