JPH01220910A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To vary the frequency over a wide range by applying weight to electrode fingers so as to maximize the cross length in the exciting electrode closest to a grating reflector and arranging a dummy elecotrode in opposition to the crossing part. CONSTITUTION:The cross length of interdigital electrodes in an exciting electrode 6 is designed longer symmetrically to both sides toward the vicinity of the grating reflector 7 and maximized at the both ends and the electrode fingers are so shaped that the outer shape of the cross part is part of a cosine curve. Moreover, the dummy electrode 8 is arranged respectively in opposition to the crossed electrode fingers at parts except the cross part between interdigital electrodes 6a, 6b to prevent the disturbance in the phase of a surface acoustic wave. Thus, the resonance is caused without occurrence of a high-order lateral mode spurious radiation. Then the frequency is varied over a wide band in case of applying the device to an oscillator component of a voltage controlled oscillator.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a surface acoustic wave resonator.

(Prior Art) In recent years, surface acoustic wave resonator devices have been developed as oscillators and narrowband filters for VHF and UHF bands.

As shown in FIG. 3, an excitation electrode (interdigital electrode) 2 formed by intersecting a pair of comb-teeth electrodes 2a and 2b is formed on a piezoelectric substrate 1, and this excitation electrode 2
A grating reflector 3 is formed on both sides of the piezoelectric substrate 1 so that surface acoustic waves excited from the excitation electrode 2 and propagating in both directions on the piezoelectric substrate 1 are formed on both sides of the grating reflector 3.
It is configured to be repeatedly reflected between the two to generate a standing wave.

By the way, when such a resonant device is used as an oscillator of a voltage controlled oscillator, a device with a small capacitance ratio and low impedance is desired in order to enable wide band variation, and for this reason, the comb in the excitation electrode 2 is Tooth electrode 2 a s
2. It is necessary to lengthen the crossing length of the electrode fingers in b.

In general, a device in which the crossing length of each electrode finger of the excitation electrode 2 is constant is called a regular type surface acoustic wave resonator. There is a problem in that a high-order transverse mode occurs, and this has an adverse effect as spurious on the high-frequency side of the resonance frequency.

In order to solve this problem, recently, as shown in FIG. , and a surface acoustic wave resonator has been developed in which the electrode fingers are weighted by the aboding method so that the outer shape of the intersection draws a cosine curve.

In such a surface acoustic wave resonator, by changing the excitation efficiency in the excitation electrode 2 so that it is maximum at the center, the surface acoustic wave energy distribution is made to match the fundamental mode, and higher-order transverse modes are generated. It is configured not to. In addition, in the short crossing length, a dummy electrode 4 that does not contribute to excitation is provided facing the electrode fingers of the wJm electrode 2 a s 2 b so that the phase of the surface acoustic wave is not disturbed. It is configured so that the surface waves experience a metal film of a certain area.

Furthermore, conventionally, as shown in FIG.
A surface acoustic wave resonator is also known in which weighting is performed so that the outer shape of the intersection of the electrodes 2b draws a symmetrical cosine curve from the center of the excitation electrode 2 toward both sides.

However, in the surface acoustic wave resonator shown in FIGS. 3 and 4, although the high-order transverse mode spurious that appeared in the regular surface acoustic wave resonator is suppressed, As shown in Figure 5, the excitation electrode 2
Since an asymmetric electric field is applied between the end of the grating reflector 3 and the grating reflector 3, there is a problem in that an asymmetric high-order transverse mode is excited. As shown in the graph showing the pass characteristics of the resonator in Figure 6, the excitation of such a high-order transverse mode is caused by a high-order transverse mode excited in the normal type on the high-frequency side near the resonance frequency. Spurious (S) is generated at a frequency closer to the resonant frequency than the spurious of , degrading the characteristics. Therefore, when using such a surface acoustic wave resonator as an oscillator in a voltage-controlled oscillator and utilizing the inductance region, that is, between the resonance and anti-resonance frequencies, the above-mentioned spurious (S) occurs, so Furthermore, there was a problem that the frequency variable width became narrow.

(Problems to be Solved by the Invention) As mentioned above, in conventional surface acoustic wave resonators, spurious signals occur near the high frequency side of the resonant frequency, so when used in a voltage controlled oscillator, broadband There was a problem that the frequency could not be varied.

The present invention was made to solve such problems, and an object of the present invention is to provide a surface acoustic wave resonator for a voltage-controlled oscillator that suppresses the generation of high-order transverse mode spurious and is capable of wide-band frequency tuning. do.

[Structure of the Invention] (Means for Solving the Problems) The surface acoustic wave resonator of the present invention includes a piezoelectric substrate and a comb-shaped electrode formed on the piezoelectric substrate, which is made up of a plurality of electrode fingers. In a surface acoustic wave resonator device comprising an excitation electrode and a grating reflector formed on the substrate and arranged to sandwich the excitation electrode, the excitation electrode closest to the grating reflector has a maximum intersection length. It is characterized in that the electrode fingers are weighted so that the dummy electrodes are placed opposite the intersections.

(Function) In the surface acoustic wave resonator of the present invention, weighting is performed so that the intersection length of the comb-teeth electrodes is maximized at the electrode closest to the grating reflector. The electrode fingers from the front block most of the electrode opening length. Therefore, asymmetrical excitation does not occur between the excitation electrode and the grating reflector, and spurious higher-order transverse modes are suppressed.

Further, since the dummy electrode is arranged on the piezoelectric substrate at a position opposite to the intersecting portion of the comb-teeth electrodes, the phase of the propagating surface acoustic wave is not disturbed.

(Example) Hereinafter, details of an example of the present invention will be described based on the drawings.

FIG. 1 is a front view showing an embodiment of the surface acoustic wave resonator of the present invention.

In the same figure, the code 5 is LITaO3, LINbo 3
Alternatively, a piezoelectric substrate made of crystal or the like is shown, and on this piezoelectric substrate 5, a comb-teeth electrode 6 made of a plurality of electrode fingers (BQI)
An excitation electrode 6 formed by crossing a and 6b is formed by photo-etching a deposited film such as Aβ, and grating reflectors 7 are formed on both sides of this excitation electrode 6 by a similar method. .

The line width of each grating reflector 7 is 1/4 of the wavelength λ of the surface acoustic wave, and such fingers are arranged at a pitch of 1/2 of the wavelength λ. Furthermore, the intersection length of the comb-teeth electrodes in the excitation electrode 6 increases symmetrically on both sides as it approaches the grating reflector 7 from the center, and is maximized at both ends. The electrode fingers are shaped like a part of a curve.

Furthermore, in the area other than the intersection between the comb-teeth electrodes 6a and 6b,
In order to prevent phase disturbance of the surface acoustic waves, dummy electrodes 8 are placed opposite the crossed electrode fingers.

In the surface acoustic wave resonator of the embodiment configured as described above, the electrode fingers at the end of the excitation electrode 6 are the longest and extend occupying most of the length of the electrode opening. Asymmetrical excitation is not caused between the excitation electrode 6 and the grating reflector 7, and the generation of spurious waves due to higher-order transverse modes is suppressed. therefore,
When this surface acoustic wave resonator device is used as an oscillator of a voltage controlled oscillator, the range in which it can be used as an inductance is expanded, and the variable range of frequency can be expanded.

FIG. 2 shows another embodiment of the present invention, in which the excitation electrode 6
The comb-teeth electrodes have a symmetrical crossing length that decreases on both sides from the center to a predetermined position, and are weighted so that the crossing length increases again toward the ends, so that the crossing length is maximum at the center and both ends. The electrode fingers are weighted by the avoidization method so that the outer shape of the intersection draws a cosine curve.

By using a surface acoustic wave resonator with such a configuration, the comb-shaped electrodes with long crossing lengths are located in areas where the degree of electromechanical coupling is strong, resulting in good excitation efficiency and without deteriorating the performance of the resonator. The generation of spurious signals due to higher-order transverse modes can be suppressed.

In the above embodiment, a structure in which the excitation electrode 6 and the grating reflector 7 are electrically insulated has been described, but the present invention is not limited to such a structure. A structure may be used in which the finger-side bus bar and the short bus bar of the grating reflector are electrically connected.

In a resonant device having such a structure, the electrode fingers at the end of the excitation electrode and the grating reflector are at the same potential, so no excitation occurs between them, and spurious waves in higher-order transverse modes are completely suppressed.

Furthermore, in the surface acoustic wave resonator of the present invention, the same effect can be achieved even when the grating reflector has a shape in which the reflection electrode fingers are not connected by short bus bars.

[Effects of the Invention] As is clear from the above description, according to the surface acoustic wave resonator of the present invention, resonance can be generated without generation of higher-order transverse mode spurious.

Therefore, when this is used as an oscillator of a voltage controlled oscillator, it becomes possible to vary the frequency over a wide band.

[Brief explanation of the drawing]

FIG. 1 is a simplified plan view showing one embodiment of the surface acoustic wave resonator of the present invention, FIG. 2 is a simplified plan view showing another embodiment of the surface acoustic wave resonator of the present invention, and FIGS. Figure 4 is a simplified plan view of a conventional surface acoustic wave resonator, Figure 5 is a diagram for explaining modes of excitation energy distribution in a conventional surface acoustic wave resonator, and Figure 6 is a diagram of a conventional surface acoustic wave resonator. It is a graph showing the passage characteristics of excitation energy in the device. 1.5...Piezoelectric substrate 2aq 2bq 6a, 6b--Comb electrode 2.6-
......Excitation electrode 3.7...Grating reflector 4.8
・・・・・・・・・Dummy electrode applicant Toshiba Corporation Patent attorney Sa-Shin Suyama Figure 3 Figure 4

Claims (1)

[Scope of Claims] A piezoelectric substrate, an excitation electrode formed on the piezoelectric substrate and formed by intersecting comb-shaped electrodes made of a plurality of electrode fingers, and an excitation electrode formed on the substrate and arranged to sandwich the excitation electrode. In a surface acoustic wave resonator device comprising a grating reflector, the excitation electrode closest to the grating reflector has electrode fingers weighted so that the intersection length is maximized, and A surface acoustic wave resonator characterized by arranging dummy electrodes.