JPH03238909A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To suppress the effect of the diffraction effect of simple electrode structure by expanding a cross width of an electrode finger opposite to each other so as to suppress the effect of a diffraction wave without changing a surface acoustic wave exciting strength between the electrode finger pairs opposite to each other. CONSTITUTION:Electrode fingers 1, 3 prolonged from upper and lower buses 4, 5 and a stray electrode 2 have cross parts whose cross width is W1 and W2 independently electrically of an adjacent opposite electrode. Since a level of a floating electrode 2 has an intermediate level between a level of the electrode finger 1 and a level of the electrode finger 3, the phase of a surface acoustic wave element is an exciting phase obtained at the two cross parts whose cross width is W1 and W2 to give excitation of the strength proportional to the cross width W1 and W2. Thus, the excitation of the surface acoustic wave of the desired strength is obtained by selecting properly the cross width W1 and W2 because the surface acoustic wave excited by the floating electrode 2 and the electrode fingers 1, 3 is propagated while being cancelled together. Thus, the diffraction effect is suppressed.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention relates to a surface acoustic wave filter that suppresses the influence of diffracted waves caused by narrow crossing width portions when cross width weighting is applied. .

[Prior Art] In order to obtain desired frequency characteristics, some surface acoustic wave filters use interdigital electrodes in which the width of intersection of pairs of opposing electrode fingers is weighted. FIG. 3 is a plan view of an example of such a conventional interdigital electrode, and in FIG. 1, numerals 4 and 5 are upper and lower common busbars that connect the respective groups of electrode fingers that connect the opposing electrodes. At the intersection of the electrode fingers of each counter electrode, surface acoustic waves with an intensity proportional to the width of each intersection are excited. Elastic garment g excited between a pair of electrode fingers with a narrow crossing width
There is a problem in that the power intensity of the waves is concentrated only at short intersections in the length direction of the opposing electrodes, resulting in the influence of single diffracted waves.

As a countermeasure to the above problem, Japanese Patent Laid-Open No. 50-24050 discloses that the opposing IE [I is 2 in the length direction! It is disclosed that a counter electrode section is formed by connecting floating intermediate electrodes obtained by dividing i or three or more in series and connecting them in parallel. FIG. 4 shows that by this technique, the electrode films were arranged almost uniformly in the excitation part.

A plan view of the blind-shaped tIi is shown. This technique has the advantage that the power intensity distribution is approximately uniform in the length direction of the opposing electrode.

[Problems to be Solved by the Invention] However, with the above conventional technology, when the electrode structure becomes am and the number of divisions increases, only a modest excitation power can be obtained from a wide electrode area, and a piezoelectric substrate with a large area is required. There are problems such as becoming

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the above-mentioned conventional techniques and to provide a surface acoustic wave filter that has a relatively simple one-pole structure and suppresses the influence of diffraction effects.

[Means for solving all] In order to achieve the above object in the present invention.

In the conventional method of surface acoustic wave filters, there is a floating electrode finger that is electrically separated from the adjacent electrode at the point where the crossing width of the pair of opposing electrode fingers becomes narrow.

a pair of upper and lower electrode fingers extending from each of the upper and lower common busbars connecting each of the electrode finger groups constituting the counter electrode;
Two upper and lower intersections having different intersection widths are formed.

[Effect] The floating electrode has an intermediate potential between the upper and lower electrodes tW.

As a result, the directions of the electric fields generated at the two upper and lower intersections are opposite to each other, so the surface acoustic waves excited at each of the two intersections cancel each other out, resulting in a narrow intersection width corresponding to the difference in the intersection width between the two intersections. The intensity is similar to that of a conventional surface acoustic wave excited at one intersection between opposing electrode fingers. Since the intersection width of each of the two intersections can be made longer than in the conventional case, the influence of the single diffracted wave can be suppressed.

[Example] 1st JI shows the plane @ of the interdigital electrode of the present invention-implementation man, electrode fingers 1 and 3 extending from the upper and lower common bus lines 4 and 5
The floating electrode 2 and the floating electrode 2 form an intersection having intersection widths Wl and W at two locations, electrically independent of the counter electrode in contact with II. Since the potential of the floating electric current li2 has an intermediate potential between the potential of the electrode finger l and the potential of the electrode finger 3, the crossing width W and w
The phases of the excitation of the surface acoustic waves obtained at the intersections of the two locations in 2 are opposite in phase, giving excitations with intensities proportional to the intersection widths W and W□, respectively. Therefore, the surface acoustic waves excited by the floating electrode 2 and the electrode fingers 1 and 3 are
Since the surface acoustic waves excited at Il are canceled and propagated, by appropriately selecting the intersections 11wt and w2, it is possible to obtain surface acoustic wave excitation with a desired intensity.

2nd @ shows a plan view of the interdigital electrode of another embodiment of the invention using split electrodes, the floating electrode 7 intersects only with the electrode fingers 6 and 8 extending from the common electrodes 4 and 5 . In this way, each floating electrode intersects only the electrode fingers of IIIL extending from the upper and lower common roots 1[4 and 5,
11, so the degree of freedom in selecting the crossing width is further increased. In FIG. 5, the frequency characteristic of the insertion loss of the filter of this embodiment is shown by a solid line, and the frequency characteristic of the conventional filter using interdigital Sa is shown by a broken line. As described above, according to this embodiment, the degree of suppression of frequency characteristics outside the high frequency band is improved by 5 dB compared to the conventional one.

[Effects of the Invention] As explained above, according to the present invention, without changing the desired surface acoustic wave excitation intensity, the conventional cross-width weighting is applied at the portion where the cross-width becomes narrow in the electrode, and the weighting of the opposing electrode fingers is Since the intersection width can be expanded, the effect of suppressing the 9-diffraction effect can be obtained.

Furthermore, even if the externally applied voltage is increased, the crossing width of the opposing electrode fingers can be increased, thereby suppressing the single diffraction effect and achieving the effect of reducing the chip size.

[Brief explanation of drawings]

Fig. 1 is a plan view of a transducer-shaped surface acoustic wave filter according to an embodiment of the present invention; Fig. 2 is a plan view of a transducer-shaped electrode of a surface acoustic wave filter according to an embodiment of the present invention;
The figure and the fourth wing are plan views of interdigital electrodes of a conventional surface acoustic wave filter, respectively different from each other. FIG. 5 is a diagram showing a comparison of the insertion loss frequency characteristics of the example filter shown in FIG. 2 and the conventional filter. 3.6.8... Electrode fingers extending from the common bus bar, 2.
7...Floating side 1 4.5...Common bus bar.

Claims (4)

[Claims] 1. In a surface acoustic wave filter in which a plurality of interdigital electrodes are disposed on a piezoelectric substrate for input and output, and the intersecting width of the opposing electrode fingers of at least one of these interdigital electrodes is weighted, the intersecting width of the opposing electrode fingers is weighted. A surface acoustic wave filter characterized in that the influence of diffracted waves is suppressed by expanding the crossing width of a pair of opposing electrode fingers without changing the surface acoustic wave excitation intensity between the pairs. 2. 2. The surface acoustic wave according to claim 1, wherein the intersection between a pair of opposing electrode fingers that are electrically independent from adjacent opposing electrodes is divided into two intersections in the length direction of the electrode fingers. filter. 3. Floating electrode fingers electrically isolated from adjacent electrodes
2. A pair of upper and lower electrode fingers each extending from an upper and lower common generatrix that connects each of the electrode finger groups constituting the counter electrode, forming two upper and lower intersection portions having different intersection widths. surface acoustic wave filter. 4. 3. The surface acoustic wave filter according to claim 2, wherein the interdigital electrodes are of a split type.