JPH10284988A - Surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To permit the passing band characteristic of a filter to be the flat one without inserting a reactance element so as to make it small by respectively differring the electrode logarithms of input/output IDTs in the first-stage and the second-stage triple mode surface wave filters of a two-stage vertical connection vertical coupling triple mode surface wave filter. SOLUTION: Two IDTs 2 and 3 are arranged along a surface acoustic wave propagating direction and a grating 14 is arranged between them on the main surface of a piezo-electric substrate 1. Then, reflecting equipments 5a and 5b are arranged at the both sides of IDTs 2 and 3 so as to form a filter A. Two IDTs 6 and 7 are arranged along the surface acoustic wave propagating direction and the grating 8 is arranged between them at distance without the occurrence of acoustic interference with the filter A on the piezo-electric substrate 1. Then, the reflecting equipment 9a and 9b are arranged at the both sides of IDTs 6 and 7 so as to form the filter B. When the electrode logarithms of input side IDT2 and output side IDT 3 in the filter A are respectively adopted as Ni and Nm1, Ni≠Nm1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a longitudinally coupled multi-mode surface acoustic wave filter, and more particularly to a two-stage cascaded multi-mode surface acoustic wave filter having reduced in-band ripples caused when the filters are cascaded.

[0002]

2. Description of the Related Art In recent years, there have been remarkable small-size, low-loss, low-cost multi-mode SAW filters, which have greatly contributed to the spread of portable telephone terminals and the like. Multi-mode SA
One type of W filter is a vertically coupled triple mode SAW filter (hereinafter, referred to as a triple mode filter) as shown in FIG. In the triple mode filter shown in FIG. 4, two IDTs 21 and 22 are arranged on the main surface of the piezoelectric substrate 20 along the propagation direction of the surface acoustic wave, and a grating 23 is provided between the two IDTs 21 and 22. 24a, 2
The filter A in which 4b is arranged is referred to as a filter A. Further, two IDTs 25 along the propagation direction of the elastic surface at a distance that does not cause acoustic interference with the filter A on the piezoelectric substrate 20.
26 and a grating 27 in between, and the ID
A filter in which reflectors 28a and 28b are arranged on both sides of T25 and T26 is referred to as a filter B.

Each of the IDTs 21 and 22 forming the filter A is constituted by a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween. IDT21, 22
Is connected to ground potential and the other is electrically connected to an input or output.
The IDTs 25 and 26 forming the filter B are formed in the same manner as the IDTs 21 and 22 of the filter A. One of the comb-shaped electrodes of the IDT 22 of the filter A and one of the comb-shaped electrodes of the IDT 25 of the filter B are connected by a lead electrode, so that the filter A and the filter B are connected in cascade. Note that the IDT 21 in the filter A,
22 are equal in number of electrode pairs, and the distance G1 between the centers of the innermost electrode fingers facing the IDT 21 and the grating 23 is G1.
In general, the center-to-center spacing G2 between the innermost electrode fingers that face the IDT 22 and the grating 23 is equal. Also, the electrode pattern configuration of the filter B is configured in the same manner as the filter A.

The IDTs 21 and 22 of the filter A shown in FIG.
As a result, the vibration energy of the surface wave excited by the grating 23 is confined between the reflectors 24a and 24b.
The next mode, the vertical second mode and the vertical third mode are vigorously excited. It is well known that a triple mode filter can be configured using the resonance frequencies of these resonance modes and their phases. The operation of the filter B is the same as the operation of the filter A. In general, the filter A and the filter B are electrically connected in cascade to increase the attenuation gradient and increase the stop band attenuation. ing.

[0005]

However, in the conventional triple mode filter as described above, for example,
A 45 ° X-cut Z-propagating lithium tetraborate substrate is used as the piezoelectric substrate 20, the number of electrode pairs of the IDT 21 of the filter A is 40, the number of electrode pairs of the IDT 22 is 40, the number of electrode fingers of the grating 23 is 18, and the reflector 24a is used. , 24b each have 100 electrode fingers, and the gap G1 between the IDT 21 and the grating and the gap G2 between the IDT 22 and the grating are equal to 0.450λ (λ is the electrode finger cycle of the IDTs 21 and 22). Filter A for Filter B
Use the same parameters as The filter characteristic in which the filter A and the filter B prototyped using the above parameters are connected in cascade has a problem that a large ripple occurs in the pass band as shown in FIG. In order to remove this ripple, the lead electrodes connecting filters A and B
When a reactance element is inserted electrically in parallel with the DTs 22 and 25 and the reactance value is adjusted, the ripple on the low frequency side in the pass band as shown in FIG. There is a problem that the ripple on the high frequency side is larger than the ripple before the reactance is inserted. It is an object of the present invention to provide a two-stage cascaded triple mode filter which has a flat characteristic without having to insert a reactance element into the passband characteristic of a two-stage cascade triple mode filter.

[0006]

According to a first aspect of the present invention, there is provided a surface acoustic wave filter according to the present invention, wherein a surface acoustic wave filter is provided on a main surface of a piezoelectric substrate along a propagation direction of a surface acoustic wave.
A grating is disposed between two IDTs and an intermediate portion thereof,
In a two-stage cascaded longitudinally-coupled triple-mode surface-wave filter configured by juxtaposing two longitudinally-coupled triple-mode surface-wave filters each having a reflector disposed on both sides of the DT, the first-stage triple-mode surface-wave filter includes The number of electrode pairs of the input / output IDT is Ni and Nm1, respectively, and the number of electrode pairs of the input / output IDT of the second-stage triple mode surface acoustic wave filter is Nm.
2. When No, a surface acoustic wave filter characterized by Ni ≠ Nm1 or No ≠ Nm2. According to a second aspect of the present invention, there is provided a vertically coupled triple having two IDTs and a grating disposed therebetween on the main surface of the piezoelectric substrate along the propagation direction of the surface acoustic wave, and reflectors disposed on both sides of the IDTs. In a two-stage cascade longitudinally coupled triple mode surface wave filter configured by juxtaposing two mode surface wave filters,
In the first stage, the center-to-center spacing between the innermost electrode fingers facing the input / output side IDT and the grating of the longitudinally coupled triple mode surface wave filter is G11, G12, G2 is the distance between the centers of the innermost electrode fingers facing the input / output side IDT and the grating.
1, a surface acoustic wave filter characterized by G11 ≠ G12 or G21 ≠ G22, where G22.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the drawings. FIG. 1 shows a second embodiment of the present invention.
FIG. 4 is a diagram showing an example of an embodiment of a cascaded triple mode filter, in which two IDTs 2, 3 and a grating 4 are arranged on the main surface of the piezoelectric substrate 1 along the propagation direction of the surface acoustic wave, Reflectors 5a, 5 on both sides of the IDTs 2, 3
A filter in which b is arranged is referred to as a filter A. Further, two IDTs 6, 7 along the propagation direction of the elastic surface at a distance that does not cause acoustic interference with the filter A on the same piezoelectric substrate 1.
And a grating 8 between them and the IDT 6,
A filter in which reflectors 9a and 9b are arranged on both sides of the filter 7 is referred to as a filter B.

The IDTs 2 and 3 forming the filter A are each composed of a pair of comb-shaped electrodes having a plurality of electrode fingers interposed therebetween. One of the IDTs 2 and 3 is connected to the ground potential, and the other is electrically connected to an input or an output. And
One comb-shaped electrode of IDT2 of filter A and filter B
The IDT 6 is connected to one of the comb-shaped electrodes using a lead electrode, and the filter A and the filter B are connected in cascade. The distance between the centers of the innermost electrode fingers that face the IDT 2 and the grating 4 is G11, and the distance between the centers of the innermost electrode fingers that face the IDT 3 and the grating 4 is G12. In the filter B, the distance between the centers of the innermost electrode fingers facing the IDT 6 and the grating 8 is G2.
1. The center-to-center distance between the innermost electrode fingers facing the IDT 7 and the grating 8 is defined as G22.

The vibration energy of the surface wave excited by the IDTs 2 and 3 and the grating 4 of the filter A shown in FIG. 1 is confined between the reflectors 5a and 5b.
Acoustically coupled to three resonant modes, the longitudinal first mode,
The vertical secondary mode and the vertical tertiary mode are strongly excited. A triple mode filter can be configured using the resonance frequencies and phases of these resonance modes. The operation of the filter B is the same as the operation of the filter A.
As described above, the filter and the filter B are electrically connected in cascade to increase the attenuation gradient of the filter and increase the amount of attenuation in the stop band.

The inventor studied various design parameters to flatten the passband characteristics of the filters without externally providing a reactance element between the two filters A and B. As a result, 1) the number of electrode pairs of the input side IDT2 of the filter A is Ni,
When the number of electrode pairs of the output side IDT3 is Nm1, the number of electrode pairs of the input side IDT6 of the filter B is Nm2, and the number of electrode pairs of the output side IDT7 is No, the condition of Ni ≠ Nm1 (1) or Nm2 ≠ No (2) Or 2) as shown in FIG.
2. The center-to-center spacing of the innermost electrode fingers facing the IDT 3 and the grating 4 is G11 and G12, respectively, and the center-to-center spacing of the innermost electrode fingers facing the input / output sides IDT6 and IDT7 of the filter B and the grating 8 is G11 and G12. Each G
21 and G22, when the condition of G11 ≠ G12 (3) or G21 ≠ G22 (4) is satisfied, without externally providing a reactance element between the two triple mode filters, it is possible to reduce It has been found experimentally that the characteristics can be made flat.

For example, as a first embodiment, a piezoelectric substrate 1
45 ° X-cut Z-propagating lithium tetraborate substrate was used as the filter A, and the number of electrode pairs Ni of the IDT 2 was 40 as the filter A.
The number of electrode pairs Nm1 of the IDT3 is 28, the number of electrode fingers of the grating 4 is 52, the number of electrode fingers of the reflectors 5a and 5b is 100, and the gaps G11 and G12 between the IDTs 2 and 3 and the grating 4 are 0. .460λ,
0.525λ, and as a filter B, IDT
No. 6 electrode pairs Nm2, 28 IDT7 electrode pairs No.
, The number of electrode fingers of the grating 8 is 52, the number of electrode fingers of the reflectors 9a and 9b is 100, and the ID is
Filters A and G22 at intervals G21 and G22 between T6 and T7 and grating 8 were set to 0.525λ and 0.460λ, respectively.
FIG. 2 shows the filtering characteristics when the filter and the filter B are connected in cascade. As is clear from the figure, a flat characteristic in the pass band was obtained. Moreover, a large ripple on the high frequency side in the pass band, which was caused when the conventional triple mode filters were cascaded in two stages and the reactance elements were connected in parallel between the stages, could be eliminated. At this time, the relationship between Ni and Nm1 and N
The relationship between o and Nm2 satisfies equations (1) and (2), respectively. Further, the relationship between G11 and G12 and G21 and G2
The relationship with 2 satisfies equations (3) and (4).

As a second embodiment, the piezoelectric substrate 1
Using a 5 ° X-cut Z-propagating lithium tetraborate substrate, as the filter A, the number of electrode pairs Ni of the IDT 2 was 35,
The number of electrode pairs Nm1 of T3 is 35 pairs, the number of electrode fingers of grating 4 is 38, the number of electrode fingers of reflectors 5a and 5b is 100 each, G11 is 0.455λ, and G12 is 0.
550λ, and as the filter B, the number of electrode pairs Nm2 of the IDT 6 is 35 and the number of electrode pairs of the IDT 7 is 35
In contrast, the number of electrode fingers of the grating 8 is 38, and the reflector 9
a and 9b each having 100 electrode fingers, G21 being 0.550λ, and G22 being 0.455λ, filter A
FIG. 3 shows the filtering characteristics when the filter and the filter B are connected in cascade. As is clear from the figure, a flat characteristic in the pass band was obtained. In this case, the relationship between Expressions (1) and (2) is not satisfied, but the relationship between Expressions (3) and (4) is satisfied.

[0013] Not only the above two examples but also the formulas (1) to
According to the present invention, it is possible to flatten the characteristics in the pass band without inserting an external reactance element between stages when cascading a triple mode filter by using any of the relations of (4). Confirmed experimentally. In the above two examples, a lithium tetraborate substrate was used as the piezoelectric substrate. However, the present invention is effective for other types of piezoelectric substrates, such as quartz, lithium tantalate, and lithium niobate. Needless to say,

[0014]

As described above, the present invention is constructed as described above. Therefore, when a vertically coupled triple mode filter is cascaded in two stages to form a filter, a reactance element is not externally provided between the stages, and the pass band can be reduced. Can be flattened, so that an excellent effect that a filter having a small size and excellent filtering characteristics can be easily realized is exhibited.

[Brief description of the drawings]

FIG. 1 is a diagram showing an electrode pattern of an embodiment of a two-stage cascade connection triple mode surface acoustic wave filter according to the present invention.

FIG. 2 is a diagram showing a filtering characteristic of a filter according to a first embodiment, which is prototyped according to the present invention.

FIG. 3 is a diagram showing the filtering characteristics of a prototyped filter according to a second embodiment based on the present invention.

FIG. 4 is a schematic diagram showing an electrode pattern of a conventional two-stage cascade-connected longitudinally coupled triple-mode surface acoustic wave filter.

5A and 5B are diagrams showing the filtering characteristics of a conventional two-stage cascade-connected longitudinally-coupled triple-mode surface acoustic wave filter, wherein FIG. 5A shows a case where no reactance element is inserted between connection stages, and FIG. 5B shows a case where a reactance element is inserted. This is the case.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Piezoelectric substrate 2, 3, 6, 7 ... IDT 4, 8 ... Grating 5a, 5b, 9a, 9b ... Reflector G11, G12, G21, G22 ... IDT and grating Center-to-center spacing of innermost electrode fingers facing each other Filter A, Filter B: longitudinally coupled triple mode surface acoustic wave filter

Claims (2)

[Claims] 1. A longitudinally coupled triple mode surface wave comprising two IDTs and a grating disposed between the two IDTs on the principal surface of the piezoelectric substrate along the propagation direction of the surface acoustic wave, and reflectors disposed on both sides of the IDTs. In a two-stage cascaded longitudinally coupled triple-mode surface acoustic wave filter configured by arranging two filters in parallel, the number of electrode pairs of input / output IDTs of the first-stage triple-mode surface acoustic wave filter is Ni and Nm1, respectively. A surface acoustic wave filter characterized in that Ni ≠ Nm1 or No ≠ Nm2 when the number of electrode pairs of the input / output IDT of the triple mode surface acoustic wave filter is Nm2 and No, respectively. 2. A longitudinally coupled triple mode surface wave comprising two IDTs and a grating disposed between the two IDTs along the propagation direction of the surface acoustic wave on the main surface of the piezoelectric substrate and reflectors disposed on both sides of the IDTs. In a two-stage cascade-connected longitudinally-coupled triple-mode surface acoustic wave filter constituted by arranging two filters in parallel, the input / output side I of the longitudinally-coupled triple-mode surface acoustic wave filter in the first stage
The centers of the innermost electrode fingers that face the DT and the grating are G11 and G12, respectively, and the input / output side IDT of the second-stage longitudinally coupled triple mode surface acoustic wave filter and the innermost electrode finger that faces the grating. A surface acoustic wave filter characterized in that G11 ≠ G12 or G21 ≠ G22 when the center-to-center spacing is G21 and G22, respectively.