JP2009130831A - Tunable filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tunable filter that achieves a wide range of variable frequency. <P>SOLUTION: This invention relates to the tunable filter where there are provided a series arm resonator having a fixed resonance frequency (frs), a parallel arm resonator having a fixed resonance frequency (frp), and a variable capacitor connected to the series arm resonator and parallel arm resonator. In the tunable filter, the ratio frs/frp of the fixed resonance frequency (frs) provided in the series arm resonator to the fixed resonance frequency (frp) in the parallel arm resonator is 1.0<frs/frp<1.1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a tunable filter.

  In recent years, development of mobile communication systems corresponding to a plurality of communication standards or communication modes has been promoted. In the mobile communication system, a mounted filter device includes a filter bank including a plurality of filters corresponding to a plurality of bands, and switches a filter to be used for each required frequency band. For example, in the DCS / RX mode used in the UMTS (Universal Mobile Telecommunications System), which is a European third generation mobile communication system, the pass band of the filter mounted on the mobile terminal is set to 1805 MHz to 1880 MHz. The Similarly, in the PCS / RX mode, the pass band of the filter is set to 1930 MHz to 1990 MHz. In the case of the W-CDMA system using the 2.1 GHz band, the pass band of the filter is set to 2110 MHz to 2170 MHz. However, the filter bank device naturally has problems such as an increase in device size and complicated switching control, so switching of the pass band required in these modes / systems is realized. Therefore, a tunable filter having a wide variable pass band is required.

  Regarding a filter with a variable pass band, for example, Patent Literature 1 below describes a ladder-type variable filter. The variable filter described in this document is characterized by a feedback configuration in which the circuit constant of a variable capacitor or an inductor element is monitored and the capacitance of the variable capacitor is controlled based on information on the circuit constant. That is, the pass band is varied by controlling the capacity based on the feedback.

JP 2005-217852 A

  However, the filter used for the RF front end of next-generation wireless communication (Multi-band Multi-mode, Software wireless, etc.) that can be adapted to a plurality of communication standards requires a device for realizing a wide variable range of the passband. is there. Therefore, realization of a tunable filter having a wide bandwidth, a steep cutoff characteristic, and a wide frequency variable range is urgently required.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide a new and improved tunable filter capable of maintaining a wide variable frequency range of the passband. It is to provide.

  In order to solve the above problems, according to an aspect of the present invention, a series arm resonator having a fixed resonance frequency frs, a parallel arm resonator having a fixed resonance frequency frp, the series arm resonator, and A tunable filter comprising a variable capacitor connected to the parallel arm resonator is provided. In particular, the tunable filter has a ratio frs / frp between a fixed resonance frequency frs of the series arm resonator and a fixed resonance frequency frp of the parallel arm resonator of 1.0 <frs / frp <1. 1 may be sufficient.

  In the tunable filter, a ratio frs / frp between a fixed resonance frequency frs of the series arm resonator and a fixed resonance frequency frp of the parallel arm resonator is 1.025 <frs / frp <1. 1 may be sufficient.

  In the tunable filter, a ratio frs / frp between a fixed resonance frequency frs of the series arm resonator and a fixed resonance frequency frp of the parallel arm resonator is 1.07 <frs / frp <1. It may be 08.

  The tunable filter may be formed in a plurality of stages by connecting two or more basic sections formed by one series arm resonator, one parallel arm resonator, and the variable capacitor. However, in each basic section, the ratio frs / frp between the fixed resonance frequency frs of the series arm resonator and the fixed resonance frequency frp of the parallel arm resonator is 1.0 <frs / frp <1. 1, 1.025 <frs / frp <1.1, or 1.07 <frs / frp <1.08. Further, the ratio frs / frp belonging to any one of the above ranges may be set in common in all of the plurality of basic sections, or the ratio belonging to any one of the above ranges for each basic section. frs / frp may be set. Of course, the ratio frs / frp in which only some of the plurality of basic sections belong to any of the above ranges may be used.

  As described above, the ratio between the fixed resonance frequency frs of the series arm resonator and the fixed resonance frequency frp of the parallel arm resonator is set as described above, so that a predetermined pass characteristic (low loss) is obtained. While maintaining the above, a wide variable frequency range can be realized. In particular, the range of the resonance frequency ratio takes into account a decrease in the variable frequency range as the ratio increases. That is, by defining the upper limit value as described above, a wide variable frequency range is realized while maintaining a predetermined pass characteristic. For example, the above technique can be applied to a mobile terminal or a base station that can cover a communication band defined by a plurality of communication standards. Further, the configuration of the resonator can be realized by using, for example, a surface acoustic wave device or a boundary acoustic wave device.

  As described above, according to the present invention, a variable frequency range having a wide passband can be realized.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

[Frequency characteristics of ladder filter]
First, prior to describing an embodiment according to the present invention, a frequency characteristic of a ladder filter related to technical matters of the embodiment described below will be briefly described.

  FIG. 1 shows a circuit configuration of a ladder type filter 10. As shown in FIG. 1, the filter 10 includes a plurality of resonators (series arm resonator 12 and parallel arm resonator 14) that form a series arm and a parallel arm in the circuit configuration. The series arm resonator 12 and the parallel arm resonator 14 include, for example, two capacitors (C0, C1) connected in parallel and one of the capacitors (C1) as shown by reference numeral 16 in FIG. The inductor element (L1) connected in series is formed. Note that the circuit configuration of reference numeral 16 is very simplified for convenience of explanation. Therefore, it should be noted that it actually includes a resistance component and other components. In addition, the configuration of the resonator described in the following embodiments is not limited to this.

  For example, consider a case where an RF signal having a frequency f is applied from the terminal 1 to the terminal 1 ′ and the frequency f is increased. When the frequency f of the RF signal increases, the resonator described by the circuit configuration of reference numeral 16 in FIG. 1 enters a resonance state and becomes an AC conductive state. In the following description, this resonance frequency fr may be referred to as a resonance point. When the frequency f is further increased, the resonator described with reference to the circuit configuration of reference numeral 16 in FIG. 1 enters an anti-resonance state and a high impedance state. Therefore, an RF signal attenuation pole is formed in the vicinity of the resonance frequency fa. In the following description, this resonance frequency fa may be called an antiresonance point or an antiresonance frequency.

  Further, in order to obtain bandpass characteristics, a circuit like the filter 10 shown in FIG. 1 is composed with the circuit configuration of reference numeral 16 as a series arm and a parallel arm. According to this circuit configuration, admittance characteristics and pass characteristics as shown in FIG. 2 can be obtained. However, the resonance frequency (frs) of the series arm resonator 12 and the anti-resonance frequency (fap) of the parallel arm resonator 14 are set to coincide with each other in order to obtain good pass characteristics and steep cut-off characteristics.

  For example, consider a case where the frequency f of the RF signal applied from the input terminal is increased. When this frequency f reaches the resonance frequency (frp) of the parallel arm resonator 14, the parallel arm resonator 14 enters a resonance state and its impedance becomes zero. Then, the parallel arm resonator 14 is short-circuited, and the RF signal does not reach the output terminal. As a result, an attenuation pole on the low frequency side is formed at the resonance frequency frp of the parallel arm resonator 14.

  When the frequency of the RF signal is further increased to reach the anti-resonance frequency fap of the parallel arm resonator 14, the parallel arm resonator 14 enters an anti-resonance state and enters a high impedance state. Further, since the resonance frequency frs of the series arm resonator 12 and the anti-resonance frequency fap of the parallel arm resonator 14 coincide with each other, the series arm resonator 12 enters a resonance state and becomes conductive. When the frequency f of the RF signal is further increased to reach the antiresonance frequency (fas) of the series arm resonator 12, the series arm resonator 12 enters an antiresonance state and enters a high impedance state. Therefore, transmission of the RF signal to the output terminal is blocked, and an attenuation pole on the high frequency side is formed at the antiresonance frequency fas of the series arm resonator 12.

  Based on the principle described above, a pass characteristic as shown in FIG. 2 is obtained. Based on the above description, embodiments of the present invention will be described below.

<First Embodiment>
First, the configuration of the tunable filter 100 according to the first embodiment of the present invention and the variable frequency range control method will be described with reference to FIG. FIG. 3 is an explanatory diagram illustrating a configuration of the tunable filter 100 according to the present embodiment.

  As shown in FIG. 3, the tunable filter 100 includes a variable series arm resonator 102 and a variable parallel arm resonator 104. The variable series arm resonator 102 includes variable capacitors 1022 and 1024 and a resonator 1026. Further, the variable parallel arm resonator 104 includes variable capacitors 1042 and 1044 and a resonator 1046. The resonator 1026 is an example of a series arm resonator. The resonator 1046 is an example of a parallel arm resonator.

  As shown in FIG. 3, the variable capacitor 1022 is connected in series with the resonator 1026. The variable capacitor 1024 is connected in parallel to the resonator 1026. The variable capacitor 1042 is connected in series with the resonator 1046. The variable capacitor 1044 is connected in parallel to the resonator 1046. The capacitances of the variable capacitors 1022, 1024, 1042, and 1044 are controlled by, for example, a control unit (not shown).

(Variable series arm resonator 102)
The variable series arm resonator 102 forms a series arm of the tunable filter 100, and has a variable resonance frequency frs ′ and a variable anti-resonance frequency fas ′. The resonator 1026 constituting the variable series arm resonator 102 has a fixed resonance frequency frs and a fixed anti-resonance frequency fas. Further, the capacitances of the variable capacitors 1022 and 1024 are expressed as Css and Csp, respectively. The resonator 1026 may be a resonator having a fixed resonance frequency frs and a fixed anti-resonance frequency fas, and is not limited to the configuration indicated by reference numeral 16 above.

  Note that the resonance frequency frs ′ of the variable series arm resonator 102 increases as the capacitance Css of the variable capacitor 1022 increases. On the other hand, the anti-resonance frequency fas ′ of the variable series arm resonator 102 decreases as the capacitance Csp of the variable capacitor 1024 increases. However, it should be noted that the resonance frequency frs and the anti-resonance frequency fas of the resonator 1026 do not change regardless of the capacitance change of the variable capacitors 1022 and 1024.

(Variable parallel arm resonator 104)
The variable parallel arm resonator 104 forms a parallel arm of the tunable filter 100, and has a variable resonance frequency frp ′ and a variable anti-resonance frequency fap ′. The resonator 1046 constituting the variable parallel arm resonator 104 has a fixed resonance frequency frp and a fixed anti-resonance frequency fap. Further, the capacitances of the variable capacitors 1022 and 1024 are expressed as Cps and Cpp, respectively. The resonator 1046 only needs to be a resonator having a fixed resonance frequency frp and a fixed anti-resonance frequency fap, and is not limited to the configuration indicated by reference numeral 16 above.

  Note that the resonance frequency frp ′ of the variable parallel arm resonator 104 increases as the capacitance Cps of the variable capacitor 1042 increases. On the other hand, the anti-resonance frequency fap ′ of the variable parallel arm resonator 104 decreases as the capacitance Cpp of the variable capacitor 1044 increases. However, it should be noted that the resonance frequency frp and the antiresonance frequency fap of the resonator 1046 do not change regardless of the capacitance change of the variable capacitors 1042 and 1044.

(About passband characteristics)
As described above, the tunable filter 100 can vary the resonance characteristics of the variable series arm resonator 102 and the variable parallel arm resonator 104. Therefore, the pass characteristic of the tunable filter 100 will be described based on the knowledge about the pass characteristic of the filter 10 already described.

  The pass band of the tunable filter 100 can be adjusted by varying the resonance characteristics of the variable series arm resonator 102 and the variable parallel arm resonator 104. At this time, the pass band is formed between the resonance frequency frp ′ of the variable parallel arm resonator 104 and the anti-resonance frequency fas ′ of the variable series arm resonator 102. However, since the pass characteristic (attenuation amount) depends on the anti-resonance frequency fap ′ of the variable parallel arm resonator 104 and the resonance frequency frs ′ of the variable series arm resonator 102, the resonance frequencies frs and frp of the resonators 1026 and 1046 are determined. The set values of the antiresonance frequencies fas and fap are important. Hereinafter, this point will be described.

  First, the pass characteristic of the tunable filter 100 will be described with reference to FIG. FIG. 4 is an explanatory diagram showing admittance characteristics and pass characteristics of the tunable filter 100. Note that the frequency decreasing direction is expressed as A direction and the increasing direction is expressed as B direction. A chain line in the figure represents an admittance characteristic of the variable parallel arm resonator 104. An alternate long and short dash line represents the admittance characteristic of the variable series arm resonator 102. Further, the solid line in the figure represents the pass characteristic of the RF signal by the tunable filter 100.

  In FIG. 4, admittance characteristics and pass characteristics when the fixed resonance frequency frs of the resonator 1026 and the fixed resonance frequency frp of the resonator 1046 are adjusted as Case 1 (frs = frp). It is shown. Further, as Case 2, the admittance characteristic when the resonance frequency frs ′ of the variable series arm resonator 102 and the antiresonance frequency fap ′ of the variable parallel arm resonator 104 are adjusted to coincide with each other (frs ′ = fap ′). , And pass characteristics are shown.

  First, focus on Case 1 (frs = frp). The resonance frequency frs ′ of the variable series arm resonator 102 can be shifted in the B direction by increasing the capacitance Css of the variable capacitor 1022. Similarly, the resonance frequency frp ′ of the variable parallel arm resonator 104 can be shifted in the B direction by increasing the capacitance Cps of the variable capacitor 1042. Further, by reducing the capacitance Csp of the variable capacitor 1024, the antiresonance frequency fas' of the variable series arm resonator 102 can be shifted in the A direction. Similarly, the antiresonance frequency frs' of the variable parallel arm resonator 104 can be shifted in the A direction by decreasing the capacitance Cpp of the variable capacitor 1044.

  Therefore, as in Case 1, when the fixed resonance frequency frp of the resonator 1046 and the fixed resonance frequency frs of the resonator 1026 are matched, the variable series arm resonator 102 and the variable parallel arm resonator over a wide frequency range. The resonance / anti-resonance frequency of 104 can be varied. This means that in order to realize a low-loss tunable filter, the resonance frequency frs ′ of the variable series arm resonator 102 and the antiresonance frequency fap ′ of the variable parallel arm resonator 104 are set near the center frequency of the passband. Although it is necessary to match, the variable frequency range of the resonance frequency frs ′ of the variable series arm resonator 102 and the anti-resonance frequency fap ′ of the variable parallel arm resonator 104 can be adjusted widely, so that the variable range of the center frequency of the passband can be adjusted. Is widely adjustable. As an example, Case 2 in the figure shows a state after adjustment so that the resonance frequency frs 'and the anti-resonance frequency fap' coincide with each other.

  Further, the bandwidth of the pass band can be adjusted by adjusting the anti-resonance frequency fas 'of the variable series arm resonator 102 and / or the resonance frequency frp' of the variable parallel arm resonator 104. Here, the pass band refers to a frequency band in which a loss during passing takes a value of 2 dB or less. The bandwidth refers to the width of the pass band.

  For comparison with the above, attention is again directed to FIG. As described above, FIG. 2 shows a state where the fixed resonance frequency and anti-resonance frequency are frs = fap. This is a general setting of a ladder type filter used conventionally. As shown in FIG. 2, when the fixed anti-resonance frequency fap of the resonator 1046 and the fixed resonance frequency frs of the resonator 1026 match, the resonance frequency frs ′ of the variable series arm resonator 102 has a large frequency. The anti-resonance frequency fap ′ of the variable parallel arm resonator 104 can be shifted only in the direction in which the frequency decreases (corresponding to the A direction in FIG. 4).

  For this reason, the resonance frequency frs 'of the variable series arm resonator 102 and the antiresonance frequency fap' of the variable parallel arm resonator 104 cannot be adjusted in the vicinity of the center frequency of the passband as in the present embodiment. Although it may be possible to adjust the center frequency of the pass spectrum by adjusting the respective resonance frequency and anti-resonance frequency, the pass loss is very large. In other words, the effective variable frequency range for the low-loss passband is very limited.

  Thus, the variable frequency range of the passband must be an effective variable frequency range in which a loss amount equal to or less than a predetermined value is realized. Therefore, further discussion will be made focusing on the relationship between the resonance frequency frs of the resonator 1026 and the resonance frequency frp of the resonator 1046.

By the way, the variable frequency range of the pass band may depend on parameters other than those described above. For example, a variable frequency range of the pass band can be widened by increasing the coupling coefficient K ² of the resonator. However, increasing the coupling coefficient K ² of the resonator, blocking characteristics deteriorate. In addition, there is a problem that the coupling coefficient K ² is larger resonators are very limited. Therefore, the conditions under which good characteristics can be obtained under realistic conditions are considered.

(Dependence on fixed resonance frequency ratio)
First, a change in the variable frequency range of the passband according to the resonance frequency ratio (frs / frp) will be described by comparing FIG. 5 and FIG. FIG. 5 shows the variable frequency range of the passband when the resonance frequency frs of the series arm resonator 1026 and the resonance frequency frp of the parallel arm resonator 1046 coincide (resonance frequency ratio frs / frp = 1.0). It is explanatory drawing shown. On the other hand, FIG. 6 is an explanatory diagram showing the variable frequency range of the passband when the resonance frequency ratio is 1.075.

  In the example of FIG. 5 (frs / frp = 1.0), the variable frequency range of the pass band has a width of about 13.1%. On the other hand, in the example of FIG. 6 (frs / frp = 1.075), the variable frequency range of the pass band has a width of about 19.9%. That is, the range is slightly larger than the condition (see FIG. 4: Case 1) that maximizes the variable frequency range of the anti-resonance frequency fap ′ of the variable series arm resonator 102 and the resonance frequency frs ′ of the variable parallel arm resonator 104. The result is that the effective variable frequency range can be expanded by narrowing. Here, the variable frequency range described as Tuning Range in the figure is the lowest frequency side when the pass band is changed with a specific standardized bandwidth (hereinafter referred to as bandwidth BW). The frequency difference between the lowest frequency at 2 dB of the passband located at 2 and the highest frequency at 2 dB of the passband located at the highest frequency side is standardized at 2 GHz. The bandwidth BW is a bandwidth standardized by the center frequency of the pass band.

  Therefore, a more detailed evaluation was performed on the dependency of the variable frequency range on the resonance frequency ratio (frs / frp). The result is shown in FIG.

FIG. 7 is an explanatory diagram showing a resonance frequency ratio (frs / frp) characteristic in the variable frequency range. Since there is dependency on the bandwidth BW, FIG. 7 also shows the evaluation results regarding the conditions of the bandwidth BW = 4% and 10% in addition to the normal bandwidth BW = 6%. . Further, the result of FIG. 7 is evaluated under the condition that the coupling coefficient K ² of the resonator is 41%.

  Referring to FIG. 7, it can be seen that the variable frequency range is expanded as the resonance frequency ratio (frs / frp) increases until a predetermined resonance frequency ratio is reached. However, it can be seen that when the predetermined resonance frequency ratio is exceeded, the variable frequency range is rapidly reduced.

  For example, when the bandwidth BW = 6% (solid line), the variable frequency range is rapidly reduced when the resonance frequency ratio (frs / frp) = 1.075 is exceeded (P1). Similarly, in the case of the bandwidth BW = 4% (one-dot chain line), the variable frequency range is rapidly reduced after P1. Further, when the bandwidth BW = 10% (chain line), the variable frequency range is reduced when the resonance frequency ratio (frs / frp) = 1.1 is exceeded (P2).

  As described above, although depending on the bandwidth BW, at least when the bandwidth BW = 4%, 6%, and 10%, the variable frequency range is reduced from the point where the predetermined resonance frequency ratio (frs / frp) is exceeded. You can see that Therefore, in order to maintain a desired effective variable frequency range, it is necessary to set the resonance frequency ratio (frs / frp) within a predetermined range.

  For example, when the bandwidth BW = 10%, it is preferable to limit the resonance frequency ratio (frs / frp) to a range W1 in which the variable frequency range increases. Also, under this condition, a variable frequency range of about 10% or more is maintained for all bandwidths BW shown in the figure. At this time, the range W1 is 1.0 ≦ frs / frp ≦ 1.1.

  As another example, assuming a practical bandwidth BW = 6%, it is preferable to limit the resonance frequency ratio (frs / frp) to a range W2 in which a variable frequency range of about 13% or more can be maintained. At this time, the range W2 is 1.025 ≦ frs / frp ≦ 1.1. Further, when the resonance frequency ratio (frs / frp) is set in a range of 1.05 ≦ frs / frp ≦ 1.08, a variable frequency range of 15% or more can be secured under all conditions of a bandwidth of 4% to 10%. Therefore, it is considered more preferable.

  In order to cover the three UMTS schemes already described, a variable frequency range of at least about 18.4% is required. In order to satisfy this condition, for example, it is preferable to set the resonance frequency ratio (frs / frp) within the range W3 assuming a practical bandwidth BW = 6%. The range W3 is 1.07 ≦ frs / frp ≦ 1.08.

The above evaluation results were obtained under the condition of the resonator coupling coefficient K ² = 41%. The above properties are somewhat dependent on the coupling coefficient K ² of the resonator. Therefore, the result evaluated under the condition of the coupling coefficient K ² = 53% is also shown (FIG. 8). FIG. 8 shows resonance frequency ratio (frs / frp) characteristics in the variable frequency range when the resonator coupling coefficient K ^{2 is} 53%.

Referring to FIG. 8, even when the coupling coefficient K ² of the resonator is 53%, the resonant frequency ratio of the variable frequency range (frs / frp) characteristics shows the same tendency as in FIG. 7 In some cases, peaks (P1, P2) are observed at a certain resonance frequency ratio.

Referring to the characteristics of the range W1 to W3 with respect to the bandwidth BW = 6%, when the resonator coupling coefficient K ² = 53%, under the condition that the resonance frequency ratio is set so as to be included in the range W2, the UMTS It can be seen that the variable frequency range of about 18.4% necessary to cover the three systems is almost exceeded. Even if the error is taken into consideration, if the resonance frequency ratio (frs / frp) is set in the range of 1.025 ≦ frs / frp ≦ 1.08, the three methods can be covered almost certainly.

As already mentioned, since the variable frequency range as the coupling coefficient K ² is increased in the resonator is increased, the above-described results. Therefore, by setting so as to satisfy the preferred range W1~W3 under the conditions of the coupling coefficient K ^{2 =} 41%, desired characteristics can be obtained to a variable frequency range under conditions of the coupling coefficient K ² ≧ 41%. Examples of the resonator capable of realizing the above coupling coefficient include a boundary acoustic wave element using KNbO ₃ (Japanese Patent Application No. 2007-252925), a known surface acoustic wave element, and the like.

  The circuit configuration of the tunable filter 100 according to the present embodiment and the method for controlling the variable frequency range have been described above. The above characteristics relate to the configuration of one section shown in FIG. 3, and the configuration may be extended to a ladder-type tunable filter in which this is connected in multiple stages. Hereinafter, as the second embodiment, a technique related to the expansion will be described.

Second Embodiment
Next, the configuration of the tunable filter 200 according to the second embodiment of the present invention and the control method of the variable frequency range will be described with reference to FIG. FIG. 9 is an explanatory diagram showing the configuration of the tunable filter 200 according to the present embodiment. In the present embodiment, the tunable filter 100 according to the first embodiment is extended to a multistage configuration. Therefore, detailed description of components having substantially the same functional configuration is omitted.

  As shown in FIG. 9, the tunable filter 200 includes variable series arm resonators 102 and 202 and variable parallel arm resonators 104 and 204. The variable series arm resonator 102 includes variable capacitors 1022 and 1024 and a resonator 1026. The variable series arm resonator 202 includes variable capacitors 2022 and 2024 and a resonator 2026. Further, the variable parallel arm resonator 104 includes variable capacitors 1042 and 1044 and a resonator 1046. The variable parallel arm resonator 204 includes variable capacitors 2042 and 2044 and a resonator 2046. The resonators 1026 and 2026 are examples of series arm resonators. The resonators 1046 and 2046 are examples of parallel arm resonators. Furthermore, a unit composed of one variable series arm resonator and one variable parallel arm resonator is expressed as a basic section.

For the same reason as in the first embodiment, the ratio frs ₁ / frp ₁ between the fixed resonance frequency frs ₁ of the resonator 1026 and the fixed resonance frequency frp ₁ of the resonator 1046 is 1.0. _{<frs 1 / frp 1 <1.1,1.025} <frs 1 / frp 1 <1.1, _or, preferably a _{1.07 <frs 1 / frp 1 <} 1.08. Further, the ratio frs ₂ / frp ₂ between the fixed resonance frequency frs ₂ included in the resonator 2026 and the fixed resonance frequency frp _{2 included in} the resonator 2046 is 1.0 <frs ₂ / frp ₂ <1.1, 1 It is preferable that 0.025 <frs ₂ / frp ₂ <1.1 or 1.07 <frs ₂ / frp ₂ <1.08.

  As shown in FIG. 9, the variable capacitor 2022 is connected in series with the resonator 2026. The variable capacitor 2024 is connected in parallel to the resonator 2026. The variable capacitor 2042 is connected in series with the resonator 2046. The variable capacitor 2044 is connected in parallel to the resonator 2046. Note that the capacitances of the variable capacitors 1022, 1024, 1042, 1044, 2022, 2024, 2042, and 2044 are controlled by control means (not shown).

(Frequency characteristics of tunable filter 200)
Next, the frequency characteristics of the tunable filter 200 according to the present embodiment will be described with reference to FIG. FIG. 10 is an explanatory diagram showing frequency characteristics of the tunable filter 200 according to the present embodiment.

  Basically, it is the same as in the first embodiment, but the positions of the resonance point and antiresonance point that determine the variable frequency range are different. In the case of the configuration shown in FIG. 9, the variable frequency range TR1 is between the fixed resonance frequency frs (104) of the resonator 1046 and the fixed anti-resonance frequency fap (202) of the resonator 2026. FIG. 10 shows a variable frequency range TR2 realized by the variable series arm resonator 102 and the variable parallel arm resonator 104, and a variable frequency realized by the variable series arm resonator 202 and the variable parallel arm resonator 204. Range TR3 is also shown. Naturally, the variable frequency range TR1 having a multi-stage configuration is wider than the variable frequency ranges TR2 and TR3. Therefore, if the conditions of the ranges W1 to W3 shown in the first embodiment are applied, a desired variable frequency range can naturally be secured.

(Modification)
The tunable filter 200 according to the present embodiment can be modified to have a configuration like the tunable filter 300 shown in FIG. FIG. 11 is an explanatory diagram showing a configuration of a tunable filter 300 according to a modification of the present embodiment. Although the basic components are substantially the same as those of the tunable filter 200, the arrangement order of the variable series arm resonators 102 and 202 and the variable parallel arm resonators 104 and 204 is changed. Even if the arrangement order of the variable series arm resonators 102 and 202 and the variable parallel arm resonators 104 and 204 is changed as in the tunable filter 300, for example, the transmission characteristics substantially as shown in FIG. Since the same characteristics can be obtained, such modifications are also included in the technical scope of the present embodiment. Naturally, even if the arrangement order of the variable series arm resonators 102 and 202 and the variable parallel arm resonators 104 and 204 is changed to another order, it is substantially the same, and a modified example in which three or more basic sections are connected. Is also included in the technical scope of the present embodiment.

  As mentioned above, although preferred embodiment of this invention was described referring an accompanying drawing, it cannot be overemphasized that this invention is not limited to the example which concerns. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope of the claims, and these are naturally within the technical scope of the present invention. Understood.

It is explanatory drawing which shows the structure of a ladder type tunable filter. It is explanatory drawing which shows the frequency characteristic of a ladder type tunable filter. It is explanatory drawing which shows the tunable filter which concerns on 1st Embodiment of this invention. It is explanatory drawing which shows the frequency characteristic of the tunable filter which concerns on the same embodiment. It is explanatory drawing which shows the frequency characteristic of the tunable filter which concerns on the same embodiment. It is explanatory drawing which shows the frequency characteristic of the tunable filter which concerns on the same embodiment. It is explanatory drawing which shows the frequency characteristic of the tunable filter which concerns on the same embodiment. It is explanatory drawing which shows the frequency characteristic of the tunable filter which concerns on the same embodiment. It is explanatory drawing which shows the tunable filter which concerns on 2nd Embodiment of this invention. It is explanatory drawing which shows the frequency characteristic of the tunable filter which concerns on the same embodiment. It is explanatory drawing which shows the tunable filter which concerns on the modification of the embodiment.

Explanation of symbols

100, 200, 300 Tunable filter 102, 202 Variable series arm resonator 104, 204 Variable parallel arm resonator 1022, 1024, 1042, 1044, 2022, 2024, 2042, 2044 Variable capacitor 1026, 1046, 2026, 2046 Resonator

Claims (4)

A series arm resonator having a fixed resonant frequency frs;
A parallel arm resonator having a fixed resonant frequency frp;
The series arm resonator, and a variable capacitor connected to the parallel arm resonator;
With
The ratio frs / frp between the fixed resonance frequency frs of the series arm resonator and the fixed resonance frequency frp of the parallel arm resonator is 1.0 <frs / frp <1.1. , Tunable filter.   The ratio frs / frp between the fixed resonance frequency frs of the series arm resonator and the fixed resonance frequency frp of the parallel arm resonator is 1.025 <frs / frp <1.1. The tunable filter according to claim 1.   A ratio frs / frp between a fixed resonance frequency frs of the series arm resonator and a fixed resonance frequency frp of the parallel arm resonator is 1.07 <frs / frp <1.08. The tunable filter according to claim 2.   The basic section formed by one of the series arm resonators, one of the parallel arm resonators, and the variable capacitor is connected in two or more to be formed in a plurality of stages. The tunable filter according to any one of the above.

Images