JPS63136706A - Surface acoustic wave transducer - Google Patents

Abstract

PURPOSE:To decrease the effect of the fluctuation of an external load by setting the area of an electrode finger part causing the acoustic reflection within a range of a specific value of the area of an electrode finger part in the electrode cross region of a transducer so as to make a radiation conductance of a transducer in the pass band nearly constant. CONSTITUTION:The quantity of the acoustic reflection of the titled transducer depends on the ratio of the total area SE (=SE1+SE2) of the electrode region between two electrode fingers connected together to the total area SA (=SA1+SA2) of the electrode region between two electrode fingers connected together in the former cross region, and in this embodiment, the ratio of decided in the range of 0.6 <= SA/SE <= 0.9. Thus, the distribution of the weighting of the acoustic reflection is in a form of roll-off. Since the weighting of the acoustic reflection is applied by the rate of the electrode finger part causing the acoustic reflection really in existing in the direction of the cross width, the value A is selected to the maximum electrode cross width. The electroacoustic conversion loss of the transducer designed as above depands oN the degree of the cancellation between the electric reflection and the acoustic reflection.

Description

[Detailed description of the invention] [Purpose of the invention] (Industrial application field) The present invention relates to a surface acoustic wave device.

(Prior Art) Surface acoustic wave transducers formed by forming interdigital electrodes on a piezoelectric substrate have been put to practical use in intermediate frequency filters of television receivers and the like.

In surface acoustic wave devices configured using such interdigital electrodes, spurious (hereinafter referred to as reflected spurious) caused by reflection of surface acoustic waves by the comb-like electrodes is always a problem. This reflected spurious appears in the form of triple transit echo (TTE) when a filter or delay line is actually constructed, and causes a significant deterioration of the characteristics. Therefore, research has been conducted to reduce this reflected spurious.

The causes of reflected spurious are the reflection component (hereinafter referred to as acoustic reflection) caused by the acoustic impedance between the electrode part of the comb-shaped electrode and the gap between these electrode parts, and the electrical re-excitation of the comb-shaped electrode. It is known that there are two types of reflected components (hereinafter referred to as electrical reflections) caused by this. Therefore, focusing on these two reflected components, a surface acoustic wave transducer with a configuration as shown in FIG. (No. 145716, 1983).

This surface acoustic wave transformer is constructed as shown in the figure.

When the electrode crossing width is uniform, the weighting of the acoustic reflection is uniform in the propagation direction as shown in Figure 3(a), and the portion that causes acoustic reflection is the transducer's electrode crossing. Distributed throughout the area.

Furthermore, the frequency characteristics of the radiation conductance of the transducer, in which the weighting of the electrode crossing width and the acoustic reflection are uniform, exhibit bimodal characteristics as shown in FIG. 4(a).
It varies greatly within the passband.

On the other hand, in a surface acoustic wave transducer in which the amount of weighting of acoustic reflections is uniform in the propagation direction, the weighting of acoustic reflections as shown in Fig. 2(b) is applied in order to further improve reflection spurious. A surface acoustic wave transducer has also been proposed in which the quantity is given in a triangular distribution as shown in the self-composite IJ, -Shin function of the electrode crossing width function, i.e., in Fig. 1409
No. 17).

Furthermore, when the electrode crossing width is uniform, the frequency characteristic of the radiation fundance exhibits a single peak as shown in FIG. 4(e), and similarly changes greatly within the pass band.

(Problem to be Solved by the Invention) In this way, in a transducer with a uniform electrode crossing width, the weighting of acoustic reflections is uniform in the propagation direction or the self-constituent of the electrode crossing width function = tr IJ age. When given by a function of There is a drawback that the frequency characteristics of the loss and therefore the pass characteristics are affected when a filter is configured.

In addition, when the weighting of acoustic reflection is determined by a constant value or a self-composite function, the electro-acoustic conversion loss of the transducer is determined by controlling the film thickness of the electrode fingers, etc. This is done when changing the reflectance, but it also has the disadvantage that as the film thickness increases, it becomes difficult to manufacture the transducer due to the electrode width, etc.

The present invention provides a surface acoustic wave transducer in which the radiation conductance has a substantially constant value within the passband and is less affected by changes in external load by changing the amount of weighting of acoustic reflections from the conventional one. The purpose is to

[Structure of the invention]

(Means for Solving the Problems) In order to solve the above problems, the present invention provides a surface acoustic wave transducer in which interdigital electrodes consisting of first and second comb-shaped electrodes are formed on a piezoelectric substrate. The second comb-shaped electrode is configured such that three electrode fingers are interposed between adjacent electrode fingers of the first comb-shaped electrode, and among these three electrode fingers, Parts of the two are connected to each other,
When the total area of the electrode crossing area between the two connected electrode fingers is calculated as Sp+, the total area SA of the connecting areas within this electrode crossing area is 0.6≦S people/SB≦0.
．． Provided is a surface acoustic wave transducer characterized in that the surface acoustic wave transducer is in the range of 9.

(Function) According to the configuration of the present invention, the amount of weighting of the acoustic reflection becomes an intermediate value between the conventional case where the acoustic reflection is weighted uniformly in the propagation direction of the surface acoustic wave and the case where it is weighted in a triangular shape. As a result, the frequency characteristic of the radiation conductance also becomes an intermediate characteristic between the conventional bimodal and unimodal characteristics, that is, a nearly constant characteristic within the pass band.

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

FIG. 1 shows an embodiment of the surface acoustic wave transducer of the present invention. In the figure, a first comb-like electrode 11 constituting an interdigital electrode has electrode fingers each having a width of λ/8 and arranged at a period of λ. The second comb-shaped electrode 12 basically has three electrode fingers each having a width of λ/8 between two adjacent electrode fingers of the first comb-shaped electrode.
The electrode fingers are arranged at intervals of , but two of the electrode fingers are connected to each other at a part of the electrode fingers.

Therefore, in this connecting portion, the electrode fingers have a width of 3λ/8.

The amount of acoustic reflection of this device is the total area SE (=8Bm+S
B, ) and the total area of the connection area of the two electrode fingers connected within this intersection area S A (=S A, +84)
In the present invention, this is determined by the ratio of 0.6≦SA/3.
, E≦0.9. Therefore, the weighting of the acoustic reflection has a roll-off distribution as shown in FIG. 3(C).

(Function) Therefore, consider the amount of weighting in a sinusoidal roll-off shape as shown in FIG. 3(C). The electrode finger 1 of the interdigital electrode is in the range from t=-a to t=a, and when the roll-off rate is α, the weighting amount Ar(t) is as follows.

Here, since the acoustic reflection is weighted by the ratio of the electrode finger portions that are present in the crossing width direction and cause a net acoustic reflection, A is set to the maximum electrode crossing width. The area of the part surrounded by the envelope of the intersection width distribution discretely given as the area 871 of the electrode intersection part, and the area SR of the part (connection area) that causes acoustic reflection, are weighted by the amount Ar(t). Considering the area, it becomes . It can be seen that the change in radiation conductance is small when SA is 0.7SB.

Further, the electro-acoustic conversion loss of such a transducer is determined by the degree of cancellation between electrical reflection and acoustic reflection. The time-axis characteristic of the electrical reflection of a transducer with a uniform electrode crossing width is approximately triangular, and its frequency characteristic is (-) dew-shaped around the center frequency.

On the other hand, the frequency characteristic of acoustic reflection is Ar(t) where a is 2a
is given by its Fourier transform as . The magnitude of acoustic reflection at the center frequency is obtained by equation 0 when w=Q. That is, the magnitude of the acoustic reflection at the center frequency is determined by the integral value of the weighting of the acoustic reflection, and therefore by the area 8A of the portion that causes the acoustic reflection. From this, it can be seen that the conversion loss at the center frequency l increases as the number of people increases.

However, when 8 people are increased to be equal to SK, the conventional acoustic reflection becomes constant and the influence of reflected spurious appears as shown in Figure 4(a), and when SA is 0.98, the effect of reflected spurious appears. The effect is not noticeable.

Furthermore, when SA is in the range from o, ssE to 0.68B, the pass characteristics when the filter is configured are almost the same as when the filter is triangular, and the effect of changing the amount of acoustic reflection weighting is Not so much.

Therefore, SA is practically set in the range of 0.68B to 0.98. Fig. 4 Φ) and (d) show the frequency characteristics of the radiation conductance when 8A=0.98n and 5A=0.68B, respectively. In addition, Fig. 4 (a
') to (e') are the pass characteristics of a filter configured with transducers having the characteristics of (a) to (e) as input/output transducers.

〔Effect of the invention〕

As described above, in the present invention, the area of the electrode finger portion that causes acoustic reflection in a surface acoustic wave transducer is reduced by the area S of the electrode finger portion of the electrode crossing region of the transducer.
By setting it in the range of 0.6 to 0.9 times R,
The radiation conductance of the transducer within the passband is kept almost constant to reduce the influence of external load fluctuations.

Alternatively, if a filter is configured, the electro-acoustic conversion loss at the center frequency can be improved, and the insertion loss can be improved.

Furthermore, if the magnitude of the acoustic reflection of the transducer is the same, almost the same electro-acoustic conversion loss can be obtained, and the magnitude of the acoustic reflection at the center frequency is determined by the area Sn of the part where acoustic reflection occurs and the electrode It is determined by the acoustic reflectance at

Conventionally, SE was determined by the number of electrode pairs, maximum electrode crossing width, crossing width function, etc., and the magnitude of acoustic reflection was controlled by changing the electrode film thickness. The present invention has the advantage of increased design freedom, such as being able to control the magnitude of acoustic reflection by the area of Sx even if the electrode film thickness is constant, and obtaining the same acoustic reflection even if the number of electrode pairs etc. is changed. .

In Figure 5, the weighting of acoustic reflections that result in the same acoustic reflection at the center frequency (the area of the shaded area is the same) when the range in which the transducer exists is changed is the distribution of the magnitude of the quantity in the propagation direction. shows.

Note that the weighting of acoustic reflections in the present invention is not limited to the roll-off shape shown in FIG. Although this has been described above, it can also be applied to cases where the crossing width changes along the propagation direction.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a conventional surface acoustic wave device, FIG. 3 is a diagram showing the distribution of the amount of weighting of acoustic reflection, and FIG. Figure 5 shows the frequency characteristics of the radiation conductance of a surface acoustic wave device and the passage characteristics of a filter configured as an input/output transducer. The distribution within is shown. 11.12...Comb-shaped electrode, SE, ,SE,...
Electrode crossing area area, SA,, SA2... connection area area.

Claims (1)

[Scope of Claim] A surface acoustic wave transducer comprising an interdigital electrode consisting of first and second comb-shaped electrodes formed on a piezoelectric substrate, wherein the second comb-shaped electrode is connected to the first comb-shaped electrode. The comb-shaped electrode is configured such that three electrode fingers are interposed between adjacent electrode fingers, and two of these three electrode fingers are partially connected to each other. When the total area of the electrode crossing area between two connected electrode fingers is SE, the total area SA of the connecting areas within this electrode crossing area is 0.6≦SA/S
A surface acoustic wave transducer characterized in that E≦0.9.