Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-01-30
2002-10-15
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R, C310S31300R
Reexamination Certificate
active
06466108
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to Japanese Patent Application No. 2000-296713 filed on Sep. 28, 2000, whose priority is claimed under 35 USC §119, the disclosure of which is incorporated by reference in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave resonator and a surface acoustic wave filter using the same, particularly a ladder type filter.
2. Description of the Related Art
A surface acoustic wave filter and a resonance circuit using a surface acoustic wave resonator can be provided with a compact size and a low cost. Therefore, a surface acoustic wave resonator is one of the necessary constitutional elements for reducing the size of recent communication equipments, such as a portable phone.
FIG. 17
is a constitutional diagram showing a conventional ordinary surface acoustic wave resonator.
The surface acoustic wave resonator comprises a piezoelectric substrate
1
having thereon a interdigital transducer (IDT)
2
formed with an aluminum alloy having a period corresponding to a desired frequency, and reflectors
3
-
1
and
3
-
2
reflecting a surface acoustic wave excited by the interdigital transducer
2
. The electrode period pi of the interdigital transducer
2
can be obtained from the velocity vi of the surface acoustic wave on the substrate at the interdigital transducer and the desired frequency fi by the following equation:
pi=vi/fi
The surface acoustic wave resonator shown in
FIG. 17
is a single terminal pair resonator, in which one of the end parts of the interdigital transducer
2
is an input electrode
2
-
1
, to which an input signal is applied, and the other thereof is an output electrode
2
-
2
, from which an output signal is taken out. The reflectors
3
-
1
and
3
-
2
are generally formed with a grating having periodicity.
While the grating can be formed by making grooves on the piezoelectric substrate, an aluminum alloy grating is generally used, which can be formed simultaneously with the interdigital transducer.
The grating period pr can be obtained, as similar to the case of the interdigital transducer, from the velocity vr of the surface acoustic wave at the reflector and the desired frequency fr by the following equation:
2×
pr=vr/fr
In general, as fi=fr, assuming that vi and vr are substantially the same as each other, the design is often made with pi=2×pr.
Herein, twice the grating period pr is sometimes referred to as a period of the reflector. In this case, the reflector is sometimes referred to as “a half-period reflector”.
In general, the interdigital transducer
2
has been formed with a single electrode comprising two electrode fingers within the electrode period pi. The reflector has also been generally formed with a single electrode as similar to the interdigital transducer
2
since two grating electrode fingers
3
-
3
are present within twice the grating period pr, which is the same as the electrode period pi.
The single electrode herein has such a constitution that the electrode fingers of the interdigital transducer are arranged where one electrode finger extending from the end part of the input electrode
2
-
1
and one electrode finger extending from the end part of the output electrode
2
-
2
are alternately arranged. That is, one electrode finger extending from the end part of the output electrode
2
-
2
is necessarily arranged between two adjacent electrode fingers extending from the end part of the input electrode
2
-
1
.
The electrode fingers thus alternately arranged each are referred to as a single electrode finger.
FIG. 18
is a constitutional diagram showing a conventional double terminal pairs resonator comprising plural interdigital transducers, in which numerals
2
-
3
and
2
-
4
denote ground terminals.
FIG. 19
is a diagram showing the simplest electrically equivalent circuit of a single terminal pair surface acoustic wave resonator formed on a piezoelectric substrate
1
, such as quartz and LiTaO
3
. A single terminal pair surface acoustic wave resonator is used by electrically connected in serial or in parallel as shown in FIGS.
20
(
a
) and
20
(
b
) or FIGS.
21
(
a
) and
21
(
b
).
In
FIG. 19
, symbol R
1
denotes a resistance, C
0
and C
1
denote capacitance, Li denotes an inductance, Ti denotes a terminal of the input electrode
2
-
1
, and To denotes a terminal of the output electrode
2
-
2
.
Herein, R
1
, C
1
and L
1
are such values that are determined by the material of the piezoelectric substrate, and C
0
is a value varying depending on the number of pairs of the interdigital transducers.
In the case of the serial connection shown in FIG.
20
(
a
), a single terminal pair surface acoustic wave resonator R is arranged in serial between the input Ti and the output To as shown in FIG.
20
(
b
). In the case of the parallel connection shown in FIG.
21
(
a
), a single terminal pair surface acoustic wave resonator R is arranged in parallel between the pair of the input Ti and the output To, and the ground G as shown in FIG.
21
(
b
).
FIG. 22
is a diagram showing general frequency characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in serial. Herein, the abscissa indicates the frequency (Hz), and the ordinate indicates the attenuation amount (dB). According to the diagram, the attenuation amount exhibits the maximum value at a certain frequency, which is referred to as an antiresonance frequency fas.
FIG. 23
is a diagram showing impedance characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in serial. Herein, the abscissa indicates the frequency, and the ordinate indicates the absolute value of impedance (logarithmic value). According to the diagram, double resonance characteristics are observed, in which a resonance frequency frs, at which the impedance shows the minimum, appears on the low frequency side, and an antiresonance frequency fas, at which the impedance shows the maximum, appears on the high frequency side.
FIG. 24
is a diagram obtained by overlapping FIG.
22
and FIG.
23
. In this figure, a part to be a pass band of the ladder type filter and a part to be an attenuation band of the ladder type filter are shown.
FIG. 25
is a diagram showing general frequency characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in parallel, and
FIG. 26
is a diagram showing impedance characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in parallel. Herein, the ordinate of
FIG. 25
indicates the absolute value of admittance (logarithmic value).
In these figures, the frequency, at which the attenuation amount becomes minimum, is the resonance frequency frp, the frequency, at which the admittance becomes maximum, is the resonance frequency frp, and the frequency, at which the admittance becomes minimum, is the antiresonance frequency fap. In the case of the parallel connection, the double resonance characteristics having two resonance frequencies frp and fap are exhibited.
The surface acoustic wave resonator of this type is used singly or as a combination of plurality thereof as a ladder type filter.
FIG. 27
is a constitutional diagram showing an example of the ladder type filter. In the ladder type filter as shown in
FIG. 27
, several surface acoustic wave resonators (S
1
, S
2
, R
1
and R
2
) are connected in parallel and serial. At this time, the interdigital transducers of the respective resonators are designed in such a manner that the antiresonance frequency fap of the parallel resonators R
1
and R
2
substantially agree with the resonance frequency frs of the serial resonators S
1
and S
2
.
FIG. 28
is a diagram showing general frequency characteristics of a ladder type filter. The ladder type filter is a band pass filter passing a certain frequency band.
Characteristic values demanded in a band pass filter include the pass band w
Ikata Osamu
Inoue Shogo
Matsuda Takashi
Tsutsumi Jun
Armstrong Westerman & Hattori, LLP
Fujitsu Limited
Pascal Robert
Summons Barbara
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