JPS61154212A - Surface acoustic wave filter - Google Patents

Abstract

PURPOSE:To suppress a spurious surface wave without giving adverse effect on the excitation for a prescribed surface wave by constituting the remote end electrode finger at the opposite electrode side of a weight electrode with electrde fingers formed by connecting electrically plural lectrode fingers with a specific interval. CONSTITUTION:A weighted comb-line electrode 2 being an input electrode is formed on a piezoelectric substrate 1 and a normal comb-line electrode 3 being an output electrode is formed from the electrode 2 with a prescribed interval. An elctroder finger 7 of the side of the opposite electrode 3 in the remotest electrode finger pairs 6, 7 in the hot side comb-line electrode 4 of the weighted electrode 2 consists of an electrode finger part 7a of the electrode 2 arranged with a prescribed pitch and exciting a desired surface wave and an electrode finger part 7b shifted toward the opposite electrode 3f by #10/2 in the propagat ing direction of the wave from the finger 7a and not contributing to the excita tion of the surface wave. the electrode fingers 7a, 7b have a nearly equal length a/2, the total length (a) is formed nearly equal to one electrode finger and they are connected electrically.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a surface acoustic wave filter, and more particularly to a surface acoustic wave filter with flat passband characteristics and a wide fractional bandwidth.

(Prior art) In general, a surface acoustic wave filter has input and output side comb-like electrodes formed on a piezoelectric substrate, separated by a predetermined distance, and utilizes surface waves propagating from the input side electrode to the output side electrode. In particular, the frequency characteristics based on the electrode shape are used to realize various characteristics required for, for example, a VIP filter for a TV. In such a surface acoustic wave filter, in addition to desired surface waves, TTE, edge reflected waves,
Unnecessary waves such as bulk waves are also generated. Conventionally, in order to eliminate the influence of unnecessary waves, each device had a split electrode structure, was coated with sound-absorbing material, or had its bottom surface processed.

However, unlike TV VIP filters, for example, the center frequency is 70 MHz and the 3 dB passband width is 23 MHz.
When configuring a filter with a very wide fractional bandwidth such as Hz, in addition to the above-mentioned unnecessary waves, spurious waves called surface wave type spurious waves become relatively large, and within the passband, for example, 1NbOG So fo = 70M days 7.3 (
jB Bandwidth -23M+-111 Propagation distance -1, θII
III7), the amplitude is 0.9d B .

Large undulations with a period of about 6M1-1z can be seen. This is due to the wide fractional bandwidth, which reduces the number of logarithms of the electrode, resulting in a smaller inductance, which results in a smaller inductance than the desired surface wave. Type spurious becomes relatively large, and interference due to phase shift due to difference in propagation distance between two waves occurs.
This is because it becomes noticeable.

A means for reducing such surface wave type spurious is disclosed in, for example, Japanese Patent Laid-Open No. 122215/1983, and a representative example thereof is shown in FIG. FIG. 4 shows electrode fingers 2, 22, 23, and 2 having different lengths at the ends of the comb-like electrode.
4 are arranged in the wave propagation direction so that the repetition period is λ/4.

With this configuration, the unnecessary surface wave responses caused by the hatched portions of the electrode fingers 21 and 23 have a difference of λ/2, so that they are in opposite phases and cancel each other out.

Unwanted surface wave responses caused by the shaded portions of electrode fingers 22, 24 are similarly canceled out.

(Problems to be Solved by the Invention) However, in the configuration shown in FIG. 4, the electrode fingers 2, which have different qualities,
22, 23, and 24 are formed along the wave propagation direction, the propagation time of the surface waves differs in the direction perpendicular to the propagation direction, resulting in a curved wave front and a worsening of the frequency characteristics. have.

(Means for resolving the problem) For weighting, connect the extreme electrode fingers on the opposing electrode side to each other (N+
1/2) It is composed of electrode fingers formed by electrically connecting a plurality of electrode finger parts separated by a distance of λ0, and the weighting of the plurality of electrode finger parts is from the odd-numbered electrode finger from the electrode side. This is a surface acoustic wave filter in which the sum of the finger lengths of the sections and the sum of the finger lengths of even-numbered electrode finger sections are approximately equal.

(Function) Due to the extreme electrode fingers shifted by (N+1/2)λO, unnecessary waves seen from the opposing electrodes have equal magnitudes and mutually opposite phases, so that they cancel each other out on the opposing electrode side. Moreover, the ratio of the area with electrodes and the area without electrodes does not change due to this endmost electrode finger.

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

In FIG. 1, on a piezoelectric substrate 1, for example, a weighted comb-shaped electrode 2, which serves as an input electrode, is formed, and a regular comb-shaped electrode 3, which serves as an output electrode, is formed at a predetermined distance from this electrode 2. . The weight is that the electrode 2 has predetermined pass characteristics, for example, a center frequency of 70 MHz and a 3 dB pass band width of 2.
In order to obtain the transmission characteristics of 3M l-1z, a cross-top pattern is applied, and a split electrode consisting of a pair of electrode fingers is used for the purpose of TTE removal. This weighting is such that among the pair of extreme electrode fingers 6.7 on the counter electrode 3 side of the hot-side comb-tooth electrode 4 of the electrode 2, the electrode fingers 7 on the counter electrode 3 side are arranged at a predetermined pitch so that they fall on the electrode 2. An electrode finger portion 7a that is oriented and excites the desired surface wave, and an electrode finger portion 7b that is shifted toward the J counter electrode 3 side and does not contribute to the excitation of the surface wave. It consists of

In addition, these electrode finger portions 7a and 7b have approximately equal lengths a/2, are configured such that the total length a is approximately equal to one electrode finger, and are electrically connected. has been done. In this embodiment, even in the ground-side comb-like electrode 5 of the electrode 2, the outer electrode finger 9 of the outermost electrode finger pair 8.9 is the uppermost electrode finger portion. Similar to a and 7b, two electrode finger portions 9a separated by a distance of λ0/2
, 9b.

With this configuration, the surface waveform spurious generated from the endmost electrode finger 7 is generated at the output side electrode with a difference between the spurious based on the electrode finger portion 7a and the spurious based on the electrode finger portion 7b, which corresponds to λ0/2. Hail to 3, so
They will cancel each other out. Further, although the spurious level is relatively small at the terminal end of the comb-shaped electrode 5 on the ground side, it is canceled out in the same way. For example, in the past, waviness with an amplitude width of 0.9 dB and a period of about 6M1-1z was generated within the passband, but according to this embodiment, the waviness is 0.5 dB.
dBPl [waviness, and practically sufficient flatness is 1q
It will be done. Moreover, since the electrode finger portion 7a located on the center side opposite to the electrode bus bar also contributes to the original excitation of the surface wave as a sub-electrode, the pass band can be flattened without adversely affecting the frequency characteristics. can be measured.

FIG. 2 shows a second embodiment, which differs from the above embodiments in that a dummy electrode finger is used as the endmost electrode finger.

That is, the electrode 2 has a dummy end electrode finger 10.11 added to the end of the electrode 2 on the side of the counter electrode 2, that is, further outside the electrode finger 7.9 closest to the counter electrode 2. It is being This endmost dummy electrode finger 10.11 has approximately the same electrode finger length as the adjacent electrode finger 7.9, and has approximately the same length as the adjacent electrode finger 7.9, and λo/2 in the wave propagation direction at approximately the center thereof.
It is constructed by bending at a distance of .

FIG. 3 shows a third embodiment, in which the endmost electrode fingers 12.13 each have three horn electrode finger portions 12a, 12b, 1.
2c.

and 13a, 13b, and 13c.

Electrode finger portions 12a, 12b, and 12c are finger portion 1
Finger portions 12b and 12c are arranged successively from 2a toward the counter electrode 3 side at intervals of λ0/2. Similarly, the electrode finger portions 13a, 13b, and 13c on the ground side are also spaced at intervals of λo/2. It is composed of

(Effects) According to the present invention, even in a filter with a wide fractional bandwidth, surface waveform spurious can be suppressed without adversely affecting the excitation of a predetermined surface wave, and a flatter passband characteristic can be obtained. In particular, it does not disturb the wavefront of surface waves.

[Brief explanation of the drawing]

FIG. 1, FIG. 2, and FIG. 3 each show an embodiment of a surface acoustic wave filter according to the present invention, and FIG. 4 shows a conventional example. 1 is a piezoelectric substrate, 2 is a weighted electrode, 3 is a regular type electrode, 7.
10.12 are the extreme electrode fingers, 7a, 7b, 10a,
10b, 12a, 12b, 12c are electrode finger portions.

Claims (2)

[Claims] (1) In a surface acoustic wave filter in which at least one of the input and output comb-shaped electrodes is constituted by a weighted electrode, at least the most extreme electrode finger on the opposite electrode side of the hot-side comb-shaped electrode of the weighted electrode An electrode finger formed by electrically connecting a plurality of electrode finger portions separated from each other by a distance of (N+1/2)λ_0, and an odd-numbered electrode from the weighted electrode side of the plurality of electrode finger portions. 1. A surface acoustic wave device characterized in that the total length of the finger portions is approximately equal to the total length of the even-numbered electrode finger portions. However, N=0, 1, 2..., λ_0 is the wavelength of the surface wave at the center frequency. (2) The surface acoustic wave filter according to claim 1, wherein the weighted electrode is constituted by a split electrode consisting of a pair of electrode fingers.