JP6439328B2 - Variable resonance circuit and variable filter circuit - Google Patents

Description

  The present invention provides a variable resonance circuit in which a resonance frequency can be adjusted by controlling a variable capacitor connected to a resonator, and a variable resonance circuit in which one or more variable resonance circuits are connected between input and output terminals so that the filter characteristics can be adjusted. The present invention relates to a filter circuit.

  As a conventional variable filter circuit, a resonator in which a resonator is connected in a ladder type and a variable capacitor is connected in series and in parallel to each of the resonators is used (for example, see Patent Document 1). Each resonator has a frequency at which the impedance is minimized (hereinafter referred to as a resonance frequency or a resonance point), and a variable capacitor connected in series with the resonator controls the resonance point of the variable filter circuit. Is provided. The resonator has a frequency at which the impedance is maximized (hereinafter referred to as an anti-resonance frequency or an anti-resonance point), and the variable capacitor connected in parallel to the resonator has an anti-resonance point of the variable filter circuit. Provided to control. In the variable filter circuit, the attenuation characteristic between the pass band and the stop band is adjusted on the filter characteristic by controlling the variable capacitor connected in parallel to the resonator and the variable capacitor connected in series to the resonator. In addition, band adjustment of the pass band and stop band can be performed.

JP 2009-130831 A

  In the conventional variable filter circuit, it is necessary to connect two variable capacitors in order to adjust the resonance point and antiresonance point of each resonator, respectively, and a large number of variable capacitors are required as a whole circuit. For this reason, a complicated control system for individually controlling a large number of variable capacitors has been required. In addition, the variable capacitor has a larger element size than other circuit elements, and the conventional variable filter circuit tends to have a larger circuit size.

  Therefore, in the conventional variable filter circuit, it can be considered to omit one of the two variable capacitors connected to the resonator. However, one of the two variable capacitors corresponds to the adjustment of the resonance point, and the other corresponds to the adjustment of the anti-resonance point. For this reason, if one of the two variable capacitors is omitted, only one of the resonance point and the antiresonance point can be adjusted. For this reason, in the filter characteristics, for example, when adjusting the pass band or the stop band, the attenuation characteristics greatly change between the pass band and the stop band, and there is an adjustment range in which a desired attenuation characteristic can be obtained. It was easy to narrow. Further, for example, the adjustable frequency range of the pass band and the stop band itself tends to be narrow.

  Therefore, an object of the present invention is to provide a variable resonance circuit and a variable filter circuit that can suppress the number of variable capacitors and that can simultaneously adjust a plurality of resonance points and antiresonance points by controlling one variable capacitor. It is to provide.

  The variable resonance circuit of the present invention includes a first resonator, a second resonator connected in series to the first resonator, and a parallel connection to the first resonator. The resonance frequency of the first resonator and the resonance frequency of the second resonator are the anti-resonance frequency of the first resonator and the anti-resonance frequency of the second resonator. The anti-resonance frequency of the first resonator is between the resonance frequency and the anti-resonance frequency of the second resonator.

  In this variable resonance circuit, since the first resonator and the second resonator are connected in series, the resonance point corresponding to the resonance frequency of the first resonator or the second resonator on the impedance characteristic. And a first antiresonance point corresponding to the antiresonance frequency of the first resonator and a second antiresonance point corresponding to the antiresonance frequency of the second resonator are generated. Further, since the anti-resonance frequency of the first resonator is between the resonance frequency and the anti-resonance frequency of the second resonator, the frequency band between the first anti-resonance point and the second anti-resonance point. Then, a new resonance point (hereinafter referred to as a sub-resonance point) is generated on the impedance characteristic by combining the capacitive impedance of the first resonator and the inductive impedance of the second resonator. When the variable capacitor connected in parallel to the first resonator is controlled, the frequency of the first antiresonance point moves on the impedance characteristic, and the frequency of the sub-resonance point moves according to the first antiresonance point. Thereby, the frequency of the first antiresonance point and the frequency of the sub-resonance point can be adjusted simultaneously by controlling one variable capacitor.

  Alternatively, the variable resonance circuit of the present invention includes a first resonator, a second resonator connected in series to the first resonator, and a parallel connection to the second resonator. A variable capacitor connected thereto, wherein a resonance frequency of the first resonator and a resonance frequency of the second resonator are an anti-resonance frequency of the first resonator and an anti-resonance frequency of the second resonator. The anti-resonance frequency of the first resonator is between the resonance frequency and the anti-resonance frequency of the second resonator.

  In this variable resonance circuit, when the variable capacitor connected in parallel to the second resonator is controlled, the frequency of the second antiresonance point moves on the impedance characteristic, and the frequency of the subresonance point becomes the second antiresonance point. Move according to. As a result, the sub-resonance point and the second anti-resonance point can be adjusted simultaneously by controlling one variable capacitor.

  The variable capacitor may be connected in parallel to a series circuit of the first resonator and the second resonator. Accordingly, the first antiresonance point, the sub-resonance point, and the second antiresonance point can be adjusted simultaneously by controlling one variable capacitor.

  Alternatively, in the variable resonance circuit, a variable capacitor connected in parallel to the second resonator is defined as a second variable capacitor, and separately from the second variable capacitor, the variable resonator is connected to the first resonator. A first variable capacitor connected in parallel, and a parallel circuit of the first resonator and the first variable capacitor; a parallel circuit of the second resonator and the second variable capacitor; , May be connected in series. Thus, the first antiresonance point, the subresonance point, and the second antiresonance point can be adjusted with a higher degree of freedom.

  Each of the above-described variable resonance circuits may further include a third variable capacitor connected in series to the first resonator and the second resonator separately from the variable capacitor. By controlling such a third variable capacitor, it is possible to adjust the resonance point corresponding to the resonance frequency of the first resonator or the second resonator. The third variable capacitor is preferably connected in series to a parallel circuit of the resonator and the variable capacitor. In this way, the resonance point corresponding to the resonance frequency of the first resonator or the second resonator can be adjusted without substantially affecting the adjustment of the sub-resonance point by controlling the first variable capacitor. .

  Alternatively, the variable resonance circuit of the present invention includes a first resonator, a second resonator connected in series to the first resonator, the first resonator, and the second resonator. A variable capacitor connected in series to the resonator, wherein the resonance frequency of the first resonator and the resonance frequency of the second resonator are the anti-resonance frequency of the first resonator and the resonance frequency of the first resonator. The antiresonance frequency of the first resonator is lower than the antiresonance frequency of the second resonator, and the antiresonance frequency of the first resonator is between the resonance frequency and the antiresonance frequency of the second resonator.

  In this variable resonance circuit, when the variable capacitance connected in series to the first and second resonators is controlled, the frequency of the resonance point of the first resonator and the second resonator moves on the impedance characteristic, and the sub-resonance circuit also has a sub-frequency. The frequency of the resonance point moves according to the resonance point of the first resonator and the second resonator. Thereby, the resonance point and the sub-resonance point can be adjusted simultaneously by controlling one variable capacitor.

  Alternatively, the variable resonance circuit according to the present invention includes a first resonator, a second resonator connected in parallel to the first resonator, and the first resonator in series. An anti-resonance frequency of the first resonator and an anti-resonance frequency of the second resonator, the anti-resonance frequency of the first resonator and the anti-resonance frequency of the second resonator. The resonance frequency of the first resonator is higher than the resonance frequency, and the resonance frequency of the first resonator is between the resonance frequency and the anti-resonance frequency of the second resonator.

  In this variable resonance circuit, since the first resonator and the second resonator are connected in parallel, the first resonance point corresponding to the resonance frequency of the first resonator on the impedance characteristic, and A second resonance point corresponding to the resonance frequency of the second resonator is generated, and an antiresonance point corresponding to the antiresonance frequency of the first resonator or the second resonator is generated. In addition, since the resonance frequency of the first resonator is between the resonance frequency and the anti-resonance frequency of the second resonator, the first resonance point is in the frequency band between the second resonance point and the first resonance point. By combining the capacitive impedance of the first resonator and the inductive impedance of the second resonator, a new antiresonance point (hereinafter referred to as a subantiresonance point) is generated on the impedance characteristics. When the variable capacitor connected in series to the first resonator is controlled, the frequency of the first resonance point moves on the impedance characteristic, and the frequency of the secondary antiresonance point moves according to the first resonance point. Thus, the sub-antiresonance point and the first resonance point can be adjusted simultaneously by controlling one variable capacitor.

  Alternatively, the variable resonance circuit of the present invention includes a first resonator, a second resonator connected in parallel to the first resonator, and the second resonator in series. An anti-resonance frequency of the first resonator and an anti-resonance frequency of the second resonator, the anti-resonance frequency of the first resonator and the anti-resonance frequency of the second resonator. The resonance frequency of the first resonator is higher than the resonance frequency, and the resonance frequency of the first resonator is between the resonance frequency and the anti-resonance frequency of the second resonator.

  In this variable resonance circuit, when the variable capacitor connected in series with the second resonator is controlled, the frequency of the second resonance point moves on the impedance characteristic, and the frequency of the secondary antiresonance point follows the second resonance point. Move. Thereby, the second resonance point and the secondary antiresonance point can be adjusted simultaneously by controlling one variable capacitor.

  The variable capacitor may be connected in series to a parallel circuit of the first resonator and the second resonator. Thus, the first resonance point, the secondary antiresonance point, and the second resonance point can be adjusted simultaneously by controlling one variable capacitor.

  Alternatively, in the variable resonance circuit, a variable capacitor connected in series to the second resonator is defined as a second variable capacitor, and separately from the second variable capacitor, the variable resonator is connected to the first resonator. A first variable capacitor connected in series, and a series circuit of the first resonator and the first variable capacitor; a series circuit of the second resonator and the second variable capacitor; , May be connected in parallel. Thus, the first resonance point, the secondary antiresonance point, and the second resonance point can be adjusted with a higher degree of freedom.

  The variable filter circuit of the present invention includes an input terminal and an output terminal, and a resonance circuit connected over one or more stages between the input terminal and the output terminal, and the resonance circuit includes a variable resonance circuit. It is preferable to include.

  When the variable filter circuit is configured using the variable resonance circuit as described above, if the passband corresponding to the resonance point or antiresonance point adjusted by the control of the variable capacitance and the stopband are adjacent to each other, the passbands thereof are passed. The band and stop band can be adjusted over a wide frequency range. In this case, even if the band adjustment between the pass band and the stop band is performed, the attenuation characteristic between the pass band and the stop band is hardly changed. Further, when two resonance points and two anti-resonance points are adjusted, the steepness on the high frequency side and the steepness on the low frequency side of the passband and stopband can be adjusted.

  According to the present invention, the number of variable capacitors constituting the variable resonance circuit and the variable filter circuit can be suppressed. In addition, in the variable resonance circuit and the variable filter circuit of the present invention, a sub-resonance point and a sub-anti-resonance point are generated in addition to a resonance point and an anti-resonance point generated near the resonance frequency and anti-resonance frequency of the two resonators It is possible to adjust not only one resonance point and antiresonance point but also a subresonance point and subantiresonance point by controlling one variable capacitor.

1 is a circuit diagram of a variable filter circuit according to a first embodiment. It is the circuit diagram and impedance characteristic figure of the series circuit of the 1st resonator and 2nd resonator which concern on 1st Embodiment. It is the circuit diagram and impedance characteristic figure of the parallel circuit of the 1st resonator and variable capacitor which concern on 1st Embodiment. It is a circuit diagram and an impedance characteristic diagram of a variable resonance circuit concerning a 1st embodiment. It is a filter characteristic view of the variable filter circuit according to the first embodiment. FIG. 6 is a circuit diagram of a variable filter circuit according to a second embodiment. It is an impedance characteristic view of the variable resonance circuit according to the second embodiment. It is a filter characteristic view of the variable filter circuit according to the second embodiment. FIG. 6 is a circuit diagram of a variable filter circuit according to a third embodiment. It is an impedance characteristic view of the variable resonance circuit according to the third embodiment. It is a filter characteristic view of the variable filter circuit according to the third embodiment. It is a circuit diagram of the variable resonance circuit which concerns on a 1st modification. FIG. 10 is a circuit diagram of a variable filter circuit according to a fourth embodiment. It is an impedance characteristic view of the variable resonance circuit according to the fourth embodiment. It is a filter characteristic view of the variable filter circuit according to the fourth embodiment. It is a circuit diagram of the variable resonance circuit which concerns on a 2nd modification.

<< First Embodiment >>
FIG. 1 is a circuit diagram of a variable filter circuit 10 according to a first embodiment of the present invention.

  The variable filter circuit 10 includes a variable resonance circuit 11. The variable filter circuit 10 includes a signal line connected between the input / output terminal P1 and the input / output terminal P2, and a ground line branched from the signal line and connected to the ground connection terminal P3. The variable resonance circuit 11 is inserted in series with the ground line of the variable filter circuit 10. That is, in the variable filter circuit 10, the variable resonance circuit 11 is connected to the shunt between the input / output terminal P1 and the input / output terminal P2, and the variable resonance circuit is connected between the input / output terminal P1 and the input / output terminal P2. 11 is connected to only one stage.

  The variable filter circuit 10 may be configured by connecting the variable resonance circuit 11 to a shunt across a plurality of stages between the input / output terminal P1 and the input / output terminal P2. The variable filter circuit 10 may be configured by connecting the variable resonance circuit 11 in series between the input / output terminal P1 and the input / output terminal P2. The variable filter circuit 10 may have a ladder configuration by alternately connecting a plurality of variable resonance circuits 11 in series and parallel. The variable filter circuit 10 may be configured by adding another resonance circuit as long as at least one variable resonance circuit 11 is provided. The additional resonance circuit is a variable resonance circuit having a configuration different from that of the variable resonance circuit 11. There may be.

  The variable resonance circuit 11 includes a first resonator SAW1, a second resonator SAW2, and a first variable capacitor C1. One end of the first resonator SAW1 is connected to the signal line. One end of the second resonator SAW2 is connected to the ground connection terminal P3. The other end side of the first resonator SAW1 and the other end side of the second resonator SAW2 are connected to each other. Accordingly, the first resonator SAW1 and the second resonator SAW2 are connected in series by the ground line. The first variable capacitor C1 is connected in parallel to the first resonator SAW1, and is connected in series to the second resonator SAW2. The first resonator SAW1 and the second resonator SAW2 can be configured as surface acoustic wave resonant elements. The first resonator SAW1 and the second resonator SAW2 may be composed of other resonators such as a surface acoustic wave resonator, for example, a bulk acoustic wave resonator (BAW), an LC resonance circuit, or the like. it can.

  The variable filter circuit 10 and the variable resonance circuit 11 are configured as described above, and the number of variable capacitors is smaller than the conventional one. Therefore, the variable filter circuit 10 and the variable resonance circuit 11 can suppress the circuit size, and can simplify the system required for controlling the variable capacitance.

  Here, the impedance characteristic of the series circuit 12 of the first resonator SAW1 and the second resonator SAW2 will be described. FIG. 2A is a circuit diagram of the series circuit 12. FIG. 2B is an impedance characteristic diagram of each of the first resonator SAW1 and the second resonator SAW2. FIG. 2C is an impedance characteristic diagram of the series circuit 12.

  As shown by a solid line in FIG. 2B, the first resonator SAW1 has a resonance frequency at which the impedance is minimized and an anti-resonance frequency at which the impedance is maximized on a higher frequency side than the resonance frequency. doing. Similarly, as indicated by a broken line in FIG. 2B, the second resonator SAW2 also has a resonance frequency at which the impedance is minimized, and an anti-resonance frequency at which the impedance is maximized on the higher frequency side than the resonance frequency. have.

  Here, the resonance frequency of the first resonator SAW1 and the resonance frequency of the second resonator SAW2 are both set to be approximately the same frequency of about 755 MHz. The anti-resonance frequency of the first resonator SAW1 is set to a frequency higher than the resonance frequency (about 755 MHz) of the first resonator SAW1 and the second resonator SAW2 so as to be about 785 MHz. Yes. The anti-resonance frequency of the second resonator SAW2 is about 845 MHz so that the resonance frequency (about 755 MHz) of the first resonator SAW1 and the second resonator SAW2 and the first resonator SAW1 A frequency higher than the antiresonance frequency (about 785 MHz) is set.

  That is, the resonance frequency of the first resonator SAW1 and the resonance frequency of the second resonator SAW2 are lower than the antiresonance frequency of the first resonator SAW1 and the antiresonance frequency of the second resonator SAW2. Is set. The antiresonance frequency of the first resonator SAW1 is set between the resonance frequency and the antiresonance frequency of the second resonator SAW2.

  The first resonator SAW1 has inductive impedance in the frequency band (about 755 to 785 MHz) on the high frequency side of the resonance frequency and the low frequency side of the antiresonance frequency, and the frequency band (˜about 755 MHz) and a frequency band on the high frequency side of the anti-resonance frequency (about 785 MHz) has capacitive impedance.

  The second resonator SAW2 also has an inductive impedance in the high frequency side of the resonance frequency and the low frequency side of the antiresonance frequency (about 755 to 845 MHz), and the low frequency side of the resonance frequency (about approx. 755 MHz) and a frequency band on the high frequency side of the anti-resonance frequency (about 845 MHz) has capacitive impedance.

  The series circuit 12 in which the first resonator SAW1 and the second resonator SAW2 having such impedance characteristics are connected in series has the impedance characteristic of the first resonator SAW1 and the impedance of the second resonator SAW2. FIG. 2C, which is a combination of the characteristics, has impedance characteristics indicated by a solid line.

  Specifically, in the impedance characteristic of the series circuit 12 indicated by a solid line in FIG. 2C, the impedance is minimal at a frequency of about 755 MHz that matches the resonance frequency of the first resonator SAW1 and the second resonator SAW2. A resonance point fr1 is generated. In addition, a first anti-resonance point fa1 at which the impedance is maximized occurs at a frequency of about 785 MHz that matches the anti-resonance frequency of the first resonator SAW1. In addition, a second antiresonance point fa2 at which the impedance is maximized occurs at a frequency of about 845 MHz that matches the antiresonance frequency of the second resonator SAW2. Further, between the first anti-resonance point fa1 and the second anti-resonance point fa2, the capacitive impedance of the first resonator SAW1 and the inductive impedance of the second resonator SAW2 are combined. The combined impedance is minimized at a frequency at which the impedance waveform of the first resonator SAW1 and the impedance waveform of the second resonator SAW2 cross each other, and a sub-resonance point fr2 is generated at this frequency.

  Next, the impedance characteristic of the parallel circuit 13 of the first resonator SAW1 and the variable capacitor C1 will be described. FIG. 3A is a circuit diagram of the parallel circuit 13. FIG. 3B is an impedance characteristic diagram of the parallel circuit 13.

  The solid line shown in FIG. 3B indicates the impedance characteristic of the parallel circuit 13 when the capacitance value of the variable capacitor C1 is adjusted to 0 pF. This impedance characteristic is almost equivalent to the impedance characteristic of the first resonator SAW1 alone. If the capacitance value of the variable capacitor C1 is increased from this state, the resonance frequency in the parallel circuit 13 hardly changes. However, the anti-resonance frequency in the parallel circuit 13 gradually moves to the low frequency side and approaches the resonance frequency. Go.

  Here, paying attention to the amount of change in impedance at the same frequency, the amount of change in impedance accompanying the change in the capacitance value of the variable capacitor C1 increases as the anti-resonance frequency is approached. For this reason, the frequency at which the impedance waveform of the parallel circuit 13 and the impedance waveform of the second resonator SAW2 alone indicated by a dotted line in FIG. 3B cross (hereinafter referred to as the cross frequency) is the capacitance of the variable capacitor C1. As the value is increased, it moves to the lower frequency side.

  Note that even if the capacitance value of the variable capacitor C1 changes, the impedance waveform itself of the second resonator SAW2 hardly changes. For this reason, when the capacitance value of the variable capacitor C1 is controlled by the variable filter circuit 10 and the variable resonance circuit 11 shown in FIG. 1, the sub-resonance point fr2 exhibits the same movement as the above-described movement of the cross frequency.

  Next, the impedance characteristic of the variable resonance circuit 11 will be described. FIG. 4A is a circuit diagram of the variable resonance circuit 11. FIG. 4B is an impedance characteristic diagram of the variable resonance circuit 11.

  The solid line shown in FIG. 4B corresponds to the case where the capacitance value of the variable capacitor C1 is adjusted to 0 pF. This impedance characteristic is almost equivalent to the impedance characteristic of the series circuit 12.

  From this state, even if the capacitance value of the variable capacitor C1 is increased, the resonance frequency corresponding to the first resonator SAW1 hardly changes as described above. Therefore, a broken line and an alternate long and short dash line in FIG. Also in the impedance characteristic of the variable resonance circuit 11 shown by (4), the resonance point fr hardly changes.

  Further, in the impedance characteristic of the variable resonance circuit 11, the second antiresonance point fa2 corresponding to the antiresonance frequency of the second resonator SAW2 is hardly changed even if the capacitance value of the variable capacitor C1 is increased.

  On the other hand, as described above, the anti-resonance frequency corresponding to the first resonator SAW1 gradually moves to the low frequency side as the capacitance value of the variable capacitor C1 is increased. Also in the impedance characteristic of the variable resonance circuit 11 indicated by the broken line and the alternate long and short dash line, the first antiresonance point fa1 gradually moves to the low frequency side.

  As described above, the cross frequency between the impedance waveform corresponding to the first resonator SAW1 and the impedance waveform corresponding to the second resonator SAW2 increases the capacitance value of the variable capacitor C1. Since it moves greatly toward the low frequency side, even in the impedance characteristic of the variable resonance circuit 11 indicated by the broken line and the alternate long and short dash line in FIG. 4B, the sub-resonance point fr2 decreases as the capacitance value of the variable capacitor C1 increases. Will move a lot to the side.

  Therefore, in the variable resonance circuit 11, the first antiresonance point fa1 and the sub-resonance point fr2 can be adjusted simultaneously by controlling the single variable capacitor C1.

  Next, the filter characteristics of the variable filter circuit 10 shown in FIG. 1 will be described. FIG. 5 is a filter characteristic diagram of the variable filter circuit 10.

  FIG. 5A corresponds to a state in which the capacitance value of the variable capacitor C1 is adjusted to 0 pF. FIG. 5B corresponds to a state in which the capacitance value of the variable capacitor C1 is adjusted to 3 pF. FIG. 5C corresponds to a state in which the capacitance value of the variable capacitor C1 is adjusted to 10 pF.

  Since the variable resonance circuit 11 is inserted in the ground line in the variable filter circuit 10, the frequency band near the resonance point fr1 and the sub resonance point fr2 where the impedance of the variable resonance circuit 11 is minimized is blocked by the variable filter circuit 10. It becomes a band. On the other hand, the frequency band in the vicinity of the first antiresonance point fa1 and the second antiresonance point fa2 where the impedance of the variable resonance circuit 11 is maximized is the passband of the variable filter circuit 10.

  When the capacitance value of the variable capacitor C1 is increased from the state where the capacitance value of the variable capacitor C1 is 0 pF, both the frequencies of the first antiresonance point fa1 and the sub-resonance point fr2 are on the low frequency side as described above. In this case, the frequency interval between the first anti-resonance point fa1 and the sub-resonance point fr2 is not so narrow. On the other hand, the frequency interval between the resonance point fr1 and the first anti-resonance point fa1 is greatly narrowed, and the frequency interval between the sub-resonance point fr2 and the second anti-resonance point fa2 is greatly expanded.

  Therefore, in this variable filter circuit 10, the pass band corresponding to the first anti-resonance point fa1 and the stop band corresponding to the sub-resonance point fr2 can be adjusted by controlling one variable capacitor C1, Even if these band adjustments are performed, it is possible to suppress changes in attenuation characteristics between the two bands. In addition, since the passband corresponding to the first antiresonance point fa1 and the stopband corresponding to the sub-resonance point fr2 simultaneously move in the same direction, the adjustable frequency range of these bands becomes wide.

<< Second Embodiment >>
FIG. 6 is a circuit diagram of the variable filter circuit 20 according to the second embodiment of the present invention.

  The variable filter circuit 20 includes a variable resonance circuit 21. The variable resonance circuit 21 is connected to a shunt between the input / output terminal P1 and the input / output terminal P2. The variable resonance circuit 21 includes a first resonator SAW1, a second resonator SAW2, and a variable capacitor C2. In the present embodiment, the variable capacitor C2 is connected in parallel to the second resonator SAW2, and is connected in series to the first resonator SAW1.

  FIG. 7A is an impedance characteristic diagram of a series circuit of the first resonator SAW1 and the second resonator SAW2.

  As shown by the broken line and the alternate long and short dash line in FIG. 7A, in the impedance characteristic of the first resonator SAW1 alone and the impedance characteristic of the second resonator SAW2 alone, the resonance frequency of the first resonator SAW1 The resonance frequency of the second resonator SAW2 is set to be lower than the antiresonance frequency of the first resonator SAW1 and the antiresonance frequency of the second resonator SAW2. The antiresonance frequency of the first resonator SAW1 is set between the resonance frequency and the antiresonance frequency of the second resonator SAW2.

  As shown by the solid line in FIG. 7A, the impedance characteristics of the series circuit of the first resonator SAW1 and the second resonator SAW2 are the same as in the first embodiment. A resonance point fr1 is generated between the resonance frequency of the SAW1 and the resonance frequency of the second resonator SAW2. In addition, a first anti-resonance point fa1 at which the impedance is maximized occurs at a frequency that matches the anti-resonance frequency of the first resonator SAW1. In addition, a second anti-resonance point fa2 at which the impedance is maximized occurs at a frequency that matches the anti-resonance frequency of the second resonator SAW2. In addition, the sub-resonance point is at a frequency at which the impedance waveform of the first resonator SAW1 and the impedance waveform of the second resonator SAW2 cross between the first anti-resonance point fa1 and the second anti-resonance point fa2. fr2 is generated.

  FIG. 7B is an impedance characteristic diagram of a parallel circuit of the second resonator SAW2 and the variable capacitor C2.

  The solid line shown in FIG. 7B corresponds to the case where the capacitance value of the variable capacitor C2 is adjusted to 0 pF. This impedance characteristic is almost equivalent to the impedance characteristic of the second resonator SAW2 alone. When the capacitance value of the variable capacitor C2 is increased from this state, the resonance frequency hardly changes, but the anti-resonance frequency gradually moves to the low frequency side and approaches the resonance frequency. The cross frequency between the impedance waveform of the parallel circuit of the second resonator SAW2 and the variable capacitor C2 and the impedance waveform of the first resonator SAW1 alone indicated by a dotted line in FIG. 7B is the variable capacitor C2. As the capacitance value increases, it moves greatly toward the low frequency side.

  Even if the capacitance value of the variable capacitor C2 changes, the impedance waveform itself of the first resonator SAW1 hardly changes. For this reason, when the capacitance value of the variable capacitor C2 is controlled by the variable filter circuit 20 or the variable resonance circuit 21 shown in FIG. 1, the sub-resonance point fr2 exhibits the same movement as the above-described movement of the cross frequency.

  FIG. 7C is an impedance characteristic diagram of the variable resonance circuit 21.

  The solid line shown in FIG. 7C corresponds to the case where the capacitance value of the variable capacitor C2 is adjusted to 0 pF. This impedance characteristic is almost equivalent to the impedance characteristic of the series circuit of the first resonator SAW1 and the second resonator SAW2.

  Even if the capacitance value of the variable capacitor C2 is increased from this state, the resonance point fr and the first antiresonance point fa1 corresponding to the antiresonance frequency of the first resonator SAW1 hardly change. On the other hand, the sub-resonance point fr2 and the second anti-resonance point fa2 move to the low frequency side as the capacitance value of the variable capacitor C2 increases. Therefore, in the variable resonance circuit 21, the sub-resonance point fr2 and the second anti-resonance point fa2 can be adjusted simultaneously by controlling the single variable capacitor C2.

  FIG. 8 is a filter characteristic diagram of the variable filter circuit 20.

  FIG. 8A corresponds to a state in which the capacitance value of the variable capacitor C2 is adjusted to 0 pF. FIG. 8B corresponds to a state in which the capacitance value of the variable capacitor C2 is adjusted to 3 pF. FIG. 8C corresponds to a state in which the capacitance value of the variable capacitor C2 is adjusted to 10 pF.

  Since the variable resonance circuit 21 is inserted in the ground line in the variable filter circuit 20, the frequency band near the resonance point fr 1 and the sub resonance point fr 2 where the impedance of the variable resonance circuit 21 is minimized is blocked by the variable filter circuit 20. It becomes a band. On the other hand, the frequency band in the vicinity of the first anti-resonance point fa1 and the second anti-resonance point fa2 where the impedance of the variable resonance circuit 21 is maximized is the pass band of the variable filter circuit 20. Therefore, by controlling the capacitance value of the variable capacitor C2, the stop band corresponding to the sub-resonance point fr2 and the pass band corresponding to the second anti-resonance point fa2 can be adjusted. Since the stop band corresponding to the sub-resonance point fr2 and the pass band corresponding to the second anti-resonance point fa2 simultaneously move in the same direction, the adjustable frequency range of these bands becomes wide.

<< Third Embodiment >>
FIG. 9 is a circuit diagram of the variable filter circuit 30 according to the third embodiment of the present invention.

  The variable filter circuit 30 includes a variable resonance circuit 31. The variable resonance circuit 31 is connected to a shunt between the input / output terminal P1 and the input / output terminal P2. The variable resonance circuit 31 includes a first resonator SAW1, a second resonator SAW2, and a variable capacitor C3. In the present embodiment, the variable capacitor C3 is connected in series to the first resonator SAW1 and the second resonator SAW2.

  FIG. 10A is an impedance characteristic diagram of a series circuit of the first resonator SAW1 and the second resonator SAW2.

  As indicated by the broken line and the alternate long and short dash line in FIG. 10A, the resonance frequency of the first resonator SAW1 and the second resonance frequency of the first resonator SAW1 and the impedance characteristic of the second resonator SAW2 The resonance frequency of the resonator SAW2 is set to be lower than the antiresonance frequency of the first resonator SAW1 and the antiresonance frequency of the second resonator SAW2. The antiresonance frequency of the first resonator SAW1 is set between the resonance frequency and the antiresonance frequency of the second resonator SAW2.

  Therefore, as indicated by the solid line in FIG. 10A, the impedance characteristics of the series circuit of the first resonator SAW1 and the second resonator SAW2 are equal to the resonance frequency of the first resonator SAW1 and the second resonance frequency. A resonance point fr1 is generated in the vicinity of the frequency overlapping with the resonance frequency of the resonator SAW2. In addition, a first anti-resonance point fa1 at which the impedance is maximized occurs at a frequency that matches the anti-resonance frequency of the first resonator SAW1. In addition, a second anti-resonance point fa2 at which the impedance is maximized occurs at a frequency that matches the anti-resonance frequency of the second resonator SAW2. In addition, the sub-resonance point is at a frequency at which the impedance waveform of the first resonator SAW1 and the impedance waveform of the second resonator SAW2 cross between the first anti-resonance point fa1 and the second anti-resonance point fa2. fr2 is generated.

  FIG. 10B is an impedance characteristic diagram of a series circuit of the second resonator SAW2 and the variable capacitor C3.

  The solid line shown in FIG. 10B corresponds to the case where the capacitance value of the variable capacitor C3 is adjusted to a sufficiently large value. This impedance characteristic is almost equivalent to the impedance characteristic of the second resonator SAW2 alone. If the capacitance value of the variable capacitor C3 is decreased from this state, the anti-resonance frequency of this series circuit hardly changes, but the resonance point of this series circuit gradually increases from the resonance frequency of the second resonator SAW2. Move sideways and approach the anti-resonance frequency. The frequency at which the impedance waveform of the series circuit of the second resonator SAW2 and the variable capacitor C3 and the impedance waveform of the first resonator SAW1 alone indicated by a dotted line in FIG. 10B cross is variable. As the capacitance value of the capacitor C3 is lowered, it moves greatly toward the high frequency side.

  Note that even if the capacitance value of the variable capacitor C3 changes, the impedance waveform itself of the first resonator SAW1 hardly changes. For this reason, when the capacitance value of the variable capacitor C3 is controlled by the variable filter circuit 30 or the variable resonance circuit 31 shown in FIG. 9, the sub-resonance point fr2 exhibits the same movement as the above-described movement of the cross frequency.

  FIG. 10C is an impedance characteristic diagram of the variable resonance circuit 31.

  The solid line shown in FIG. 10C corresponds to the case where the capacitance value of the variable capacitor C3 is adjusted to a sufficiently large value. This impedance characteristic is almost equivalent to the impedance characteristic of the series circuit of the first resonator SAW1 and the second resonator SAW2.

  Even if the capacitance value of the variable capacitor C3 is decreased from this state, the first antiresonance point fa1 and the second antiresonance point hardly change. On the other hand, the resonance point fr1 and the sub-resonance point fr2 move to the high frequency side as the capacitance value of the variable capacitor C3 decreases. Therefore, in the variable resonance circuit 31, the resonance point fr1 and the sub resonance point fr2 can be adjusted simultaneously by controlling the single variable capacitor C3.

  FIG. 11 is a filter characteristic diagram of the variable filter circuit 30.

  FIG. 11A corresponds to a state in which the capacitance value of the variable capacitor C2 is adjusted to a sufficiently large value. FIG. 11B corresponds to a state in which the capacitance value of the variable capacitor C3 is adjusted to 3 pF. FIG. 11C corresponds to a state in which the capacitance value of the variable capacitor C3 is adjusted to 1 pF.

  Since the variable resonance circuit 31 is inserted in the ground line in the variable filter circuit 30, the frequency band in the vicinity of the resonance point fr1 and the sub resonance point fr2 where the impedance of the variable resonance circuit 31 is minimized is blocked by the variable filter circuit 30. It becomes a band. On the other hand, the frequency band in the vicinity of the first antiresonance point fa1 and the second antiresonance point fa2 at which the impedance of the variable resonance circuit 31 is maximized is the passband of the variable filter circuit 30. Therefore, by controlling the capacitance value of the variable capacitor C3, the stop band corresponding to the resonance point fr1 and the stop band corresponding to the sub resonance point fr2 can be adjusted. Since the pass band corresponding to the first anti-resonance point fa1 and the pass band corresponding to the second anti-resonance point fa2 do not change, the two pass bands can be increased by controlling the capacitance value of the variable capacitor C3. Adjustment can be made so as to switch between a state in which the frequency side has high steepness and a state in which the low frequency side has high steepness.

<< First Modification >>
FIG. 12 is a circuit diagram of a variable resonance circuit according to a first modification.

  A variable resonance circuit 11A shown in FIG. 12A includes a first resonator SAW1, a second resonator SAW2, and a variable capacitor C1. The first resonator SAW1 and the second resonator SAW2 are connected in series. The variable capacitor C1 is connected in parallel to the series circuit of the first resonator SAW1 and the second resonator SAW2.

  In the variable resonance circuit 11A, the first capacitor anti-resonance point fa1 corresponding to the first resonator SAW1, the sub-resonance point fr2, and the second resonator SAW2 corresponding to the second resonator SAW2 are controlled by the variable capacitor C1. The antiresonance point fa2 can be simultaneously adjusted in the same direction. Therefore, when a variable filter circuit is configured using the variable resonance circuit 11A, a pass band and a stop band corresponding to the first antiresonance point fa1, the sub resonance point fr2, and the second antiresonance point fa2 are set in a wide frequency range. Can be adjusted. Furthermore, even if adjustment is performed, a predetermined attenuation level can be maintained among the first antiresonance point fa1, the sub-resonance point fr2, and the second antiresonance point fa2.

  12B includes a variable resonance circuit 11B, a first resonator SAW1, a second resonator SAW2, a first variable capacitor C1, and a second variable capacitor C2. The first resonator SAW1 and the second resonator SAW2 are connected in series. The first variable capacitor C1 is connected in parallel with the first resonator SAW1 and in series with the second resonator SAW2. The second variable capacitor C2 is connected in series with the first resonator SAW1 and in parallel with the second resonator SAW2.

  In the variable resonance circuit 11B, the first antiresonance point fa1 and the second antiresonance point fa2 are individually controlled by controlling the first variable capacitor C1 and the second variable capacitor C2. Can be adjusted. Therefore, when the variable filter circuit is configured using the variable resonance circuit 11B, the first antiresonance point fa1, the sub resonance point fr2, and the second antiresonance point fa2 can be adjusted with a higher degree of freedom. Can do.

  A variable resonance circuit 11C shown in FIG. 12C includes a first resonator SAW1, a second resonator SAW2, a first variable capacitor C1, and a third variable capacitor C3. The first resonator SAW1 and the second resonator SAW2 are connected in series. The first variable capacitor C1 is connected in parallel with the first resonator SAW1 and in series with the second resonator SAW2. The third variable capacitor C3 is connected in series to the first resonator SAW1 and the second resonator SAW2. In the variable resonance circuit 11C, the resonance point fr1 can be adjusted by controlling the third variable capacitor C3. The third variable capacitor C3 is connected in series to the parallel circuit of the first resonator SAW1 and the first variable capacitor C1, so that the sub-resonance point fr2 by the control of the first variable capacitor C1. Almost no influence on the adjustment of.

<< Fourth Embodiment >>
FIG. 13 is a circuit diagram of a variable filter circuit 40 according to the fourth embodiment of the present invention.

  The variable filter circuit 40 includes a variable resonance circuit 41. The variable resonance circuit 41 is connected to a shunt between the input / output terminal P1 and the input / output terminal P2. The variable resonance circuit 41 includes a first resonator SAW1, a second resonator SAW2, and a first variable capacitor C1. The first resonator SAW1 and the first variable capacitor C1 are connected in series. The second resonator SAW2 is connected in parallel to the series circuit of the first resonator SAW1 and the first variable capacitor C1.

  FIG. 14A is an impedance characteristic diagram of a parallel circuit of the first resonator SAW1 and the second resonator SAW2.

  As shown by the broken line and the alternate long and short dash line in FIG. 14A, the anti-resonance frequency of the first resonator SAW1 and the impedance of the first resonator SAW1 and the impedance characteristic of the second resonator SAW2 The antiresonance frequency of the second resonator SAW2 is set higher than the resonance frequency of the first resonator SAW1 and the resonance frequency of the second resonator SAW2. The resonance frequency of the first resonator SAW1 is set between the resonance frequency and the antiresonance frequency of the second resonator SAW2.

  Then, in the impedance characteristic of the parallel circuit of the first resonator SAW1 and the second resonator SAW2 indicated by a solid line in FIG. 14A, the antiresonance frequency and the second resonance of the first resonator SAW1 are shown. Between the anti-resonance frequency of the child SAW2, there is an anti-resonance point fa1 'where the impedance is maximized. In addition, a first resonance point fr1 'at which the impedance is minimized occurs at a frequency that matches the resonance frequency of the first resonator SAW1. In addition, a second resonance point fr2 'at which the impedance is minimized occurs at a frequency that matches the resonance frequency of the second resonator SAW2. In addition, between the second resonance point fr2 ′ and the first resonance point fr1 ′, the capacitive impedance of the first resonator SAW1 and the inductive impedance of the second resonator SAW2 As a result of the synthesis, the synthesized impedance is maximized at a frequency at which the impedance waveform of the first resonator SAW1 and the impedance waveform of the second resonator SAW2 cross each other, and a sub-antiresonance point fa2 ′ is generated at this frequency.

  FIG. 14B is an impedance characteristic diagram of a series circuit of the first resonator SAW1 and the variable capacitor C1.

  The solid line shown in FIG. 14B corresponds to the case where the capacitance value of the variable capacitor C1 is adjusted to a sufficiently large value. This impedance characteristic is almost equivalent to the impedance characteristic of the first resonator SAW1 alone.

  When the capacitance value of the variable capacitor C1 is decreased from this state, the antiresonance frequency hardly changes, but the resonance frequency gradually moves to the high frequency side and approaches the antiresonance frequency. The frequency at which the impedance waveform of the series circuit of the first resonator SAW1 and the variable capacitor C1 and the impedance waveform of the second resonator SAW2 alone indicated by a dotted line in FIG. 14B cross is variable. As the capacitance value of the capacitor C1 is lowered, it moves greatly toward the high frequency side.

  Note that even if the capacitance value of the variable capacitor C1 changes, the impedance waveform itself of the second resonator SAW2 hardly changes. Therefore, when the variable filter circuit 40 or the variable resonance circuit 41 shown in FIG. 13 controls the capacitance value of the variable capacitor C1, the secondary antiresonance point fa2 ′ exhibits the same movement as the above-described movement of the cross frequency. .

  FIG. 14C is an impedance characteristic diagram of the variable resonance circuit 41.

  The solid line shown in FIG. 14C corresponds to the case where the capacitance value of the variable capacitor C1 is adjusted to a sufficiently large value. This impedance characteristic is almost equivalent to the impedance characteristic of the parallel circuit of the first resonator SAW1 and the second resonator SAW2.

  Even if the capacitance value of the variable capacitor C1 is decreased from this state, the antiresonance point fa1 'and the second resonance point fr2' do not change much. On the other hand, the secondary anti-resonance point fa2 'and the first resonance point fr1' move greatly toward the high frequency side as the capacitance value of the variable capacitor C1 decreases. Therefore, in the variable resonance circuit 41, the sub antiresonance point fa2 'and the first resonance point fr1' can be adjusted simultaneously by controlling the single variable capacitor C1. For this reason, when a variable filter circuit is configured using the variable resonance circuit 31, it is possible to adjust the passband and stopband corresponding to the sub-antiresonance point fa2 ′ and the first resonance point fr1 ′. And the adjustable frequency range can be expanded.

  FIG. 15 is a filter characteristic diagram of the variable filter circuit 40.

  FIG. 15A corresponds to a state in which the capacitance value of the variable capacitor C1 is adjusted to a sufficiently large value. FIG. 15B corresponds to a state in which the capacitance value of the variable capacitor C1 is adjusted to 3 pF. FIG. 15C corresponds to a state in which the capacitance value of the variable capacitor C1 is adjusted to 1 pF.

  Since the variable resonance circuit 41 is inserted in the ground line in the variable filter circuit 40, the frequency band in the vicinity of the first resonance point fr1 ′ and the second resonance point fr2 ′ at which the impedance of the variable resonance circuit 41 is minimized is This is the stop band of the variable filter circuit 40. On the other hand, the frequency band in the vicinity of the anti-resonance point fa <b> 1 ′ and the sub-anti-resonance point fa <b> 2 ′ where the impedance of the variable resonance circuit 41 is maximized is the pass band of the variable filter circuit 40.

  Therefore, by controlling the variable capacitor C1, the stop band corresponding to the sub-antiresonance point fa2 ′ and the pass band corresponding to the first resonance point fr1 ′ are attenuated between these stop bands and the pass band. Adjustment can be made without greatly changing the characteristics.

<< Second Modification >>
FIG. 16 is a circuit diagram of a variable resonance circuit according to a second modification.

  A variable resonance circuit 41A shown in FIG. 16A includes a first resonator SAW1, a second resonator SAW2, and a second variable capacitor C2. The first resonator SAW1 and the second resonator SAW2 are connected in parallel. The second variable capacitor C2 is connected in parallel to the first resonator SAW1 and in series to the second resonator SAW2.

  In the variable resonance circuit 41A, the second resonance point fr2 ′ corresponding to the second resonator SAW2 and the sub-antiresonance point fa2 ′ are simultaneously adjusted in the same direction by controlling the second variable capacitor C2. be able to. Therefore, when a variable filter circuit is configured using the variable resonance circuit 41A, the passband and stopband corresponding to the second resonance point fr2 ′ and the sub-antiresonance point fa2 ′ can be adjusted in a wide frequency range. . Furthermore, even if adjustment is performed, a predetermined attenuation level can be maintained between the second resonance point fr2 'and the sub-antiresonance point fa2'.

  A variable resonance circuit 41B shown in FIG. 16B includes a first resonator SAW1, a second resonator SAW2, a first variable capacitor C1, and a second variable capacitor C2. The first resonator SAW1 and the first variable capacitor C1 are connected in series. The second resonator SAW2 and the second variable capacitor C2 are connected in series. The series circuit of the first resonator SAW1 and the first variable capacitor C1 and the series circuit of the second resonator SAW2 and the second variable capacitor C2 are connected in parallel.

  In the variable resonance circuit 41B, the first resonance point fr1 ′ and the second resonance point fr2 ′ are individually adjusted by controlling the first variable capacitor C1 and the second variable capacitor C2. can do. Therefore, when the variable filter circuit is configured using the variable resonance circuit 41B, the first resonance point fr1 ′, the secondary antiresonance point fa2 ′, and the second resonance point fr2 ′ are adjusted with a higher degree of freedom. can do.

  A variable resonance circuit 41C illustrated in FIG. 16C includes a first resonator SAW1, a second resonator SAW2, and a first variable capacitor C1. The first resonator SAW1 and the second resonator SAW2 are connected in parallel. The first variable capacitor C1 is connected in series to a parallel circuit of the first resonator SAW1 and the second resonator SAW2.

  In the variable resonance circuit 41C, by controlling the first variable capacitor C1, the first resonance point fr1 ′, the secondary antiresonance point fa2 ′, and the second resonance point fr2 ′ are simultaneously set in the same direction. Can be adjusted. Therefore, when a variable filter circuit is configured using the variable resonance circuit 41C, the first resonance point fr1 ′, the secondary antiresonance point fa2 ′, and the passband and stopband corresponding to the second resonance point fr2 ′ are obtained. It can be adjusted over a wide frequency range. Therefore, the characteristics of the variable filter can be adjusted with a higher degree of freedom.

  As described above, the present invention can be implemented. It should be noted that the present invention can be implemented with any other configuration as long as the configuration falls within the scope of the claims as long as it is the configuration described in each of the above-described embodiments.

10, 20, 30, 40 ... variable filter circuits 11, 11A, 11B, 11C, 21, 31, 41, 41A, 41B, 41C ... variable resonance circuit 12 ... series circuit 13 ... parallel circuit SAW1 ... first resonator SAW2 ... second resonator C1 ... first variable capacitor C2 ... second variable capacitor C3 ... third variable capacitor

Claims (8)

A first resonator;
A second resonator connected in series with the first resonator;
A variable capacitor connected in parallel to the first resonator;
A variable resonance circuit comprising
The resonance frequency of the first resonator and the resonance frequency of the second resonator are on the lower frequency side than the anti-resonance frequency of the first resonator and the anti-resonance frequency of the second resonator,
Anti-resonance frequency of the first resonator, Ri mania the resonant frequency and the antiresonant frequency of the second resonator,
The variable resonance circuit is:
A first resonance point corresponding to a resonance frequency of the first resonator;
A second resonance point on the higher frequency side than the first resonance point and on the lower frequency side than the anti-resonance frequency of the second resonator;
A first anti-resonance point on a higher frequency side than the first resonance point and a lower frequency side than the second resonance point;
A second antiresonance point corresponding to the antiresonance frequency of the second resonator;
Have
The frequency of the first anti-resonance point and the frequency of the second resonance point simultaneously change in the same frequency direction as the capacitance value of the variable capacitor changes.
Variable resonant circuit. A first resonator;
A second resonator connected in series with the first resonator;
A variable capacitor connected in parallel to the second resonator;
A variable resonance circuit comprising
The resonance frequency of the first resonator and the resonance frequency of the second resonator are on the lower frequency side than the anti-resonance frequency of the first resonator and the anti-resonance frequency of the second resonator,
Anti-resonance frequency of the first resonator, Ri mania the resonant frequency and the antiresonant frequency of the second resonator,
The variable resonance circuit is:
A first resonance point corresponding to a resonance frequency of the first resonator;
A second resonance point on the higher frequency side than the first resonance point and on the lower frequency side than the anti-resonance frequency of the second resonator;
A first anti-resonance point corresponding to the anti-resonance frequency of the first resonator;
A second anti-resonance point on a higher frequency side than the first anti-resonance point and on a lower frequency side than the anti-resonance frequency of the second resonator;
Have
The frequency of the second resonance point and the frequency of the second anti-resonance point simultaneously change in the same frequency direction as the capacitance value of the variable capacitor changes.
Variable resonant circuit. The variable capacitor is connected in parallel to a series circuit of the first resonator and the second resonator.
The variable resonance circuit according to claim 2. A variable capacitor connected in parallel to the second resonator is defined as a second variable capacitor.
In addition to the second variable capacitor, the first variable capacitor connected in parallel to the first resonator,
The parallel circuit of the first resonator and the first variable capacitor and the parallel circuit of the second resonator and the second variable capacitor are connected in series.
The variable resonance circuit according to claim 2. In addition to the variable capacitor, the device further comprises a third variable capacitor connected in series to the first resonator and the second resonator.
The variable resonance circuit according to claim 1. The variable resonance circuit according to claim 5, wherein the third variable capacitor is connected in series to a parallel circuit of the first resonator or the second resonator and the variable capacitor. A first resonator;
A second resonator connected in series with the first resonator;
A variable capacitor connected in series to the first resonator and the second resonator;
A variable resonance circuit comprising
The resonance frequency of the first resonator and the resonance frequency of the second resonator are on the lower frequency side than the anti-resonance frequency of the first resonator and the anti-resonance frequency of the second resonator,
Anti-resonance frequency of the first resonator, Ri mania the resonant frequency and the antiresonant frequency of the second resonator,
The variable resonance circuit is:
A first resonance point on a higher frequency side than a resonance frequency of the first resonator and on a lower frequency side than an anti-resonance frequency of the first resonator;
A second resonance point on a higher frequency side than the antiresonance frequency of the first resonator and on a lower frequency side than the antiresonance frequency of the second resonator;
A first anti-resonance point corresponding to the anti-resonance frequency of the first resonator;
A second antiresonance point corresponding to the antiresonance frequency of the second resonator;
Have
The variable resonance circuit , wherein the frequency of the first resonance point and the frequency of the second resonance point simultaneously change in the same frequency direction as the capacitance value of the variable capacitor changes . Input and output terminals;
A resonant circuit connected over one or more stages between the input terminal and the output terminal;
With
The resonant circuit includes the variable resonant circuit according to any one of claims 1 to 7 .
Variable filter circuit.