JPH0351130B2 - - Google Patents

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to a surface acoustic wave device.

[Technical background of the invention and its problems]

A surface acoustic wave device is known in which a pair of electrode fingers is formed by interlocking a first comb-shaped electrode and a second comb-shaped electrode on a piezoelectric substrate, thereby configuring a surface acoustic wave transducer. There is. FIG. 1 shows the basic structure of this type of transducer. In the figure, the width of the first electrode finger 4, the width of the second electrode finger 5, and the width of the first electrode finger 4 and the second electrode of the input transducer 2 and the output transducer 3 formed on the piezoelectric substrate 1, respectively. The gap 6 between the finger 5 and the finger 5 has a wavelength of 1/4 of the surface acoustic wave propagating on the piezoelectric substrate 1, respectively. However, with this electrode finger structure, surface acoustic waves are reflected at both ends of each electrode finger due to the difference in acoustic impedance between the location where the electrode finger is present and the location where the electrode finger is not present, and furthermore, the phases of these reflected waves are all in the same phase. Therefore, there is a problem that this appears as spurious in the output signal. Therefore, when a surface acoustic wave filter is constructed using a transducer having this electrode finger structure, ripples appear in the pass band due to the influence of spurious acoustic reflected waves. As a means to eliminate this drawback, the comb-shaped electrodes of the transducer may be replaced with split-shaped electrodes having a width of 1/8 wavelength, as described in U.S. Pat. No. 3,727,155, or the
As shown in No. 56-1805, the structure is a combination of a 5/8 wavelength width electrode and a 1/8 wavelength width electrode, and the phase of the acoustic reflected waves at both ends of the electrode finger is reversed, and the composite wave is It is known that it makes 0. When a surface acoustic wave filter is configured with such non-reflective electrodes, the composite wave of acoustic reflected waves inside the transducer becomes zero, but if this is done alone, the following phenomenon still occurs, causing the wave to pass through. Ripples occur within the band. In other words, as mentioned above,
Acoustic reflection occurs at both ends of the electrode finger because the acoustic impedance is different between the place where the electrode finger is present and the place where the electrode finger is not present. Similarly, acoustic reflection occurs at both ends of the electrode finger. There is also a difference in acoustic impedance, which causes acoustic reflections at both ends of the transducer. As a means to eliminate this acoustic reflection, as shown in FIG.
In 1972, a surface acoustic wave device with a structure that formed
Reported at UItrasonics Symposium PP.373-376. However, this structure still has the disadvantage that acoustic reflection occurs at the end of the transducer on the sound absorbing material side due to the surface acoustic waves 24 and 25 propagating toward the sound absorbing material, causing spurious waves. Furthermore, even in a transducer in which the width of the first electrode finger is 5/8 wavelength and the width of the second electrode finger is 1/8 wavelength, the propagation path between the input and output transducers is simply 1/8 wavelength. Placing a 5/8 wavelength wide strip does not improve acoustic reflections at both ends of the input and output transducers.

[Purpose of the invention]

This invention improves the above-mentioned drawbacks of the conventional surface acoustic wave device, and the electrode fingers of the comb-shaped electrodes are interlocked with each other so that the composite wave of acoustic reflected waves inside the transducer becomes zero. The present invention provides a surface acoustic wave device that can improve acoustic reflection at both ends of an input/output transducer by setting the width and the space between electrode fingers.

[Summary of the invention]

The present invention provides a surface acoustic wave device in which a transducer is constructed by interlocking first and second electrode fingers, in which a synthesized wave of acoustic reflected waves inside the transducer becomes zero, and Set the width dimensions of the first and second electrode fingers, the space between these electrode fingers,
Further, strip electrodes are provided on both sides of the transducer so that the acoustic impedance of the transmission path in the direction of propagation of the surface acoustic wave matches the period of the acoustic impedance distribution of the transducer.
This improves acoustic reflection at both ends of the input/output transducer.

〔Effect of the invention〕

According to the present invention, it is possible to improve the acoustic reflected waves at both ends of the input/output transducer, thereby achieving the effect that spurious waves due to the acoustic reflected waves do not appear in the output signal.

[Embodiments of the invention]

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 3 shows an embodiment of the present invention, in which an input transducer 31 and an output transducer 32 are formed on a piezoelectric substrate 30. In FIG. The input transducer 31 and the output transducer 32 each have first electrode fingers 31a, 32a with a 5/8 wavelength width and second electrode fingers 3 with a 1/8 wavelength width.
1b and 32b are arranged with gaps 31c and 32c of 1/8 wavelength in between. On the left side of the input transducer 31 in the figure, on the right side of the output transducer 32 in the figure, and on the surface wave propagation path between the input and output transducers 31 and 32, 5/
Strip electrodes 33, 34 and 35 are formed of strips of 8 wavelength width and 1/8 wavelength width. The strip electrode 33 is used to improve acoustic reflection at the left end of the input transducer 31 in the figure.
The acoustic impedance of the propagation path is formed to match the period of the acoustic impedance distribution of the electrode fingers of the input transducer 31. That is, first, the leftmost electrode finger 3 of the input transducer 31
Since 1b has a width of 1/8 wavelength, a strip 33a of 5/8 wavelength is placed with a gap 33c of 1/8 wavelength from the electric finger on the left side. Further, from the strip 33a with a width of 5/8 wavelength to the gap 33c with a width of 1/8 wavelength.
Strips 33b each having a width of 1/8 wavelength are arranged at intervals of 1/8 wavelength. A large number of strips having such a structure having a 5/8 wavelength width 33a and a 1/8 wavelength width 33b are arranged, and a sound absorbing material 36 is provided at an appropriate position on the strip electrode 33 to absorb unnecessary reflected waves. . Similarly, for the strip electrode 35 between the input and output transducers 31 and 32, the acoustic impedance of the propagation path between the input and output transducers 31 and 32 corresponds to the period of the acoustic impedance distribution of the input and output transducers 31 and 32. The distribution is formed to match. In other words, first input transducer 3
Since the rightmost electrode finger 31b of No. 1 has a width of 1/8 wavelength, a strip 35a of 5/8 wavelength width and a strip 35 of 1/8 wavelength width are formed with a gap 35c of 1/8 wavelength between them.
b are arranged periodically in the direction of the output transducer. Since the leftmost electrode finger 32b of the output transducer 32 has a 1/8 wavelength width, 1/8
It ends with a strip 35a having a width of 5/8 wavelength with a wavelength gap 35c therebetween. Furthermore, the right strip electrode 34 of the output transducer 32
is formed to match the period of the acoustic impedance distribution of the output transducer 32.
That is, since the width of the rightmost electrode finger 32b of the output transducer is 1/8 wavelength, this electrode finger 3
A strip 32a having a width of 5/8 wavelength is arranged from the strip 2b with a gap 32c having a width of 1/8 wavelength.
2b is placed. A sound absorbing material 37 for absorbing unnecessary surface acoustic waves is provided at an appropriate position on the strip electrode 34 made up of such a repeating structure. Note that the strip electrodes 33, 34
and 35 are each grounded.

According to the structure described above, first of all, for the surface acoustic wave propagating to the left among the surface acoustic waves generated by the input transducer 31, the electrode finger of the input transducer is placed on the left side of the input transducer 31. Since the strip electrode 33 is provided that matches the period of the acoustic impedance distribution, there is no reflection of surface acoustic waves at the left end of the input transducer 31 due to mismatch in acoustic impedance. Furthermore, since the sound absorbing material 36 is formed on the left end of the strip electrode 33,
Reflection of the surface acoustic wave at the left end of 3 is prevented.

Regarding the surface acoustic waves propagating to the right side of the input transducer 31, a strip electrode 3 that matches the acoustic impedance distribution of the input transducer is connected between the input and output transducers 31 and 32.
5 is provided, so the input transducer 3
Reflection of surface acoustic waves at the right end of 1 disappears. Furthermore, since the strip electrode 35 matches the acoustic impedance distribution of the output transducer 32, reflection of surface acoustic waves at the left end of the output transducer 32 can also be prevented.

Furthermore, regarding the surface acoustic waves that pass through the output transducer 32 without being converted into electrical signals,
Since a strip electrode 34 whose acoustic impedance distribution matches that of the output transducer 32 is provided on the right side of the output transducer 32, reflection of surface acoustic waves at the right end of the output transducer 32 is eliminated. Further, since a sound absorbing material 37 is provided on the right end of the strip electrode 34, reflection of the surface acoustic waves passing through the strip electrode 34 at the right end of the strip electrode 34 is prevented.

Therefore, according to the present invention, it is possible to realize a surface acoustic wave device in which spurious noise due to acoustic reflection does not appear in the output signal.

[Other embodiments of the invention]

FIG. 4 shows that the present invention has N pairs of electrode fingers (N=1, 2,
3..., N = 2 in the figure) is applied to a surface acoustic wave device equipped with an input/output transducer configuration.
To explain the strip electrodes 33, 34, and 35 with reference to the configuration shown in FIG. 3, the arrangement order of the 5/8 wavelength width strips 33a, 34a, 35a and the 1/8 wavelength width strips 33b, 34b, and 35b is input/output. The input and output transducers 31, 3 are arranged to match the acoustic impedance distribution of the transducer.
It has been changed according to the structure of 2.

FIG. 5 shows that the present invention is applied to a surface acoustic wave device in which the phase of the electrically reflected wave that is re-excited due to a mismatch between the phase of the acoustically reflected wave generated within the transducer and the external load is in an antiphase relationship. Showing the applied configuration. The transducer of this type of surface acoustic wave device has the effect of improving unnecessary reflected waves by equalizing the magnitude of acoustic reflection and electrical reflection. However, even in this transducer structure, acoustic reflection occurs at both ends of the transducer. The transducer has a pair of electrode fingers 50a with a 1/8 wavelength width and an electrode finger 50b with a 5/8 wavelength width, and a pair of electrode fingers 50c with a 1/8 wavelength width and an electrode finger 50a with a 3/8 wavelength width. It is made up of interlocking elements. In the input/output transducer shown in Figure 5, the acoustic impedance gradually changes in the direction of the aperture from the center of the transducer, and the electrode finger structures at both ends of the transducer and the electrode finger structure at the center are different. It's on. In such a structure, it is important to distribute the acoustic impedance of the surface acoustic wave propagation path so that it is continuous with the period of the acoustic impedance distribution near both ends of the transducer.

FIG. 6 shows the structure of a tap delay line having the transducer shown in FIG. 5, but like the surface acoustic wave device shown in FIG. 5, the composite wave of unnecessary acoustic reflection waves is made to be zero. .

[Brief explanation of drawings]

Figure 1 shows the basic transducer structure in which the electrode fingers are solid-shaped, and Figure 2 shows the propagation path of the surface acoustic wave between the input and output transducers. FIG. 3 is a diagram showing a conventional surface acoustic wave device in which a strip having the same width as the electrode finger is arranged to improve acoustic reflection; FIG. 3 is a diagram showing an embodiment of the surface acoustic wave device of the present invention; FIG. 5 shows an example in which the present invention is applied to a surface acoustic wave device having an input/output transducer structure with N pairs of electrode fingers, and FIG. 5 shows an example in which the present invention is applied to a surface acoustic wave device having another electrode structure. FIG. 6 is a diagram showing an example in which the present invention is applied to a tap delay line transducer. 30... Piezoelectric substrate, 31... Input transducer, 32... Output transducer, 33, 34,
35... Strip electrode, 36, 37... Sound absorbing material.

Claims (1)

[Claims] 1. Comb-shaped first electrode fingers and second electrode fingers on a piezoelectric substrate.
In a surface acoustic wave device comprising a transducer in which the first electrode fingers of the transducer are engaged with each other to form a transducer, the first , the width of the second electrode finger and the space between the electrode fingers are set, and the acoustic impedance of the propagation path in the traveling direction of the surface acoustic wave is set on both sides of the transducer so that the acoustic impedance of the transducer is equal to the acoustic impedance of the transducer. A surface acoustic wave device characterized in that a strip electrode is provided to match the period of distribution. 2. The elasticity according to claim 1, characterized in that a sound absorbing material is provided further outside the strip electrodes provided outside the first and second electrode fingers so as to face the strip electrodes. surface wave device.