JPH11205072A - Weighting electrode for surface acoustic wave filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a weighing electrode with few deteriorations in an amplitude characteristic by means of a surface acoustic wave diffraction effect, even when a same weighting coefficient is repeatedly used twice in weighing coefficient data of a λ(surface acoustic wave length)/4-interval sampling. SOLUTION: Intersecting parts are provided in two electrode fingers 11 and 14 by λ/2-interval and a floating electrode 15 is provided in a part corresponding to the intersecting parts at a position at equal distance from the centers of the two electrode fingers. The size of surface acoustic wave excited in the intersecting parts which are constituted by the both adjacent electrode fingers 11 and 14 and the float electrode 15 is equal and arbitrary weighing coefficient where the same weighting coefficient is repeated twice by λ/4-interval sampling can be expressed. Also the intersecting parts of the both adjacent electrode fingers 11 and 14 and the float electrode 15 are the same in positions in a direction being vertical with respect to a surface acoustic wave propagating direction so that the width of the intersecting part corresponding to the max. weighting coefficient is made to be provided with opening length W. The width of the other intersecting part is widened so that the deterioration of the filter characteristic by the diffraction effect is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface acoustic wave filter, and more particularly, to a weighting electrode of a surface acoustic wave filter.

[0002]

2. Description of the Related Art As a weighting electrode used in a conventional surface acoustic wave filter, an apodized electrode whose crossing width as shown in FIG. 3 is changed in proportion to a weighting function depending on a location is mainly used. In the figure, 41 and 42 are bus electrodes, 31 to 34, 36 and 37 are electrode fingers, W is an opening length, w1 and w2 are cross lengths, and λ is a wavelength of a surface acoustic wave. Examples of this type of electrode are disclosed in JP-A-60-264110, JP-A-5-63488, and JP-A-7-106.
908, JP-A-6-29771 and JP-A-3
-108808.

[0003]

In the conventional apodized electrode shown in FIG. 3, the same weighting coefficient is repeatedly used twice for weighting coefficient data of λ / 4 interval sampling. In this case, since the maximum weighting coefficient is used twice in succession with the same sign, the cross width corresponding to the maximum weighting coefficient is about の of the opening length W of the apodized electrode, and the other weighting coefficients correspond. Since the cross width is also narrow, there is a disadvantage that the amplitude characteristic of the surface acoustic wave filter is greatly deteriorated due to the diffraction effect of the surface acoustic wave.

The present invention has been made in consideration of the above circumstances, and in the case where the same weighting coefficient is used twice repeatedly with the weighting coefficient data of λ / 4 interval sampling, the deterioration of the amplitude characteristic due to the diffraction effect of the surface acoustic wave. It is an object of the present invention to provide a weighting electrode for a surface acoustic wave filter having a small number of components.

[0005]

According to a first aspect of the present invention, there is provided a weighting electrode for a surface acoustic wave filter, wherein the weighting electrode for a surface acoustic wave filter is composed of interdigital electrodes on a piezoelectric substrate. Adjacent to each other, and having a plurality of electrode fingers connected to the bus electrode, between each of the plurality of electrode fingers adjacent to each other, from the electrode finger adjacent to one side and the electrode finger adjacent to the other side. Floating electrodes are provided at equidistant positions and at positions corresponding to intersections of electrode fingers adjacent to one side and electrode fingers adjacent to the other side. According to a second aspect of the present invention, in the first aspect, the floating electrode is included in the same plane as a plane including the electrode fingers. According to a third aspect of the present invention, in the first or second aspect, the floating electrode is provided at a distance from the electrode finger in the plane.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a plan view of a weighting electrode for a surface acoustic wave filter according to an embodiment. The surface acoustic wave filter weighting electrode has a plurality of electrode fingers 11 to 14 adjacent to each other at an interval of λ / 2, and has a floating electrode 15 disposed between the electrode fingers 11 to 14. The electrode fingers 11 and 13 are connected to the bus electrode 21, and the electrode fingers 12 and 14 are connected to the bus electrode 22.

The floating electrode 15 is connected to the electrode finger 1 on the left.
They are provided at positions equidistant from the electrode fingers 1 and 12 and the right electrode fingers 13 and 14 and at positions corresponding to the intersections of the left electrode finger 11 and the right electrode finger 14. The electrode finger 1
Reference numerals 6 and 17 denote dummy electrode fingers.
6 is connected to one bus electrode 21, and the other dummy electrode finger 17 is connected to the other bus electrode 22.

Next, the operation will be described with reference to FIGS. The surface acoustic wave is excited by applying a voltage between two electrode fingers having a crossing, and its magnitude is proportional to the capacitance of the crossing and is also proportional to the applied voltage. Therefore, the magnitude of the surface acoustic wave to be excited is proportional to the amount of charge stored in the intersection.
The polarity of the surface acoustic wave to be excited is determined by the direction of the electric field due to the applied voltage.

The electrode finger 11 and the floating electrode 15 have an intersection having a width w1. Similarly, the floating electrode 15 and the electrode finger 14 have an intersection having a width w2. Are set equal. In addition, the floating electrode 15 is
And the electrode finger 14 at the same distance. Therefore, the capacitance of these intersections becomes equal.

When a voltage is applied between the bus electrode 21 and the bus electrode 22, that is, between the electrode finger 11 and the electrode finger 14, since the capacitance of these intersections is equal, these intersections are formed. The amounts of charges stored in the sections become equal. Also,
Since the direction of the electric field at these intersections is the same, the magnitude k1 of the surface acoustic wave excited by the electrode finger 11 and the floating electrode 15 and the elastic surface wave excited by the floating electrode 15 and the electrode finger 14 The wave magnitudes k2 are equal. And
The same applies to the magnitude of the surface acoustic wave excited after the other taps, and the magnitude of the surface acoustic wave excited at each tap, that is, the weighting coefficient k1, k2, k3, k of each tap.
4 is as shown in FIG.

[0011] Therefore, when sampling at λ / 4 interval, 2
It is possible to express an arbitrary weighting coefficient in which the same weighting coefficient is repeated twice. Further, since each of the intersections formed by the floating electrode 15 and the electrode fingers 11 and 14 on both sides thereof has the same position in the direction perpendicular to the propagation direction of the surface acoustic wave, the intersection of the intersection corresponding to the maximum weighting coefficient is used. Since the width can be set to the opening length W and the width of the other intersections can be increased, the deterioration of the filter characteristics due to the diffraction effect can be reduced.

[0012]

As described above, according to the present invention,
Since the positions of the intersections formed by the floating electrode and the electrode fingers on both sides thereof are the same in the direction perpendicular to the propagation direction of the surface acoustic wave, the width of the intersection corresponding to the maximum weighting coefficient is defined as the opening length W. The width can be made the same, and the width of the other crossing portions can be widened, whereby the deterioration of the filter characteristics due to the diffraction effect, which occurs in the conventional apodized electrode, can be reduced.

[Brief description of the drawings]

FIG. 1 is a plan view showing one embodiment of a weighting electrode for a surface acoustic wave filter according to the present invention.

FIG. 2 is an explanatory diagram showing weighting coefficients of one embodiment of a weighting electrode for a surface acoustic wave filter according to the present invention.

FIG. 3 is a plan view showing a conventional apodized electrode.

[Explanation of symbols]

λ: wavelength of surface acoustic wave 11 to 14, 16, 17, 31 to 34, 36, 37: electrode finger 15: floating electrode 21 to 22, 41 to 42: bus electrode W: opening length w1, w2: cross width k1 , K2, k3, k4 ... weighting factors

Claims (3)

[Claims] 1. A weighting electrode for a surface acoustic wave filter comprising interdigital electrodes on a piezoelectric substrate, wherein, when the wavelength of the surface acoustic wave is λ, they are adjacent to each other at an interval of λ / 2, and a bus electrode. Having a plurality of electrode fingers connected to each other, between each of the plurality of electrode fingers adjacent to each other, at a position equidistant from the electrode finger adjacent to one side and the electrode finger adjacent to the other side, and A weighting electrode for a surface acoustic wave filter, wherein a floating electrode is provided at a position corresponding to an intersection of an electrode finger adjacent to one side and an electrode finger adjacent to the other side. 2. The weighting electrode for a surface acoustic wave filter according to claim 1, wherein the floating electrode is included in the same plane as a plane including the electrode fingers. 3. The weighting electrode for a surface acoustic wave filter according to claim 1, wherein the floating electrode is provided at a distance from the electrode finger in the plane.