Synthesis and Design of Quasi-Canonical Planar Filters Comprising Cascaded Frequency-Variant Blocks

This article discusses the synthesis of a new class of cross-coupled blocks incorporating frequency-variant couplings, that is, the frequency-variant blocks (FVBs), and their planar implementations based on an analytical approach. The proposed FVBs require fewer coupling elements with regard to traditional blocks, while can realize a quasi-canonical characteristic (i.e., realizing k-1 transmission zeros for a block of kth order). Derivation of the FVBs relies on a uniform synthesis process, where each transformation is mathematically simplified as a non-reciprocal matrix. An Nth-order quasi-canonical filter (possessing either symmetric or asymmetric TZs) can thereafter be synthesized by properly cascading the FVBs. Moreover, a unique planar technique is introduced to construct the quasi-canonical filters (with TZs above passband) using parallel-coupled microstrips, where a complete design procedure from synthesis to circuit parameters will be given. Validity of this work is eventually demonstrated by three illustrative examples, whose synthesis, simulation, and test results perform desired responses and are well-matched with each other.