The present invention generally relates to a piezoelectric ceramic resonator performing thickness resonant mode vibrations accruing from thickness shear and thickness extension.
Generally, in a relatively high frequency range of 1 MHz or more, resonators are operated in thickness resonant mode mainly accruing from thickness shear and thickness extension. The reason is that if a contour extensional mode accruing from the extensional vibration of a disk is used, dimensions of the resonator becomes too small to machine and support it.
Especially, it is known that a satisfactory resonant response without spurious signals around the resonance frequency can be obtained by the thickness resonant mode resonator, so-called trapped energy resonator. The resonance frequency of the thickness mode resonator is, however, determined substantially by the thickness of the piezoelectric ceramic sheet or base plate. Since the resonance frequency is an inverse proportion to the thickness of the sheet, it is difficult to adjust the resonance frequency once the thickness of the sheet is determined after mounting the electrodes, and this is a common drawback of this type of resonators.
However, the adjustment of frequency is not absolutely impossible, but a method is known in which the frequency is controlled by precisely increasing the thickness of the electrode in a vapor deposition apparatus as in the case of manufacturing some kind of crystal vibrators. However, this method is unsuitable for mass production, causing high manufacturing cost and thus limitation of use.
In manufacturing thickness resonant mode piezoelectric ceramic resonators in the past, it could not be expected to adjust the frequency after deposition of the electrodes because of the cost problem as mentioned above. Therefore, the resonance frequency of piezoelectric ceramic chips was measured before mounting the electrodes to select a chip having a resonance frequency within a predetermined range without the electrodes. Then, a number of piezoelectric ceramic resonators were produced from the thus selected chip by providing the electrodes.
In this case, the resonance frequency of each resonator is irregular due to the difference of the sound velocity at different locations in the piezoelectric ceramic chip and the irregularity in the thickness of the chip, in size of deposited electrodes, and in thickness of deposited electrodes. Accordingly, it was quite difficult to manufacture piezoelectric resonators with resonance frequency irregularity less than 0.5%.
Therefore, in manufacturing narrow-band filters having a fractional band width of 1% or less, the frequency must be adjusted at least as accurate as 0.1%, and very poor yield rate results when fabricating narrow-band filters by merely using piezoelectric ceramic resonators produced in the conventional manufacturing processes as described above.
Therefore, in order to realize a narrow-band filter by use of a thickness resonant mode piezoelectric ceramic resonator, the frequency must somehow be adjusted.
However, it is difficult to adjust the frequency of the thickness resonant mode resonator per se as mentioned above.
On the other hand, it is known that by connecting a reactive element such as an inductor or a capacitor in series with a piezoelectric resonator, the resonance frequency of a piezoelectric resonator unit consisting of the piezoelectric resonator and the reactive element can be varied. For the reactive element, a capacitor is more preferable than an inductor, since the latter has large dimensions and a small Q-value.
However, the performance of a thickness resonant mode piezoelectric ceramic resonator cannot be enhanced by merely connecting a capacitor to it at random.
In order to materialize a practically valuable and high-quality and highly reliable thickness resonant mode piezoelectric ceramic resonator unit, the following points must be taken into consideration:
(1) The resonance frequency fr is easy to adjust;
(2) The resonant response is sharp within the range of adjustment of the resonance frequency;
(3) Piezoelectric ceramic is advantageously a high electro-mechanical coupling factor material. However, it also has a disadvantage that it is very difficult to obtain compatibility between temperature characteristics and aging characteristics for the resonance frequency. That is to say, it is very difficult to find a piezoelectric ceramic material having a nearly zero temperature coefficient for the resonance frequency and also satisfactory aging characteristics for the resonance frequency. If the resonator has a poor stability in these characteristics, it is impossible to fabricate a highly reliable resonator unit or filter using resonator units. Accordingly, it is necessary to obtain excellent temperature and aging characteristics for the resonance frequency of the unit; and
(4) There has been widely used for the circuit arrangement of a piezoelectric ceramic filter a ladder type circuit and a Jaumann type circuit. It is known that when the mechanical quality factor Qm of the piezoelectric ceramic resonator incorporated in such a circuit varies, the filter insertion loss also varies depending on it. Typically, the piezoelectric ceramic resonator used for the piezoelectric ceramic resonator unit or filter has a mechanical quality factor Qm of 300 to 2000. Piezoelectric ceramic resonators with a small temperature dependent Qm within the temperature range from -20.degree. C. to 60.degree. C. can easily be obtained when the material has a Qm of 500 or less, whereas it is difficult to obtain such resonators if Qm is larger than 500. However, when a narrow-band filter having a fractional band width of 1% or less and using an piezoelectric ceramic material having a Qm of 500 or less, the filter insertion loss tends to increase, and it is difficult to obtain a filter having satisfactory characteristics. It is also difficult to obtain a filter having sharp frequency selectivity characteristics. Therefore, in order to make a narrow-band filter or a filter having sharp frequency selectivity characteristics, it is necessary to use a piezoelectric ceramic resonator having a Qm of 500 or more. However, when a filter is constructed using a piezoelectric resonator having a Qm of 500 or more, the value of Qm depends too much on the temperature and thus the filter insertion loss also depends too much on the temperature. Therefore, the magnitude of the output signal from such filters varies disadvantageously depending on the temperature. For the reasons set forth above, there is a need of suppressing variations of Qm with temperature.