JPH01137718A - Transducer for surface acoustic wave - Google Patents

Abstract

PURPOSE:To thin the thickness of a metallic film of an electrode and to reduce reflection spurious by setting up the width of the electrode to a value in the vicinity of electrode width giving a maximum acoustic reflection variable. CONSTITUTION:A pair A of electrode fingers is constituted of an electrode with about 1/8lambda width, a pair B of electrode fingers is constituted of an electrode with about 1/8lambda width and an electrode with about 1/4lambda width and the electrode with 1/4lambda width is set up in the vicinity of the electrode width so that an acoustic reflection variable is maximized in the same film thickness. Since the phase of electric reflection is reversed from the phase of acoustic reflection is said constitution, both the reflection elements are canceled each other and the reflection spurious is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a low-loss surface acoustic wave transducer with improved reflection spurious.

(Prior Art) Conventionally, surface acoustic wave transducers in which interdigitated electrodes (hereinafter referred to as IDTs) are provided on the surface of a piezoelectric substrate have been put into practical use. The IDT is constructed by interlocking first and second comb-shaped electrodes with a predetermined gap. By changing the pitch crossing width of the electrode strips of this IDT, various frequency characteristics can be realized.

In various surface acoustic wave devices configured using the above-mentioned IDT, there is a spurious reflection of a surface wave caused by the IDT, which always poses a problem. This reflected spurious is
When actually configuring filters and delay lines, triple
This appears in the form of transit echo (hereinafter referred to as TTE) and causes a significant deterioration of characteristics.

The cause of this reflected spurious is the reflected component (
There are two components: a reflected component (hereinafter referred to as Rε) caused by electrical re-excitation of the IDT.

Therefore, in order to remove the reflected components R^ and Rε, the reflected components R^ and Rε should be set in opposite phases to each other so that they cancel each other out.

Conventionally, based on the above-mentioned idea, there is a transducer shown in FIG. 5 as a means for canceling out the reflected component R^ caused by the difference in acoustic impedance and the reflected component Rt caused by electrical re-excitation. 60-140917
(see publication).

In the same figure, reference numeral 1 denotes a first
2 is a comb-shaped electrode, and the reference numeral 2 indicates a second comb-shaped electrode, and this first comb-shaped electrode 1 and second comb-shaped electrode 2
A surface acoustic wave transducer is constructed by meshing the respective electrodes.

The electrode finger pairs of this transducer are a first type of electrode finger pair A in which only electrical reflections RE occur and acoustic reflections R^ are zero, and a second type of electrode finger pair A in which both electric and acoustic reflections occur. It consists of a type of electrode finger pair B. 1st
The width dimension of the second type electrode finger pair A is set to 178 of the wavelength λ of the surface acoustic wave, and the width dimension of the second type electrode finger pair B is set to a combination of 1/8λ and 3/8λ. There is.

FIG. 6 is a modification of FIG. 5, and corresponds to the case where the regions B are dispersed in the cross width direction. In Figure 6,
The reason why the region of electrode type B has a triangular shape is explained as follows. That is, in the case of the figure, since electrical reflection occurs from both areas A and B, the time axis waveform of the overall electrical reflection characteristic of this IDT has a triangular shape, which is a convolution of rectangles. Therefore, in order to make these electrical reflections cancel each other out at all points on the time axis, it is sufficient to make the acoustic reflection waveforms the same triangular shape. If considered on the frequency axis, this means that the frequency amplitude characteristics of the acoustic reflection R^ and the frequency amplitude characteristics of the electrical reflection Rt are made equal over a wide band.

That is, it is suggested that when a filter is constructed using an IDT having such a structure, TTE can be suppressed over a wide band.

(Problems to be Solved by the Invention) However, such conventional surface acoustic wave transducers have the following problems. That is, the amount of electrical reflection of a surface acoustic wave transducer changes depending on the value of the load connected to it. Therefore, it is necessary to generate an amount of acoustic reflection commensurate with the amount of electrical reflection.

The amount of acoustic reflection is adjusted by changing the metal film thickness of the electrode fingers of the comb-shaped electrode. This is gold 11IM
This is due to the fact that the amount of acoustic reflection changes due to the mass effect of No. 4. Therefore, for example, when the amount of electrical reflection increases depending on the order of loads, it is necessary to increase the thickness of the electrode fingers to increase the mass effect and increase the amount of acoustic reflection.

However, when the electrode amount IfL film is made thick in this manner, a portion of the surface wave energy is often converted into the bulk wave mode, and ripples appear in the output signal due to so-called bulk spurious. This phenomenon becomes more pronounced as the thickness of the comb-shaped electrode increases and as the number of electrode fingers increases.

The present invention has been made in order to solve the above-mentioned problems.The present invention has been made in order to solve the above-mentioned problems. The purpose is to provide a transducer.

[Configuration of the Invention (Means for Solving Problems) The surface acoustic wave transducer of the present invention is of the first type in which only electrical reflection occurs on the piezoelectric substrate and acoustic reflection is zero. It is formed by combining an electrode finger pair and a second type electrode finger pair that causes both electrical reflection and acoustic reflection, and the first type electrode finger pair is formed by combining approximately 1 of the surface acoustic waves propagating through the substrate. The second type of electrode finger pair is formed by an electrode finger pair having a width dimension of approximately 1/8 of the wavelength and near the electrode width where the amount of acoustic reflection is maximum on the piezoelectric substrate. It is characterized by being formed.

(Function) Figure 4 shows the relationship between the ti width and acoustic reflectance of lithium niobate (jiNbot) (Reference: I
EEE TRANSACTIONS ON IILTR
AsONIcs.

FLRRQELECTRIC8,AND FREQIJ
ENCY C0NTR0L, VOL.

UFFC-33, NO, 4JULY 1986 P, 3
70).

In the figure, the horizontal axis is the electric power W standardized by the wavelength of surface acoustic waves.
1 width and the vertical axis is the amount of acoustic reflection, Olo, 01.
0.02! llte standardized shirikin! E a thickness is shown. As is clear from the figure, even if the metal film thickness is the same, the amount of acoustic reflection varies depending on the electrode width. Conventionally, in the example disclosed in Japanese Patent Application Laid-Open No. 60-145716, an electrode width of approximately 3/8 of the wavelength of the surface acoustic wave was used, but this does not necessarily mean that the electrode width can provide the maximum amount of acoustic reflection. However, if the electrode width is set close to the electrode width where the maximum amount of acoustic reflection is obtained, the electrode metal film thickness can be made thinner than in the conventional example, and a sufficient amount of acoustic reflection can be obtained from the scales. Can be done. In particular, when a surface acoustic wave filter with a narrow band pass characteristic in which the number of pairs of comb-teeth electrodes increases, the proportion of surface wave energy converted into bulk wave mode decreases. Therefore, it is possible to improve insertion loss and reduce ripple due to the reduction in propagation loss. Moreover, in this case, compared to the conventional technology, it can be realized with a thinner film thickness, so there is an advantage that the manufacturing process is also easier.

(Example) An example of the present invention will be described below with reference to FIGS. 1 to 3.

FIG. 1 is a diagram showing a surface acoustic wave transducer according to an embodiment of the present invention, and this example shows an example in which lithium niobate (LiNbOy) is used as a piezoelectric substrate.

FIG. 2 shows the relationship between the acoustic reflectance and the comb-shaped electrode metal film thickness of a surface acoustic wave device constructed using interdigital electrodes. As is clear from the figure,
The thicker the metal film is than the predetermined film thickness T, the greater the acoustic reflection obtained by the action of the mass effect.On the other hand, if the metal film is thinner than the film thickness T, the electric field short circuit effect acts and the acoustic reflection obtained by the mass effect is increased. Acoustic reflection with a phase opposite to that of is obtained, and its value increases as the film thickness becomes thinner. Further, at the film thickness T, the mass effect and the electric field short circuit effect have the same magnitude, and no acoustic reflection occurs.

In this way, acoustic reflection can be caused not only by the action of the mass effect but also by the action of the electric field short circuit effect by reducing the thickness of the metal film to a predetermined amount or less. The electrode film thickness should be such that acoustic reflection occurs. That is, as shown in FIG. 1, the first type of electrode finger pair, in which only electrical reflection occurs and zero acoustic reflection, is different from the second type in which both electrical reflection and acoustic reflection occur. Let B be the type of electrode finger pair. Electrode finger pair A consists of electrodes with a width of approximately 1/8λ, where λ is the wavelength of the surface acoustic wave propagating through the substrate, and electrode finger pair B consists of electrodes with a width of approximately 1/8λ and electrodes with a width of approximately 1/4λ. It consists of electrodes. Said 17
The electrode having a width of 4λ was set near the electrode width at which the amount of acoustic reflection was maximized at the same film thickness from FIG. 4 shown in the above-mentioned operation. Further, the 174λ width electrode in FIG. 1 corresponds to the 3/8λ width electrode in FIG. 6 shown in the conventional example, but the positions of the electrodes are shifted by 174λ. this is,
This is because the phase of the acoustic reflection due to the electric field short-circuit effect is opposite to the phase of the acoustic reflection obtained due to the mass effect. With this configuration, the phase of electrical reflection and the phase of acoustic reflection are opposite to each other. Therefore, the two cancel each other out, and as a result, reflected spurious can be reduced.

In this way, acoustic reflection is generated by the action of the electric field short-circuit effect, and the acoustic reflection and electric reflection are made to have opposite phases and cancel each other out, thereby removing the reflected spurious. Furthermore, the electrode width is designed to maximize acoustic reflection. Therefore, according to the present invention, a surface acoustic wave transducer with less reflected spurious can be obtained. Furthermore, since the thickness of the electrode metal film can be reduced, the rate of mode conversion of surface wave energy into bulk waves is reduced, so bulk spurious can also be reduced.

In this example, the electrode finger pair B was composed of electrodes with a width of approximately 1/8λ and approximately 1/4λ, but the electrode width is not limited to a width of 174λ, and the width of the electrodes is not limited to a width of 174λ. (For example, in the case of lithium niobate, the electrode width can be set to 1/10 λ to 1/4 λ as shown in Fig. 4.) In addition, in this example, lithium niobate is used as the substrate material. However, it goes without saying that the present invention can also be applied to piezoelectric substrates made of other substrate materials.

Further, according to the present invention, by forming groups 12 on the piezoelectric substrate 11 as shown in FIG. 3, it is also possible to obtain even greater acoustic reflection by combining them.

[Effects of the Invention] As explained above, according to the surface acoustic wave transducer of the present invention, the electrode metal film thickness can be made thinner than in the past, so surface wave energy is mode-converted to bulk waves. The proportion of Therefore, bulk wave spurious can also be reduced, and insertion loss can also be improved. This effect is particularly noticeable in cases where electrical reflection increases with the load value and the amount of acoustic reflection must be increased to cancel this out. Furthermore, since it can be realized with a thin film thickness, the manufacturing process is easy, and therefore frequency control is also easy, so the manufacturing yield can be increased.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an electrode in an embodiment according to the present invention;
FIG. 2 is a diagram showing the relationship between electrode film thickness and acoustic reflectance, FIG. 3 is a diagram showing another embodiment of the present invention, FIG. 4 is a diagram showing the relationship between electrode width and acoustic reflectance, and FIG. 5 and 6 are diagrams showing the configuration of conventional electrodes. 1......First comb-shaped electrode 2...
. . . Second comb-shaped electrode 11 . . . Piezoelectric substrate 12 . . . Group applicant Toshiba Corporation Representative Patent attorney Satoshi Suyama - Figure 1, Figure 2 Figure] 1. Figure 3. Figure 4.

Claims (5)

[Claims] (1) A first type electrode finger pair in which only electrical reflection occurs on the piezoelectric substrate and zero acoustic reflection, and a second type electrode finger pair in which both electric reflection and acoustic reflection occur. The first type of electrode finger pair is formed by electrode fingers having a width approximately 1/8 of the surface acoustic wave propagating on the substrate, and the second type of electrode finger pair is formed by a width of approximately 1/8 of the surface acoustic wave propagating on the substrate. A transducer for surface acoustic waves, characterized in that it is formed by a pair of electrode fingers with a width dimension of approximately 1/8 and a width near the electrode width at which the amount of acoustic reflection is maximum on a piezoelectric substrate. (2) The piezoelectric substrate is lithium niobate (LiNbO_3
), and the second type of electrode finger pair has a wavelength of about 1
2. The surface acoustic wave transducer according to claim 1, wherein the surface acoustic wave transducer is constituted by a pair of electrode fingers having a width dimension of approximately 1/10 to 1/4. (3) The surface acoustic wave transducer according to claim 1 or 2, wherein each electrode film has a thickness that causes acoustic reflection due to an electric field short circuit effect. (4) The surface acoustic wave transducer according to claim 1, wherein the first type of electrode finger pairs and the second type of electrode finger pairs are distributed in the cross width direction. (5) The first type of electrode finger pair and the second type of electrode finger pair are formed to be distributed in the propagation direction of the surface wave. Transducer for surface acoustic waves.