JPH0210671Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an energy-trapping multi-mode ceramic filter that utilizes thickness vibrations such as longitudinal thickness vibrations and thickness sliding vibrations, and its purpose is to suppress spurious waves caused by harmonics. The aim is to improve the filtering characteristics.

For example, an energy trapping ceramic filter using thickness longitudinal vibration has input/output electrodes 3, 5 divided on the main surface of a ceramic plate 1 polarized in the thickness direction, as shown in FIGS. 1 and 2, on the back surface. A device in which opposing ground electrodes 7 are formed is generally known. Note that 11 is a terminal for connection to an external circuit, and 9 is a lead electrode connecting the terminal to the electrodes 3, 5, and 7, which are provided on the ceramic plate 1.

As shown in Fig. 2, this ceramic filter has vibration displacement distributions of symmetrical mode A and oblique symmetrical mode B. In particular, the vibrational displacement distribution of symmetrical mode A becomes large near the area between input and output electrodes 3 and 5, and The symmetrical mode A has a larger vibration displacement distribution than the obliquely symmetrical mode B. In addition to the fundamental vibration, such ceramic filters tend to generate harmonic harmonics that are odd multiples of the fundamental vibration.Moreover, the harmonic harmonics are concentrated near the input/output electrodes, that is, in the area where the vibration displacement distribution of symmetrical mode A is large. distribution, worsening spurious characteristics. FIG. 5 is a filtering characteristic diagram of a conventional ceramic filter.

The present invention is intended to improve the above-mentioned drawbacks, and examples thereof will be described below.

FIG. 3 shows an embodiment of the present invention, in which split input/output electrodes 3 and 5 are formed on the main surface of a ceramic plate 1 which is polarized in the thickness direction, and a ground electrode 7 is formed on the back surface thereof. In the energy trapping type ceramic filter due to thickness longitudinal vibration,
The polarization Q of the ceramic plate 1 corresponding to the input/output electrodes 3, 5 is determined between the input/output electrodes 3, 5 and the earth electrode 7.
This ceramic filter is characterized in that the polarization in the thickness direction of the ceramic plates between the two is different. The external shape of the ceramic filter is the same as the planar shape shown in FIG.

FIG. 4 shows another embodiment, in which a ceramic filter using thickness-sliding vibration using a ceramic plate 1 polarized P' in the plane direction is used. This is a ceramic filter in which the polarization direction of the electrode 1 is different from the polarization direction between the input/output electrodes 3 and 5 and the ground electrode 7, and the electrode shape is the same as that in FIG.

In the ceramic filter of the present invention, the direction of the polarization Q of the ceramic plate 1 corresponding to the input/output electrodes 3 and 5 is oblique, and the direction of the polarization Q in the thickness direction and the polarization P' in the plane direction of the ceramic plate 1 are oblique. make a difference,
In other words, by creating an oblique relationship,
The vibration displacement changes as explained below.

Figures 7 and 8 are three diagrams for explaining vibration displacement.
Represents the orthogonal coordinate axes of a dimension. First, in FIG. 7, consider the case where an electric field _E3 is applied in the same direction as the polarization direction to the ceramic plate 1 which has been polarized P in the thickness direction to excite longitudinal vibration in the thickness. Note that the thickness direction is made to coincide with the direction of the three axes. When stress T = 0, strain in three axial directions directly related to longitudinal thickness vibration
S ₃ is expressed by the following piezoelectric equation.

S ₃ =d ₃₃ E ₃ (1) d ₃₃ is a piezoelectric strain constant, that is, a component of the d constant.

Next, consider a new coordinate system consisting of the 1', 2', and 3' axes by rotating it around one axis by an angle θ as shown in FIG. The ceramic plate 1 is also rotated in the same way, but in the new coordinate system it is assumed that the polarization Q is tilted by an angle θ in the thickness direction.

Since the thickness direction of the ceramic plate 1 and the 3'-axis direction in the new coordinate system match, of course, the direction of the polarization Q is inclined by an angle θ from the 3'-axis direction. When the electric field E' ₃ in the thickness direction is made equal to the electric field E ₃ described above, the strain S' ₃ in the thickness direction in the new coordinate system is expressed by the following equation (2) in the same way as equation (1).

S′ ₃ =d′ ₃₃ E′ ₃ =d′ ₃₃ E ₃ (2) Here, d′ ₃₃ is expressed by equation (3) using the component of the d constant in the first coordinate system.

d′ ₃₃ = d ₃₃ cos ⁴ θ+d ₂₂ sin ⁴ θ+ (2d ₂₃ +4d ₄₄ ) ・sin ² θcos ² θ−4 (d ₃₄ cos ² θ+d ₂₄ sin ² θ) ・sin θcosθ (3) In piezoelectric ceramics, d ₂₂ = _d23 = _d44 =
d ₃₄ =0, and if the component d ₂₄ that does not directly participate in the thickness longitudinal vibration is ignored, it is expressed by equation (4).

d' ₃₃ = d ₃₃ cos ⁴ θ≦d ₃₃ (4) Then, the following relationship in equation (5) is derived.

|S' ₃ |≦|S ₃ | (5) Therefore, in a thickness-longitudinal vibrator in which polarization P is applied in the thickness direction of the ceramic plate 1, the strain decreases when the polarization direction is tilted. In other words, the vibration displacement distribution is reduced.

This can also be discussed in the same way for a thickness wandering oscillator that has a polarization axis in the in-plane direction.

That is, when applying an electric field perpendicular to the polarization direction to the ceramic plate 1, which is polarized P' in a direction parallel to the main surface, to excite thickness-traversing vibration, 1, 2, 3 as shown in FIG. If we choose a coordinate system consisting of the axes, the strain S ₄ that is directly involved in thickness sliding vibration is expressed by the following piezoelectric equation.

S ₄ =d ₂₄ E ₂ (6) Note that E ₂ is an electric field that coincides with the direction of the two axes.

Next, consider a new coordinate system consisting of the 1', 2', and 3' axes by rotating it by an angle θ around the 1 axis in FIG.

As in the case of longitudinal thickness vibration, compare the strain S′ ₄ in the new coordinate system with the strain S ₄ in the original coordinate system.
S′ ₄ is expressed by equation (7).

S' ₄ = d' ₂₄ E' ₂ = d' ₂₄ E ₂ (7) Here, if d' ₂₄ is expressed using the component of the d constant in the first coordinate system, it becomes as shown in equation (8).

d′ ₂₄ = {d ₃₃ sin ² θ−d ₂₂ cos ² θ+2(d ₃₄ −d ₂₄ )・
sinθcosθ}sinθcosθ+{d ₂₄ cos ² θ+d ₃₄ sin ² θ+(
_d23
+2d ₄₄ )・sinθcosθ}・(cos ² θ−sin ² θ) (8) For piezoelectric ceramics, d ₂₂ = d ₂₃ = d ₃₄ =
If d ₄₄ =0 and the component d ₃₃ that is not directly involved in the thickness sliding vibration is ignored, equation (8) becomes equation (9).

d′ ₂₄ = d ₂₄ cos ² θ (cos ² θ−sin ² θ)−2d ₂₄・
sin ² θcos ² θ=d ₂₄ cos ² θ (cos ² θ−3sin ² θ)=d ₂₄ cos
² θ.
(4cos ² θ−3)=d ₂₄ cosθ・cos(3θ)≦d ₂₄ (9). Then, the same equation (10) as in the case of thickness longitudinal vibration is derived.

｜S′ ₄ ｜≦｜S ₄ ｜ (10) Figures 9 and 10 show the cross-sectional views of the conventional and inventive ceramic filters, respectively, along with the vibration displacement distributions of the fundamental vibration of the symmetric mode and the harmonics. There is. As mentioned above, since the vibration displacement distribution is reduced by tilting the polarization direction, the input/output electrodes 3 and 5
If such a polarization Q is applied between them, the displacement distribution of the fundamental vibration of the symmetric mode A ₁ and the vibration displacement distribution of the harmonics
As shown in FIG. 10, A ₂ is decreased compared to FIG. 9. However, since the harmonic vibration displacement distribution A ₂ is more concentrated between the input and output electrodes 3 and 5, it is significantly lower than the fundamental vibration displacement distribution A ₁ . Therefore, according to the present invention, the displacement distribution _A1 of the fundamental vibration is kept within a range that does not cause any practical problems, and
Only the vibration displacement distribution A ₂ of the undesirable harmonics can be sufficiently reduced.

FIG. 6 is a filtering characteristic diagram of the embodiment shown in FIG. 3, and shows that harmonics can be suppressed significantly. In particular, the third harmonic has been improved by about 30dB.

By the way, as in the present invention, the input and output electrodes 3 and 5
As a method of making the direction of polarization Q of the ceramic plate corresponding to the gap different from the polarization direction of the entire ceramic plate 1, the following method can be considered. First, input and output electrodes 3,
5 and a ground electrode 7 are formed, and then an electric field is applied between the electrodes within a range where the polarization in the thickness direction is not reversed.
For example, an electric field E is applied between the input and output electrodes 3 and 5.
At this time, an electric field of 1/2E is applied between the input electrode 3 and the earth electrode 7 and between the output electrode 5 and the earth electrode 7, so it is necessary to carry out 1/2E at a level below the coercive electric field.

Further, the present invention is not limited to the case where the polarization direction of the ceramic plate corresponding to the input/output electrodes 3 and 5 is obliquely crossed with respect to the polarization P and P' applied to the ceramic plate 1 as in the above-mentioned embodiment. The reduction in harmonics, which is the objective of the present invention, can also be achieved by changing the direction, such as by reversing the direction. Of course, in this case as well, there is no problem with the basic vibration.

Now, in the above embodiment, one ceramic filter was explained, but it is also possible to form a plurality of ceramic filters on the same ceramic plate, and each ceramic filter can be formed not only by a pair of input/output electrodes 3 and 5. It can be widely implemented in multimode filters consisting of multiple electrodes.

As explained above, the present invention is a ceramic filter using thickness vibration, which has input/output electrodes divided on the main surface of a ceramic plate 1 polarized in the thickness direction or surface direction, and a ground electrode opposite to the back surface thereof. Since this is a ceramic filter in which the polarization direction of the ceramic plate between the input and output electrodes is different from the polarization direction between the input and output electrodes and the earth electrode, a filter with good spurious characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1: A plan view showing a conventional ceramic filter. FIG. 2: A cross-sectional view showing a conventional ceramic filter. FIG. 3: A cross-sectional view showing one embodiment of the ceramic filter of the present invention. FIG. 4: A cross-sectional view showing another embodiment of the ceramic filter of the present invention. FIG. 5: A filtering characteristic diagram of the ceramic filter shown in FIG. 2. FIG. 6: A filtering characteristic diagram of the ceramic filter shown in FIG. 3. FIGS. 7 and 8: Three-dimensional orthogonal coordinate axes for explaining vibration displacement according to the present invention. FIG. 9: A cross-sectional view showing the vibration displacement distribution of a conventional ceramic filter. FIG. 10: A cross-sectional view showing the vibration displacement distribution of the ceramic filter of the present invention. FIG. 11: Three-dimensional orthogonal coordinate axes for explaining another vibration displacement according to the present invention. 1: Ceramic board, 3, 5: Input/output electrode, 7:
Earth electrode, P, P', Q: polarization.

Claims (1)

[Scope of utility model registration request] In a ceramic filter that uses thickness longitudinal vibration or thickness sliding vibration, the input and output electrodes are divided on the main surface of a ceramic plate polarized in the thickness direction or in the plane direction, and a ground electrode is formed on the back surface of the ceramic plate to face the input and output electrodes. A ceramic filter characterized in that the direction of polarization of the ceramic plate in the gap located between the electrode and the output electrode is partially different from the direction of polarization.