JPH054848B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an electrode structure of a surface acoustic wave multimode filter, and particularly to a structure for adjusting coupling between resonators.

[Conventional technology]

When two surface acoustic wave (SAW) resonators are placed close together and coupled elastically, two modes of vibration are created. That is, there are two vibration modes, a symmetric mode and an anti-symmetric mode, and the resonant frequency of the symmetric mode and the anti-resonant frequency of the anti-symmetric mode are matched to form a dual mode SAW filter.

There are also types that generate two vibration modes within a single resonator, and there are also multimode filters that have three or more such resonators.

Such a multimode resonator type surface acoustic wave filter is described in the Institute of Electronics and Communication Engineers Ultrasound Research Group Material US-77.
−33 etc. There is also a two-port type in which two interdigital electrodes are arranged in each resonator.

In the surface acoustic wave multimode filter as described above, if the interdigital electrodes of the two resonators are placed close to each other, the coupling becomes too strong in some areas. As a result, as shown by the broken line 45 in FIG. 4, the output of a filter with a center frequency of 82 MHz increases by 2 dB at a specific frequency within the passband, making it impossible to obtain flat passband characteristics.

In order to improve this problem, the resonators may be placed apart from each other, but the problem arises that the chip size increases.

〔assignment〕

The present invention provides a surface acoustic wave multimode filter that prevents coupling from becoming too strong at a specific frequency even when resonators are placed close to each other and provides flat bandpass characteristics.

It is also an attempt to reduce the resistance of interdigital electrodes connected in multiple stages.

[Means to solve the problem]

The present invention solves the above problem by connecting interdigital electrodes of a resonator with a stripline.

That is, a resonator consisting of an interdigital electrode and a reflector sandwiching the interdigital electrode on the propagation path of the surface acoustic wave excited by the interdigital electrode is installed on a piezoelectric substrate. In surface acoustic wave multimode filters arranged side by side in a direction perpendicular to the propagation direction, the bus bars of the interdigital electrodes of the resonator are connected by a plurality of striplines of equal width arranged at the same interval. , and the ratio of the conductor to the surface of the interdigital electrode in the propagation direction of the surface acoustic wave is larger than the ratio of the conductor to the surface of the stripline in the propagation direction of the surface acoustic wave. .

The period of the stripline can be made the same as the period of the interdigital electrode, or the stripline can also be formed between the reflectors.

[Effect]

The portion of the stripline between the resonators has a smaller conductor ratio than the conductor portion of the interdigital electrode of the resonator or the reflector electrode. Therefore, the speed of sound in this portion increases, and it is possible to prevent energy from concentrating between the resonators.

As a result, the bonding state of each component is made uniform, and only a specific component is not bonded extremely strongly.

Furthermore, by connecting multiple strip lines, the resistance can be significantly lowered.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view showing an embodiment of the present invention. On the surface of the substrate 10 having piezoelectricity such as crystal,
The electrode is formed from a metal film such as aluminum.
Interdigital electrode 11 that excites surface acoustic waves
reflectors 12a and 12b are arranged in the propagation direction of the surface acoustic waves on both sides thereof, thereby forming two resonators. One of the busbars of the interdigital electrodes 11a and 11b of the resonator is connected to each other, and this connection is made by a stripline 13.

The stripline 13 is formed of the same conductive material as the interdigital electrode 11 at the same time as the resonator. Its width is narrower than the width of the interdigital electrode, that is, it is formed thin.

The period of this stripline 13 can be set arbitrarily. However, the ratio of the conductor in the propagation direction of the surface acoustic wave must be made smaller than the ratio of the electrodes in the interdigital electrode portion and the reflector portion.

In the portion between the resonators where the stripline 13 is formed, interdigital electrodes 11a, 11b
The area ratio of the conductor is smaller than that of the
The sound speed of surface acoustic waves increases. Therefore, the energy concentrated in the stripline 13 portion is relatively small.

In order to increase the sound speed of surface acoustic waves, the state should be close to a free surface (a surface without a conductive film), but the busbars connecting interdigital electrodes have become very thin and have to be placed adjacent to each other. The resistance between the interdigital electrodes of the resonator becomes very large. Strip lines are formed to reduce resistance and prevent energy concentration.

FIG. 2 is a plan view showing another embodiment of the invention. An electrode is formed using a metal film such as corrugated aluminum on the surface of a piezoelectric substrate 20 such as crystal. Interdigital electrodes 21a and 21b that excite surface acoustic waves are formed, respectively, and reflectors 22a and 22b are provided on both sides in the propagation direction of the surface acoustic waves.
are arranged to form two resonators. Reflectors 22 on one side and on the same side of the busbars of the interdigital electrodes 21a, 21b of the resonator
a and 22b are connected, and this connection is made by a stripline 23.

The stripline 23 is formed at the same time as the resonator with the same conductive material as the interdigital electrodes 21a, 21b and the reflectors 22a, 22b. Its width is narrower than the width of the interdigital electrode and the electrode fingers of the reflector, that is, it is formed thin.

FIG. 3 is a plan view showing another embodiment of the present invention, in which interdigital electrodes 31a, 31b and reflectors 32a, 32b are the same as in the previous example, but
All the strip lines 33 are not connected,
Some striplines have a sectioned structure in which the middle section is removed. In this way, when the stripline 33 is not uniform, the ratio of conductors changes in each part, and the speed of sound changes accordingly, so the concentration state of energy changes and the state of coupling can be varied.

〔effect〕

According to the present invention, it is also possible to equalize the coupling between the resonators, and as shown by the solid line 46 in FIG.
Flat characteristics can be obtained within the passband with the center frequency at . This example shows a case where the width of the stripline is half the width of the electrode fingers of the interdigital electrodes, and the spacing is equal to that of the interdigital electrodes. This provides a broadband surface acoustic wave multimode filter.

Furthermore, the interdigital electrodes of adjacent resonators can be connected with low resistance, and a large output can be obtained.

Furthermore, since the coupling can be adjusted, the length of the electrode fingers can be increased, and the impedance of the interdigital electrode can be lowered.

[Brief explanation of drawings]

1 to 3 are plan views showing embodiments of the present invention, and FIG. 4 is an explanatory diagram of the characteristics of a conventional filter and a filter according to the present invention. 11a, 11b, 21a, 21b, 31a, 3
1b...Interdigital electrode, 12a, 12
b, 22a, 22b, 32a, 32b... reflector, 13, 23, 33... stripline.

Claims (1)

[Claims] 1. A resonator consisting of an interdigital electrode and a reflector provided on the propagation path of a surface acoustic wave excited by the interdigital electrode with the interdigital electrode sandwiched therebetween is provided on a piezoelectric substrate. In a surface acoustic wave multi-mode filter arranged side by side so that the surface acoustic wave multimode filters are located in a direction perpendicular to the propagation direction of the surface acoustic wave, the bus bars of the interdigital electrodes of the resonator have a plurality of equal-width strips arranged at the same interval. The conductor is connected by a stripline, and the ratio of the conductor to the surface of the interdigital electrode in the direction of propagation of surface acoustic waves is larger than the ratio of the conductor to the surface of the stripline in the direction of propagation of the surface acoustic wave. surface acoustic wave multimode filter. 2. The surface acoustic wave multimode filter according to claim 1, wherein the stripline has the same period as the electrode fingers of the interdigital electrode and has a narrower width than the electrode fingers. 3. A resonator consisting of an interdigital electrode and a reflector sandwiching the interdigital electrode on the propagation path of the surface acoustic wave excited by the interdigital electrode is installed on the piezoelectric substrate. In surface acoustic wave multimode filters that are arranged in parallel in a direction perpendicular to the direction, a plurality of widths are arranged at equal intervals between the bus bars of the interdigital electrodes of the resonator and between the reflectors of the resonator. The ratio of the conductor to the surface of the interdigital electrode and the reflector in the propagation direction of the surface acoustic wave is equal to the ratio of the conductor to the surface of the stripline in the propagation direction of the surface acoustic wave. A surface acoustic wave multi-mode filter characterized in that it is larger than . 4. the stripline has the same period as the electrode fingers of the interdigital electrode and the electrode fingers of the reflector;
4. The surface acoustic wave multimode filter according to claim 3, wherein the surface acoustic wave multimode filter is formed to have a width narrower than the electrode fingers.