Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2000-02-23
2001-12-04
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S193000, C333S195000
Reexamination Certificate
active
06326864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a surface acoustic wave filter used in a communications device for a high-frequency band, and more particularly, relates to a ladder-type surface acoustic wave filter.
2. Description of the Related Art
In general, a ladder-type surface acoustic wave filter includes one-port resonators arranged alternately in series arms and parallel arms, and achieves low insertion loss and a wide band, which are extremely good characteristics for a filter. In such a ladder-type surface acoustic wave filter, the anti-resonant frequency (hereinafter, in the present specification, the anti-resonant frequency will be referred to as the anti-resonant point) of the surface acoustic wave resonator connected in parallel (hereinafter abbreviated as parallel arm resonator) is matched with the resonant frequency (hereinafter, in the present specification, the resonant frequency will be referred to as the resonant point) of the surface acoustic wave resonator connected in series (hereinafter abbreviated as series arm resonator). As a consequence, a bandpass filter having the resonant point of the parallel arm resonator and the anti-resonant point of the series arm resonator as its attenuation poles, and having the anti-resonant point of the parallel arm resonator and the resonant point of the series arm resonator as its center frequency, is formed, and is widely used in filters for mobile telephones and other similar devices.
There is increasing use in recent mobile telephones of systems in which the transmitter side frequency band and the receiver side frequency band are close together. As a result, it becomes important to improve the steepness near the pass band. To meet these market demands, technology has been proposed to raise the steepness near the pass band, and especially in the high-region side.
For example, EP0795958A2 discloses a method of improving the steepness near the high-region side of the pass band as well as improving the amount of attenuation, by generating multiple anti-resonant points in the series arm resonators. More specifically, in this method, the gap between the reflector and the interdigital transducer (IDT) electrode in the series arm resonator is shifted from 0.5&lgr; (where &lgr; is the wavelength of the surface acoustic waves, determined according to the pitch of the reflector), thereby generating a new anti-resonant point. According to this prior art reference, in the case where the gap between the IDT electrode of the series arm resonator and the reflector is within a range of (n/2+0.55)&lgr; to (n/2+0.81)&lgr; (where n is zero or a positive integer), it is possible to generate a new anti-resonant point which is at an appropriate position with respect to the original anti-resonant point and to improve the amount of attenuation in the high band side of the pass band of the ladder-type surface acoustic wave filter.
Furthermore, when the gap between the IDT electrode and the reflector is changed, the position of the newly generated anti-resonant point is changed. That is, using this technique makes it possible to move the anti-resonant point closer to the pass band, and to increase the steepness very close to the high band side of the pass band.
FIG.
1
and
FIG. 2
show resonance characteristics of such a conventional surface acoustic wave filter. Here,
FIG. 1
shows the relationship between impedance and frequency, and
FIG. 2
shows the relationship between transmission characteristics and frequency.
As shown in
FIG. 1
, the conventional surface acoustic wave filter has two anti-resonant points (M
1
and M
2
). As a consequence, the obtained surface acoustic wave filter has two attenuation poles (A
1
and A
2
) on the high band side of the pass band, as shown in FIG.
2
. The two anti-resonant points M
1
and M
2
in
FIG. 1
correspond to the two attenuation poles A
1
and A
2
in FIG.
2
.
In conventional surface acoustic wave filters, multiple attenuation poles are generated in the high band side of the pass band, thereby improving the attenuation characteristics near the high band side of the pass band. However, the inventors of the present application have found from the research that, in the method for raising steepness near the high band side of the pass band by generating multiple anti-resonant points, the rebound (hereinafter referred to in the present specification as sub-resonant point) between the anti-resonant points M
1
and M
2
cannot be ignored. That is, as shown in
FIG. 2
, a spurious high Q is generated at a point B
1
corresponding to M
3
. Consequently, although the better steepness is achieved near the high band side of the pass band, there is the disadvantage of the effect of the spurious high Q adversely affecting the amount of attenuation. Furthermore, as shown in
FIG. 3
, this spurious high Q tends to increase as the attenuation pole A
1
, corresponding to the anti-resonant point M
1
, moves closer to the pass band. That is, it can be said that there is a trade-off between the steepness near the high band side of the pass band and the actual amount of attenuation.
Furthermore, there are conventional methods other than the method of generating multiple anti-resonant points as described above, for instance, a method of forming at least one of the gaps between the reflector electrodes in a reflector having multiple reflector electrodes to have a different size as compared to the other gaps, or a method of making at least one of the gaps between the electrode fingers of an IDT electrode multiple electrode fingers a different value from the other gaps. However, even in these and other methods for generating multiple anti-resonant points, there is still the disadvantage of generation of a spurious high Q that deteriorates the amount of attenuation.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter, a duplexer, and a communications device in which a steepness near the pass band on the high band side is maintained while improving the amount of attenuation in the pass band on the high band side.
According to one preferred embodiment of the present invention, a ladder-type surface acoustic wave filter includes at least two surface acoustic wave resonators provided in a series arm, and at least one parallel arm surface acoustic wave resonator in a parallel arm. At least one of the series arm surface acoustic wave resonators has a plurality of anti-resonant points and a sub-resonant point provided between the anti-resonant points, and the sub-resonant point of that series arm resonator substantially matches an anti-resonant point of the other series arm resonators. Consequently, the anti-resonant points of the other series arm resonators cancel the spurious high Q caused by the effect of the sub-resonant point, enabling the amount of attenuation at the high band side of the pass band to be improved while maintaining steepness.
In a preferred embodiment of the present invention, a surface acoustic wave filter includes a ladder type circuit having a series arm and at least one of parallel arms, first and second series arm surface acoustic wave resonators provided in the series arm, and at least one parallel arm surface acoustic wave resonator provided in each of the parallel arms, wherein the first series arm surface acoustic wave resonators has a plurality of anti-resonant points and a sub-resonant point provided therebetween, and the sub-resonant point substantially matches an anti-resonant point of the second series arm surface acoustic wave resonator.
In other preferred embodiments, the sub-resonant point is adjusted by varying the pitch or gap width of at least one electrode finger in the interdigital transducer, or the reflector, or between the outermost electrode fingers of the interdigital transducer and the reflector relative to the other pitches. Note that the pitch can be varied by changing the thickness of the electrode finger.
Other features, element
Takamine Yuichi
Taniguchi Norio
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Pascal Robert
Summons Barbara
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