| US 7,486,158 B2 | ||
| Branching filter | ||
| Toshiaki Takata, Ishikawa-ken (Japan) | ||
| Assigned to Murata Manufacturing Co., Ltd., Kyoto (Japan) | ||
| Filed on Jun. 21, 2007, as Appl. No. 11/766,233. | ||
| Application 11/766233 is a continuation of application No. PCT/JP2005/021862, filed on Nov. 29, 2005. | ||
| Claims priority of application No. 2004-373986 (JP), filed on Dec. 24, 2004. | ||
| Prior Publication US 2007/0247259 A1, Oct. 25, 2007 | ||
| Int. Cl. H03H 9/72 (2006.01) | ||
| U.S. Cl. 333—133 [333/193] | 6 Claims |
| 1. A branching filter comprising:
a first band-pass filter having a relatively low frequency passband; and
a second band-pass filter having a relatively high frequency passband; wherein
the first and second band-pass filters each include a surface acoustic wave filter having a ladder circuit structure including
series resonators and parallel resonators, and ends of the first and second band-pass filters are connected to one common
terminal;
the second band-pass filter is a polar filter including:
a first parallel resonator having a relatively high impedance Z1 and a relatively high resonant frequency f1;
a second parallel resonator having a relatively low impedance Z2 and a relatively low resonant frequency f2, the second parallel resonator being connected to a parallel arm that is different from that to which the first parallel
resonator is connected, a common-connection ground terminal commonly connected to ends of the first parallel resonator and
the second parallel resonator, the ends being connected to a ground potential;
a first inductance connected between the common-connection ground terminal and the ground potential;
a third parallel resonator connected to a parallel arm that is different from those to which the first and second parallel
resonators are connected; and
a second inductance connected between the third parallel resonator and the ground potential;
a relationship f1>f0>f2 is satisfied, where f0 denotes an average of resonant frequencies of all the parallel resonators; and
the first inductance and the second inductance are arranged to suppress an occurrence of a ripple at a high frequency side
of the passband of the first band-pass filter, thereby achieving a sufficient pass band width.
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