JP4640412B2 - Elastic wave filter - Google Patents

Description

  The present invention relates to an acoustic wave filter such as a surface acoustic wave filter device or a boundary acoustic wave filter device, and more specifically, at least one inductor is disposed on a series arm connecting an input end and an output end, The present invention relates to an acoustic wave filter in which a parallel arm resonator is connected between a series arm and a ground potential.

  Conventionally, a surface acoustic wave filter has been used as an RF stage bandpass filter for communication devices such as mobile phones. For example, in Patent Document 1 below, an inductance is connected between the input and output terminals, and parallel arm resonators are provided between one end of the inductance and the ground potential and between the other end of the inductance and the ground potential, respectively. A connected bandstop filter is disclosed.

  FIG. 15A is a circuit diagram showing a circuit configuration of the band rejection filter 100. An inductor 101 is connected between the input terminal IN and the output terminal OUT. A parallel arm resonator P1 is connected between the input terminal IN and the ground potential. A parallel arm resonator P2 is connected between the output terminal OUT and the ground potential.

The parallel arm resonators P1 and P2 are each composed of a surface acoustic wave resonator formed by forming an IDT electrode on a piezoelectric substrate. Further, the resonance frequencies F1 and F2 of the parallel arm resonators P1 and P2 are substantially equal, and are located in the inhibition region. FIG. 15B is a diagram showing the frequency characteristics of the band rejection filter 100.
JP 2004-129238 A

  As shown in FIG. 15B, in the band rejection filter 100, the trap is provided in the vicinity of 880 MHz. This trap is arranged on the high band side of the pass band. That is, the band rejection filter 100 is used to provide a trap on the high band side of the pass band.

  However, in the band rejection filter 100 described in Patent Document 1, as indicated by the arrow X, the steepness of the filter characteristics in the low frequency region within the trap band is not sufficient. For this reason, there has been a problem that the insertion loss tends to increase in the vicinity of the end portion on the high band side in the pass band.

Object of the present invention to solve the problems described above, an elastic wave filter having a pass band which is provided adjacent to the low-frequency side of the trap band and the trap band, low-frequency side of the trap band To provide an acoustic wave filter in which the steepness of the filter characteristics on the frequency band side is enhanced, thereby making it possible to reduce the insertion loss in the vicinity of the high band side end in the pass band. .

According to the present invention, an elastic wave filter having a trap band and a pass band provided adjacent to the outside of the low band of the trap band, and disposed in a series arm connecting an input terminal and an output terminal At least one first inductor, at least one first parallel arm resonator provided between one end of the portion where the first inductor is provided and the ground potential, and at least one A second parallel arm resonator provided between the other end of the portion where the first inductor is provided and a ground potential, and a first inductor adjacent to the second parallel arm resonator when there are a plurality of the first inductors Between the input terminal and the trap circuit part and between the output terminal and the trap circuit part. Small A filter circuit portion provided on at least one of the series arm, and the filter circuit portion is disposed between the input terminal or the output terminal and the trap circuit portion in the series arm; and A second inductor connected between one end or the other end of the series arm resonator and a ground potential, and the resonance frequency of the series arm resonator is a high-frequency side end of the passband of the acoustic wave filter An elastic wave filter is provided, characterized in that it is substantially matched with

  In a specific aspect of the acoustic wave filter according to the present invention, the filter circuit portion is provided both between the input terminal and the trap circuit portion and between the output terminal and the trap circuit portion.

  In another specific aspect of the present invention, a resonance frequency of a resonance circuit including the capacitance of the series arm resonator and the second inductor is lower than a lower end of the pass band. Has been.

  According to still another specific aspect of the acoustic wave filter according to the present invention, the acoustic wave filter further includes a piezoelectric substrate on which the parallel arm resonator and the series arm resonator are formed, and the piezoelectric substrate is provided on or in the piezoelectric substrate. First and second inductors are formed.

In still another specific aspect of the acoustic wave filter according to the present invention, the capacitance of the series arm resonator is Cs, the inductance value of the first inductor is Ls, the capacitance of the parallel arm resonator is Cp, and the second inductor Cs / Cp> 1 and Lp / Ls> 1 where Lp is the inductance value.
(The invention's effect)

  In the elastic wave filter according to the present invention, the trap circuit portion is provided in the case where there are a plurality of the at least one inductor, the first parallel arm resonator, the second parallel arm resonator, and the inductor. The parallel arm resonators of the first and second parallel arm resonators or the first to third parallel arm resonators have a large attenuation and form traps.

  However, steepness is not sufficient in the frequency region on the low frequency side in the trap formed in this way. Therefore, in the present invention, the filter circuit portion having the second inductance and the series arm resonator is provided between at least one of the input terminal and the trap circuit portion and between the output terminal and the trap circuit portion. The resonance frequency of the series arm resonator is substantially coincident with the high band side end of the passband of the acoustic wave filter. Therefore, the steepness of the filter characteristics in the low frequency range within the trap band can be enhanced, and the insertion loss near the high band side end in the pass band of the acoustic wave filter can be reduced.

  That is, the impedance of the series arm resonator is an inductive impedance between the resonance frequency and the anti-resonance frequency in the lower frequency range within the trap band. The value of the inductive impedance varies greatly depending on the frequency. Therefore, by making the resonance frequency of the series arm resonator substantially coincide with the end of the pass band on the high band side, the steepness of the filter characteristics on the low band side in the trap band can be effectively enhanced, and It is possible to reduce the insertion loss in the vicinity of the end portion on the high side of the passband.

Therefore, according to the present invention, in the acoustic wave filter in which the pass band is provided adjacent to the low band outside of the trap band, the steepness of the low frequency band in the trap band is increased, and the high band side in the pass band is increased. The insertion loss in the vicinity of the end can be effectively reduced, and an elastic wave filter having good filter characteristics can be provided.

  In the present invention, when the filter circuit part is provided both between the input terminal and the trap circuit part and between the output terminal and the trap circuit part, the high frequency side in the pass band It becomes possible to further effectively reduce the insertion loss in the vicinity of the end.

  In the present invention, when the resonance frequency of the resonance circuit composed of the capacitance of the series arm resonator and the second inductor is lower than the lower end of the pass band, the pass band It is possible to make the attenuation amount in a lower frequency range sufficiently large.

  In the case where a piezoelectric substrate on which a parallel arm resonator and a series arm resonator are formed is further provided and the first and second inductors are formed on or in the piezoelectric substrate, the first and second Since the inductor is formed on or in the piezoelectric substrate, the acoustic wave filter can be reduced in size.

  When the capacitance of the series arm resonator is Cs, the first inductance is Ls, the capacitance of the parallel arm resonator is Cp, and the inductance of the second inductor is Lp, Cs / Cp> 1 and Lp / Ls> 1. In this case, it is possible to provide an acoustic wave filter with a small insertion loss.

FIG. 1 is a circuit diagram of a surface acoustic wave filter according to a first embodiment of the present invention. FIG. 2A is a diagram illustrating a circuit configuration of a low-pass LC filter including a series inductor and a parallel capacitor, and FIG. 2B is a diagram schematically illustrating the frequency characteristics thereof. FIG. 3A is a diagram illustrating a circuit configuration of an LC filter including a series capacitor and a parallel inductance, and FIG. 3B is a diagram schematically illustrating the frequency characteristics thereof. FIG. 4 is a diagram illustrating impedance characteristics of the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 used in the first embodiment. FIG. 5 is a diagram showing the frequency characteristics of the trap circuit portion used in the first embodiment and the frequency characteristics of the filter circuit portion composed of the series arm resonator and the second inductance, as solid lines or broken lines, respectively. is there. FIG. 6 is a diagram illustrating frequency characteristics of the surface acoustic wave filter according to the first embodiment. FIG. 7 is a schematic diagram illustrating a structure in which the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 are formed on the lower surface of the piezoelectric substrate in the surface acoustic wave filter according to the first embodiment, with the piezoelectric substrate being seen through. FIG. FIG. 8 is a plan view schematically showing the electrode structure of the surface wave resonator. FIG. 9 is a plan view of a package substrate on which the piezoelectric substrate shown in FIG. 7 is mounted. FIG. 10A is a schematic plan view for explaining an inductor externally attached to a piezoelectric substrate, and FIG. 10B schematically shows a structure in which a surface acoustic wave chip is mounted on a package substrate. It is a typical front sectional view shown. FIG. 11 is a plan view schematically showing a structure in which the first and second inductances are formed as electrodes on the piezoelectric substrate in the first embodiment, and schematically shows the electrode structure on the lower surface of the piezoelectric substrate. FIG. FIG. 12 is a circuit diagram of a surface acoustic wave filter according to the second embodiment of the present invention. FIG. 13 is a diagram illustrating frequency characteristics of the surface acoustic wave filter according to the second embodiment. FIG. 14 is a circuit diagram for explaining a modification of the surface acoustic wave filter of the present invention. FIG. 15A is a circuit diagram for explaining an example of a conventional surface acoustic wave filter, and FIG. 15B is a diagram showing frequency characteristics thereof.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Surface acoustic wave filter 11 ... Surface acoustic wave chip 12 ... Piezoelectric substrate 13a ... IDT electrode 13b, 13c ... Reflector 14a-14f ... Metal bump 15 ... Package substrate 16a-16e ... Electrode land 21 ... Surface acoustic wave filter L1 ... 1st inductance P1, P2 ... 1st, 2nd parallel arm resonator P3 ... 3rd parallel arm resonator S1, S2 ... Series arm resonator L2a, L2b ... 2nd inductance

  The present invention will be clarified by describing specific embodiments of the present invention with reference to the drawings.

  FIG. 1 is a circuit diagram of a surface acoustic wave filter according to a first embodiment of the present invention. In the surface acoustic wave filter 1 of the present embodiment, the first inductor L1 is connected between the input terminal IN and the output terminal OUT. A first parallel arm resonator P1 is connected between one end of the first inductor L1 and the ground potential, and a second parallel arm resonator is connected between the other end of the first inductor L1 and the ground potential. P2 is connected. The trap circuit portion A having the first inductor L1 and the first and second parallel arm resonators P1 and P2 is substantially the same as the conventional band rejection filter 100 shown in FIG.

  In the surface acoustic wave filter 1 according to the present embodiment, inductors L2a and L2b are connected in series to both sides of a trap circuit portion A composed of the first inductor L1 and the first and second parallel arm resonators P1 and P2. Filter circuit portions B1 and B2 including arm resonators S1 and S2 are provided.

  That is, the series arm resonator S1 is connected between the input terminal IN and the first inductor L1, and the second point is between the connection point between the input terminal IN and the series arm resonator S1 and the ground potential. Inductor L2a is connected.

  The series arm resonator S2 is also connected between the first inductor L1 and the output terminal OUT, and between the connection point between the output terminal OUT and the series arm resonator S2 and the ground potential. The second inductor L2b is connected.

  In the surface acoustic wave filter 1, the series arm resonator S1, the first inductor L1, and the series arm resonator S2 are arranged in this order on the series arm that connects the input terminal IN and the output terminal OUT. It will be.

  By the way, as shown in FIG. 2A, when an inductance Lx is connected in series between the input terminal IN and the output terminal OUT, and a parallel capacitor C1 is connected between the input terminal IN and the ground potential, A low-pass band type filter characteristic as shown in FIG. 2B is obtained by the LC circuit of the inductance Lx and the capacitor C1.

  On the other hand, as shown in FIG. 3A, when a series capacitor C2 is connected in series between the input terminal IN and the output terminal OUT, and an inductance Ly is connected between the input terminal IN and the ground potential. The LC circuit composed of the capacitor C2 and the inductance Ly provides the high-pass band type filter characteristics shown in FIG.

  In the surface acoustic wave filter 1 shown in FIG. 1, the low-pass band-type filter includes the inductances of the inductors L1, L2a, and L2b and the capacitances of the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2, respectively. Alternatively, a high-pass filter is configured, and as a whole, a characteristic having a pass band is obtained.

  More specifically, by the trap circuit portion A having the first inductor L1 and the parallel arm resonators P1 and P2, as in the conventional band-stop filter 100 shown in FIG. A pass band outside the low band is formed.

  In the present embodiment, the filter circuit portion including the series arm resonator S1 and the second inductor L2a and the filter circuit portion B2 including the series arm resonator S2 and the second inductor L2b are included in the trap circuit portion A. Since the resonance frequencies of the series arm resonators S1 and S2 are connected to both sides and substantially coincide with the high band side end of the pass band of the surface acoustic wave filter 1, the low band frequency band within the trap band And the insertion loss at the end of the high passband side is effectively reduced.

  As an example of the surface acoustic wave filter 1, let us consider an example applied to a surface acoustic wave filter for a terrestrial digital one-segment tuner mounted on a mobile phone. In this case, the pass band is 470 to 770 MHz, and it is necessary to configure the transmission band of the mobile phone 830 to 845 MHz as the trap band on the higher frequency side than the pass band. When a surface acoustic wave filter having such a trap band and a pass band is configured, the impedance characteristics shown in FIG. 4 of the series arm resonators S1, S2 and the parallel arm resonators P1, P2 are used.

  In the impedance characteristics of the surface acoustic wave resonator, in a frequency range lower than the resonance frequency, it becomes a capacitive impedance, and the impedance changes gently with respect to the change in frequency. On the other hand, in the frequency region between the resonance frequency and the anti-resonance frequency, the impedance becomes an inductive impedance, and the impedance changes greatly with respect to the change in frequency.

  In the trap circuit portion A, the low band side in the trap band is configured by using impedance characteristics in a frequency band lower than the resonance frequency of the parallel arm resonators P1 and P2, that is, capacitive impedance. Therefore, the steepness in the low frequency region in the trap band is not sufficient, and the conventional surface acoustic wave filter 100 tends to have a large insertion loss in the vicinity of the end portion on the high band side in the pass band. .

  On the other hand, in the present embodiment, the resonance frequencies of the series arm resonators S1 and S2 are substantially matched with the end portion on the high side of the passband. Therefore, the impedances of the series arm resonators S1 and S2 are inductive impedances in the low frequency range within the trap band, and the impedance changes greatly with respect to the change. Therefore, the steepness of the filter characteristics in the low frequency region in the trap band can be enhanced, and the insertion loss of the surface acoustic wave filter 1 in the vicinity of the end on the high frequency band side in the pass band can be significantly reduced.

  When the resonance resistance of the resonator is small, the capacitive impedance and the inductive impedance can be regarded as capacitive and inductive reactance, respectively. Therefore, in the low frequency region within the trap band, the voltage at the connection point between the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 is the voltage applied to the input terminal × [parallel arm resonator P1. Or, P2 capacitive reactance / (capacitive reactance of parallel arm resonator P1 or P2 + inductive reactance of series arm resonator S1 or S2)]. Since the capacitive reactance and the inductive reactance have opposite polarities, it can be seen that the voltage at the connection point is equal to or higher than the input terminal voltage, and insertion loss on the high passband side can be reduced.

  Since the impedance values of the inductors L1, L2a, and L2b are large impedances in the trap band, the influence of the impedances of the inductors L1, L2a, and L2b can be ignored.

  More specifically, the solid line C in FIG. 5 shows the characteristics of the trap circuit portion A composed of the inductor L1 and the parallel arm resonators P1 and P2. On the other hand, the characteristics of the high-pass filter circuit portions B1 and B2 constituted by the capacitance of the series arm resonator S1 or S2 and the inductor L2a or L2b are indicated by a broken line D. The cut-off frequency in the frequency characteristics of the filter circuit portions B1 and B2 is a resonance frequency due to resonance between the capacitance of the series arm resonator S1 or S2 and the inductance of the inductor L2a or L2b. By making the resonance frequency fr of the series arm resonator S1 or S2 substantially coincide with the end portion on the high side of the pass band, the steepness in the low frequency range in the trap band can be enhanced, and the pass band in the pass band can be increased. The insertion loss in the vicinity of the high frequency side end can be reduced.

  FIG. 6 is a diagram showing attenuation frequency characteristics indicating the characteristics of the surface acoustic wave filter 1. The characteristics shown in FIG. 6 are as follows. The equivalent circuit constants of the resonators S1 and S2 and the parallel arm resonators P1 and P2 are as shown in Table 1 below, the inductance value of the inductor L1 is 13 nH, and the inductor L2a. The inductance value is 27 nH, and the inductance value of the inductor L2b is 27 nH.

  As can be seen from FIG. 6, since the attenuation in the trap band can be 50 dB or more and the steepness in the low frequency region in the trap band can be increased, the insertion loss is 2 dB or less over the entire pass band. It can be understood that

  In the surface acoustic wave filter 1 of this embodiment, preferably, the capacitances of the series arm resonators S1 and S2 are Cs, the inductance value of the first inductor L1 is Ls, and the capacitances of the parallel arm resonators P1 and P2 are respectively set. When Cp and the inductance value of the second inductor are set to Lp, Cs / Cp> 1 and Lp / Ls> 1 are set, whereby the insertion loss can be further reduced. This will be described below.

  Now, the resonance frequency of the LC resonance circuit composed of the inductor L1 and the parallel arm resonators P1 and P2 is higher than the resonance frequency of the LC circuit composed of the series arm resonator S1 or S2 and the second inductor L2a or L2b. It has become. Therefore, the following formula (1) is established.

  The characteristic impedance of the trap circuit portion A and the filter circuit portions B1 and B2 are both 50Ω. Therefore, the following formula (2) is established.

  When formula (1) is transformed,

Thus, when equation (2) is modified, Ls / Cp = Lp / Cs. Therefore, when Lp = (Cs / Cp) · Ls and Cp = Cs · (Ls / Lp) are substituted into Equation (3), CsCp> 1 and Lp / Ls> 1.

  Therefore, by setting Cs / Cp> 1 and Lp / Ls> 1, the characteristic impedances of the trap circuit portion A and the filter circuit portions B1 and B2 are made uniform, and the insertion loss can be reduced.

  In the above embodiment, the resonance frequencies of the series arm resonators S1 and S2 are substantially coincident with the end portion on the high side of the passband, but it is not always necessary to completely coincide with the coincidence. The frequency at the end of the region should be in the range of -5% to 0%. In addition, in order to reduce the insertion loss in the passband to 2 dB or less, it has been confirmed by the inventor's experiment that the frequency at the end of the passband high band side may be in the range of -5% to 0%. .

  Therefore, the resonance frequency of the series arm resonators S1 and S2 is preferably in the range of -5% or more and 0% or less of the frequency at the high band side end of the pass band of the surface acoustic wave filter 1.

  In the above embodiment, the filter circuit portions B1 and B2 are connected to both the input side and the output side of the trap circuit portion A, but the filter circuit portion may be connected to only one of them. That is, only the filter circuit portion B1 including the series arm resonator S1 and the inductor L2a may be connected to the trap circuit portion A, or only the filter circuit portion B2 including the series arm resonator S2 and the inductor L2b may be trapped. It may be connected to the circuit part A.

  In the surface acoustic wave filter 1 of the present embodiment, the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 preferably form interdigital electrodes (IDT electrodes) on a single piezoelectric substrate. It is comprised by the surface acoustic wave resonator comprised by these. FIG. 7 is a plan view schematically showing the electrode structure formed on the lower surface of the piezoelectric substrate 12 in the surface acoustic wave chip 11 through the piezoelectric substrate 12.

  The piezoelectric substrate 12 is made of ceramics or a piezoelectric single crystal, and IDT electrodes are formed on the lower surface thereof to form series arm resonators S1 and S2 and parallel arm resonators P1 and P2. FIG. 7 schematically shows a portion where the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 are formed. FIG. 8 schematically shows a specific structure of the IDT electrode. Show. As shown in FIG. 8, reflectors 13b and 13c are formed on both sides of the IDT electrode 13a to constitute one surface acoustic wave resonator. Each of the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 includes an IDT electrode and reflectors disposed on both sides of the IDT electrode. However, as shown in Table 1 described above, the specifications of the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 are different depending on the required characteristics.

  The surface acoustic wave chip 11 is provided with metal bumps 14a to 14f. The surface acoustic wave chip 11 is mounted on the package substrate 15 by the flip chip bonding method using the metal bumps 14a to 14f. The package substrate 15 is made of an appropriate insulating material, and has electrode lands 16a to 16e on the upper surface thereof. The metal bump 14a is connected to the electrode land 16a, the metal bump 14b is connected to the electrode land 16b, the metal land 14c is connected to the ground potential, the metal bumps 14c and 14d are connected to the electrode land 16d, and the metal bump 14f is connected to the electrode land 16c. 16e, respectively.

  FIG. 10B is a schematic front sectional view of a structure in which the surface acoustic wave chip 11 is mounted on a package substrate 15.

  As schematically shown in FIG. 10A, external elements constituting the inductors L1, L2a, and L2b are appropriately connected to the electrode lands 16a to 16e outside the package substrate 15.

  In the surface acoustic wave chip 11 shown in FIG. 7, only the series arm resonators S1 and S2 and the parallel arm resonators P1 and P2 made of surface acoustic wave resonators are formed on the piezoelectric substrate 12, but FIG. As shown in the figure, the inductors L1, L2a, and L2b may also be provided by forming electrodes having a meandering shape, that is, a meandering shape, on the lower surface of the piezoelectric substrate 12.

  FIG. 11 is a plan view schematically showing electrodes formed on the lower surface of the piezoelectric substrate 12 through the piezoelectric substrate 12, as in FIG. 7. Here, the inductors L1, L2a, and L2b are each configured by forming electrodes in a meander shape. The inductors L1, L2a, and L2b are not necessarily configured by forming a meandering electrode pattern on the lower surface of the piezoelectric substrate 12. For example, a coiled electrode pattern may be formed on the lower surface of the piezoelectric substrate 12.

  As shown in FIG. 11, the surface acoustic wave filter 1 can be miniaturized by forming the inductors L1, L2a, and L2b by applying electrode materials integrally to the surface of the piezoelectric substrate.

  FIG. 12 is a circuit diagram of a surface acoustic wave filter according to the second embodiment of the present invention. The surface acoustic wave filter 21 according to the second embodiment includes a trap circuit portion A configured similarly to the trap circuit portion A illustrated in FIG. 1 between the input terminal IN and the output terminal OUT. The trap circuit portion A includes a first inductor L1, a parallel arm resonator P1 connected between one end of the first inductor L1 and the ground potential, and the other end of the first inductor L2 and the ground potential. A parallel arm resonator P2 connected in between.

  In the second embodiment, on both sides of the trap circuit portion A, the filter circuit portion B3 and the series arm resonator S12 including the series arm resonator S11 and the second inductor L2a and the second inductor L2b are provided. The filter circuit portions B4 are connected to each other. The difference from the first embodiment is that one ends of the inductors L2a and L2b are connected to connection points 22 and 23 between the series arm resonators S11 and S12 and one end or the other end of the first inductor L1. There is.

  Also in the present embodiment, the resonance frequency of the series arm resonators S11 and S12 is made to substantially coincide with the end portion on the high band side of the passband, so that the resonance frequency between the resonance frequency and the antiresonance frequency of the series arm resonators S11 and S12 By using the inductive impedance, the steepness of the filter characteristics in the low frequency region of the trap band can be enhanced. Therefore, it is possible to reduce the insertion loss in the vicinity of the end portion on the high band side in the pass band.

  As apparent from the first and second embodiments, the second inductor in the filter circuit portion is connected to the series arm resonator as long as it is connected between one end or the other end of the series arm resonator and the ground potential. It may be connected to either one end or the other end.

  FIG. 13 is a diagram showing the frequency characteristics of the surface acoustic wave filter 21 shown in FIG. As is apparent from FIG. 13, also in the present embodiment, in the pass characteristics, the steepness in the low frequency region within the trap band is enhanced, and the insertion loss near the high frequency side end in the pass band is small. It can be seen that the insertion loss is 2 dB or less over the entire passband. The equivalent circuit constants of the series arm resonators S11 and S12 and the parallel arm resonators P1 and P2 are as shown in Table 2 below. The inductance value of the inductor L1 is 11 nH, the inductance value of the inductor L2a is 16 nH, and the inductance value of the inductor L2b is 13 nH.

  In the surface acoustic wave filters 1 and 21 of the first and second embodiments shown in FIGS. 1 and 12, the trap circuit portion A is connected to the first inductor L1 and both ends of the first inductor L1. Although the parallel arm resonators P1 and P2 are provided, the trap circuit portion may have a structure in which a plurality of first inductors are connected.

  That is, as in the modification shown in FIG. 14, the trap circuit portion A may include first inductors L1a, L1b, and L1c. In this case, the third parallel arm resonator P3 is connected between the adjacent inductors L1a and L1b, between the adjacent inductors L1b and L1c, and the ground potential.

  Thus, in the present invention, the number of stages of the LC circuit including the inductor and the parallel arm resonator in the trap circuit portion including the first inductor and the parallel arm resonator is not particularly limited.

  Moreover, although the said embodiment demonstrated the surface acoustic wave filter using a surface acoustic wave, this invention can also be applied to the boundary acoustic wave filter using a boundary acoustic wave.

Claims (5)

An acoustic wave filter having a trap band and a pass band provided adjacent to the lower outside of the trap band,
Provided between at least one first inductor disposed in a series arm connecting the input terminal and the output terminal, and one end of a portion where at least one first inductor is provided, and a ground potential. A first parallel arm resonator, a second parallel arm resonator provided between the other end of the portion where at least one of the first inductors is provided and a ground potential, and the first A plurality of inductors, a trap circuit portion having a third parallel arm resonator connected between adjacent first inductors and a ground potential;
A filter circuit portion provided between at least one of the input terminal and the trap circuit portion and between the output terminal and the trap circuit portion;
The filter circuit portion is a series arm resonator disposed between the input terminal or the output terminal and the trap circuit portion in the series arm;
A second inductor connected between one end or the other end of the series arm resonator and a ground potential;
The elastic wave filter according to claim 1, wherein a resonance frequency of the series arm resonator is substantially coincident with a high band side end of a pass band of the elastic wave filter.   The acoustic wave filter according to claim 1, wherein the filter circuit portion is provided both between the input terminal and the trap circuit portion and between the output terminal and the trap circuit portion.   The resonance frequency of the resonance circuit comprised by the capacity | capacitance of the said series arm resonator and the said 2nd inductor is made into the frequency lower than the low-frequency side edge part of the said pass band. Elastic wave filter. A piezoelectric substrate on which the parallel arm resonator and the series arm resonator are formed;
The elastic wave filter according to any one of claims 1 to 3, wherein the first and second inductors are formed on or in the piezoelectric substrate.   When the capacitance of the series arm resonator is Cs, the inductance value of the first inductor is Ls, the capacitance of the parallel arm resonator is Cp, and the inductance value of the second inductor is Lp, Cs / Cp> The elastic wave filter according to claim 1, wherein 1 and Lp / Ls> 1.

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