Tunable filter including a variable capacitor connected with a surface acoustic wave resonator
A tunable filter includes a circuit configuration in which variable capacitors are connected with surface acoustic wave resonators. The surface acoustic wave resonators each include a piezoelectric substrate made of LiTaO 3 or LiNbO 3 , an IDT electrode made of an electrode material filled in a recess in an upper surface of the piezoelectric substrate, and a SiO 2 film arranged to cover the piezoelectric substrate.
1. A tunable filter, comprising:
a surface acoustic wave resonator including:
a piezoelectric substrate made of LiNbO 3 with Euler angles of (0°, 80° to 130°, 0°) and including a recess provided in an upper surface thereof; and
an IDT electrode including a metal material that is primarily made of at least a material selected from a group consisting of Au, Pt, W, Ta, Ag, Cu, Mo, and Ni and embedded in the recess; wherein
when λ is a wavelength of a surface acoustic wave, a relationship among the metal from which the IDT electrode is primarily made, a lower limit of a film thickness of the electrode, and a duty is a relationship shown in Table 1, an upper limit of the film thickness of the electrode is a value shown in Table 1, and the duty is in a range from about 0.15 to about 0.85; and
a variable capacitor connected with the surface acoustic wave resonator:
TABLE 1
(X: Duty)
Upper limit
Electrode
Lower limit of film thickness (λ)
of film thickness (λ)
Au
0.0381 − 0.0211X − 0.0222X 2
0.25
Pt
0.0501 − 0.0445X − 0.0031X 2
0.25
W
0.0748 − 0.0978X + 0.0444X 2
0.25
Ta
0.0739 − 0.0789X + 0.0222X 2
0.25
Ag
0.1287 − 0.1767X + 0.0667X 2
0.25
Cu
0.1883 − 0.23X + 0.0667X 2
0.25
Mo
0.1497 − 0.0990X
0.25
Ni
0.22112 − 0.13613X + 0.0439X 2
0.25.
2. The tunable filter according to claim 1 , wherein the relationship among the metal from which the IDT electrode is primarily made, the film thickness of the electrode, and the duty is a relationship shown in Table 3, and the upper limit of the film thickness of the electrode is a value shown in Table 3:
TABLE 3
(X: Duty)
Upper
Electrode
Lower limit of film thickness (λ)
limit of film thickness (λ)
Au
0.0343 − 0.0190X − 0.0200X 2
0.25
Pt
0.0451 − 0.0401X − 0.0028X 2
0.25
W
0.0673 − 0.0880X + 0.040X 2
0.25
Ta
0.0665 − 0.0710X + 0.020X 2
0.25
Ag
0.1158 − 0.1590X + 0.060X 2
0.25
Cu
0.1695 − 0.207X + 0.060X 2
0.25
Mo
0.1347 − 0.0891X
0.25
Ni
0.1997 − 0.1259X + 0.0358X 2
0.25.
3. The tunable filter according to claim 1 , wherein
the surface acoustic wave resonator includes a plurality of surface acoustic wave resonators, and the plurality of surface acoustic wave resonators are connected in series between an input terminal and an output terminal;
the variable capacitor includes first and second variable capacitors, the first variable capacitor being connected in series with at least one of the plurality of surface acoustic wave resonators, the second variable capacitor being connected in parallel with at least one of the plurality of surface acoustic wave resonators; and
the tunable filter further comprises:
an inductance connected between a node of the plurality of surface acoustic wave resonators and a ground; and
matching capacitors respectively connected between the input terminal and the ground and between the output terminal and the ground.
4. The tunable filter according to claim 3 , wherein the matching capacitors and the inductance have impedances in a range from about 20Ω to about 100Ω.
5. A tunable filter, comprising:
a surface acoustic wave resonator including:
a piezoelectric substrate made of LiNbO 3 with Euler angles of (0°, 80° to 130°, 0°) and including a recess provided in an upper surface thereof; and
an IDT electrode including a metal material that is primarily made of at least a material selected from a group consisting of Au, Pt, W, Ta, Ag, Cu, Mo, and Ni and embedded in the recess; wherein
when λ is a wavelength of a surface acoustic wave, a relationship among the metal from which the IDT electrode is primarily made, a lower limit of a film thickness of the electrode, and a duty is a relationship shown in Table 2, an upper limit of the film thickness of the electrode is a value shown in Table 2, and the duty is greater than about 0.5; and
a variable capacitor connected with the surface acoustic wave resonator:
TABLE 2
(X: Duty)
Upper limit
Electrode
Lower limit of film thickness (λ)
of film thickness (λ)
Au
0.0381 − 0.0211X − 0.0222X 2
0.25
Pt
0.0501 − 0.0445X − 0.0031X 2
0.25
W
0.0748 − 0.0978X + 0.0444X 2
0.25
Ta
0.0739 − 0.0789X + 0.0222X 2
0.25
Ag
0.1287 − 0.1767X + 0.0667X 2
0.25
Cu
0.1883 − 0.23X + 0.0667X 2
0.25
Mo
0.1497 − 0.0990X
0.25
Ni
0.22112 − 0.13613X + 0.0439X 2
0.25.
6. The tunable filter according to claim 5 , wherein
the surface acoustic wave resonator includes a plurality of surface acoustic wave resonators, and the plurality of surface acoustic wave resonators are connected in series between an input terminal and an output terminal;
the variable capacitor includes first and second variable capacitors, the first variable capacitor being connected in series with at least one of the plurality of surface acoustic wave resonators, the second variable capacitor being connected in parallel with at least one of the plurality of surface acoustic wave resonators; and
the tunable filter further comprises:
an inductance connected between a node of the plurality of surface acoustic wave resonators and a ground; and
matching capacitors respectively connected between the input terminal and the ground and between the output terminal and the ground.
7. The tunable filter according to claim 6 , wherein the matching capacitors and the inductance have impedances in a range from about 20Ω to about 100Ω.
8. A tunable filter, comprising:
a surface acoustic wave resonator including:
a piezoelectric substrate made of LiNbO 3 with Euler angles of (0°, 80° to 130°, 0°) and including a recess provided in an upper surface thereof; and
an IDT electrode in which Al or an alloy primarily made of Al is embedded in the recess; wherein
when λ is a wavelength of a surface acoustic wave, a normalized film thickness H/λ of the IDT electrode is in a range from about 0.036 to about 0.4, a relationship between an upper limit of a normalized film thickness of Al and a duty is preferably a relationship shown in Table 4, a lower limit of the normalized film thickness of Al is a value shown in Table 4, and a duty is less than about 0.5 or greater than about 0.5; and
a variable capacitor connected with the surface acoustic wave resonator:
TABLE 4
(Y: Al film thickness, X: Duty)
Electrode
Lower limit
Upper limit
Al
0.036 λ
Y = −0.9X + 0.766.
9. The tunable filter according to claim 8 , wherein a relationship among the metal from which the IDT electrode is primarily made, the upper limit of the film thickness of the electrode, and the duty is a relationship shown in Table 5, and the lower limit of the film thickness of the electrode is a value shown in Table 5:
TABLE 5
(Y: Al film thickness, X: Duty)
Electrode
Lower limit
Upper limit
Al
0.036 λ
Y = −0.81X + 0.689.
10. The tunable filter according to claim 8 , wherein
the surface acoustic wave resonator includes a plurality of surface acoustic wave resonators, and the plurality of surface acoustic wave resonators are connected in series between an input terminal and an output terminal;
the variable capacitor includes first and second variable capacitors, the first variable capacitor being connected in series with at least one of the plurality of surface acoustic wave resonators, the second variable capacitor being connected in parallel with at least one of the plurality of surface acoustic wave resonators; and
the tunable filter further comprises:
an inductance connected between a node of the plurality of surface acoustic wave resonators and a ground; and
matching capacitors respectively connected between the input terminal and the ground and between the output terminal and the ground.
11. The tunable filter according to claim 10 , wherein the matching capacitors and the inductance have impedances in a range from about 20Ω to about 100Ω.
12. A tunable filter, comprising:
a surface acoustic wave resonator including:
a piezoelectric substrate made of LiNbO 3 with Euler angles of (0°, 80° to 130°, 0°) and including a recess provided in an upper surface; and
an IDT electrode in which a metal material that is primarily made of at least a material selected from a group consisting of Au, Pt, W, Ta, Ag, Cu, Mo, and Ni is embedded in the recess; wherein
when λ is a wavelength of a surface acoustic wave, a thickness H/λ of the electrode that is primarily made of Au, Pt, W, or Ta is about 0.006 or less, a thickness H/λ of the electrode that is primarily made of Ag is about 0.01 or less, or a thickness H/λ of the electrode that is primarily made of Cu, Mo, or Ni is about 0.013 or less; and
a variable capacitor connected with the surface acoustic wave resonator.
13. The tunable filter according to claim 12 , wherein
the surface acoustic wave resonator includes a plurality of surface acoustic wave resonators, and the plurality of surface acoustic wave resonators are connected in series between an input terminal and an output terminal;
the variable capacitor includes first and second variable capacitors, the first variable capacitor being connected in series with at least one of the plurality of surface acoustic wave resonators, the second variable capacitor being connected in parallel with at least one of the plurality of surface acoustic wave resonators; and
the tunable filter further comprises:
an inductance connected between a node of the plurality of surface acoustic wave resonators and a ground; and
matching capacitors respectively connected between the input terminal and the ground and between the output terminal and the ground.
14. The tunable filter according to claim 13 , wherein the matching capacitors and the inductance have impedances in a range from about 20Ω to about 100Ω.