A piezoelectric filter using a thickness-shear vibration of a piezoelectric substrate or the like is generally arranged such that the piezoelectric substrate is provided on one surface thereof with a pair of input/output electrodes (split electrodes) with a predetermined gap provided, and on the other surface thereof with a grounding electrode opposite to the pair of input/output electrodes. For example, when such a piezoelectric filter is to be incorporated in a communication device or the like, the following structure is often adopted in order to simplify the mounting process or the like. That is, the piezoelectric filter is inserted in a container of the surface mounting type made of ceramics or the like and the piezoelectric substrate is supported in the container at such a suitable position as not to prevent the piezoelectric substrate from being vibrated.
To assure high attenuation characteristics, a plurality of such piezoelectric filters are required to be connected in cascade in multiple stages. However, when a plurality of piezoelectric filters each housed in one surface-mounting-type container are connected in multiple stages, this results in increase in mounting surface. This contributes to the failure to make the device in a compact design.
To overcome the problem above-mentioned, there is conventionally proposed and put in practical use a so-called multiple-stage-connected multiple-mode piezoelectric filter unit in which two electrode groups each comprising a pair of input/output electrodes and a grounding electrode, are formed on one piezoelectric substrate and two piezoelectric filters are substantially formed on a single piezoelectric substrate and housed in a single container.
Such a multiple-stage-connected multiple-mode piezoelectric filter unit is finished as a product by connecting the output electrode of one electrode group, out of two electrode groups formed on the piezoelectric substrate, to the input electrode of the other electrode group. However, when one electrode group is excited, the other electrode group also resonates due to the acoustic and electromagnetic coupling between the electrode groups. It is therefore substantially impossible to independently adjust the respective filters, causing the final product to be adjusted with much difficulty.
In this connection, a variety of proposals have been made as follows. In a piezoelectric substrate at its position where two electrode groups are partitioned, a slit is formed to alleviate the acoustic coupling between the electrode groups, and an electromagnetism intercepting material is secured to the container and passes through the slit to prevent the electromagnetic coupling between the electrode groups (Japanese Patent Laid-Open Publication No. 59-24910). To partition the two electrode groups on the same piezoelectric substrate, an intercepting groove is formed in the piezoelectric substrate by cutting the same from a peripheral edge thereof and a band-like metallic layer is disposed on each of both sides of the intercepting groove (Japanese Patent Publication No. 57-56810). On a piezoelectric substrate at its position where two electrode groups are partitioned, a linear electrode to be connected to a grounding electrode is formed and a damping material such as silver paste or the like is applied on the linear electrode (Japanese Patent Laid-Open Publication No. 61-17826).
However, none of these proposals provides a sufficient ability of preventing the acoustic and electromagnetic coupling between the electrode groups. Thus, the final adjustment is disadvantageously still difficult.
Recently, in a communication device, particularly, a portable-type communication device, it is increasingly required to minimize the mounting area of each of the incorporated circuit elements. Accordingly, as a multiple-stage-connected multiple-mode piezoelectric filter unit, too, there is required a structure in which a small container of the surface mounting type houses an element having two electrode groups formed on a small-area piezoelectric substrate. However, when a piezoelectric substrate having two electrode groups formed thereon is housed in a small container, the distance between these two electrode groups is very small in the structure according to each of the proposals above-mentioned. It is therefore difficult to avoid interferences of the lines of electric force generated based on the electric fields formed by two pairs of input/output electrodes. Disadvantageously, this not only provides poor guarantee attenuation characteristics but also makes the final adjustment difficult due to electromagnetic coupling resulting from the lines of electric force.
In view of the foregoing, the present invention is proposed with the main object of providing, in a multiple-stage-connected multiple-mode piezoelectric filter unit, a piezoelectric filter which is higher in the ability of preventing the acoustic and electromagnetic coupling between the electrode groups than each of the proposals of prior art, thus facilitating the final adjustment of the filter.
It is another object of the present invention to provide a piezoelectric filter in which, even though the mounting area is minimized to cause the two electrode groups to get near each other, the influences of lines of electric force from the electrode groups are hardly exerted, such that the piezoelectric filter is small in size and easy in final adjustment and capable of exhibiting excellent guarantee attenuation characteristics.