Electric filter, and duplexer using the same
View Patent ↗An electric filter according to an embodiment of the present invention is an electric filter having a piezoelectric resonator 10 to obtain a signal of a predetermined resonance frequency from a bulk wave propagating inside a piezoelectric film 15. This electric filter is configured to satisfy k 2 >(BP/0.6f), where k 2 is the square of an effective electromechanical coupling coefficient of the piezoelectric resonator 10, BP a pass bandwidth of the electric filter, and f a center frequency of a target pass band of the electric filter.
1. A duplexer comprising:
a transmitting-end filter including an electric filter and having a predetermined pass frequency band; and
a receiving-end filter including the electric filter and having a pass frequency band different from that of the transmitting end filter, wherein the electric filter includes:
a piezoelectric resonator for obtaining a signal of a predetermined resonance frequency from a bulk wave,
wherein the piezoelectric resonator comprises:
a first electrode;
a second electrode placed opposite to the first electrode; and
a piezoelectric film disposed between the first electrode and the second electrode, in which the bulk wave propagates,
the electric filter satisfying k 2 >(BP/0.6f),
where k 2 is the square of an effective electromechanical coupling coefficient of the piezoelectric resonator, BP a target pass bandwidth of the electric filter, and f a center frequency of the target pass band of the electric filter.
2. The electric filter according to claim 1 , wherein the center frequency f is not less than 1 GHz and wherein the pass bandwidth BP is not less than 1 MHz.
3. The electric filter according to claim 1 , comprising a plurality of piezoelectric resonators as defined.
4. The electric filter according to claim 3 , having a ladder structure, wherein the plurality of piezoelectric resonators include:
at least one piezoelectric resonator placed on a first wiring portion connecting an input electrode and an output electrode; and
a piezoelectric resonator placed on at least one second wiring portion formed between the first wiring portion and an earth electrode, and having a resonance frequency to form the target pass band, together with an antiresonance frequency of the piezoelectric resonator placed on the first wiring portion.
5. The electric filter according to claim 1 , wherein the piezoelectric film is ZnO or AlN.
6. The electric filter according to claim 1 , wherein the piezoelectric resonator is an SMR type piezoelectric resonator in which the piezoelectric film and, the first electrode and the second electrode for applying a voltage to the piezoelectric film are formed on an acoustic multiplayer film.
7. The electric filter according to claim 1 , wherein a thickness of the piezoelectric film is not more than 10 μm.