Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-05-27
2001-07-10
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R, C310S31300R
Reexamination Certificate
active
06259336
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter device that is employed for example in a mobile communication instrument and so on such as a cellular-phone. Further, the present invention relates to, in particular, a resonant surface acoustic wave filter device that is provided with an inter-digital transducer (IDT) and reflectors. Still further, the present invention relates to a structure of the IDT in a resonant surface acoustic wave filter device that comprises the IDT and the reflectors.
2. Description of the Related Art
Recently, surface acoustic wave filter devices, being small in size, light in weight, highly reliable and excellent in signal attenuation characteristics outside a pass band, are used in large part in the field of mobile communication. In particular, resonant filter devices that utilize the resonance characteristics of surface acoustic wave can be made small in size, and are capable of producing the resonant filter devices for VHF band and UHF band and further excellent in their filter characteristics. Therefore, they are in the mainstream of the surface acoustic wave filter device technology for mobile communication.
The resonant surface acoustic wave filter device employs a surface acoustic wave resonant element such as illustrated schematically in FIG.
40
. The surface acoustic wave resonant filter device comprises IDTs and grating reflectors (GRs) formed on a piezoelectric substrate, and in the device, two grating reflectors disposed an appropriate distance apart form a standing wave resonator of the surface acoustic wave. And IDTs are disposed between the two grating reflectors so as to couple well with the standing wave.
Accompanying further progress of communication instruments for such as vehicular communication, still higher characteristics are demanded for the surface acoustic wave filter devices that are used for such communication instruments. Lower loss in a pass band frequency range and in broader frequency ranges, larger attenuation outside a pass band and excellent cut-off characteristics outside the pass band are desired as frequency characteristics of the surface acoustic wave filter devices.
In resonant surface acoustic wave filter devices, surface acoustic wave reflections by IDT electrode fingers are positively utilized in addition to the reflections due to the GRs. An existing IDT comprises solid electrodes in which polarity of the electrode fingers alternates every adjacent electrode finger. Accordingly, characteristics of the resonant surface acoustic wave filter device are determined by a number of the IDT fingers and GRs, a pitch and a thickness thereof and so on.
However, in an existing method in which the characteristics of the filter are obtained by selecting a number, a pitch and a thickness of IDT fingers and GR, there are restrictions due to a chip size and a limitation in the photo-etching process. Further, in the existing method, the impedance of the filter device varies with the change of these parameters, although it is preferable to have the impedance matching between the filter device and an external circuit connected to the filter device. Thus, there is a problem that a resonant surface acoustic wave filter device having desired characteristics is difficult to be obtained.
In addition, an existing surface acoustic wave filter device having steep cut off characteristics can be constituted in multi-stage. In this case, the filter devices of multi-stage are tuned by use of an external circuit element such as a coil or a capacitor, or tuning elements disposed between filter elements so as to make susceptance components of the filter devices of multi-stage small and favorably zero or flat with respect to frequency dependence to obtain preferable filter device characteristics. There are further problems such that the addition of these external tuning elements makes the external circuit complicated when the filter device is employed and makes the filter device itself large.
Furthermore, a surface acoustic wave filter device of the type to which the present invention relates shows improved filter cut off characteristics in which an IDT of the filter device comprises electrode fingers having a different finger pitch or width from the pitch or width for other ordinary electrode fingers. However, in order to provide surface acoustic wave filters which satisfy recent demands, further improvements on filter cut off characteristics are desired.
SUMMARY OF THE INVENTION
To solve above mentioned problems, this invention provides a resonant surface acoustic wave filter device. An object of the present invention is to obtain a surface acoustic wave filter device having desirable frequency characteristics. Another object of the present invention is to provide a surface acoustic wave filter device in which impedance matching can be maintained when the surface acoustic wave filter device characteristics are varied.
Another object of the present invention is to provide a surface acoustic filter device having low loss in a pass band, having excellent attenuation characteristics outside of the pass band and being capable of responding flexibly to complicated system requirements.
In the first aspect of the present invention, a resonant surface acoustic wave filter device comprises a piezoelectric substrate, an IDT having a first electrode area that is formed on the piezoelectric substrate and excites a first surface acoustic wave and a second electrode area that excites a second surface acoustic wave of inverted phase with respect to the first surface acoustic wave excited at the first electrode area, and GRs formed on the piezoelectric substrate disposed at both sides of the IDT along the direction of propagation of the surface acoustic waves that are excited by the IDT.
Here, an IDT is for example a pair of comb shaped metal thin film electrodes having electrode fingers arranged such that each electrode finger having one polarity is placed in a space between electrode fingers having the other polarity. In a structure of the resonant surface acoustic wave filter device, the GRs made of metallic thin film are disposed on both sides of the IDT so as to put the IDT between the GRs along the direction of propagation of the surface acoustic waves that are excited by the IDT.
The resonant surface acoustic wave filter device of the present invention can have a plurality of IDTs placed between the reflectors. Further, the resonant surface acoustic wave filter device of the present invention may comprise resonant surface acoustic wave filter devices having above mentioned IDTs connected in two stages or more.
A pitch of the electrode fingers array in the second electrode area of the IDT of the resonant surface acoustic wave filter device can be set approximately equal to that of the electrode fingers array in the first electrode area, and thereby, the first surface acoustic wave in frequency region of a pass band of the filter device can be partially counteracted by the second surface acoustic wave of inverted phase to make pass band frequency characteristics of the filter device flat.
In addition, the pitch of the electrode fingers array in the second electrode area of the IDT can be set larger than that of the electrode fingers array of the first electrode area, and thereby, the first surface acoustic wave at low frequency outside of the filter device pass band is counteracted by the second surface acoustic wave of inverted phase with respect to the first surface acoustic wave to increase attenuation at the low frequency outside of the filter device pass band.
Further, the pitch of the electrode fingers array in the second electrode area of the IDT can be set smaller than that of the electrode fingers array of the first electrode area, thereby, the first surface acoustic wave at high frequency outside of the filter device pass band is counteracted by the second surface acoustic wave of inverted phase with respect to the first surface acoustic wave to increase attenuation at th
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pascal Robert
Summons Barbara
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