JPS6110310A - Surface acoustic wave transducer - Google Patents

Abstract

PURPOSE:To eliminate the effect of reflection spurious wave over a wide band by constituting the titled transducer through the combination of an electrode finger pair causing only electric reflection and an electrode finger pair causing electric reflection and acoustic reflection and having different acoustic reflection. CONSTITUTION:A surface acoustic wave transducer is constituted to a surface acoustic wave propagating section of a piezoelectric substrate 11 by crossing the electrode finger pairs 14, 15 of the 1st and 2nd comb-line electrodes 12, 13. Only the electric reflection component is caused to the electrode finger pairs 14, 15 and the acoustic reflection is nearly zeroed in the 1st type electrode 14 and both the reflections are caused in the 2nd type electrode finger pair 15, and they are combined. The width of the electrode fingers 14a, 14b of the electrode finger pair 14 and the electrode fingers 15a, 15b of the electrode finger pair 15 is set nearly to 1/8 of the wavelength of the propagated surface acoustic wave. Further, the electrode fingers 15a, 15b are constituted to have different acoustic reflection. Thus, the reflection spuriousness is reduced over the wide band.

Description

DETAILED DESCRIPTION OF THE INVENTION 1. Technical Field of the Invention The present invention relates to a low-loss surface acoustic wave transducer with improved reflection spurious.

[Technical Background of the Invention] As a surface acoustic wave transducer, one in which interdigitated electrodes (IDT) are provided on the surface of a piezoelectric substrate is in practical use.

The IDT is constructed by having first and second comb-shaped electrodes facing each other, and the electrode fingers intersecting each other with a certain gap.

By changing the pitch, crossing width, etc. 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 a filter or delay line, it appears in the form of a triple transit echo (T-+-E),
This causes the characteristics to deteriorate significantly.

There are two factors contributing to this reflected spurious: an acoustic reflection component R^ caused by the difference in acoustic impedance between the electrode portion and the gap portion of the IDT, and an electrical reflection component RE caused by electrical re-excitation of the IDT.

To remove this reflection spurious, the acoustic reflection component R
^ and the electrical reflection component RE may be generated in opposite phases to each other so that they cancel each other out.

Conventionally, based on this idea, a second A surface acoustic wave transducer as shown in the figure is known.

In the same figure, a first
A comb-shaped electrode 2 and a second comb-shaped electrode 3 are formed by intersecting their respective electrode fingers with a certain gap between them.

These comb-shaped electrodes 2.3 are provided with electrode finger pairs 4 each consisting of a first electrode finger 4a and a second electrode finger 4b at a predetermined pitch in the longitudinal direction.

Each of these electrode fingers 4a and 4b is formed so that its width is approximately 1/8 of the wavelength λ of the surface acoustic wave propagating through the piezoelectric substrate 1, and the gap between each electrode finger is also approximately 1/8 λ. has been done.

These electrode fingers 4b are formed so that the acoustic reflection at the ends of the electrode fingers is greater than that of the electrode fingers 4a.

Adjustment of the acoustic reflection of such a pair of electrode fingers can be achieved, for example, by forming each electrode finger with an aluminum thin film and then laminating a chromium film only on the second electrode finger 4b to form a two-layer structure. This is realized by At this time, the second electrode finger 4b mainly contributes to the acoustic reflection component R^ caused by the acoustic impedance mismatch between the electrode finger part and the gap, and the acoustic reflection component RE caused by electrical reexcitation The reflected components R^ are generated in a relationship of opposite phases to each other, and cancel each other out.

[Problems with the Background Art] By the way, when looking at the impulse response of the reflection characteristics of this transducer on the time axis, as shown in Fig. 3, the impulse response of the acoustic reflection component R^ has a rectangular shape as shown by the broken line. The electrical reflection component RE
The impulse response appears in a triangular shape that changes over time, as shown by the dashed line in the figure.

Therefore, the acoustic reflection component R^ and the electrical reflection component RE
The composite wave R changes as shown by the solid line R in the figure, and the impulse response becomes zero in a narrow range and does not become zero over the entire time axis t.

When this is replaced on the frequency axis f, as shown in Figure 4, the intensity of the acoustic reflection component R^ and the electrical reflection component R
It can be seen that the range in which the intensities of E are approximately equal is limited to the very vicinity of the center frequency of the transducer.

For this reason, in conventional surface acoustic wave transducers, the frequency range in which the sum of reflected spurious waves becomes zero is extremely narrow, and large reflected spurious waves occur on both sides of the band, making it impossible to sufficiently remove the influence of reflected spurious waves over the entire band. The problem was that I couldn't do it.

[Object of the Invention] The present invention has been made in order to solve the above-mentioned conventional drawbacks, and an object of the present invention is to provide a surface acoustic wave transducer that is free of reflection spurious over a wide band.

[Summary of the Invention] That is, the surface acoustic wave transducer of the present invention has a first type of electrode finger pair on a piezoelectric substrate that causes only electrical reflection and zero acoustic reflection, and a pair of electrode fingers that causes only electrical reflection and zero acoustic reflection. It is formed by combining a second type of electrode finger pair that causes both physical and external reflection, and each of the first type of electrode finger pair has a wavelength of approximately 1/8 of the wavelength of the surface acoustic wave propagating through the piezoelectric substrate. The second electrode finger is composed of a first electrode finger and a second electrode finger with a width dimension.
The type of electrode finger pair has a third electrode finger and a fourth electrode finger, each having a width dimension approximately 1/8 of the wavelength of the surface acoustic wave propagating through the piezoelectric substrate.
It is characterized in that the third electrode finger or the fourth electrode finger is formed to have different acoustic reflections from each other.

[Embodiments of the Invention] Examples of the present invention will be described in detail below with reference to the drawings.

In FIG. 1, lithium niobate (Li Nb 03 ), lithium tantalate (L
In the surface acoustic wave propagation path of the piezoelectric substrate formed of a piezoelectric material such as iTa 03 ), electrode finger pairs 14 and 15 of the first comb-shaped electrode 12 and the second comb-shaped electrode 13 are crossed. A surface acoustic wave transducer is constructed.

These electrode finger pairs 14 and 15 are, as shown in the figure, a first type electrode finger pair 14 in which only an electrical reflection component RE occurs and an acoustic reflection component R[ is almost zero, and a second type electrode finger pair 14 which will be described later. It consists of a combination with a 29th type electrode finger pair 15 formed so that electrical reflection RE and acoustic reflection R^ are caused by the structure.

Electrode fingers 14a, 1 constituting the first type of electrode finger 14
The width dimension of each electrode finger 4b is set to approximately ⅛ of the wavelength λ of the surface acoustic wave propagating through the piezoelectric substrate 11, and the thickness of the aluminum film constituting these electrode fingers is the same.

Further, the electrode fingers 1 constituting the second type electrode finger pair 15
The width dimensions of the electrode fingers 5a and 15b are similarly formed from an aluminum film having the same thickness as the first type electrode finger pair 14, and are each set to a width approximately 1/8 of the wavelength λ.
A chromium film is further formed on b to form a two-layer structure of aluminum and chromium. That is, the second electrode finger pair 15 consists of an aluminum film portion 15a and an aluminum-chromium two-layer structure portion 15b.

Further, the cross width direction of the aluminum-chromium two-layer structure portion 15b of the second type electrode finger pair 15 is as follows:
Along the propagation direction of the surface acoustic wave, it increases and decreases in a diamond shape (indicated by broken lines in the figure) so that it is maximum at the center of the transducer and zero at both ends.

The impulse response of the reflection characteristics of this surface acoustic wave transducer is plotted on the time axis [As shown above, as shown in FIG. Corresponding to the shape of the layered structure portion 15b, it changes into a triangular shape as shown by the broken line in the figure.

On the other hand, in the electrical reflection component RE, waves successively generated from each electrode due to electrical re-excitation overlap, and the thickness of this overlap becomes a reflection level, and the change in this level changes in a triangular shape as shown by the dashed line in the figure.

If the pattern of the two-layer structure portion 15b of the electrode finger of the second type electrode finger pair 15 is changed so that the acoustic reflection component R^ and the electrical reflection component RE cancel each other out in this way, the reflection The sum of waves can be made zero over the entire frequency bandwidth.

As a result, reflected spurious signals can be removed and the characteristics of the surface acoustic wave transducer can be significantly improved.

FIG. 6 is a plan view showing another embodiment of the present invention. In the following figures, parts common to those in FIG. 1 are given the same reference numerals.

The surface acoustic wave transducer of this embodiment has a width of 1/8λ on the outside of the outermost pair of electrode fingers of the comb-shaped electrode of the embodiment shown in FIG. 1
It is constructed by adding an auxiliary electrode finger 16 having a length of /2. In the surface acoustic wave transducer configured in this way, compared to the embodiment shown in FIG. As a result, the reflected waves from the upper half of the comb-shaped electrode and the reflected waves from the lower half become opposite in phase and cancel each other out.

FIG. 7 is a plan view showing a further embodiment of the present invention.

In this transducer, the first type electrode finger pair 14 and the second type electrode finger pair 15 in the embodiment shown in FIG. It is distributed and provided. The transducer of this embodiment has the effect of making the amplitude of the acoustic reflection component R^ uniform in the cross width direction.

As a result, it becomes possible to obtain the reflection canceling effect of the acoustic reflection component R^ and the electrical reflection component RE even in the surface wave beam width direction, and the degree of freedom in selecting the other side IDT when configuring a filter etc. It has the advantage of increasing

FIG. 8 is a plan view showing still another embodiment of the present invention.

In this embodiment, a first type of electrode finger pair 14 and a second type of electrode finger pair 14 are used.
It is constructed by distributing electrode finger pairs 15 of this type in the propagation direction of surface acoustic waves. In a transducer with this structure, the slope of the excitation efficiency can be controlled by the so-called extraction electrode method, and in this case, the distribution pattern of the electrode finger pairs that cause acoustic reflection can be adjusted using surface acoustic waves in response to the electrical reflection waveform. It can be controlled by changing the density in the propagation direction.

According to this embodiment, both the electrical reflection component RE and the acoustic reflection component R^ have a uniform power distribution along the surface wave beam width direction, so that the electrical reflection component RE and the acoustic reflection R^ are canceled out. The effect can be almost completely achieved, and
Since there is no so-called avocado loss, it is possible to reduce the loss. When the above-mentioned IDT is applied to a filter, the effect is particularly remarkable for a narrowband filter with a fractional bandwidth of about 3% or less.

In the above embodiments, the case where the intersecting width of the electrode fingers is all constant is described, but the present invention can also be applied when the intersecting width changes along the surface acoustic wave propagation direction as shown in FIG. can do. In this case, the electrical reflection component R^
is generated from the intersecting region of the electrode fingers, whereas the acoustic reflection component R^ is generated from the non-intersecting region 17 (dummy electrode head 1).
1) It is also possible to generate it in the area (outside the area surrounded by the broken line in the figure). At this time, the pattern of the second electrode 15b of the electrode finger pair that causes acoustic reflection can be roughly determined as follows. In other words, after determining the impulse response corresponding to the intersection region (the region surrounded by the broken line in the figure), find its self-convolution,
The acoustic reflection area may be provided so as to correspond to a waveform whose time width is compressed to 1/2. In this case, as shown in Fig. 9, in addition to the normal region (region C in the figure) where electrically reflected waves and acoustically reflected waves have opposite phases to each other, electrically reflected waves and acoustically reflected waves are in phase with each other (region D Vi in the figure)
may occur. In addition, in the comb-shaped electrode having the configuration shown in FIG. 9, the acoustic reflecting portion is arranged in the cross-width direction in order to achieve the effect of canceling the electrical reflection component RE and the acoustic reflection component R^ also in the surface wave beam width direction. A non-uniform distribution is also possible.

In the above description, strictly speaking, the electrically reflected waveform changes depending on the load conditions of the external circuit, and multiple reflections between the electrode fingers must also be taken into account for the acoustically reflected wave. However, these can be analyzed using an IDT equivalent circuit model, etc., and the final pattern can be determined based on the analysis results.

Although the present invention has been described above with reference to several embodiments, the present invention is not limited to these embodiments in any way. It can be used in parallel, applied to piezoelectric thin films, and used as a group (full) as an acoustic reflection part.
Various modifications are possible without departing from the spirit of the invention.

[Effects of the Invention] According to the present invention, compared to conventional surface acoustic wave transducers, a range in which acoustic reflection components and electrical reflection components cancel each other out is formed, and a surface acoustic wave transducer with less reflection spurious over a wide band can be achieved. Users can be realized.

[Brief explanation of drawings]

FIG. 1 is a plan view of an embodiment of the present invention, FIG. 2 is a plan view of a conventional surface acoustic wave transducer, and FIGS.
The figures are respectively an impulse characteristic diagram and a frequency characteristic diagram, FIG. 5 is an impulse characteristic diagram, and FIGS. 6 to 9 are plan views showing other embodiments of the present invention. 1.2... Comb tooth-shaped electrode 7... Electrode area 9... Auxiliary electrode fingers 14...
......First type electrode finger pair 15...
...Second type electrode finger pair 15a...
...Aluminum film portion 15b...
...Partial representative patent attorney for aluminum-chrome two-layer structure Sasu Suyama --- Figure 1 Figure 2 φ Figure 3 Figure 4

Claims (6)

[Claims] (1) A first type of electrode finger pair that causes only electrical reflection and zero acoustic reflection, and a second type of electrode finger pair that causes both electrical and acoustic reflection on the piezoelectric substrate. The first type of electrode finger pair each has a wavelength approximately 1/1/2 of the wavelength of the surface acoustic wave propagating through the piezoelectric substrate.
The pair of electrode fingers of the second type each has a width dimension of approximately 1/8 of the wavelength of the surface acoustic wave propagating through the piezoelectric substrate. A surface acoustic wave comprising a third electrode finger and a fourth electrode finger, and wherein the third electrode finger or the fourth electrode finger is formed so that acoustic reflections differ from each other. transducer. (2) The outermost pair of electrode fingers has a width that is approximately 1/8 of the wavelength of the surface acoustic wave propagating on the piezoelectric substrate on the outside, and a length that is approximately 1/2 of the intersecting width of the electrode fingers. A surface acoustic wave transducer according to claim 1, characterized in that the surface acoustic wave transducer has auxiliary electrode fingers. (3) 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. (4) The surface acoustic wave transducer according to claim 1, characterized in that the first type electrode finger pairs and the second type electrode finger pairs are distributed in the propagation direction of the surface acoustic wave. . (5) The surface acoustic wave transducer according to claim 1, wherein the third electrode finger or the fourth electrode finger is formed of a laminate of two or more types of conductors. (6) In addition to the first type of electrode finger pair and the second type of electrode finger pair, the device is characterized by having a third type of electrode finger pair (dummy electrode part) that does not contribute to electrical reflection. A surface acoustic wave transducer according to claim 1.