Ladder type surface acoustic wave filter and duplexer using same
View Patent ↗An object of the present invention is to improve the passing characteristic at high temperature in a ladder-type elastic wave filter and a duplexer including the filter. The ladder-type elastic wave filter of the present invention includes a piezoelectric substrate, a first series elastic-wave resonator formed on the piezoelectric substrate and connected in series between the input and output terminals of the filter, a parallel elastic-wave resonator formed on the piezoelectric substrate and connected in parallel between the series elastic-wave resonator and the ground terminal, and a dielectric film formed on the piezoelectric substrate so as to cover the first series elastic-wave resonator. The piezoelectric substrate is formed of a material with a negative temperature coefficient. The dielectric film is formed of a material with a positive temperature coefficient and its film thickness is formed thicker than that with which the frequency-temperature coefficient of the first series elastic-wave resonator becomes 0.
1. A ladder-type elastic wave filter comprising:
a piezoelectric substrate formed of a material with a negative temperature coefficient;
a first series elastic-wave resonator formed on the piezoelectric substrate and connected in series between an input terminal and an output terminal, the first series elastic-wave resonator having a first capacitance;
a second series elastic-wave resonator connected in series between the input terminal and the output terminal and having a second capacitance, the first capacitance of the first series elastic-wave resonator being higher than the second capacitance of the second series elastic-wave resonator;
a parallel elastic-wave resonator formed on the piezoelectric substrate and connected in parallel between the first series elastic-wave resonator and a ground terminal; and
a dielectric film formed on the piezoelectric substrate the dielectric film covering the first series elastic-wave resonator and the second series elastic-wave resonator, the dielectric film formed of a material with a positive temperature coefficient, a thickness of the dielectric film being thicker than a film thickness with which a frequency-temperature coefficient of a passing characteristic of the first series elastic-wave resonator becomes 0.
2. The ladder-type elastic wave filter of claim 1 ,
wherein the thickness of the dielectric film is formed thicker than a film thickness with which respective frequency-temperature coefficients of the passing characteristic of the first series elastic-wave resonator and the second series elastic-wave resonator become 0.
3. A duplexer comprising:
the ladder-type elastic wave filter of claim 1 ; and
a filter through which a signal with a frequency higher than a passband of the ladder-type elastic wave filter passes.
4. The ladder-type elastic wave filter of claim 1 wherein a passband of the first series elastic-wave resonator shifts to a high frequency side as temperature rises.
5. The ladder-type elastic wave filter of claim 1 wherein the ladder-type elastic wave filter is configured to pass signals in a transmission band between 1.85 GHz and 1.91 GHz and suppress signals in a reception band between 1.93 GHz and 1.99 GHz.
6. The ladder-type elastic wave filter of claim 5 further comprising a reception filter configured to suppress signals in a transmission band between 1.85 GHz and 1.91 GHz and pass signals in a reception band between 1.93 GHz and 1.99 GHz.
7. The ladder-type elastic wave filter of claim 6 wherein the ladder-type elastic wave filter is a WCDMA standard compliant filter.
8. The ladder-type elastic wave filter of claim 1 further comprising an antenna terminal connected to an output terminal of the ladder-type elastic wave filter.
9. The ladder-type elastic wave filter of claim 1 wherein the parallel elastic-wave resonator is electrically connected between the first series elastic-wave resonator and the second series elastic-wave resonator.
10. The ladder-type elastic wave filter of claim 1 wherein the dielectric film includes a protrusion part formed on a top surface of the dielectric film.
11. The ladder-type elastic wave filter of claim 1 wherein the dielectric film includes silicon dioxide.
12. A ladder-type elastic wave filter comprising:
a piezoelectric substrate formed of a material with a negative temperature coefficient;
a series elastic-wave resonator formed on the piezoelectric substrate and connected in series between an input terminal and an output terminal;
a first parallel elastic-wave resonator formed on the piezoelectric substrate and connected in parallel between the series elastic-wave resonator and a ground terminal, the first parallel elastic-wave resonator having a first capacitance;
a second parallel elastic-wave resonator connected between the series elastic-wave resonator and the ground terminal and having a second capacitance, the first capacitance of the first parallel elastic-wave resonator being lower than the second capacitance of the second parallel elastic-wave resonator; and
a dielectric film formed on the piezoelectric substrate, the dielectric film covering the first parallel elastic-wave resonator and the second parallel elastic-wave resonator, the dielectric film formed of a material with a positive temperature coefficient, a thickness of the dielectric film being thinner than a film thickness with which a frequency-temperature coefficient of a passing characteristic of the first parallel elastic-wave resonator becomes 0.
13. The ladder-type elastic wave filter of claim 12
wherein the thickness of the dielectric film is formed thinner than respective film thicknesses with which respective frequency-temperature coefficients of the passing characteristic of the first parallel elastic-wave resonator and the second parallel elastic-wave resonator become 0.
14. A duplexer comprising:
the ladder-type elastic wave filter of claim 12 ; and
a filter through which a signal with a frequency lower than a passband of the ladder-type elastic wave filter passes.
15. The ladder-type elastic wave filter of claim 12 wherein the ladder-type elastic wave filter is configured to pass signals in a transmission band between 1.85 GHz and 1.91 GHz and suppress signals in a reception band between 1.93 GHz and 1.99 GHz.
16. The ladder-type elastic wave filter of claim 15 further comprising a reception filter configured to suppress signals in a transmission band between 1.85 GHz and 1.91 GHz and pass signals in a reception band between 1.93 GHz and 1.99 GHz.
17. The ladder-type elastic wave filter of claim 12 wherein a passband of the first parallel elastic-wave resonator shifts to a low frequency side as temperature increases.
18. The ladder-type elastic wave filter of claim 12 further comprising a second series elastic-wave resonator formed on the piezoelectric substrate and connected in series with the first series elastic-wave resonator between the first parallel elastic-wave resonator and the second parallel elastic-wave resonator.
19. The ladder-type elastic wave filter of claim 12 wherein a resonance frequency of the first parallel elastic-wave resonator shifts as temperature rises.
20. The ladder-type elastic wave filter of claim 19 wherein a resonance frequency of the second parallel elastic-wave resonator does not shift as temperature rises.