Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2001-02-07
2003-03-18
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R
Reexamination Certificate
active
06535080
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave filters and more particularly, to a surface acoustic wave filter having a ladder circuit construction that includes a balanced input/output.
2. Description of the Related Art
A plurality of conventional surface acoustic wave filters have been used as band-pass filters in mobile communication devices. Particularly, a variety of surface acoustic wave filters have been proposed which have a ladder circuit construction with low insertion loss and wide bandwidth.
FIG. 27
is a circuit diagram showing a basic construction of the surface acoustic wave filter which has the ladder circuit construction. A signal line which establishes connection between an input terminal
201
and an output terminal (not shown) defines a series arm. The series arm is connected to a series resonator
202
. The series arm and the ground potential define a parallel arm therebetween. The parallel arm is connected to a parallel arm resonator
203
. Although the filter circuit having a single stage is shown in
FIG. 27
, the surface acoustic wave filter usually has a multistage construction in which a plurality of series-arm resonators and parallel-arm resonators is provided.
The series-arm resonator and the parallel-arm resonator are constructed using one-port surface acoustic wave resonator. The one-port surface acoustic wave resonator is provided with an IDT electrode disposed in the center of the propagation direction of a surface acoustic wave and a reflector disposed on both sides of the IDT electrode in the propagation direction of the surface acoustic wave.
FIG. 28
shows one example of typical filtering characteristics of the surface acoustic wave filter having the above-described ladder circuit construction.
In EP0541284A2, a surface acoustic wave filter is disclosed having the ladder circuit construction in which an increase in the bandwidth as well as in the attenuation is achieved by adding an inductance component to the parallel arm resonator in series. Particularly, by constructing this inductance component via wire bonding, the size of a chip and its package are not increased.
That is, in an unbalanced surface acoustic wave filter in which a surface acoustic wave filter element and the package are electrically connected via a bonding wire, the inductance component of the bonding wire produces filtering characteristics thereof having a wider band as compared with filtering characteristics of a solely surface acoustic wave filter.
However, in the surface acoustic wave filter disclosed in EP0541284A2, the surface acoustic wave filter operates normally in an unbalanced-input-to-unbalanced-output circuit while not operating normally in a balanced-input-to-balanced-output circuit.
To solve this problem, a surface acoustic wave filter having the balanced ladder circuit construction corresponding to the balanced circuit is disclosed in U.S. Pat. No. 5,499,003. In
FIG. 29
, the circuit construction of the surface acoustic wave filter disclosed in this related art is shown.
Here, series arm resonators S
1
to S
4
and series arm resonators S
5
to S
8
are connected, respectively, to a first series arm defined by a signal line between a hot-side input terminal
211
and a hot-side output terminal
212
and a second series arm defined by a signal line between a ground-side input terminal
213
and a ground-side output terminal
214
. In addition, a plurality of parallel arms are connected between the first and second series arms. Parallel arm resonators P
1
, P
2
, and P
3
are connected to corresponding parallel arms.
As described above, by connecting parallel arms between the first and second series arms, a balanced surface acoustic wave filter is obtained.
Furthermore, a ladder surface acoustic wave filter corresponding to the unbalanced input/output is disclosed in Japanese Unexamined Patent Application Publication No. 6-69750. The ladder surface acoustic wave filter disclosed therein corresponds to the unbalanced input/output by optimizing the electrode capacitance of the series arm resonator and that of the parallel arm resonator.
In a balanced ladder surface acoustic wave filter disclosed in U.S. Pat. No. 5,499,003, an inductance component is not added to the parallel arm resonator. Accordingly, it is difficult to increase the bandwidth and the attenuation due to the above-described addition of the inductance component. In the surface acoustic wave filter described in this related art, the parallel arm resonator is limited to the surface acoustic wave resonator, and the band-pass width depends on a piezoelectric substrate. Consequently, the degree of freedom of band-pass width design is greatly reduced.
Furthermore, in the balanced ladder surface acoustic wave filter described in U.S. Pat. No. 5,499,003, electrical characteristics thereof can be measured only when the balanced circuit is connected thereto. Generally, network analyzers, which are used for evaluating electrical characteristics of the filter, are unbalanced circuits. Therefore, to measure electrical characteristics of the above surface acoustic wave filter using the network analyzer, a balance-to-unbalance transformer must be inserted therebetween.
One example of the above-described measuring circuit is shown in FIG.
30
. In
FIG. 30
, reference numeral
221
represents a surface acoustic wave filter and reference numerals
222
and
223
represent balance-to-unbalance transformers. However, due to loss caused by the resistance and reflection included in these balance-to-unbalance transformers
222
and
223
, highly accurate evaluation of filtering characteristics is very difficult to achieve. Furthermore, the operating frequency bands of the balance-to-unbalance transformers
222
and
223
are generally narrow and simultaneous evaluation of a wide frequency band is very difficult. In addition, variations in the balance-to-unbalance transformers
222
and
223
make it difficult to accurately and repeatedly evaluate the filter characteristics.
SUMMARY OF THE INVENTION
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a surface acoustic wave filter having a ladder circuit construction which eliminates the problems with the foregoing related art, is capable of corresponding to the balanced input/output, has a wide frequency band and low loss, has a high degree of freedom of band width design, and is capable of accurately and easily having the filter characteristics thereof measured.
According to a first preferred embodiment of the present application, a surface acoustic wave filter includes a piezoelectric substrate and a plurality of surface acoustic wave resonators provided on the piezoelectric substrate wherein a ladder filter circuit including the plurality of surface acoustic wave resonators is constructed. The filter circuit includes input-side balanced signal terminals A
1
and A
2
connected to an external balanced circuit, output-side balanced signal terminals B
1
and B
2
connected to an external balanced circuit, at least one first series arm surface acoustic wave resonator connected in series between the input-side balanced signal terminal A
1
and the output-side balanced signal terminal B
1
, at least one second series arm surface acoustic wave resonator connected in series between the input-side balanced signal terminal A
2
and the output-side balanced signal terminal B
2
, and at least one impedance element connected between a first signal line connecting the input-side balanced signal terminal A
1
to the output-side balanced signal terminal B
1
and a second signal line connecting the input-side balanced signal terminal A
2
to the output-side balanced signal terminal B
2
.
In a specific aspect of the surface acoustic wave filter according to the first preferred embodiment, at least one third surface acoustic wave resonator connected in series with the impedance element is provided.
In another specific aspect of the surface acoustic wave f
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Summons Barbara
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