Ceramic filter using multiple thin piezoelectric layers

A ceramic filter including a piezoelectric ceramic and input and output electrode groups disposed within the ceramic body. The input electrode group and the output electrode group extend toward each other from opposing sides of the piezoelectric ceramic. They do not meet, however, and a predetermined spacing, along the thickness dimension of the ceramic, separates the two electrode groups. The input and output electrode groups are each composed of a plurality of electrodes which overlap each other with ceramic layers interposed therebetween.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improvements in the construction of a ceramic 
filter making use of bulk waves wherein within a ceramic, an input 
electrode group and an output electrode group are spaced a predetermined 
distance apart from each other and overlap each other in the direction of 
the thickness of the ceramic. 
2. Description of the Prior Art 
Japanese Patent-Laying Open Gazette No. 85613/1983 discloses an example of 
such ceramic filter. The ceramic filter disclosed in said Japanese 
Patent-Laying Open Gazette No. 85613/1983 is shown in a perspective view 
in FIG. 1. This multi-layered type ceramic filter 1 comprises a 
piezoelectric ceramic 2 treated for polarization in the direction of the 
thickness, i.e., in the P direction, and two electrodes groups 3 and 4 
disposed within the piezoelectric ceramics 2. 
One electrode group 3 forms an input electrodes group, while the other 
electrodes group 4 forms an output electrodes group. The electrode groups 
3 and 4 are composed of a plurality of interdigital electrodes 3a, 3b, 3c, 
3d and 4a, 4b, 4c, 4d, respectively, overlapping each other in the 
direction of the thickness of the piezoelectric ceramic 2. In the input 
electrodes group 3, the electrodes 3a and 3c are interconnected and 
brought out together to the periphery of the ceramic. The other electrodes 
3b and 3d are likewise interconnected and routed. This manner of lead-out 
is also applied to the other electrodes group. 
When a voltage is applied across the input electrodes group 3, vibrations 
are transmitted in the direction of the thickness, i.e., in the vertical 
direction since the electrodes 3a, 3b, 3c and 3d overlap each other in the 
direction of the thickness. These vibrations are transmitted to the 
mechanically coupled electrodes group 4. That is, vertical vibrations 
produced by the input electrodes group 3 are converted into transverse 
vibrations which are then transmitted. The transmitted vibrations cause a 
displacement of acoustic waves in the direction of the thickness of the 
device, i.e., in the vertical direction in the output electrodes group 4, 
whereby a voltage is produced. The produced voltage is available the 
electrodes 4a and 4c and the electrodes 4b and 4d of the output electrodes 
group 4. 
With the conventional multi-layered type ceramic filter 1, where vibrations 
are transmitted from the input electrodes group 3 to the output electrodes 
group 4, the direction of displacement is changed twice, as described 
above, and, consequently, the efficiency of the device is very low. 
Another drawback is that with the transverse displacement, i.e., 
transverse effect, an unnecessary spurious vibration is produced in the 
output waveform. 
Japanese Patent-Laying Open Gazette No. 85614/1983 discloses a ceramic 
filter usable in the high frequency region. FIG. 2 show this ceramic 
filter in a schematic front view. Within the piezoelectric ceramics 5, two 
electrodes groups 6 and 7 are disposed. The electrodes group 6 forms an 
input electrodes group, while the electrodes group 7 forms an output 
electrodes group. The electrodes groups 6 and 7 are composed of a 
plurality of electrodes 6a . . . 6d and 7a . . . 7d, respectively, 
overlapping each other, the ceramic layers between adjacent electrodes 
being polarized in opposite directions as indicated by arrows. In the 
electrodes groups 6 and 7, input and output terminals are connected to the 
two electrodes 6a and 6d and the two electrodes 7a and 7d, respectively, 
positioned on the opposite outer sides close to the piezoelectric ceramic 
surfaces. Because of such construction, a ceramic filter is attained which 
is capable of effectively suppressing spurious vibrations due to primary 
resonance. 
With the conventional ceramic filter shown in FIG. 2, however, because 
vibrations are transmitted from the input electrodes group 6 to the output 
electrodes group 7, since the direction of displacement is changed twice, 
and the efficiency remains is very low. Another drawback is that with the 
transverse displacement, i.e., transverse effect, an unnecessary spurious 
vibration is produced in the output waveform. 
SUMMARY OF THE INVENTION 
Accordingly, a principal object of this invention is to provide a ceramic 
filter which eliminates the previously discussed drawbacks of prior art 
ceramic devices by being highly efficient and by producing a minimum of 
unnecessary spurious vibrations. 
According to a broad aspect of this invention, there is provided a ceramic 
filter making use of bulk waves and comprising a piezoelectric ceramic, 
and an input electrode group of an output electrode group which are spaced 
a predetermined distance within the ceramic layers in the direction of the 
thickness and overlap each other. Each input and output electrode groups 
include a plurality of internal electrodes which overlap each other in the 
direction of the thickness of the ceramic layers which separate adjacent 
electrodes. The electrode arrangement is such that vibrations produced by 
a voltage applied across the input electrodes group are propagated in the 
direction of the thickness of the ceramics to reach the output electrodes 
group, whereby an output is derived from the output electrodes group. 
Because of the arrangement described above, no change of direction of 
vibration is made in this invention and hence the efficiency is increased 
and since the vibrations are trapped in the ceramics, it is possible to 
provide a ceramic filter suitable for use as a chip component. 
The ceramic layers may be polarized uniformly in one direction or 
preferably, in order to increase the efficiency of trapping vibration 
energy, only part of the ceramic may be polarized. Further, in the 
plurality of internal electrode portions, each ceramic layer between 
adjacent internal electrodes may be polarized so that adjacent ceramic 
layers are polarized in opposite directions along the direction of the 
thickness. Further, an arrangement may be made wherein each internal 
electrode consists of a plurality of divisional electrodes and first and 
second adjacent electrodes in the divisional electrodes are opposed at 
least partly to a third electrode adjacent thereto through another ceramic 
layer. 
Further, the piezoelectric ceramic may be polarized in the direction at 
right angles to the direction of the thickness, in which case thickness 
shear vibrations are utilized. 
These objects and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a schematic sectional view for explaining the principle of the 
invention. According to the invention, within a piezoelectric ceramic 8, 
an input electrode group 9 and an output electrodes group 10 are spaced a 
predetermined distance apart from each other in the direction of the 
thickness and overlap each other. 
The input and output electrodes groups 9 and 10 are respectively composed 
of a plurality of internal electrodes 9a . . . 9g and 10a . . . 10g, 
overlapping each other in the direction of the thickness of the ceramic 8 
through its many ceramic layers. As a result of such arrangement, 
according to the invention, bulk waves produced by a voltage applied 
across the input electrodes group 9 are propagated in the direction of the 
thickness as indicated by arrow A to reach the output electrodes group 10, 
whereby an output is derived from the output electrodes group 10. 
Since bulk waves vibrating in the direction of the thickness propagate as 
such in the direction of the thickness and are interrupted by the output 
electrode group 10, there is no change of direction in the accoustical 
vibrations as in the conventional ceramic filters shown in FIGS. 1 and 2. 
Therefore, a highly efficient filter is realized. The details of each 
embodiment will now be described. 
FIG. 4 is a perspective view showing a multi-layered type piezoelectric 
ceramic used in a first embodiment of this invention. The multi-layered 
type ceramics 11 is formed by applying an internal electrode forming paste 
to a number of ceramic green sheets and firing them together. Examples of 
piezoelectric ceramic materials are lead zirconate titanate, lead 
titanate, barium titanate and other substances which can modify the 
previous materials. 
Examples of the internal electrode forming pastes are palladium, 
silver-palladium alloy and other high-melting metals and alloys. In this 
embodiment, a composition of Pb.sub.0.85 La.sub.0.1 TiO.sub.3 +0.5 wt. % 
MnO.sub.2 was used as the piezoelectric ceramic material. 
The ceramic green sheets are constructed by mixing calcinated powder of the 
aforesaid composition with a binder and shaping the mixture by the doctor 
blade method. The ceramic green sheets thus obtained are used for applying 
thereon, by a printing method, palladium paste for forming internal 
electrodes. They are then pressed together and sintered at 1200.degree. C. 
for two hours, whereby a multi-layered type ceramic 11 is obtained. 
Subsequently, polarization electrodes 12 and 13 are formed on the upper and 
lower surfaces of the multi-layered type ceramic 11. Then the ceramic 11 
is subjected to a polarization treatment so that it is polarized in the 
direction of its thickness, i.e., in the direction of arrow P. As is seen 
from FIG. 4, the internal electrodes are divided into two electrode groups 
14 and 15. The electrode groups 14 and 15, in this embodiment, comprise 
five electrodes 14a . . . 14e and 15a . . . 15e, respectively. The 
distance between adjacent electrodes is 200 .mu.m. Further, the distance 
between the electrode groups 14 and 15, i.e., between the electrodes 14e 
and 15a is 1 mm according to the presently shown embodiment of the 
invention. As is clear from FIG. 4, the electrodes 14a . . . 14e and 15a . 
. . 15e of the electrodes groups 14 and 15 are alternately led out to the 
lateral surfaces 16 and 17. Therefore, the electrodes 14a . . . 14e and 
15a . . . 15e are in the form of interdigital electrodes. 
Subsequently, the lateral surfaces 16 and 17 of the piezoelectric ceramic 
11 are formed with input electrodes 21 and 22 and output electrodes 23 and 
24, whereby an embodiment of this invention is obtained, as shown in a 
perspective view in FIG. 5. In this case, the electrode group 14 forms the 
input electrodes group, while the electrodes group 15 forms the output 
electrodes group, but it is understood that the electrodes group 14 may be 
used as the output electrodes when the electrode group 15 are the input 
electrodes. 
In this embodiment, when a voltage is applied across the input external 
electrodes 21 and 22, the input electrode group 14 is vibrated along of 
the thickness of the device and elastic waves due to these vibrations are 
transmitted through the multi-layered type ceramic 11 to the output 
electrodes group 15. The electrodes 15a . . . 15e of the output electrodes 
group 15 vibrate as a result and an output is generated at the output 
external electrodes 23 and 24 which are connected to the output electrode 
group 15. 
Because there is no change of direction in the acoustic wave as in the 
conventional multi-layered type ceramic filter, the illustrated 
multi-layered type ceramic filter, according to the invention, is highly 
efficient. Similarly, since no unnecessary vibrations due to transverse 
effects occur, spurious outputs are reduced. Further, in the electrode 
groups 14 and 15 of the multi-layered type ceramics 11, the vibrations are 
trapped in the bulk centered at a region where the electrodes 14a-14e and 
the electrodes 15a-15e overlap each other, so that the lateral surfaces 16 
and 17 can be fixed and hence a chip component can be easily made. 
Further, in this embodiment, the frequency pass band can be easily changed 
without changing the ceramic material. That is, by changing the thickness 
of green sheets in forming the multi-layered type ceramic 11, the 
interelectrode distance can be changed and hence the frequency of sound 
waves allowed to pass can be changed. 
Let "v" be the sound velocity, "f" be the frequency of sound waves that 
propagate through, ".lambda." be the wave length, and "t" the thickness of 
each layer, then the relation holds: 
EQU v=f.lambda.=f.multidot.2t 
Thus, if the sound velocity is constant, then the frequency "f" can be 
changed by changing the thickness "t". The sound velocity of the bulk 
waves of this material system is about 5200 m/sec. Further, by changing 
the distance between the electrode groups 14 and 15 in FIG. 5, it is very 
easy to change the delay time between input and output electrodes so that 
the assembly can be used as a delay element. 
Other embodiments of the present invention will now be described in 
connection with actual data obtained in reducing the invention to 
practice. 
FIG. 6 is a perspective view showing a second embodiment of the invention. 
In this case, one electrode group 34 and another electrode group 35 
include composed of six electrodes 34a . . . 34f and 35a . . . 35f, 
respectively, with electrodes 36a, 36b, and 36c formed between the 
electrode groups 34 and 35. The electrode groups 34 and 35 are formed with 
external electrodes 21, 22, 23, and 24, as in the case of the embodiment 
shown in FIG. 5. Thus, in the embodiment shown in FIG. 6, since the 
electrode groups 34 and 35 comprise six electrodes 34a . . . 34f and 35a . 
. . 34f, respectively, the number of electrode pairs, n, is: 
EQU 2n+1=6 
Thus, n=2.5. The frequency characteristic of this embodiment is shown in 
FIG. 8. As is clear from FIG. 8, the spurious level is low. 
FIG. 7 is a perspective view showing a third embodiment of this invention. 
In this case, the electrode groups 44 and 45 are composed of seven 
electrodes 44a . . . 44g and 45a . . . 45g, respectively, with a single 
electrode 46 is disposed between the electrodes groups 44 and 45. Thus, 
this embodiment differs from the one shown in FIG. 6 only in the number of 
electrodes. The rest of the arrangement is the same. In this case, since 
the electrodes groups 44 and 45 include seven electrodes 44a . . . 44g and 
45a . . . 45g, respectively, if follows from 2n+1=7 that the number it 
electrode pairs, n, is 3. 
The frequency characteristic of the embodiment shown in FIG. 7 is shown in 
FIG. 9. A comparison between the frequency characteristics shown in FIGS. 
9 and 8 indicates that the width of pass band can be easily changed by 
changing the number of electrode pairs. 
As is clear from the second and third embodiments, of the invention, the 
electrodes 36a . . . 36c, and 46 which are not connected to the external 
electrodes are not formed at each electrodes group, and no electrodes may 
be formed between the electrode groups 14 and 15 of the embodiment shown 
in FIG. 5. 
Further, in order to suppress the reflection of induced bulk waves at the 
end surfaces (e.g., 12 and 13 in FIG. 4), the end surfaces are provided at 
positions remote from the electrodes groups. The end surfaces are formed 
with undulations equal to or greater than the wave length or the 
interelectrode distance in the electrodes groups. Atternately the end 
surfaces are inclined with respect to the electrodes, whereby the spurious 
component is reduced. 
FIG. 10 is a perspective view showing a ceramic filter 41 according to a 
fourth embodiment of this invention. The ceramic filter 41 of this 
embodiment has almost the same arrangement as the one shown in FIG. 6. 
Thus, correspondingly identical parts are marked with corresponding 
reference numerals, and will not be described. 
The ceramic filter 41 of this embodiment is characterized in that the 
piezoelectric ceramic is subjected only partially, but not totally, to a 
polarization treatment. That is, by forming the polarization electrodes on 
only a portion of the end surfaces of the piezoelectric ceramics aligned 
in the direction of the thickness, (for example, a shaded portion 44) (and 
a similar portion, not shown, on the end surface 43), and by effecting 
polarization, the ceramic portion between the overlapping portions of the 
polarization electrodes is polarized. 
In the example of FIG. 10, the width of the polarization electrodes, as 
shown at a shaded portion 44, is the same as the width of the overlapping 
portions of the internal electrodes 34a . . . 34f and 35a . . . 35f 
forming the input and output electrodes groups and hence the ceramics 
between the overlapping portion of the internal electrodes is polarized. 
Thus, since only the ceramics between the overlapping portion of the 
internal electrodes is vibrated, trapping efficiency of thickness 
vibration can be fully increased, with the result that it becomes possible 
to obtain a response having less spurious vibrations. 
FIG. 11 shows the attenuation-frequency characteristic of the ceramic 
filter shown in FIG. 10. When FIG. 11 is compared with FIG. 8 which shows 
the characteristic of the embodiment shown in FIG. 6, it can be seen that 
the ceramic filter of the embodiment shown in FIG. 10 provides an improved 
response. That the vibration envelope is smooth because spurious output 
are suppressed in the pass band region. 
In addition, in the embodiment shown in FIG. 10, it is preferred that only 
the portions between the overlapping portions of the internal electrodes 
34a . . . 34f and 35a . . . 35f be treated for polarization. It is pointed 
out, however, that it cannot be avoided that the portion to be treated for 
polarization extends somewhat. 
FIG. 12 is a perspective view showing a ceramic filter according to a fifth 
embodiment of this invention. In this case, the right and left lateral 
surfaces of a piezoelectric ceramics 51 formed with internal electrodes 
52a . . . 52e and 53a . . . 53e are formed with polarization electrodes 54 
and 55, and polarization treatment is applied thereto. With polarization 
treatment, as shown in FIG. 12, the ceramic layers between the internal 
electrodes 52a . . . 52e and 53a . . . 53e are polarized in opposite 
directions, as indicated by arrows. 
Subsequently, as shown in FIG. 13, the upper and lower surfaces of the 
piezoelectric ceramic 51 are formed with electrodes 56 and 57. The 
electrodes 56 and 57 parallelly overlap a plurality of internal electrodes 
52a . . . 52e and 53a . . . 53e. Thus, the electrode 56 and electrodes 52a 
through 52e form one electrode group, while the electrodes 53a through 53e 
and electrode 57 form another electrode group. Thereafter, the 
polarization electrodes 54 and 55 are removed by cutting or grinding, and 
external electrodes 58 and 59 for contacting the electrodes 52e and 53e 
are formed on one lateral surface of the ceramics 51. As shown in FIG. 13, 
the ceramic filter, wherein the electrodes are connected in series, can be 
obtained. 
In the embodiment shown in FIG. 13, if the upper electrode group is the 
input electrode group, a voltage will be applied across the electrode 56 
and internal electrode 52e. Thereby, vibrations are produced at the upper 
electrode group in the vertical direction of the thickness and propagate 
within the piezoelectric ceramics 51 to reach the lower electrode group, 
so that an output is derived from the electrode 57 and internal electrode 
53e. Thus, since there is no change of direction of displacement involved 
in contrast to the case of the conventional ceramic filter shown in FIG. 
2, it is seen that a highly efficient ceramic filter can be attained. 
Similarly, since undesired vibrations attendant on transverse effect are 
not produced, the spurious level is also very low. Further, since only the 
electrodes 56, 52e, 53a, and 57 positioned on opposite outer sides of the 
electrodes groups among the electrodes 56, 52a . . . 52e, 57, and 53a . . 
. 53e forming the electrodes groups are led out, the electrodes are not 
connected in parallel as in the conventional ceramic filter shown in FIG. 
1. Thus, even if the number of electrodes is increased to utilized 
vibration modes in a higher frequency region, sharp decreases in impedance 
can be avoided and no problem of impedance matching is incountered. The 
impedance-frequency characteristic and attenuation-frequency 
characteristic of the embodiment shown in FIG. 13 are shown in FIGS. 14 
and 15. In addition, for comparison purposes, the characteristics of a 
ceramic filter of the type in which the internal electrodes are connected 
in parallel are shown in broken lines in FIGS. 14 and 15. The number of 
electrode pairs, n, in the embodiment shown in FIG. 13 is 4.5 as computed 
from 2n+1=10. 
FIG. 16 is a perspective view showing a sixth embodiment of the invention. 
In the present embodiment, in addition to the configuration of the 
embodiment shown in FIG. 13, one or more internal electrodes 60 are formed 
between the input and output electrodes groups. In this way, one or more 
internal electrodes 60 may be formed between the input and output 
electrode groups. Thus, the ceramic filter shown in FIG. 16 can be 
obtained by preparing a piezoelectric ceramic with a number of internal 
electrodes arranged in parallel overlapping relation, and utilizing part 
of the internal electrodes to form two electrodes groups. That is, two 
electrodes including a number of ceramic layers partitioned by the 
internal electrodes are connected to the outside, whereby an electrode 
group can be formed. Further, if the internal electrodes 60 shown in FIG. 
16 are grounded, the stray capacity is decreased. 
FIGS. 17 through 26 collectively illustrate a seventh embodiment of the 
invention. The ceramic filter in this embodiment is intended to increase 
impedance as in the ceramic filters of the fifth and sixth embodiments 
described above. 
FIG. 17 is a sectional view schematically showing one of the input and 
output electrode groups in the seventh embodiment of the invention. In 
FIG. 17, the internal electrodes 114a . . . 119a and 114b . . . 119b 
overlap each other in the direction of thickness and are formed in three 
or more layers, e.g., six layers. Between these layers of electrodes, 
piezoelectric ceramic layers 120 . . . 124 are interposed. The 
piezoelectric ceramic layers 120 . . . 124 are constructed by sintering 
when the layers are sintered with the electrodes 115a . . . 118a and 115b 
. . . 118b disposed therebetween. 
The formation of the ceramic layers and internal electrodes involves the 
same materials and methods as used in the first embodiment described above 
with reference to FIGS. 4 and 5. If the electrodes 114a, 114b, 119a, and 
119b positioned on the outermost sides are made of palladium or 
silver-palladium alloy, they would be easily oxidized when subjected to 
high temperatures and the resistance would rise. Thus, though not shown, 
it is preferable that ceramic layers be formed to cover the electrodes 
114a, 114b, 119a, 119b before firing or that after firing, the electrodes 
114a, 114b, 119a, and 119b be formed by baking thereon silver paste, for 
example. 
As described above, external terminals 125 and 126 are connected to the 
electrodes of the respective layers. External terminal 125 is connected to 
the electrodes 114a, 116a, and 118a, while external terminal 126 is 
connected to the electrodes 115a, 117a, and 119a. 
The presently discussed embodiment is characterized in that the electrodes 
positioned at layers which include the electrodes which are externally 
connected are referred to as divisional electrodes. In this embodiment, 
the electrodes connected to the external electrodes are the electrodes 
114a through 119a, and what is referred to as the electrodes positioned at 
the layers along which these electrodes extend includes all the 
electrodes, and hence it follows that all the electrodes are divisional 
electrodes. For example, the electrodes 114a and 114b are divisional 
electrodes. 
Further, the seventh embodiment is also characterized in that, first and 
second adjoining electrodes, of the divisional electrodes, are separated 
at least partially, by a third electrode which extends through a 
piezoelectric ceramic layer. The first and second adjoining electrodes 
are, for example, the electrodes 114a and 114b, and the third electrode is 
the electrode 115b. These first, second, and third electrodes are 
relatively determined; thus, if the electrodes 115a and 115b are the first 
and second electrodes, then the third electrode is the aforesaid electrode 
114b, and at the same time since the electrode 116b lies oppositely at 
least partly, to the electrodes 115a and 115b through a piezoelectric 
ceramic layer 121, it may be said to be the third electrode. 
Because of the aforesaid arrangement, the successive connection of the 
first electrode, piezoelectric ceramic layer, third electrode, 
piezoelectric ceramic layer, and second electrode forms a series-connected 
capacitor forming part. The ceramic layer as such has a dielectric 
characteristic and a piezoelectric characteristic. Thus capacitance is 
derived from the ceramic layer between electrodes. Therefore, the term 
"capacitor forming part" as used herein means a part having a dielectric 
capacitance. The relationship between the various parts of the aforesaid 
arrangement will now be described with reference to a particular portion 
of the embodiment as an example. 
The electrode 114a, (first electrode), piezoelectric ceramic layer 120, 
electrode 115b (third electrode), piezoelectric ceramic layer 120, and 
electrode 114b (second electrode) are successively connected to form a 
series-connected capacitor forming part. In addition, in this embodiment, 
further connected to the electrode 114b serving as the third electrodes 
are the piezoelectric ceramic layer 120 and electrode 115a. 
In this embodiment, the piezoelectric layers are treated to be polarized in 
the direction of their thickness. Further, this embodiment is 
characterized in that the portion positioned between the first and third 
electrodes and the portion positioned between the second and third 
electrodes are mutually oppositely polarized. More particularly, the 
piezoelectric ceramic layers 120 through 124 are polarized in the 
direction of the thickness. Focusing on ceramic layer 120, for example, it 
is seen that the portion positioned between the electrode 114a (first 
electrode) and the electrode 115b (third electrode) is in the direction of 
an arrow opposite to the portion positioned between the electrode 114b 
(second electrode) and the electrode 115b (third electrode); thus, these 
portions are mutually oppositely polarized. 
If an electric field is applied across the external terminals 125 and 126 
so that, for example, as shown in FIG. 7, the external terminal 125 is at 
a positive potential and the external terminal 126 is at a negative 
potential, all the portions in the piezoelectric ceramic layers 120, 122, 
and 124 are contracted (squeezed together), while all the portions in the 
piezoelectric ceramic layers 121 and 123 are expanded. That is, mutually 
opposite vibration displacements appear in the piezoelectric ceramic 
layers on the opposite sides of any one layer of electrode. And since for 
example three series-connected capacitors are formed between the external 
terminals 125 and 126, the capacity is lowered and the impedance is 
increased. 
FIG. 18 shows the frequency-impedance characteristic of the arrangement 
shown in FIG. 17. It can be observed that the curve S indicating the 
characteristic of the embodiment of FIG. 17 is greater in impedance than 
the curve T indicating the characteristic of an arrangement having no 
divisional electrodes. 
FIGS. 19 and 20 show modifications of the seventh embodiment, which differ 
from the foregoing example in the number of series-connected capacitors. 
In addition, in FIGS. 19 and 20, two layers of electrodes and one 
intervening piezoelectric ceramic layer appear. 
In FIG. 19, two electrodes 127a and 127b forming one layer, one electrode 
128 forming another layer, and a piezoelectric ceramic layer 129 
interposed therebetween are shown. External terminals 130 and 131 are 
connected to the electrodes 127a and 127b, respectively. Arrows shown in 
the piezoelectric ceramic layer 129 indicate the direction of 
polarization. In this embodiment, the electrodes to be connected to the 
external terminals 130 and 131 are the electrodes 127a and 127b, which 
are, therefore, divisional electrodes. Thus, the electrode 129a 
corresponds to the first electrode, the electrode 127b corresponds to the 
second electrode, and the electrode 128 corresponds to the third 
electrode. In addition, the electrode 128 having no external terminal 
connected thereto is not a divisional electrode. 
In FIG. 20, four series-connected capacitors are formed. In this 
embodiment, three electrodes 132a, 132b, and 132c form one layer, while 
two electrodes 133a and 133b form another layer, with a piezoelectric 
ceramic layer 134 interposed therebetween. Arrows shown in the 
piezoelectric ceramic layer 134 indicate the direction of polarization. 
The electrode 132a has an external terminal 135 connected thereto, while 
the electrode 132c has an external terminal 136 connected thereto. The 
electrodes positioned at the layer where the electrodes, which have the 
external terminal 135 and 136 connected thereto, extend are divisional 
electrodes, as are electrodes 132a, 132b, and 132c. In this embodiment, 
the electrodes 133a and 133b, which are not connected to the external 
terminals, are also divisional electrodes. There is no contradiction here 
because there is no stipulation that electrodes not connected to external 
terminals are not divisional electrodes. 
In the embodiments of FIGS. 19 and 20, the number of series-connected 
capacitors is arbitrary and may be selected in connection with a desired 
impedance value. 
The method for creating the polarization in the product and the external 
terminal forming method in the actual production of the structure shown in 
FIG. 17 will now be described. 
As shown in FIG. 17, the piezoelectric ceramic layers 120 . . . 125 
differently configured and vary in accordance with their position in the 
direction of polarization. Thus, after sintering, the procedure of simply 
applying a high DC electric field is not sufficient to obtain such 
polarized state. For this reason, the following methods are used. 
In FIG. 21, a polarization method for each piezoelectric ceramic layer is 
shown. For example, when a piezoelectric ceramic layer 121 is treated to 
polarize it, a positive potential is applied to electrode 116a and a 
negative potential is applied to electrode 115a, so that an electric field 
is applied successively to the electrodes 116a, 115b, 116b, and 115a, to 
create polarization in opposite directions. 
Another possible method, as shown in FIG. 22, is to form slits in 
electrodes positioned oppositely to two electrodes through one 
piezoelectric ceramic layer in order to separate the electrodes. In the 
range shown in FIG. 22, the electrodes 114b and 115b are formed with slits 
137. At ends of the electrodes 114b and 115b adjacent the slit 137 are 
lead-out portions 138a and 138b extending to an edge end of the 
piezoelectric ceramic layer 120 or 121. Such arrangement is also employed 
in the other electrodes. 
FIG. 23 shows the relationship between the electrodes where the electrodes 
shown in FIG. 22 are stacked. As seen at FIG. 23, three separate sets of 
electrodes are arranged in the direction of the thickness. Thus, if a 
voltage shown in FIG. 23 is applied to each electrode groups, the 
piezoelectric ceramic layers 120 . . . 124 are polarized in the direction 
indicated by arrows. 
In FIG. 24, the external appearance of the sintered body 139 after it is 
polarized with the electrode groups illustrated in FIG. 23 is shown in 
perspective. 
On the surface of the sintered body 139, as shown in FIG. 25, are formed 
two connecting electrodes 140a and 140b and the external terminal 125 and 
126. As is clear from FIGS. 24 and 25, a connection terminal 140a is 
connected to the lead-out portions 138a and a connection electrode 140b is 
connected to the lead-out portions 138b. In this manner, the electrode 
114b-119b separated by the slits 137, after being polarized, are again 
electrically reconnected. After the external terminals 125 and 126 are 
formed, the electrically connected state shown in FIG. 17 is attained. 
FIG. 26 shows a ceramic filter of the seventh embodiment of the invention 
using electrodes produced in the manner described above. In this case, the 
two electrode groups, i.e., the electrode groups 147 and 148 are disposed 
within a ceramic and spaced a predetermined distance apart from each other 
in the direction of the thickness. 
FIGS. 27 through 32 are helpful in describing an eighth embodiment of the 
invention. In this embodiment, there is provided a ceramic filter 
utilizing thickness shear vibration, not longitudinal thickness vibration 
as described before. It is important to note that it has heretofore been 
very difficult to produce a multi-layered type ceramic filter utilizing 
such thickness shear vibration. The production process for such a filter 
will be described. 
First, carbon paste is applied by a printing process to portions of ceramic 
green sheets at areas where electrodes are to be formed. Then the sheets 
are laminated. (This carbon paste may contain a ceramic powder, preferably 
a ceramic powder of the same type as the ceramic green sheets. In such a 
case, the ceramic powder remains at the sintering stage, so that the 
cavities are supported by this ceramic powder and thereby prevented from 
deforming.) This state is shown in FIG. 27. In this figure, layers of 
carbon are disposed in parallel overlapped relationship to one another 
within the ceramic 201 in its direction of thickness. 
The ceramic 201 containing the layers of carbon 206 is then sintered. In 
this sintering step, the carbon 206 burns out, leaving cavities at 
positions where the layers of carbons 201 were present. This state is 
shown in FIG. 28. In FIG. 28 the cavities are referenced by reference 
numeral 207. In this state, electrodes, of silver for example, are baked 
on opposite sides of the ceramics 201 to form polarization electrodes 204 
and 205, and a polarization treatment is performed so that polarization 
takes place in a direction which extends at right angles to the direction 
of the thickness of the ceramics 201. Since there is no internal electrode 
in the ceramics 201, the ceramic is uniformly polarized. Thereafter, the 
polarization electrodes 204 and 205 are removed. 
While the polarization electrodes 204 and 205 have been shown to be located 
on the right and left sides of the ceramic 201, they may be positioned on 
the front and back sides. In such a case, if the polarization electrodes 
204 and 205 are porous electrodes covering the openings of the cavities 
207, then in the step of filling metal electrodes to be described later 
with reference to FIG. 29, there is an advantage that the filled molten 
metal will remain where aplied even during the pulling up of the ceramic 
201 from the molten metal. In addition, if such form of polarization 
electrodes is used, a formation of external electrodes to be later 
described with reference to FIG. 30 is not necessary. 
After polarization, the cavities 207 are filled with metal electrodes by 
means of a molten metal filling device 208 shown in a schematic sectional 
view in FIG. 29. The filling is performed as follows. 
First, an electrode metal 210 in a molten state is stored in a tank 209. 
Examples of electrode metals 210 are 40/60 solder mixture, metal tin, and 
metal lead. Next, the pressure in the tank 209 is reduced by evacuating 
means 211. Under this reduced pressure, the ceramic 201 with its cavities 
207 is immersed in the electrode molten metal 210. Subsequently, the 
evacuating means 211 are disabled, and pressurizing means 212 are 
activated to pressurize the interior of the tank 209. As a result, the 
cavities 207 of the ceramics 201 are filled with the internal electrode 
metal. In addition, to prevent depolarization of the ceramics 201, the 
filling of the internal electrode metal is effected at temperatures below 
the curie point. For example, the filling temperature is 170.degree. C. 
for 40/60 solder, and 200.degree. C. for metal tin. Finally, the ceramics 
201 is pulled up out of the molten metal 210, and the electrode metal 210 
present in the cavities 207 is allowed to cool and solidify. Thereby a 
piezoelectric element capable of achieving the objects of this embodiment 
is obtained. 
FIG. 30 is a perspective view of a piezoelectric element 213 obtained by 
the aforesaid production method. The ceramic 201 has been uniformly 
polarized in the direction at right angles to the direction of the 
thickness, with a plurality of electrodes 210 arranged in the direction of 
the thickness in parallel overlapping relation. 
Finally, the front and back sides of the piezoelectric element shown in 
FIG. 30 are formed with external electrodes 217 and 218, whereby the 
ceramic filter shown in FIG. 31 is obtained. In this case, thermosetting 
electrically conductive paste which thermally sets at temperatures which 
do not cause depolarization, e.g., up to 200.degree. C. is used to form 
the external electrodes 217 and 218. FIG. 32 shows the 
attenuation-frequency characteristic of the ceramic filter thus obtained. 
As shown in FIG. 32, the spurious response level is much lower than that 
of the ceramic filter utilizing the longitudinal thickness vibration 
described previously. 
Though not shown, an advantageous arrangement which can be applied to all 
the embodiments described above will now be described. In this invention, 
as described above, bulk waves are transmitted within the piezoelectric 
ceramic in the direction of the thickness of the piezoelectric ceramic. On 
the other hand, in such piezoelectric devices as piezoelectric resonators 
and piezoelectric filters, it is required to adjust the center frequency 
or resonant frequency according to specific applications. Thus, in this 
invention, since bulk waves propagate in the direction of the thickness of 
the piezoelectric ceramic and because the internal electrodes are disposed 
to overlap one another in the direction of the thickness of the 
piezoelectric ceramics, a desired center frequency for a filter can be 
obtained by adjusting the thickness of the ceramic layer between the 
internal electrode positioned on the outermost side and the end surface of 
the piezoelectric ceramics close to the internal electrode. For example, 
in the first embodiment shown in FIG. 5, a desired center frequency can be 
obtained by controlling the thickness between the internal electrode 14a 
and the end surface of the piezoelectric ceramic 11 adjacent thereto in 
the direction of the thickness thereof and/or the thickness of the 
ceramics between the internal electrode 15e and the end surface of the 
piezoelectric ceramics 11 adjacent to the internal electrode 15a in the 
direction of the thickness of the piezoelectric ceramics 11. Preferably, a 
voltage is applied across the external electrodes 21 and 22 in the 
embodiment shown in FIG. 5 to derive an output from the external 
electrodes 23 and 24, i.e., to activate the ceramic filter. Under this 
condition the end surfaces of the piezoelectric ceramic 11 in the 
direction of the thickness are processed or coated with a damping agent to 
increase its mass, whereby the central frequency can be easily and 
reliably adjusted by actual measurement on real time. Thus, a ceramic 
filter in which the thickness between the internal electrodes which is 
positioned on the outermost side and the end surface of the piezoelectric 
ceramics in the direction of the thickness, is also made possible by the 
present invention. 
FIG. 33 is a schematic front view useful for explaining the effects of 
reflected waves namely triple transit echo waves (T.T.E.). In a ceramic 
filter of the present invention according to an embodiment where bulk 
waves propagate within the piezoelectric ceramics 302 in the direction of 
the thickness and where the filter is formed by stacking a number of 
ceramic green sheets and electrode patterns and sintering the laminate, a 
very small-sized bulk wave device can be constructed another advantage 
offered by the ceramic of the invention is that it is possible to obtain 
bulk wave devices having various impedances by merely changing the 
internal electrode forming method and the polarization method. 
However, because of the large difference in density between the ceramics 
302 and air, a bulk wave C propagate as shown in a schematic front view in 
FIG. 33. It is seen that the wave is easily reflected back and forth from 
the end surfaces 302a and 302b of the ceramic 302 along the thickness of 
the ceramic. Thus, a reflected wave D called triple transit echo (T.T.E) 
is generated. 
Accordingly, in the ceramic filter of the first embodiment shown in FIG. 5, 
for example, the ceramic layers positioned outwardly of the internal 
electrodes 14a and 14e do not positively vibrate. Therefore, even if the 
waves are reflected from the end surfaces 302a and 302b of FIG. 33, the 
vibration mode of the reflected waves differs from that of the bulk wave 
C; therefore, it is possible that propagation of reflected bulk waves 
other than the bulk waves of the intended frequency interferes with the 
frequency characteristic such that the desired performance is not 
obtained. 
FIG. 34 is a perspective view of a ninth embodiment of the invention which 
addresses and solves the problem noted above. A filter 311 includes a 
ceramic 312 treated for polarization in the direction of the thickness, 
and internal electrodes 313 and 314 consisting of a plurality of 
electrodes 313a . . . 313f and 314a . . . 314f disposed within the 
ceramics 312 in parallel overlapping relation in the direction of the 
thickness. The electrodes 313a . . . 313f and 314a . . . 314f forming the 
internal electrodes 313 and 314 are alternately led out to opposite 
lateral surfaces of the ceramics 312 and connected to external electrodes 
315a, 315b and 316a, and 316b. This embodiment is characterized in that 
the electrodes 313a and 314f positioned on the outermost sides in the 
internal electrodes 313 and 314 are respectively formed on the end 
surfaces 312a and 312b of the ceramic 312 in the direction of the 
thickness. 
Thus, if the internal electrode 313 is used as the input electrode and a 
voltage is applied across the external electrodes 315a and 315b, bulk 
waves propagates in the direction of the thickness of the ceramic 312 and 
transmitted on the other internal electrode 314, and an output is develops 
at the external electrodes 316a and 316b. In this case, the outermost 
ceramic layers 317 and 318 are also positively vibrated, so that bulk 
waves propagating in the direction of the thickness of the ceramics 312 
are trapped between the electrodes 313a and 313f. Thus, even if bulk waves 
propagating within the ceramic 312 in the direction of the thickness are 
reflected from the electrodes 313a and 314f owing to the difference in 
density between the electrodes 313a and 314f and air outside the ceramic 
312, the reflected bulk waves resulting from the reflection will have the 
same vibration mode as the bulk waves propagating from the internal 
electrode 313 to the internal electrode 314. Therefore, only the bulk 
waves of the intended frequency will propagate within the ceramic 312 in 
the direction of the thickness. The amplitude characteristics will be 
greatly improved, no T.T.E. will occur, and the insertion loss will be low 
in accordance with design goals. 
In addition, it should be noted that the present form of the internal 
electrodes on the outermost end surfaces of the piezoelectric ceramics in 
the direction of the thickness can be used in all the other embodiments 
described so far. 
FIGS. 35 and 36 are a fragmentary sectional view and a bottom view which 
show a tenth embodiment of the invention. This embodiment is also arranged 
to control the thickness of the ceramic layers positioned outwardly of the 
outermost electrodes in the direction of the thickness of the ceramic 
described previously and, as in the ninth embodiment, to eliminate bulk 
waves which cause the spurious effects. As seen in FIG. 35 (the internal 
electrodes are omitted from the illustration) and FIG. 36, a plurality of 
grooves 421, useful for eliminating unnecessary bulk waves, are formed at 
one end surface 402b of a piezoelectric ceramic in the direction of the 
thickness. 
The bottoms 422 of the grooves 421 are parallel to the end surface 402b, as 
shown and the depth d of the grooves 421 is such that when the wave length 
of bulk waves is denoted by .lambda., the relation 
EQU 2d=(n+1/2).lambda. 
(where n=0, 1, 2 . . . ) is satisfied. Thus, bulk waves E reflected from 
the end surface 402b are one half wavelength out of phase with bulk waves 
F reflected from the bottoms 422 of the grooves 421, so that the reflected 
waves E and F cancel each other. Thus, the reflected bulk waves are 
effectively eliminated. 
In the embodiment shown in FIGS. 35 and 36, the grooves 421 have been 
formed as illustrated. Preferably, however, the total area of the bottoms 
422 of the grooves 421 is made equal to the remainder of the area of the 
end surface 402b, so that the bulk waves reflected from the end surface 
402b and the bottoms 422 of the grooves 421, respectively, are equalized 
and hence eliminated. 
However, since it is only through those ceramic portions where the internal 
electrodes overlap each other that bulk waves propagate in the direction 
of the thickness, the grooves 421 may be formed in the region of the 
ceramic corresponding to the overlapping portions of the internal 
electrodes. It is to be pointed out that the shape of the grooves 421 and 
the manner in which they are distributed are not limited as shown and may 
assume other shapes. 
FIG. 37 is a schematic front view for explaining an eleventh embodiment of 
this invention. As is clear from FIG. 37, shows that for bulk wave 
elimination, one end surface 402b of a ceramics 402 in the direction of 
the thickness is shaped as a non-parallel plane with respect to the 
internal electrodes 404i and 404j (shown in phantom lines). Thus, a bulk 
wave G is reflected by the end surface 402b as shown, which means that it 
is reflected in a direction where the internal electrodes 404i and 404j 
are not present. Thus, it is seen that spurious vibration due to reflected 
bulk waves can be effectively reduced. 
In addition, between the angle of inclination .theta. of the end surface 
402b in the embodiment shown in FIG. 37 and the reflectance, there is a 
relation shown in the graph of FIG. 38. Therefore, it is seen that if 
.theta..gtoreq.10.degree., then the reflectance is 80% smaller than when 
.theta.=0.degree.. Thus, it is preferable that the angle of inclination 
.theta. be at least 10.degree.. 
In addition, in the eleventh embodiment, not only can the end surface 402b 
be inclined as shown in FIG. 37, but it can also be shaped in various 
manners so long as the bulk waves are reflected in a direction away from 
the internal electrodes. For example, as shown in FIG. 39, the end surface 
402b is curved like the lateral surface of a cylinder or as a spherical 
surface. 
FIG. 40 illustrates another embodiment of the invention. In the embodiment 
of FIG. 40, one end surface 402b of a ceramic 402 in the direction of the 
thickness is randomly formed with recesses 431 in order to eliminate bulk 
waves. The average depth x of the recesses 431 is selected so that 
x.gtoreq..lambda./2 where .lambda. is the wavelength of bulk waves. The 
formation of the grooves 431 ensures that bulk waves traveling straight to 
the end surface 402b are scattered. Thus, bulk waves reflected in the 
direction of the internal electrodes can be effectively eliminated. 
In addition, the grooves 431 may be formed after sintering or they may be 
formed simultaneously with the stacking and pressing of ceramic green 
sheets before sintering. 
FIG. 41 illustrates a thirteenth embodiment of the invention. In the 
embodiment of FIG. 41, a sound absorbing layer 441 having substantially 
the same acoustic impedance as a ceramic 402 is attached to one end 
surface 402b of the ceramics 402 in the direction of the thickness. The 
layer 441 eliminates bulk-waves. If the ceramic 402 is of the PZT ceramics 
with a specific gravity of about 8, the sound absorbing layer 441 will be 
formed of a sound absorbing material mixed with PZT powder or lead or 
other metal powder having relatively high specific gravity or an oxide 
such as PbO or Nb.sub.2 O.sub.5, whereby substantially the same magnitude 
of acoustic impedance as that of the ceramic 402 can be obtained. 
In the present embodiment, since the sound absorbing layer 441 having 
substantially the same magnitude of acoustic impedance as that of the 
ceramic is attached to one end surface 402b of the ceramic 402 in the 
direction of the thickness, bulk waves propagating toward one end surface 
402b in the direction of the thickness can be effectively absorbed by the 
sound absorbing layer 441. Therefore, the production of undesirable bulk 
waves is prevented. 
In addition, the sound absorbing layer 441 is not necessarily attached to 
the entire area of the end surface 402 in the direction of the thickness 
and instead it may be attached only to the portion where bulk waves are 
expected . 
The four types of bulk wave eliminating means of the tenth, eleventh, 
twelfth and thirteenth embodiments can be suitably combined to further 
improve elimination. To show this, test results from a test performed on a 
combination of bulk wave eliminating means used in the embodiments shown 
in FIGS. 40 and 41 will be provided below. 
A ceramic filter was fabricated using a lead titanate ceramic 402 and a 
plurality of internal electrodes 1 mm in diameter, the electrodes were 
arranged within the. The interelectrode distance was fixed at 100 .mu.m, 
the propagation distance was 200 .mu.m, and the number of pairs of 
electrodes was 4.5. One end surface 402b of the ceramics 402 in the 
direction of the thickness was randomly formed with recesses whose average 
depth x was greater than .lambda./2, while a sound absorbing layer 441 was 
attached to the ceramic. It was found that whereas the ceramic filter 
before being treated produced a great amount of spurious vibrations, the 
ceramic filter according to the invention provided a characteristic curve 
which was substantially improved in accordance with the design goals. 
It is to be pointed out that since the embodiments shown in FIGS. 10 
through 13 have their end surfaces treated in the direction of thickness, 
they can be used in the first through eighth embodiments. 
In the ceramic filter of the invention, bulk waves propagate from the input 
electrodes group to the output electrodes group and vice versa and hence 
an electrical-mechanical conversion loss of 3 dB for each of the input and 
output electrode groups cannot be avoided. Thus, a total 
electrical-mechanical conversions loss of 6 dB due to bidirectionally is 
present. In a fourteenth embodiment shown in FIG. 42 and in a fifteenth 
embodiment shown in FIGS. 43 and 44 bidirectional propagation loss is 
eliminated. 
FIG. 42 illustrates a fourteenth embodiment of the invention. In FIG. 42, 
only the input electrode group side, i.e., the excitation side of a bulk 
wave filter serving as a bulk wave device is shown. The excitation section 
comprises first and second excitation parts 517a and 517b. The first and 
second excitation parts 517a and 517b comprise a plurality of internal 
electrodes 513a . . . 513d and 513e . . . 513i, respectively. Then are 
spaced 1/4.lambda. apart in the direction of the thickness of a 
piezoelectric ceramic 512. The internal electrodes 513a . . . 513i of the 
first and second excitation parts 517a and 517b are respectively connected 
to external electrodes 515a, 515b, and 515c formed on opposite sides of 
the piezoelectric ceramic 512. As shown, in the second excitation part 
517b, a coil 519 is connected between the external electrodes 515a and 
515c. The second excitation part 517b is adapted to reflect bulk waves 
which propagate from the first excitation part 517a. The coil 519 serves 
to increase the efficiency of reflecting bulk waves coming from the first 
excitation part 517a and is not absolutely necessary. It does, however, 
from part of the bulk wave direction control means of the present 
invention. 
In the embodiment shown in FIG. 42, when a voltage is applied across the 
external electrodes 515a and 515b as to activate the first excitation part 
517a, bulk waves propagate on both sides of the first excitation part 
517a, i.e., in opposite directions along the thickness of the 
piezoelectric ceramic 512. However, since the first and second excitation 
parts 517a and 517b are spaced 1/4.lambda. apart, bulk waves generated by 
the first excitation part 517a are transmitted to the second excitation 
part 517b with a delay of 1/4.lambda.. Further, the transmitted bulk waves 
are reflected by the second excitation part 517b and are further delayed 
by 1/2.lambda. passing through 517b and also delayed by 1/4.lambda. upon 
returning from the second excitation part 517b to the first excitation 
part 517a. Thus, the bulk waves propagated from the first excitation part 
517a to the second excitation part 517b return to the first excitation 
part 517a with a delay of .lambda.. Thus means that they are propagated in 
phase with bulk waves which are propagated in the upper portion of FIG. 
42, i.e., on the side opposite to the second excitation part 517b. 
Therefore, the embodiment of FIG. 42, produces a unidirectional excitation 
part. 
In addition, though not shown, an output electrode group is constructed as 
in the case of the bulk wave filters described so far. 
Also, in the embodiment of FIG. 42, the first and second excitation parts 
517a and 517b have been located at a distance of 1/4.lambda. from each 
other. However, it should be noted that they may be spaced 
n.lambda.+1/4.lambda. (where n is an integer) apart from each other. 
FIG. 43 is a schematic circuit block diagram which explains a fifteenth 
embodiment of the invention. In this embodiment, bulk waves are not 
reflected as they are in the case of the embodiment shown in FIG. 42; they 
are instead delayed by .lambda./4. Thus, a unidirectional bulk wave device 
is formed. That is, two excitation parts 521 and 522 are arranged 
n.lambda.+.lambda./4 apart from each other. This arrangement is the same 
as that shown in FIG. 42. Further, in this embodiment, a .lambda./4 delay 
circuit 523 is connected between the two excitation parts 521 and 522. For 
the .lambda./4 delay circuit 523, any desired arrangements may be used, 
but one comprising capacitors and resistors is preferable. This is because 
the .lambda./4 delay circuit can be constructed integrally with the 
piezoelectric ceramic base. In the embodiment shown in FIG. 43, since the 
first and second excitation parts 521 and 522 are spaced at a distance of 
n.lambda.+.lambda./4 and since the .lambda./4 delay circuit 523 is 
connected between the first and second excitation parts 521 and 522, the 
phase of bulk waves generated by the first excitation part 521 lags by 
.lambda./4 behind bulk waves generated by the second excitation part 522, 
whereby the bulk waves generated by the second excitation part 522 are 
transmitted to the first excitation part 521 with a delay of .lambda./4. 
Thus, they are transmitted in phase with the bulk waves propagating at the 
first excitation part 521 in the direction H in FIG. 43, their direction 
of propagation being also the same, or H. On the other hand, some of the 
bulk waves generated by the first excitation part 521 which propagate 
toward the second excitation part 522 arrive with a delay of .lambda./4 
but are canceled because they are in reverse phase with the bulk waves 
generated by the second excitation part 522. Thus, in the embodiment shown 
in FIG. 43, bulk waves are propagated only in the direction of arrow H. 
an example of a reduction to practice of the embodiment described above 
with reference to FIG. 43 will now be described with reference to the 
perspective view of FIG. 44. In this example, a first excitation part 537a 
and a second excitation part 537b are spaced .lambda.+.lambda./4 apart 
from each other in the direction of the thickness of a piezoelectric 
ceramic 532. The excitation parts 537a and 537b include a plurality of 
internal electrodes 533a . . . 533h and 533i . . . 533q, respectively. The 
internal electrodes 533a . . . 533q are respectively connected to external 
electrodes 535a, 535b, and 535c and are led out to opposite sides. A delay 
circuit J is connected between the external electrodes 535b and 535c so 
that the first excitation part 535a may generate bulk waves lagging by a 
phase difference of .lambda./4 behind bulk waves generated by the second 
excitation part 537b. In addition, FIG. 44, the numeral 541 denotes a 
matching network, and 542 is a power source. In the bulk device shown in 
FIG. 44, since the first and second vibrating parts 537a and 537b are 
spaced .lambda.+1/4.lambda. apart from each other and since the first 
excitation part 537a is adapted to produce bulk waves lagging by a phase 
difference of .lambda./4, as described above, the bulk waves produced are 
propagated only upwardly in the direction of the thickness of the 
piezoelectric ceramics 532, i.e., to the opposite side from the first 
excitation part 537a to the second excitation part 537b. 
In addition, in the embodiment shown in FIG. 42, the first and second 
excitation parts have been shown spaced .lambda./4 apart from each other, 
but in the case where 3-phase unidirectional transducer having a phase 
delay of 120.degree.-120.degree.-120.degree., it is also possible to 
propagate bulk waves only in one direction. 
It is to be pointed out that since the fourteenth and fifteenth embodiments 
described above relate to the arrangement and construction of input 
electrode groups, i.e., construction of excitation parts, they are also 
applicable to all other embodiments described so far. 
FIGS. 45 through 52 show embodiments in which internal electrodes are 
weighted to obtain a desired pass band characteristic. These embodiments 
are characterized in that the internal electrodes are weighted in a 
multi-layered type filter such as the one shown in FIG. 5. Since the rest 
of the arrangement is the same as in the multi-layered type ceramic filter 
described above, only the weighting structure of the internal electrodes 
will be explained. 
FIG. 45 is a sectional view for explaining the structure for weighting 
interdigital electrodes in the pertinent embodiments of the invention. 
FIG. 45 shows the formation of interdigital electrodes 612a . . . 612j 
disposed in the direction of the thickness in overlapping relation within 
a ceramic 11. The interdigital electrodes 612a . . . 612j, as seen in FIG. 
45, are formed so that their transverse lengths as viewed in the figure 
overlap each other. Therefore, as is clear from the respective shapes of 
envelope lines U and V, it is seen that this construction is weighted. 
The particular configuration shown in FIG. 45 can be obtained, as shown in 
a perspective view in FIG. 46 by stacking ceramic green sheets 616a . . . 
616k formed with electrode patterns serving as the interdigital electrodes 
612a . . . 612j, and sintering the same. 
Though not shown in FIG. 45, it is possible to form external electrodes on 
the right and left end surfaces of the ceramic 611, to extend the ceramics 
611 in the direction of the thickness or in the transverse direction, and 
to form another interdigital electrode group. The latter interdigital 
electrode group can be formed with a pair of external electrodes, to 
thereby provide any of the filters described so far. The filter 
construction weighted in the manner shown in FIG. 45 provides a filter 
having a negligible side lobe response and almost no ripple, i.e., having 
a greatly improved selectivity characteristics. 
FIG. 47 is a sectional view showing another example of electrode weighting 
in a manner which corresponds to the arrangement of FIG. 45. In this case 
too, interdigital electrodes 622a . . . 622s are disposed to overlap each 
other in the direction of the thickness of a ceramic 621, but weighting is 
applied so that the interdigital electrodes 622a . . . 622s are 
distributed unevenly in the direction of the thickness. Also in the case 
where the weighting shown in FIG. 45 is used, since the interdigital 
electrodes are weighted, it is possible to form a filter having a good 
selectivity characteristic. 
The weighting shown in FIG. 47 is obtained, as shown in a perspective view 
in FIG. 48, by stacking a ceramic green sheet 626a, ceramic green sheets 
626b and 626c having interdigital electrodes 622a and 622b formed thereon, 
ceramic green sheets 626d, 626e, and 626f having no electrode formed 
thereon, ceramic green sheets 626g and 626f having interdigital electrodes 
622c and 622d formed thereon, an so on until on end ceramic green sheet 
626t is reached, in accordance with the above order. The ceramic green 
sheets 626a . . . 626m are stacked in the reverse order on the ceramic 
green sheet 626t, and are sintered. In this case also, the ceramic 621 of 
the construction shown in FIG. 47 is extended in the direction of the 
thickness, another electrode group is formed, and external electrodes are 
formed, whereby a multi-layered type ceramic filter is produced. 
FIGS. 49 and 50 show a front sectional view and a right-hand side view, 
respectively, of a third example of weighted electrode arrangement 
according to the invention. Interdigital electrodes 632a . . . 632s 
overlap each other in the direction of the thickness of a ceramic 631 and 
alternately emerge at opposite sides as shown in FIG. 51, for example. The 
structure being obtained by stacking ceramic green sheets 636a and 636b 
having the electrodes 632a and 632b of the same shape formed thereon. In 
this case, weighting is attained not by the shape of the interdigital 
electrodes but by controlling voltages impressed between the individual 
interdigital electrodes 632a . . . 632s. 
As shown in FIg. 50, the interdigital electrodes 632a, 632c . . . 632s 
which emerge to the right have resistors 637a, 637c . . . 637s 
respectively connected thereto, and one external electrode 634b is 
connected through said resistors 637a, 637c . . . 637s. Thus, by setting 
the resistors 637a, 637c . . . 637s at different resistance values, it is 
possible to control voltages impressed between individual interdigital 
electrodes 632a . . . 632s. Therefore, where weighting is used to 
construct the filter, the selectivity characteristic can be greatly 
improved. 
Furthermore, the interdigital electrodes 632b, 632d . . . 632r are 
connected to the external electrode 634a on the left-hand side surface as 
viewed in the figure. In FIG. 50, the electrodes 632i and 632k are a 
common electrode connected to the external electrode 634b through a 
resistance wire 637j. 
FIG. 52 illustrates a fourth example of a weighted electrode arrangement 
according to the invention. In the example shown in FIG. 52, a ceramic 
green sheet 646a having no electrode formed thereon, and ceramic green 
sheets 646b, 646c, 646d, and 646e having interdigital electrodes 642a, 
642b, 642c and 642d in divisional electrode patterns thereon, are 
successively stacked. Voltages are impressed between the individual 
electrodes in a changing fashion, whereby weighting is obtained. FIG. 52 
schematically illustrates only some ceramic green sheets in order to show 
the divided shape of the electrode patterns, but actually, a desired 
number of ceramic green sheets having variously divided electrodes formed 
thereon are stacked, and another electrodes group is formed, whereby a 
ceramic filter is produced. 
It is to be noted that the weighting methods described with reference to 
FIGS. 45 through 52 are also applicable to the first through fifteenth 
embodiments described previously. 
The above description refers to ceramic filters. However, when the ceramic 
filters of the fourth through sixteenth embodiments are used as resonators 
by using only one of the input and output electrodes groups, advantages 
not found in the prior art are realized. In the case where a resonator is 
constructed, the portion above or below the dash-dot line W shown in FIGS. 
10, 13, 16, 26, 31, and 34 is used whereby a resonator of novel 
construction is obtained. 
Although the present invention has been described in connection with 
preferred embodiments thereof, other modifications and variations will now 
become apparent to persons skilled in the art. It is preferred, therefore, 
that the invention be limited not by the specific embodiments disclosed 
herein but only by the following claims.