The Doublet: A New Building Block for Modular Design of Elliptic Filters

The paper presents a new building block for modular design of elliptic filters. The building block, called doublet, contains two resonators which generate one transmission zero on either side of the passband. Higher-order filters can be designed by cascading doublets similarly to cascade triplets and quads. The doublet exhibits some interesting zero-shifting features which are not present in triplets and quads. Some of these features are discussed in connection with all-inductive doublet filters in rectangular waveguides. Higher-order filters in rectangular waveguide technology are also presented along with suggestions for implementation in other technologies such as stripline and microstrip.