Direct coupled quad-band bandstop filter synthesis using frequency transformation

In this paper we propose a design of quadband bandstop filter for the suppression of interfering signals in broadband applications. There are different approaches to implement multiband filters among which frequency transformation techniques is advantageous than other methods in terms of complexity to find the transfer functions. So it is applied here for finding the locations of poles and zeros of a direct coupled quadband bandstop filter. By properly combining several single-band filtering functions, the multiband filtering function of similar bandwidth are generated. For easy tuning, filter has been synthesized in cul-de-sac topology which eliminates the need for diagonal couplings. Finally 16-pole symmetric quadband bandstop filters have been synthesized to validate the proposed method.