1. Field of the Invention
The present invention relates to surface acoustic wave devices, and more specifically, to a surface acoustic wave device including a plurality of surface acoustic wave filters contained in a single package.
2. Description of the Related Art
In recent years, multiband cellular phones compatible with two or more communications systems have been designed with the aim of improving functionality of mobile communications devices. A cellular phone of this type requires a broadband filter. However, it has been difficult to implement a broadband filter which covers two or more bands and which causes only small loss.
Accordingly, surface acoustic wave devices incorporating a plurality of surface acoustic wave filters contained in a single package, as shown in FIGS. 18 and 19, have been widely used.
In the surface acoustic wave device 100 shown in FIG. 18, surface acoustic wave filters 101 and 102 having different center frequencies are contained in a package 103. In the surface acoustic wave filter 101, a signal is input to an unbalanced input terminal 104, and a signal is output from an unbalanced output terminal 105. Similarly, in the surface acoustic wave filter 102, a signal is input from an unbalanced input terminal 106, and a signal is output from an unbalanced output terminal 107.
That is, the surface acoustic wave device shown in FIG. 18 outputs two unbalanced output signals corresponding to two unbalanced input signals.
In another surface acoustic wave device of the related art, a surface acoustic wave device 200 shown in FIG. 19, two surface acoustic wave filters 201 and 202 having different center frequencies are contained in a package 203. The surface acoustic wave filters 201 and 202 share a common unbalanced input terminal 204. That is, the surface acoustic wave filters 201 and 202 receive a signal input to the unbalanced input terminal 204, and output signals respectively to unbalanced output terminals 205 and 206. In this example, because the impedance on the side of the common unbalanced input terminal 204 is capacitive, an inductor 207 is connected in parallel to the unbalanced input terminal 204. The inductor 207 is arranged internally or externally with respect to the package 203. A surface acoustic wave device of this type is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 11-68511.
When two surface acoustic wave filters with unbalanced input and output terminals, having different center frequencies, are contained in a single package, there has been a problem that stopband attenuation is reduced due to electromagnetic or capacitive mutual interference between the two surface acoustic wave filters, which results in the filter failing to provide a sufficient attenuation. The mutual interference can be reduced to a certain extent when the electrical interconnection between the surface acoustic wave filters and electrode pads of the package is achieved by wire bonding, by optimizing the layout of the electrode pads, bonding conditions, and other methods. However, it has been difficult to reduce the mutual interference particularly when the surface acoustic wave filters are electrically connected to and contained in the package by the face-down manufacturing method compared with the case of wire bonding.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device including a plurality of surface acoustic wave filters contained in a single package, in which the effect of mutual interference between the plurality of surface acoustic wave filters is minimized so as to provide a sufficient stopband attenuation.
According to one preferred embodiment of the present invention, a surface acoustic wave device includes a package, and a plurality of surface acoustic wave filters having different center frequencies and contained in the package, wherein one of the input terminal and the output terminal of at least one of the plurality of surface acoustic wave filters is a balanced terminal and the other of the input terminal and the output terminal is an unbalanced terminal.
As a result of this unique arrangement, stopband attenuation is increased compared with the surface acoustic wave device 100 according to the related art shown in FIG. 18 in which a plurality of surface acoustic wave filters with an unbalanced input and an unbalanced output is contained in a single package.
Preferably, in the surface acoustic wave device, one of the input terminal or the output terminal of each of the plurality of surface acoustic wave filters is a balanced terminal and the other is an unbalanced terminal.
Accordingly, stopband attenuation is increased for all of the surface acoustic wave filters included in the surface acoustic wave device.
Furthermore, the unbalanced terminal may be shared among the plurality of surface acoustic wave filters.
Accordingly, stopband attenuation is increased compared with the surface acoustic wave device 200 with a single unbalanced input and two unbalanced outputs according to the related art shown in FIG. 19.
The surface acoustic wave device according to another preferred embodiment of the present invention preferably includes an impedance matching element attached to the shared unbalanced terminal.
Preferably, the impedance matching element is an inductor attached in parallel to the unbalanced terminal. Accordingly, frequency characteristics are even more improved.
Furthermore, at least one of the plurality of surface acoustic wave filters is preferably a surface acoustic wave filter of the type using cascaded resonators.
Furthermore, at least one of the plurality of surface acoustic wave filters preferably has a different electrode thickness from the other surface acoustic wave filters.
Accordingly, the filter characteristics of each of the surface acoustic wave filters can be optimized by varying the electrode thicknesses thereof, so as to achieve favorable frequency characteristics.
The plurality of surface acoustic wave filters may be disposed on a single piezoelectric substrate.
Accordingly, the size of the surface acoustic wave device can be minimized.
According to another preferred embodiment of the present invention, a communications device includes a surface acoustic wave device according to preferred embodiments described above, which surface acoustic wave device defines a band-pass filter.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.
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