JPH0728195B2 - Surface acoustic wave resonator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a surface acoustic wave formed by forming an interdigital electrode for exciting a surface acoustic wave and a grating reflector on a piezoelectric substrate. Revision of resonator.

(Prior Art) In recent years, surface acoustic wave resonators (hereinafter referred to as "SAW resonators") have been developed as resonators for the VHF and UHF bands and narrow band filters. As shown in FIG. 7, this SAW resonator is formed by forming interdigital electrodes 2 on a piezoelectric substrate 1 and grating reflectors 3 and 4 on both sides of the interdigital electrodes 2. There is.
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 normal type. The grating reflectors 3 and 4 are formed by periodically arranging a large number of strips at a pitch of 1/2 of the surface acoustic wave wavelength. A standing wave is formed by repeatedly reflecting between 4 and 4. 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 lengthen the electrode opening length W of the interdigital electrodes 2, that is, the cross length of the interdigital electrodes 2. However, if the electrode opening length is simply increased,
There is a problem that the spurious 5 is generated on the high frequency side of the resonance frequency as shown by the resonance passage characteristic in the figure. This is because the distribution of the 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) is performed. Higher-order transverse modes 7 and 8 other than the 0th-order transverse mode 6 (fundamental wave) are affecting.

Therefore, it is also known that the interdigital electrode 2 is weighted by the apodization method so that the outer shape of the intersecting portion of the electrodes draws an arc line of a cosine as shown in FIG. In this SAW resonator, the excitation efficiency in the interdigital electrode 2 is changed so as to be maximized in the central portion, so that the surface acoustic wave energy distribution is made to coincide with the 0th-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 such a configuration, conventionally, as is clear from the Smith chart of FIG. 11, spurious is generated on the side lower than the resonance frequency, and the characteristics are deteriorated. Such characteristic deterioration is caused by continuously changing the oscillation frequency, especially when the extension coil is connected and a frequency lower than the resonance frequency is used in order to enable wide-band tuning as an oscillator of the oscillator, particularly the voltage controlled oscillator. It becomes a big problem that the frequency jump phenomenon occurs.

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

The present invention was made under such circumstances,
An object of the present invention is to provide a SAW resonator in which spurious is not generated over a wide band centered on the oscillation frequency.

[Structure of the Invention] (Means for Solving Problems) In the present invention, an interdigital electrode, which is formed on a piezoelectric substrate and converts an electric signal into a surface acoustic wave, approaches the grating reflector from its center. As a result, the apodization is performed so that the cross width decreases symmetrically, and the dummy electrode is arranged in a portion that does not cross.

(Operation) The inventors of the present invention performed a simulation on the frequency characteristic of impedance when the shape of the interdigital electrode was changed variously. As a result, the aforementioned spurious is due to the reflection of the surface acoustic wave from the dummy electrode, and the tip shape of the dummy electrode, that is, the intersection of the interdigital electrodes is reduced symmetrically as it approaches the grating reflector. It was confirmed that it is possible to suppress the noise or move the spurious to the low frequency side. Although this mechanism is not clear yet,
Based on this fact, the above-described shape of the interdigital electrode propagates only the 0th-order transverse mode energy and reduces the influence of reflection from the dummy electrode.

Embodiments Embodiments of the present invention will be described below 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-XLiTaO ₃ substrate that propagates surface acoustic waves. On the piezoelectric substrate 1, surface acoustic wave excitation interdigital electrodes 2 made of, for example, a 0.5 μm thick aluminum thin film are provided, and grating reflectors 3 and 4 are provided on both sides thereof.
The grating reflectors 3 and 4 are made of aluminum strip of 2.8 μm, which has a line width of 1/4 of the surface acoustic wave wavelength, at 200 wavelength intervals.
The books are arranged one by one. The period length of the interdigital electrode 2 is set to be about 1% smaller than the period length of the grating reflectors 3 and 4.

The crossing length of the interdigital electrodes 2 becomes shorter as it gets closer to the grating reflectors 3 and 4 on both sides from the center. The outer shape of the intersecting portion has a shape in which the tips of double cosine curves are symmetrically arranged. In addition, in order to prevent the phase of the surface acoustic wave from being disturbed, a dummy electrode 9 that does not contribute to the excitation is formed in a portion other than the intersecting portion.

The frequency characteristics of the impedance of the SAW resonator thus constructed is shown in FIG. Comparing the conventional characteristics shown in FIG. 11, the spurious 10 near the resonance frequency (low range) is sufficiently suppressed, and further the characteristics are improved by moving to the low range side. Even when it is used as an oscillating element of a Colpitts type voltage controlled oscillator, the extension coil can be inserted in series and the capacitive region of the resonator can be utilized, so that the control sensitivity and the frequency variable width can be expanded.

FIG. 3 shows another embodiment of the present invention. In this case, the outer shape of the intersection of the interdigital electrodes 2 has a shape in which the curve of the base end of the cosine curve is symmetrically arranged. The frequency characteristics of the impedance of this SAW resonator are shown in FIG. As is clear from this figure, the spurious 10 is suppressed and is moving to the low frequency side.
Therefore, also in this case, the capacity area can be expanded.

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

It should be noted that the present invention is not particularly limited to the outer shape of the intersecting portion of the interdigital electrode 2 along the cosine curve, and various modifications may be considered as long as the intersecting length is symmetrically shortened on both sides. To be

[Advantages of the Invention] As described above, according to the present invention, SA in which spurious is not generated over a wide band centered on the oscillation frequency
A W resonator can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view showing a configuration of a SAW resonator according to an embodiment of the present invention, FIG. 2 is a Smith chart showing frequency characteristics of impedance of the resonator, and FIG. FIG. 5 is a plan view showing a configuration of a SAW resonator according to another embodiment of the present invention, FIG. 4 is a Smith chart showing frequency characteristics of impedance of the resonator, and FIG.
FIG. 7 is a plan view showing a structure of a SAW resonator according to still another embodiment of the present invention, FIG. 6 is a Smith chart showing frequency characteristics of impedance of the resonator, and FIG. 7 is a conventional normal type SAW resonator. Fig. 8 is a plan view showing the configuration of Fig. 8, Fig. 8 is a diagram showing the pass characteristic (amplitude) of the resonator, Fig. 9 is a diagram for explaining each mode of energy distribution of the SAW resonator, and Fig. 10 is a conventional diagram. FIG. 11 is a plan view showing the structure of the SAW resonator by the apodization method, and FIG. 11 is a Smith chart showing the frequency characteristic of impedance of the resonator. 1 ... Piezoelectric substrate, 2 ... Interdigital electrodes, 3,4
...... Grating reflector, 9 ... Dummy electrode.

Claims (2)

[Claims] 1. A piezoelectric substrate, interdigital electrodes formed on the piezoelectric substrate for converting electric signals into surface acoustic waves, and grating reflectors formed on both sides of the interdigital electrodes for reflecting surface acoustic waves. In the surface acoustic wave resonator consisting of, the interdigital electrode is apodized so that the crossing width decreases symmetrically from the center thereof toward the grating reflector, and the dummy electrode is arranged in the non-crossing part. A surface acoustic wave resonator characterized by being formed. 2. The surface acoustic wave resonator according to claim 1, wherein the interdigital electrode has an outer shape of at least a part of an arc line of a cosine at an intersecting portion.