Wideband Bandstop Filter With Extreme Sharp Skirt Selectivity

A novel and simple wideband bandstop filter with extreme sharp skirt selectivity is proposed. The wideband bandstop response with multi-transmission zeros and large skirt-band attenuation rate is realized based on the modified coupled-line sections with open-/short-circuited termination and two quarter-wavelength transmission lines. In order to simultaneously improve stopband rejection (SR) and passband return loss, the gray boxes with different circuit structures are introduced. For demonstration, a designed prototype operating at 1 GHz with 64% relative bandwidth of 17-dB SR is simulated, fabricated, and measured.