Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-06-18
2003-06-17
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C333S193000
Reexamination Certificate
active
06580199
ABSTRACT:
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-294748, filed Oct. 18, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a surface acoustic wave apparatus having an interdigital transducer formed on a piezoelectric substrate.
A surface acoustic wave apparatus is mainly used as an intermediate frequency filter for electronic devices. These devices include television sets, communication devices, and cellular phones (of, for example, the CDMA system), etc. The surface acoustic wave apparatus is characterized in that it is compact and lightweight, and thus can be utilized to the full when utilized in a cellular phone.
Low loss, a narrow bandwidth and a steep cut-off frequency are demanded by the intermediate frequency filter for use in a cellular phone. As a filter serving as an intermediate frequency filter of this type, a surface acoustic wave apparatus having an interdigital transducer (IDT) as a main structural element has been developed.
In this surface acoustic wave apparatus, to meet the demands of narrow bandwidth and steep cut-off frequency characteristics, a piezoelectric substrate is used which is formed of a material, such as crystal, having characteristics of low fluctuation in oscillation irrespective of temperature changes.
In the surface acoustic wave apparatus, it is known that an internal reflected wave (a reflected acoustic wave, a reflected electric wave) occurs since the apparatus utilizes a mechanical vibration such as a surface acoustic wave (SAW). The internal reflected wave adversely influences the fundamental wave of the surface acoustic wave, thereby causing its amplitude attenuation or phase distortion, etc. To reduce the influence of the reflected wave upon the fundamental wave and to adjust the direction of transmission of the fundamental wave to a predetermined direction, a technique for adjusting or trimming the width of an electrode finger in the IDT section of the surface acoustic wave apparatus has been developed. This technique is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 54-17647.
In the disclosed apparatus, the adverse influence of the internal reflected wave upon the fundamental wave is not completely removed. In view of the transmission function characteristic of a signal passing band in the surface acoustic wave apparatus, the adverse influence appears as a characteristic curve asymmetric with respect to the basic frequency at the center of the curve, or as a distorted characteristic curve.
A description will now be given of the phase relationship between the exited wave and the reflected wave in the surface acoustic wave apparatus, which the inventors of this invention have especially focused on.
FIGS.
1
A and
FIG. 1C
show cross sections of essential parts of the surface acoustic wave apparatus. The electrode fingers of the surface acoustic wave apparatus include pairs of electrode fingers of a narrow width of &lgr;/16, and pairs of electrode fingers of a wide width of 3&lgr;/16. Further, a plurality of pairs of electrode fingers projecting from a first common electrode and a plurality of pairs of electrode fingers projecting from a second common electrode are alternately arranged. The interval between each pair of adjacent electrode fingers is set at &lgr;/8.
FIG. 1B
shows the phase relationship between the excited wave and the internal reflected wave that advances to the left in the figure, while
FIG. 1D
shows the phase relationship between the excited wave and the internal reflected wave that advances to the right in the figure. Suppose that the phase of the excited wave is a reference value, and the clockwise direction with respect to the phase of the vector of the excited wave is the direction of a phase delay in the reflected wave.
Referring first to
FIGS. 1A and 1B
, the relationship between the reflected wave that advances to the left in the figures, and the excited wave will be described.
Suppose that P
1
represents any randomly-selected excitation point at which the surface acoustic wave is excited, A represents a reflected wave reflected from one EA of the edges of an electrode finger of a width (&lgr;/16) closest to the excitation point, and B represents a reflected wave reflected from the other edge EB of the electrode finger. Further, suppose that C represents a wave from the excitation point as a reference point, which is passed through the electrode finger closest to the excitation point, and reflected from one EC of the edges of an electrode finger of a width of 3&lgr;/16 adjacent to the first-mentioned electrode finger, and D represents a reflected wave reflected from the other edge ED of the second-mentioned electrode finger.
Concerning the reflected wave A, a phase delay element equal to a distance of 0.125&lgr; with respect to the phase of the excited wave at the excitation point occurs, and hence a phase delay of 45° occurs with respect to the phase of the excited wave. In other words, supposing that a delay =X, X=45° is given by the following equation:
(({fraction (0.125/2)})×2)&lgr;:X=&lgr;:360°
(×2) included in this equation indicates a coefficient for obtaining a double distance.
Further, concerning the reflected wave B, a phase delay element equal to a distance of (0.125+(⅛)) &lgr; with respect to the phase of the excited wave at the excitation point P
1
occurs, and phase inversion ((½)&lgr;) occurs at the edge EB. Accordingly, a phase delay of 270° occurs with respect to the phase of the excited wave. In other words, supposing that a delay=X, X=360×(0.125+(⅛)+(½))=270° is given by
{(({fraction (0.125/2)})×2)+(({fraction (1/16)})×2)+(½)}&lgr;:X=&lgr;:360
(×2) included in this equation indicates a coefficient for obtaining a double distance, and (½) indicates the amount of phase inversion at the edge EB.
Concerning the reflected wave C, a phase delay element equal to a distance of (0.125+(⅛)+(¼)) &lgr; with respect to the phase of the excited wave at the excitation point P
1
occurs, and hence a phase delay of 180° occurs with respect to the phase of the excited wave. In other words, supposing that a delay=X, X=360×(0.125+(⅛)+(¼))=180° is given by
{(({fraction (0.125/2)})×2)+(({fraction (1/16)})×2)+((⅛)×2)}&lgr;:X=&lgr;:360
(×2) included in this equation indicates a coefficient for obtaining a double distance.
Concerning the reflected wave D, a phase delay element equal to a distance of (0.125+(⅛)+(¼)+(⅜)) &lgr; with respect to the phase of the excited wave at the excitation point P
1
occurs, and phase inversion occurs at the edge ED. Accordingly, a phase delay of 135° occurs with respect to the phase of the excited wave. In other words, supposing that a delay=X, X=360×((⅛)+(⅛)+(¼)+(⅜)+(½))=495°=135° is given by
{(({fraction (0.125/2)})×2)+(({fraction (1/16)})×2)+((⅛)×2)+(
{fraction (3/16)})×
2+(½)}&lgr;:X=&lgr;:360
(×2) included in this equation indicates a coefficient for obtaining a double distance, and (½) indicates the amount of phase inversion at the edge ED.
The result of synthesizing the vectors of the reflected waves A, B, C and D corresponds to the phase delay amount of the internal reflected wave relative to the excited wave. As shown in
FIG. 1B
, the phase of the vector obtained by synthesizing the vectors of the internal reflected waves A, B, C and D is 157.5°. This value indicates that the phase is deviated by 22.5° from a phase (180°) in a direction in which the excited wave is offset.
The synthesized vector is obtai
Budd Mark O.
Kabushiki Kaisha Toshiba
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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