WO2000057548A1

WO2000057548A1 - Surface acoustic wave filter

Info

Publication number: WO2000057548A1
Application number: PCT/JP1999/005208
Authority: WO
Inventors: Jun Tsutsumi; Takashi Matsuda; Osamu Ikata; Yoshio Satoh
Original assignee: Fujitsu Limited; Fujitsu Media Devices Limited
Priority date: 1999-03-24
Filing date: 1999-09-22
Publication date: 2000-09-28
Also published as: JP2000278083A

Abstract

A surface acoustic wave filter comprises a piezoelectric substrate, a plurality of surface acoustic wave channels arranged on the piezoelectric susbtrate, and reflector arranged across the surface acoustic wave channels. An input interdigital transducer is arranged on at least one of the surface acoustic wave channels. An output interdigital transducer is arranged on at least one of the other surface acoustic wave channels. The input interdigital transducer and output interdigital transducer are arranged on the same side with respect to the reflector. As a result, this surface acoustic wave filter is short in the direction of propagation and has a frequency characteristic of good squareness ratio.

Description

Description Surface acoustic wave filter Technical field

The present invention relates to a surface acoustic wave filter, and more particularly, to a surface acoustic wave filter having a plurality of surface acoustic wave transmission paths. Background art

2. Description of the Related Art In recent years, in mobile communication systems such as mobile phones, a new digital system called a CDMA (Code Division Multiple Access) system has been adopted in addition to a TDMA (Time Division Multiple Access) system.

The IF (Intermediate Frequency) filter used in the CDMA system is required to have frequency characteristics that are extremely excellent in squareness as compared with conventional mobile phone systems. Here, as shown in Fig. 10, the squareness is the ratio of the bandwidth (the first bandwidth and the second bandwidth) at a certain two attenuations (the second bandwidth / the first bandwidth). Width). The bandwidths for two attenuations are, for example, a 3 dB bandwidth and a 10 dB bandwidth. It is said that the closer the ratio of the bandwidths of these two attenuation amounts to 1, the better the squareness, and therefore, the better the squareness indicates that the filter characteristics change sharply.

Figure 9 shows a transversal filter, which is one of the conventionally used surface acoustic wave filters. One of the filters is an IDT (In-Digital Digital Transducer) for signal input, and the other is an IDT for signal output. The IDT on the right side of Fig. 9 extends vertically. The IDT on the left is an apodized weighted electrode in which the length of each electrode finger differs according to a fixed rule. In the past, the squareness of the frequency characteristics of a surface acoustic wave filter was improved by weighting the IDT, as in the case of such an apodized weighted electrode.

However, in order to realize a surface acoustic wave filter with sufficiently excellent squareness by weighting the IDT like an apotize weighted electrode, an extremely large number of electrode pairs is required, and the propagation direction of the surface acoustic wave ( In the case of 9, there is a problem in that the length (in the horizontal direction of the paper) becomes long. Therefore, in mobile phones, etc., it is necessary to reduce the surface acoustic wave filter in order to satisfy its portability and miniaturization, but if the size of the surface acoustic wave filter is increased in order to improve the squareness, However, this is contrary to the demand for miniaturization. Therefore, it is difficult to realize a surface acoustic wave filter having a small size and excellent squareness characteristics by weighting the conventional IDT as shown in FIG. 9 to improve the squareness. Disclosure of the invention

An object of the present invention is to provide a surface acoustic wave filter having a small length in the direction of propagation of a surface acoustic wave and having excellent frequency characteristics with excellent squareness, using an IDT having a small number of electrode pairs. And

The present invention includes a piezoelectric substrate, a plurality of surface acoustic wave propagation paths arranged on the piezoelectric substrate, and a reflector arranged so as to traverse the plurality of surface acoustic wave propagation paths. An incoming digital transmitter is arranged on the surface acoustic wave propagation path, and an outgoing digital transducer is arranged on at least one of the other surface acoustic wave propagation paths. Digital Transducer and Output In The term "digital transducer" provides a surface acoustic wave filter which is disposed on the same side as the reflector. According to this, it is possible to provide a small surface acoustic wave filter having frequency characteristics with excellent squareness. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a configuration diagram of a surface acoustic wave filter according to a first embodiment of the present invention. FIG. 2 is a configuration diagram of a surface acoustic wave filter according to a second embodiment of the present invention. FIG. 3 is a configuration diagram of a surface acoustic wave filter according to a third embodiment of the present invention. FIG. 4 is a configuration diagram of a surface acoustic wave filter according to a fourth embodiment of the present invention. FIG. 5 is a configuration diagram of a surface acoustic wave filter according to a fifth embodiment of the present invention. FIG. 6 shows a surface acoustic wave filter according to a second embodiment of the present invention.

FIG. 9 is a configuration diagram when the length of T is different.

FIG. 7 is a configuration diagram of a specific example of the surface acoustic wave filter according to the present invention. FIG. 8 is a frequency characteristic diagram of a specific example of the surface acoustic wave filter of FIG. 7 of the present invention.

FIG. 9 is a configuration diagram of a conventional transversal surface acoustic wave filter.

FIG. 10 is an explanatory diagram of squareness. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the interdigital transducer is called IDT (Interdigital Transduser), and the surface acoustic wave is called S AW (Surface Acoustic Wave).

In the surface acoustic wave filter of the present invention, a plurality of interdigital transducers (IDTs) are provided, of which electric signals are input. The input interdigital transducer (input IDT) used for inputting and the output digital converter (output IDT) used for outputting an electric signal need only be one each. The number is not limited to one, and a plurality may be provided as needed.

Further, in the present invention, the input IDT and the output IDT are arranged so as to be parallel to a direction perpendicular to the propagation direction of the surface acoustic wave.

Further, the present invention comprises a piezoelectric substrate and a plurality of surface acoustic wave propagation paths arranged on the piezoelectric substrate, and each of the surface acoustic wave propagation paths has one IDT and a reflection at a predetermined interval. A surface acoustic wave, wherein each IDT is arranged in a direction perpendicular to the direction of propagation of the surface acoustic wave and on the same side with respect to each of the reflectors. It offers a Phil evening.

Here, in particular, one of the IDTs may be an input IDT and one of the other IDTs may be an output IDT. However, the number of input IDTs and output IDTs is not limited to one, and there may be more than one.

Further, a surface acoustic wave waveguide may be formed on the piezoelectric substrate between any IDT formed on the surface acoustic wave propagation path and the reflector.

Also, at least one of the input IDT and the output IDT may be weighted.

For example, as the weight, there is “apotize weight” or “thinning weight”. Weighting can improve the squareness of the frequency characteristics of the surface acoustic wave filter.

Further, a unidirectional IDT may be used for at least one of the input IDT and the output IDT. Elastic surface with unidirectional IDT Wave fill loss can be reduced.

Weighting such as thinning weighting may be applied to the reflector. Further, one reflector may be divided into a plurality in the propagation direction of the surface acoustic wave. This weighting and division can improve the squareness of the frequency characteristics of the surface acoustic wave filter. Further, twice the period of the electrode of the reflector may be slightly different from the period of the electrode fingers of the input ID Τ and the output ID Τ. By doing so, the frequency characteristics of the surface acoustic wave filter can be improved.

Hereinafter, the present invention will be described in detail based on an embodiment shown in the drawings. However, the present invention is not limited by this.

FIG. 1 shows a configuration diagram of a surface acoustic wave filter according to a first embodiment of the present invention. Figure 1 shows the configuration of a surface acoustic wave filter composed of four surface acoustic wave propagation paths (first propagation path 5, second propagation path 6, third propagation path 7, and fourth propagation path 8). I have. However, the present invention is not limited to this. In general, η (η ≥2) surface acoustic wave propagation paths may be provided.

Further, a plurality of IDTs 1, 2, and 3 made of a metal film (such as Cu and A1) and a reflector 4 are formed on a piezoelectric substrate 9 such as a quartz crystal.

The reflector 4 is arranged so as to traverse all the surface acoustic wave propagation paths 5 to 8, and the IDT is arranged on each propagation path. The IDT does not need to be placed on all propagation paths, and it is sufficient if there is at least one input IDT1 and at least one output IDT2 on any propagation path. The other IDTs 3 may be for input or output.

The input IDT 1 and the output IDT 2 are arranged in a direction perpendicular to the SAW propagation direction, and are arranged on the same side with respect to the reflector 4. However, the distance between the input IDT and the reflector 4 and the distance between the output IDT 2 and the reflector 4 need not necessarily be the same. The four surface acoustic wave propagation paths 5 to 8 are areas in which the SAW excited by the input IDT 1 propagates in the left and right direction on the paper, but the two adjacent propagation paths are separated by a predetermined distance, and the piezoelectric substrate 9 Present on.

Each IDT 1, 2, 3 is composed of two comb-shaped electrodes having a large number of electrode fingers, and each electrode finger extends in a direction substantially perpendicular to the SAW propagation direction. Are arranged alternately from above and below.

In addition, although IDTS 1, 2, and 3 in FIG. 1 are so-called normal electrodes, they may be weighted by thinning out or weighted by apoptosis if necessary. The reflector 4 includes a plurality of electrode fingers having a grating structure. FIG. 2 shows a configuration diagram of a surface acoustic wave filter according to a second embodiment of the present invention. FIG. 2 shows an embodiment of the special form of FIG. 1, that is, a surface acoustic wave filter having the simplest configuration.

The DT is composed of one input IDT 1 and one output IDT 2. The input IDT 1 is connected to the reflector 4 on the first propagation path 5 and the output IDT 2 is connected to the reflector 4 on the second propagation path 6. Formed on the same side. The reflector 4 is disposed transversely so as to straddle the two propagation paths 5 and 6.

Here, the principle of propagation of surface acoustic waves (SAW) of the surface acoustic wave filter of the present invention will be described with reference to FIG.

First, when an input signal is externally applied to the electrode of the input IDT 1, the SAW is excited, and the SAW proceeds in the left-right direction of the input IDT 1.

The SAW intensity distribution 11 immediately after being radiated from the input I D T 1 is confined inside the first propagation path 5 as shown in FIG. The SAW emitted to the right of the input IDT 1 travels on the surface of the piezoelectric substrate 9 in the direction 13 of the reflector 4.

When the SAW reaches the reflector 4, the reflector 4 is straddled on the two propagation paths 5 and 6, so that the 38 intensity distributions 12 spread over the entire area of the reflector 4. I get cramped. That is, the SAW is also propagated on the second propagation path 6. Then, a part of the SAW that has propagated on the second propagation path 6 is reflected by the reflector 4, travels toward the output IDT 2 (in the direction of reference numeral 14), and reaches the output IDT 2. Detected as a signal.

In addition, the input IDT 1, the output IDT 2, and the reflector 4 each have a frequency characteristic of easily transmitting only a signal in a specific frequency band depending on the logarithm and period of the electrode finger. Since SAW having characteristics obtained by synthesizing the frequency characteristics of IDT 1 and reflector 4 is input from reflector 4 to output IDT 2, a filter having excellent squareness characteristics can be obtained. FIG. 6 shows a SAW filter having the same configuration as that of FIG. 2, but in the case where the length of the input IDT 1 in the SAW propagation direction is longer than the length of the output IDT 2 in the SAW propagation direction. FIG.

In the conventional transversal filter shown in FIG. 9, the length in the SAW propagation direction is mainly the sum of the length L 1 of the input IDT 1 and the length L 2 of the output IDT 2 (L 1 + L 2) Determined by

On the other hand, in the surface acoustic wave filter of the present invention shown in FIG. 6, the length L 3 of the longer IDT of the input IDT 1 (length L 1) or the output IDT 2 (length L 2) in the S AW propagation direction is mainly used. And the length L4 of the reflector in the SAW propagation direction (L3 + L4). Here, L 3 is L 1 or L 2.

Therefore, when the length L 4 of the reflector in the SAW propagation direction is shorter than the shorter IDT of the SADT in the SAW propagation direction, the length of the SAW filter of the present invention in the SAW propagation direction is the same as that of the conventional transformer. It is shorter than the Versal-type Phil evening.

FIG. 3 shows a configuration diagram of a surface acoustic wave filter according to a third embodiment of the present invention. Here, as in the first and second embodiments, the input IDT 1 and the output IDT 2 are It is arranged in a direction perpendicular to the SAW propagation direction (left-right direction on the paper), but differs in that a plurality of reflectors are divided and arranged on each propagation path. That is, a plurality of reflectors 411, 412, and 4-2 are arranged on the same side in the SAW propagation direction with respect to IDTs 1 and 2. In the third embodiment, a configuration having two input IDTs is shown.

Thus, even when the reflector 4 is divided, the surface acoustic waves radiated from the two inputs IDT 1 are reflected from the reflectors 411 and 413 due to the acoustic coupling between the reflectors. To the output unit 4-2, and further to the output IDT2.

FIG. 3 shows an embodiment including three IDTs and reflectors, but the number of IDTs and the like is not limited to this.

Also, at least one input IDT 1 and one output IDT 2 are required, and the number of input IDTs is not limited to two as shown in FIG. FIG. 4 shows a surface acoustic wave filter according to a fourth embodiment of the present invention, which shows a minimum configuration of the surface acoustic wave filter shown in FIG. Here, one input I DT 1 and one output I DT 2 are arranged on the same side in the SAW propagation direction of each IDT, and the reflectors 4 — 1, 4-2 arranged on the SAW propagation path It is composed of

Similarly to the surface acoustic wave filters shown in FIGS. 1 and 2, the surface acoustic wave filter having the configuration shown in FIGS. 3 and 4 can make the length in the SAW propagation direction smaller than that of the transversal type filter. Excellent fill evening. In Fig. 4, the input IDT 1 and the reflector 4-1 and the output IDT 2 and the reflector 4-2 are located on the SAW propagation path that is not displaced in the vertical direction of the drawing in terms of reducing loss. Is preferred. However, if a SAW waveguide is provided between the input ID Τ1 and the reflector 4-1 and between the output ID Τ2 and the reflector 4_2, the input IDT1 and the reflector 4 1, output The DT 2 and the reflectors 4-1 and 2 need not necessarily be on the same SAW propagation path. For example, the input IDT 1 and the reflectors 4-1 may be arranged at positions shifted vertically in the drawing. You may.

FIG. 5 shows a configuration diagram of a surface acoustic wave filter according to a fifth embodiment of the present invention. Here, the configuration of the IDT and the reflector 4 is the same as that shown in FIG. 1. In each SAW propagation path between each IDT 1, 2, 3 and the reflector 4, the SAW waveguide 21 1, The difference is that 22, 23, and 24 are formed.

However, the SAW waveguide need not be formed in the entire region between the IDT and the reflector, and the SAW waveguide may be provided in any region between the IDT and the reflector. This SAW waveguide can avoid SAW diffraction loss on the propagation path from the input IDT 1 to the reflector 4 or on the propagation path from the reflector 4 to the output IDT 2, reducing insertion loss at the SAW filter. can do.

Also, in FIGS. 2, 3, and 4, a SAW waveguide may be provided in a region between each IDT and each reflector. The SAW waveguide can be formed of a metal film with a uniform surface structure, an insulating film with a uniform surface structure, a metal film with a grating structure, or an insulating film with a grating structure.

FIG. 7 shows a specific example of the surface acoustic wave filter according to the present invention.

This surface acoustic wave filter includes one input IDT 1, one output IDT 2, one reflector 4, and two SAW waveguides 21, 22. Here, the input IDT 1 and the output IDT 2 are both assumed to have a period of electrode fingers 1 = 3 Q jum and the number of pairs of electrode fingers = 55 pairs.

The reflector 4 has a grating structure, the period of the electrode fingers 2 = 18 ja, and the number of electrode fingers = 160.

Between input IDT 1 and reflector 4 and between output IDT 2 and reflector 4 The SAW waveguides 21 and 22 each having a grating structure are arranged in the first stage. The period of this grating structure is 14 m.

The distance between the input IDT 1 and the output IDT 2 is about 100 μm, and the input IDT 1 and the reflector 4 have a structure that is slightly displaced in the vertical direction from the same SAW propagation path. However, in order to avoid loss in the area between the input IDT 1 and the reflector 4, the SAW waveguide 21 is bent halfway so as to connect the SAW propagation path connecting the input IDT and the reflector. It has a shape.

The same applies to the positional relationship between the output IDT 2 and the reflector 4 and the shape of the SAW waveguide 22.

The input I DT 1, the output I DT 2, the reflector 4, and the SAW waveguides 21, 22 are all formed of a metal film having a thickness of about 1 zm. Paining is done. As the material of the metal film, for example, aluminum can be used.

The surface acoustic wave filter having the above structure can be formed as a chip with a length of about 1.5 mm in the vertical direction and about 7.5 mm in the horizontal direction.

FIG. 8 shows a graph of the insertion loss versus frequency characteristic in the specific example of the surface acoustic wave filter of the present invention shown in FIG.

According to this graph, the center frequency was set to 85.5 MHz, and a surface acoustic wave filter having excellent frequency characteristics with squareness was obtained. For reference, in the conventional transversal type filter shown in Fig. 9, when the input IDT is weighted by the apoise and the output IDT 2 is weighted by thinning, the elasticity with the same frequency characteristics as in Fig. 8 is obtained. In order to use a surface acoustic wave filter, the number of input IDT electrode pairs = 295 pairs and the number of output IDT electrode pairs = 139 pairs are required. At this time, the length of the input IDT in the S AW propagation direction = 10 · 62 mm, and the length of the output IDT in the S AW propagation direction = 5.04 mm. The total length in the AW propagation direction is 15.624 mm, which is considerably longer than the length of the specific example shown in FIG. 7 of the present invention shown in FIG.

Therefore, in the surface acoustic wave filter having the structure of the present invention, the length in the SAW propagation direction can be reduced as compared with the related art, and the surface acoustic wave filter itself can be reduced in size.

According to the present invention, it is possible to provide a surface acoustic wave filter that can reduce the length of the surface acoustic wave in the propagation direction as compared with the related art and has a frequency characteristic with excellent squareness.

Claims

The scope of the claims

1. At least one surface acoustic wave comprising a piezoelectric substrate, a plurality of surface acoustic wave propagation paths arranged on the piezoelectric substrate, and a reflector arranged to traverse the plurality of surface acoustic wave propagation paths An input interdigital transducer is arranged on the propagation path, an output interdigital transducer is arranged on at least one of the other surface acoustic wave propagation paths, and the input interdigital transducer is further arranged. The surface acoustic wave filter and the output transducer are arranged on the same side with respect to the reflector.

2. The surface acoustic wave filter according to claim 1, wherein the input and output digital transducers and the output and output digital transducer are one each.

3. A surface acoustic wave waveguide is formed on at least one of the piezoelectric substrates between the input interdigital transducer and the reflector, and between the output interdigital transducer and the reflector. Surface acoustic wave filter according to claim 1 or 2.

4. Composed of a piezoelectric substrate and a plurality of surface acoustic wave propagation paths arranged on the piezoelectric substrate, and each of the surface acoustic wave propagation paths is provided with one interdigital transducer at a predetermined interval. A reflector is arranged, and each interdigital transducer is arranged in a direction perpendicular to the propagation direction of the surface acoustic wave and on the same side as each of the reflectors. Surface acoustic wave fill

5. The surface acoustic wave filter according to claim 4, wherein the interdigital transducer comprises one input interdigital transducer and one output interdigital transducer. evening.

6. The surface acoustic wave waveguide according to claim 4 or 5, wherein a surface acoustic wave waveguide is formed on a piezoelectric substrate between any interdigital transducer formed on the surface acoustic wave propagation path and a reflector. Surface wave fill evening.

7. The surface acoustic wave waveguide is formed using any one of a metal film having a uniform surface structure, an insulating film having a uniform surface structure, a metal film having a grating structure, and an insulating film having a grating structure. 7. The surface acoustic wave filter according to claim 6, wherein: