JPS639407B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a piezoelectric filter formed by electrically differentially connecting three-terminal piezoelectric vibrators that use thickness longitudinal vibration for excitation.

The most widely used piezoelectric filters with differential connections include vibrating bars, tuning forks, longitudinal vibrators,
It is well known that there are mechanical filters that utilize the following. However, these conventional mechanical filters constitute one system as a mechanical vibration system.

One of the biggest reasons for this is that -1:1 and 1:1 transformers can be easily obtained by simply gluing piezoelectric ceramic plates to the front and back surfaces of a constant elastic metal such as Erinva, paying attention to the polarization direction. One example is that a filter can be realized simply by making a purely electrical differential connection.

FIG. 1 shows an example of a conventional three-terminal mechanical vibrator in which piezoelectric ceramic plates 12a and 12b, which have been polarized in advance, are bonded to constant elastic metals 11a and 11b, such as Erinvar, using a deflection mode.

In the figure, arrows indicate polarization directions. Shown on the right are electrical equivalent circuits of the three-terminal mechanical resonators shown on the left, where La and Lb are dynamic inductances, Ca and Cb are dynamic capacitances, and Cda and Cdb are damping capacitances.

That is, it can be seen that 1:1 and -1:1 transformers can be easily obtained by simply bonding piezoelectric ceramic plates with care in the polarization direction.

By differentially connecting the three-terminal mechanical vibrators shown in FIG. 1 constructed in this way, a filter circuit as shown in FIG. 2 can be obtained.

In the circuit of FIG. 2, C _d =C _da +C _db .

Such a differentially connected filter is not only a medium- and wide-band filter, but also has a resonant frequency f _ra of the resonator shown in Figure 1a and a resonant frequency of the resonator shown in Figure 1b.
A common advantage is that a narrowband filter can be easily realized by bringing f _rb close to each other.

However, with mechanical filters made of piezoelectric ceramic plates bonded to constant elastic metals such as Erinba,
Practical frequency bands are 500Hz to 10KHz with a tuning fork, 500Hz to 60KHz with a tone piece, and about 40KHz to 300KHz even when using a vertical vibrator, and it is completely unrealizable above 1MHz.

The present invention aims to provide a piezoelectric filter that functions in a high frequency band of 1 MHz or higher.

Now, as a filter used in a high frequency band of 1 MHz or more, as shown in FIG.
There is a monolithic filter that utilizes energy confinement and has a structure in which a ground electrode 33 is provided on the back surface.

However, the monolithic filter shown in FIG. 3 has the disadvantage that once the electrodes are attached, the bandwidth cannot be adjusted. Furthermore, if a narrowband filter is to be realized, the distance between the electrodes 31 and 32 needs to be considerably widened, which makes it susceptible to variations in sound velocity and polishing precision of the materials, and the gap between the electrodes 31 and 33 increases. resonant frequency
f _r1 and the resonance frequency between electrode 32 and electrode 33
The variation in f _r2 becomes large, and the yield becomes extremely poor. In order to adjust f _r1 and f _r2 in order to obtain a high-quality filter, there has been an attempt to independently adjust the film thickness of electrode 31 and electrode 32 using a vacuum evaporation device, and a monolithic film using a quartz plate has actually been proposed. This method is used to adjust the frequency of the filter. However, manufacturing a filter by adjusting the frequency in a vacuum evaporation apparatus has the disadvantage that it is not suitable for mass production and is extremely expensive.

The present invention has been made to eliminate the above-mentioned drawbacks, and aims to provide a piezoelectric filter that can function satisfactorily as a narrow band filter in a high frequency band of 1 MHz or higher. One of its features is the two components that make up the piezoelectric filter.
The two piezoelectric vibrators belong to mutually independent mechanical vibration systems, and each piezoelectric vibrator is provided with a ground electrode inside.

Figures 4a, b, and c show a three-terminal piezoelectric vibrator used in the piezoelectric filter of the present invention that utilizes the energy trapping of thickness longitudinal vibration; are diagrams each showing the plane thereof. FIG. 4a shows a substantially circular earth partial electrode 43 in the center of the piezoelectric ceramic plate 40a in the thickness direction.
an upper partial electrode 41a with a lead taken out toward the left side, and an upper partial electrode 41a with a lead taken out toward the left side, and an upper partial electrode 41a facing to the left so that its circular portion overlaps the upper side of the earth partial electrode 43a, and the circular portion thereof overlaps the lower side of the earth partial electrode 43a. A lower partial electrode 42a with a lead taken out in the right direction is provided, and the piezoelectric ceramic plate 40a is polarized from one end face to the other end face in the thickness direction, for example, in the direction shown by the arrow in the figure. A three-terminal piezoelectric vibrator is shown. This is Figure 1a
It can be represented by an electrical equivalent circuit with a 1:1 transformer as shown in FIG. The reason why this 1:1 transformer is necessary for the piezoelectric filter of the present invention is as follows. First, assume that a voltage is applied between the electrode 41a and the electrode 43a, and as a result, the thickness of the piezoelectric ceramic plate 40a increases. Next, if we forcibly increase the thickness of the piezoelectric ceramic plate 40a by applying an external force, a voltage will be generated whose phase is 180 degrees different from the previous one. Therefore, if a voltage is applied between the electrodes 41a and 43a to excite longitudinal thickness vibration, in the polarization direction shown in FIG. 4a, an output in phase with the signal applied to the electrode 41a will be obtained at the electrode 42a. That's why.

In FIG. 4b, the input and output electrodes 4 on the piezoelectric ceramic plate 40b
1b, 42b and a ground electrode 43b, and has a structure similar to that shown in FIG. It is polarized toward the electrode 43b.

For the reason stated above, the equivalent circuit of the three-terminal piezoelectric vibrator shown in Figure 4b, which transfers the excitation of this thickness longitudinal vibration, is the equivalent circuit shown in Figure 1b with a -1:1 transformer. be equal. Similarly, as shown in FIG. 4c, an electrode 41 is attached to the piezoelectric ceramic plate 40c.
c, 42c, and 43c, and the three-terminal piezoelectric vibrator polarized from the ground electrode 43c toward the input/output electrodes 41c and 42c as shown in the figure also has a -1:1 transformer (b) in Fig. 1. This is equivalent to the equivalent circuit shown in .

Therefore, by simply electrically differentially connecting the vibrators shown in FIGS. 4a and 4b or the vibrators shown in FIGS. 4a and 4c, they can be configured independently of each other as a mechanical vibration system. In addition, the filter circuit shown in FIG. 2 is obtained.

Although the input and output electrodes of each piezoelectric vibrator are shown buried inside the piezoelectric ceramic plate, there is no problem even if they are exposed on the surface of the piezoelectric ceramic plate.

Furthermore, when manufacturing the piezoelectric filter of the present invention,
It is possible to utilize the technology of ceramic multilayer capacitors. Furthermore, the piezoelectric filter of the present invention can also be realized by bonding two piezoelectric ceramic plates provided with electrodes and having piezoelectric properties. In this case, not only thickness longitudinal vibration but also thickness sliding vibration can be used.

Next, as an example of the implementation of the present invention, as shown in FIGS. 4a and 4b, the distance between the input electrode and the ground electrode in the thickness direction and the distance between the output electrode and the ground electrode in the thickness direction are both 0.23 mm. The input electrode, the ground electrode, and the output electrode have a diameter of 2.2 mm and a plate thickness of approximately 0.65 mm, and have the same dimensions but different polarization directions.
We prototyped a terminal vibrator. First, we prototyped a differentially connected filter with a center frequency of 3.5MHz, which is easy to implement with a piezoelectric filter and has a characteristic bandwidth of 3%, which is commonly used in consumer products. Obtained.

Then, for the purpose of obtaining a narrow band filter, FIG.
By polishing the porcelain surface, the resonant frequency f _rb of the three-terminal resonator shown in FIG. 4 was brought close to the resonant frequency f _ra of the three-terminal resonator shown in FIG. As a result, we were able to easily obtain a characteristic with a fractional bandwidth of 0.5%. Here, the value of the fractional bandwidth of 0.5% is difficult to achieve with a monolithic type ceramic filter, but according to the present invention, it can be easily achieved by simply bringing the resonant frequencies of the two piezoelectric vibrators closer together. can. It goes without saying that similar results can be obtained by using the piezoelectric vibrator shown in FIG. 4c instead of the piezoelectric vibrator shown in FIG. 4b.

The properties of the piezoelectric ceramic used in the prototype are Poisson's ratio σ' = 0.42, which enables thickness longitudinal energy confinement, thickness longitudinal coupling coefficient k _t = 0.40, relative permittivity ε ^T ₃₃ = 850, and the electrodes are made of platinum. Using.

As described in detail above, according to the present invention, a differentially connected piezoelectric filter using thickness longitudinal vibration can be manufactured easily and at low cost, and has great industrial value.

[Brief explanation of the drawing]

Figures 1a and 1b show an example of a conventional three-terminal mechanical resonator using the flexural vibration mode;
1a and 11b are constant elastic metals such as Erinba, 12
a and 12b are piezoelectric ceramic plates, the arrow indicates the polarization direction, and the right side is an electrical equivalent circuit of a 3-terminal mechanical resonator, L _a and L _b are dynamic inductances,
C _a and C _b are dynamic capacities, and C _da and C _db are braking capacities. Figure 2 shows a filter circuit in which the three-terminal mechanical resonator shown in Figure 1 is electrically connected, where C _d and C _da are connected.
It becomes the sum of C _db . FIG. 3 shows an example of a conventional monolithic piezoelectric filter, in which 30 is a piezoelectric ceramic plate;
31 is an input electrode, 32 is an output electrode, and 33 is a ground electrode. FIGS. 4a, b, and c show a three-terminal piezoelectric vibrator that is a component of the present invention that utilizes the energy trapping of thickness longitudinal vibration, and in each figure, A is a cross-sectional view, and B is a plan view. 40a, 40b, 4
0c is a piezoelectric ceramic plate, 41a, 42a, 43a, 4
1b, 42b, 43b, 41c, 42c, 43c
indicates the electrode, and the arrow indicates the polarization direction.

Claims (1)

[Claims] 1 A piezoelectric filter in which first and second three-terminal piezoelectric vibrators, which constitute an independent system as a mechanical vibration system, are differentially connected as an electrical system, and the first three-terminal piezoelectric vibrator A ground partial electrode is provided in the center of the piezoelectric ceramic plate in the thickness direction, an upper partial electrode is provided opposite to the upper side of the earth partial electrode, and a lower partial electrode is provided opposite to the lower side of the earth partial electrode. and polarization is applied from the ground partial electrode toward the upper partial electrode and the lower partial electrode, or from the upper partial electrode and the lower partial electrode toward the earth partial electrode, and the thickness The second three-terminal piezoelectric vibrator is a three-terminal piezoelectric vibrator that uses longitudinal vibration for excitation, and the second three-terminal piezoelectric vibrator is
A ground partial electrode is provided in the center of the piezoelectric ceramic plate in the thickness direction, an upper partial electrode is provided opposite to the upper side of the earth partial electrode, and a lower partial electrode is provided opposite to the lower side of the earth partial electrode, A piezoelectric filter characterized in that it is a three-terminal piezoelectric vibrator that polarizes a piezoelectric ceramic plate from one end surface in the thickness direction to the other end surface and utilizes thickness longitudinal vibration for excitation.