JPH0318768B2 - - Google Patents

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 method of obtaining asymmetric frequency response characteristics with a single interdigital transducer is
One known method is to change the distance between the centers of adjacent electrode fingers (hereinafter referred to as electrode pitch) and the crossing width of the electrodes along the surface acoustic wave propagation direction. 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 do this is to make the intersection width (surface wave excitation/reception area) between adjacent electrode fingers proportional to the size of each peak point (indicated by an arrow) in the impulse response, and to adjust the electrode pitch to 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 electrode is also non-uniform, and as a result, the interdigital electrode has a variable pitch type.

The conventional method described above can satisfy the desired characteristics, but since the electrodes are unevenly pitched, it is difficult to design the electrode pattern, and the electrodes are designed for high frequencies because they create thick and thin electrodes. It has the disadvantage of being easily short-circuited.

In order to solve the above-mentioned problems, attempts have been made to obtain asymmetric frequency response characteristics using equally spaced interdigital electrodes, and methods called the odd-even 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 H ₁ (ω) is a linear representation of the desired frequency response characteristic, then H ₁ (ω
This is a method that assumes H ₂ (ω) such that -ω ₀ )=H ₂ (ω ₀ -ω). The relationship between H ₁ (ω) and H ₂ (ω) is shown in Figure 2. Let the even component be H _R (ω) and the odd component H _I (ω), and define H _R (ω) and H _I (ω) as follows, then their functions will be as shown in Figure 3. Become.

H _R (ω)=H ₁ (ω)+H ₂ (ω)/2 (1) H _I (ω)=H ₂ (ω)−H ₁ (ω)/2j (2) Also, H ₁ (ω) From equations (1) and (2), H ₁ (ω)=H _R (ω)−jH _I (ω) (3).

The impulse response is the Fourier transform of equation (3), h(t)=h _R (t)−jh _I (t)=∫H _R (ω)e ^j(2 〓 ^ft+
〓 ⁾ df+∫−jH _I (ω)e ^j(2 〓 ^ft+ 〓 ⁾ df(4).

When the impulse responses shown by h _R (t) and -jh _I (t) in equation (4) are determined at t=1/2f ₀ , they become as shown by the solid line and broken line in FIG. 4, respectively. The two impulse response curves in the same figure are both uniform in that the time between the peak points is 1/2f ₀ (λ ₀ /2 when expressed in terms of wavelength), and the peak points of both curves are within the distance between the peak points on the other side. located in the middle of The interdigital electrodes corresponding to the impulse response shown by the solid line constitute the even component, and the impulse response represented by the dashed line constitute the odd component. The interdigital electrode is divided into two stages based on the two impulse responses shown in Fig. 4, and the electrode pattern shown in Fig. 6 is electrically connected in parallel. "A Design Method for Filters" (issued September 28, 1972, materials from the 172nd Acoustic Engineering Research Meeting, Institute of Electrical Communication, Tohoku University). In FIG. 6, one interdigital electrode 1 is composed of two interdigital electrodes 2 and 3 arranged perpendicular to the propagation direction, with electrode 3 exciting and receiving the even component and electrode 2 exciting and receiving the odd component. (the reverse is also possible), two electrodes 2,
The other interdigital electrode 4 is formed to cover the propagation path 3.

However, with the electrode 1 shown in Fig. 6 above, it is possible to realize an asymmetrical frequency response characteristic with equal pitch, but since two interdigital electrodes are arranged in a direction perpendicular to the propagation direction, the surface wave excitation and reception area is expanded, and the surface wave The disadvantage is that the wave board becomes wider. Further, the central portion where the surface wave excitation/reception intensity is high is divided into both sides, and the central portion of the electrode serves as 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 this, an electrode pattern can be constructed as shown in FIGS. 7a and 7b. In the figure, one interdigital electrode 5 has main electrode fingers 6, 7, 8, and 9 having an electrode width of 1/8λ ₀ arranged at an electrode pitch of 1/4λ, and two adjacent main electrode fingers 6 and 7, 8 and 9 are connected at common parts with different potentials, and the lengths of the two main electrode fingers are different, and each of the main electrode fingers 6, 7,
Auxiliary electrode finger 1 with a width of 1/8λ ₀ facing the free ends of 8 and 9 and connected to a common part of different potentials.
0, 11, 12, and 13 are arranged at an electrode pitch of 1/4λ ₀ . According to this interdigital electrode, even components are excited and received in the area where adjacent main electrode fingers 7 and 8 of different potentials intersect (shaded area on the upper right), and the adjacent main electrode fingers 6 and 8 intersect with each other.
In the area where 9 and the auxiliary electrode fingers 11 and 12 intersect (shaded area downward to the right), the odd component is excited and received at a distance of λ/4 from the even component. If such interdigital electrodes are used, the electrode width in the direction perpendicular to the surface wave propagation direction can be narrowed, and the surface wave substrate can be made smaller.
Since the surface waves are excited and excited even in the area (cross diagonal line) where 9 and 9 intersect, an error occurs in the frequency response characteristic, and it is very troublesome to design with this error in mind in advance. In addition, in order to reduce the influence of excitation and vibration between the main electrode fingers 6 and 8 and between 7 and 9 to a negligible extent, the electrode fingers 7 and 11, 8 and 1 located between them are
If the tips of the fingers 7 and 11, 8 and 12 are brought closer to each other to reduce the cross-hatched area, there is a risk that the fingers 7 and 11, 8 and 12 will be short-circuited at the tips during pattern formation.

The latter reflection method or mirror method is
Letting the center frequency of a given frequency characteristic be f ₀ ,
This method assumes a virtual image with a center frequency of 3f ₀ that is line symmetric with respect to 2f ₀ , and the impulse response obtained is the same as in the case of the odd-even function method described above, and the electrode pattern is also similar to that shown in Figures 6 and 7. It is determined in the same way as those in Figures a and b, and has 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. The content is such that 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 earlier application,
In addition to the effects obtained in the previous application, it is possible to reduce the substrate size and suppress diffraction loss.

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

In FIG. 8, input/output side interdigital electrodes 21 and 22 are formed at a predetermined distance apart on a surface wave substrate 20 made of LiNbO ₃ , PZT, a ZnO film on a glass substrate, or the like. One interdigital electrode 21 is composed of first and second interdigital electrodes 23 and 24. 1st
The electrodes 23 are cross-width weighted in the usual way based on the impulse response (solid line) defining the even component in FIG.
One of them 23b is formed so as to substantially follow the weighted envelope line. The second electrode 24 is an interdigital electrode that defines the odd component in FIG.
After setting in advance so that the vibration source located near 3C becomes zero, the common electrode part 23 of the first electrode part 23
b, that is, a non-intersecting region of even components, and is formed on the propagation path of the other side electrode 22. This electrode 24 is a common electrode portion 23b of the first electrode 23.
and a separate common electrode section 24a formed close to the vibration source 23C of the maximum intersecting width of the first electrode 23, with electrode fingers protruding from the common electrode section 24a. The common electrode portion 24 a of the second electrode 24 and the common electrode portion 23 a of the first electrode 23 are coupled by a shield electrode 25 . Terminal electrodes 26, 27, 28, and 29 are formed, for example, at each corner of the substrate 20, and are connected to respective predetermined common electrode portions. In this embodiment, no other vibration source is disposed outside the vibration source 23C having the maximum intersecting width of the first electrode 23, and the width dimension of the substrate can be reduced accordingly.

Next, a method for reducing the vibration source located near the vibration source 23C of the maximum width of the first electrode 23 in the second electrode 24 to zero will be briefly described.

As mentioned above, when the desired frequency characteristic is expressed by equation (3), the impulse response is h(t)=h _R (t)−jh _I (t)=∫H _R (ω)e ^j(2 〓 ^ft+
〓 ⁾ df+∫−jH _I (ω)e ^j(2 〓 ^ft+ 〓 ⁾ df(4), and the characteristics are as shown in Fig. 12. 1st
For convenience of explanation, FIG. 2 does not necessarily correspond to FIGS. 1, 4, and 5. According to a method such as the reflection method, data is sampled at a time interval of t=1/4f ₀ (solid arrow) in FIG. The interdigital electrodes 24 of odd components are constructed based on data corresponding to odd numbers. 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 odd component are changed, and, for example, the source of the odd component near the maximum value of the even component is increased. That is, when sampling is performed at a time interval of t'=1/4 (f ₀ +Δf), the source of the odd component becomes smaller as shown by the broken line in FIG. Furthermore, if the maximum value of the excitation intensity is set to 1 on a relative scale, an excitation source of, for example, 0.02 or less is forcibly set to zero. Of course, if the vibration source is forcibly set to zero, correction should be made using another vibration source. With this configuration, the conventional electrode pattern becomes 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 closer to the position where the excitation intensity is maximum in the electrode finger direction orthogonal to the surface wave propagation direction, for example, the center. The electrode configuration shown in FIG. 9 can be realized. With this arrangement, no other vibration source is placed outside the vibration source 23C 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 odd component are formed separately on both sides of the first electrode 23. That is, another common electrode section 23 of the first electrode 23
Another common electrode part 24a' is also provided, which is curved so as to almost follow the weighted envelope, and extends near the maximum cross width of the first electrode 23.
An interdigital electrode 24' is constructed by having electrode fingers protrude from and 24a'. Common electrode part 2
3b and the common electrode portion 24a' are connected through the outside of the electrode. The electrode 24 and the electrode 24' constitute a second electrode that defines the odd component.

In each of the above embodiments, asymmetric frequency characteristics can be constructed with a single electrode, so compared to the conventional variable pitch method or the mirror method (or reflection method) shown in FIG. 7, the frequency characteristics are much higher with the same electrode width. A frequency filter can be realized.

FIG. 11 shows yet another embodiment, which differs from the above two embodiments in that the electrodes 23 and 24 are configured in the form of split electrodes in order to have the effect of removing TTE. According to this embodiment, the electrode fingers of a pair of split electrodes can be configured to have the same length, and calculation errors are reduced compared to the conventional mirror method (or reflection method).

Although the electrodes in the above embodiments have very simple envelopes, the present invention is applicable to electrodes having any envelope. Moreover, the even component and odd component as used in this specification collectively refer to the even component and odd component in the odd-even function method, the symmetric component and the asymmetric component in the reflection method, and the like. moreover,
When the design is performed with the phase corrected, the maximum value of the crossing width may be on the odd function side, and in that case, the above-mentioned method may be applied on the even function side.

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

[Brief explanation of the drawing]

Fig. 1 is an impulse response characteristic diagram of a conventional variable pitch type electrode, Figs. 2 to 5 are diagrams used to explain the conventional example and the present invention, and Fig. 2 shows the characteristics of H ₁ (ω) and H ₂
(ω) frequency characteristic diagram, Figure 3 shows H _R (ω) and jH _I
(ω) frequency characteristic diagram, Figure 4 shows H _R (ω) and jH _I
(ω) impulse response characteristic diagram, Figure 5 shows H _R (ω)
Impulse response characteristic diagram that combines and jH _I (ω),
FIG. 6 is a diagram showing a conventional filter, FIG. 7a is a diagram showing another conventional filter, and FIG. 7b is a partially enlarged view.
FIG. 8, FIG. 9, FIG. 10, and FIG. 11 are diagrams each showing a filter according to the present invention, and FIG. 12 is an impulse response characteristic diagram used for explaining the present invention.

Claims (1)

[Scope of 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 a center frequency, wherein at least one electrode is weighted in cross width. A first interdigital electrode that defines an even component (or an odd component) of the frequency response characteristic, and a frequency response that is mainly arranged in a non-intersecting region of the first interdigital electrode and that weights the intersection width. the second which specifies the odd component (or even component) of the characteristic;
and an interdigital electrode, and the second interdigital electrode is configured such that at least one vibration source near the vibration source, which is the maximum crossing width of the first interdigital electrode, becomes zero. A surface acoustic wave filter characterized by: 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. 2. The surface acoustic wave filter according to item 1.