JPH088689A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To provide the surface acoustic wave device having the IIDT structure which has the out-band attenuation characteristic improved while maintaining a sufficiently small insertion loss. CONSTITUTION:Plural IDTs 21 to 25 for input and plural IDTs 31 to 36 for output are alternately arranged on a piezoelectric substrate 10, and plural IDTs 21 to 25 for input and plural IDTs 31 to 36 for output are connected in parallel to constitute the IIDT structure. Plural IDTs 21 to 25 for input and plural IDTs 31 to 36 for output consist of plural kinds of IDTs different by logarithms, and logarithms of IDTs have relations of about an integral multiple value obtained by dividing fo by ¦fo-fr¦ where fo is the passage frequency and fr is the suppression frequency to be especial suppressed. A relatively wide trap is brought about in the vicinity of the suppression frequency fr.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a plurality of IDs on a piezoelectric substrate.
The present invention relates to a surface acoustic wave device having T formed therein, and more particularly to a surface acoustic wave filter suitable for use as a front end filter of a wireless reception device.

[0002]

2. Description of the Related Art In recent years, a so-called front-end filter, which is a filter that passes a target frequency fo, has been used in a receiving section of a mobile communication device or the like. As the front-end filter, a filter having a small insertion loss in the pass band and a high attenuation outside the pass band is desired.

A surface acoustic wave filter having an IIDT structure is used as a suitable one for such a front end filter. The surface acoustic wave filter having the IIDT structure is
A plurality of input IDTs and a plurality of output IDTs are alternately arranged, and a plurality of input IDTs and a plurality of output IDTs are connected in parallel. The surface acoustic wave filter having the IIDT structure has a characteristic that the insertion loss is small, but it is difficult to obtain a large attenuation characteristic outside the band. Considering this point, for example, the logarithmic ratio of the input IDT and the output IDT is set to 1 so that the peak of the output IDT coincides with the trap of the radiation conductance of the input IDT.
By setting the ratio to 0.7, a relatively flat attenuation characteristic outside the band is realized.

[0004]

However, such a surface acoustic wave filter having an IIDT structure may not be able to obtain a sufficient attenuation characteristic mainly outside the band. Therefore, weighting such as thinning or apodization is performed on the IDT, or two or more electrode structure rows are connected in cascade.

However, if the IDT is thinned or weighted by apodization, the insertion loss in the pass band increases, and if two or more electrode structure rows are connected in cascade, the insertion loss may increase. There is a problem that the substrate itself of the surface acoustic wave device becomes large. It is an object of the present invention to provide a surface acoustic wave device having an IIDT structure, which has improved out-of-band attenuation characteristics while maintaining a sufficiently low insertion loss.

[0006]

The inventor of the present application has made it possible for the IDT to perform weighting such as thinning or apodization on the IDT or cascade connection of two or more electrode structure rows.
As a result of diligent research on how to improve the out-of-band attenuation characteristics of the surface acoustic wave filter having the structure, if it is possible to sufficiently attenuate the specific frequency instead of all the frequencies outside the band, For example, I conceived that the characteristics as a front-end filter could be improved. That is, if the image frequency in the super-heterodyne reception system or a specific frequency such as a frequency with a strong interfering radio wave can be attenuated,
The present invention was conceived to improve the reception characteristics of a receiving unit such as a mobile communication device using such a surface acoustic wave device as a front end filter.

Therefore, an object of the present invention is to provide a piezoelectric substrate, a plurality of input IDTs formed on the piezoelectric substrate and connected in parallel with each other, and a plurality of the input IDTs formed on the piezoelectric substrate. In a surface acoustic wave device having a plurality of output IDTs connected in parallel with each other, the plurality of input IDTs and the plurality of output ITs are provided.
The DT is composed of a plurality of types of IDTs whose logarithms are different from each other, and the pass frequency is fo, and in particular, the suppression frequency that suppresses is f.
As r, the relationship between the logarithms of the plurality of types of IDTs is f
It is achieved by a surface acoustic wave device characterized in that the relationship is a value obtained by dividing o by | fo-fr |, which is an integer multiple.

In the surface acoustic wave device, the plurality of types of IDTs are at least three types of logarithmic IDTs.
Is desirable. Further, in the above-described surface acoustic wave device, there are four or more sets of input IDTs and output IDTs, respectively, and it is desirable that they are of a normal type without apodization. The logarithm of the input IDT and the output IDT may deviate from the above-described calculated value by about the width of the pass band, or about 5%. Further, the input IDT and the output IDT may be replaced with each other.

If there are two or more suppression frequencies to be suppressed, the passing frequency is fo, and the suppressing frequencies to be particularly suppressed are fr1, fr2, ... And the passing frequency fo is Δ.
fr1 (= | fo-fr1 |), Δfr2 (= | fo-
fr2 |), ..., It is desirable to associate the values k1, k2, ...

Further, a pair of reflectors may be provided outside the input IDT and the output IDT.

[0011]

According to the present invention, a plurality of input IDTs and a plurality of output IDTs are provided as a plurality of types of IDs having different logarithms.
The relation between the logarithms of a plurality of types of IDTs is an integer multiple of the value obtained by dividing fo by | fo-fr |, where T is the pass frequency, and fr is the suppression frequency for suppression. Therefore, it is possible to improve the out-of-band attenuation characteristic by attenuating a specific unnecessary suppression frequency while securing a sufficiently low insertion loss as the IIDT structure.

In the surface acoustic wave device, if the plurality of types of IDTs are IDTs of at least three types of logarithms, sufficient attenuation characteristics can be realized for specific frequencies outside the band. Further, in the above-described surface acoustic wave device, if the input IDT and the output IDT are made normal without apodizing, the insertion loss can be further reduced while maintaining sufficient out-of-band attenuation characteristics. it can.

Further, the relationship between the logarithms of a plurality of types of IDTs is set to be an integer multiple of a value close to the least common multiple of values k1, k2, ... Which are obtained by dividing the pass frequency fo by Δfr1, Δfr2 ,. It is possible to effectively suppress signals having two or more suppression frequencies. Furthermore, if a pair of reflectors are provided outside the input IDT and the output IDT, the insertion loss can be further reduced.

[0014]

【Example】

[Embodiment 1] FIG. 1 shows a surface acoustic wave device according to this embodiment. The surface acoustic wave device of the present embodiment, as shown in FIG.
An electrode structure array 20 is formed on a piezoelectric substrate 10 made of a 36 ° rotated Y plate-X propagation LiTaO ₃ . The electrode structure array 20 includes five input IDTs 21 to 25 having different logarithms.
And six output IDTs 31 to 36 having different logarithms, and a pair of reflectors 37 and 38. Between the six output IDTs 31 to 36, the five input IDTs 21 to
25 are arranged so as to be respectively inserted, and the five input IDTs 21 to 25 and the six output IDTs 31 to 36 are connected in parallel. These input IDT21
˜25 and the output IDTs 31 to 36 are a pair of reflectors 37,
It is sandwiched by 38.

Input IDTs 21 to 25, output IDT 3
1 to 36 and the reflectors 37 and 38 are made of an aluminum alloy having a normalized film thickness h / λ of about 3%. The opening length of the input IDTs 21 to 25 and the output IDTs 31 to 36 is 10λ. In this embodiment, the pass frequency fo = 1.
00, the suppression frequency fr = 0.75.
Frequency difference Δfr (= pass frequency fo and suppression frequency fr)
| Fo-fr |) is 0.25, and the value k (= fo / Δfr) obtained by dividing the passing frequency fo by the frequency difference Δfr is 4.

Therefore, the logarithm of the input IDTs 21 to 25 and the logarithm of the output IDTs 31 to 36 are set to values close to multiples of 4, in this embodiment, values close to 20, 24 and 28. Specifically, set the input IDT 23 at the center to 2
8.5 pairs, the output IDTs 33 and 34 on both outer sides are 28.5 pairs, and the input IDTs 22 and 24 on both outer sides.
24.5 pairs, and output IDTs 32 and 3 on both outer sides
5 is 24.5 pairs, and the input IDTs 21 on both outer sides are
25 for 20.5 pairs, and output IDT3 on both outer sides
1 and 36 are 20.5 pairs. Since the surface acoustic wave leaks from the end IDT, the logarithm of the outer IDT is reduced in both the input IDTs 21 to 25 and the output IDTs 31 to 36.

As shown in FIG. 2, for a pass frequency fo = 1, at a frequency fr = 0.75, 1.25, a relatively wide trap (suppression) having an attenuation length of 10 dB or more larger than the surroundings Region), and the average value of attenuation outside the band is about 30 dB. The minimum insertion loss in the pass band is 1.8 dB or less. Incidentally, the reflectors 37, 38
Even when no is provided, the attenuation characteristic outside the band can be obtained similarly. [Embodiment 2] A surface acoustic wave device according to this embodiment is shown in FIG.

The surface acoustic wave device of this embodiment is the same as the surface acoustic wave device of the first embodiment except that the input IDTs 21 to 25 and the output IT are used.
Only the logarithm of the DTs 31 to 36 is different, and the other configurations are the same. In this embodiment, the first suppression frequency fr1 = 0.875 with respect to the pass frequency fo = 1.000, and the second frequency
The suppression frequency fr2 is set to 0.750. Frequency difference Δfr1 (= | f between pass frequency fo and suppression frequency fr1
o-fr1 |) becomes 0.125, and the frequency difference Δfr2 (= | fo-fr) between the pass frequency fo and the suppression frequency fr2.
2 |) becomes 0.250.

The value k1 (= fo / Δfr1) obtained by dividing the passing frequency fo by the frequency difference Δfr1 becomes 8, and the value k2 (= fo / Δf) obtained by dividing the passing frequency fo by the frequency difference Δfr2.
r2) becomes 4. Since the least common multiple of k1 and k2 is 8, the logarithm of the input IDTs 21 to 25 and the output IDT3.
The logarithm of 1 to 36 is a value close to a multiple of 8, which is 1 in this embodiment.
It is set to a value close to 6, 24 and 32. Specifically, the central input IDT 23 is 32.5 pairs, the outer output IDTs 33 and 34 are 32.5 pairs, and the outer input IDTs 22 and 24 are 24.5 pairs. 24.5 pairs of output IDTs 32 and 35 on both outer sides,
The input IDTs 21 and 25 on both outer sides are 16.5 pairs, and the output IDTs 31 and 36 on both outer sides are 16.5 pairs.

Also in this embodiment, in the input IDTs 21 to 25 and the output IDTs 31 to 36, the logarithm of the outer IDT is smaller. FIG. 4 shows the attenuation characteristics of the surface acoustic wave device according to this example. As shown in FIG. 2, for the pass frequency fo = 1, the frequency fr = 0.625,
0.875, 0.750, 1.125, 1.250,
At 1.375, a relatively wide trap (suppression region) having an attenuation of 10 dB or more larger than that of the surrounding area can be obtained. The average value of the attenuation outside the band is about 30 dB.
The minimum insertion loss in the pass band is 1.8 dB or less. Comparative Example FIG. 5 shows a surface acoustic wave device according to a comparative example.

The surface acoustic wave device of this comparative example comprises the surface acoustic wave devices of Examples 1 and 2 and input IDTs 21 to 2.
5 and the output IDTs 31 to 36 are different only in logarithm, and other configurations are the same. Input IDTs 21 to 25 and output I
Although the logarithmic ratio of DT31 to 36 is about 1: 0.7,
The input IDTs 21 to 25 have the same logarithm, and the output IDTs 31 to 36 have the same logarithm. That is, the input IDTs 21 to 25 are all 20.5 pairs, and
The DTs 31 to 36 are all 30.5 pairs.

FIG. 6 shows the attenuation characteristics of the surface acoustic wave device of the comparative example.
Shown in As shown in FIG. 2, the attenuation outside the band is 30
There is a portion that does not reach dB, and there is no trap (suppression region) as in the first and second embodiments. The average value of the attenuation outside the band is about 30 dB. The minimum insertion loss in the pass band is 1.8 dB.

[0023]

As described above, according to the present invention, a plurality of input IDTs and a plurality of output IDTs are composed of a plurality of types of IDTs having different logarithms, and the pass frequency is fo,
In particular, the suppression frequency to be suppressed is fr, and a plurality of types of ID
Since the relationship between the logarithms of T is such that it is an integer multiple of the value obtained by dividing fo by | fo-fr |, a sufficiently low insertion loss is ensured as the IIDT structure, and a specific unnecessary suppression is performed. The frequency can be attenuated to improve the out-of-band attenuation characteristic.

[Brief description of drawings]

FIG. 1 is a diagram showing a surface acoustic wave device according to a first embodiment.

FIG. 2 is a graph showing the attenuation characteristics of the surface acoustic wave device of Example 1.

FIG. 3 is a diagram showing a surface acoustic wave device according to a second embodiment.

FIG. 4 is a graph showing the attenuation characteristics of the surface acoustic wave device of Example 2.

FIG. 5 is a diagram showing a surface acoustic wave device of a comparative example.

FIG. 6 is a graph showing attenuation characteristics of a surface acoustic wave device of a comparative example.

[Explanation of symbols]

 10 ... Piezoelectric substrate 20 ... Electrode structure array 21-25 ... Input IDT 31-36 ... Output IDT 37, 38 ... Reflector

Claims (2)

[Claims] 1. A piezoelectric substrate, a plurality of input IDTs formed on the piezoelectric substrate and connected in parallel to each other, and formed on the piezoelectric substrate and inserted between the plurality of input IDTs. Multiple output IDs connected in parallel with each other
A surface acoustic wave device having T and the plurality of input IDTs and the plurality of output IDTs.
Is composed of a plurality of types of IDTs having different logarithms, and the relationship between the logarithms of the plurality of types of IDTs is fo |
A surface acoustic wave device having a relationship of being an integer multiple of a value divided by fo-fr |. 2. The surface acoustic wave device according to claim 1, wherein the plurality of types of IDTs have at least three types of logarithmic I.
A surface acoustic wave device characterized by being a DT.