A wideband dual-band bandstop filter with flexible control of stopband rejection, bandwidth and centre frequency ratio

A novel wideband dual-band bandstop filter with multi-transmission zeros and multi-reflection zeros is presented. It consists of two pairs of symmetric open-ended coupled lines at two sides and (2N − 1) pairs of coupled-line sections with an open-circuited stub in the middle. Through even- and odd-mode analyses, general simultaneous equations for characteristic impedances are derived with the proposed algorithm. By suitably selecting all the design parameters, the proposed topology could not only provide a controllable equal-ripple response in the passband but also control the stopband rejection, bandwidth (BW), and centre frequency ratio in the stopband flexibly. For verification, one prototype circuit is finally fabricated and measured in the experiment. Good agreement between the measured and simulated results is attained so as to successfully validate the correctness of the proposed design method.