US Patent 7295090 Filter circuit

A filter circuit includes a plurality of resonators connected in parallel and each having loaded Q deviation equal to allowable deviation of a group delay, a divider to divide an input signal to the resonators, a combiner to combine output signals of the resonators, and an opposite phase unit for making signals passing two resonators of the resonators an approximately opposite phase in an output of the combiner, the two resonators having resonance frequencies adjacent to each other, respectively.