JPH06260880A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To reduce the loss and the in-band ripple and to miniaturize a device by alternately arranging input and output transducers, which have plural electrode fingers arranged at regular intervals, on the same surface acoustic wave propagation line. CONSTITUTION:A surface acoustic wave wavelength lambda is determined by electrode finger pitches of input transducers 11 to 14 and output transducers 21 to 23. Reflectors 31 and 32 are arranged on both of outermost sides of groups of transducers 11 to 14 and 21 to 23. Since respective electrode finger pitches of transducers 11 to 14 and 21 to 23 are equal and they are alternately arranged on the same surface acoustic wave propagation line, the loss is reduced. Since reflectors 31 and 32 have plural short strip conductors and are arranged on both of outermost sides of transducers 11 to 14 and 21 to 23, the device is miniaturized. Relations between the wave-length lambda and distances (d) from the centers of electrode fingers adjacent to reflectors 31 and 32 to the centers of short strip conductors of reflectors 31 and 32 are selected to reduce the in-band ripple.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-electrode type surface acoustic wave filter, and more particularly, to an optimum distance between a surface acoustic wave converter arranged on the outermost side and a reflector arranged adjacent to the surface acoustic wave converter. Involved in the range.

[0002]

2. Description of the Related Art In recent years, there has been a demand for reduction in size and weight of mobile communication devices such as mobile phones and cordless phones. Aiming to be applied to this mobile communication device, small-sized and low-loss high-frequency surface acoustic wave filters have been actively researched.

Conventionally, in order to reduce the loss of a surface acoustic wave filter, a multi-electrode surface acoustic wave filter having a plurality of surface acoustic wave converters arranged on the same propagation path and alternately used as input and output converters. Is being considered. As prior art documents,
For example, U.S. Pat. No. 3,582,840 is known.

It is known that in multi-electrode surface acoustic wave filters, the loss decreases with the number of transducers. For example, in the above-mentioned US Pat. No. 3,582,840, assuming that the number of input side converters is N and the number of output side converters is (N-1), loss = 10 log [2N / (2N−2)]. Therefore, in order to reduce the loss, it is desired that the number of input / output converters is large. Normally,
Often a total of 13 converters are used for input and output.

However, when attempting to construct a filter having a relatively narrow band, it is necessary to increase the number of pairs of electrode fingers of the interdigital transducer formed in one transducer as the band is narrowed. This is because the relative bandwidth Δf / f0 obtained by normalizing the bandwidth Δf with the filter center frequency f0 is almost inversely proportional to the number of electrode finger pairs. Therefore, in the above-mentioned electrode configuration, there is a problem that the device becomes large in size when the bandwidth is narrowed.

As a means for solving this problem, the use of a reflector used in a surface acoustic wave resonator can be considered. Even in the multi-electrode surface acoustic wave filter, by installing a reflector on the outermost side, the surface acoustic wave energy leaking outside the converter can be reflected and confined inside the converter again, thus achieving low loss. It is thought to be done. As an example of a prior art document in which a reflector is combined with a multi-electrode surface acoustic wave filter, for example, Japanese Patent Laid-Open No. 3-27
There is 0309 publication.

[0007]

As a structure of a reflector to be combined with a multi-electrode surface acoustic wave filter, an open strip type reflector, a short strip type reflector, and the like have been studied. However, no consideration was given to the position where the reflector should be installed. In particular, when a short strip reflector is used, a large ripple occurs in the band, which is a serious problem.

Therefore, a first object of the present invention is to provide a low-loss multi-electrode surface acoustic wave filter.

A second object of the present invention is to provide a small multi-electrode surface acoustic wave filter.

A third object of the present invention is to provide a multi-electrode type surface acoustic wave filter with reduced ripples within the band.

[0011]

To solve the above problems, the present invention provides a surface acoustic wave filter having an input converter, an output converter and a reflector on the surface of a piezoelectric substrate,
The input converter has a plurality of electrode fingers, the electrode fingers are arranged at intervals, and the output converter has a plurality of electrode fingers, and the electrode fingers are arranged at intervals. The input converter and the output converter are alternately arranged on the same surface acoustic wave propagation path, the reflector has a plurality of short strip conductors, and the input converter and the output The first strip conductor of the reflector, located on the outermost sides of the group of transforms, from the center of the electrode fingers of the input or output transducers adjacent to the reflector. Where d is the distance to the center of and the wavelength of the surface acoustic wave is λ, {(n + 0.2) · λ / 2} ≧ d ≧ {(n-0.2) · λ / 2} (n is Positive integer) is satisfied.

[0012]

The input transducer has a plurality of electrode fingers spaced apart, the output transducer has a plurality of electrode fingers spaced apart, and the input transducer and the output transducer have the same elastic surface. Since they are alternately arranged on the wave propagation path, a low-loss multi-electrode surface acoustic wave filter can be obtained.

Since the reflector has a plurality of short strip conductors and is arranged on both outermost sides of the group of input converters and output converters, it is smaller than the one without a reflector.

Moreover, the distance from the center of the electrode finger adjacent to the reflector among the electrode fingers of the input converter or the output converter to the center of the first strip conductor of the reflector is d, and the wavelength of the surface acoustic wave is set. Is expressed as λ, {(n + 0.2) · λ / 2} ≧ d ≧ {(n-0.2) · λ / 2} (n is a positive integer) is satisfied. It is possible to reduce the ripple in the band, which has been a problem in the surface wave filter.

[0015]

1 is a diagram schematically showing the structure of a surface acoustic wave filter according to the present invention, and FIG. 2 is a partially enlarged view of the surface acoustic wave filter shown in FIG. In the figure, 11 to 14 are input converters, 21 to 23 are output converters, and 31 and 32 are reflectors. These are formed on the surface of a piezoelectric substrate (not shown). A 36 ° rotation Y-cut lithium tantalate substrate can be used as the piezoelectric substrate. Input converter 11
2 to 14 are a plurality of electrode fingers a1 as shown in FIG.
Have a9. The electrode fingers a1 to a9 are arranged with a space w therebetween. The electrode fingers a <b> 1 to a <b> 9 are combined in a comb shape, and are divided into a set of an electrode grounded every other and an electrode connected to the input terminal 4. The number of electrodes a1 to a9 can be increased or decreased.

The output converters 21 to 23 also have a plurality of electrode fingers b.
1 to b9, and electrode fingers b1 to b9 are arranged at intervals. The electrode fingers b1 to b9 are combined in a comb shape, and are divided into a set of an electrode grounded at every other electrode and an electrode connected to the output terminal 5. The number of electrodes b1 to b9 can be increased or decreased.

Input converters 11-14 and output converter 21
23 are alternately arranged on the same surface acoustic wave propagation path X.

The wavelength λ of the surface acoustic wave is determined by the input converters 11 to 11.
14 or the electrode finger pitch of the output converters 21 to 23, the interval w between the electrode fingers a1 to a9 and b1 to b9 is determined in consideration of the wavelength λ of the surface acoustic wave.

The reflectors 31, 32 have a plurality of short strip conductors and are arranged on both outermost sides of the group of input converters 11-14 and output converters 21-23. Figure 1
, The reflector 31 is arranged adjacent to and outside the input converter 11, and the reflector 32 is arranged adjacent to and outside the input converter 14. Although not shown, the reflector 31 may be disposed adjacent to the output converter when the output converter is located on the outermost side.

Input converters 11-14 and output converters 21-
The electrodes of 23 and the reflectors 31 and 32 can be formed by applying a general photolithography technique as a semiconductor process. Aluminum or aluminum alloy is used as the electrode material.

The input converters 11 to 14 have a plurality of electrode fingers a.
1 to a9 are arranged at intervals w, the output converters 21 to 23 are arranged at a plurality of electrode fingers b1 to b9 at intervals, and the input converters 11 to 14 and the output converter 21 are arranged. Since Nos. 23 to 23 are alternately arranged on the same surface acoustic wave propagation path X, a low-loss multi-electrode surface acoustic wave filter can be obtained.

The reflectors 31, 32 have a plurality of short strip conductors and are arranged on both outermost sides of the group of input converters 11-14 and output converters 21-23.
The size is smaller than that without the reflectors 31 and 32.

Next, referring to FIG. 3, the input converter 11
Of the electrode fingers a1 to a9, the distance from the center of the electrode finger a1 adjacent to the reflector 31 to the center of the first short strip conductor c1 of the reflector 31 is d, and the wavelength of the surface acoustic wave is λ. At this time, {(n + 0.2) · λ / 2} ≧ d ≧ {(n−0.2) · λ / 2} (n is a positive integer) is satisfied. Input converter 14 and reflector 3
The same relationship is also established between the two.

As described above, the relationship between the distance d from the center of the electrode finger a1 adjacent to the reflector 31 to the center of the first short strip conductor c1 of the reflector 31 and the wavelength λ of the surface acoustic wave is described above. By selecting as described above, it is possible to reduce the ripple in the band, which has been a problem in the related art in the multi-electrode surface acoustic wave filter in which the reflectors are combined.

Next, the operation and effect of the present invention will be described in more detail with reference to the experimental data shown in FIGS.

FIG. 4 shows frequency characteristics when reflectors are not installed on both outermost sides of the converter. The structure is the same as that of FIG. 1 except that the reflector is not provided. The number of input converters 11 to 14 is four, and the number of output converters 21 to 23 is three. The wavelength λ of the surface acoustic wave was set to λ = 4.7 μm.

As shown in FIG. 4, when the reflectors are not installed on both outermost sides of the converter, the frequency characteristics in which the in-band characteristics are rounded and the loss is low are narrow,
Not preferred.

FIG. 5 shows the surface acoustic wave filter shown in FIG. 1 in which the reflectors 31 and 32 are installed and the distance d is d = 1.7.
It is a frequency characteristic when it is set to 5λ. Input converter 11
The number of the output converters 21 to 23 is four, and the number of the output converters 21 to 23 is three, and the reflectors 31 and 32 including 55 short strip conductors are installed on the outermost sides of both. The wavelength λ of the surface acoustic wave was set to λ = 4.7 μm.

D = 1.75λ is a value when n = 4 in d = (n-0.5) · λ / 2. Alternatively, it is a value when n = 3 in d = (n + 0.5) · λ / 2. Therefore, d = (n-0.5) * [lambda] / 2 <(n-0.2) * [lambda] / 2 d = (n + 0.5) * [lambda] / 2> (n + 0.2) * [lambda] / 2 The range of the present invention {(n + 0.2) · λ / 2} ≧ d ≧ {(n−0.2) · λ / 2} is not satisfied.

As shown in FIG. 5, a large ripple is seen in the band, which is not preferable as a high frequency low loss filter.

FIG. 6 shows the surface acoustic wave filter shown in FIG. 1 in which reflectors 31 and 32 are provided on both outermost sides of the converter.
It is a frequency characteristic when the distance d is d = 2.0λ.
The number of input converters 11 to 14 is four, and the number of output converters 2 is 4.
The number of 1 to 23 is 3, and the reflectors 31 and 32 including 55 short strip conductors are installed on the outermost sides of both. The wavelength λ of the surface acoustic wave was set to λ = 4.7 μm. However, the wavelength λ is not limited to this value.

D = 2.0λ in this embodiment is a value when n = 4 in d = (n + 0) λ / 2. Therefore, {(n + 0.2) · λ / 2} ≧ {(n + 0) · λ / 2} ≧ {(n−0.2) · λ / 2} and {(n + 0.2) · λ / 2 } ≧ d ≧ {(n-0.2) · λ / 2} is satisfied.

As shown in FIG. 6, it is understood that the surface acoustic wave filter according to the present invention has a flat band and a wide frequency range in which a low loss is obtained, and a desirable characteristic as a high frequency low loss filter is obtained. .

As a result of the above experiment, in the multi-electrode type surface acoustic wave filter using the short strip type reflector, the position d of the reflector is half the value of λ that can be set by the electrode finger pitch of the transducer. It has been found that a filter having an expanded low loss region in the band can be obtained by setting the filter so that the filter is installed in the vicinity of an integral multiple of λ / 2.

Although the embodiment of the present invention has been described above, the present invention is not limited to this, and those skilled in the art can make various modifications and changes within the scope of the claims. Would be obvious.

[0036]

As described above, according to the present invention,
The following effects can be obtained. (A) It is possible to provide a low-loss multi-electrode surface acoustic wave filter. (B) It is possible to provide a small multi-electrode surface acoustic wave filter. (C) It is possible to provide a multi-electrode surface acoustic wave filter with reduced ripple in the band.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a configuration of a surface acoustic wave filter according to the present invention.

FIG. 2 is a partially enlarged view of the surface acoustic wave filter shown in FIG.

FIG. 3 is a partially enlarged view of the surface acoustic wave filter shown in FIG.

FIG. 4 is a frequency characteristic diagram of a conventional surface acoustic wave filter without a reflector.

FIG. 5 is a frequency characteristic diagram of a surface acoustic wave filter in which a reflector is installed at a position of d = 1.75λ.

FIG. 6 is a frequency characteristic diagram of a surface acoustic wave filter according to the present invention in which a reflector is grounded at a position of d = 2.0λ.

[Explanation of symbols]

 11-14 Input converter 21-23 Output converter 31, 32 Reflector

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuo Sato 1-13-1 Nihonbashi, Chuo-ku, Tokyo Inside TDC Corporation

Claims (1)

[Claims] 1. A surface acoustic wave filter having an input transducer, an output transducer and a reflector on a surface of a piezoelectric substrate, wherein the input transducer has a plurality of electrode fingers, and the electrode fingers are spaced apart from each other. The output transducer has a plurality of electrode fingers, the electrode fingers are arranged at intervals, the input transducer and the output transducer are the same surface acoustic wave propagation. Alternating on the road, the reflector having a plurality of short strip conductors,
The input converter and the output converter are arranged on both outermost sides of the group of the output converters, of the electrode fingers of the input converter or the output converter, from the center of the electrode finger adjacent to the reflector, When the distance to the center of the first short strip conductor of the reflector is d and the wavelength of the surface acoustic wave is λ, {(n + 0.2) · λ / 2} ≧ d ≧ {(n-0.2) A surface acoustic wave filter satisfying λ / 2} (n is a positive integer).