Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-10-28
2001-06-12
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06246150
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a surface acoustic wave device (hereinafter called SAW filter) having corresponding comb-like electrodes.
In generally, an SAW filter is small in size, light in weigh and having high reliability. The SAW filter is used in a cellular phone and the like in a filed of a mobile communication system.
An intermediate frequency (IF) filter which is one of constituent components of the cellular phone is required to have narrow-band steep frequency characteristics with low loss, flat group delay characteristics and phase characteristics. In this SAW filter, an interdigital transducer (IDT) of a transversal filter type filter is used for obtaining the flat group delay characteristics.
Further, since narrow-band steep characteristics are required, a piezoelectric substrate in which the temperature dependence of frequency characteristic is small such as a crystal is used.
Further, it is known to set the transmitting direction of surface acoustic wave in the IDT into one direction to reduce insertion loss.
One type of unidirectional transducers is described in Jpn. Pat. Appln. KOKAI Publication No. 61-6917, for example. The unidirectional transducer described in Jpn. Pat. Appln. KOKAI Publication No. 61-6917 is of a structure so-called split electrodes or double electrodes in which a pair of comb-like electrodes are opposed on a piezoelectric substrate.
This unidirectional transducer comprises first and second opposed comb-like electrodes each including a large number of sets of electrode fingers, while each set including two electrode fingers. The first and second comb-like electrodes are arranged such that the sets of the comb-like electrodes are interdigitaly overlapped with each other. Here, when the two electrode fingers (first set) of the first comb-like electrodes are interdigitaly overlapped with the two electrode fingers (second set) of the second comb-like electrodes, a maximum distance in widthwise direction of disposition area of the four electrode fingers is &lgr; (wavelength of center frequency of surface acoustic wave). By selecting the width of the electrode finger within a range of this &lgr;, a propagation direction of the surface acoustic wave is emphasized. However, in the case of the SAW filter, in addition to the selection of the propagation direction of the surface acoustic wave, it is necessary to pay attention to the fact that amplitude ripple and group delay ripple in a using frequency band must be suppressed. That is, in IDT, since triple transit echo (TTE: electric spurious multiple reflection) is generated, it is necessary to suppress the TTE.
In the above-described conventional SAW, measures for deleting the amplitude ripple and the group delay ripple in the using frequency band are insufficient.
BRIEF SUMMARY OF THE INVENTION
Thereupon, the present invention has been accomplished in view of the above problem, and it is an object of the invention to provide an SAW filter which excellently reduces amplitude ripple and group delay ripple in a using frequency band.
To achieve the above object, a surface acoustic wave device of the present invention comprises:
a piezoelectric substrate,
a first common electrode formed on the piezoelectric substrate,
a first comb-like electrode finger including a plurality of first electrode finger groups, the groups comprising a plurality of electrode fingers having distributed positive and negative deviations wx(wx=w1−(&lgr;/8), &lgr; is surface acoustic wavelength and w1 is a width of electrode finger) with respect to &lgr;/8 in one finger group, and the plurality of first electrode finger groups connected with the first common electrode,
a second common electrode provided in parallel to the first common electrode on the piezoelectric substrate, and
a second com-like electrode finger including a plurality of second electrode finger groups, the groups comprising a plurality of electrode fingers having distributed positive and negative deviations wx(wx=w1−(&lgr;/8), &lgr; is surface acoustic wavelength and w1 is a width of electrode finger) with respect to &lgr;/8 in one finger group, and the plurality of second electrode finger groups connected with the second common electrode, the second electrode finger groups being interdigitaly overlapped with the first electrode finger groups.
a second electrode fingers including a plurality of second finger groups which are interdigitaly overlapped with the plurality of first finger groups of the first comb-like electrode finger and which are projected from the second common electrode, each of the finger groups including therein a plurality of electrode fingers having positive and negative deviations wx with respect to &lgr;/8 (&lgr; is surface acoustic wavelength), the deviations being formed differently.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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SPUDT-Based Filters, Thomson Microsonics, URSI 25th General Assembly, Sep. 1996.*
Acoustic Surface Wave Filters, Tancrell et al., Proceedings of IEEE, vol. 59, No. 3, Mar. 1971.
Dougherty Thomas M.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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