Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-12-11
2002-10-15
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S348000
Reexamination Certificate
active
06466107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ladder filter including ladder-connected serial resonators and parallel resonators.
2. Description of the Related Art
A two-stage ladder filter used for communication equipment is shown in FIG.
1
. In this ladder filter, two serial resonators
3
and
4
are connected in series between an input terminal
1
and an output terminal
2
. A parallel resonator
5
is connected between the midpoint of the serial resonators
3
and
4
and a ground, and a parallel resonator
6
is connected between the output terminal
2
and the ground.
FIG. 2A
is a vertical sectional view showing the specific construction of a conventional ladder filter, and
FIG. 2B
is a horizontal sectional view thereof. This ladder filter
11
includes an input terminal plate
12
, a ground terminal plate
16
, an output terminal plate
18
, an internal connection terminal plate
14
which is bent into a U-shape, piezoelectric resonators
13
and
19
for defining serial resonators
3
and
4
utilizing extensional vibration (hereinafter referred to as “extensional resonator”), and piezoelectric resonators
15
and
17
for defining parallel resonators
5
and
6
utilizing extensional vibration (extensional resonators). The input terminal plate
12
, the ground terminal plate
16
, and the output terminal plate
18
have lead feet
12
a
,
16
a
, and
18
a
, respectively. The extensional resonators
13
,
15
,
17
, and
19
each perform extensional vibrations wherein the expansion toward the outer periphery direction and the contraction toward the center direction are repeated by the application of an electrical signal. Nodes are located at the centers of the main surfaces of each of these extensional resonators
13
,
15
,
17
, and
19
.
As shown in
FIGS. 2A and 2B
, this ladder filter is defined by stacking the above-described components in the order of the input terminal plate
12
, the extensional resonator
13
, one electrode
14
a
of the internal connection terminal plate
14
, the extensional resonator
15
, the ground terminal plate
16
, the extensional resonator
17
, the output terminal plate
18
, the extensional resonator
19
, the other electrode
14
b
of the internal connection terminal plate
14
. Herein, protrusions provided on the input terminal plate
12
, the ground terminal plate
16
, the output terminal plate
18
, and the electrode plates
14
a
and
14
b
of the internal connection terminal plate
14
are each abutted against the central portions which are the nodes of the extensional resonators
13
,
15
,
17
, and
19
. The lead feet
12
a
,
16
a
, and
18
a
of the respective input terminal plate
12
, the ground terminal plate
16
, the output terminal plate
18
are each inserted into holes of a bottom lid
21
. The holes are filled with a resin
22
, and sealed by providing a cover
20
thereon.
Such a ladder filter, however, not only has a complicated structure and is difficult to assemble, but also must be redesigned each time the number of stages thereof is increased. This redesign process is both time consuming and costly. For example, since the two-stage ladder filter and the three-stage ladder filter have very different terminal structures (particularly, in the structure of the internal connection terminal), it is impossible to design a three-stage or four-stage ladder filter on the basis of the structure of a two-stage ladder filter, and there is a need for redesigning whenever the number of stages is changed.
FIG. 3A
shows the construction of an extensional resonator used as a serial resonator or a parallel resonator in the ladder filter as described above.
FIG. 3B
shows the directions of the polarization axis and the electric-field axis thereof. This extensional resonator
7
is provided with surface electrodes
9
on the main surfaces of a single-layered piezoelectric layer
8
having a square shape, and the entire piezoelectric layer
8
is polarized in a direction that is perpendicular to both main surfaces. Since the direction of an electric field applied across the surface electrodes
9
(the electric-field axis) is also perpendicular to both main surfaces, the electric-field axis is parallel with the polarization axis. In such an extensional resonator
7
, once a signal is applied between the surface electrodes
9
, the piezoelectric layer
8
expands and contracts with respect to the outer periphery direction, in the planes parallel with both main surfaces.
In the extensional resonator
7
, the product of the length Ls of one side thereof and resonance frequency fr is substantially constant as expressed by:
Ls×fr=As
(1)
where, As is a constant (frequency constant), and approximately equal to 2100 mmkHz. For example, when trying to obtain a resonator having a resonance frequency fr=450 kHz, the length of one side thereof will be Ls=4.67 mm.
However, since there is an increasing need to miniaturize electronic components, it is difficult for such an extensional resonator to meet the needs for reductions in the size and weight, and further for cost reduction. That is, the dimensions as described above cannot be substantially reduced and maintain the desired resonance frequency.
FIG. 4
shows attenuation characteristics of the ladder filter having a two-stage configuration. As characteristics of such a ladder filter, the guaranteed attenuation value Att. shown in
FIG. 4
must be as large as possible. Denoting the inter-terminal capacities of the serial resonators
3
and
4
as C
1
and C
1
, respectively, and the inter-terminal capacities of the parallel resonators
5
and
6
as C
2
and C
2
, respectively, the guaranteed attenuation value Att. of the ladder filter of a two-stage configuration is expressed by:
Att.=
2×20 log (
C
2
/
C
1
) (2)
To increase the guaranteed attenuation value, therefore, it is necessary to increase the inter-terminal capacities C
2
and C
2
of the respective parallel resonators
5
and
6
, and to decrease the inter-terminal capacities C
1
and C
1
of the respective serial resonators
3
and
4
. However, when extensional resonators as described above are used as the parallel resonators
5
and
6
, it has been difficult to increase the inter-terminal capacity C
2
, for the reasons described hereinafter.
Denoting the length of one side of the extensional resonator
7
shown in
FIG. 3A
as Ls, the dielectric constant of the piezoelectric layer
8
as &egr;, and the thickness thereof as t, the inter-terminal capacity Cs thereof is expressed by the following equation:
Cs
=(&egr;×&egr;
0
×Ls
2
)/
t
(3)
where, &egr;
0
is a permittivity in a vacuum, and &egr;
0
=8.854×10
−12
.
Since the length of one side of the extensional resonator
7
is determined if the resonance frequency fr of the extensional resonator
7
is determined (see the equation (1)), the inter-terminal capacity is determined only by the thickness t and the dielectric constant &egr; of the piezoelectric layer
8
.
To increase the inter-terminal capacity Cs of the extensional resonator
7
, it is necessary to increase the dielectric constant &egr; of the piezoelectric layer
8
, or reduce the thickness t thereof. However, the dielectric constant &egr; of the piezoelectric layer
8
is a constant inherent in the material of the piezoelectric layer
8
, and cannot be optionally selected. If the piezoelectric material is changed to increase the dielectric constant &egr;, other characteristics are affected. On the other hand, if the thickness t of the piezoelectric layer
8
is reduced, the strength thereof will decrease, and the extensional resonator
7
becomes more susceptible to failure, so that the range of selection of the thickness t is substantially limited.
Therefore, although a resonator having a large inter-terminal capacity has been required as a parallel resonator for a ladder filter, it has been difficult to obtain a resonator having a large inter-termina
Keating & Bennett LLP
Pascal Robert
Takaoka Dean
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