Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-04-16
2004-05-18
Cunningham, Terry D. (Department: 2816)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000
Reexamination Certificate
active
06737942
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave apparatuses, such as a longitudinally-coupled-resonator-type surface acoustic wave filter in which a plurality of interdigital transducers (hereinafter called IDTs) each having a plurality of electrode fingers are arranged in a direction in which a surface acoustic wave propagates, and also relates to communication apparatuses including such surface acoustic wave apparatuses.
2. Description of the Related Art
Surface acoustic wave filters (surface acoustic wave apparatuses) are widely used as bandpass filters in RF stages of portable telephones. The performance required for the bandpass filters includes a low loss, a high attenuation, and a wide bandwidth. Many surface acoustic wave filters satisfying the performance have been invented.
Among these surface acoustic wave filters, as a method for extending the bandwidth of a longitudinally-coupled-resonator-type surface acoustic wave filter, a method has been widely used in which the arrangement of resonant modes is optimized by a condition in which electrode fingers are arranged periodically through two adjacent IDTs, more specifically, by shifting the distance between the centers of adjacent electrode fingers positioned across the space between two adjacent IDTs from half the wavelength determined by the pitch of electrode fingers, such as that disclosed, for example, in Japanese Unexamined Patent Publication No. 05-267990.
In the conventional method, although the bandwidth of a filter can be extended, an insertion loss deteriorates.
In other words, when the distance between adjacent electrode fingers positioned across the space between two adjacent IDTs is shifted from half the wavelength determined from the pitch of electrode fingers, periodic continuity deteriorates in a propagation path of a surface acoustic wave at that portion. Especially in a surface acoustic wave filter including a piezoelectric substrate which uses a leaky surface acoustic wave, such as a 36-degree Y-cut X-propagation LiTaO
3
substrate or a 64-degree Y-cut X-propagation LiNbO
3
, a loss caused by the emission of a bulk wave increases. As a result, although the bandwidth of the filter is extended, an insertion loss (propagation loss) deteriorates.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a longitudinally-coupled-resonator-type surface acoustic wave apparatus which has a wide pass bandwidth, has a small insertion loss and a small VSWR, and does not cause deterioration of the dielectric strength, and for which electrodes are easily formed, and to provide a communication apparatus which includes such a novel surface acoustic wave apparatus.
According to one preferred embodiment of the present invention, a surface acoustic wave apparatus of a longitudinally-coupled-resonator type includes a plurality of interdigital transducers each having a plurality of electrode fingers, arranged along the propagation direction of a surface acoustic wave on a piezoelectric substrate, wherein the pitch of an electrode finger located in an end portion facing another interdigital transducer is smaller than that of electrode fingers at the other portions in at least one interdigital transducer, and the duty (also called a metallization ratio) of the electrode finger of which the pitch is made smaller is smaller than that of the electrode fingers at the other portions.
According to the above-described unique structure, in a longitudinally-coupled-resonator-type surface acoustic wave filter having a plurality of interdigital transducers, the pitch of an electrode finger located in an end portion where two IDTs face each other is smaller than that of electrode fingers at the other portions in the IDT, and the duty of the electrode finger (hereinafter called a narrow-pitch electrode finger) of which the pitch has been made smaller is smaller than that of electrode fingers at the other portions. Therefore, the filter has a bandwidth that is substantially equal to or wider than that obtained by a conventional technology, has a small insertion loss, does not cause deterioration of the dielectric strength, and its electrodes are easily formed.
In the above-described surface acoustic wave apparatus, it is preferred that both IDTs adjacent to each other be provided with an electrode finger having a different pitch, and the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs be made substantially equal to about 0.5&lgr;I
2
, where &lgr;I
2
indicates the wavelength determined by the pitch of the electrode fingers of which the pitch has been made different, and &lgr;I
1
indicates the wavelength determined by the pitch of the other electrode fingers.
According to the above-described unique structure, when both IDTs adjacent to each other are provided with an electrode finger having a different pitch, the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs is made almost equal to 0.5&lgr;I
2
. Therefore, since the continuity of a surface acoustic wave propagating between the IDTs is ensured, the deterioration of an insertion loss is reliably prevented.
In the above-described surface acoustic wave apparatus, it is further preferred that one of IDTs adjacent to each other be provided with an electrode finger having a different pitch, and the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs be made almost equal to 0.25&lgr;I
1
+0.25&lgr;I
2
, where &lgr;I
2
indicates the wavelength determined by the pitch of the electrode finger of which the pitch has been made different, and &lgr;I
1
indicates the wavelength determined by the pitch of the other electrode fingers.
According to the above-described unique structure, when one of IDTs adjacent to each other is provided with an electrode finger having a different pitch, the distance between the centers of adjacent electrode fingers positioned across the space between two IDTs is made almost equal to 0.25&lgr;I
1
+0.25&lgr;I
2
. Therefore, since the continuity of a surface acoustic wave propagating between the IDTs is ensured, the deterioration of an insertion loss is reliably prevented.
In the surface acoustic wave apparatus, it is further preferred that, in the interdigital transducer in which the pitch of the electrode finger located in the end portion facing the another interdigital transducer is made different from that of the other electrode fingers in the interdigital transducer, the distance between the centers of adjacent electrode fingers one of which is an electrode finger of which the pitch has been made different and the other of which is an electrode finger of which the pitch has not been made different be made almost equal to 0.25&lgr;I
1
+0.25&lgr;I
2
.
According to the above-described unique structure, since the distance between the centers of adjacent electrode fingers one of which is an electrode finger of which the pitch has been made different and the other of which is an electrode finger of which the pitch has not been made different is made almost equal to 0.25&lgr;I
1
+0.25&lgr;I
2
, the continuity of a surface acoustic wave propagating between the electrode finger of which the pitch has been made different and the electrode finger of which the pitch has not been made different is ensured, and the deterioration of an insertion loss is prevented.
The surface acoustic wave apparatus may be configured such that the duty of an electrode finger of which the pitch has been made smaller is smaller than that of electrode fingers in the other portions in the IDT and is substantially equal to or larger than about 0.45. According to the above-described unique structure, since the duty of an electrode finger of which the pitch has been made smaller is smaller than that of electrode fingers in the other portions in the IDT and is substantially equal to or large
Takamine Yuichi
Watanabe Hiroki
Cunningham Terry D.
Keating & Bennett LLP
Murata Manufacturing Co. LTD
Tra Quan
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