JPH11330904A - Resonator type surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To adjust the passing bandwidth of the resonator type surface acoustic wave filter. SOLUTION: Serial arm elements 30S-1, 30S-23, and 30S-4 pass signals of frequencies above a resonance frequency and below an antiresonance frequency well and do not pass frequencies above the antiresonance frequency well. Parallel arm elements 30P-12 and 30P-34 pass frequencies above the resonance frequency and below the antiresonance frequency well and do not pass frequencies below the resonance frequency well. Consequently, this resonator type SAW filter pass signals of frequencies from the resonance frequency of the parallel arm elements 30P-12 and 30P-34 to the antiresonance frequency of the serial arm elements 30S-1, 30S-23, and 30S-4 to serve as a band-pass filter. A piezoelectric substrate 31 has a large electromechanical coupling coefficient, so the bandwidths of serial arm resonators 40S-1, 40-S-23, and 40S-4 and parallel arm resonators 40P-12 and 40P-34 become unnecessarily wide. Therefore, capacitors 50S-1, 50S-23, 50S-4, 50S-12, and 50P-34 are connected to those respective resonators in series to reduce and adjust the bandwidth.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave (Surface Acoustic) which is used in various circuits of, for example, an automobile telephone, a portable telephone, a communication device and the like, and in particular, has a configuration in which a signal passing bandwidth can be adjusted. Wave, "SA
W ”).

[0002]

2. Description of the Related Art Generally, a SAW device is an interdigital transducer for exciting a SAW.
The SAW device can have various characteristics and functions by processing the IDT. Conventionally, SAW devices are often mainly referred to as SAW filters, and
Among the filters, a multi-electrode SAW filter played a leading role. However, recently, in addition to the multi-electrode SAW filter,
Research and development of resonator-type SAW filters has also become active, and SA
Speaking of the W filter, it has not always meant a multi-electrode SAW filter.

[0003] A resonator type SAW filter is a filter using a SAW resonator configured based on a classical electric filter design method. The main body of the SAW resonator is an IDT with interdigital electrodes facing each other.
Reflectors may be provided on the left and right of the camera. Reflector ID
Like T, it is composed of interdigital electrodes, all of which may be electrically short-circuited or open. The reflector mainly reflects the SAW leaking to the left and right of the IDT acoustically. Even if all the electrode fingers are electrically short-circuited or opened, the influence on the characteristics of the SAW resonator is not affected. Almost no. Since the impedance characteristics of the SAW resonator are very similar to the impedance characteristics of the LC resonator, the design method of the electric filter can be applied.

FIG. 2 is a plan view showing a conventional SAW resonator. The SAW resonator 10 includes, for example, lithium tantalate (LiTaO ₃ ), lithium niobate (LiNb
O ₃ ) or quartz (SiO ₂ ).
An IDT 14 having an input terminal 12, an output terminal 13, and a plurality of electrode fingers 14a is formed on the piezoelectric substrate 11. I
Reflectors 15L and 15R are formed on both sides of DT14.
Incidentally, the reflectors 15L and 15R are not formed if they are unnecessary.

FIGS. 3 (a) to 3 (c) show the reflector 15 shown in FIG.
FIGS. 5A and 5B are plan views showing L, 15R, wherein FIG. 5A shows a schematic diagram, FIG. 5B shows a short-circuit type, and FIG. The number of electrode fingers 15a of each reflector 15L, 15R is 50
About 100 is suitable. Generally, each reflector 15L, 15
The R electrode finger 15a is electrically short-circuited as shown in FIG. 3B, or has an open shape as shown in FIG. 3C. The positions where the reflectors 15L and 15R are arranged are set at various positions in order to obtain a desired impedance.
The center of the outermost electrode finger 14a of the IDT 14 and the reflector 15L,
The distance from the center of the electrode finger 15a on the IDT 14 side of the 15R is often set to be a distance of about 前後 wavelength of the SAW to be excited. In the manufacturing process of the SAW resonator 10, the IDT 14 and the reflectors 15L and 15R are formed at the same time, and therefore have the same material and thickness. IDT14
Is generally a pure Al or an alloy mainly containing the pure Al, but depending on the case, pure Au, pure Ti, or an alloy mainly containing these metals may be used. The thickness of the IDT 14 is set to several hundreds A to several thousands A. Since the SAW resonator 10 configured as described above has a reactance characteristic similar to that of an LC resonator, an equivalent circuit is often represented approximately by the LC resonator.

FIGS. 4A and 4B are diagrams showing an equivalent circuit and reactance characteristics of the SAW resonator 10 shown in FIG. In the equivalent circuit of FIG. 4 (a), the inductor 16 of inductance L, a capacitor 17 and a resistor of resistance value r 18 of the electrostatic capacitance c ₁ are connected in series between the input terminal 12 and output terminal 13, these parallel capacitor 19 of capacitance C ₀ is connected to. In the reactance characteristic of FIG. 4B, the frequencies at which the reactance characteristic X0 becomes 0Ω are the resonance frequency Fr1 and the antiresonance frequency Fa1, and approximately the following equations (1) and (2).
Each is represented by (2). Fr1 = 1 / 2π√ (Lc ₁ ) (1) Fa1 = (1 / 2π) √ {(c ₁ + C ₀ ) / (c ₁ C ₀ L)} (2) Such reactance A method of designing an electric filter using an element having characteristics has been known for a long time.

When a filter is formed using the SAW resonator 10 shown in FIG. 2, a one-stage ladder circuit is a basic circuit. FIG.
(A), (b) is a block diagram showing two types of examples of a conventional one-stage ladder type circuit. One stage ladder type circuit 20A of FIG.
5B and the one-stage ladder type circuit 20B of FIG. 5B both have a parallel arm (arm) resonator 10P and a series arm resonator 10S and are symmetrical. That is, the impedance of the ladder circuit 20A viewed from the left terminals 21-0 and 22-0 of the ladder circuit 20A is equal to the impedance of the ladder circuit 20B from the right terminals 21-1 and 22-1 of the ladder circuit 20B. The impedance when viewing the ladder-type circuit 20A from the right side terminals 21-1 and 22-1 of the ladder-type circuit 20A is equal to the impedance of the ladder-type circuit 20B.
It is equal to the impedance when the ladder type circuit 20B is viewed from -0, 22-0. When a filter is formed using the ladder circuits 20A and 20B, the ladder circuit 20A or 20B is selected in consideration of the impedance between the ladder circuits 20A and 20B. In the ladder type circuits 20A and 20B, if the anti-resonance frequency of the parallel arm resonator 10P and the resonance frequency of the series arm resonator 10S are very close to or coincide with each other, the matching state of the input terminal and the output terminal of the entire system will be improved. Very good bandpass filter characteristics are obtained.

FIG. 6 is a diagram showing reactance characteristics and transmission characteristics of the one-stage ladder type circuits 20A and 20B of FIG. In FIG. 6, the reactance characteristic of the parallel arm resonator 10P and the reactance characteristic of the series arm resonator 10S are shown by characteristic curves Xp and Xs, and have resonance frequencies Frp and Frs and antiresonance frequencies Fap and Fas, respectively. As a result, the insertion loss characteristic of the single-stage ladder type circuit 20A or 20B is represented by a characteristic curve I0,
The return loss characteristics are as shown in a characteristic curve R0.

The insertion loss characteristic I0 is the insertion loss characteristic of the band-pass filter. The attenuation of the components other than the pass band is significantly increased by increasing the number of stages of the ladder circuits 20A and 20B, but the insertion loss of the pass band also increases. . Therefore, the number of stages required for the configuration of the filter is set according to the condition of the characteristics of the filter, for example, the level of the insertion loss in the pass band and the level of the out-of-band attenuation. The number of stages contributes to a certain extent to the pass band width BW of the filter, but is not a decisive factor. The pass bandwidth BW is determined by the bandwidth of the series arm resonator 10S and the parallel arm resonator 10P constituting the filter, that is, the resonance frequencies Frp and Frs of the respective SAW resonators and the frequencies of the anti-resonance frequencies Fap and Fas. The interval is the frequency interval between the resonance frequency Frs of the series arm resonator 10S and the anti-resonance frequency Fap of the parallel arm resonator 10P. FIG. 6 shows the reactance characteristics of each SAW resonator constituting the resonator type SAW filter. The pass bandwidth BW of the filter is different from the resonance frequency Frp of the parallel arm resonator 10P and the series arm resonator 10S.
Within the anti-resonance frequency Fas. That is, the pass bandwidth BW of the filter cannot be wider than the frequency interval between the resonance frequency Frp of the parallel arm resonator 10P and the anti-resonance frequency Fas of the series arm resonator 10S. In FIG. 6,
Although the anti-resonance frequency Fap and the resonance frequency Frs are shown to match, if the frequencies Fap and Frs do not match, for example, if the anti-resonance frequency Fap is lower than the resonance frequency Frs, the pass bandwidth BW becomes If the anti-resonance frequency Fap is higher than the resonance frequency Frs, the pass bandwidth BW becomes narrow.
In any case, if the two frequencies do not match, the larger the amount of deviation, the larger the ladder-type circuit
The matching state in 20A and 20B deteriorates, and as a result, the filter characteristics deteriorate.

[0010] However, conventionally, the frequency band used by law for various communication devices is not necessarily the pass band width when the matching state in the ladder type circuits 20A and 20B is good.
Does not match BW. Therefore, the pass bandwidth BW must be adjusted to match the usable frequency band determined by law. Therefore, in the resonator type SAW filter, the anti-resonance frequency Fap of the parallel arm resonator 10P and the resonance frequency Frs of the series arm resonator 10S are shifted from each other even if the matching state in the ladder circuits 20A and 20B is sacrificed. The bandwidth BW must be adjusted to match the allocated frequency band. In this case, the insertion loss characteristic and the reflection loss characteristic of the resonator type SAW filter are deteriorated, and the reduction of the filter loss is prevented.

On the other hand, the resonator type SA actually used
The number of stages of the W filter is generally three or more, but the basic configuration of the filter is not different from the case of the single stage as described above. Also, a ladder type circuit of a SAW resonator constituting a filter
The number of stages of 20A and 20B has some influence on the pass bandwidth BW, but is not a decisive factor. However, for the sake of simplicity, the configuration of a resonator type SAW filter composed of a multi-stage ladder type circuit will be described for convenience. As the number of stages of the ladder type circuits 20A and 20B constituting the resonator type SAW filter increases, the number of resonators 10P and 10S also increases in proportion to the number of stages.

FIG. 7 is a block diagram showing a conventional four-stage resonator type SAW filter formed using the ladder type circuits 20A and 20B of FIG. In this resonator type SAW filter, FIG.
Using the one-stage ladder type circuits 20A and 20B of (a) and (b), terminals having the same impedance are connected so that the impedance between the stages is matched, and a filter composed of a total of eight resonators is formed. ing. That is, the first stage ladder type circuit
20-1 parallel arm resonator 10P-1, second stage ladder type circuit 20
-2 parallel arm resonator 10P-2, third stage ladder type circuit 20-3
Parallel arm resonator 10P-3 and fourth stage ladder type circuit 20-4
The parallel arm is constituted by the parallel arm resonator 10P-4 of the ladder type circuit 20-1, the series arm resonator 10S-1 of the ladder type circuit 20-1, the series arm resonator 10S-2 of the ladder type circuit 20-2,
The series arm resonator 10S-3 of the ladder type circuit 20-3 and the series arm resonator 10S-4 of the ladder type circuit 20-4 are incorporated.

[0013] Two resonators connected in series or two resonators connected in parallel can be combined into one resonator. This composite resonator is characterized in that it has impedance characteristics substantially equal to those of the two resonator systems.
For example, the series arm resonator 10S-1 of the ladder type circuit 20-1 is synthesized by combining the series arm resonator 10S-2 of the ladder type circuit 20-2 and the series arm resonator 10S-3 of the ladder type circuit 20-3. And ladder circuit 20
And the parallel arm resonator 10P-3 of the ladder type circuit 20-4 and the parallel arm resonator 10P-4 of the ladder type circuit 20-4. As a result, FIG.
(A) and (b) of FIG.
become that way. FIGS. 8A and 8B are configuration diagrams showing the combined four-stage resonator type SAW filter of FIG. 7, in which FIG. 8A is an equalization circuit diagram, and FIG. FIG.

In the equalizer circuit shown in FIG. 8A, a new parallel arm resonator 10P-12 is formed by combining the resonators of the parallel arm resonators 10P-1 and 10P-2, and a series arm resonator is formed.
A new series arm resonator 10S-23 is formed by combining the resonators of 10S-2 and 10S-3, and a new parallel arm resonator is formed by combining the resonators of the parallel arm resonators 10P-3 and 10P-4.
10P-34 is configured. Therefore, in FIG. 7, eight resonators 10
While P-1, 10P-2, 10P-3, 10P-4, 10S-1, 10S-2, 10S-3, 10S-4 were required, in FIG. 8, five resonators 10P- 12,10P
-34, 10S-1, 10S-23 and 10S-4 constitute a resonator type SAW filter having the same transmission characteristics and impedance characteristics as those in FIG.

In the plan view of FIG. 8B, this resonator type SAW filter includes an input terminal 21-0 and an output terminal 21-4 formed on a piezoelectric substrate 11, and resonators 10S-1 and 10S-. 23,10S-
4,10P-12,10P-34 and each series arm resonator 10S-1,10S-2
3, 10S-4, transmission line pattern 23 connecting between parallel arm resonators 10P-12, 10P-34, and parallel arm resonators 10P-12, 10P-34
And an earth pattern 24. Resonator type SA
Since the W filter is generally a bandpass filter, the passband width BW is one of the important elements of the characteristic as well as the insertion loss and the out-of-band attenuation of the passband. However, the passband width BW is determined by the electromechanical coupling coefficient k ² of the piezoelectric substrate 11 to be used in basically the filter configuration, and since there are only several kinds also the material of the piezoelectric substrate 11, the pass band width BW to the desired value It is difficult to do. Here, the electromechanical coupling coefficient k ² is
It is a constant indicating the degree of coupling between an electric system and a mechanical system for a piezoelectric material or a magnetostrictive material. When Em is the electric energy Ei externally applied to a sample and stored as mechanical energy, k ² = It is given by Em / Ei.

For example, when a resonator type SAW filter as shown in FIG. 8 is manufactured by using the piezoelectric substrate 11 of LiTaO ₃ , in order to obtain a sufficient amount of out-of-band attenuation, a ladder-type circuit 20 is required.
Since the number of stages A and 20B is required to be four or more, if the anti-resonance frequency Fap of the parallel arm resonator 10P and the resonance frequency Frs of the series arm resonator 10S are made to match as shown in FIG. It is about 20MHz. But the pass bandwidth BW
Is 25 MHz, so to satisfy this condition, the anti-resonance frequency Fap of the parallel arm resonator 10P constituting the filter is
Must be shifted to 2.5 MHz or more in the low band, and the resonance frequency Frs of the series arm resonator 10S must be shifted to 2.5 MHz in the high band. As a result, the insertion loss in the pass band is degraded by about 1.0 dB.

Also, the electromechanical coupling coefficient k ² is LiTaO ₃
(K ^2, for example 0.66%) LiNbO ₃ to about 9 times greater than
When a resonator type SAW filter as shown in FIG. 8 is manufactured using a piezoelectric substrate (k ² is, for example, 5.5%), the pass band width BW is too large compared with the specification, and the ladder type The anti-resonance frequency Fap of the parallel arm resonator 10P constituting the circuits 20A and 20B is set to a high range of 10.0 MHz or more, and the series arm resonator
The resonance frequency Frs of 10S must be shifted to the low band by 10.0 MHz. As a result, a considerable impedance mismatch occurs between the parallel arm resonator 10P and the series arm resonator 10S, and the insertion loss in the pass band deteriorates by several decibels.
As described above, in the conventional resonator-type SAW filter, indispensable characteristics are preferentially realized depending on use conditions, but other characteristics are sacrificed.

[0018]

The conventional S as shown in FIG.
There are various issues in improving the performance of AW filters,
Emphasis is mainly placed on low loss (that is, reduction of passband insertion loss), high attenuation outside the passband, high power handling, and the like. However, in recent years, various types of mobile communication devices have been increased due to the development of mobile communication technology, and the management of frequency bands, which are often in short supply, has become stricter. In addition to the performance of filters with low loss, high out-of-band attenuation, and high power handling, there is an added requirement that these performances be maintained within tightly defined frequency bands. In this regard, S
AW filters are disadvantaged over other filters.

That is, since the pass band width BW of the SAW filter is substantially proportional to the electromechanical coupling coefficient k ² of the piezoelectric substrate 11 of the filter, the degree of freedom in design is limited. As described above, there are only a few types of materials for the piezoelectric substrate 11 that can be used as the substrate of the SAW filter.
O ₃ , LiNbO ₃ , SiO ₂ , potassium niobate (K
NbO ₃ ), langasite (La ₃ Ga ₅ SiO ₁₄ ), and the like. Since the respective electromechanical coupling coefficients k ² are also determined, a SAW filter as shown in FIG. The bandwidth BW is almost fixed. In this case, if the angle between the cut surface and the crystal axis is adjusted when cutting the piezoelectric substrate 11 in the process of processing the substrate, the electromechanical coupling coefficient k ² can be slightly changed.
The pass band width BW of the AW filter can be finely adjusted by this. However, since the angle between the cut plane and the crystal axis has a dominant role in the insertion loss of the SAW filter, it cannot be easily changed, and if the angle between the cut plane and the crystal axis is adjusted to change the pass bandwidth BW, However, there is a problem that the insertion loss of the filter pass band is deteriorated.

The present invention eliminates such inconsistency and provides Li
In the case of manufacturing a resonator type SAW filter with large piezoelectric substrate of the electromechanical coupling coefficient k ^2, such as NbO _3, etc., the cut surface of the piezoelectric substrate, such as deteriorating the filter passband insertion loss of the crystal axis It is an object of the present invention to provide a resonator type SAW filter having a configuration capable of reducing a pass band width BW of a filter to a desired value without changing an angle.

[0021]

Means for Solving the Problems] bandwidth BW pass of the SAW resonator, since substantially determined by electromechanical coupling coefficient k ² of the piezoelectric substrate of the SAW resonator, simply by changing only the structure of the SAW resonator Can not. LiTaO ₃ and SiO _{2, which} are inexpensive and have good temperature characteristics, have been conventionally used as the piezoelectric substrate of the SAW resonator.
² is small, so that a SAW resonator having a wide pass bandwidth BW or S
Not suitable for AW filters. In particular, when a resonator type SAW filter is configured using a SAW resonator, if the matching state between the stages of the ladder type circuit is set to the best state, the pass band width BW of the filter becomes considerably narrower than the specified width. I will. On the other hand, when the electromechanical coupling coefficient k ² is LiTaO ₃ or Si
If a material (for example, LiNbO ₃ ) larger than O ₂ is used as the piezoelectric substrate, the pass band width BW of the SAW resonator becomes too wide, and is not suitable for a bandpass filter. Therefore, in the invention according to claim 1 of the present invention, in the resonator type SAW filter, a predetermined electromechanical coupling coefficient k ²
A piezoelectric substrate having: an input terminal formed on the piezoelectric substrate for inputting an input signal; an output terminal formed on the piezoelectric substrate for outputting an output signal; and an input terminal formed on the piezoelectric substrate and receiving the input signal. One or more series arm resonators connected in series between a terminal and an output terminal and configured by a SAW resonator having predetermined impedance characteristics, and one each for the one or more series arm resonators One or a plurality of first capacitors connected in series with each other, and a SAW resonator formed on the piezoelectric substrate and connected in a ladder shape to each of the series arm resonators and having a predetermined impedance characteristic. One or a plurality of parallel arm resonators, and one or a plurality of second capacitors serially connected to the one or a plurality of parallel arm resonators, respectively. There.

By adopting such a configuration, when an input signal is input from the input terminal, each series arm resonator passes a signal having a frequency equal to or lower than the anti-resonance frequency and transmits a signal having a frequency equal to or higher than the anti-resonance frequency. Reflects frequency. Further, each parallel arm resonator passes a signal having a frequency equal to or higher than its resonance frequency and reflects a frequency equal to or lower than the resonance frequency. Therefore, the resonator-type SAW filter passes a signal having a frequency ranging from the resonance frequency of the parallel arm resonator to the anti-resonance frequency of the series arm resonator, and functions as a bandpass filter. For example, as a piezoelectric substrate, an electromechanical coupling coefficient k ²
In the case of using LiNbO ₃ , the bandwidth of each SAW resonator is unnecessarily wide, so that each first and second capacitor is connected in series to each series arm resonator and parallel arm resonator. Thereby, the resonance frequency is increased and the anti-resonance frequency is decreased. Therefore, the signal passing frequency bandwidth is reduced and adjusted to a desired value. According to the second aspect of the invention, the first and second aspects of the first aspect are provided.
Are composed of a capacitive element connected to the outside of the piezoelectric substrate, an interdigital electrode type capacitive element formed on the piezoelectric substrate, or a multilayer electrode type capacitive element formed on the piezoelectric substrate. Thereby, each S constituting the filter is
Capacitors are connected in series to the AW resonators, and by narrowing the frequency interval or bandwidth between the resonance frequency and the antiresonance frequency of the obtained new SAW resonator, the pass band width BW of the filter is determined by the value of the capacitance of these capacitors. Is adjusted to the desired bandwidth.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment FIGS. 9A and 9B are configuration diagrams of a SAW resonator according to a first embodiment of the present invention, and FIG. 9A is a plan view. , And FIG. 3B shows an equalizing circuit. In the plan view of FIG. 9A, the SAW resonator 30 is, for example, LiNbO ₃
Of the piezoelectric substrate 31, the input terminal 32 on the piezoelectric substrate 31,
An output terminal 33, a SAW resonator 40 and an interdigital electrode type capacitor 50 are formed. The SAW resonator 40 and the interdigital electrode type capacitor 50 are connected in series between the input terminal 32 and the output terminal 33. The SAW resonator 40 has an IDT 41 having a plurality of electrode fingers 41a, and reflectors 42L and 42R formed on both sides of the IDT 41. Capacitor 50
Has an interdigital electrode 50a. This capacitor 50
Is structurally the same as a SAW resonator without a reflector, but the purpose is not to perform SAW excitation, but to obtain capacitance. The number of pairs of electrodes and the intersection length are set based on the required capacitance value.

In the equivalent circuit of FIG.
A SAW resonator 40 and a capacitor 50 are connected in series between 32 and the output terminal 33. In the SAW resonator 40, an inductor 40a having an inductance L, a capacitor 40b having a capacitance c ₁ , and a resistor 40c having a resistance value r are connected in series between the capacitor 50 and the output terminal 33. ₀ capacitor 40d is connected.

Next, the operation of FIG. 9A will be described. When a high-frequency input signal is input from the input terminal 32, the input signal passes through the capacitor 50 and reaches the SAW resonator 40. The capacitance of the capacitor 50 is C ₅₀ and the angular frequency of the input signal is ω. Then, since the impedance of the capacitor 50 becomes (−1 / C ₅₀ ω), the impedance 1 / C ₅₀ ω is subtracted from the impedance of the SAW resonator 40. On the other hand, a voltage difference is generated between the electrode fingers of all the IDTs constituting the SAW resonator 40, and the SAW is excited.
Numeral 40 indicates impedance characteristics like a quartz resonator or a conventional LC resonator.

FIG. 10 is a characteristic diagram of the reactance of the SAW resonator 30 shown in FIG. However, FIG. 10 also shows the characteristic curve X0 in FIG. 4B for comparison.
In FIG. 10, the characteristic curve Xv indicates the reactance characteristic of the SAW resonator 30 in FIG. Resonant frequency F
r1 and anti-resonance frequency Fa1 are the same as those of the conventional SAW resonator 10 shown in FIG.
The resonance frequency Fr2 and the anti-resonance frequency Fa2 are the values of the SAW resonator 30 in FIG. Since the impedance of the capacitor 50 is negative, if it is connected in series with the SAW resonator 40, the reactance of the entire system (that is, the SAW resonator 30) will decrease in the negative direction. As a result, as shown in FIG. 10, (Fr2−Fa2) is smaller than (Fr1−Fa1). That is, by connecting the capacitor 50 to the SAW resonator 40 in series,
Is smaller than that of the conventional SAW resonator 30 shown in FIG. In other words, the bandwidth of the SAW resonator 40 that is determined by the electromechanical coupling coefficient k ² of the piezoelectric substrate 31 (Fr1-Fa1) is the bandwidth by serially connecting a capacitor 50 (Fr2-F
a2) has been reduced.

FIGS. 1A and 1B are configuration diagrams of a four-stage resonator type SAW filter of the present embodiment using the SAW resonator shown in FIG. 9, and FIG. 1A is a plan view and FIG. FIG. 1B is an equalization circuit diagram. In the plan view of FIG. 1A, this resonator type SAW filter has a piezoelectric substrate 31, on which an input terminal 41-0, an output terminal 41-4, a series arm resonator 40S-1, 40S-23, 40S-4, parallel arm resonator 40P-1
2, 40P-34 and interdigital electrode type capacitors 50S-1,5
0S-23, 50S-4, 50P-12 and 50P-34 are formed. Further, on the piezoelectric substrate 31, a transmission line pattern 6 connecting the series arm resonator 40S-1 and the capacitors 50S-23 and 50P-12 is formed.
1, Series arm resonator 40S-23 and capacitors 50S-4, 50P-3
Transmission line pattern 62 connecting between 4 and parallel arm resonator 4
A transmission line pattern 63 connecting between 0P-12 and the capacitor 50P-12, a transmission line pattern 64 connecting between the parallel arm resonator 40P-34 and the capacitor 50P-34, and a parallel arm resonator 40
Earth patterns 65 of P-12 and 40P-34 are formed.

In the equivalent circuit of FIG. 1B, the series arm resonators 40S-1, 40S-23, 40S-4, the parallel arms 40P-12,
Capacitors 50S-1, 50S-23, 50S-4, 50P-12, 40P-34
50P-34 are connected in series respectively, and the series arm element 30S-
1, 30S-23, 30S-4 and parallel arm element 30P-12, 30P-34
Is configured. The series arm element 30S-1 is connected between the input terminal 41-0 and the node N1, and the parallel arm element 30P-12 is connected between the node N1 and the ground pattern 65. Series arm element 30S-23 between node N1 and node N2
Are connected, and the parallel arm element 30P-34 is connected between the node N2 and the ground pattern 65. Series arm element 30S-4 is connected between node N2 and output terminal 41-4.

Next, the operation of FIG. 1 will be described. Each of the series arm elements 30S-1, 30S-23, and 30S-4 passes a signal well at a frequency equal to or higher than the resonance frequency and equal to or lower than the anti-resonance frequency, and does not pass a frequency higher than the anti-resonance frequency. Also, the parallel arm elements 30P-12 and 30P-34 pass frequencies higher than their respective resonance frequencies and lower than the antiresonance frequency well, and do not pass much frequencies below the resonance frequency. As a result, this resonator type SAW filter passes signals of frequencies from the resonance frequency of the parallel arm elements 30P-12, 30P-34 to the anti-resonance frequency of the series arm elements 30S-1, 30S-23, 30S-4. And acts as a bandpass filter. However, in the present embodiment, since using the larger LiNbO ₃ of the electromechanical coupling coefficient k ² as the piezoelectric substrate 31, series arm resonators 40S-1,40S-23,40S-4 and the parallel arm resonator 40P-12, 40P-34 bandwidth is wider than necessary. Therefore, a capacitor 50 is connected to each of these resonators.
S-1, 50S-23, 50S-4, 50P-12, 50P-34 are connected in series, respectively, and the bandwidth is adjusted to a desired value by reducing the bandwidth.

FIG. 11 is a characteristic diagram showing the insertion loss characteristics of the resonator type SAW filter of FIG. However, FIG.
For comparison, each resonator in FIG.
The characteristic curve I1 when the 1,50S-23, 50S-4, 50P-12, and 50P-34 are not connected is also displayed. 11, a characteristic curve Iv indicates the insertion loss characteristic of the resonator type SAW filter of FIG. 1, and a series arm resonator constituting this filter.
40S-1,40S-23,40S-4 and parallel arm resonator 40P-12,40P-3
FIG. 4 shows that the pass bandwidth BW is reduced by connecting the capacitors 50S-1, 50S-23, 50S-4, 50P-12, and 50P-34 in series. The pass band width BW of the resonator type SAW filter shown in FIG. 1 configured by using the SAW resonator 30 shown in FIG. 9 is about の of the bandwidth of the SAW resonator 30. Thus, for example, an electromechanical coupling coefficient k ² of the large cavity with a piezoelectric substrate 31 SA such as LiNbO ₃
Even if a W filter is configured, the series arm resonators 40S-1, 40S-
By connecting capacitors 50S-1, 50S-23, 50S-4, 50P-12, and 50P-34 in series to 23, 40S-4 and parallel arm resonators 40P-12, 40P-34, respectively, the passband is too wide. The width BW is adjusted to a desired value.

As described above, in the first embodiment,
For example, when a resonator-type SAW filter using a piezoelectric substrate 31 having a large electromechanical coupling coefficient k ^2, such as LiNbO _3, the series arm resonator 40S-1,40S-23,40S-4 and the parallel arm resonator 40P-12,40P-34 and capacitor 50S-1,50
By connecting S-23, 50S-4, 50P-12, and 50P-34 in series, the excessively wide pass bandwidth BW can be adjusted to a desired value.

Second Embodiment FIG. 12 is a cross-sectional view of a SAW resonator showing a second embodiment of the present invention, and is common to the elements in FIG. 9A showing the first embodiment. Are denoted by the same reference numerals. In this SAW resonator, a capacitor 60 having a different configuration is provided in place of the capacitor 50 in FIG. This capacitor 60 has a multilayer electrode type capacitor structure,
A conductor thin film 61 and a dielectric thin film (for example, S
iO ₂ ) 62, conductive thin film 63, dielectric thin film 64 and conductive thin film 65
Are sequentially laminated. The conductive thin film 65 is connected to the input terminal 32. The capacitance of the capacitor 60 is
It is determined by the intersection area of the conductor thin films 61, 63, 65 and the film thickness and the dielectric constant of the dielectric thin films 62, 64. When a resonator type SAW filter 0 similar to that of FIG. 1 is configured using this SAW resonator, substantially the same operation as in the first embodiment is performed. The multilayer electrode type capacitor 60 requires more time for the manufacturing process than the IDT-type capacitor 50 of the first embodiment, but has a large degree of freedom in setting the capacitance.
May be adopted for design reasons. As described above, in the second embodiment, the series arm resonators 40S-1,
A multilayer electrode type capacitor 60 is used as a capacitor connected in series to the 40S-23, 40S-4 and the parallel arm resonators 40P-12, 40P-34.
Is adopted, the degree of freedom in setting the capacitance can be increased in addition to the advantages of the first embodiment.

The present invention is not limited to the above embodiment,
Various modifications are possible. For example, there are the following modifications. (A) In the embodiment, a four-stage resonator type SAW filter has been described, but the number of stages may be increased or decreased according to the required specification of the amount of attenuation. (B) In the configuration of the resonator type SAW filter, if the circuit configuration shown in FIG. 7 is used, the number of components is increased, but the same operation and effect as in FIG. 1 can be obtained. (C) The reflectors 42L and 42R in FIG. 9 may be deleted if the loss may be large. (D) The capacitor 50 in FIG.
It may be formed between the resonator 40. (E) Capacitor 50 in FIG. 9 and capacitor in FIG.
50S-1, 50S-23, 50S-4, 50P-12, 50P-34 are all of the IDT type, but the dimensions of the piezoelectric substrate 31, the SAW resonator 40
Depending on the position, size, etc. of the piezoelectric substrate 31, a multilayer electrode type or a mixture of an interdigital electrode type and a multilayer electrode type may be used.
May be connected to the outside. Thereby, the resonator type SAW
The degree of freedom in filter design can be increased.

[0034]

As described above in detail, according to the first aspect of the present invention, each capacitor is connected in series to each of the series arm resonators and each of the parallel arm resonators constituting the resonator type SAW filter. Thereby, the passband BW that is too wide can be adjusted to a desired value. According to the second aspect of the present invention, a capacitance element connected to the outside of the piezoelectric substrate as a capacitor connected in series to each of the series arm resonators and the parallel arm resonators, and an interdigital electrode type capacitance formed on the piezoelectric substrate Since the element or the multilayer electrode type capacitance element is employed, the degree of freedom in setting the capacitance can be increased in addition to the effect of the invention according to claim 1.

[Brief description of the drawings]

FIG. 1 shows a four-stage resonator type SAW according to a first embodiment of the present invention.
It is a block diagram of a filter.

FIG. 2 is a plan view of a conventional SAW resonator.

FIG. 3 is a plan view of reflectors 15L and 15R in FIG.

FIG. 4 is a diagram showing an equivalent circuit and reactance characteristics of the SAW resonator 10 of FIG.

FIG. 5 is a configuration diagram of a conventional one-stage ladder type circuit.

6 is a transmission characteristic diagram of FIG.

FIG. 7 shows a conventional ladder circuit constructed using the ladder type circuit of FIG.
It is a block diagram of a step resonator type SAW filter.

FIG. 8 is a configuration diagram of a resonator-type SAW filter after synthesis.

FIG. 9 is a configuration diagram of a SAW resonator according to the first embodiment of the present invention.

FIG. 10 is a characteristic diagram of the reactance of FIG. 9;

11 is a characteristic diagram of the insertion loss characteristic of FIG.

FIG. 12 is a sectional view of a SAW resonator according to a second embodiment of the present invention.

[Explanation of symbols]

30,40 SAW resonator 30S-1,30S-23,30S-4 Series arm element 30P-12,30P-34 Parallel arm element 40S-1,40S-23,40S-4 Series arm resonator 40P-12,40P -34 Parallel arm resonator 31 Piezoelectric substrate 32,41-0 Input terminal 33,41-4 Output terminal 41 IDT 42L, 42R Reflector 50,50S-1,50S-23,50S-4,50P-12,50P- 34,60 capacitors

Claims (2)

[Claims] A piezoelectric substrate having a predetermined electromechanical coupling coefficient; an input terminal formed on the piezoelectric substrate for inputting an input signal; and an output terminal formed on the piezoelectric substrate for outputting an output signal. One or more series arm resonators formed on the piezoelectric substrate, connected in series between the input terminal and the output terminal, and configured by surface acoustic wave resonators having predetermined impedance characteristics, One or more first capacitors respectively connected in series to the one or more series arm resonators, and one or more first capacitors formed on the piezoelectric substrate and in a ladder form with respect to each of the series arm resonators One or more parallel arm resonators connected to each other and configured by a surface acoustic wave resonator having a predetermined impedance characteristic, and one series connection to each of the one or more parallel arm resonators A resonator-type surface acoustic wave filter comprising one or more second capacitors. 2. The capacitor according to claim 1, wherein the first and second capacitors are capacitive elements connected to the outside of the piezoelectric substrate, IDT-type capacitive elements formed on the piezoelectric substrate, or formed on the piezoelectric substrate. 2. A resonator type surface acoustic wave filter according to claim 1, wherein said surface acoustic wave filter comprises a multilayer electrode type capacitive element.