JPS62200814A - Surface acoustic wave resonator - Google Patents

Abstract

PURPOSE:To prevent a spurious radiation from being caused over a wide band around the oscillated frequency by applying apodizing to an interdigital electrode so that the cross width is decreased symmetrically toward the grating reflector from the center. CONSTITUTION:The surface acoustic wave exciting interdigital electrode 2 made of, e.g., an aluminum thin film of 0.5mum thick is provided on a piezoelectric substrate 1 and the grating reflectors 3, 4 are provided at its both sides. The cross length of the interdigital electrode 2 is made shorter toward the grating reflectors 3, 4 from the center. The outer shape of the cross part is formed in a shape at the center. The outer shape of the cross part is formed in a shape that the tips of two cosine curves are arranged symmetrically to the right/left. Further, a dummy electrode 9 not contributing the excitation is formed to parts other than the cross part in order to prevent the phase of the surface acoustic wave from being disturbed.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) This invention provides an elastic surface formed by forming interdigital electrodes for surface acoustic wave excitation and a grating reflector on a piezoelectric band plate. Concerning improvement of wave resonators.

(Prior art) In recent years, surface acoustic wave resonators (hereinafter referred to as rsAW resonators) have been used as zero oscillators and narrowband filters for VHF and UHF bands.
development is underway. As shown in FIG. 7, this SAW co-reflector is constructed by forming an interdigital electrode 2 on a piezoelectric substrate 1, and forming grating reflectors 3.4 on both sides of the interdigital electrode 2. ing. The interdigital electrode 2 is a transducer that converts an electric signal into a surface acoustic wave, and one having a uniform crossing width as shown in the figure is called a regular type. The grating reflector 3.4 is composed of a large number of strips arranged periodically at a pitch of 1/2 of the surface acoustic wave wavelength. A standing wave is formed by repeated reflection between . therefore,
With this configuration, a one-terminal pair type resonator can be configured.

By the way, in order to obtain a low impedance SAW resonator, it is necessary to increase the electrode opening length W of the interdigital electrodes 2, that is, the crossing length of the interdigital electrodes 2. However, if we simply increase the electrode aperture length, the 8th
As shown in the resonance pass characteristics shown in the figure, there was a problem in that spurious noise 5 was generated on the high frequency side of the resonance frequency. This is because the distribution of surface acoustic wave energy excited by the interdigital electrode 2 has a rectangular shape as shown in FIG. 9(a), and the Fourier series expansion of the energy distribution shown in FIG. 9(b) The ^-order transverse modes 7.8 other than the 0-order transverse mode 6 (fundamental wave) are influential.

Therefore, it is also known that the interdigital electrode 2 is weighted by the avoidization method so that the outer shape of the intersecting part of the electrodes draws a cosine arc, as shown in FIG. 10, for example. In this SAW resonator, the excitation efficiency in the interdigital electrode 2 is changed to be maximum at the center, so that the distribution of surface acoustic wave energy matches the above-mentioned zero-order transverse mode 6. In this case, a dummy electrode 9 that does not contribute to excitation is provided so that the phase of the surface acoustic wave is not disturbed at a short intersection, so that the surface acoustic wave uniformly experiences the metal film of the electrode.

However, even with this configuration, conventionally the 11th
As is clear from the Smith chart in the figure, spurious signals were generated on the side lower than the resonant frequency, deteriorating the characteristics. This kind of characteristic deterioration is caused by the fact that the oscillation frequency cannot be continuously varied when an extension coil is connected to use a frequency lower than the resonant frequency in order to enable wide-band variation as an oscillator for an oscillator, especially a voltage-controlled oscillator. This poses a major problem in that it causes a frequency jump phenomenon.

(Problems to be Solved by the Invention) As described above, the conventional SAW resonator has a problem in that it cannot be tuned over a wide range because it generates spurious on the low frequency side near the resonant frequency.

This invention was made based on these circumstances,
It is an object of the present invention to provide a SAW resonator in which no noise spurious occurs in a wide band around the oscillation frequency.

[Structure of the Invention] (Means for Solving the Problems) In the present invention, an interdigital electrode formed on a piezoelectric substrate and converting an electric signal into a surface acoustic wave approaches the grating reflector from its center. The abodization is performed so that the crossing width decreases symmetrically as the electrodes cross, and dummy electrodes are arranged in the portions where they do not intersect.

(Function) The present inventors performed simulations on the frequency characteristics of impedance when the shape of the interdigital electrode was changed in various ways. As a result, the above-mentioned spurious is due to the reflection of surface acoustic waves from the dummy electrode, and the spurious can be suppressed by reducing the tip shape of the dummy electrode, that is, the intersection of the interdigital electrodes symmetrically as it approaches the grating reflector. It was confirmed that it is possible to suppress the spurious response or move the spurious response to the lower frequency side. This mechanism is not yet clear, but based on this fact, only the energy of the zero-order transverse mode is propagated by the above-mentioned shape of the interdigital electrode.
The influence of reflection from the dummy electrode is reduced.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 shows a surface acoustic wave resonator according to an embodiment of the present invention, which is an example suitable for a voltage controlled oscillator of a synthesizer local oscillator in a 400 MHz band mobile radio, for example.

In the figure, the piezoelectric substrate 1 is, for example, a 36'' Y-XLiTaO31 plate that propagates surface acoustic waves.

On this piezoelectric substrate 1, there is an interdigital electrode 2 for excitation of surface acoustic waves made of a thin aluminum film with a thickness of 0.5 mm, for example.
is provided, and furthermore, grating reflectors 3 are provided on both sides thereof.
．． 4 are provided. The grating reflectors 3 and 4 are formed by arranging 200 aluminum strips each having a line width of 1/4 of the surface acoustic wave wavelength and having a line width of 1/4 at 1/2 wavelength intervals. The period length of the interdigital electrode 2 is
The period length is set to be about 1% smaller than the period length of the grating reflectors 3 and 4.

The intersection length of the interdigital electrode 2 is 3! from the center to both sides of the grating reflection. ! ! The closer it gets to 3.4, the shorter it gets. The outer shape of the intersection is a shape in which the tips of double cosine curves are arranged symmetrically. In addition, a dummy electrode 9 that does not contribute to excitation is formed at a location other than this intersection in order to prevent the phase of the surface acoustic wave from being disturbed.

FIG. 2 shows the impedance frequency characteristics of the SAW resonator configured in this manner. Compared to the conventional characteristics shown in FIG. 11, the spurious 10 near the resonance frequency (low frequency) is sufficiently suppressed and moved to the lower frequency side, resulting in a significant improvement in the characteristics. Even when used as an oscillation element in a Colpitts-type voltage controlled oscillator, an extension coil is inserted in series,
Since the capacitive region of the resonator can also be used, the control ms can be
The frequency and frequency variable range can be expanded.

FIG. 3 shows another embodiment of the invention.

In this case, the outer shape of the intersection of the interdigital electrodes 2 has a shape in which the base end curve of a cosine curve is arranged symmetrically. FIG. 4 shows the impedance frequency characteristics of this SAW resonator. As is clear from this figure, the spurious 10 is suppressed and has moved to the lower frequency side. Therefore, in this case as well, the capacitive area can be expanded.

FIG. 5 shows still another embodiment of the present invention, in which the outer shape of the intersecting portion of the interdigital electrodes 2 is a cosine curve itself arranged symmetrically. In this case, as shown in FIG. 6, although the effect of suppressing the spurious 10 is small, it has moved to a lower level, and it can be seen that the capacitive region is also expanded.

It should be noted that the present invention is not particularly limited to the shape of the interdigital electrode 2 where the cross section follows a cosine curve; various modifications can be considered as long as the cross length is shortened symmetrically on both sides. It will be done.

[Effects of the Invention] As described above, according to the present invention, the S
An AW resonator can be provided.

[Brief explanation of drawings]

FIG. 1 is a plan view showing the configuration of a SAW resonator according to an embodiment of the present invention, FIG. 2 is a Smith diagram showing the frequency characteristics of the impedance of the resonator, and FIG. 3 is a plan view showing the structure of a SAW resonator according to an embodiment of the present invention. A plan view showing the configuration of the SAW resonator according to the example, FIG. 4 is a Smith diagram showing the frequency characteristics of the impedance of the resonator,
FIG. 5 is a plan view showing the configuration of a SAW resonator according to still another embodiment of the present invention, FIG. 6 is a Smith diagram showing the frequency characteristics of the impedance of the resonator, and FIG. 7 is a conventional regular type A plan view showing the configuration of the SAW resonator, FIG. 8 is a diagram showing the pass characteristics (amplitude) of the resonator, FIG. 9 is a diagram explaining each mode of the energy distribution of the SAW resonator, and FIG. 10 1 is a plan view showing the configuration of a SAW resonator based on the conventional abodization method, and FIG. 11 is a Smith chart showing the frequency characteristics of impedance of the resonator. 1... Piezoelectric substrate, 2... Interdigital electrode,
3.4... Grating reflector, 9... Dummy electrode. Applicant's representative Patent attorney Takehiko Suzue No. 11! ! n Figure 2 Figure 3 s4 Figure 6 Figure s7 Figure 118 Figure 9 Figure 11

Claims (2)

[Claims] (1) A piezoelectric substrate, an interdigital electrode formed on the piezoelectric substrate to convert an electric signal into a surface acoustic wave, and a grating reflector formed on both sides of the interdigital electrode to reflect the surface acoustic wave. In the surface acoustic wave resonator, the interdigital electrodes are
A surface acoustic wave resonator characterized in that the surface acoustic wave resonator is apodized so that the crossing width decreases symmetrically as one approaches the grating reflector from the center thereof, and dummy electrodes are arranged in the portions where they do not intersect. (2) The surface acoustic wave resonator according to claim 1, wherein the interdigital electrode has an outer shape of at least a part of a cosine arc at an intersection.