JPH0244171B2 - DANSEIHYOMENHAFUIRUTA - 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) along the surface acoustic wave propagation direction. This is a so-called variable pitch interdigital electrode.
It is as described below. That is, when the frequency response characteristic is inversely Fourier transformed, for example, the first
The impulse response shown in the figure 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 between adjacent electrode fingers (surface wave excitation/reception area) 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 the peak points in .
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 techniques 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. Here, let the even component be H _R (ω) and the odd component H _I (ω), and if H _R (ω) and H _I (ω) are defined as follows, their functions are 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)− _j h _I (t) =∫H _R (ω)e ^j2 〓 ^ft df +∫− _j H _I (ω)e ^j2 〓 ^ft df (4).

The impulse responses represented by h _R (t) and _−j h _I (t) in equation (4) are shown as the solid line and broken line in FIG. 4, respectively. The two impulse response curves in the same figure are uniform in that the time between their peak points is 1/2f ₀ (λ ₀ / 2 when expressed in terms of wavelength), and the peak points of both curves are equal to each other's peak points. Located in the middle. 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 electrode pattern shown in Fig. 6 consists of interdigital electrodes divided into two stages based on the two impulse responses shown in Fig. 4, and electrically connected in parallel.
This is disclosed in "A Design Method for Surface Acoustic Wave Filters" by Nakamura and Shimizu (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 2 exciting and receiving the even component and electrode 3 exciting and receiving the odd component. The other interdigital electrode 4 is formed so as to cover the propagation path of the two electrodes 2 and 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, 9 with an electrode width of 1/8λ ₀ arranged at an electrode pitch of 1/4λ ₀ , and two adjacent main electrodes Fingers 6 and 7, 8 and 9 are connected at common parts with different potentials, and the lengths of these two main electrode fingers are different. It is formed by arranging auxiliary electrode fingers 10, 11, 12, and 13 having a width of 1/8λ ₀ at an electrode pitch of 1/4λ ₀ and connected to a common part of different potentials. According to this interdigital electrode, an even component is excited and received in the region where the adjacent main electrode fingers 7 and 8 of different potentials intersect (upper right region), and the adjacent main electrode fingers 6 and 9 and the auxiliary electrode finger 11 , 12 (down-right region), the odd component is excited and received with a distance of λ/4 from the even component. By using such interdigital electrodes, 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 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 reduce the influence of excitation and vibration between electrode fingers 6 and 8 and between electrode fingers 7 and 9 to a negligible extent, the finger tips of electrode fingers 7, 11, 8, and 12 located between them are brought close to each other and crossed. If the shaded area is made smaller, there is a risk that both figures 7 and 11, 8 and 12 will be short-circuited at their tips during pattern formation.

The latter reflection method or mirror method is
Letting the center frequency of a given frequency characteristic be f ₀ ,
Line symmetry with respect to 2f ₀ . This method assumes a virtual image with a center frequency of 3f 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 the same as in Figures 6 and 7 a and b. It has been decided to do so, and has the same problems as mentioned above.

The present invention eliminates all the drawbacks of the prior art described above, can be configured with the same substrate dimensions as the case shown in FIG. It is an object of the present invention to provide a surface acoustic wave filter which can be obtained using a split electrode or a single electrode without the complexity of calculation which was a major drawback of the mirror method.

That is, the present invention provides 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 of the input and output side electrodes is
A first interdigital electrode that performs intersection width weighting to define an even component of the frequency response characteristic, a common electrode formed along the envelope of the first interdigital electrode, and a second interdigital electrode with this common electrode sandwiched therebetween. 1
On the side opposite to the first interdigital electrode, a relatively large portion of the first interdigital electrode is disposed in the non-intersecting region, and the first interdigital electrode is weighted by the intersection width and defines the odd component of the frequency response characteristic. 2
an interdigital electrode, an electrode finger connected to one potential of the first and second interdigital electrodes is connected to the common electrode, and the other potential of the first and second interdigital electrodes is connected to the common electrode. The electrode fingers connected to each other are electrically connected to each other.

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. On the other hand, the interdigital electrode 21 is composed of first and second interdigital electrodes 23 and 24. The first electrode 23 is cross-width weighted in the usual manner based on the impulse response (solid line) that defines the even component in FIG. It is formed approximately along the envelope of The second electrode 24 is the first electrode 23
It is formed outside the common electrode portion 23b, that is, in a non-intersecting region, and in a region including the propagation path of the first electrode 23 or a range slightly protruding from it. This second electrode 24 is a common electrode part 23b of the first electrode 23.
and a separate common electrode part 24a formed close to the maximum crossing width part of the first electrode 23, with electrode fingers protruding from the odd component (dashed line) in FIG.
Cross-width weighting is applied in the usual way based on the impulse response that defines . Common electrode part 24a of the second electrode 24 and common electrode part 23 of the first electrode 23
a and is coupled to each other by a shield electrode 25. Terminal electrodes 26, 27, 28, 29 are connected to the substrate 20
, and are connected to predetermined common electrode portions, respectively.

FIG. 9 shows another embodiment, and the difference from the above embodiment is that the second electrode constituting the odd component is
This is because it was formed on both sides of 3. That is, another common electrode portion 23a of the first electrode 23
Another common electrode part 24a' is provided which is curved so as to almost follow the weighted envelope and extends near the maximum crossing 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. That is, in the embodiment of FIG. 9, when focusing on the common electrode part 23b along one envelope of the first electrode 23, the first and second electrodes 23, which are connected to a different potential from the common electrode part 23b, 24 electrode fingers are connected to the common electrode part 23a and the common electrode part 24a,
These common electrode portions 23a and 24a are connected to each other via a shield electrode 25, and are further electrically connected to a terminal electrode 26. Further, when focusing on the common electrode section 23a along the other envelope of the first electrode 23, the common electrode section of the first and second electrodes 23 and 24' arranged on both sides of the common electrode section 23a Electrode fingers connected to a different potential from 23a are electrically connected to each other at terminal electrode 27 via common electrode section 23b and common electrode section 24a'. That is, in the embodiment of FIG. 9, the configuration of the present invention is also provided on the common electrode portion 23a side provided along the other envelope of the first electrode 23.

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

FIG. 10 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 the calculation difference is 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.

As explained above, according to the present invention, the desired frequency characteristics can be reliably obtained without errors even with the same substrate dimensions as the mirror method, and the complicated calculations at the time of design can be reduced, and even with a single electrode. A split electrode can also be used.

[Brief explanation of drawings]

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 _j H _I
(ω) frequency characteristic diagram, Figure 4 shows H _R (ω) and _j H _I
(ω) impulse response characteristic diagram, Figure 5 shows H _R (ω)
Impulse response characteristic diagram that combines and _j H _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.
FIGS. 8, 9 and 10 each illustrate a filter according to 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 of the input and output side electrodes is crossed. a first interdigital electrode that performs width weighting to define an even component of the frequency response characteristic; a common electrode formed along the envelope of the first interdigital electrode; and a first interdigital electrode with the common electrode in between. On the side opposite to the interdigital electrodes, a relatively large portion is arranged in the non-intersecting region of the first interdigital electrode, and intersection width weighting is applied to define the odd component of the frequency response characteristic. a second interdigital electrode; an electrode finger connected to one potential of the first and second interdigital electrodes is connected to the common electrode; A surface acoustic wave filter characterized in that electrode fingers connected to the other potential are electrically connected to each other.