Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2000-03-28
2002-07-16
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S187000, C310S366000
Reexamination Certificate
active
06420945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piezoelectric resonators utilizing extensional vibration. The present invention also relates to piezoelectric components and ladder filters for surface-mounting. Further, the present invention relates to ladder filters having piezoelectric resonators utilizing extensional vibration.
2. Description of the Related Art
The structure of a conventional piezoelectric resonator
1
utilizing extensional vibration is shown in FIG.
1
. In the piezoelectric resonator
1
, a piezoelectric plate
2
is polished to have a thickness T and electrode films
3
a
and
3
b
are provided on the principal planes of the piezoelectric plate
2
such that the piezoelectric plate
2
is polarized in the thickness direction by polarization processing. When T is the thickness of the piezoelectric resonator
1
, and L
1
and L
2
are lengths of edges thereof, the capacitance across terminals (capacitance between electrodes) Cf is defined by the following equation (1),
Cf=(&egr;
o
·&egr;
s
·L
1
·L
2
)/T (1),
wherein: &egr;
o
=dielectric constant under vacuum; and &egr;
s
=specific dielectric constant of the piezoelectric plate.
The resonance frequency fr of the extensional vibration of the piezoelectric resonator
1
is defined by the following equation (2) under the condition L
1
≅L
2
,
fr=V/L
1
(2),
wherein V=wave motion speed propagating in the piezoelectric plate
2
, and V≅2000 m/sec. Therefore ,when the required resonance frequency fr is determined, lengths of edges L
1
=L
2
of the piezoelectric resonator
1
are determined according to the equation (2).
Accordingly, in order to increase the capacitance across terminals Cf of the piezoelectric resonator
1
at a predetermined resonance frequency fr, a piezoelectric material having a large specific dielectric constant &egr;
s
needs to be selected or the thickness T of the piezoelectric plate
2
needs to be reduced according to the above equation (1).
However, when the specific dielectric constant &egr;
s
of the piezoelectric material used for the piezoelectric plate
2
is increased, other piezoelectric constants such as a piezoelectric quality constant Qm and an electromechanical constant k are also changed, so that only the capacitance across terminals Cf cannot be increased without changing other piezoelectric constants. On the other hand, when the thickness T of the piezoelectric plate
2
is excessively reduced, the piezoelectric resonator
1
is liable to crack when subjected to external impacts caused for example by dropping. Thus, this method of reducing the thickness of the piezoelectric plate
2
has limitations. When L
1
≅L
2
≅4.5 mm (fr=450 kHz), the thickness T at the limit of the strength required is empirically and approximately 300 &mgr;m.
Piezoelectric components used for surface-mount ladder filters are disclosed in Japanese Unexamined Patent Publication No. 8-18382 and Japanese Unexamined Patent Publication No. 7-176977. In these piezoelectric components, a plurality of piezoelectric resonators and metallic terminals which are aligned in a line alternating in the vertical direction are accommodated in a case and external electrodes are provided by bending lead portions of the metallic terminals so as to extend outside of the case.
However, these piezoelectric components have a large external thickness, so that the piezoelectric component protrudes greatly from a surface of a substrate when being mounted on the substrate, thereby causing difficulties for use as a circuit substrate in a low-profile device and preventing a device from being modified into a low profile device.
In these piezoelectric components, the required number of metallic terminals is increased. For example, when a component includes four piezoelectric resonators, four to five metallic terminals are required. Thereby, the cost of materials is increased and the assembly time is increased, resulting in added costs necessary to produce the piezoelectric component.
The circuit diagram of a conventional four-element type (two-stage) ladder filter
201
is shown in FIG.
14
. The ladder filter
201
has two series-connected resonators
204
S and
205
S connected together in series between an input-terminal
202
and an output-terminal
203
and two parallel-connected resonators
206
P and
207
P respectively inserted between each of the output-sides of the respective resonators
204
S and
205
S and the ground. When the ladder filter is assembled to produce an actual component, the combination of two series-connected resonators and two parallel-connected resonators are vertically sandwiched together via a terminal board, and is accommodated into a case. (For example, Japanese Unexamined Utility Model Publication No. 4-76724).
The guaranteed attenuation ATT
o
of such a ladder filter is defined by the following equation (3), when the capacitance across terminals of the series-connected resonators
204
S and
205
S is Cf
so
and the capacitance across terminals of the parallel-connected resonators
206
P and
207
P is Cf
po
,
ATT
o
2×20Log(Cf
so
/Cf
po
) (3).
Referring to
FIG. 15
, in the conventional series-connected resonators
204
S and
205
S and the parallel-connected resonators
206
P and
207
P utilizing extensional vibration, on both surfaces of a piezoelectric plate
208
having a square shape electrodes
209
are provided. When L
s
is the edge length of the series-connected resonators
204
S and
205
S, T
s
is the thickness thereof, &egr;
s
is the specific dielectric constant thereof, and &egr;
o
is the dielectric constant under vacuum, the capacitance Cf
so
across terminals of the series-connected resonators
204
S and
205
S is defined by the following equation (4),
Cf
o
=(&egr;
o
·&egr;
s
·L
s
2
)/T
s
(4).
Likewise, when Lp is the edge length of the parallel-connected resonators
206
p
and
207
p
, T
p
is the thickness thereof, &egr;
p
is the specific dielectric constant thereof, and &egr;
c
is the dielectric constant under vacuum, the capacitance Cf
po
across terminals of the parallel-connected resonators
206
p
and
207
p
is defined by the following equation (5),
Cf
po
=(&egr;
o
·&egr;
p
·L
p
2
)/T
p
(5).
Therefore, substituting the values of the equations (4) and (5) into the above equation (3) yields the following equation (6) for obtaining the guaranteed attenuation ATT
o
,
ATT
o
=2×20Log [(&egr;
s
·L
s
2
·T
p
)/(&egr;
p
·L
p
2
·T
s
)] (6).
Furthermore, the resonance frequency of the series-connected resonators
204
S and
205
S and the resonance frequency of the parallel-connected resonators
206
p
and
207
p
are both determined by their respective sizes (edge lengths L
s
and L
p
). When a ladder filter having the required frequency is manufactured, the sizes of the series-connected resonators
204
S and
205
S and the parallel-connected resonators
206
p
and
207
p
are the same (L
s
=L
p
), so that the guaranteed attenuation ATT
o
(<0) is defined by the following equation (7),
ATT
o
=2×20Log [(&egr;
s
·T
p
)/(&egr;
p
·T
s
)] (7).
Therefore, in order to increase the guaranteed attenuation ATT
o
(the absolute value), it is necessary that the specific dielectric constant &egr;
s
of the series-connected resonators
204
S and
205
S is reduced while the specific dielectric constant &egr;
p
of the parallel-connected resonators
206
p
and
207
p
is increased. It is also necessary that the thickness T
s
of the series-connected resonators
204
S and
205
S is increased while the thickness T
p
of the parallel-connected resonators
206
p
and
207
p
is decreased.
However, when considering other piezoelectric characteristics such as a piezoelectric quality constant Qm, an electromechanical constant k, and temperature characteristics, it is preferable that the same piezoelectric material be used for the series-connected resonat
Funaki Hirofumi
Yamamoto Takashi
Keating & Bennett LLP
Murata Manufacturing Co. Ltd
Pascal Robert
Summons Barbara
LandOfFree
Piezoelectric resonator having internal electrode films,... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Piezoelectric resonator having internal electrode films,..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Piezoelectric resonator having internal electrode films,... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2834742