JPH0158892B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a monolithic ceramic filter that utilizes high-order thickness vibration.

Ceramic filters that are generally used at high frequencies of several MHz or higher use thickness vibration of a thin plate whose surface is sufficiently wide relative to its thickness as the vibration mode.

Since the resonant frequency of thickness vibration is inversely proportional to the thickness, it is necessary to reduce the thickness in order to use it at a high frequency. However, when the thickness is less than 100 μm, the mechanical strength decreases, making parallel plane polishing difficult and also making it difficult to hold the vibrator.

Therefore, even in a monolithic ceramic filter using the thick vertical vibration mode with the highest sound speed, if the operating frequency exceeds 20 MHz, the plate thickness must be approximately 100 μm or less, making manufacturing difficult. Monolithic ceramic filters using basic thickness vertical vibration are limited to materials whose Poisson's ratio σ' is larger than 1/3.

Materials that meet this condition are limited to some zircon/lead titanate-based piezoelectric ceramic materials, which have excellent temperature stability,
In terms of aging properties, it is said to be significantly superior to zircon/lead titanate-based piezoelectric ceramic materials.
A basic thickness vertical vibration mode monolithic ceramic filter using PbTiO ₃ -based piezoelectric ceramic material is impossible to realize because the value of σ′ is less than 1/3.

On the other hand, high-order mode monolithic ceramic filters have been put into practical use that use odd-numbered harmonics with the same plate thickness to obtain a resonant frequency that is an odd-numbered multiple of the fundamental wave. An example of a conventional high-order mode monolithic ceramic filter is shown in FIG. FIG. 1A is a plan view of the monolithic ceramic filter, and FIG. 1B is a front view of the piezoelectric ceramic plate 10.
Energy trapping electrodes 11, 11' on the front and back surfaces of
and 12, 12' are provided. As is well known, the operating principle of this filter is that vibration energy is transferred to the electrodes 11 and 1 due to the piezoelectric reaction and the mass effect of the electrodes.
Because it is confined between 1' and 12, 12',
The electrode portion operates as a resonator, and the electrodes 11,1
The intermediate portion between 1' and 12, 12' becomes an elastic connecting portion, and a filter can be constructed.

However, for the n-th higher mode, the capacitance ratio is n ²
increases in proportion to When the capacitance ratio of the vibrator is γ, the ratio between the interval between the resonant frequency and the anti-resonant frequency and the resonant frequency is approximately 1/2γ. Therefore,
In the conventional high-order mode monolithic ceramic filter as shown in FIG. 1, first of all, due to an increase in the capacitance ratio, the passband width of the filter is too narrow to be of practical use. Second, in monolithic ceramic filters that utilize higher-order modes, the fundamental resonance frequency on the low-frequency side becomes spurious, and since this spurious has a much smaller capacitance ratio than the higher-order modes used, it is excited even more strongly. , it has been impossible to suppress the spurious caused by this fundamental resonance. Thirdly, the electrodes 11 and 1 exposed on the surface of the piezoelectric ceramic plate 10 shown in FIG.
In the filter provided with electrodes 1', 12, and 12', it is impossible to adjust the frequency by parallel plane polishing after electrode formation. For this reason, frequency adjustment has been carried out by controlling the film thickness of electrodes using a vapor deposition apparatus, and the manufacturing cost associated with frequency adjustment has been extremely high.

On the other hand, JP-A-55-18189 proposes a monolithic ceramic filter having a structure in which two opposing energy trapping electrodes are located inside a piezoelectric ceramic. A monolithic ceramic filter having this internal electrode has good energy trapping characteristics even if the electrode is inside the piezoelectric ceramic due to the extremely large piezoelectric reaction of the piezoelectric ceramic.

However, although it is possible to adjust the frequency by parallel plane polishing in a filter with this structure, when using a higher-order mode, as in the case of the conventional higher-order mode filter shown in Fig. 1, spurious waves occur and fundamental resonance occurs. It has the disadvantage that it is impossible to suppress it, and it is also impossible to achieve small capacitance ratios in the higher-order modes used. Furthermore, even when using the basic thickness vertical vibration mode, higher frequencies cannot be achieved unless the vibrating portion is made thinner, and the various drawbacks mentioned at the beginning cannot be solved at all. In contrast, the present invention aims to solve all the drawbacks of the conventional high-order mode monolithic piezoelectric ceramic filters mentioned above, and to provide a monolithic ceramic filter that can be frequency adjusted and can be used at a higher frequency than the conventional one. There is.

The monolithic ceramic filter of the present invention has a structure in which a plurality of electrode groups each consisting of a plurality of electrodes that overlap each other in the thickness direction of the piezoelectric ceramic plate and are parallel to each other are formed inside or on the inside and surface of the piezoelectric ceramic plate. ing. Furthermore, in these electrode groups, the piezoelectric ceramic layers sandwiched between the electrodes and adjacent to each other via the electrodes are polarized in opposite directions. Further, among the plurality of electrode groups, the number of electrodes in at least two electrode groups connected to the input and output terminals is three or more and equal to each other, and furthermore, the number of electrodes in the two electrode groups connected to the abutting input and output terminals is three or more, and the number of electrodes is equal to each other. It has a structure in which input and output terminals are respectively connected to two electrodes located on both outer sides near the surface of the piezoelectric ceramic among the electrodes of each group.

The structure described above will be explained in detail with reference to the drawings.

An example of a monolithic ceramic filter having two sets of electrode groups is shown in FIGS. 2 and 3. Figure 2 shows
FIG. 3 is an exploded view showing the laminated structure of the filter. Energy trapping electrodes 21, 21', 22, 22' are provided on a sheet 20 made of piezoelectric ceramic powder and an organic material so as to overlap each other in the thickness direction, and are connected to the energy trapping electrodes to form polarization lead electrodes 2.
3, 23', 24, 24' are provided. Further, the uppermost and lowermost ceramic sheets provided with electrodes are provided with lead electrodes 25, 25', 26, 26' for electrical terminals from energy trapping electrodes. FIG. 3 shows a monolithic ceramic filter having a structure in which the laminated structure of FIG. 2 is integrated in the thickness direction and fired. In FIG. 3, A is a plan view and B is a sectional view. External electrodes 27, 27' for polarization are provided on the outside of the piezoelectric ceramic plate 20', which is formed by integrating and firing the sheet 20, and are connected to the lead electrodes 23, 23', 24, 24' for polarization. It is connected to the electrode every other layer. Also, among the energy trapping electrodes, the electrodes located close to the surface of the piezoelectric ceramic plate on both sides are connected to the electrical terminal lead electrodes 25, 25', 26, 2.
6' are provided with external electrodes 28, 28',
29, 29'.

The present invention is characterized by a structure in which only two electrodes located on both outer sides near the surface of the piezoelectric ceramic plate in two sets of electrode groups connected to the input and output terminals are connected to the input and output terminals. That is, after polarization, the lead electrodes to be connected to the input and output terminals among the external polarization electrodes 27, 27' are connected to the electrical terminals, leaving only the portions that are in contact with them 27,
It would be possible to delete other parts of 27', but this would be quite difficult considering the overall shape. Therefore, in the example shown in FIG. 3, lead electrodes 25, 25', 26, 26' for electronic terminals are provided separately, and after polarization, all external electrodes 27, 27' for polarization are removed, and external electrodes 28, 28' are provided. , 29, 29', the electrical terminals 30, 30', 31, 31' are connected to the electrical terminal lead electrodes 25, 25', 26, 26'. In addition, polarization is performed by applying a DC high electric field to the external electrodes 27 and 27' in FIG. polarized.

In the monolithic filter of the present invention having such a structure, the polarization directions are opposite to each other with respect to a certain electric field direction between adjacent energy trapping electrodes that are stacked so as to overlap each other in the thickness direction. Therefore, vibration displacements in opposite directions appear between electrodes adjacent to each other in the thickness direction, and a specific higher-order mode (3 in the case of Fig. 3) appears.
Only the following mode) can be strongly excited. In other words, being able to strongly excite only a specific higher-order mode means that a small capacitance ratio can be achieved in the specific higher-order mode used, and the fundamental mode that causes spurious is suppressed by canceling the charge. It has the excellent feature of being possible.

Since piezoelectric ceramics are essentially high-coupling materials, the piezoelectric reaction is extremely large. Therefore, the cutoff frequency of the electrode portion is considerably lower than that of the non-electrode portion, and the monolithic filter of the present invention shown in FIG. Even if the surface of the electrode is covered with a small amount of porcelain layer, this will not impair energy confinement. By using a structure with such internal electrodes, in order to adjust to the desired frequency, it is sufficient to simply polish the surface porcelain layer, and the electrical terminals 30, 30', 31, 3 provided in advance can be adjusted to the desired frequency.
1', it is possible to know the accurate frequency characteristics.

Further, parallel plane polishing can be performed until the energy trapping electrode closest to the porcelain surface is exposed to the porcelain surface. Also, the number of electrodes connected to the input and output terminals must be equal. If the number of electrodes is different, the vibration mode excited at the input side electrode section will not resonate sufficiently at the output side electrode section. Furthermore, in order to excite higher-order modes of second order or higher, three or more of the above-mentioned electrodes are required.

On the other hand, as shown in FIG. 4, a monolithic filter can also be considered, which is uniformly polarized in the direction of the arrow in the thickness direction.

However, because the input and output electrodes are all electrically connected in parallel due to the structure, the impedance becomes 1/n ₂ when the n-th mode is used compared to the monolithic filter of the present invention. In other words, in an attempt to reach a higher frequency range, the higher the order of the higher mode is raised, the lower the impedance becomes, which may make it difficult to match the impedance with an external circuit.

In the monolithic filter of the present invention, FIG.
As is clear from Fig. 3, the structure is such that the electrical terminal lead electrodes 25, 25', 26, 26' are taken only from the energy trapping electrode closest to the surface of the piezoelectric ceramic plate, resulting in this reduction in impedance. can't happen.

As described above, the monolithic filter of the present invention eliminates the various drawbacks of conventional monolithic filters using higher-order modes.

In order to achieve even steeper frequency selection characteristics than the structure shown in Figure 3, it is necessary to confine energy between the input side and output side electrode groups in the longitudinal direction of the piezoelectric ceramic plate, as shown in Figure 5. 1 electrode group
This can be easily achieved by forming the above and short-circuiting the top and bottom surfaces of these electrode groups. (5th
(In the figure, the arrow indicates the polarization direction.) When using the structure shown in Figure 5, vibrational energy is trapped not only in the input and output electrodes but also in the short-circuited intermediate electrode, and these parts move as resonators. . These resonators are placed in cascade via elastic coupling parts (parts without electrodes), so the signal must pass through many resonators between input and output. Therefore, frequency selectivity is improved.

In the structure shown in Fig. 3, only the third-order thick vertical vibration mode is strongly excited, but in the case of the second-order case, the number of laminated ceramic sheets on which electrodes are applied in Fig. 2 is three. , 5 for the fourth order
By changing the number of laminated layers to n+1 in the case of a general n-th order, it is possible to strongly excite other higher-order modes, and it goes without saying that spurious due to fundamental resonance can be suppressed.

Next, as an embodiment of the present invention, FIGS.
This article describes a monolithic filter for mobile radio with a center frequency of 27MHz that uses third-order thickness vertical vibration and has the structure shown in the figure.

A method for manufacturing the prototype monolithic filter will be explained with reference to FIG. First, a slurry containing piezoelectric ceramic powder, an organic binder, and an organic bath medium was prepared using a porcelain lamination technique, and a film was formed using a casting method to obtain a green sheet 20. Energy is trapped in this raw sheet and the electrodes 21, 21', 22, 2
2', polarization lead electrodes 23, 23', 24, 2
4' and electrical terminal lead electrodes 25, 25', 2
6,26' was printed. Next, these green sheets were cut into predetermined sizes, laminated, and pressed together to produce a laminate. A monolithic filter obtained by firing this laminate is shown in FIG. Further, silver paste was applied to predetermined positions as external electrodes 27, 27', 28, and 28' and baked. Thereafter, a DC high electric field was printed on this external electrode to perform polarization treatment. The piezoelectric material has a thick vertical coupling coefficient kt=0.45
The thickness of _the plate after firing is 330 μm, the distance between adjacent energy trapping electrodes in the thickness direction is approximately equal, and the outer dimensions of the monolithic filter are 6.4 × 8.4 mm.
Next, the frequency was adjusted by polishing the surface porcelain layer, and when the plate thickness was 261 μm, the center frequency of the filter was 27 MHz. The operating attenuation characteristics of the filter at this time around 27MHz are shown in FIG. A 3dB specific bandwidth of 3.0% has been obtained, and the amount of operational attenuation near the spurious due to basic thickness vibration has been suppressed to about 20dB. On the other hand, in the conventional monolithic piezoelectric ceramic filter using third-order thickness vertical vibration shown in FIG.
Despite using PbTiO ₃ -based piezoelectric ceramics, not only was it possible to obtain a 3 dB specific bandwidth of only about 1%, but also the amount of operational attenuation in the vicinity of spurious waves due to basic thickness vibration was about 5 dB. In other words, when comparing the characteristics of both filters using thickness vertical vibration, the monolithic filter of the present invention can achieve a much wider band than the conventional one, and has a lower basic thickness that causes spurious waves. vibration can be suppressed.

As detailed above, the high-order mode monolithic filter of the present invention has the advantages of being able to strongly excite only a specific high-order mode, suppressing fundamental mode vibrations that become spurious, and being easy to adjust the frequency. It has the following characteristics.

[Brief explanation of drawings]

FIG. 1 shows a conventional monolithic ceramic filter, in which A is a plan view and B is a front view. Second
The figure shows the laminated structure of the monolithic ceramic filter of the present invention. FIG. 3 shows a cross-sectional view of the monolithic ceramic filter of the present invention. Arrows indicate polarization directions. FIG. 4 shows a sectional view of a monolithic ceramic filter having a structure different from that of the present invention. FIG. 5 is a diagram showing an example in which four electrode groups are formed among the embodiments of the monolithic ceramic filter of the present invention. FIG. 6 is a diagram showing the characteristics of the monolithic ceramic filter of the present invention. In the above figures, 10 and 20' are piezoelectric ceramic plates,
11, 11', 12, 12' are electrodes, and 20 is a sheet made of piezoelectric ceramic powder and organic material. 21, 21',
22, 22' are internal electrodes for energy confinement;
23, 23', 24, 24' are lead electrodes for polarization,
25, 25', 26, 26' are lead electrodes for electric terminals, 27, 27' are external electrodes for polarization, 28, 2
8', 29, 29' are external electrodes for electrical terminals, 30,
30', 31, 31' are electrical terminals.

Claims (1)

[Claims] 1 Inside or on the inside and surface of the piezoelectric ceramic plate,
A plurality of electrode groups are formed, each consisting of a plurality of electrodes that are parallel to each other and overlap each other in the thickness direction of the piezoelectric ceramic plate, and the piezoelectric ceramic layers that are sandwiched between the electrodes and that are adjacent to each other through the electrodes are opposite to each other. The number of electrodes in the two electrode groups connected to at least the input and output terminals among the plurality of electrode groups is 3 or more and equal to each other, and the number of electrodes is equal to each other, and the number of electrodes is equal to each other, and A monolithic ceramic characterized by having a structure in which input and output terminals are connected to two electrodes located on both outer sides near the surface of a piezoelectric ceramic plate among the electrodes constituting two sets of electrode groups. filter.