JPS59123305A - Elastic surface wave filter - Google Patents

Abstract

PURPOSE:To reduce the size of a substrate of an elastic surface wave filter and to suppress the diffraction loss by setting an interdigital electrode so that an exciting source existing in the vicinity of another exciting source forming a maximum crossing width of other interdigital electrodes goes to zero. CONSTITUTION:Input/output side interdigital electrodes 21, 22 are formed on the elastic surface wave substrate 20 with a prescribed interval. The interdigital electrode 21 consists of interdigial electrodes 23, 24. The crossing width weighting is applied to the electrode 23 on the basis of an impulse response specifying an even number component, and an electrode section 23b of two common electrode sections 23a, 23b is formed so as to be placed nearly along with the envelope for weighting. The electrode 24 specifies an odd number component. After the electrode 24 is set in advance so that the exciting source positioned near the exciting source 23c forming the maximum crossing width of the electrode 23 goes to zero, the electrode 24 is formed at the non-crossing region of the even number component and on a propagating line of the other electrode 22.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an electrode pattern to obtain a frequency response characteristic that is asymmetric with respect to a center frequency.

Conventionally, one way to obtain asymmetric frequency response characteristics with a single interdigital transducer is to set the distance between the centers of adjacent electrode fingers (hereinafter referred to as electrode pitch) and the crossing width of the electrodes in the surface acoustic wave propagation direction. There is a known method for changing it along the following lines. This is a so-called variable pitch interdigital electrode, as described below.

That is, when the frequency response characteristic is subjected to inverse Fourier transform, an impulse response as shown in FIG. 1, for example, is obtained.

Since this impulse response has an asymmetric frequency response characteristic, it includes an imaginary part as a result of inverse Fourier transform, and the time intervals between peak points at which the imaginary part becomes zero are non-uniform. Then, by forming interdigital electrodes corresponding to the obtained impulse response, the desired frequency response characteristics can be achieved with these electrodes. The way to deal with this is to
The intersection width between the Nuffingers (surface wave receiving area) is made proportional to the size of each peak in the impulse response (indicated by the arrow), and the electrode pitch is adjusted to the impulse response (indicated by the arrow).
It may be done in proportion to the time between peak points. However, since the time between peak points is non-uniform, the electrode pitch of the interdigital electrodes is also non-uniform, and as a result, the interdigital electrodes have a variable pitch type.

The conventional method described above can satisfy the desired characteristics, but since the electrodes have uneven pitches, it is difficult to design the electrode pattern, and it is difficult to design the electrode pattern for thick and thin electrodes. 'It has the disadvantage that the electrodes are easily short-circuited.

In order to solve the above-mentioned problems, attempts have been made to obtain asymmetric frequency response characteristics using interdigital electrodes of equal pitch, and techniques called the odd-corner function method and the mirror method or reflection method, which will be described later, have been proposed. .

In the former odd-even function method, if Hl (ω) is a linear representation of the desired frequency response characteristic, then Hl(ω-ωo
) = H2(ω0-(/J)) This is a method that assumes H2 (ω). The relationship between Hl (ω) and 1-12(ω) is as shown in Figure 2. Here, the even component is 1”R(ω)
, the blue component is Hl (ω), and HR (ω) and HI (ω
) are defined as follows, their functions become as shown in Figure 3.

Also, Hl (ω) is f−h from equations (1) and (2)
(ω)=HR(ω)-jl-1+(ω) (3).

Then, the impulse response becomes a Fourie expression (3).

i in equation (4)? , (Mo> Y--3!z(E), which shows one impulse line and a broken line.The two impulses in the same figure (λo/2 when expressed in terms of wavelength) are uniform, and both curves are uniform. The peak points are located in the middle between the peak points of the opposite side.The interdigital electrodes corresponding to the impulse responses shown by the solid line constitute the even component, and the impulse responses shown by the dashed line constitute the blue component. The electrode pattern shown in Figure 6 consists of ink-digital electrodes divided into two stages based on the two impulse responses and electrically connected in parallel. A design method for wave filters J (published September 28, 1972, 172nd Sound Engineering Research Group, Institute of Electrical Communication, Tohoku University).In Fig. 6, one of the interdigital electrodes 1 is It consists of two interdigital electrodes 2.3 arranged perpendicular to the propagation direction, and is configured so that electrode 3 excites the even component and electrode 2 excites the blue component (the reverse is also possible). The other interdigital electrode 4 is formed to cover the propagation path of electrode 2.3.

However, with electrode 1 shown in Fig. 6 above, asymmetric frequency response characteristics can be achieved with a small pitch, but since two interdigital electrodes are arranged in a direction perpendicular to the propagation direction, the excitation region of the surface wave is expanded. , the disadvantage is that the surface wave substrate becomes wider. Further, the center portion where the surface wave excitation/reception intensity is high is divided into both sides, and the center portion of the electrode becomes a common electrode, which is not a preferable electrode pattern.

In order to eliminate the above-mentioned problem and realize an asymmetric frequency response characteristic with one equally pitched interdigital electrode, the two impulse responses in Fig. 4 are synthesized as shown in Fig. 5, and this synthesized impulse response is Based on Figure 7 (a
), the electrode pattern can be configured as shown in (b). In the same figure, the main electrode fingers 6.7.
8.9 l.

The two adjacent main electrode fingers 6a'3 and 7 and Bi3 and 9 are connected at common parts of different potentials, and the lengths of these two main electrode fingers are different, and It is formed by arranging co-junkers 10.11-2.13, which are separated from the free end of each main electrode finger d and 6.7.8.9 and have different potentials, at an electrode pitch of λo. According to this interdigital electrode, an even component is excited and received in the region where adjacent main electrode fin kerfs 8 of different potentials intersect (shaded region on the upper right), and the adjacent main electrode fingers 6.9 and auxiliary electrode fin kerfs 11
．． The area where 12 intersect (lower right λ diagonal shaded area> 1-odd component is not offset from the even component of 7, and the excitation and reception fins are generated. When such interdigital electrodes are used, the direction of surface wave propagation and The electrode width in the perpendicular direction can be narrowed, and the surface wave substrate can be made too small.
However, since surface waves are excited and received, errors occur in the frequency response characteristics, and it is extremely troublesome to design with these errors taken into consideration in advance. In addition, in order to minimize the influence of excitation and reception between the electrode fingers 6.8 and 7.9 to a negligible extent, the finger tips of the electrode fingers 7 and 11.8j5 and 12 located between them are brought close together. If the cross-hatched area is made smaller, there is a risk that both fingers 7 and 11.8 and 12 will be short-circuited at their tips during pattern formation.

The latter glue flexicon method or mirror method is a method that assumes a virtual image with a center frequency of 3fO that is line symmetric with respect to 2fo, assuming that the center frequency of a predetermined frequency characteristic is [0], and the resulting impulse response is as described above. The results are the same as in the odd-even function method, and the electrode patterns are as shown in Figures 6 and 7.
Figures (a) and (b) are determined in the same way as later and have the same problems as described above.

The present inventor previously filed an application in Japanese Patent Application No. 57-104391 for a surface acoustic wave filter that eliminates the drawbacks of the prior art described above. In this case, a common electrode is provided along the envelope of interdigital electrodes constituting even components, and interdigital electrodes having odd components are constructed on one or both sides of this common electrode.

The present invention is a further improvement of the above-mentioned prior application, and in addition to the effects obtained in the prior application, it is possible to achieve reduction in substrate size and suppression of diffraction loss.

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

J-3 in Figure 8, l-i Nl) 03, P':
T, surface wave substrate 2 consisting of a ZnO film on a glass substrate, etc.
0, input/output side interdigital electrodes 21 and 22 are formed spaced apart by a predetermined distance. One interdigital voltage (21) is connected to the first +13 and second interdigital 7-
It is composed of intal electrodes 23 and 24. 10th thunder (
Order 23 is the impulse response (
Based on the solid line), one of the two common electrode portions 23a and 231), 23b, is
is formed so as to roughly follow the envelope of the weight. The second electrode 24 is an interdigital electrode that defines the odd component in FIG. [In addition, the common electrode part 2 of the first electrode 8X23
3b, that is, a non-intersecting region of even components, and is formed on the propagation path of the used electrode 22. This electrode 24 includes a common electrode portion 23b of the first electrode 23 and a common electrode portion 23b of the first electrode 23.
A separate common electrode portion 24a is formed close to the vibration source 23C with the maximum intersecting width of the electrode finger. Common electrode part 24a of second electrode 24 and first electrode 23
is coupled to the common electrode portion 23a by a shield electrode 25. Terminal electrodes 26, 21, 28, and 29 are formed, for example, at each corner of the substrate 20, and are respectively connected to a predetermined common electrode portion. In this embodiment, the first electrode 23
No other vibration sources are placed outside the single source 23C having the maximum intersecting width, and the width of the substrate can be reduced accordingly.

Next, the second electrode 24 is deflected (briefly about the method of zeroing out the vibration source located near the maximum crossing width fl!23G of the first electrode 23).

As mentioned above, when the desired frequency characteristic is expressed by equation (3), the impulse response is h (t ) = lI
R(t)-jhI(j), and the characteristics are as shown in FIG. Figure 12 is for convenience of explanation.
．． 4.5 The sampling does not necessarily match the sample shown in Figure 5 (solid line arrow), the even component interdigital electrode 23 is formed based on the data corresponding to the even number, and the interdigital electrode 23 of the white component is formed based on the data corresponding to the odd number. electrode 24
It consists of The same electrode configuration is used for the odd-even function method. However, in this embodiment, by slightly changing the sampling time interval, the even component and the white component are changed, and, for example, the oscillation source of the white component at most one nearest neighbor of the even component is reduced. In other words, when sampling at a time interval of t'-4-(previously bf), the source of the white component becomes smaller as shown by the broken line in Fig. , for example, the source of 0.02 or less is forcibly set to zero.Of course, if the source is forcibly set to zero, it must be corrected with another source.By configuring in this way, The conventional electrode pattern results in the electrode configuration shown in FIG. 8, which is one embodiment described above.

This embodiment can be further advanced by arranging each vibration source of the second electrode 24 at a retarded position, for example, at the center, where the excitation intensity is maximum in the electrode finger direction perpendicular to the surface wave propagation direction. The electrode configuration shown in FIG. 9 can be realized. By arranging it in this manner, other vibrations are not arranged outside the vibration source 23G having the maximum intersecting width of the first electrode 23, and the width of the substrate can be reduced accordingly. Furthermore, since the second electrode 24 is concentrated in the center where the excitation intensity is originally high, the influence of diffraction loss and the like is eliminated. Since the other configurations are almost the same as the embodiment shown in FIG. 8, the explanation thereof will be omitted.

FIG. 10 shows another embodiment, which differs from the above embodiment in that the second electrodes constituting the white component are formed separately on both sides of the first electrode 23. That is, the other common electrode portion 23a of the first electrode 23 is also curved so as to substantially follow the weighted envelope, and the maximum crossing width of the first electrode 23 is
Another common electrode section 24a' is provided extending nearby, and an interdigital electrode 24' is formed by protruding electrode fingers from the common electrode sections 23a and 24a'. Common electrode part 2
3b and the common electrode portion 248' are connected through the outside of the electrode. The white component is defined by the electrodes 24 and 24' (
A second electrode is configured.

In each of the above embodiments, it is possible to configure asymmetric frequency characteristics of the single electrode C, so compared to the conventional barrier pull pitch method and the mirror method (or reflection method) shown in FIG.
A much higher frequency filter can be realized with the same electrode width.

FIG. 11 roughly shows another embodiment, and the difference from the above two embodiments is that the electrodes 23 and 24 are configured in a split electrode type in order to enhance the rTE removal effect. According to this embodiment, the electrode fingers of the pair of split electrodes can be configured to have the same length, and the gl calculation error is reduced compared to the conventional mirror method (or reflection method).

Although the electrodes in the above-mentioned embodiments have very simple envelopes, the present invention is also applicable to electrodes with different envelopes. Furthermore, the even component and white component in the Water Quantity II book collectively refer to the even component and white component in the odd-even function method, the symmetrical component d3 and the asymmetrical component in the reflection method, and the like. Furthermore, if the design is designed with the phase corrected,
There are cases where the maximum value of the intersection width is on the odd function side, in which case the above-mentioned method may be applied on the even function side.

As explained above, according to the present defense, the desired frequency characteristics can be reliably obtained without error with a substrate size comparable to or smaller than that of the mirror method, and the complicated planning at the time of design can be reduced. It can be configured with either a single electrode or a split electrode, and furthermore, the influence of diffraction loss can be minimized.

[Brief explanation of the drawing]

Figure 1 is an impulse response characteristic diagram of a conventional variable pitch electrode, Figures 2 to 5 are diagrams used to explain the conventional example and the present invention, and Figure 2 is a diagram of th (ω) and H2 (ω). Frequency characteristic diagram, Figure 3 is the frequency characteristic diagram of HR (ω) and jHx (ω), Figure 4 is the impulse response characteristic diagram of HR (ω) and jHI (ω), Figure 5 is the HR (ω) and An impulse response characteristic diagram synthesized with JHI(ω), FIG. 6 shows a conventional filter, FIG. 7 (a) shows another conventional filter, and FIG. 7 (b) shows a partially enlarged view. Figure 8, Figure 9, Figure 1
0 and 11 are diagrams showing filters according to the present invention, respectively, and FIG. 12 is an impulse response characteristic diagram used in the 31st filter of the present invention. Patent application Mura 1 Co., Ltd. (J Manufacturing Co., Ltd.

Claims (2)

[Claims] (1) A surface acoustic wave filter having at least input and output side electrodes for obtaining a frequency response characteristic asymmetrical with respect to the center frequency, in which at least one electrode is weighted with an intersection width to obtain a frequency response characteristic. A first interdigital electrode that defines an even component (or an odd component) and an odd component of the frequency response characteristic that is arranged mainly in a non-intersecting region of the first interdigital electrode and weighted by the intersection width. (or an even component);
The elasticity of the second interdigital electrode is set such that at least one vibration source near the vibration source, which is the maximum crossing width of the first interdigital electrode, becomes zero. surface wave filter. (2) The remaining vibration sources of the second interdigital electrode are arranged close to the position where the excitation intensity is maximum in the direction orthogonal to the surface wave propagation direction of the first interdigital electrode. The surface acoustic wave filter according to item (1).