JPS583414B2 - surface acoustic wave filter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a surface acoustic wave filter, and more specifically, it is provided with an electrode consisting of a pair of comb-shaped electrode portions on each of the input and output sides on a piezoelectric body, and on the input and output sides of the piezoelectric body. The electrode on at least one side of the electrode is divided into two or more in the width direction of the electrode to form two or more surface wave propagation paths, and a phase difference is created between each surface wave signal in each surface wave propagation path. The present invention relates to a surface acoustic wave filter that can exhibit various excellent effects, such as being able to perform predetermined weighting, thereby making it possible to perform weighting that approximates theory.

It is known that a filter can be constructed by providing a surface wave propagation path on a piezoelectric body with a pair of input and output electrodes.

By the way, as a conventional surface acoustic wave filter, ■
There are several types, including those that use the electrode overlap width weighting method (apodization method), those that use the @ electric field weighting method, those that use the zero capacitance weighting method, and those that use the ○electrode spreading method. However, each of these had its own drawbacks as described below.

Note that in the above, "weighting" refers to a change in the surface wave (sound wave) generation intensity depending on the location.

First, as shown in Fig. 1, the electrode overlap width W (location where surface wave signals are generated) of the pair of comb-shaped electrodes 1 and 1' arranged across the piezoelectric body is measured on the surface. The filter characteristics are changed according to a predetermined pattern along the wave propagation direction Y, and weighting is performed based on this change to obtain a predetermined filter characteristic.

However, in the above case, the energy distribution of the surface waves in the direction perpendicular to the surface wave propagation direction Y, that is, in the vertical direction in the figure, is not uniform, so phase disturbance occurs in the surface wave signal.
The problem was that small ripples were generated in the bandpass signal.

Additionally, if the electrodes of the above method (■) are applied to both the input and output terminals, the design becomes extremely difficult in order to obtain the desired filter characteristics, so it is usually necessary to use only one of the input and output terminals. It could only be applied to one side.

In addition, as shown in FIG. 2 a, b, and c, the item D is as follows:
A pair of electrodes 2, 2' is divided into two parts (b in the same figure) or three parts (
It is constructed by dividing it into an appropriate number of parts as shown in C) of the same figure.

The electric field between the electrodes weakens in inverse proportion to the number of divisions.
Weighting is performed by taking advantage of the fact that the generated intensity of surface waves changes.

However, since the energy distribution of the surface waves in the direction perpendicular to the surface wave propagation direction Y is relatively uniform, the above can be applied as electrodes for both the input and output parts, but due to the division of the electrodes, the weight Since the attachment is also discontinuous, there is a large discrepancy between the actual characteristics and the theory, creating a problem in the design.

Furthermore, in the case of O, a capacitor of a different value is connected in series to each of the toothed portions of the comb-shaped electrode to vary the power supplied to each of the toothed portions, thereby changing the intensity of surface wave generation depending on the location. It was designed to let you do so.

However, since the above-mentioned device additionally connects a plurality of capacitors, it is difficult to manufacture the device, and furthermore, the cost is increased.

Next, the above-mentioned @ is designed to change the generated intensity of surface waves depending on the location by arranging electrodes at appropriate positions from the regular electrode structure.

However, since the change in the generated intensity becomes digital, it is extremely difficult to construct a filter with desired characteristics.

The present invention is intended to provide a surface acoustic wave filter capable of solving the various technical problems described above.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on an embodiment shown in the figure in which the present invention is applied to a sinX type bandpass filter.

In FIG. 3, the symbol S is a piezoelectric material formed of zinc oxide (Zno), etc., and E1 and E2 are electrodes on the input side and output side, respectively.These electrodes E1 and E2 are each made of a vacuum-deposited thin film of Al or the like, for example. It is formed by patterning.

Each of the electrodes E1 and E2 has a plurality of comb-shaped electrode portions E1'..., E2'..., E2'..., E2'...
A pair of comb-shaped electrodes are provided, respectively, and the electrodes E1 and E2 are configured such that the comb-shaped electrode portions E1' and E1'' and E2' and E2' are opposed to each other and intersect with each other.

In addition, in the case of Fig. 3, the input side has comb-shaped electrode parts E1' and E.
1' is divided into two parts in the width direction as shown by symbols A and B, and an electrode (hereinafter referred to as a weighting electrode) E1 which is weighted in a sin This shows a case where electrodes E2 are arranged with all the electrodes having the same interval between electrodes.

Note that when configuring a surface acoustic wave filter, it is not limited to the case where weighting electrodes are placed on the human power side and the regular electrodes are placed on the output side as described above, but both input and output electrodes are used. It is sufficient to dispose a weighting electrode only on at least one of them.

That is, contrary to the illustrated example in FIG. 3, the normal electrode E2 is placed on the input side.
Weighting electrodes E1 may be arranged on the output side, or both input and output electrodes may be configured with weighting electrodes E1.

The filter characteristics can be changed depending on the arrangement of the electrodes.

Next, referring to FIGS. 4a, b and 5, the above si
An electrode configuration that produces nX-type weighting will be described.

First, for convenience of explanation, FIG. 4 corresponds to a part of the electrodes shown in FIG. It corresponds to the part E1′, E1″ taken out, and also corresponds to b
corresponds to an arbitrary pair of comb-shaped electrode portions E3', E3' of a conventional regular electrode.

Note that in proceeding with the description, the electrode width of each comb-shaped electrode portion itself will not be taken into consideration.

Furthermore, in the same figure a, the reference numeral 1 in each section A and B, which is obtained by dividing the electrodes E1' and E1'' into two sections, indicates the surface wave propagating through each surface wave propagation path A and B with respect to the wavelength of the center frequency of the filter. This is a distance equivalent to a phase difference that shifts the wave signal by a predetermined phase θ (radians).

And each comb-shaped electrode part E1' in both sections A and B,
The arrangement position of B1'' is shifted to the left by 1 in the figure in the A section with respect to the arrangement position of the regular electrodes E3' and E3'', and on the other hand, it is shifted to the right in the B section. Deploy.

In other words, the surface wave propagation path in each section of AB! The surface wave signal propagating through the normal electrode portion is delayed by θ radians in the A section and advances by θ radians in the B section.

When the normal electrode E2 (broadband characteristic) is arranged on the output side as shown in Fig. 3, the output signal taken out from the output side electrode E2 propagates through the respective propagation paths A and B. This is the sum of the signal waves on both surfaces.

Therefore, each surface wave signal propagating through the surface wave propagation path in each section A and B can be expressed as follows: A section side: 1 /2 cos (ωt - θ) B section side: 1 /2 cos (ωt + θ) In this case, the output signal is as follows.

That is, 2 output signals = 1/2cos(ωt-θ) + 1/2cos
(ωt+θ) where the sign ω is the central angular frequency of the filter,
t is time.

Therefore, from the above output signal equation, if the value of θ is changed between 0 and π, cos θ is changed between 1 and −1.

In other words, it is possible to weight from 1 to -1.

Next, FIG. 5 shows five configuration examples of weighting electrodes as indicated by reference numeral E1 in FIG. The electrode period, n (o to 10) indicates the comb-shaped electrode portion, that is, the so-called comb number, and 11 to 110 indicate the phase difference equivalent distance, respectively.

Here, when L=1 and the total number of comb trees is N,
The phase difference equivalent distances 11 to 110 can be calculated using the following equations.

Based on each value of 11 to 110 calculated by the above formula, each comb-shaped electrode part is set as shown in FIG. With respect to the arrangement position of each electrode part in the regular electrode shown by the imaginary line, one section is shifted to the right in the figure, and the other section is arranged so as to be shifted to the left.

With this arrangement, a sinX type weighting electrode can be constructed.

FIG. 6 is a graph of the equation of History 3 above.

Further, when the above sinx type weighting is applied, the frequency characteristic K(jω) of the filter can be expressed by the following equation.

Here C is a constant.

FIG. 7 is a graph representing the above-mentioned equation for K(jω), and shows the ideal frequency characteristic when the weighting of sinX can be completely done.

In the same figure, the symbol f0 is the center frequency of the filter, and f1~
f2 is the bandwidth, and the center frequency f0 is determined by the electrode period L,
Further, the bandwidth f1 to f2 is approximately determined by the number N of electrodes, etc.

Then, the weighting electrode E constructed by the method described above
1 on the input side as shown in FIG. 3, a bandpass filter having desired characteristics can be constructed.

Note that the number of electrodes between x=0 and π is different between the comb-shaped electrode portions E1' and E2'' of the sinX-shaped weighting electrode E1 in FIG. 3 and the comb-shaped electrode portion of the same-shaped weighting electrode in FIG. can be set as appropriate when setting the filter characteristics arbitrarily.

Next, to describe the operation of the surface acoustic wave filter which is an embodiment of the present invention configured as described above, first, when an AC electric signal is input to the input side electrode E1, a surface wave signal is generated due to the piezoelectric effect of the piezoelectric body S. However, this surface wave signal propagates in a direction perpendicular to the comb-shaped electrode portions E1' and E2' (±X direction).

When the above-mentioned surface wave signal is generated, the input side electrode E1 is weighted in a sinX type.
The surface wave signals generated from the respective comb-shaped electrode portions E 1' and E 1'' are different from the surface wave signals generated from the regular electrodes by the phase difference θ set for each electrode portion.
Advances and delays occur in both categories A and B, respectively.

Then, as mentioned above, both surface wave signals having a phase difference generated in both sections A and B are transmitted to the output side electrode E, which has broadband characteristics.
2, and a signal within a predetermined frequency band f1 to f2 is extracted as an output according to the filter characteristics as shown in FIG. 7 specified by the surface acoustic wave filter.

Next, other embodiments of the present invention will be described with reference to FIGS. 8 to 11, respectively.

First, in the case of Fig. 8, the comb-shaped electrode parts E4', E4'' are connected to A'B.
', C'..., and the arrangement of the comb-shaped electrode portions between adjacent sections is alternately reversed by a phase difference equivalent distance 1 with respect to the arrangement position 3 of the regular electrode. They are arranged so as to be shifted in the direction (left and right in the figure).

If the electrodes E4' and E4'' are configured in a multi-segmented configuration as described above, the energy of the surface wave signal is uniformly distributed and propagated over the entire surface wave propagation path (the entire width in the direction perpendicular to the surface wave traveling direction). Therefore, this is an extremely suitable electrode structure when desired filter characteristics are obtained by disposing weighting electrodes on both the input and output sides.

Next, in the case of FIG. 9, the fifth case shown in FIG.
The A section and the B section of the weighting electrode E0 corresponding to the figure are A'' and B'' at the top and bottom in the figure, as shown in FIG.
The weighting electrode E5 is constructed by dividing the electrode into two parts as shown in FIG.

In the same figure (b), the weighting electrode E5 configured as described above is used as the input side electrode, and on the output side, corresponding to this input side electrode E5, the normal shape electrode is similarly divided into two parts AM and B''. This figure shows a surface acoustic wave filter configured by disposing the divided normal shape electrode E6.

S' is a piezoelectric material.

Desired filter characteristics can be achieved even by configuring the weighting electrodes etc. in a split type as described above. When configured in this way, the electrodes on the A// section side and the B" section side are equally divided. Even if they are formed on separate piezoelectric bodies and connected, they can have the same effect as those formed on the same piezoelectric body, so it is possible to provide diversity in the structure of the surface acoustic wave filter. Demonstrate the effect of what you can do.

The example shown in FIG. 10 shows a double electrode type weighting electrode DE in which each comb-shaped electrode part DE', DE' is formed of a double electrode (double electrode). By forming it with a double electrode, it is possible to effectively prevent electrode reflection in the case of a so-called single electrode.
This has the effect of further reducing deterioration of filter characteristics.

Furthermore, in the case of FIG. 11, as an example, a weighting electrode E7 is arranged on the input side of the piezoelectric body S', and a normal shape electrode E8 is arranged on the output side. This figure shows a surface acoustic wave filter with a multi-strip coupler in which a multi-strip coupler 5 is disposed between output electrodes E7 and E8.

The above-mentioned multi-strip coupler 5 is composed of a plurality of electrically isolated elongated metal strips, and when this multi-strip coupler is installed between input and output, Since it is possible to average the surface wave energy distribution in the direction perpendicular to the surface wave propagation direction (vertical direction in the figure), it is possible to obtain the desired filter characteristics by arranging weighting electrodes on both the input and output sides. The present invention is extremely suitable for use in the present invention, and also has the effect of reducing bulk wave spurious and making it possible to construct a filter with excellent characteristics.

Note that the weighting electrodes in the above-mentioned embodiment are sin x
Although only the weighting type has been described, the present invention is not limited to the sin x type, but can also be applied to other weighting types such as the (sin x/X)2 type.

Furthermore, in the above embodiment, ZnO is used as the material of the piezoelectric body.
However, not only ZnO but also lithium niobate (
Other piezoelectric materials such as LiNbO3), aluminum nitride (AIN), cadmium sulfide (CdS), and zinc sulfide (ZnS) may also be applied;
It may be formed in the form of a thin film on a substrate such as AI2O3.

As described in detail above, according to the present invention, each of the input and output sides of the piezoelectric body is provided with an electrode consisting of a pair of intersecting comb-shaped electrode portions, and at least one of the input and output sides The electrode on this side is divided into two or more parts in the width direction of the electrode to form two or more surface wave propagation paths, and a phase difference is created between the respective surface wave signals in each surface wave propagation path to provide a predetermined phase difference. Since the weighting is performed in such a manner that the weighting is performed with a higher degree of approximation to the theoretical design value than in the conventional example, it is possible to obtain a filter with desired characteristics, which is an excellent effect.

In addition, the comb-shaped electrodes are not arranged at equal intervals, but are arranged at different intervals corresponding to the distance equivalent to the phase difference, which reduces electrode reflection of surface waves and reduces ripples in bandpass signals. It has an excellent effect of reducing

Furthermore, since the energy distribution of the surface waves in the direction perpendicular to the propagation direction of the surface waves can be made uniform, a surface acoustic wave filter with excellent efficiency can be constructed even when weighting electrodes are provided on both the input and output sides. This has the excellent effect of allowing for greater diversity in the design of filter characteristics.

Furthermore, compared to conventional apodized electrodes, surface acoustic wave generation efficiency is higher, and the filter shape can be made smaller, thereby providing a surface acoustic wave filter with excellent usability. It's something you get.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a conventional apodized weighting electrode, FIGS. 2a, b, and c are partial plan views for explaining an electric field weighting electrode, which is also a conventional example, and FIG. 3 is a plan view of a conventional apodized weighting electrode. Plan view of a surface acoustic wave filter as an example, FIG. 4a
, b is a partial plan view for explaining the construction method of the weighting electrode applied to the surface acoustic wave filter as above, in which figure a shows the weighting electrode, figure b shows the normal electrode, and figure 5 shows the same as the above. Figure 6 is a plan view showing an example of a shape weighting electrode configured based on the construction method, Figure 6 is a characteristic curve diagram showing a graph of the formula, and Figure 7 is a diagram showing a weighting electrode weighted in the same shape as above. A characteristic diagram showing frequency characteristics? 8 to 11 respectively show other embodiments of the present invention or examples of electrodes applied to other embodiments, and FIG. Figure 9 is a plan view for explaining an example of a surface acoustic wave filter consisting of segmented electrodes. Figure a is a non-divided weighting electrode (corresponding to Figure 5), and Figure b is a plan view for each of the input and output sides. A surface acoustic wave filter configured by arranging a divided weighting electrode and a divided normal shape electrode is shown. FIG. 10 is a plan view of a main part showing an example of a dual electrode type weighting electrode, and FIG. 11 is a plan view of a surface acoustic wave filter with a multi-strip force puller. A, B, A'~E'. A'', B//,
A', B': Electrode classification, E0, El, E4
, E7: Weighting electrode, E2, E3 'Normal electrode, E1', E1'', E4', E,''
: Comb-shaped electrode part E2', E in the weighting electrode
2": comb-shaped electrode part in the regular electrode, E,: split weighting electrode, E5', E5": comb-shaped electrode part in the same electrode as above, E6: split regular electrode, E6', E6": comb shape in the same electrode Electrode part, DE: double electrode type weighting electrode, DE', DE'': comb-shaped electrode part in the same electrode as above, S, S', S'': piezoelectric body, 5: multi-strip coupler.

Claims (1)

[Scope of Claims] 1. Electrodes formed by opposing and intersecting a pair of comb-shaped electrodes each having two parts of a plurality of comb-shaped electrodes are disposed on the input and output sides of the piezoelectric body, and the input and output sides 2 electrodes on at least one side of the electrode in the width direction of the electrode.
The above divisions form two or more surface wave propagation paths, and the arrangement position of each comb-shaped electrode part in each division is determined according to the distance from the comb-shaped electrode part located at the center. The position of each electrode part in the regular electrode is shifted by a distance corresponding to the required phase difference for the signal, and the direction of this position shift is opposite to each other between the adjacent sections, and each of the above-mentioned A surface acoustic wave filter characterized in that a phase difference is provided between each surface wave signal in a surface wave propagation path and predetermined weighting is performed. 2. The surface acoustic wave filter according to claim 1, wherein the weighting is siax (X is a distance in the surface wave propagation direction). 3. The surface acoustic wave filter according to claim 1 or 2, wherein the number of sections in the width direction of the electrode is two. 4. The surface acoustic wave filter according to claim 1, 2, or 3, wherein the comb-shaped electrode portion is constituted by a double electrode.