Patent ID: 8040203

Claim:
A filter circuit comprising: an input terminal configured to input a signal input having a certain band; a four-port device configured to receive the signal input from the input terminal at a terminal A, divide and transmit the received signal through terminals B and C, combine signals supplied to the terminals B and C and transmit the combined signal through the terminal A when the signals are in-phase with each other and through a terminal D when the signals are reversed-phase with each other; an output terminal configured to output the signal transmitted through the terminal D; a first band stop filter configured to have a center frequency of the input signal within a stopband, reflect the stopband signal and back the signal to the terminal B, and pass a passband signal; a second band stop filter configured to have the same stopband as the first band stop filter, reflect the stopband signal and back the signal to the terminal C, and pass a passband signal; a first bandstop resonators circuit having a plurality of resonators configured to reflect signals of a desired band within the passband of the first band stop filter; a second bandstop resonators circuit having s a plurality of resonators having the same frequency as the first bandstop resonators circuit, configured to reflect signals of a desired band within the passband of the second band stop filter; a first open end connected in parallel to the first bandstop resonators circuit; and a second open end connected in parallel to the second bandstop resonators circuit, wherein a signal reflected by the first band stop filter and supplied to the terminal B is reversed-phase with a signal reflected by the second band stop filter and supplied to the terminal C, a signal reflected by the first bandstop resonators circuit and supplied to the terminal B is reversed-phase with a signal reflected by the second bandstop resonators circuit and supplied to the terminal C, and a signal reflected by the first open end and supplied to the terminal B is in-phase with a signal reflected by the second open end and supplied to the terminal C.