WO2007023643A1

WO2007023643A1 - Elastic wave filter

Info

Publication number: WO2007023643A1
Application number: PCT/JP2006/315098
Authority: WO
Inventors: Kazumasa Haruta
Original assignee: Murata Manufacturing Co., Ltd.
Priority date: 2005-08-23
Filing date: 2006-07-31
Publication date: 2007-03-01
Also published as: JPWO2007023643A1; JP4640412B2

Abstract

There is provided an elastic wave filter having a passing band at a lower outer side of the trap band. Steepness of the filter characteristic in the low frequency range in the trap band is increased and the insert loss at the pass band high range side end portion is sufficiently reduced. The elastic wave filter (1) includes: a trap circuit section (A) having a first inductance L1 and a first and a second parallel arm resonator (P1, P2); and a filter circuit section having series arm resonators (S1, S2) and a second inductance (L2a or L2b), and connected between an input terminal and/or an output terminal and the trap circuit section (A). The series arm resonators (S1, S2) have a resonance frequency which substantially coincides with the pass band high range side end portion.

Description

Specification

Elastic wave filter

Technical field

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 provided in 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.

Background art

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 resonance is performed between one end of the inductance and the ground potential and between the other end of the inductance and the ground potential. A bandstop filter with a child connected 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 OU T and the ground potential.

[0004] The parallel arm resonators PI 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 PI and P2 are substantially equal to each other and are located in the stop band. FIG. 15B is a diagram showing the frequency characteristics of the band rejection filter 100.

Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-129238

Disclosure of the invention

[0005] As shown in Fig. 15 (b), in the band rejection filter 100, a 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 frequency side of the pass band. [0006] 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 band within the trap band is not sufficient. . For this reason, there is a problem that the insertion loss tends to be large in the vicinity of the high end of the passband in the passband.

[0007] An object of the present invention is an elastic wave filter that eliminates the above-mentioned drawbacks of the prior art and has a pass band and a pass band provided on the low band side of the trap band. To provide an acoustic wave filter in which the steepness of the filter characteristic on the frequency band side is enhanced, thereby making it possible to reduce the insertion loss in the vicinity of the end on the high band side in the passband. It is in.

[0008] According to the present invention, the acoustic wave filter has a trap band and a pass band provided outside the low band of the trap band, and is disposed on a series arm that connects the input terminal and the output terminal. At least one first inductor and at least one first parallel arm resonator provided between one end of the portion where the first inductor is provided and a ground potential; and 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 plurality of the first inductors are adjacent to each other. A trap circuit portion having a third parallel arm resonator connected between the inductors of 1 and a ground potential, between the input terminal and the trap circuit portion, and between the output terminal and the trap circuit portion. At least between A filter circuit part provided on the other side, and the filter circuit part is arranged in the series arm between the input terminal or the output terminal and the trap circuit part, and A second arm 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 the elastic wave filter is substantially matched.

[0009] 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. It has been.

[0010] 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 an end portion on a lower band side of the passband. Low It is said to be a frequency.

[0011] 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. The first and second inductors are formed in the piezoelectric substrate.

[0012] 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, When the inductance value of inductor 2 is Lp, CsZCp> l and LpZLs> l.

(The invention's effect)

[0013] In the elastic wave filter according to the present invention, the trap circuit portion includes the at least one inductor, the first parallel arm resonator, the second parallel arm resonator, and a plurality of inductors. The third parallel arm resonator is provided, and at the resonance frequency of the first and second parallel arm resonators or the first to third parallel arm resonators, the amount of attenuation increases and a trap is formed. It is.

[0014] However, the steepness is not sufficient in the low frequency region 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 elastic wave filter. Accordingly, the steepness of the filter characteristics in the low frequency band within the trap band can be enhanced, and the insertion loss near the high band side end in the pass band of the elastic 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 portion on the high band side of the pass band, the steepness of the filter characteristics on the low band side within the trap band can be effectively enhanced, and the frequency within the pass band can be effectively increased. It is possible to reduce the insertion loss in the vicinity of the end portion on the high side of the passband.

[0016] Therefore, according to the present invention, the bullet having the pass band provided outside the lower band of the trap band. In an acoustic wave filter, it is possible to increase the steepness of the low frequency region in the trap band, effectively reduce the insertion loss in the vicinity of the high-frequency side end in the pass band, and to provide an elastic wave having good filter characteristics. A filter can be provided.

In the present invention, when 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, It becomes possible to further effectively reduce the insertion loss in the vicinity of the high frequency side end.

[0018] In the present invention, the resonance frequency of the resonance circuit composed of the capacitance of the series arm resonator and the second inductor is set to be lower than that of the low-frequency side end of the pass band. In this case, the amount of attenuation in the frequency range lower than the pass band can be made sufficiently large.

[0019] 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. Since the first and second inductors are formed on or in the piezoelectric substrate, the acoustic wave filter can be reduced in size.

[0020] 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, CsZCp> l and LpZLs> l. In such a case, it is possible to provide an elastic 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. 2 (a) is a diagram showing a circuit configuration of a low-pass LC filter composed of a series inductor and a parallel capacitor, and (b) is a diagram schematically showing its frequency characteristics.

[FIG. 3] FIG. 3 (a) is a diagram showing a circuit configuration of an LC filter composed of a series capacitance and a parallel inductance, and (b) is a diagram schematically showing its frequency characteristics.

FIG. 4 is a diagram showing impedance characteristics of series arm resonators SI and S2 and parallel arm resonators PI and P2 used in the first embodiment.

FIG. 5 shows the frequency characteristics of the trap circuit used in the first embodiment and the direct Each of the frequency characteristics of the filter circuit portion composed of the column arm resonator and the second inductance has a solid line! Is a diagram indicated by a broken line.

FIG. 6 is a diagram showing frequency characteristics of the surface acoustic wave filter according to the first embodiment.

FIG. 7 shows a structure in which the series arm resonators SI and S2 and the parallel arm resonators PI and P2 are formed on the lower surface of the piezoelectric substrate in the surface acoustic wave filter according to the first embodiment. It is a typical top view shown with permeability.

FIG. 8 is a plan view schematically showing an electrode structure of a surface wave resonator.

FIG. 9 is a plan view of a knock board on which the piezoelectric substrate shown in FIG. 7 is mounted.

[FIG. 10] FIG. 10 (a) is a schematic plan view for explaining an inductor externally attached to a piezoelectric substrate, and FIG. 10 (b) is a surface acoustic wave chip mounted on a cage substrate. FIG. 3 is a schematic front sectional view schematically showing the structure.

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. FIG. 3 is a plan view schematically showing an electrode structure.

FIG. 12 is a circuit diagram of a surface acoustic wave filter according to a second embodiment of the present invention.

FIG. 13 is a diagram showing 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. 15 (a) is a circuit diagram for explaining an example of a conventional surface acoustic wave filter, and FIG. 15 (b) is a diagram showing its frequency characteristics.

Explanation 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, Ρ2 ···· First and second parallel arm resonators

Ρ3 ··· 3rd parallel arm resonator

SI, S2 series arm resonator

L2a, L2b ... Second inductance

BEST MODE FOR CARRYING OUT THE INVENTION

[0023] 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 according to this embodiment, the first inductor L 1 is connected between the input terminal IN and the output terminal OUT. The first parallel arm resonator P1 is connected between one end of the first inductor L1 and the ground potential, and the second parallel arm is connected between the other end of the first inductor L1 and the ground potential. Resonator P2 is connected. The trap circuit portion A having the first inductor L1 and the first and second parallel arm resonators PI and P2 is substantially the same as the conventional band rejection filter 100 shown in FIG.

[0025] In the surface acoustic wave filter 1 of the present embodiment, inductors L2a, L2a are arranged on both sides of a trap circuit portion A composed of the first inductor L1 and the first and second parallel arm resonators PI, P2. Filter circuit parts Bl and B2 consisting of L2b and series arm resonators SI and S2 are provided.

That is, the series arm resonator S 1 is connected between the input terminal IN and the first inductor L 1, and the connection point between the input terminal IN and the series arm resonator S 1 and the ground potential The second inductor L2a is connected between the two.

[0027] The series arm resonator S2 is also connected between the first inductor L1 and the output terminal OUT, and the connection point between the output terminal OUT and the series arm resonator S2 and the ground potential are connected. of A second inductor L2b is connected between them.

In the surface acoustic wave filter 1, the series arm resonator Sl, the first inductor L1, and the series arm resonator S2 are arranged in this order on the series arm connecting the input terminal IN and the output terminal OUT. Will be placed.

[0029] By the way, as shown in Fig. 2 (a), 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. When connected, an LC circuit with an inductance Lx and a capacitance C1 provides low-pass filter characteristics as shown in Fig. 2 (b).

On the other hand, as shown in FIG. 3 (a), 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. In such a case, the high-pass filter characteristics shown in Fig. 3 (b) can be obtained by the LC circuit consisting of the capacitor C2 and the inductance Ly.

[0031] In the surface acoustic wave filter 1 shown in FIG. 1, the low-pass filter includes the inductances of the inductors LI, L2a, and L2b and the capacitances of the series arm resonators SI and S2 and the parallel arm resonators PI and P2, respectively. A band-pass filter or a high-pass filter is configured, and as a whole, characteristics having a pass band can be obtained.

More specifically, the trap band portion A having the first inductor L 1 and the parallel arm resonators PI and P 2 is used in the trap band as in the conventional band-stop filter 100 shown in FIG. As a result, a pass band outside the lower band of the trap band is formed.

[0033] 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 described above. Since it is connected to both sides of the trap circuit part A and the resonance frequency of the series arm resonators SI and S2 is substantially the same as the high band side end of the pass band of the surface acoustic wave filter 1, The steepness of the filter characteristics in the low frequency band is enhanced, and the insertion loss at the end of the high pass band is effectively reduced.

As an example of the surface acoustic wave filter 1, let us consider an example in which the present invention is applied to a surface acoustic wave filter for a terrestrial digital one-segment tuner mounted on a mobile phone. In this case, the passband is 470 to 770 MHz, and the mobile phone transmission band is higher than the passband. 830 to 845 MHz must be configured as the trap 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 SI and S2 and the parallel arm resonators P 1 and P2 are used.

[0035] 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.

[0036] In the trap circuit portion A, the low frequency side in the trap band is configured using the impedance characteristic in the frequency range lower than the resonance frequency of the parallel arm resonators PI and P2, that is, the capacitive impedance. Become. For this reason, in the conventional surface acoustic wave filter 100 in which the steepness in the low frequency region within the trap band is not sufficient, the insertion loss tends to be large near the high band side end of the pass band. .

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

[0038] 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, the voltage at the connection point between the series arm resonators SI and S2 and the parallel arm resonators P1 and P2 in the low frequency range within the trap band is the voltage X [parallel arm resonance The capacitive reactance Z of the child P1 or P2 (capacitive reactance of the parallel arm resonator P1 or P2 + inductive reactance of the series arm resonator S1 or S2)]. Since the capacitive reactance and the inductive reactance have opposite polarities, the voltage at the above-mentioned connection point is higher than the input terminal voltage, which indicates that the insertion loss on the high passband side can be reduced. [0039] Since the impedance values of the inductors LI, L2a, and L2b have a large impedance in the trap band, the influence of the impedances of the inductors LI, 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 PI and P2. In contrast, the characteristics of the high-pass filter circuit parts Bl and B2 composed of the capacitance of the series arm resonator S1 or S2 and the inductor L2a or L2b are indicated by a broken line D. The cutoff frequency in the frequency characteristics of the filter circuit parts Bl 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 of the high side of the pass band, the steepness in the low frequency range within the trap band can be increased and the pass in the pass band can be increased. The insertion loss in the vicinity of the band high band 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 SI and S2 and the parallel arm resonators PI and P2 are as shown in Table 1 below, and the inductance value of the inductor L1 is 13 nH. The inductance value of L2a is 27nH, and the inductance value of inductor L2b is 27nH.

[0042] [Table 1]

[0043] As is apparent from Fig. 6, the attenuation in the trap band can be set to 50 dB or more, and the steepness in the low frequency region within the trap band can be increased. It can be seen that the loss can be 2 dB or less.

In the surface acoustic wave filter 1 of the present embodiment, preferably, the capacitance of the series arm resonators SI and S2 is Cs, the inductance value of the first inductor L1 is Ls, and the parallel arm resonators PI and P2 When the capacitance is Cp and the inductance value of the second inductor is Lp, Cs / Cp> 1 and LpZLs> l, so that the insertion loss can be further reduced. The This will be described below.

[0045] Now, the resonance frequency of the LC resonance circuit composed of the inductor L1 and the parallel arm resonators PI and P2 is the resonance frequency of the LC circuit composed of the series arm resonator S1 or S2 and the second inductor L2a or L2b. Is getting higher than. Therefore, the following formula (1) is established.

[0046] [Equation 1]

• ■ Formula (1)

[0047] The characteristic impedances of the trap circuit portion A and the filter circuit portions Bl and B2 are both 50 Ω. Therefore, the following formula (2) is established.

[0048] [Equation 2]

[0049] By transforming equation (1),

[0050] [Equation 3] ((L p ■ Cs) / (L s ■ Cp))> 1 ■ ■ ■ Equation (3)

[0051] When Equation (2) is transformed, LsZCp = Lp / Cs. Therefore, substituting Lp = (Cs / Cp) -Ls and Cp = Cs ′ (Ls / Lp) into equation (3) results in CsCp> l and LpZLs> l.

[0052] Therefore, by setting CsZCp> l and LpZLs> l, the characteristic impedances of the trap circuit portion A and the filter circuit portions Bl and B2 can be made uniform, and the insertion loss can be reduced.

[0053] In the above embodiment, the resonance frequency of the series arm resonators Sl, S2 is not necessarily the same as the force that is substantially coincident with the end of the passband high band side, and substantially coincidence. The frequency at the end of the high side of the pass band is within the range of 5% or more and 0% or less. In addition, in order to reduce the insertion loss in the passband to 2 dB or less, it has been confirmed by the inventors' experiment that the frequency at the end of the high end of the passband should be in the range of 5% to 0%. . Therefore, preferably, the resonance frequency of the series arm resonators SI and S2 is in the range of 5% or more and 0% or less of the frequency at the high-frequency side end of the passband of the surface acoustic wave filter 1. .

[0055] In the above embodiment, the filter circuit portion is connected to only one of the forces to which the filter circuit portions Bl and B2 are connected to both the input side and the output side of the trap circuit portion A. Also good. That is, only the filter circuit part B1 consisting of the series arm resonator S1 and the inductor L2a may be connected to the trap circuit part A, or only the filter circuit part B2 consisting of the series arm resonator S2 and the inductor L2b. It may be connected to the trap circuit part A.

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

[0057] The piezoelectric substrate 12 is made of ceramics or a piezoelectric single crystal, and the IDT electrodes are formed on the lower surface thereof to form the series arm resonators SI and S2 and the parallel arm resonators PI and P2. . In Fig. 7, the force at which the series arm resonators SI and S2 and the parallel arm resonators PI and P2 are formed is schematically shown. Fig. 8 schematically shows the specific structure of this IDT electrode. . 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 SI and S2 and the parallel arm resonators P1 and P2 includes an IDT electrode and reflectors disposed on both sides of the IDT electrode. In all cases, as shown in Table 1 above, the specifications of the series arm resonators SI and S2 and the parallel arm resonators PI and P2 are different depending on the required characteristics.

[0058] The surface acoustic wave chip 11 is provided with metal bumps 14a to 14f. The surface acoustic wave chip 11 is mounted on the knock board 15 by flip chip bonding using these metal bumps 14a to 14f. The knock substrate 15 is made of an appropriate insulating material, and has electrode lands 16a to 16e on the upper surface thereof. Metal bump 14a on electrode land 16a, metal bump 14b on electrode land 16b, electrode land 16c connected to ground potential Metal bump 14c, 14d force Metal bump 14e on electrode land 16d, metal bump 14 f is joined to the electrode land 16e.

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

Then, as schematically shown in FIG. 10 (a), external elements constituting the inductors LI, L2a, and L2b are appropriately connected to the electrode lands 16a to 16e outside the package substrate 15. The

In the surface acoustic wave chip 11 shown in FIG. 7, only the series arm resonators SI and S2 and the parallel arm resonators PI and P2, which are surface acoustic wave resonator forces, are formed on the piezoelectric substrate 12. FIG. Further, as shown in the plan view, the inductors LI, L2a, and L2b may also be provided by forming a meander-shaped electrode, that is, a meandering electrode on the lower surface of the piezoelectric substrate 12.

FIG. 11 is a plan view schematically showing the electrodes formed on the lower surface of the piezoelectric substrate 12 through the piezoelectric substrate 12 as in FIG. Here, the inductors LI, L2a, and L2b forces are configured by forming the 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.

[0063] As shown in FIG. 11, the surface acoustic wave filter 1 can be reduced in size by forming the inductors LI, L2a, and L2b by applying the electrode material 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 has a trap circuit portion A configured in the same manner as the trap circuit portion A shown 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. And a parallel arm resonator P2 connected between the two.

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 are provided on the both sides of the trap circuit portion A. A filter circuit portion B4 including the inductor L2b is connected to each other. First implementation The difference from the configuration is that one end of the inductors L2a and L2b is connected to connection points 22 and 23 between the series arm resonators Sl l and S12 and one end or the other end of the first inductor L1. .

[0066] Also in the present embodiment, the resonance frequency of the series arm resonators Sl l, S12 is made to substantially coincide with the end portion on the high side of the passband so that the resonance frequency of the series arm resonators Sl l, S12 and the anti-resonance By using the inductive impedance with the frequency, 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 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 limited as long as it is connected between one end or the other end of the series arm resonator and the ground potential. The series arm resonator may be connected to one end or the other end of the displacement.

FIG. 13 is a diagram showing frequency characteristics of the surface acoustic wave filter 21 shown in FIG. As is apparent from FIG. 13, also in this 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 also achieved. 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 Sl l and S12 and the parallel arm resonators PI and P2 are as shown in Table 2 below. Inductor L1 has an inductance value of 11ηΗ, inductor L2a has an inductance value of 16nH, and inductor L2b has an inductance value of 13ηΗ.

[0069] [Table 2]

In the surface acoustic wave filters 1 and 21 of the first and second embodiments shown in FIG. 1 and FIG. 12, the trap circuit portion Α includes the first inductor L1 and the first inductor L1. Although the parallel arm resonators PI and P2 connected to both ends 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 force first inductor L1 You may have a, Lib, and Lie. In this case, the third parallel arm resonator P3 is connected between the adjacent inductors Lla and Lib, between the adjacent inductors Lib and Lie, and the ground potential.

[0072] Thus, in the present invention, the number of stages of the LC circuit which is the inductor and the parallel arm resonator force in the trap circuit portion including the first inductor and the parallel arm resonator is not particularly limited. Absent.

In the above embodiment, the surface acoustic wave filter using a surface acoustic wave has been described. However, the present invention can also be applied to a boundary acoustic wave filter using a boundary acoustic wave.

Claims

The scope of the claims

[1] An elastic wave filter having a trap band and a pass band provided outside a low band of the trap band,

Provided between at least one first inductor arranged in a series arm connecting the input terminal and the output terminal, and one end of the portion where at least one first inductor is provided, and the ground potential. The first parallel arm resonator provided, the second parallel arm resonator provided between the other end of the portion where at least one of the first inductors is provided and the ground potential, and the first parallel arm resonator, A trap circuit portion having a third parallel arm resonator connected between adjacent first inductors and a ground potential when there are a plurality of inductors, and the input terminal and the trap circuit portion. And at least one of the filter circuit portion provided 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 substantially coincides with a high band side end of a pass band of the elastic wave filter.

[2] The electrostatic 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.

[3] The resonance frequency of the resonance circuit including the capacitance of the series arm resonator and the second inductor is lower than the lower end of the passband. The elastic wave filter according to 2.

[4] The parallel arm resonator and the series arm resonator are formed !, further comprising a piezoelectric substrate,

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.

[5] The capacitance of the series arm resonator is Cs, and the inductance value of the first inductor is Ls. The force according to any one of claims 1 to 4, wherein CsZCp> l and LpZLs> l, where Cp is a capacitance of the parallel arm resonator and Lp is an inductance value of the second inductor. Elastic wave filter.