JPS6313573B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a piezoelectric ceramic vibrator that performs thickness vibration. In general, piezoelectric ceramic resonators that vibrate through thickness (hereinafter simply referred to as piezoelectric ceramic resonators) are used in the band of several hundred KHz to several tens of MHz, and have a high coupling coefficient and are used in filters, etc. with a relatively wide band. has been made into However, compared to piezoelectric single crystals such as quartz and metal mechanical resonators such as Elinva, piezoelectric ceramic resonators have a disadvantage in that they are less stable with respect to aging of the resonance frequency, so they are used in communication filters, etc. In cases where highly stable aging characteristics are required, it is difficult to construct a filter using a piezoelectric ceramic vibrator due to material considerations.

The piezoelectric ceramic resonator is polarized in advance by applying a high DC voltage, and aging of the resonator constants such as the resonant frequency rp, the anti-resonant frequency ap, and the parallel capacitance Co starts immediately after polarization. In general, when the resonant frequency of a piezoelectric vibrator immediately after polarization is rpo, and the resonant frequency during aging is rp, the normalized resonant frequency change amount Δrp (= rp−rpo) is calculated by taking the number of days as the horizontal axis. The result is as shown in FIG. That is, it can be seen that the resonant frequency of the piezoelectric ceramic vibrator increases with aging, that the change in the resonant frequency is larger immediately after polarization, and that the change in the resonant frequency becomes smaller as time passes.

The change in resonance frequency is greater the closer it is to immediately after polarization, so conventionally, as a countermeasure against aging, general piezoelectric ceramic resonators have been heated at temperatures of 150℃ to 200℃ after polarization.
It is well known that the aging characteristics are improved by performing forced aging in advance by heat treatment at high temperatures. but,
Even if forced aging is performed in this way, a new aging process will begin once the forced aging ends, and with conventional piezoelectric ceramic resonators, no matter how highly stable the material is, the improvement can only be achieved by 0.03%/decade at most. It was hot.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the disadvantages of conventional piezoelectric ceramic vibrators regarding the stability of aging of the resonant frequency, and to obtain a piezoelectric vibrator unit having even better aging characteristics of the resonant frequency.

The details of the present invention will be explained below. As shown in the example shown in FIG. 3, the piezoelectric ceramic vibrator unit of the present invention is a piezoelectric ceramic vibrator in which electrodes (shaded areas in FIG. 3) are installed on a polarized piezoelectric ceramic 31. and a ceramic capacitor 32 connected in series to this piezoelectric ceramic vibrator. The ceramic capacitance 32Ca used here is the ratio of the parallel capacitance Co that exists parasitically to the piezoelectric ceramic vibrator.
It is a porcelain capacitor with aging characteristics in which Ca/Co gradually increases over time.

As is well known, piezoelectric ceramic vibrators have a resonant frequency rp
It is represented by an equivalent circuit in which a part represented by a series connection of dynamic inductance and dynamic capacitance that gives , and a part consisting of a parallel capacitance Co are connected in parallel. Here, as mentioned above, the dynamic inductance and dynamic capacitance are the resonance frequencies at which the impedance of the piezoelectric ceramic vibrator becomes minimum.
Anti-resonant frequency where r and impedance are maximum
It is uniquely determined by giving a.

Let the resonant frequency of the piezoelectric ceramic vibrator be rp, the anti-resonant frequency be ap, the parallel capacitance be Co, and the ceramic capacitor connected in series with this piezoelectric vibrator be Ca. The resonant frequency r of the piezoelectric ceramic vibrator unit of the invention is expressed by the following equation.

r ² = 1/1 + (Ca/Co) {(Ca/Co) r ² p + a ² p} From the above formula, when Ca/Co is infinite, r = rp, but as Ca/Co becomes smaller, It can be seen that the resonant frequency r of the piezoelectric ceramic vibrator unit gradually increases and approaches the anti-resonant frequency of the piezoelectric vibrator.

Regarding the aging characteristics of the resonant frequency rp of a piezoelectric ceramic resonator, we have already shown in Figure 1 that it gradually increases with aging, but the anti-resonant frequency ap may in rare cases gradually decrease with aging depending on the material. However, increasingly expensive materials are common.

As can be seen from the above equation, if rp increases over time, r will also change over time in the same way as rp, assuming that Ca/Co remains unchanged over time. However, as in the present invention, Ca/
As Co increases over time, r ² gradually approaches r ² p over time. r is always rp
Therefore, the amount of change in r ² over time is smaller than the amount of change in r ² p over time. That is, the aging characteristics of the piezoelectric ceramic vibrator unit of the present invention are superior to those of conventional piezoelectric ceramic vibrators.

For example, if the aging characteristic of the parallel capacitance Co of a piezoelectric ceramic resonator is negative and the antiresonance frequency ap decreases with aging, then the aging characteristic of the resonant frequency r of the piezoelectric ceramic resonator unit is simply the parallel capacitance The improvement is achieved simply by connecting a capacitor Ca that has the same aging characteristics as Co in series, and furthermore, if a capacitor Ca whose aging characteristics are sufficiently smaller than that of the parallel capacitor Co is connected immediately after,
This means that the aging characteristics of the resonance frequency r are further improved.

However, it is rather common for ap to be a oscillator that increases over time due to the material.

Next, as an embodiment of the present invention, an energy trapping type thickness sliding oscillator 31 as shown in FIG.
A piezoelectric ceramic vibrator unit having a configuration in which a ceramic capacitor 32 is connected in series with the piezoelectric ceramic vibrator unit will be explained. The energy trapping type thickness sliding piezoelectric ceramic vibrator 31 uses NEPEC-31 manufactured by Tohoku Metals as the piezoelectric ceramic material.
It has external dimensions of 12 mm in length, 0.552 mm in thickness, and 0.32 mm in width, and extends across the entire width from each end of the piezoelectric ceramic on the front and back sides to the center as shown by diagonal lines.
Cr--Au electrodes are provided, and the corresponding overlap between the electrodes is 2.5 mm. The parallel capacitance Co of this resonator 31 is 13.41PF, and its aging characteristics are shown by the solid line in Figure 2 (normalized by the initial value).
has. In addition, the external ceramic capacitor 32 is unpolarized.
Using NEPEC-31, its aging characteristics at 40.3PF have the physical properties (normalized by the initial value) shown by the dotted line in Figure 2. The aging characteristics of a piezoelectric ceramic resonator that has been heat-treated at 150°C in advance, normalized to the resonance frequency rpo immediately after the heat treatment, is shown by the dotted line in Figure 4. In addition, the aging characteristics of the resonant frequency of the piezoelectric ceramic resonator unit of the present invention, which uses a piezoelectric ceramic resonator heat-treated under the same conditions as the piezoelectric resonator shown by the dotted line, are normalized by rpo and are shown as a solid line in Fig. 4. Indicated by

From the above, it is said to be extremely stable.
Even a piezoelectric ceramic vibrator using NEPEC-31,
It can be seen that while the aging characteristic of the resonant frequency is about 0.03%/decade, it is improved to about 0.015%/decade in the piezoelectric ceramic vibrator unit according to the present invention.

Further, as an application of the piezoelectric ceramic resonator unit of the present invention, as shown in FIG. 53 consisting of NEPEC-31 unpolarized capacitors
We prototyped a narrowband filter with a 2.0MHz band specific bandwidth of 0.5%. As a result, the aging characteristics of the center frequency of the filter were significantly improved from 0.03%/decade when using a conventional piezoelectric ceramic vibrator alone to 0.013%/decade. This value of 0.013%/decade was a value that was considered impossible with conventional piezoelectric ceramic vibrators.

As explained above, according to the present invention, it is possible to provide a piezoelectric vibrator unit with extremely excellent aging characteristics, which has great industrial value.

[Brief explanation of the drawing]

FIG. 1 shows an example of normalized aging characteristics of the resonant frequency of a conventional piezoelectric ceramic vibrator. Figure 2 shows an example of aging characteristics of a normalized capacitor, where the solid line is the aging characteristic of a parallel capacitor of a thick piezoelectric ceramic resonator, and the dotted line is an unpolarized capacitor used for a ceramic capacitor connected in series with a piezoelectric ceramic resonator. This is the aging characteristic of capacity. FIG. 3 shows, as an embodiment of the present invention, a piezoelectric ceramic resonator unit in which an unpolarized ceramic capacitor is connected in series with an energy-trapped thickness-stretching piezoelectric vibrator, and the arrow indicates the polarization direction. In FIG. 4, the dotted line shows the aging characteristic of the resonant frequency of the conventional piezoelectric ceramic vibrator, and the solid line shows the aging characteristic of the resonant frequency of the piezoelectric vibrator unit according to the embodiment of the present invention. FIG. 5 shows an example of a filter to which the piezoelectric ceramic vibrator unit of the present invention is applied. In the figure, 31 is a thickness-stretching piezoelectric ceramic vibrator;
32 is a ceramic capacitor, 50 is a piezoelectric ceramic vibrator unit, 51 is an unpolarized ceramic capacitor, 52 is a piezoelectric ceramic vibrator, and 53 is an interstage capacitor.

Claims (1)

[Claims] 1 A piezoelectric ceramic vibrator that uses thickness vibration and a ceramic capacitor Ca connected in series to the piezoelectric ceramic vibrator
The magnetic capacitance Ca has an aging characteristic such that the ratio Ca/Co of the magnetic capacitance Ca and the parallel capacitance Co parasitic to the piezoelectric ceramic vibrator gradually increases with the passage of time. A piezoelectric ceramic vibrator unit characterized by a magnetic capacitance.