JPH02170711A - Surface acoustic wave device - Google Patents

Abstract

PURPOSE:To equalize the exciting power distribution of an interdigital electrode (IDT) and to eliminate unnecessary spuriousness by inverting the tilt of an apodized type electrode including a main lobe and a side lobe due to an impulse response at every side lobe and main lobe. CONSTITUTION:Input side electrodes 22a and 22b respectively consist of the apodized type IDT, and output side electrodes 23a to 23b respectively consist of a normal type IDT. The constitution of the input side electrodes 22a and 22b is weighted according to the impulse response obtained by a combination. with a desired frequency characteristic, and it is weighted so as to include a main lobe part M and a side lobe part N. The tilt shape of each IDT is basically a straight line, whereas, the side lobe N bends to the main lobe part M as shown in a figure, and it tilts in an inverted direction. Thus, the exciting power distribution of the IDT is equalized, and the unnecessary spuriousness can be eliminated.

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) Conventionally, electrode configurations in surface acoustic wave devices used as filters include LiNb03, LIT
On the surface of a piezoelectric substrate made of piezoelectric material such as AO 3,
A first comb-shaped electrode (hereinafter referred to as an IDT) serving as an input-side electrode and a second comb-shaped electrode serving as an output-side electrode are arranged to face each other, and the electrode fingers are crossed with each other at a predetermined interval. has been done.

In the electrode structure of this IDT, if both the input and output electrodes are regular type electrodes with a constant crossing width, the frequency characteristic is as follows: If the number of electrode pairs is N, the frequency bandwidth is Δf -0,88/N and the sidelobe level is -26 dB. However, in order to further satisfy the desired frequency characteristics, such as flat frequency characteristics within the passband and suppression of sidelobe levels, generally an apodization method is applied to either the input side electrode or the output side electrode. An electrode structure weighted according to the following is used.

FIG. 7 is a diagram showing such a conventional surface acoustic wave device.
FIG. 7(a) is a configuration diagram of a conventional bidirectional surface acoustic wave device, and FIG. 7(b) is a configuration diagram of a surface acoustic wave (SA) device shown in FIG.
It is a figure showing excitation power distribution of W). Broken line 1 in the same figure
In a surface acoustic wave device in which weighting by the apodization method is applied almost symmetrically with respect to the central axis of the aperture length of electrode 1, as shown in Figure 2, the SAW excitation power is concentrated at the center in the cross width direction, and the power in the surrounding area is small. Put it away. In the surface acoustic wave device having such an electrode configuration, the surface acoustic wave 4 radiated from the approximate center of the apodized electrode 2 in the cross width direction, that is, the portion where the SAW excitation power is concentrated, is received by the regular electrode 3. At this time, in the regular electrode 3, a region B where the power of the surface acoustic wave is not concentrated becomes a load with respect to a region A where the power of the surface acoustic wave is concentrated, and a new surface acoustic wave 5 is radiated from the region B. Furthermore, this surface acoustic wave 5 is reflected by the apodized electrode 1 for the same reason, and a triple medium transit echo (hereinafter referred to as TTE) is generated. As a result, there is a problem in that unnecessary spurious signals are generated as TTE. A known method for solving this problem is to apply tilt to make the excitation pulse power uniform.

FIG. 8(a) is a diagram showing the structure of the electrode 6 in which linear tilt is applied to the apodization of the Hamming function, and FIG. 8(b)
FIG. 4 is a diagram showing the excitation power distribution in the aperture length direction in the case of the weighting shown in FIG.

As shown in FIG. 8(b), by applying tilt, the uniformity of the excitation power is improved, and a filter with good characteristics can be realized. However, such a tilt shape
A high-loss filter composed of bidirectional electrodes as shown in the figure was effective even if the excitation intensity was not sufficiently uniform, but a low-loss filter was effective due to slight non-uniformity of the excitation power. Since TTE occurs, resulting in unnecessary spurious signals and adversely affecting the characteristics, sufficient uniformity of excitation power is required.

Furthermore, as shown in FIG. 9, when the electrode 7 is weighted so that the apodization includes side lobes, the excitation power in the side lobes becomes small when tilted in a linear or humming manner, making it difficult to achieve sufficient uniformity. It was impossible to obtain this, and was the cause of TTE.

FIG. 10 is a diagram showing a low loss filter having such an electrode structure. On the piezoelectric substrate 11, an input side electrode 12a
, 12b and output side electrodes 13a, 113b, 13c are arranged alternately, and each input side electrode 12a, 12b
are connected by a bonding pad 14, and each output side electrode 13a, 13b, 13c is connected by a bonding pad 15, respectively. In addition, each input side electrode 12a, 1
Reference numeral 2b is an apodized comb-teeth electrode having the electrode structure shown in FIG. 9, and each of the output side electrodes 13a, 13b, and 13c is a regular comb-teeth electrode.

The frequency characteristic of such a low loss filter is 10th
As shown in Figure (b), a ripple 17 occurs within the passband, which causes a serious problem of deteriorating the characteristics.

(Problem to be Solved by the Invention) As described above, in an apodized IDT including a main lobe and side lobes due to an impulse response, the SAW excitation power is non-uniform, so reflections from the regular electrode at the position of low power occur. This causes a problem in that unnecessary spurious signals are generated as a TTE. Particularly in low-loss filters such as multi-electrode type filters, the unnecessary spurious signals caused by the TTE significantly deteriorate the characteristics of the filter.

The present invention was made in order to solve the above-mentioned conventional problems, and an object of the present invention is to provide a surface acoustic wave device in which the excitation power distribution of an IDT weighted to remove unnecessary spurious components is sufficiently uniform. shall be.

[Structure of the Invention] (Means for Solving the Problems) A first surface acoustic wave device of the present invention includes a piezoelectric substrate, a comb-shaped input side electrode formed on one main surface of the piezoelectric substrate, and a comb-shaped input side electrode. In a surface acoustic wave device having a tooth-shaped output side electrode, either the input side electrode or the output side electrode is configured with an apodized electrode having a main lobe and a side lobe based on an impulse response, and the main lobe The invention is characterized in that the direction of the tilt inclination of the side lobe is reversed with respect to the direction of the tilt inclination of the side lobe.

A second surface acoustic wave device of the present invention includes a piezoelectric substrate, and a comb-shaped input electrode and a comb-shaped output electrode formed on one main surface of the piezoelectric substrate. wherein either the input side electrode or the output side electrode is configured with an apodized electrode having a main lobe and a plurality of side lobes based on an impulse response, and the side lobes are arranged in the direction of the tilt inclination of the main lobe. The present invention is characterized in that the direction of the tilt slope is reversed for each side lobe.

(Function) By reversing the tilt of the apodized electrode, which includes a main lobe and side lobes in the impulse response, for each side lobe and main lobe, the excitation power distribution of the IDT becomes sufficiently uniform, and as a result, the surface acoustic wave Unnecessary spurious noise due to TTE caused by non-uniformity of excitation power is removed.

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

FIG. 1 is a diagram showing the electrode structure of a surface acoustic wave device according to an embodiment, and a broken line A1 in the diagram depicts a weighting envelope. On one main surface of the piezoelectric substrate 21 made of a piezoelectric material such as LiNb01, LITaO3, quartz, etc., a comb-shaped input side electrode 22a made of AJ2 or the like is formed by a thin film forming technique such as photo-etching of a vapor deposited film. 22
b and output side electrode 23 a s 23 b s 23
c is provided.

Input side electrodes 22as 22b and output side electrodes 23a, 23b, 23c in this elastic wave device are arranged alternately, respectively, and each input terminal M pole 22a, 22b is connected to each output side electrode 23a, 23b by a bonding pad 24. ,
23c is connected by a bonding pad 25. The input side electrodes 22a, 22b are apodized comb-teeth electrodes, and the output side electrodes 23a, 23b, 23c are regular comb-teeth electrodes.

As shown in FIG. 2, the structure of the input side electrodes 22 a s 22 b is weighted to correspond to the impulse response obtained in combination with the desired frequency characteristics, and the main lobe part M and the side lobe part N are weighted. It is weighted to include The tilt shape of each comb-teeth electrode is basically a straight line, but as shown in FIG. In this case, the slope width of the side lobe portion N is 1/2.

Note that the line width of the electrode fingers 26 is 1/1 of the wavelength λ of the surface acoustic wave.
4, and such an electrode finger 26 has a wavelength of λ.
They are arranged at a pitch of 1/2 of that of .

Further, in areas other than the intersections between the comb-teeth electrodes, dummy electrodes 27 are arranged opposite to the crossed electrode fingers 26 in order to prevent surface acoustic waves from being disturbed.

In the surface acoustic wave device of the embodiment configured as described above,
It operates as a low-loss filter that passes a certain frequency band. In such an electrode structure, FIGS. 3(a) and (b)
As shown in , since the excitation power distribution 28 in the aperture length direction is sufficiently uniform, the regular comb-teeth electrodes 23a, 23b, 23, which are the output side electrodes for the excited surface acoustic waves,
In C, the load is almost the same at any part in the aperture length direction, and no reflection occurs from the regular electrode. Therefore, TTE due to reflection from the output side electrodes 23a, 23b, and 23c can be removed, and a surface acoustic wave device with good characteristics without unnecessary spurious within the pass band can be obtained, as shown in FIG. 3(c). be able to.

In this example, the electrode line width is l/4λ, but the present invention is not limited to such a line width.
As shown in Figures 4(a) and (b), the 1/8 split electrode configuration or the 1/8 split electrode configuration that does not cause acoustic reflection due to the electrode
It can also be adapted to an 8λ-5/8λ electrode configuration.

FIG. 5 is a diagram showing an embodiment in which the present invention is applied to a surface acoustic wave device having another electrode structure, in which each input side electrode 31a, 3
1b and the respective output side electrodes 23a, 123b, 23c are arranged in the same manner as in the embodiment described in Section 1 above. The envelopes of the input side electrodes 31a and 31b in this surface acoustic wave device have a structure in which the slope of the side lobe portion is similar to the slope of the main lobe portion, and the envelope is sloped along the length of the opening, as shown by the broken line A1.

FIG. 6 is a diagram showing another embodiment of the present invention. Even when each input side electrode 41a, 41b is weighted to include a plurality of side lobes, the tilt does not change as shown by the broken line A3 in the figure. By applying a tilt in which the direction of the slope is alternately changed for each main lobe and side lobe, the excitation power can be made sufficiently uniform.

Furthermore, as a method of realizing a low-loss surface acoustic wave device, there is also a method of imparting directionality to the electrodes through multiphase excitation or the like. The electrode structure described in the above embodiment can also be applied to such a directional transducer.

[Effects of the Invention] As explained above, according to the surface acoustic wave device of the present invention, the excitation power distribution of the comb-shaped electrode becomes sufficiently uniform, and as a result, non-uniformity of the excitation power of the surface acoustic wave is reduced. This makes it possible to remove unnecessary spurious signals caused by TTE.

[Brief explanation of the drawing]

FIG. 1 is a front view showing an embodiment of the surface acoustic wave device of the present invention, FIG. 2 is a diagram showing the structure of the apodized comb-teeth electrode of FIG. 1, and FIGS. 3(a) and (b) are A diagram for explaining the excitation power distribution of the apodized comb-teeth electrode of the example, the same figure (c
) is a graph showing the frequency characteristics of the surface acoustic wave device of the present invention, FIG. 4 is a diagram showing another electrode structure in the surface acoustic wave device of the embodiment, and FIGS. 5 and 6 are graphs showing other embodiments of the present invention. An electrode configuration diagram showing an example, FIG. 7(a) is a front view showing a conventional surface acoustic wave device, FIG. 7(b) is a diagram for explaining the excitation power distribution of FIG. 7(a), Figures 8 and 9
The figure shows a detailed view and excitation power distribution diagram of a conventional apodized comb electrode structure, Figure 10(a) is a configuration diagram of a low-loss filter using the electrode structure of Figure 9, and Figure 10(b) is the same. It is a graph which shows the frequency characteristic of the conventional surface acoustic wave device of FIG. 21...Piezoelectric substrate 22a% 22b...Input side electrode (apodized comb electrode) 23a, 23b, 23c... Output side electrode (regular type comb-teeth electrode) A I , A 2 , A 3 ...... Apodized weighting envelope 28 ...... Excitation power distribution curve Applicant Toshiba Corporation Agent Person Patent Attorney Suyama Sa - (a) (b) Figure 7 (a) Child 7- (b) Figure 8

Claims (2)

[Claims] (1) In a surface acoustic wave device having a piezoelectric substrate, and a comb-shaped input side electrode and a comb-shaped output side electrode formed on one main surface of the piezoelectric substrate, the input side electrode or the output side Either one of the electrodes is constituted by an apodized electrode having a main lobe and a side lobe due to an impulse response, and the direction of the tilt inclination of the side lobe is inverted with respect to the direction of the tilt inclination of the main lobe. A surface acoustic wave device characterized by: (2) In a surface acoustic wave device having a piezoelectric substrate, and a comb-shaped input side electrode and a comb-shaped output side electrode formed on one main surface of the piezoelectric substrate, the input side electrode or the output side Either one of the electrodes is composed of an apodized electrode having a main lobe and a plurality of side lobes based on an impulse response, and the direction of the tilt inclination of the side lobe is different for each side lobe with respect to the direction of the tilt inclination of the main lobe. A surface acoustic wave device characterized by having an inverted slope.