JPS63272214A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To reduce the periodic spurious radiation and to make the spurious radiation flat by folding a part of a common electrode parted from other comb shape electrode opposed to the common electrode in a direction where the length of the non-crossing part of the electrode finger is decreased gradually, and forming at least one ridge of the folded part to be of sawtooth shape in a surface acoustic wave device provided with the comb shape electrode. CONSTITUTION:The part of the apotized type comb shape electrode 31 of tapered electrode structure parted from a normal comb shape electrode 33 of the common electrode 32 is folded in a direction where the length of the non-crossing part of the electrode digit 31a is decreased gradually from the position of the acoustic center A to form the folded part 34. The inner ridge 34A and the outer ridge 34B of the folded part 34 are formed to be saw-tooth shape. The angle R of the saw-tooth blade is set to 45 deg.. Surface waves W1, W2' are generated from the electrode digits 15' and 31a' due to the sudden interruption of the electric field applied periodically and momentarily, but the spurious radiation at the period of f1 can be reduced as the length of both the electrode digits is decreased.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a surface acoustic wave device including comb-shaped electrodes.

(Technical background of the invention and its problems) This type of bone surface wave device is used as a filter, delay line, etc., but especially when used as a filter in a repeater of an image transmission device, it is used in multiple stages. Therefore, transmission distortion occurring at each stage is sequentially added up, causing deterioration of one image. Therefore, for a surface acoustic wave device used as a filter for an image transmission device, the in-band spurious amplitude and group delay are 0 and 1 dB (
peak to peak), characteristics that satisfy ±2 to 3 g5ec are required.

FIG. 8 shows an example of a surface acoustic wave device, in which a 7-potized comb-shaped electrode 2 having an inclined electrode structure is provided on a substrate 1'2. This comb-like electrode 2 has a plurality of electrode fingers 2a and a common electrode 2b commonly connected to these electrode fingers, and a signal source 4 is connected to the negative common electrode 2b. In addition, this comb-shaped electrode 2 is attached to the substrate l''2.
An IE vermilion-shaped comb-shaped electrode 3 is provided opposite to.

This comb-like electrode 3 has electrode fingers 3a and a common electrode 3b connected in common to the electrode fingers 3a, and a load 5 is connected to one side of the electrode 3b.

By the way, in the apotized comb-shaped electrode 2 described in (■-), the non-intersecting portions included in the areas a, , a2 of the electrode finger group 2a located on the right side of the acoustic center A and away from the regular comb-shaped electrode 3 are as follows. , is unnecessary as a surface wave propagation section.

Therefore, a sound-absorbing material is applied to the area a2 to reduce spurious noise. In this case, if there are variations in the application state of the sound absorbing material to the region a2, spurious waves of different magnitudes will occur in each surface acoustic wave device, so the sound absorbing material needs to be applied to the region a2 with high precision. However, when the surface acoustic wave device is used as a filter in a high frequency band of 100 MHz or higher, the comb electrode 2 described in 2 is formed with a width of about 2 mm. Therefore, mass production is impossible. Also, area a2
Since the electrode fingers 2a may be broken, the yield will also be reduced.

FIG. 9 shows another conventional surface acoustic wave device.

That is, in this surface acoustic wave device, an apotized comb-shaped electrode 2 having an inclined electrode structure is provided on a substrate IU made of a piezoelectric material such as 128° Y-X lithium niobate, and this comb-shaped 'It electrode 2 has a plurality of electrode fingers 2a that intersect at an angle, and common electrodes 2b and 2c that are commonly connected to these electrode fingers 2a. A signal source 4 is connected to one common electrode 2c, and a conductive film 6 made of an aluminum film with a thickness of about 200 OA is integrally formed. This conductive film 6 covers the non-intersecting portions of the electrode finger group 2a located on the right side of the acoustic center A in the figure.

A regular comb-shaped electrode 3 faces the comb-shaped electrode 2 with a distance of about 1 mm from the pole 2. This comb-shaped electrode 3 has electrode fingers 3a that are uniformly crossed, and these electrode fingers 3a.
A has coplanar electrodes 3b, 3b commonly connected to a, one common electrode 3b is grounded via a load 5, and a sound absorbing material 7 is placed near the comb-shaped electrode 2.3. are arranged. These sound absorbing materials 7 are made of epoxy adhesive,
To eliminate unnecessary reflection of surface waves at the edge of a substrate 1, etc., and to prevent unnecessary excitation of bulk waves. In addition, in the figure, 8 is a dummy electrode.

When the frequency characteristics of the surface acoustic wave device having the above configuration were measured, the results shown in FIG. 10(A) were obtained.

That is, FIG. 10(A) shows the amplitude frequency characteristic of the relative gain, and periodic spurious signals were generated in the passband region A. As shown in Figure 10 (B), this spurious is generated with a magnitude of approximately 0.3 dB (peak to peak), and is a combination of a spurious with a large period f1 and a spurious with a small period f2. there were. Then, it was found that the period number interval of each spurious with f and f2 periods has a relationship of f+''2fz.Also,
The unnecessary surface wave corresponding to the f4-period spurious propagates with a time difference equal to the time required to propagate half the length of the comb-shaped electrode 2 with respect to the surface wave of the 11-ring response. found.

(Even if the bottom surface of the plate 1 is roughened by providing unevenness, the above f+,
The fz periodic spurious was only slightly reduced.

Therefore, it can be seen that the f+ and fz same-period spurious waves are not directly caused by bulk waves, and that these spurious waves are reduced by scattering of a portion of the bulk waves due to the unevenness of the bottom surface.

In addition, as shown in Fig. 8, in another surface acoustic wave device having a similar structure, the length dimension parallel to the propagation direction of the surface wave of the main response in the aborted comb-shaped electrode with the inclined electrode structure is determined by the center frequency. When the wavelength was measured and the frequency characteristics were examined, it was found that the above-mentioned periodic spurious was also generated.

By the way, the manner in which surface waves are generated by the surface acoustic wave device shown in FIG. 9 can be shown as shown in FIG. 11. That is, in the apotized comb-shaped electrode 2 with the inclined electrode structure, r
A surface wave Wo, which is the main response, is generated from the f-acoustic center A, and a surface wave W1 is generated from the electrode finger 2a at the end facing the regular comb-shaped electrode 3 (see Fig. 9). A surface wave W2 is generated from the end face of the common electrode and the electrode fingers 2a at the ends spaced apart from the comb-shaped electrode 3. Further, a surface wave generated from the acoustic center A and propagating in the opposite direction is reflected by the electrode finger 2a at the end and propagates as a surface wave W3. If the length from the acoustic center A of the comb-shaped electrode 2 to both ends is V, and the propagation speed of the surface wave on the substrate l is V, then each surface wave W
o The time difference for #W3 to reach the regular comb-shaped electrode 3 is based on the main response surface wave WO, and one surface wave W1 is l/
Advance of V seconds 1 surface wave W2 is delayed by l/V seconds, surface wave W3
is delayed by 21/V seconds. Therefore, it is considered that the f2 period spurious is generated by the surface wave W3.

In order to confirm that the surface wave W3 caused by reflection is a spurious wave with a frequency of f2, the inventors of the present invention set the common electrode 2C and part of the conductive film 6 at the wavelength of the surface wave, as shown in FIG. The length dimension was removed stepwise by 1/4, and the periodicity characteristics were measured. As a result, the f2 period spurious is approximately 0,
1 dB (peak t.

peak). In this case, the surface wave W3 reflected at the non-cutting part b2 and the surface wave W3 reflected at the cutting part b2
Since there is a cutting ζ1 modulus difference of 0/4, theoretically a phase difference of π occurs, and when the waves are received by the regular comb-shaped electrode 3, they cancel each other out. but,
Actually, as described above, a spurious signal having a frequency of f2 occurs. The reason for this is thought to be that the lengths of the surface waves W3 and W3' passing through the conductive film 6 are different between the bt section and the b2 section, and thus a phase difference occurs between these surface waves.

On the other hand, the f+ period spurious is approximately 0.25 dB (pe
ak to peak).

This is because the surface waves W2 and W2' have a phase difference of 0/4, and the lengths of the waves passing through the conductive film 6 are similarly different between the bl section and the b2 section.

Further, as shown in FIG. 13, the inventor cut the common electrode 2C and the conductive film 6 obliquely at an angle θ with respect to a direction perpendicular to the propagation direction of the surface waves. When the frequency characteristics were measured, as the cutting angle 0 increased, the amplitude of the above-mentioned spurious response became smaller, and in particular, the spurious response of the f2 period was significantly reduced as shown in FIG. 14.

In other words, the f2 period spurious has almost disappeared. "I"
From the results of -, it was found that the surface wave W3 due to reflection generates spurious waves with a period of f2.

Incidentally, the f+ same period spurious was also reduced to about 0.2 dB.

Therefore, the present inventor proposed a surface acoustic wave device shown in FIG. 15 (see patent application filed on April 22, 1985). The device has a substrate 20 made of a piezoelectric material such as the like, and an abbotized comb-shaped electrode 21 having an inclined electrode structure is provided on the substrate 20. This comb-shaped electrode 21
has a plurality of electrode fingers 21a that intersect at an angle, and common electrodes 21b and 22 that are commonly connected to these electrode fingers. One common electrode 21b is grounded, the other common electrode 22 is connected to the signal source 4, and these common electrodes 21b and 22 are provided with a dummy electrode 15 that does not intersect with the electrode finger 21a. This comb-shaped electrode 21 is opposed to a regular comb-shaped electrode 23, and this comb-shaped electrode 2
3 are electrode fingers 23a that are uniformly crossed, and common electrodes 23b and 23c that are commonly connected to these electrode fingers 23a.
One common electrode 23b is grounded, and the other common electrode 23c is grounded via a load 5.

1- The regular comb-shaped electrode 23 of the common electrode 22
The part further away from the acoustic center A position is the electrode finger 21a.
is bent in a direction that gradually reduces the length of the non-intersecting portion to form a bent portion 22A. The refracting portion 22A has an inclined edge 22a on the outer end side, which is inclined at a predetermined angle with respect to the propagation direction (see arrow) of the main response surface wave.

A sound absorbing material 25 made of epoxy adhesive is provided on the bending portion 22A, including the inclined edge 22a. Sound absorbing section 26.2 including each edge of the other common electrode 21b
7. Note that a sound absorbing material 28 made of the same material is also provided near the comb-shaped electrode 23.

In the surface acoustic wave device having the above configuration, the surface wave W3 generated from the sound center A and propagating toward the bending portion 22A of the common electrode 22 is transmitted to the central portion of the sound absorbing material 25, as shown in FIG. The sound is absorbed, and a part of it is reflected by the inclined edge 22a and absorbed (sequentially by the parts 27 and 26. Therefore, the f2 period spurious almost disappears. As shown in FIG. 16, the surface wave W2 that is not absorbed at the center of the absorbing material 25 propagates in a direction perpendicular to the edge 22a, and most of it is absorbed by the sound absorbing portion 27. Ru.

By the way, when we investigated the frequency characteristics of the surface acoustic wave device shown in Fig. 15, we found that the periodic spurious in the passband region A is as shown in Fig. 17 (A), as shown in Fig. 17 (B). Approximately 0.1 dB (peak t.

peak). However, this figure 17 (
As is clear from B), the spurious was changing rapidly. In addition, as shown in the same figure (C), the first group delay time was ±3 nsec, but the group delay characteristic shown by B in the same figure (A) suddenly changed as shown in the same figure (C). A spurious signal was occurring. When the spurious changes sharply like this, the transmission distortion characteristics depend on the rate of change of the spurious, so even if the spurious is small, the waveform distortion of the transmitted signal will become large. .

Therefore, when the present inventor repeated various experiments on the surface acoustic wave device shown in FIG. 15, the cause of the sudden change in spurious was found. That is, as shown in FIG.
Inclined edge 22a of bending portion 22A and its inner edge 22b
An unnecessary surface wave W2 is generated from the surface wave W2.
Since a part of the waveform is reflected at the inner edges of the common electrodes 21b and 22, it intersects the main response surface wave in the propagation path and disturbs the wavefront, causing the spurious to change sharply.

In addition, since the unnecessary surface wave W3 generated from the acoustic center A is also reflected at the inner edges of the common electrodes 21b and 22, the spurious waves change sharply, albeit slightly.

(Objective of the Invention) An object of the present invention is to provide a surface acoustic wave device in which the periodic spurious is extremely small.Secondly, the spurious can be flattened.

(Summary of the Invention) The present invention provides an apotized comb-like electrode in which electrode fingers are crossed, and the length of the non-intersecting part of the common electrode is gradually adjusted to It is characterized in that it is bent in the direction of making it smaller, and that at least one edge of this bent part is formed into a sawtooth shape.

(Embodiments of the Invention) Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

The surface acoustic wave device according to the present invention includes a substrate 30, as shown in FIG. The substrate 30 is made of a piezoelectric material such as 128° The comb-like electrode 31 has a plurality of electrode fingers 31a that intersect at an angle, and common electrodes 31b and 32 that are commonly connected to these electrode fingers. one common′
The electrode 31b is grounded, the other common electrode 32 is connected to the signal source 4, and the common electrodes 31b and 32 are provided with a dummy electrode 15 that does not intersect with the electrode finger 31a. A regular comb-shaped electrode 33 faces the comb-shaped electrode 31 with a gap of about 1 mm therebetween. This comb-shaped electrode 33 includes electrode fingers 33a that are uniformly crossed, a common electrode 33b that is commonly connected to these electrode fingers 33a,
33c, one common electrode 33b is grounded,
The other common electrode 33c is grounded via the load 5.

In the 7-botified comb-shaped electrode 31 with the inclined electrode structure, the length of the non-intersecting part of the electrode fingers 31a is gradually increased from the acoustic center A position in the portion of the common electrode 32 that is spaced apart from the regular comb-shaped electrode 33. The bending part 3
4 is formed. The bent portion 34 has an inner edge 34A and an outer edge 34B each formed into a sawtooth shape. That is, as shown in FIG. 2, these inner edge 34A and outer edge 34B are formed by an edge 34a parallel to the propagation direction of the surface wave of the main response and an edge 34b having an angle of R. Formed into a blade shape, the angle R is set to 45 degrees. In this case, the line segment parallel to the electrode finger 31a is 0, and the envelope connecting the 07 point and the tip of the inner edge 34A is the 7.02 point and the outer edge 3.
If the envelope connecting the tip of 4B is lz, the envelope intersection 7 and the line 2 may be curved lines or broken lines.

Also, the inclination angle Φ of the envelope 1+ with respect to the line segment 0.

The envelope 9.1 (the tip of the inner edge 34A) is the electrode finger 3.
The outer edge 34B may be set arbitrarily as long as it does not pass over the intersection of the line segment O, as shown by the broken line in FIG. It may be formed into a saw blade shape so that the tip is located above fLZ.

On the bending part 34, as shown in FIG.
A sound absorbing material 35 made of epoxy adhesive containing
is provided. This sound absorbing material 35 is formed in a substantially U-shape and has sound absorbing parts 36 and 37 including the non-bending part of the common electrode 32 and each edge of the other common electrode 31b. Note that a sound absorbing material 38 made of the same material is also provided near the comb-shaped electrode 33.

In the surface acoustic wave device of the present invention having the above configuration,
As shown in FIG. 3(A), the unnecessary surface wave W3 generated from the acoustic center A and propagating toward the bending part 34 side of the common electrode 32 is partially perpendicular to the edge 34b of the inner edge 34A. Since the sound is reflected in the direction, the sound is absorbed by the central portion of the sound absorbing material 35 and the sound absorbing portion 36.

On the other hand, the unnecessary surface wave W2 generated at the edge 34a of the inner edge 34A propagates parallel to the electrode finger 31a and is transmitted to the sound absorbing portion 36.3.
A part of the sound is reflected by the inner edge of the common electrode 31b in parallel with the electrode finger 31a, and is absorbed by the central portion of the sound absorbing material 35 and the sound absorbing portion 36.

Moreover, as shown in FIG. 3(B), the surface wave W3 is partially reflected by the edge 34b of the outer edge 34B in the right angle direction, so that the surface wave W3 is reflected at the center of the sound absorbing material 35 and the sound absorbing portion 36 Sound is absorbed. On the other hand, unnecessary surface waves W2 generated at the edge 34a of the outer edge 34B propagate parallel to the electrode finger 31a and
6.37, and a part of the sound is reflected by the inner edge of the common electrode 31b parallel to the electrode finger 31a and is absorbed by the sound absorbing material 35.
The sound is absorbed by the central part of the sound absorbing part 36 and the sound absorbing part 36.

Further, the unnecessary surface waves W2 generated at the edges 34b of the inner edge 34A and the outer edge 34B are absorbed by the sound absorbing material 35 and the sound absorbing portion 36, as shown in FIGS. 3(A) and 3(B). be done.

In this case, a portion of the surface wave W2 is reflected at the edge 34a and becomes a reflected wave W4. However, this amount of reflection is small,
In addition, since the lengths of the edges 34a and 34b are set to be as small as the wavelength of the surface waves, they can be almost ignored as a spurious response and a factor that causes the spurious response to change abruptly.

By the way, in the surface acoustic wave device shown in FIG. 1, periodic signals are detected from the electrode fingers 15' and 31a', which are located at both ends of the abotized comb-shaped electrode 31 with the inclined electrode structure and are connected to the signal source 4. Target.

Surface waves Wl, W2'' are generated due to the abrupt termination of the instantaneously applied electric field.These surface waves wt, W2' advance with the same time difference with respect to the main response surface wave, and Since there is a delay, it is added to the amplitude characteristics of the spurious in the same phase.Now, as shown in FIG. If the inclination angle of the main response of 1!iL with respect to the propagation direction of the surface wave is Φ, as this inclination angle Φ becomes thicker, that is, as the length dimension of electrode finger 15' and electrode finger 31a' becomes smaller, f , the periodic spurious is reduced.The length of the electrode finger 15' is n+, and the electrode finger 31a at the inner end facing the electrode finger 15' is
If the length of the electrode finger 15' is n2, then the length of the electrode finger 15' is n+
It was found that when the length of the electrode finger 31a' was set to about lθ% of +n2, the f1 period spurious was significantly reduced.
Of course, by designing the slope of the electrode finger 4 so that the electrode finger 31a' becomes smaller, the electrode finger 31a' can be set shorter than the electrode finger 15'.

Now, when the frequency characteristics of the surface acoustic wave device of the present invention were investigated, the results shown in FIG. 5(A) were obtained. That is, FIG. 5(A) shows the amplitude frequency characteristic of the relative gain, and periodic spurious signals were generated in the passband region A. As shown in the same figure (B), this spurious is about 0,06
dB (peak to peak), showing flat characteristics. In addition, the group delay time decreased to ±2 nsec as shown in Figure 5 (C), and the group delay characteristic shown by B in Figure 5 (A) also remained flat as shown in Figure 5 (C). Ta.

In the embodiment described above, the central part of the sound absorbing material 35 has an envelope intersection,
(See Figure 2), and the sound absorbing portions 36 and 37 need only be coated on the regular comb-shaped electrode 33 side (left side) than the virtual km la, so that the coating accuracy can be improved. is not so demanded. Therefore, it is possible to produce surface acoustic wave devices with high yield.

FIG. 6A shows another embodiment of the refracting portion 34, in which edges 34a and 34b forming an outer edge 34B are set at an angle R7 smaller than 45 degrees. Therefore, unnecessary surface waves W3 propagate to the sound absorbing material 35 side and are completely absorbed by sweat. Further, the surface waves W2 generated at the edges 34a and 34b are absorbed by the sound absorbing material 35 except for the component that becomes the reflected wave W4. Incidentally, when the inner edge is also set at the same angle R7, the surface waves W2 and Wl are absorbed in exactly the same way.

FIG. 6(B) shows still another embodiment of the refracting portion 34, in which edges 34a, 34 forming an outer edge 34B are shown.
In this embodiment, part of the unnecessary surface wave W3 is reflected by the edge 34b and becomes a reflected wave W4'. In addition, the edges 34a, 3
The surface wave W2 generated at 4b is similarly absorbed by the sound absorbing material 35 except for the component that becomes the reflected wave W4.

From the above, it can be seen that it is preferable to set the angle between the edges 34a and 34b to 45 degrees or less.

Another embodiment of the invention is shown in FIG. That is,
In this embodiment, the apotized comb-shaped electrodes 31 and 31' of the inclined electrode structure shown in FIG. The electrode 31' is used for receiving waves. The comb-like electrodes 31 and 31' are connected by a multi-strip coupler 40. Of course, the same effect can be obtained in this embodiment as well.

In each of the embodiments described above, the present invention is applied to a seven-bottomed comb-like electrode with an inclined electrode structure, but the present invention can also be applied to a normal apotized-type comb-like electrode.

In addition, in the bending part 34, the inner end edge 34A and the outer end edge 34A
At least one of B may be formed into a sawtooth shape, thereby reducing spurious and flattening the spurious.

(Effects of the Invention) According to the present invention, in an abortized electrode in which electrode fingers are crossed, the length of the non-intersecting part of the common electrode is gradually reduced in the part of the common electrode that is spaced apart from other comb-shaped electrodes. By making at least one edge of the bending part into a sawtooth shape, it is possible to significantly reduce periodic spurious waves."Second, by flattening the spurious waves, the group delay can be reduced. Characteristics can be improved. Therefore, it is possible to provide a surface acoustic wave device suitable for use as an image transmission device that needs to sufficiently suppress image transmission distortion, a filter with excellent broadband characteristics, and the like.

[Brief explanation of the drawing]

FIGS. 1 and 2 are plan views of the surface acoustic wave device according to the present invention and diagrams showing the main parts thereof, and FIGS. 3(A) and (B) are enlarged views of a part of FIG. 2, respectively. 4 is a plan view showing the relationship between the length of the electrode fingers and the periodic spurious; FIG. 4 is a plan view of the apotized electrode of the device shown in FIG. 1; A diagram showing the amplitude frequency characteristics, periodic spurious characteristics, and group delay characteristics, Figure 6 (A)
, (B) are views showing other embodiments of the refracting section according to the present invention, FIG. 7 is a schematic plan view of a device according to another embodiment of the present invention, and FIGS. 8 and 9 are views showing tilted electrodes. Plan views showing conventional surface acoustic wave devices each having an apotized comb-shaped electrode structure, Figures 1θ (A) and (B) respectively show the amplitude frequency characteristics and periodic spurious characteristics of the device shown in Figure 9. 11 is a schematic plan view illustrating the surface wave generation mode of the device shown in FIG. 9, and FIGS. 12 and 13 are
FIG.
Periodic spurious characteristic diagrams of experimental examples shown in Figures 15 and 1
Is Figure 6 the surface acoustic wave that was proposed earlier? A plan view of tFa and a schematic plan view explaining its operation, FIGS. 17(A) to (C)
15 is a diagram showing the amplitude frequency characteristic, one-period spurious characteristic, and group delay characteristic of the device shown in FIG. 15, and FIG.
FIG. 3 is a schematic plan view showing a state in which reflected waves are generated in the device shown in the figure. 30--1--------Substrate, 31.33--1--<L, bar-shaped electrode, 31a, 3
3 a ----One electrode finger, 31b, 32---One common electrode, 34-One---Bending part, 34A---Saw blade shape Inner edge, 34B--
---1---Serrated outer edge, 34a, 34b---
-One edge, 35-----1---Sound absorbing material, (1 other person) Fig. 2 1 41b 4A Fig. 4'52 Fig. 7 Fig. 5 Fig. 8 Fig. 9. Figure 10 f6 (center frequency) Figure 14 Figure 11 Figure 12 Figure 13 Figure 17 (A) (B) Figure 18

Claims (3)

[Claims] (1) In a surface acoustic wave device comprising a comb-like electrode having electrode fingers whose intersection length changes in response to an impulse response and a common electrode to which the electrode fingers are connected, the common electrodes are arranged opposite to each other, and The portion spaced apart from the comb-shaped electrode is bent in a direction that gradually reduces the length of the non-intersecting portion of the electrode fingers, and at least one edge of the bent portion is formed in a sawtooth shape. A surface acoustic wave device characterized by: (2) The surface acoustic wave device according to claim 1, wherein a sound absorbing material is disposed near and including a part of the bending section. (3) The comb-shaped electrode has an inclined electrode structure in which the intersecting position of the electrode fingers is inclined so as to shift the side lobe with respect to the main lobe. surface acoustic wave device.