Microwave Filters with Inline Topology and Transmission Zeros in the Response

The evolution of the communication technologies in the recent years has required more and more performing filtering subsystems. In particular, the very strict requirements in terms of selectivity, passband losses and compactness have required new and more effective synthesis solutions. Considering the compactness and the best use of the available footprint, the inline topology represents undoubtedly a very convenient solution. Unfortunately, in its classical implementation, it suffers from scarce selectivity (no transmission zeros can be introduced in the frequency response). Recently, various solutions have appeared in the literature, which allow introducing transmission zeros in true inline (and quasi-inline) filter topologies. This is obtained by means of new characteristic functions allowing additional degrees of freedom in the definition of the frequency response (which are exploited to impose the additional constraints necessary to make it possible the synthesis of the imposed topologies).In this talk two new frequency characteristics (namely the Reduced Chebycheff and the Bounded Chebycheff) are first introduced, and their features are illustrated and discussed (they are based on the use of complex reflection zeros in the filter response).Two types of true inline filters exploiting these characteristics are then analyzed in detail, namely the Path filters and the Extracted-zero filters. For each type, the specific synthesis procedure is described and several examples of synthesis of the prototype networks are presented. Some examples of real filters with the considered topologies (developed for 5G applications) are also presented.