Direct synthesis of a new class of bandstop filters

A direct synthesis technique of a new class of bandstop coupled resonator elliptic filters is presented. Two different coupling schemes, which both include source-load coupling are used. The first coupling and routing scheme is the standard folded structure used in implementing bandpass elliptic filters with N transmission zeros using N resonators. The second coupling scheme is identical to the multipath parallel structure, which was recently introduced for the synthesis of fully elliptic bandpass filters. In both cases, the source and load are directly coupled by an inverter in order to generate the required transmission zeros. It is shown that, for each coupling scheme, two distinct solutions that are not related by a similarity transformation exist. Furthermore, the two solutions both have realistic coupling coefficients in contrast to the bandpass case where only one solution, in general, is practical. It is also shown that the same coupling scheme can implement pseudoelliptic bandstop filters with N_z reflection zeros where 0≤N_z≤N.