Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-05-24
2001-03-27
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000
Reexamination Certificate
active
06208224
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter including a plurality of one-port surface acoustic wave resonators (one-port SAW resonators) and, more particularly, to a surface acoustic wave filter having a ladder-type circuit configuration including a plurality of one-port SAW resonators connected to each other in a ladder-type configuration.
2. Description of the Related Art
A conventional band filter includes a surface acoustic wave filter having a ladder type circuit configuration of a plurality of one-port SAW resonators connected to each other in a ladder type configuration (for example, Japanese Examined Patent Publication No. 56-19765).
The circuit of a surface acoustic wave filter of this type is partially shown in
FIG. 10
, a series arm is provided between an input terminal
51
and an output terminal (not shown), and a series resonator S
1
is included in the series arm. A parallel resonator P
1
is connected between the series arm and a reference potential, and thereby, a parallel arm is formed. Although only one series resonator S
1
and one parallel resonator P
1
are shown in
FIG. 10
, a plurality of series arms containing a plurality of series resonators and a plurality of parallel arms containing parallel resonators are arranged to extend in the direction from the input terminal
51
toward the output terminal.
Ordinarily, the one-port SAW resonators each defining the above-mentioned series resonator S
1
and the parallel resonator P
1
have an electrode structure shown in FIG.
11
. As seen in
FIG. 11
, the one-port SAW resonator has a structure in which one pair of interdigital electrodes
52
a
and
52
b
are provided on a piezoelectric substrate (not shown) whereby one interdigital transducer (IDT)
52
is produced. Grating type reflectors
53
and
54
are arranged on the opposite sides of the IDT
52
in the surface acoustic wave propagation direction.
FIG. 12
illustrates the typical filter characteristics of a surface acoustic wave device having a ladder type circuit configuration containing the above-described one-port SAW resonators defining the series resonator and the parallel resonator.
For a band filter, it is necessary to increase the attenuation in the frequency range outside of the filter pass-band. Accordingly, for the purpose of increasing the attenuation in the frequency range outside of the filter pass band, a mirror image type connection structure shown in
FIG. 13
is generally used.
A mirror image type connection structure is a structure in which a connection structure including one series resonator and one parallel resonator is connected to a connection structure including one series resonator and one parallel resonator such that both connection structures define a mirror image of each other relative to the boundary between the connection structures. More particularly, the series and parallel resonators of the connection structure shown by a broken line A in
FIG. 13
are connected to portions of the connection structure surrounded by a broken line B adjacent to the broken line A such that both connection structures have a mirror image relationship relative to a boundary therebetween. Similarly, the connection structure surrounded by a broken line C is arranged to have a mirror image relationship with the connection structure surrounded by the broken line B relative to a boundary therebetween.
More specifically, on the opposite sides of the boundary, the parallel resonators P
1
and P
2
are arranged in the boundary area of the connection structures surrounded by the broken lines A and B, and the series resonators S
1
and S
2
are arranged so as to be spaced far from the joining area of both of the connection structures, respectively.
For a surface acoustic wave filter of the above described type, it is required that its reflection characteristic (VSWR) is low in the filter pass band. It is known that in order to reduce the reflection characteristic to a value of about 2.0, which is generally required, the difference between the resonant frequency of the series resonators S
1
, S
2
, and S
3
and that of the parallel resonators P
1
, P
2
, and P
3
is adjusted. See, for example, THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS TECHNICAL REPORT, JAPAN US95-25, EMD95-21, 33 (1995-07), p39-p46.
More particularly, it is known that in the surface acoustic wave filter having a ladder type circuit configuration shown in
FIG. 13
, VSWR can be reduced by changing the difference between the resonant frequency of the series resonators S
1
, S
2
, and S
3
and that of the parallel resonators P
1
, P
2
, and P
3
.
In the above-described prior art, it is described that VSWR is reduced in a filter pass band by changing the difference between the resonant frequency of the series resonators S
1
, S
2
, and S
3
and that of the parallel resonators P
1
, P
2
, and P
3
. In this prior art, it is described that VSWR is varied as shown in
FIG. 14
when the above resonant frequency difference is changed.
More particularly, in the case that the difference between the resonant frequency of the series resonator and that of the parallel resonator is small, VSWR is large on the lower frequency side, as shown by an arrow C in FIG.
14
. On the contrary, in the case that the above frequency difference is small, VSWR is large on the higher frequency side as shown by an arrow D in FIG.
14
.
Thus, there is a problem with the prior art described above in that if VSWR on the lower frequency band side is reduced, VSWR is increased and degraded on the higher frequency band side.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having a ladder type circuit configuration containing a plurality of series resonators and parallel resonators in which VSWR can be reduced in a wide frequency range in filter pass band, and thereby, the insertion loss is significantly reduced.
The surface acoustic wave filter having a ladder type circuit configuration includes series resonators provided in a series arm located between input and output terminals and parallel resonators included in plural parallel arms, respectively, located between the series arm and a reference potential. The series resonators and the parallel resonators each include a one-port surface acoustic wave resonator, the parallel resonators are arranged alternately with the series resonators in a direction extending from the input terminal toward the output terminal, and the resonant frequency of at least one of the parallel resonators is different from the resonant frequencies of the other parallel resonators. Thus, the different resonant levels in the band can be adjusted, depending on the way the resonant frequencies of the parallel resonators are differentiated, and thereby, VSWR in the band can be significantly reduced, and the insertion loss is greatly improved in a wide frequency range in the band.
The electrode finger pitches of the interdigital transducers of the plurality of parallel resonators are preferably different from each other so that the resonant frequency of the at least one parallel resonator is different from the resonant frequencies of the other parallel resonators. Accordingly, when the series resonators and the parallel resonators are arranged to define a ladder type circuit on one piezoelectric substrate, the surface acoustic wave filter with the VSWR characteristic improved in the pass band can be easily provided by changing the electrode pattern of an IDT of one of the parallel resonators.
In a preferred embodiment of the present invention, first, second, and third parallel arms are arranged along the direction extending from the input terminal toward the output terminal in that order, and the resonant frequency of the parallel resonator included in the second parallel arm is different from the resonant frequencies of the parallel resonators included in the first and third parallel arms, respec
Taniguchi Norio
Ushiroku Tadamasa
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Pascal Robert
Summons Barbara
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