Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1997-11-18
2002-04-16
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C310S31300R, C310S31300R, C310S31300R
Reexamination Certificate
active
06373353
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a surface acoustic wave transducer for use in a surface acoustic wave device such as a SAW filter and a SAW resonator, and more particularly to a surface acoustic wave transducer comprising a piezoelectric substrate having a natural single-phase unidirectional transducer (termed as NSPUDT) property and an electrode structure having a directivity. The present invention also relates to a surface acoustic wave filter having such a transducer.
TECHNICAL BACKGROUND
Heretofore, there has been widely used a transversal type SAW filter, in which on a surface of a piezoelectric substrate, are arranged a transmitter side transducer including interdigitally arranged positive and negative electrodes to be connected to two output terminals of a single phase signal source having a phase difference of 180°, and a receiver side transducer also including interdigitally arranged positive and negative electrodes, said transmitter and receiver side transducers being separated by a given distance so as to extract a specific frequency.
In such a SAW filter, it is required to reduce an insertion loss as well as to suppress a ripple within a given frequency band. Since a conventional interdigital electrode structure shows the bidirectionality, a theoretical insertion loss amounts to 6 dB and thus the insertion loss could not be suppressed any more. Furthermore, in order to improve a performance of the surface acoustic wave device, not only the insertion loss has to be decreased, but also it is also important to flatten a phase characteristic and to improve a frequency characteristic such as a suppression of ripple within a pass band and a suppressed band.
In order to satisfy the above mentioned requirements, there has been used a unidirectional transducer, in which an insertion loss can be suppressed to a value not larger than 1 dB and good phase and frequency characteristics can be attained. For the unidirectional transducer, there have been proposed various kinds of types, which may be roughly classified into (a) multi-phase type unidirectional transducer and (b) single-phase type unidirectional transducer. Further, for the latter single-phase unidirectional transducer, there have been proposed a single-phase unidirectional transducer utilizing an internal reflection due to asymmetry of an electrode structure and mass load effect, a single-phase unidirectional transducer utilizing a reflection by a floating electrode, and a natural single-phase unidirectional transducer utilizing an anisotropy of a substrate. In surface acoustic wave devices using such unidirectional transducers, the directivity is attained by a fact that a phase difference between an exiting wave and an excited wave and a reflected wave have the same phase viewed in a propagating direction, but have opposite phased viewed in a direction opposite to the propagating direction.
In the above mentioned single-phase unidirectional transducers other than the natural single-phase unidirectional transducer, the electrode structure is complicated, and particularly an edge distance and a width of electrode fingers have to be smaller than &lgr;/4. In accordance with an increase in an operating frequency, said dimensions become extremely small, and it is difficult to manufacture accurately the electrodes having the desired dimensions.
As one solution for solving such a problem, there has been proposed a natural single-phase unidirectional transducer (NSPUDT), in which due to the anisotropy of the substrate itself, the unidirectionality can be attained even by using a conventional electrode, in which the edge distance and width of electrodes are set to &lgr;/4. In this surface acoustic wave device utilizing the NSPUDT behavior, the anisotropy of the substrate itself is utilized. Quartz substrate, LiNbO
3
substrate and LiTaO
3
substrate have been known as such a piezoelectric substrate showing the unidirectionality due to the anisotropy. The inventors have further recognized that a lithium tetraborate substrate of a special cut shows the NSPUDT property. Particularly, upon being compared with the other substrates showing the natural single-phase unidirectional transducer property, this lithium tetraborate substrate could provide an ideal surface acoustic wave device due to a fact that it has a large electromechanical coupling constant K
2
, a zero delay time temperature coefficient, a zero power flow angle and so on.
However, in the surface acoustic wave device utilizing the above mentioned NSPUDT, since the anisotropy of the substrate itself is utilized, there is a problem that it is difficult to obtain unidirectional transducers whose forward directions are opposed to each other, one for the output side, i.e. transmitter side transducer and the other for the input side, i.e. receiver side transducer. In order to solve this problem, there have been proposed various methods for reversing the directivity. For instance, the directivity of one of the transmitter and receiver side transducers may be reversed by using electrodes made of a material whose phase of reflection coefficient is different from that of the other transducer by 180°, or by providing recesses in a substrate surface and embedding electrodes in the recesses.
However, according to the generally used manufacturing process, the formation of electrodes from different electrode materials and the embedded electrodes might cause the complication in the designing and manufacturing and high cost, and thus it would be difficult to obtain a desired accuracy, so that the desired frequency and phase characteristics could not be attained.
Moreover, if a transversal type surface acoustic wave filter for extracting a signal within a specific frequency band is realized by providing transmitter and receiver side transducers on the substrate having the anisotropy, when the electrode structure of the transmitter side transducer is made of a different material than that of the receiver side transducer, a propagating velocity of a surface acoustic wave in the transmitter side transducer might be different from that in the receiver side transducer and a center frequency might shift between the transmitter side transducer and the receiver side transducer.
Further, in case of using the NSPUDT substrate having a large reflection coefficient of electrode fingers, the reflecting effect of the electrodes fingers might become too strong to control a transduction characteristic.
DISCLOSURE OF THE INVENTION
The present invention has for its object to provide a surface acoustic wave transducer and a surface acoustic wave filter, in which the above mentioned drawbacks of the known surface acoustic wave transducer having the NSPUDT property can be removed or mitigated, and superior frequency and phase characteristics can be obtained by a simple manufacturing process.
According to the invention, a surface acoustic wave transducer comprising an anisotropic piezoelectric substrate being cut to have a natural single-phase unidirectional transducer property and at least one transducer structure having an exciting electrode structure and a reflector structure formed on said substrate, characterized in that when &lgr; is a wavelength of a fundamental surface acoustic wave, said exciting electrode structure includes a positive electrode having a plurality of electrode fingers arranged at a pitch &lgr; and a negative electrode having at least one electrode finger interdigitally arranged between said electrode fingers of the positive electrode with a center distance of &lgr;/2, that said reflector transducer includes a plurality of electrode fingers arranged with a center distance of &lgr;/2, and that a distance L
g
between said exciting electrode structure and the reflector structure is set to L
g
=(2n+1)&lgr;/4 (n being a positive integer).
In a conventional piezoelectric substrate without the anisotropy, a reflection center may be situated at a center of an electrode finger although a reflection occurs at input and output side edges of the electrode finger.
Odagawa Hiroyuki
Takeuchi Masao
Tanaka Mitsuhiro
Yamanouchi Kazuhiko
Lee Benny
NGK Insulators Ltd.
Parkhurst & Wendel L.L.P
Summons Barbara
LandOfFree
Surface acoustic wave transducer using NSPUDT property... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Surface acoustic wave transducer using NSPUDT property..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface acoustic wave transducer using NSPUDT property... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2850946