JPH0247887B2 - - 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, where 1 is a piezoelectric substrate, 2 is a filter, and 2 is a piezoelectric substrate.
4 is an input transducer consisting of a pair of interdigital electrodes 3A and 3B, and 4 is a pair of interdigital electrodes 5.
The output transducer consists of A and 5B, and the signal applied from the input terminal IN is converted into a surface acoustic wave by the input transducer 2, which propagates on the surface of the piezoelectric substrate 1 as shown by the arrow. After reaching the output transducer 4, the signal is converted into an electrical signal and taken out from the output terminal OUT.

By the way, as in the surface wave device having the structure shown in FIG.
In the case of a film 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 4 to work to propagate surface waves in both left and right directions. The disadvantage is that conversion loss is unavoidable and the loss becomes large 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 FIG. 2, 6, 6A, and 6B are electrodes having a phase difference of 120° from each other, and 6 is constructed such that a gap 7 or an insulating film is interposed between it and another electrode 6A, so that the surface waves are unified. It works to propagate only in one direction.

However, this method of using 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. Further, in order to obtain desired characteristics, the design of the phase shifter itself becomes very complicated, making adjustment difficult and making it difficult to realize broadband characteristics.

On the other hand, in FIG. 3, 8A and 8B are a power feeding part and a reflecting part which are provided to constitute a part of the interdigital electrode, and are both 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 reflection section 8B connected to the reactance circuit 12 and returned to the right, and the surface wave propagated to the right and the reflected surface wave are combined in the power feeding section 8A. It will be done. As a result, when the center frequencies of surface waves are the same, the two waves overlap, but when the center frequencies deviate 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 response to the above problems, and is configured such that a regular type electrode and a chirp electrode are arranged side by side, and one electrode is used as a power feeding part and the other electrode is used as a reflecting part. An object of the present invention is to provide a surface acoustic wave device including a transducer. Embodiments of the present invention will be described below with reference to the drawings.

FIGS. 4a and 4b are a schematic top view and a schematic cross-sectional view showing a surface acoustic wave device according to an embodiment of the present invention, in which regular electrodes 14 having the same electrode period are formed on the surface of an elastic substrate 13 made of a piezoelectric material. A chirp electrode 15 formed so that the electrode period changes with respect to the propagation direction x of the surface acoustic wave is provided perpendicular to the propagation direction x, and these electrodes 14,
15 are arranged so as to be in contact with the common electrode 16 at a vertical line Y-Y'. Note that the above vertical line (contact) Y
-Y' becomes the normal electrode end when the normal electrode 14 and the chirp electrode 15 are separate bodies. Also, if the normal electrode 14 is used as a power supply section and the chirp electrode 15 is used as a reflection section, the signal source 1 is connected to the power supply section electrode 14 via a matching circuit 11.
0 is connected, and the reactance circuit 12 is connected to the reflective part electrode 15.

In the above, among the surface acoustic waves generated from the power feeding part electrode 14 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 and terminated by the reactance circuit 12. Since the part electrode 15 is present, the light is reflected by the reflection part electrode 15 and returned to the right again. At this time, let C be the point at which surface waves are generated on the power supply electrode 14 (equivalent surface wave generation point), and let l be the distance from this point C to the contact line Y-Y' between the electrodes 14 and 15. , this distance l
The transition angle θ ₀ corresponding to can be expressed as follows.

θ ₀ =2π×l/ ₀・λ ₀ =2π・N・λ ₀ /2・+λ ₀ /8/ ₀ λ ₀ =2π/ ₀ (N/2+1/8) …(1) where ₀ : surface wave Center frequency λ ₀ : Wavelength of center frequency N: Logarithm of feeder electrode: Operating frequency In addition, the reflector electrode 15 consisting of a chirp electrode is
Due to its geometrical properties, the reflection point of the surface wave differs depending on the operating frequency. Let the reflection point (equivalent surface wave reflection point) at the current operating frequency be D,
If the distance from this point D to the contact line Y-Y' is l ₁ , then the transition angle θ ₁ corresponding to this distance l ₁
can be expressed as follows.

θ ₁ =2π×l/ ₁・λ ₀ …(2) Here, the above-mentioned power feeding section electrode 14 and reflecting section electrode 1
In order to operate the transducer including 5 in a unidirectional manner, a surface wave propagating to the right from the feed electrode 14 and a surface wave propagating to the left, reflected by the reflection electrode 15, and returning to the right. It is necessary to match the phase with the wave. For this purpose, the reflection part electrode 15 (chirp electrode) must be adjusted so that the transit angle θ of the surface wave reflected and returned satisfies the following formula.
The condition is to form.

θ=(θ ₀ +θ ₁ )×2=2π(2N+1/2) (3) The following equation is derived by integrating the above equations (1) to (3).

l ₁ /λ ₀ = (N/2+1/8) 2 ₀ −/…(4) In this way, the chirp electrode that operates as the reflective part electrode can be designed to satisfy the above equations (3) and (4). As a result, the phases of both surface waves as described above can be matched over a wide operating frequency range, so that the transducer 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 reflective part electrode formed in this way with a reactance circuit, it becomes possible to easily adjust the reflective part electrode.

FIG. 5 is a schematic top view showing another embodiment of the present invention, showing a configuration in which the chirp electrode 15 is operated as a power feeding section and the regular electrode 14 is operated as a reflecting section. A signal source 10 is connected via a matching circuit 11, and a reflection section electrode 1
A reactance circuit 12 is connected to 4. In this case, the surface wave generation point of the feeder electrode 15, which is a chirp electrode, differs depending on the operating frequency due to its shape, but in the reflector electrode 14, which is a regular electrode, the surface wave generation point is generated at all operating frequencies. The reflection points will be the same. For the same reason as in the above embodiment, the surface wave propagating rightward from the power feeding part electrode 15, being reflected by the reflecting part electrode 14 and returning to the left, returns to the same position of the power feeding part electrode 15 as the generation point. Then, the phase is matched with the surface wave that propagated to the left from the beginning. Therefore, the transducer operates so as to propagate the surface wave only in one direction, to the left, so that a similar effect can be obtained.

In each of the above embodiments, the case where the chirp electrode 15 and the regular electrode 14 are in contact with each other along the vertical line (contact line) Y-Y' has been described, but even if the two electrodes are separated, the equation (3) ) and (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. 6a to 6d, a combination of a non-piezoelectric substrate 17 and a piezoelectric film 18, or a metal film 19 may be used.

As is clear from the above description, according to the present invention, the regular electrode and the chirp electrode are arranged side by side, 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 diu- sulator, it is possible to realize wideband characteristics. Furthermore, since the manufacturing process is simple, manufacturing costs can be reduced, and adjustment to obtain desired characteristics is easy, so it can be widely applied to various surface acoustic wave devices such as filters. .

[Brief explanation of the drawing]

1 to 3 are all schematic diagrams showing the conventional example, FIGS. 4a and 5 are both schematic top views showing the embodiment of the present invention, and FIG. 4b and FIGS. 6 a to d are both schematic diagrams. 1 is a schematic cross-sectional view showing an embodiment of the present invention. 10... Signal source, 11... Matching circuit, 12...
Reactance circuit, 13... Elastic substrate, 14...
...Normal electrode, 15...Chirp electrode, 16...
Common electrode, 17... Non-piezoelectric substrate, 18... Piezoelectric film, 19... Metal film.

Claims (1)

[Claims] 1. A transducer configured such that a regular electrode and a chirp electrode are arranged in parallel, one electrode is used as a power supply part, and the other electrode is used as a reflection part, The distance l from the contact line between the normal form electrode and the chirp electrode to the point corresponding to the operating frequency of the chirp electrode is l = (N/2 + 1/8)2f ₀ -f/f・λ ₀ (where N: normal form A surface acoustic wave device characterized in that a chirp electrode is formed so that the logarithm of the electrode, f: operating frequency, _f0 : center frequency, and _λ0 : wavelength of surface acoustic wave at the center frequency. 2. The surface acoustic wave device according to claim 1, wherein a signal source is connected to the power feeding part electrode, and a reactance is connected to the reflecting part electrode. 3. Claim 1 or 2, characterized in that the chirp electrode is used as a reflecting part.
The surface acoustic wave device according to any one of Items 1 to 3.