US Patent 7167065 Filter circuit

A filter circuit has a complex block and exciting portions. The complex block has: a first block end resonator; a first resonator that is coupled to the first block end resonator; a second resonator that is coupled to the first resonator; a third resonator that is coupled to the second resonator; a fourth resonator that is coupled to the third resonator; and a second block end resonator that is coupled to the fourth resonator. Couplings between the first block end resonator and the second block end resonator, between the first resonator and the fourth resonator, and between the second resonator and the third resonator are in phase. The complex block and the exciting portions are single-path-coupled.