Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-09-13
2003-11-18
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S133000, C310S31300R
Reexamination Certificate
active
06650206
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter and a communication apparatus using this.
2. Related Art of the Invention
In recent years, progress in information communication field has given rise to increase in information amount to be transmitted. Accompanied thereby, a surface acoustic wave filter that is excellent in phase linearity for a comparatively wide band range and within a passband, and conventionally, a surface acoustic wave filter of transversal type was suitable as such a surface acoustic wave filter. On the other hand, as well known, since a surface acoustic wave filter of transversal type gives rise to a large insertion loss, as technology to enhance low insertion loss, a surface acoustic wave filter of transversal type with a unidirectional electrode being utilized is being expected and considered.
A conventional surface acoustic wave filter of transversal type with a unidirectional electrode being utilized will be described below.
FIG.
10
(
a
) is a drawing showing a conventional surface acoustic wave filter of transversal type with a unidirectional electrode being utilized. Reference numeral
1001
denotes a piezoelectric substrate, and a surface acoustic wave filter is constructed by forming an input and output interdigital transducer electrode (referred to as IDT electrode hereinafter) on the piezoelectric substrate
1001
and disposing the input IDT electrode
1002
and the output IDT electrode
1003
at a predetermined distance. The input IDT electrode
1002
as well as the output IDT electrode
1003
includes a first as well as a second unidirectional electrodes
1004
and
1005
according to a prior art disclosed in the IEEE Ultrasonics symposium, 1989, PP. 77-89. Incidentally the entire disclosure of above document are incorporated herein by reference in its entirely.
FIG.
10
(
b
) shows an enlarged view of the first unidirectional electrode
1004
. As a basic construction, a basic unit is constructed by one &lgr;/4-width electrode finger
1004
a
with a quarter width of a wavelength &lgr; of a surface acoustic wave propagating on the piezoelectric substrate
1001
and two &lgr;/8-width electrode fingers
1004
b
, totaling three electrode fingers. In addition, the second unidirectional electrode
1005
and the unidirectional electrode
1004
are symmetrical. The unidirectional electrode of such type is called EWC-SPUDT (Electrode Width Controlled Single-Phase Unidirectional Transducer), and has been conventionally used as technology of lowering insertion loss.
For the surface acoustic wave filter constructed as described above, frequency characteristics of a filter are determined by thinning out electrode fingers of the input IDT electrode
1002
and the output IDT electrode
1003
and giving various kinds of weighing. In the prior arts, thus, flat filter characteristics within the passband for a wide band range and attenuation amount that is steep in the vicinity of the passband are realized.
In addition, in Japanese Patent Laid-Open No. 2000-77974 specification, a surface acoustic wave filter having a first and second channel on a piezoelectric substrate is disclosed. In
FIG. 11
, a schematic construction view of a surface acoustic wave filter having a first and a second channel according to a prior art is shown. The above described two-channel filter has two channels of a first channel
1102
and a second channel
1103
each having an input IDT electrode
1102
a,
1103
a
and an output IDT electrode
1102
b,
1103
b
provided on a piezoelectric substrate
1101
. Such a surface acoustic wave filter is constructed to have phase characteristics on the first channel
1102
and the second channel
1103
of being in phase in the passband and in opposite phase each other the stopband. Incidentally the entire disclosure of above document are incorporated herein by reference in its entirely.
In recent years, enhancement in compactness and lightness on a portable terminal is progressed and accompanied thereby, enhancement in compactness of a surface acoustic wave filter is being demanded. However, if a steep attenuation amount in the vicinity of a passband is tried to be given in the above described surface acoustic wave filter of transversal type as shown in
FIG. 10
, a sufficient weighting onto the input/output IDT electrode will become necessary to make the length IDT electrode longer, giving rise to a problem that enhancement in compactness is difficult.
In addition, the surface acoustic wave of two-channel filter construction as shown in
FIG. 11
has sharp band range characteristics and enables a compact surface acoustic wave filter to be provided, presenting, however, a problem that it lacks flatness within a passband.
SUMMARY OF THE INVENTION
Accordingly, the present invention has an object to provide a surface acoustic wave filter being compact, having wide passband and having steep attenuation characteristics in the vicinity of passband as well as a flat characteristics within the passband.
To achieve the object mentioned above, one aspect of the present invention is a surface acoustic wave filter, comprising:
a piezoelectric substrate; and
at least two filter tracks having at least an input interdigital transducer electrode and an output interdigital transducer electrode provided on said piezoelectric substrate,
wherein the respective input interdigital transducer electrodes of said at least two filter tracks are brought into connection in parallel;
the respective output interdigital transducer electrodes of said at least two filter tracks are brought into connection in parallel, and
overlapping widths of electrode fingers of said input interdigital transducer electrode and output interdigital transducer electrode are different from each other for each of said at least two filter tracks.
Another aspect of the present invention is the surface acoustic wave filter,
wherein said filter tracks are two filter tracks of a first filter track and a second filter track,
the input interdigital transducer electrodes of said first and second filter tracks are brought into connection in parallel,
the output interdigital transducer electrodes of said first and second filter tracks are brought into connection in parallel, and
overlapping width of the electrode fingers of the said input interdigital transducer electrode and output interdigital transducer electrode in said first filter track and overlapping width of the electrode fingers of the said input interdigital transducer electrode and output interdigital transducer electrode in said second filter track are different from each other.
Still another aspect of the present invention is the surface acoustic wave filter, wherein said first filter track and said second filter track have different weighting functions.
Yet still another aspect of the present invention is the surface acoustic wave filter, wherein a phase relationship between said first filter track and said second filter track is substantially in-phase within a passband, and is substantially in opposite phase outside the passband, and center frequencies of said first and second filter tracks are substantially the same.
Still yet another aspect of the present invention is the surface acoustic wave filter, wherein said first filter track has a transmission characteristic of having two peaks within the passband, said second filter track has a transmission characteristic of having one peak within the passband; and an amplitude characteristic of said first filter track and said second filter track is that frequencies of values lower by substantially 3 dB than a value of a maximum attenuation amount are substantially the same.
A further aspect of the present invention is the surface acoustic wave filter, wherein with the overlapping width of said first filter track being W
1
and the overlapping width of said second filter track being W
2
, a relationship of 0.8≦W
1
/W
2
≦0.95 is fulfilled.
A still further aspect of the present invention is the surface acoustic wave filter,
Nakamura Hiroyuki
Nishimura Kazunori
Tsuzuki Shigeru
Yamada Toru
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
Summons Barbara
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