A SAW resonator comprises two reflectors and a transducer arranged between the reflectors. A resonant space between the transducer and a respective reflector is set large enough to enable occurrence of main resonance and at least one further resonance of comparable admittance. Thus, a multiple resonant resonator is achieved that can be used as a parallel resonator in a filter circuit with a DMS track for example to improve attenuation in a stop band.

When a current flowing in a series circuit including an equivalent resistance, an equivalent inductor, and an equivalent capacitance in an electric equivalent circuit of a specific resonator in each filter is defined as an acoustic path current, under conditions that a phase of an acoustic path current of a first transmission filter at a side of a common terminal at a frequency within a first transmission band is represented as θ1.sub.Tx, a phase of an acoustic path current of the first transmission filter at the side of the common terminal at a frequency within a second transmission band is represented as θ2.sub.Tx, a phase of an acoustic path current of a first reception filter at the side of the common terminal at a frequency within the first transmission band is represented as θ1.sub.Rx, and a phase of an acoustic path current of the first reception filter at the side of the common terminal at a frequency within the second transmission band is represented as θ2.sub.Rx, a multiplexer satisfies a first condition: |(2.Math.θ1.sub.Tx−θ2.sub.Tx)−(2.Math.θ1.sub.Rx−θ2.sub.Rx)|=180°±90°, or a second condition: |(2.Math.θ2.sub.Tx−θ1.sub.Tx)−(2.Math.θ2.sub.Rx−θ1.sub.Rx)|=180°±90°.

A filter circuit has a cascaded resonator circuit with a first acoustic resonator and a second acoustic resonator connected in series on a printed circuit board (PCB). The admittances as functions of frequency of the first and second acoustic resonators are substantially identical. The filter circuit also has a composite resonator circuit formed by a capacitor connected in parallel with the second acoustic resonator on the PCB. The capacitor improves a steepness of an upper bandpass edge of the filter circuit.

A high-isolation and anti-glue-invasion Surface Acoustic Wave (SAW) duplexer includes a transmitting filter and a receiving filter. The isolation of the duplexer is improved by adjusting the positions of parallel resonance arms of the receiving filter and optimizing the distances between a grounding metal wiring of a Double Mode Structure (DMS) filter in the receiving filter and other grounding metal wirings and a distance between the grounding metal wiring of the DMS filter in the receiving filter and a signal metal wiring, and the grounding metal wiring is further arranged to surround the series resonance arms and the parallel resonance arms, so as to fill blank positions between the resonance arms and the edge of a package.

An elastic wave device includes a piezoelectric substrate, an IDT electrode on the piezoelectric substrate, and a silicon oxide film arranged on the piezoelectric substrate to cover the IDT electrode. The IDT electrode includes first and second electrode layers laminated on each other, the first electrode layer is made of metal or an alloy with a density higher than a density of metal of the second electrode layer and a density of silicon oxide of the silicon oxide film, the piezoelectric substrate is made of LiNbO.sub.3 and θ is in a range of equal to or greater than about 8° and equal to or less than about 32° with Euler Angles (0°±5°, θ, 0°±10°) of the piezoelectric substrate, and the silicon oxide film contains hydrogen atoms, hydroxyl groups, or silanol groups.

An acoustic wave resonator includes: a support substrate; a piezoelectric substrate located on the support substrate; a first amorphous layer that is in contact with the support substrate and is mainly composed of one or more constituent elements of the support substrate; a second amorphous layer that is in contact with the piezoelectric substrate and the first amorphous layer, is mainly composed of one or more constituent elements of the piezoelectric substrate, and is thinner than the first amorphous layer; and a pair of comb-shaped electrodes that is located on an opposite surface of the piezoelectric substrate from the support substrate, each of the pair of comb-shaped electrodes including electrode fingers.

When a current flowing in a series circuit including an equivalent resistance, an equivalent inductor, and an equivalent capacitance in an electric equivalent circuit of a specific resonator in each filter is defined as an acoustic path current, under conditions that a phase of an acoustic path current of a first transmission filter at a side of a common terminal at a frequency within a first pass band is represented as θ1.sub.Tx1, a phase of an acoustic path current of the first transmission filter at the side of the common terminal at a frequency within a second pass band is represented as θ2.sub.Tx1, a phase of an acoustic path current of a second transmission filter at the side of the common terminal at a frequency within the first pass band is represented as θ1.sub.Tx2, and a phase of an acoustic path current of the second transmission filter at the side of the common terminal at a frequency within the second pass band is represented as θ2.sub.Tx2, a multiplexer satisfies a first condition: |(2.Math.θ1.sub.Tx1−θ2.sub.Tx1)−(2.Math.θ1.sub.Tx2−θ2.sub.Tx2)|=180°±90°, or a second condition: |(2.Math.θ2.sub.Tx1−θ1.sub.Tx1)−(2.Math.θ2.sub.Tx2−θ1.sub.Tx2)|=180°±90°.

An acoustic wave device includes a support substrate made of silicon, a piezoelectric body provided directly or indirectly on the support substrate, the piezoelectric body including a pair of main surfaces facing each other, and an interdigital transducer electrode provided directly or indirectly on at least one of the main surfaces of the piezoelectric body, a wave length that is determined by an electrode finger pitch of the interdigital transducer electrode being λ. An acoustic velocity V.sub.Si=(V.sub.1).sup.1/2 of bulk waves that propagate in the support substrate, which is determined by V.sub.1 out of solutions V.sub.1, V.sub.2, V.sub.3 of x derived from the expression, Ax.sup.3+Bx.sup.2+Cx+D=0, is higher than or equal to about 5500 m/s.

A ladder-type filter includes a support substrate, a piezoelectric layer provided on the support substrate, a parallel resonator including first electrode fingers provided on the piezoelectric layer and having a first average pitch and a first average duty ratio, a largest first average pitch being equal to or greater than two times a thickness of the piezoelectric layer, a first end of the parallel resonator being coupled to a path between input and output terminals, a second end of the parallel resonator being coupled to a ground, and a series resonator connected in series between the input and output terminals, the series resonator including second electrode fingers provided on the piezoelectric layer and having a second average pitch and a second average duty ratio, a second average duty ratio in at least one series resonator being less than a smallest first average duty ratio.

An acoustic wave device includes a substrate including a piezoelectric layer, first and second resonators on the substrate, and a shared reflector. The second resonator is located on the substrate adjacent to the first resonator and has different frequency characteristics than the first resonator. The shared reflector is located on the substrate between the first resonator and the second resonator and is a reflector for both the first resonator and the second resonator. The first resonator includes a first interdigital transducer electrode with electrode fingers positioned with a first pitch. The second resonator includes a second interdigital transducer electrode with electrode fingers positioned with a second pitch. A lower limit frequency of a stop band of the shared reflector is between a lower limit frequency of a stop band of the first resonator and a lower limit frequency of a stop band of the second resonator. An upper limit frequency of the stop band of the shared reflector is between an upper limit frequency of the stop band of the first resonator and an upper limit frequency of the stop band of the second resonator.