JP3291860B2 - Vertical double mode surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-loss surface acoustic wave filter using vertical double mode coupling.

[0002]

2. Description of the Related Art A so-called vertical double-mode surface acoustic wave filter using two energy-trapping vibration modes generated in the propagation direction of a surface wave is known. It is used in the RF section of telephones and the like. This vertical double mode surface acoustic wave filter has a configuration in which a plurality of energy trapping type resonators are formed on a single piezoelectric substrate and are connected in multiple stages in cascade.

The characteristics of the vertical double mode surface acoustic wave filter vary greatly depending on the type of piezoelectric substrate material used and the cut orientation of the substrate. Therefore, a design suitable for the substrate material used and the cutting direction is required.

Further, in order to obtain a surface acoustic wave filter having a sufficiently wide band, it is necessary to use L of 36 ° Y rotation cut X direction propagation.
It is desirable to use a piezoelectric substrate having a large electromechanical coupling coefficient, such as an iTaO ₃ (hereinafter, 36 ° Y-XLT) piezoelectric substrate.

[0005] However, the conventional vertical double mode surface acoustic wave filter has a problem that unnecessary spurious is generated in the pass band when the filter is formed using the 36 ° Y-XLT piezoelectric substrate.

As a device capable of suppressing the unnecessary spurious, Japanese Patent Application Laid-Open No. 4-40705 discloses an interdigital transducer (hereinafter referred to as IDT) of an energy trap type resonator at a connection portion of a plurality of energy trap type resonators.
Abbreviated. ) Is electrically connected in parallel with a coupling capacitor formed by a comb electrode or the like.

[0007]

However, in the vertical type double mode surface acoustic wave filter disclosed in Japanese Patent Application Laid-Open No. 4-40705, although unnecessary spurious in the above pass band can be suppressed, it is not possible to reduce unnecessary spurious in the pass band. The insertion loss is relatively large, and reduction of the insertion loss is demanded.

An object of the present invention is to use a 36 ° Y-XLT piezoelectric substrate and not only reduce unnecessary spurious in a pass band, but also significantly reduce insertion loss. An object of the present invention is to provide a vertical double mode surface acoustic wave filter having a structure.

[0009]

A vertical double-mode surface acoustic wave filter according to the present invention comprises a Y- XLT piezoelectric substrate and a plurality of energy confining devices formed on the piezoelectric substrate and connected in a multistage cascade. Wherein each of the energy trapping type resonators has at least two IDTs disposed in close proximity to each other and reflectors disposed on both sides of the at least two IDTs. A connection part between the resonators is further provided with a comb-shaped electrode for coupling capacitance configuration, which is electrically connected in parallel with the IDT and formed in a region on the piezoelectric substrate other than on the surface acoustic wave propagation path.

Further, when the period of the electrode finger of the IDT is λ _f and the period of the electrode finger of the comb-shaped electrode is λ _c ,

[0011]

(Equation 2)

The above λ _f and λ _c are determined so as to satisfy the above. Note that the periods λ _f and λ _c are equivalent to twice the pitch between the electrode fingers of the plurality of interposed electrode fingers, that is, the IDT corresponds to the period of the surface wave excited in the IDT. Is what you do.

[0013]

According to the present invention, the coupling capacitance is connected to a connection between a plurality of energy trapping type resonators by using a 36 ° Y-XLT piezoelectric substrate.
As in the case of the vertical double-mode surface acoustic wave filter disclosed in JP-A-4-40705, spurious in the pass band is effectively suppressed.

In addition, the period of the electrode fingers of the IDT in the above-described energy-trap type resonator is λ _f , and the comb-type band for the coupling capacitance band has been confirmed by the experiments of the present inventors and will be shown in Examples described later. the period of the electrode fingers of the electrodes when the lambda _c, since the ratio λ _f / λ _c is determined so as to satisfy the above specific relation, the insertion loss is reduced.

[0015]

According to the present invention, there is provided a vertical double-mode surface acoustic wave filter constituted by using a 36 ° Y-XLT piezoelectric substrate having a large electromechanical coupling coefficient suitable for widening a band, and the coupling capacitance configuration described above. The comb-shaped electrode not only suppresses unnecessary spurious components in the pass band, but also
Since the period of the DT and the period of the comb-shaped electrode are determined so as to satisfy the above-described specific relationship, the insertion loss in the pass band can be effectively reduced.

Accordingly, a vertical double mode surface acoustic wave filter which is suitable for use in a high frequency range, can easily achieve a wide band, has excellent frequency characteristics as a filter, and has a low loss is further provided. Can be provided.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be clarified by describing non-limiting embodiments of the present invention with reference to the drawings.

FIG. 1 is a schematic plan view for explaining a vertical double mode surface acoustic wave filter to which one embodiment of the present invention is applied. Vertical double mode surface acoustic wave filter 1
Is configured using a rectangular piezoelectric substrate 2 made of 36 ° Y-XLT. By forming the following electrode structure on the piezoelectric substrate 2, first and second energy trap type resonators are formed.

That is, the first energy trap type resonator has the IDTs 3 and 4 and the reflectors 5 and 6 arranged on both sides of the IDTs 3 and 4, respectively. Each of the IDTs 3 and 4 has a pair of comb-shaped electrodes 3a and
3b and 4a, 4b, and are arranged close to each other as shown in the figure. The reflectors 5 and 6 have IDs
It has a structure in which both ends of a plurality of linear electrodes extending in parallel with the electrode fingers of T3 and T4 are connected by electrodes extending in parallel with the surface acoustic wave propagation direction.

The second energy trap type resonator has the same configuration as the first energy trap type resonator, and has IDTs 7 and 8 arranged close to the center.
The reflectors 9 and 10 are provided on both sides of the reflectors 7 and 8, respectively.

In the first energy trap type resonator, the comb electrode 3a of the IDT 3 is connected to the input terminal, and the comb electrode 3b is connected to the reference potential. Further, one comb electrode 4a of the IDT 4 is connected to the reference potential, and the other comb electrode 4b is connected to the second energy trap type resonator
It is electrically connected to one comb electrode 7a of DT7. By electrically connecting the comb electrode 4b and the comb electrode 7a, the first and second energy trap type resonators are connected in cascade.

A comb-shaped electrode 11 is electrically connected to a connection between the first and second energy trap type resonators. The comb-shaped electrode 11 is composed of a pair of comb-shaped electrodes 11a and 11b having electrode fingers interposed therebetween.

One of the interdigital electrodes 11a of the interdigital electrode 11
Are electrically connected to the connection section. The other comb electrode 11b is connected to a reference potential. The comb-shaped electrode 11 is provided at a connection portion between the first and second energy trap type resonators so as to constitute a coupling capacitor in parallel with the IDTs 4 and 7. As in the case of the surface acoustic wave filter disclosed in -40705, spurious in the passband is suppressed.

In the second energy trap type resonator, the other comb electrode 7b of the IDT 7 is connected to a reference potential. One of the IDTs 8 is connected to the reference potential, and the other IDT 8 is connected to the reference potential.
b is the output end.

As described above, the vertical double-mode surface acoustic wave filter 1 shown in FIG. 1 is structurally different from the vertical double-mode surface acoustic wave filter disclosed in Japanese Patent Laid-Open No. 40705/1992. It is almost the same. However, in the present embodiment, the period of the electrode fingers in the IDT3,4,7,8 and lambda _f, the period of electrode fingers of the comb electrodes 11 is taken as lambda _c, is the ratio λ _f / λ _c 1 .0.

FIG. 2 shows a first insertion loss-frequency characteristic of the vertical double mode surface acoustic wave filter of the present embodiment.
, And the main part thereof is shown by an alternate long and short dash line B. In addition,
The insertion loss-frequency characteristic shown by the one-dot chain line B is an enlarged view of the insertion loss-frequency characteristic curve shown by the solid line A, and shows one scale of the insertion loss in FIG. 2 as 2 dB.

For comparison, the ratio λ _f / λ _c = 3.0
The insertion loss-frequency characteristics of the vertical double-mode surface acoustic wave filter configured exactly the same as in the above embodiment except for the fact that This is indicated by a chain line D.

As is clear from the comparison between the characteristics indicated by the solid line A and the dashed line B and the characteristics indicated by the solid line C and the dashed line D, the vertical double-mode surface acoustic wave filter of this embodiment has a λ It can be seen that the insertion loss can be effectively reduced as compared with the vertical double mode surface acoustic wave filter with _f / λ _c = 3.0.

As described above, by selecting the ratio λ _f / λ _c , the insertion loss is reduced because the spurious suppression coupling capacitor ideally has only the capacitance component, It is considered that this actually has a conductance component, and the insertion loss increases when the conductance component is large.

Therefore, the capacitance of the comb-shaped electrode 11 and the capacitance
The conductance is the ratio λ_f/ Λ_cWhen you change
(The number of electrode fingers is 10 pairs,
The intersection width was 600 μm. ). The results are shown in FIG. Figure
As is apparent from FIG. _f/ Λ_cBut,

[0031]

(Equation 3)

It has been found that the conductance portion becomes smaller in the range satisfying the condition. Therefore, the coupling capacitance in such a range is constituted by the comb-shaped electrodes 11, and various vertical double-mode surface acoustic wave filters are prepared, and the relationship between the insertion loss and λ _f / λ _c is examined. FIG. 4 shows the results.

As is clear from FIG. 4, the insertion loss can be reduced in the same manner as in the above embodiment by configuring the comb-shaped electrode 11 so that the ratio λ _f / λ _c satisfies the above-mentioned specific relationship. Understand. Therefore, in the present invention, the ratio λ _f / λ
It is necessary to determine _c so as to satisfy the above specific relationship.

In the above-described embodiment, the first and second energy trapping type resonators are formed on the piezoelectric substrate 2, but the present invention is not limited to the case where three or more energy trapping type resonators are connected in cascade. The present invention can also be applied to a conventional vertical double mode surface acoustic wave filter.

Further, in the above-described embodiment, each energy trap type resonator is configured to have two IDTs. However, for example, as in the vertical double mode surface acoustic wave filter 21 shown in FIG. On the substrate 22, each of the energy trapping type resonators has three IDTs 23 to 25,2.
It may have 8 to 30. In FIG. 5, reference numerals 26, 27, 31, and 32 indicate reflectors.

In the vertical double mode surface acoustic wave filter 21 of the second embodiment, the connection of the first and second energy trap type resonators is made by the IDTs 23 and 25 of the first energy trap type resonators. One of the interdigital electrodes 23a, 25
a and IDT2 of the second energy trap type resonator
The connection is made by connecting one of the comb electrodes 28a and 30a of the first and second combs 8 and 30, and the comb electrode 33 is electrically connected to the connection portion. IDTs 23, 25, 28,
30's other comb electrode 23b, 25b, 28b, 30
b is connected to a reference potential as shown in FIG.
One of the comb electrodes 24a, 29a of the DTs 24, 29 is also connected to the reference potential. The other comb electrode 24b of the IDT 24 is connected to the input terminal, and the other comb electrode 29b of the IDT 29 is connected to the output terminal.

In the vertical double mode surface acoustic wave filter 21 of the second embodiment, in addition to the coupling capacitance added from the comb-shaped electrode 33, the ratio λ _f / λ _c falls within the specific range. With such a selection, the insertion loss can be reduced as in the first embodiment.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing a vertical double mode surface acoustic wave filter according to a first embodiment.

FIG. 2 is a diagram showing the insertion loss-frequency characteristics of a vertical double mode surface acoustic wave filter prepared for the embodiment and for comparison.

FIG. 3 shows a vertical double mode surface acoustic wave filter;
Ratio of conductance and capacitance λ _f/ Λ_cDependency on
FIG.

FIG. 4 is a diagram showing a relationship between insertion loss and a ratio λ _f / λ _c .

FIG. 5 is a schematic plan view showing a vertical double mode surface acoustic wave filter according to a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Vertical type double mode surface acoustic wave filter 2 ... Piezoelectric substrate 3, 4, 7, 8, ... IDT 5, 6, 9, 10 ... Reflector 11 ... Comb type electrode for coupling capacitance construction

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03H 9/64 H03H 9/145 H03H 9/25

Claims (1)

(57) [Claims] 1. LiTaO _{3 of} Y rotation cut X direction propagation
At least two interdigital transducers each comprising a piezoelectric substrate, and a plurality of energy-trapping resonators formed on the piezoelectric substrate and connected in multiple stages in cascade, wherein each of the energy-trapping resonators is arranged in close proximity to each other. And reflectors disposed on both sides of the at least two interdigital transducers, wherein at a connection between the plurality of energy-trapped resonators,
It further comprises a comb-shaped electrode for coupling capacitance configuration, which is electrically connected in parallel with the interdigital transducer, and is formed in a region on the piezoelectric substrate other than on the surface acoustic wave propagation path, When the period is λ _f and the period of the electrode fingers of the comb electrode is λ _c , A vertical double-mode surface acoustic wave filter in which λ _f and λ _c are determined so as to satisfy