JP2001285022A - Ladder-type ceramic filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide at a low price a ladder type ceramic filter which suppresses spurious signal by using a small number of kinds of resonators. SOLUTION: The subordinate resonance frequencies of resonators S1 to S4 which are arranged at serial branches are made different in position from one another by varying the depths of slits formed in the center parts of four sides of the resonators. Thus, the high-order subordinate resonance frequencies are dispersed, so that spurious signals can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a ladder type ceramic filter in which a plurality of resonators are combined in series and parallel.

[0002]

2. Description of the Related Art Ladder-type ceramic filters of this kind are widely used as frequency-selecting electronic components for various wireless devices because of their small size and low cost. Further, such a filter needs to suppress spurious as much as possible to secure a required guaranteed attenuation.

As a countermeasure against such spurious response, Japanese Patent Laid-Open No.
What is disclosed in 3-77439 is known. That is, a plurality of rectangular piezoelectric elements each having a different side length are used for the resonators arranged on the series branch of the ladder circuit, and a plurality of rectangular piezoelectric elements each having a different side length are used for the resonators arranged on the parallel branch. Is used to make the fundamental frequencies of the respective resonators coincide with each other so that the higher-order sub-resonance frequencies are slightly different from each other (hereinafter referred to as conventional technology).

[0004]

According to the above-mentioned prior art, since the sub-resonance frequencies of the respective resonators are slightly different from each other, the spurious is suppressed because the higher-order frequencies of the frequency characteristics when assembled as a filter are dispersed. The required guaranteed attenuation is obtained. However, in the related art, it is necessary to prepare many types of rectangular piezoelectric elements having different side lengths. For example, when three resonators are used for each of the series branch and the parallel branch, six types of rectangular piezoelectric elements having different side lengths are required. The inconvenience of having to prepare for is inevitable, which is a factor that hinders mass production,
Significant cost reduction could not be achieved.

An object of the present invention is to provide a low cost ladder type ceramic filter in which spurious is suppressed by a small number of types of resonators.

[0006]

According to the present invention, the sub-resonance of the resonator is changed by changing the depth of the slit formed in the center of the four sides of the resonator arranged in the series branch among the resonators. The frequency positions are different from each other. In this case, it is desirable to make the side lengths of the resonators equal.

[0007] In the case where the sub-resonance frequency is varied depending on the depth of the slit formed in the series branch resonator, the spurious can be suppressed because the higher-order sub-resonance frequency is dispersed. Here, the slit for adjusting the sub-resonance frequency can be easily formed by post-processing, so there is no need to prepare many types of resonators as compared with the conventional technology in which the side length of the resonator is made different. Can be greatly reduced.

Further, according to the present invention, the sub-resonant frequencies of the resonators arranged in the parallel branches are made different by making their side lengths different.

The parallel-branch resonators having different sub-resonance frequencies can disperse higher-order frequencies together with the series-branch resonators. In the case of this parallel branch resonator,
This is similar to the prior art in that the side lengths are different, but the present invention employs such means only for the parallel branch resonators, so that mass productivity is not significantly impaired unlike the prior art.

[0010]

Embodiments of the present invention will be described below with reference to embodiments shown in the drawings. FIG. 1 shows an overall perspective view of a ladder type ceramic filter, and FIG. 2 shows an internal structure thereof. FIG. 3 is an exploded perspective view of the same filter (only main parts are denoted by drawer numbers). In each of the figures, reference numeral 1 denotes a case made of a synthetic resin and having an opening on one side.
a and two inner chambers C1 and C2, respectively, and each of the inner chambers C1 and C2 has a ground terminal plate 2a and 2b, respectively.
Are arranged at required places, and the strip-shaped terminal legs 2a ₁ , 2
b ₁ is integrally fixed to the case in advance so that it protrudes outward from the other side surface of the case 1.

In one inner chamber C1 of the case 1, first and second resonators S1 and S2 arranged in series branches of a ladder circuit (see FIG. 4) and first and second resonators S1 and S2 arranged in parallel branches are arranged. A stacked body including the second resonators P1 and P2 and a plurality of terminal plates 2c to 2i that electrically and mechanically sandwich the resonators is housed. In the other inner chamber C2 of the same case, the third and fourth resonators S3 and S4 arranged in series branches, the third and fourth resonators P3 and P4 arranged in parallel branches, and A laminate comprising a plurality of terminal boards 2g, 2j to 2q for electrically and mechanically holding the terminals is stored. Here, as shown in FIG. 3, the terminal plate 2g is formed in a connection shape straddling the two chambers C1 and C2 of the case, the terminal plate 2e is formed in a three-fold connection shape, and the terminal plate 21 is formed in a two-fold connection shape. Respectively. Further, the input / output terminal plates 2d and 2p are respectively strip-shaped terminal legs 2d ₁ and 2p ₁ protruding from one side surface of the case _1.
It is configured as an input / output terminal with.

After storing a required laminate in each chamber of the case through an opening on one side thereof, the opening is sealed with a cover member 3 such as paper or a thin plastic plate and an adhesive 4,
Then, each terminal leg of the ground terminal and the input / output terminal protruding from both side surfaces of the case is bent along the lower surface of the case, and this is connected for directly connecting to a required circuit on a mounting board (not shown). Function as a terminal.

The resonators S1 to S4 arranged in the series branch of the ladder circuit and the resonators P1 to P4 arranged in the parallel branch are
In each case, a square piezoelectric element operating in the contour vibration mode is used.
As shown by A, a slit s is formed at the center of each of the four sides and the depth x is different. Further, the parallel branch resonators P1 to P4 shown in FIG. 5B have different side lengths. Such a resonator has a slightly different sub-resonance frequency as shown in FIG.

Also here, it is desired to increase the capacitance ratio between the series branch resonators S1 to S4 and the parallel branch resonators P1 to P4. FIGS. 5A and 5B show the relationship between the series branch resonator and the parallel resonator. As shown in the embodiment, the thickness of the square piezoelectric element 11 of the series resonator is set to be larger than that of the piezoelectric element 21 of the parallel branch resonator, and the electrode formed on one surface of the piezoelectric element 11 is a partial electrode 12. The surface electrode was used as the entire surface electrode 13 to reduce its capacitance. The parallel branch resonators were made thinner and the electrodes on both sides were made full electrodes 22 and 23 to increase the capacitance.

Each resonator having the frequency shown in FIG.
FIG. 7 shows the filtering characteristics of a ladder type ceramic filter configured by combining in series and parallel as shown in FIG.

For reference, as shown in FIG. 8, a series branch resonator having the same side length and the same slit depth x and a parallel branch resonator having the same side length and no slit processing are series-parallel. FIG. 9 shows the filtering characteristics of the ladder type ceramic filter combined with FIG.

As is clear from the comparison between FIG. 7 and FIG. 9, higher-order frequencies of the filter according to the present invention are dispersed and spurious are reduced, and the guaranteed attenuation is sufficiently large at 50 dB. It can be understood that the gain is improved by 10 dB.

In the present invention, the series branch resonators S1 to S
The sub-resonant frequency can be changed by changing the depth x of the slit s applied to the slit 4, but the fundamental frequency also changes slightly. Accordingly, the corner of the rectangular piezoelectric element is chamfered to correct this. 5A) can be adjusted to a predetermined fundamental frequency.

Further, in the series branch resonators S1 to S4 of the present invention, it is desirable to use the same side length, but the side lengths may be varied so as not to impair mass productivity and manufacturing cost. In the above example, as shown in FIG.
The second and fourth resonators S1, S2, and S4 are the same 4.6 m.
m, only the third resonator S3 is 4.80 m different from these.
m.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a ladder type ceramic filter of the present invention.

FIG. 2 is a sectional view taken along the line II in FIG. 1;

FIG. 3 is an exploded perspective view of FIG. 1;

FIG. 4 is an equivalent circuit diagram of a ladder-type filter.

5A is a perspective view of series branch resonators S1 to S4, and FIG. 5B is a perspective view of parallel branch resonators P1 to P4.

FIG. 6 is a table showing various characteristics such as dimensions, resonance (anti-resonance) frequency, and sub-resonance frequency of each resonator used in the present invention.

FIG. 7 is a frequency characteristic diagram of a ladder-type filter according to the present invention using each resonator shown in FIG. 6;

FIG. 8 is a table showing dimensions, resonance frequencies, and the like of respective resonators used for comparative reference.

9 is a frequency characteristic diagram of a ladder-type filter including the resonator shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Case 2a-2n, 2q Terminal board S1-S4 Series branch resonator P1-P4 Parallel branch resonator C1, C2 Inner chamber

Claims (3)

[Claims] 1. A method in which a plurality of resonators each comprising a rectangular piezoelectric element operating in a contour vibration mode are combined in series and parallel, and the sub-resonance frequencies of the respective resonators are different from each other. A ladder-type ceramic filter characterized in that the sub-resonance frequencies of the resonators are made different from each other by changing the depth of a slit provided at the center of the four sides of the resonators arranged in the series branch. 2. The ladder-type ceramic filter according to claim 1, wherein the side lengths of the resonators of the series branch are made equal. 3. The resonator according to claim 1, wherein the sub-resonance frequencies of the resonators are different from each other by changing the side lengths of the resonators arranged in parallel branches among the resonators.
A ladder-type ceramic filter as described.