Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-04-01
2004-03-23
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C310S31300R, C333S154000
Reexamination Certificate
active
06710683
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a communications apparatus using the same.
2. Description of the Related Art
Recently, advances in the information and communications fields have led to increases in the amounts of transmitted information. Consequently, there is a demand for a relatively wide-band surface acoustic wave filter which excels in phase linearity within its pass band. A conventional filter which meets this demand is a transversal surface acoustic wave filter. However, as is well known, transversal surface acoustic wave filters have large insertion losses. Thus, as a technology for reducing insertion loss, expectations are placed on, and consideration is given to, transversal surface acoustic wave filters which employ unidirectional electrodes.
Now, conventional transversal surface acoustic wave filters which employ unidirectional electrodes will be described below.
FIG. 22A
shows a first prior art example of transversal surface acoustic wave filters. Reference numeral
2001
denotes a piezoelectric substrate. A surface acoustic wave filter is constructed by forming input and output interdigital transducer electrodes
2002
and
2003
(hereinafter referred to as IDT electrodes) on the piezoelectric substrate
2001
, disposed at a certain distance from each other.
As the input IDT electrode
2002
and output IDT electrode
2003
, first and second unidirectional electrodes
2004
and
2005
are used which are based on a prior art technology disclosed in “IEEE Ultrasonics Symposium, 1989, pp. 77-89.” This type of unidirectional electrode is known as an EWC-SPUDT (Electrode Width Controlled-Single Phase Unidirectional Transducer).
FIG. 22B
shows an enlarged view of the first basic unidirectional-electrode cell
2004
. The basic unit consists of three electrode fingers: a &lgr;/4-wide electrode finger one-fourth as long as the wavelength &lgr; of surface acoustic waves propagating on the piezoelectric substrate
2001
and two &lgr;/8-wide electrode fingers. The second basic unidirectional-electrode cell
2005
is bilaterally symmetrical to the first unidirectional electrode
2004
.
In the first and second basic unidirectional-electrode cells
2004
and
2005
, the &lgr;/4-wide electrode fingers function as reflectors. By placing the reflection center of the electrode fingers asymmetrically to the excitation center of surface acoustic waves, it is possible to give directivity to the propagation of the surface acoustic waves. The first basic unidirectional-electrode cell
2004
obtains directivity running from the input IDT electrode
2002
towards the output IDT electrode
2003
while the second basic unidirectional-electrode cell
2005
obtains directivity running from the output IDT electrode
2003
towards the input IDT electrode
2002
.
FIG. 23
shows an electrode configuration disclosed in Japanese Published Unexamined Patent Application No. 2000-77973.
FIG. 23A
shows a basic section of an IDT electrode. It consists of a 3&lgr;/8-wide electrode finger and two &lgr;/8-wide electrode fingers. It is unidirectional and reflective in the right direction. The basic section here means a part of the IDT electrode which corresponds to one wavelength. The basic section of an electrode shown in
FIG. 23B
has excitation effect, but does not produce unidirectional reflections. Thus, it is bidirectional. In either case, the basic section is prescribed by the wavelength &lgr;. The basic section of an electrode shown in
FIG. 23B
has excitation effect, but substantially does not reflect waves.
FIG. 24
shows a second prior art example of surface acoustic wave filters using an electrode configuration disclosed in Japanese Published Unexamined Patent Application No. 2000-91869. In
FIG. 24A
, an input IDT electrode
2206
and output IDT electrode
2207
are mounted on a piezoelectric substrate
2201
. The input IDT electrode
2206
consists of interdigitated electrode finger pairs
2202
a
and
2202
b
interlocked or engaged with each other in a staggered configuration and extraction electrodes
2204
a
and
2204
b
which connects individual pairs
2202
a
or
2202
b
while the output IDT electrode
2297
similarly consists of interdigitated electrode finger pairs
2203
a
and
2203
b
interlocked or engaged with each other in a staggered configuration and extraction electrodes
2205
a
and
2205
b
which connects individual pairs
2203
a
or
2203
b
. Each interdigitated electrode finger pair
2202
a
,
2202
b
,
2203
a
, or
2203
b
consists of two interdigitated electrode fingers of different line widths.
FIG. 24B
shows an enlarged view of the input IDT electrode
2206
shown in FIG.
24
A. If the spacing between the narrower interdigitated electrode finger and wider interdigitated electrode finger of an interdigitated electrode finger pair
2202
b
connected to the extraction electrode
2204
b
is denoted by &ggr;, ½ the spacing between the narrower interdigitated electrode finger in the interdigitated electrode finger pair
2202
b
and the wider interdigitated electrode finger in the adjacent interdigitated electrode finger pair
2202
a
connected to the extraction electrode
2204
a
is denoted by &agr;, and ½ the spacing between the wider interdigitated electrode finger in the interdigitated electrode finger pair
2202
b
and the narrower interdigitated electrode finger in the adjacent interdigitated electrode finger pair
2202
a
connected to the extraction electrode
2204
a
is denoted by &bgr;, then &ggr;>&agr;+&bgr;.
With the unidirectional electrode shown in
FIG. 24
, if the width ratio of the wider interdigitated electrode finger to the narrower interdigitated electrode finger is defined as a line width ratio, the desirable range of the line width ratio from the viewpoint of directivity in relation to insertion loss is said to be no smaller than 3.0 and less than 5.0.
Recently, as portable terminals have become increasingly smaller and more sophisticated, smaller size and higher performance have been required of IF-stage surface acoustic wave filters. However, with the transversal surface acoustic wave filters employing any of the unidirectional electrodes described above, there is a difference in frequency between the region of a unidirectional electrode and the region of a bidirectional electrode, resulting in degradation of both in-band flatness and sharp out-of-band attenuation.
Also, with the transversal surface acoustic wave filter described above, the desirable range of the line width ratio, which is said to be no smaller than 3.0 and no larger than 5.0, is discussed only in terms of unidirectionality in relation to insertion loss, and any optimum electrode configuration which takes other elements into consideration is not suggested. Besides, actual surface acoustic wave filters often employ a combination of unidirectional and bidirectional electrodes, but no optimum electrode configuration is suggested for it either.
On the other hand, regarding the configuration consisting of only unidirectional electrodes, relationships with L2/L1 are not analyzed sufficiently.
SUMMARY OF THE INVENTION
One aspect of the present invention is a surface acoustic wave filter comprising a piezoelectric substrate and at least two interdigital transducer electrodes on the above described piezoelectric substrate,
wherein at least one of the above described interdigital transducer electrodes contains a unidirectional electrode and bidirectional electrode, and
the length &lgr;d of a basic cell of the above described unidirectional electrode is longer than the length &lgr;s of a basic cell of the above described bidirectional electrode.
Thanks to the above configuration, the present invention can provide a low-loss surface acoustic wave filter which excels in in-band flatness and attenuation characteristics.
Another aspect of the present invention is the surface acoustic wave filter wherein:
said unidirectional electrode has four electrode fingers in the basic cell prescrib
Igaki Tsutomu
Ishizaki Toshio
Matsunami Ken
Nakamura Hiroyuki
Nishimura Kazunori
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
Summons Barbara
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