Filter, duplexer and electronic device

#The present invention relates to filers, duplexers and electronic devices, and more particularly, to ladder type filters, duplexers and electronic devices.

#A ladder type filter with resonators arranged in the form of a ladder is used in radio communications such as cellular phone communications. The ladder type filter realizes a bandpass filter that passes signals having desired frequencies. The bandpass filter is required to suppress signals having frequencies away from the pass band such as harmonic components that are the second harmonics of the pass band (that is, the desired frequencies), the third harmonics and so on.

#As a method for suppressing the harmonic components of the pass band, there is known a method using a lumped constant LC resonance circuit or a distributed constant transmission line (Patent Document 1).

#FIG. 1 is a circuit diagram of a ladder type filter;

#FIG. 2(a) is a diagram of a bandpass characteristic of a series resonator, FIG. 2(b) is a diagram of a bandpass characteristic of a parallel resonator, and FIG. 2(c) is a diagram of a communication characteristic of a ladder type filter;

#FIG. 3(a) is an equivalent circuit of a resonator, and FIG. 3(b) is a diagram of a bandpass characteristic thereof;

#First, ladder type filters are described. FIG. 1 is a circuit diagram of a ladder type filter. Referring to FIG. 1, one or multiple series resonators S1˜S4 are connected in series between an input terminal In and an output terminal Out. One or multiple parallel resonators P1˜P3 are connected in parallel between the input terminal In and the output terminal Out. That is, the parallel resonators P1˜P3 are respectively connected between a node between the series resonators S1 and S2 and ground, between a node between the series resonators S2 and S3 and ground, and between a node between the series resonators S3 and S4 and ground. As described above, the series resonators S1˜S4 and the parallel resonators P1˜P3 are connected in the form of a ladder.

#Next, the operation of the ladder type filter is described. FIG. 2(a) is a diagram of a bandpass characteristic of a series resonator S. Referring to FIG. 2(a), the minimum amount of attenuation appears at a resonance point, and the maximum amount of attenuation appears at an anti-resonance point located at a higher frequency than that of the resonance point. FIG. 2(b) is a diagram of a bandpass characteristic of a parallel resonator P. Referring to FIG. 2(b), the maximum amount of attention appears at the resonance point, and the minimum amount of attention appears at the anti-resonance point. FIG. 2(c) is a diagram of a bandpass characteristic of a ladder filter composed of the series resonator S and the parallel resonator P. Referring to FIG. 2(c), the anti-resonance point of the parallel resonator P is at a frequency lower than that of the resonance point of the series resonator S. Thus, the pass band is formed by the anti-resonance point of the parallel resonator P and the resonance point of the series resonator S. Regions close to the pass band have large amounts of attenuation. However, smaller amounts of attenuation are available at a frequency 2f0 of the second harmonic and a frequency 3f0 of the third harmonic where f0 is the center frequency of the pass band of the ladder type filter.

#For example, the transmission signal of the cellular phone terminal contains not only a signal having a transmission frequency but also its harmonic components such as the second harmonic and the third harmonic. Thus, the transmission filter is required to suppress the harmonic components of the transmission frequency (that is, the harmonics of the pass band).