JP2753352B2 - Surface acoustic wave double mode filter and communication device using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention reduces the distance between two interdigital transducers arranged in parallel in a direction perpendicular to the direction of propagation of a surface acoustic wave to reduce the frequency bandwidth. And a surface acoustic wave dual mode filter.

[Conventional technology]

Regarding the double-mode surface acoustic wave filter, for example, “Narrowband double-mode double-mode surface acoustic wave filter”
Entitled "Configuration Method of SAW Filter". As described therein, to increase the bandwidth of the dual mode surface acoustic wave filter, the distance between two interdigital transducers arranged in parallel and close to each other in a direction perpendicular to the surface acoustic wave propagation direction. Needs to be reduced.

[Problems to be solved by the invention]

However, in the conventional structure, as shown in FIG. 2, a common electrode 4 exists between two interdigital transducers 3 arranged in parallel in a direction perpendicular to the surface acoustic wave propagation direction. Therefore, there is a limit in reducing the distance between these electrodes in order to increase the bandwidth.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a surface acoustic wave double mode filter in which the distance between the two parallel interdigital electrodes is reduced by a new electrode structure as compared with the related art and the frequency bandwidth is expanded.

[Means for solving the problem]

In order to achieve the above object, according to the present invention, first and second electrode fingers are arranged on a piezoelectric substrate in a comb-like shape.
The interdigital transducers are arranged facing each other in a direction perpendicular to the direction of propagation of the surface acoustic wave, and the first and second interdigital transducers provide acoustic acoustic waves in a direction perpendicular to the direction of propagation of the surface acoustic wave. In the surface acoustic wave double mode filter for generating a coupling to excite a resonance mode, each of the first and second interdigital electrodes has a tip portion whose surface has a surface acoustic wave propagation between the two interdigital electrodes. A plurality of specific electrode fingers in contact with a straight line parallel to the direction, wherein the specific electrode fingers of the first interdigital electrode and the specific electrode fingers of the second interdigital electrode alternate along the propagation direction of the surface acoustic wave. And a plurality of electrode fingers extending in a direction orthogonal to the direction of propagation of the surface acoustic wave, and a meandering electrode formed to meander between the first and second interdigital electrodes. Characterized by the provision of It is.

[Action]

As described above, the distance between the two interdigital transducers arranged in parallel in a direction perpendicular to the surface acoustic wave propagation direction is reduced by the width of the common electrode because the meander electrode is used. Further, since the ends of at least one or more electrode fingers of each of the interdigital transducers are formed so as to be in contact with a straight line drawn in parallel with the interdigital transducer between the adjacent parallel interdigital transducers, The difference between the frequencies of the surface acoustic waves in the two modes generated between the interdigital transducers was increased, thereby widening the frequency bandwidth.

〔Example〕

FIG. 1 is a schematic plan view of the first embodiment of the present invention. An ST-cut quartz crystal is used for the piezoelectric substrate 1, and a grating reflector electrode 2 and an interdigital electrode 3 are formed on the substrate by photolithography. Aluminum is used for these electrode materials, the film thickness is 3000 nm, the opening length is 100 μm, and the electrode width is
It was 2.8 μm. FIG. 2 shows a schematic plan view of a conventional dual mode filter for comparison. As shown in FIG. 8, when the surface acoustic wave resonators are arranged close to each other in a direction perpendicular to the surface acoustic wave propagation direction and in parallel, if the distance between the two resonators is sufficiently small, these resonances Acoustic coupling occurs between the devices, and two resonance modes are excited. One of them is
The two resonators are a fundamental mode (resonance frequency fa) having a symmetric displacement distribution with respect to the center, and the other is an antisymmetric mode (resonant frequency fs) having a point symmetric displacement distribution with respect to the center. These two modes have different resonance frequencies, and the difference between the resonance frequencies depends on the distance between the two resonators and the degree of confinement of vibration energy in each resonator. Specifically, it is determined by the opening length W and the gap G. FIG. 5 shows the relationship between the aperture length W and the frequency difference between the two modes using the gap G as a parameter. As can be seen from FIG. 5, in order to increase the bandwidth of the surface acoustic wave double mode filter, the frequency difference between the above two modes must be large. That is, the gap G is small,
The condition is that the opening length W is large. However, the opening length W
There is a problem that the acoustic coupling becomes weaker as the value is increased. Therefore, according to the present invention, even if the opening length is relatively large, W = 10 L
Selected. Here, L is the surface acoustic wave wavelength. Also,
Although the bandwidth can be increased by making the gap G close to zero, the conventional electrode structure shown in FIG.
There is a limit due to the presence of the common electrode 4. The common electrode 4 and the electrode fingers connected thereto have a meandering electrode 6 structure as shown in FIG. 1 so that the central common electrode 4 is eliminated, and the common electrode 4 is moved in a direction perpendicular to the surface acoustic wave propagation direction. Each of the interdigital transducers arranged close to and parallel to each other was in contact with a straight line 5-5 'drawn in parallel between the electrodes. With such a structure, the distance G of the surface acoustic wave propagation path excited by each interdigital electrode 3, that is, the gap G becomes G = 0. Therefore, the frequency difference between the two modes becomes large, and the frequency bandwidth of the surface acoustic wave double mode filter can be expanded. FIG. 7 shows frequency characteristics of a filter according to the present invention and a conventional example for comparison. It can be seen that the frequency according to the present invention is higher in the two modes than in the conventional example.

FIG. 3 is a diagram showing a second embodiment of the present invention. In the case of the first embodiment, there is a problem that the electrode finger of the meander electrode 6 becomes longer and the electrode finger resistance increases. Therefore, in the second embodiment, a bonding pad for grounding is provided in the middle of the meander electrode 6, as shown in FIG. Thereby, the influence of the electrode finger resistance can be reduced.

FIG. 4 is a diagram showing a third embodiment of the present invention. In this embodiment, in order to shorten the meander electrode, the portion of the meander electrode that is bent in parallel to the surface acoustic wave propagation direction is located at the center of the two interdigital transducers and is parallel to the two interdigital transducers. The electrode was formed so as to approach a straight line where the end of the electrode finger was in contact, and to face the end of the electrode finger.

FIG. 6 shows a fourth embodiment of the present invention, in which the electrodes of the first embodiment are connected in series. In this case, the out-of-band characteristics can be improved as compared with a single surface acoustic wave double mode filter.

Furthermore, if the surface acoustic wave dual mode filter of the present invention is used as an RF filter of a portable receiver or a pager, it is possible to use a single conversion method, thereby simplifying the circuit and miniaturizing the set.

〔The invention's effect〕

According to the present invention as described above, the frequency bandwidth of the surface acoustic wave double mode filter can be expanded,
A bandwidth 30% wider than before was obtained.

[Brief description of the drawings]

FIG. 1 is a schematic plan view of a first embodiment of the present invention, FIG. 2 is a schematic plan view of an example of a conventional surface acoustic wave double mode filter, FIG. 3 is a diagram of a second embodiment of the present invention, FIG. 4 and FIG.
FIGS. 3A and 3B are diagrams showing the third and fourth embodiments of the present invention. FIG. 5 is a characteristic diagram showing how the frequency difference between the fundamental mode and the antisymmetric mode changes depending on the aperture length, using the gap length as a parameter.
FIG. 8 is a diagram showing a comparison between the frequency characteristics of the filter of the present invention and the filter of the prior art, and FIG. 1 ... Piezoelectric substrate, 2 ... Grating reflector electrode,
3 ... interdigital electrodes, 4 ... common electrodes, 6 ... meander electrodes.

Claims (5)

(57) [Claims] A first and second interdigital transducers each having a plurality of electrode fingers arranged in a comb shape on a piezoelectric substrate so as to face each other in a direction orthogonal to a propagation direction of a surface acoustic wave. A surface acoustic wave double mode filter in which the first and second interdigital electrodes cause acoustic coupling of waves in a direction orthogonal to the propagation direction of the surface acoustic wave to excite a resonance mode; And each of the second interdigital transducers has a plurality of specific electrode fingers, each of which has a tip portion in contact with a straight line parallel to the propagation direction of the surface acoustic wave between the two interdigital transducers. The specific electrode finger of the electrode and the specific electrode finger of the second interdigital transducer are arranged alternately along the propagation direction of the surface acoustic wave, and include a plurality of electrode fingers extending in a direction orthogonal to the propagation direction of the surface acoustic wave. , The first interdigital transducer and the second interdigital transducer. SAW double-mode filter, characterized in that a meander electrode formed so as to meander between. 2. The surface acoustic wave double mode filter according to claim 1, wherein a ground bonding pad is provided at at least one portion of the meander electrode. 3. The elastic surface according to claim 1, wherein a portion of the meander electrode that is bent in parallel to a propagation direction of the surface acoustic wave faces a tip of the specific electrode finger. Wave duplex mode filter. 4. A surface acoustic wave double mode filter comprising at least two surface acoustic wave dual mode filters according to claim 1 connected in cascade. 5. A communication device using the surface acoustic wave double mode filter according to claim 1.