Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-05-24
2001-01-09
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R
Reexamination Certificate
active
06172580
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use as a band filter, and more particularly, to a surface acoustic wave filter having a ladder-type circuit configuration including a plurality of one-port surface acoustic wave resonators.
2. Description of the Related Art
An LiTaO
3
substrate is advantageous in that it has a relatively high electromechanical coefficient and the temperature characteristics thereof are relatively stable. Accordingly, the LiTaO
3
substrate is widely used as the piezoelectric substrate of a surface acoustic wave resonator or a surface acoustic wave filter.
For example, Japanese Unexamined Patent Publication No. 6-188673 discloses a surface acoustic wave filter having a ladder type circuit defined by disposing a plurality of surface acoustic wave resonators on the LiTaO
3
substrate. In this prior art, in order to realize desired impedance characteristics, it is suitable to adjust the numbers of the electrode finger pairs and the apertures (cross widths) in the surface acoustic wave resonators.
Further, Japanese Unexamined Patent Publication No. 5-183380 discloses a surface acoustic wave filter having a ladder type circuit configuration defined by a plurality of surface acoustic wave resonators. It is described in this reference that in order to obtain a desired electrode capacitance, the numbers of the electrode finger pairs and the apertures are carefully selected. The electrode capacitance of the surface acoustic wave resonator is determined substantially by the product of the number of the electrode finger pairs, the aperture, and the dielectric constant of a substrate, as described in Japanese Unexamined Patent Publication No. 5-183380. Accordingly, when the LiTaO
3
substrate is used as the piezoelectric substrate, the product of the number of the electrode finger pairs and the aperture is determined, depending on an objective electrode capacitance.
However, when the surface acoustic wave filter having a ladder type circuit configuration was produced by using a plurality of surface acoustic wave resonators, and for this purpose, the products of the numbers of the electrode finger pairs and the apertures were made constant so as to obtain an electrode capacitance having a desired value, as described above, an unsatisfactory flatness in the pass band was produced.
FIG. 17
shows the attenuation frequency characteristics of a conventional surface acoustic wave filter. As shown by an arrow A in
FIG. 17
, there is a deficiency in the left-side shoulder portion in the pass band. That is, the attenuation was insufficiently reduced in the lower frequency side range in the pass band. Accordingly, the flatness of the attenuation frequency characteristics in the pass band was unsatisfactory.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter having an excellent flatness of the attenuation frequency characteristic in the pass band and which has a wide band.
A surface acoustic wave filter according to a preferred embodiment of the present invention has a ladder type circuit configuration and includes a series resonator in a series arm provided between input and output terminals and a parallel resonator included in a parallel arm provided between the series arm and a reference potential. The surface acoustic wave filter includes a piezoelectric substrate, and a plurality of one-port surface acoustic wave resonators disposed on the piezoelectric substrate and arranged to define the series resonator and the parallel resonator. The one-port surface acoustic wave resonators include interdigital transducers. A gap length or distance between a first bus bar of the interdigital transducer of at least one of the surface acoustic wave resonators and the tip of an electrode finger connected to a second bus bar which is disposed opposite to the first bus bar has a value of up to about 1.0 &lgr; in which &lgr; represents the wave length of a surface acoustic wave.
According to preferred embodiments of the present invention, an unnecessary ripple in the pass band, due to a ripple occurring between the resonant frequency and the anti-resonant frequency of the surface acoustic wave resonator, is prevented. In addition, since the impedance characteristics of each surface acoustic wave resonator are not changed, the above-mentioned ripple is prevented from occurring while still being able to prevent any narrowing of the pass band. Thus, a surface acoustic wave filter having excellent flatness in the pass band and a very wide band is provided.
The gap length of at least one of the one-port surface acoustic wave resonators constituting the parallel resonators preferably has a value of up to about 1.0 &lgr;. In such a case, a ripple occurring between the resonant frequency and the anti-resonant frequency of the parallel resonators is prevented. As a result, the filter characteristics on the lower frequency side in the pass band are prevented from being degraded, and the flatness in the pass band is greatly improved while the pass band is prevented from being narrowed.
Further, a plurality of the parallel resonators may be provided, and the gap lengths of all the parallel resonators preferably have a value of up to about 1.0 &lgr;. In addition, the piezoelectric substrate may be an LiTaO
3
substrate. With such a unique configuration, the surface acoustic wave filter has a very wide band and very stable temperature characteristics.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
REFERENCES:
patent: 4799184 (1989-01-01), Grassl
patent: 5077545 (1991-12-01), Gopani et al.
patent: 5708403 (1998-01-01), Morozumi et al.
patent: 5793266 (1998-08-01), Allen et al.
patent: 5909156 (1999-06-01), Nishihara et al.
patent: 5963113 (1999-10-01), Ou et al.
patent: 5-183380 (1993-07-01), None
patent: 6-188673 (1994-07-01), None
Taniguchi Norio
Ushiroku Tadamasa
Keating & Bennett LLP
Lee Benny
Murata Manufacturing Co. Ltd.
Summons Barbara
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