Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2003-05-16
2004-11-23
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S187000, C310S366000, C310S328000
Reexamination Certificate
active
06822536
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric bulk wave filter which can be used as a bandpass filter, for example. More specifically, the present invention relates to a longitudinally coupled multi-mode piezoelectric bulk wave filter device, a longitudinally coupled multi-mode piezoelectric bulk wave filter, and an electronic component including such a piezoelectric bulk wave filter using a harmonic wave of vibration. The present invention also relates to a longitudinally coupled multi-mode piezoelectric bulk wave filter device and an electronic component including a piezoelectric bulk wave filter which couples harmonic waves of different orders.
2. Description of the Related Art
A variety of piezoelectric bulk wave filters are used as bandpass filters. Dual-mode piezoelectric bulk wave filters are mainly used within a frequency range of several MHz to tens of MHz because of the ease in which they can be miniaturized and their low cost.
A dual-mode piezoelectric bulk wave filter is disclosed in Japanese Unexamined Patent Application Publication No. 5-327401.
FIG. 18
is a cross-sectional view of a conventional dual-mode piezoelectric bulk wave filter using a thickness longitudinal vibration.
A piezoelectric bulk wave filter
201
includes a piezoelectric substrate
202
that is polarized in the thickness direction. A pair of exciting electrodes
203
and
204
are disposed on the top surface of the piezoelectric substrate
202
. A common exciting electrode
205
is opposed to the exciting electrodes
203
and
204
with the piezoelectric substrate
202
sandwiched therebetween.
During use, an input signal is applied between one exciting electrode
203
and the common exciting electrode
205
to excite the piezoelectric substrate
202
. When the piezoelectric substrate
202
is excited, a symmetrical mode shown in FIG.
19
A and an anti-symmetrical mode shown in
FIG. 19B
are generated. The two modes are coupled, forming a filter bandwidth. An output is picked up between the exciting electrode
204
and the ground electrode
205
.
Also known, in addition to the dual-mode piezoelectric bulk wave filter operating in the thickness longitudinal mode, is the dual-mode piezoelectric bulk wave filter which includes the piezoelectric substrate
202
polarized in a direction parallel to the top surface and operating in a shear vibration mode.
The degree of coupling between the symmetrical mode and anti-symmetrical mode in the conventional piezoelectric bulk wave filter
201
depends on the spacing between the exciting electrodes
203
and
204
. The spacing determines a frequency difference between the symmetrical mode and the anti-symmetrical mode, thereby determining a passband.
Specifically, to produce a wide band filter, the spacing between the exciting electrodes
203
and
204
must be narrowed to increase the degree of coupling between the two modes and to increase the frequency between the two modes.
The exciting electrodes
203
and
204
are typically produced using a screen printing of electrically conductive paste. The screen printing technique has limited ability to narrow the spacing between the exciting electrodes
203
and
204
. If the exciting electrodes
203
and
204
are produced using a photolithographic technique, the spacing between the exciting electrodes
203
and
204
is narrowed, but the costs involved increase.
Even if the spacing between the exciting electrodes
203
and
204
is narrowed, the capacitance between the exciting electrodes
203
and
204
increases in the piezoelectric bulk wave filter
201
, which leads to a smaller attenuation.
To attain a large attenuation, a plurality of filter elements are typically connected in the piezoelectric bulk wave filter device. As shown in
FIG. 20
, first and second piezoelectric bulk wave filter elements
213
and
214
are mounted on a substrate
212
in a piezoelectric bulk wave filter device
211
. The piezoelectric bulk wave filter element
213
and piezoelectric bulk wave filter element
214
are identical to each other in construction.
If the first and second piezoelectric bulk wave filter elements
213
and
214
are located too closely, a stray capacitance occurring between input and output of the first and second piezoelectric bulk wave filter elements
213
and
214
(as represented by an arrow A shown in
FIG. 20
) lowers the attenuation. For this reason, the first and second piezoelectric bulk wave filter elements
213
and
214
are not located too closely to each other. The whole filter device inevitably becomes large in size.
As another method to achieve a large attenuation, a piezoelectric bulk wave filter which includes a relay capacitor as shown in
FIG. 21
has been proposed. As shown, a piezoelectric bulk wave filter
221
includes first and second energy trapped piezoelectric resonators
222
and
223
disposed on a piezoelectric substrate. To form a relay capacitor between the piezoelectric resonators
222
and
223
, capacitor electrodes
224
and
225
are opposed to each other with the piezoelectric substrate sandwiched therebetween.
FIG. 22
is a circuit diagram of the piezoelectric bulk wave filter
221
shown in FIG.
21
.
The relay capacitor C is formed of the capacitor electrodes
224
and
225
as shown in FIG.
22
. Because the piezoelectric substrate forming the piezoelectric bulk wave filter
221
is polarized, an unwanted vibration occurs because of the piezoelectric effect on the portions of the piezoelectric substrate where the capacitor electrodes
224
and
225
are disposed. As a result, spurious vibrations occur.
A technique is known to partially polarize the piezoelectric substrate so that the piezoelectric substrate is not polarized in the portions where the capacitor electrodes
224
and
225
are opposed. However, if the piezoelectric substrate is partially polarized, there is a possibility that cracks may occur in the piezoelectric substrate.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a multi-mode piezoelectric bulk wave filter device, a multi-mode piezoelectric bulk wave filter, and an electronic component including the multi-mode piezoelectric bulk wave filter which has a wide bandwidth, has a large attenuation, is easy to miniaturize, and suppresses the generation of unwanted spurious vibrations. Preferred embodiments of the present invention also provide a longitudinally coupled multi-mode piezoelectric bulk wave filter that is manufactured at low costs.
In a first preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric bulk wave filter device includes first and second multi-mode piezoelectric bulk wave filters which are designed so that vibration modes of different orders of harmonic waves are excited and coupled to provide an output signal between an output electrode and an ground electrode when an input signal is input between an input electrode and the ground electrode. The longitudinally coupled multi-mode piezoelectric bulk wave filter device further includes a casing substrate on which the first and second longitudinally coupled multi-mode piezoelectric bulk wave filters are disposed on a surface thereof. Each of the first and second multi-mode piezoelectric bulk wave filters includes at least four exciting electrodes extending substantially parallel to each other and a laminated piezoelectric body, including a plurality of piezoelectric layers arranged between the exciting electrodes, and is polarized in a direction that is substantially perpendicular to or substantially parallel to the exciting electrodes.
The laminated piezoelectric body has first and second end surfaces facing in a direction that is substantially perpendicular to the plurality of the piezoelectric layers and first through fourth side surfaces connecting the first and second end surfaces. The input electrode is disposed on at least one of the first through fourth side surfaces of the laminated pie
Inoue Jiro
Nishimura Toshio
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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