Seiko Epson Corporation

Satoru Hayashi

Michiaki Takagi

Takashi Yamazaki

Masahiro Oshio

Katsuro Yoneya

The invention provides a longitudinal double-mode SAW filter that has a large reflection coefficient by the electrode conductors and having flat pass band characteristics. In the longitudinal double-mode SAW filter, nearly all of higher-order natural mode oscillation displacements ASand Aare made present in the first and second interdigital transducer regions. An electrode finger phase weighting is formed to correspond to a BPSK sign where a phase sign changes into 0 or &#x3c0; at a polarity-changing point on an electrode charge distribution function Q(x) that generates on the electrode due to natural modes of oscillation. A single pair of natural modes, which exist stationary in the direction X of propagation of the surface acoustic waves that are utilized, is selected. A symmetrical mode charge distribution (Q(x), &#x2212;Q(x)) and an obliquely symmetrical mode charge distribution (Q(x), Q(x)) are generated in order to correspond to the regions of IDTand IDTThe PTNG in the control IDTregion is set to be in the range of 1.02 to 1.04 so as to couple the two together.

Longitudinal double-mode saw filter utilizing electrode finger phase weighting

A transversally coupled double-mode SAW filter using a surface acoustic wave has two SAW resonators which have curtain-like electrodes and a pair of reflectors on both side thereof, respectively, and which are disposed in parallel on a piezoelectric flat plate in the propagation direction of the surface acoustic wave. The curtain-like electrodes of the SAW resonators have a first bus bar of electrically positive electrode which extends in the propagation direction of SAW and second and third bus bars of electrically negative electrode which extend in parallel on both sides of the first bus bar. The first bus bar has multiple electrode fingers of electrically positive electrode which face the second and third bus bars on both sides thereof and which extend in a direction perpendicular to the propagation direction of SAW so as to suitably have a space of &lgr;/2, &lgr; being the wavelength of the surface acoustic wave, and a width of &lgr;/4. The second and third bus bars have multiple electrode fingers of electrically negative electrode which correspond to the electrode fingers of electrically positive electrode, face the first bus bar and extend in a direction perpendicular to the propagation direction of SAW so as to similarly have a space of &lgr;/2 and a width of &lgr;/4. Thereby, the curtain-like electrodes are constituted to invert the direction of driving field by 180&deg; with the first bus bar therebetween, and to enable making the passband width sharply broader than before by use of the frequency difference between excited primary symmetric mode and primary anti-symmetric mode.

Transverse double mode saw filter

A multi-longitudinal-mode-coupled resonator type SAW filter having flat passband characteristic is provided. In the multi-longitudinal-mode-coupled resonator type SAW filter, an energy trapped type SAW resonator is formed on a frequency upward type piezoelectric substrate, and an IDT is divided into three IDTs, a first IDT arranged to a transmitter side, a second IDT arranged to a receiver side, and a third IDT arranged between the first IDT and the second IDT to control the amplitude of the vibration displacement in two modes for forming a filter. A frequency potential is optimized by changing an electrode period length of each IDT. Resonance amplitude strengths in the two modes are thus equalized and passband transmission characteristics are thus flattened. A filter having a relatively wide bandwidth of 1000 to 1500 ppm is fabricated by using a substrate having a small electromechanical coupling factor, such as a quartz crystal substrate.

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Okaya, Japan

Satoru Hayashi