JPH043127B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave device equipped with a unidirectional transducer that makes it possible to realize broadband characteristics.

Waves that propagate with concentrated energy along the flat surface of an elastic body, so-called surface acoustic waves, are superior in various respects to the conventionally used bulk waves, and this property can be used to create filters. It is being applied as a surface acoustic wave device to various electronic components including. Figure 1 shows a filter as an example, in which 1 is a piezoelectric substrate, 2 is a filter;
4 is an input transducer consisting of a pair of comb-shaped electrodes 3A, 3B; 4 is a pair of comb-shaped electrodes 5A, 5;
The output transducer consists of B, and the input terminal
The signal applied from IN is converted into a surface acoustic wave by the input transducer 2, propagates on the surface of the piezoelectric substrate 1 as shown by the arrow, and reaches the output transducer 4, where it is then converted into an electrical signal. It is configured such that it is converted into a signal and taken out from the output terminal OUT.

By the way, as in the surface acoustic wave device having the structure shown in FIG.
In the case of a filter in which two transducers 2 and 4 including A and 5B are arranged, an electric-mechanical system is used in order for these transducers 2 and 2 to work to propagate surface waves in both left and right directions. It has the disadvantage that conversion loss is unavoidable and the loss increases as a filter.

In order to eliminate this drawback, a so-called unidirectional transducer has been proposed, which is designed to propagate surface waves only in one direction on the surface of a piezoelectric substrate. The specific configuration of this unidirectional transducer includes a method using a 120° phase shifter as shown in Figure 2, a method using a 90° phase shifter, and a method using a reflector as shown in Figure 3. method is known.

In Figure 2, 6, 6A, and 6B are 120° to each other.
and 6 is the other electrode 6A.
A gap 7 or an insulating film is interposed between the two and serves to propagate surface waves in only one direction.

However, the method using such a phase shifter has the disadvantage that the manufacturing process is complicated because it is necessary to provide the crossing portions in the electrodes as described above. Furthermore, the design of the phase shifter itself to obtain desired characteristics is extremely complicated, making adjustment difficult and making it difficult to achieve broadband characteristics.

On the other hand, in FIG. 3, 8A and 8B are a power supply part and a reflection part provided to constitute a part of a comb-shaped electrode, both of which are regular electrodes, and 9 is a common electrode for the electrodes 8A and 8B;
10 is a signal source, 11 is a matching circuit, and 12 is a reactance circuit. In the above, the signal applied from the signal source 10 via the matching circuit 11 is
From A, it becomes a surface acoustic wave and propagates in both left and right directions. At this time, the surface wave propagated to the left is reflected by the reflecting section 8B connected to the reactance circuit 12 and returned to the right, and the surface wave propagating to the right and the reflected surface wave are combined at the feeding section 8A. will be held. As a result, when the center frequencies of surface waves are the same, both waves are superimposed, but when the center frequencies are different from each other, the waves act to cancel each other out, so the surface waves propagate in the opposite direction to the intended direction. You end up doing it. Therefore, there is a drawback that the propagation characteristics of the surface waves are limited to narrow band characteristics.

The present invention has been made in order to deal with the above problems, and the surface acoustic wave device according to the present invention is formed in contact with an elastic body, and has an electrode period that gradually increases with respect to one propagation direction of a surface wave. a first comb-shaped chirp electrode formed so as to be short; and a first comb-shaped chirp electrode formed in contact with the elastic body and arranged in parallel so as to be in contact with the first comb-shaped chirp electrode along the surface wave propagation direction. and connected to the first comb-shaped chirp electrode and a second comb-shaped chirp electrode formed so that the electrode period gradually becomes longer in the propagation direction opposite to the one propagation direction. and a signal source connected to the second comb-shaped chirp electrode, the first comb-shaped chirp electrode is used as a reflection part, and the second comb-shaped electrode is used as a power supply part. The invention is characterized in that it includes a transducer configured to

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

FIGS. 4a and 4b are a schematic plan view and a schematic sectional view showing a surface acoustic wave device according to an embodiment of the present invention.

In the figure, 13 is an elastic substrate, and on the surface of this elastic substrate 13, comb-shaped chirp electrodes 15A and 15B whose electrode period changes with respect to the propagation direction x of the surface acoustic wave are arranged in the propagation direction x. It is installed vertically. One of the comb-shaped chirp electrodes 15B is formed so that the electrode period gradually becomes shorter in the propagation direction x of the surface acoustic wave,
The other comb-shaped chirp electrode 15A is arranged in the propagation direction x
Both the comb-shaped chirp electrodes 15A and 15B are arranged in parallel so as to be in contact with each other at a vertical line Y-Y' on the common electrode 16.

The comb-shaped chirp electrode 15A is used as a power feeding section and is connected to a signal source 10 via a matching circuit 11, and the comb-shaped chirp electrode 15B is used as a reflecting section and connected to a reactance circuit 12.

In the above, among the surface acoustic waves generated from the power feeding part electrode 15A and propagated in the left-right direction on the surface of the elastic substrate 13, the surface waves propagated in the left direction are reflected in the reflection part terminated by the reactance circuit 12. Since the electrode 15B is present, the light is reflected by the reflection part electrode 15B and returned to the right direction again.

At this time, the point at which surface waves are generated differs depending on the operating frequency of the power feeding part electrode 15A, which is a comb-shaped chirp electrode, due to its shape. The generation point (equivalent surface wave generation point) at the current operating frequency is C
From this point C, both the electrodes 15A, 15B
Letting the distance to the contact line Y-Y' be l ₁ , the transition angle θ ₁ corresponding to this distance l ₁ can be expressed as follows.

θ ₁ = 2π/ ₀・l ₁ /λ ₀ ...(1) Here, : operating frequency, ₀ : center frequency, λ ₀ : wavelength of center frequency, and the reflective part electrode 15B is also composed of a comb-shaped chirp electrode. Therefore, the surface wave reflection point also differs depending on the operating frequency. Now, if the reflection point (equivalent surface wave reflection point) at the operating frequency is D, and the distance from this point D to the contact line Y-Y' is l ₂ , then the transition angle θ ₂ corresponding to this distance l ₂ is can be expressed as follows.

θ ₂ =2π/ ₀ ·l ₂ /λ ₀ ...(2) Here, in order to operate the transducer including the power feeding part electrode 15A and the reflecting part electrode 15B as unidirectional, the right side from the power feeding part electrode 15A is The surface wave propagates in the direction and the reflection part electrode 1 propagates in the left direction.
It is necessary to match the phase with the surface wave reflected by 5B and returned to the right. For this purpose, it is necessary to form the feed section electrode 15A and the reflection section electrode 15B in a comb shape (chirp electrode) so that the transit angle .theta. of the reflected and returned surface wave satisfies the following equation.

θ=2×(θ ₁ +θ ₂ )=(2π+1)π (3) The following equation is derived by integrating the above equations (1) to (3).

・(l ₁ +l ₂ )= ₀・λ ₀ (2n+1)/4 ...(4) Here, the two chirp electrodes 15A and 15B are geometrically symmetrical about the contact line Y-Y'. Then, l ₁ =l ₂ =l, and the above equation (4) becomes as follows.

・l= ₀・λ ₀ (2n+1)/8 ...(5) In this way, the two comb-shaped chirp electrodes 15A and 15B, which operate as the feeding part electrode and the reflecting part electrode, are set so as to satisfy the above equation (4). By design, it is possible to match the phase of both surface waves as described above over a wide operating frequency range, so that it can be operated as a unidirectional transducer. Furthermore, the electrode shape of the transducer that satisfies the above conditions can be easily manufactured using well-known photoetching techniques. Further, by terminating the side of the comb-shaped chirp electrodes 15A, 15B used as reflective part electrodes with a reactance circuit, the adjustment thereof can be easily performed.

In the above, two comb-shaped chirp electrodes 15
We have described the case where A and 15B are in contact with each other along the vertical line (contactability) Y-Y', but even if both electrodes are separated, as long as they are arranged within a range that satisfies the above formula (4). It can be operated as a unidirectional transducer.

Note that the elastic substrate 13 is not limited to piezoelectric material.
As shown in FIGS. 5a to 5d, it may be constructed by a combination of a non-piezoelectric substrate 17 and a piezoelectric film 18, or a metal film 19.

As is clear from the above description, according to the present invention, two comb-shaped chirp electrodes are arranged in parallel, and the unidirectional transformer is configured such that one electrode is used as a power feeding section and the other electrode is used as a reflecting section. Since it constitutes a user, wideband characteristics can be achieved. Further, since the manufacturing process is simplified, manufacturing costs can be reduced, and furthermore, it is extremely easy to design the arrangement of chirp electrodes to obtain desired characteristics.

That is, in the device of the present invention, once the necessary operating frequency bandwidth is determined, f ₀ and λ ₀ are determined in the above equation (4), so l ₁ +l ₂ at the operating frequency f can be easily determined. Since l ₁ and l ₂ are the distances from Y-Y',
The arrangement of the chirp electrodes can be easily designed.

However, if the combination of chirp electrodes does not satisfy equation (4), the design of l ₁ and l ₂ becomes quite complicated.

[Brief explanation of drawings]

1 to 3 are all schematic diagrams showing the conventional example, FIGS. 4 a and 4 b are both a schematic top view and a schematic sectional view showing the embodiment of the present invention, and FIGS. 5 a to d are both schematic diagrams showing the embodiment of the present invention. It is a schematic sectional view showing an example. 10... Signal source, 11... Matching circuit, 12...
Reactance circuit, 13...Elastic substrate, 15
A, 15B...comb-shaped chirp electrode, 16...common electrode, 17...non-piezoelectric substrate, 18...piezoelectric film, 19...metal film.

Claims (1)

[Claims] 1. A first comb-shaped chirp electrode formed in contact with an elastic body so that the electrode period gradually becomes shorter in the direction of propagation of a surface wave; a first comb-shaped chirp electrode formed so as to be in contact with the first comb-shaped chirp electrode along the surface wave propagation direction, and formed so that the electrode period gradually becomes longer with respect to the propagation direction. 2 comb-shaped chirp electrodes, a reactance connected to the first comb-shaped chirp electrode, and a signal source connected to the second comb-shaped chirp electrode, wherein the first comb-shaped chirp electrode The second comb-shaped electrode is configured to be used as a reflection part and also used as a power feeding part, and the distance l ₁ from the contact line between the two comb-shaped chirp electrodes to the equivalent generation point of the surface wave is and the distance to the equivalent reflection point l ₂ is: l ₁ + l ₂ = f ₀ · λ ₀ (2n+1)/4f (where f ₀ is the center frequency, λ ₀ is the wavelength of the surface acoustic wave at the center frequency, A surface acoustic wave device characterized in that comb-shaped chirp electrodes are formed so that n: an integer, f: an operating frequency. 2. The surface acoustic wave device according to claim 1, wherein the two comb-shaped chirp electrodes are formed geometrically symmetrically.