A Direct Synthesis Approach for Dual-Band Filters Based on Frequency-Variant Triplets

In this paper, a direct matrix method is presented to synthesize a quasi-canonical dual passband filter based on frequency-variant triplets, which is based on two low-pass prototypes and can achieve different passbands with different bandwidths, orders, and return losses. Compared with the conventional triplet, the proposed method can achieve more transmission zeros with quasi-canonical characteristics without increasing the cross-coupling paths, i.e., the frequency-variant triplet can generate two transmission zeros. Starting from the wheel topology, the process of using matrix transformation to realize the frequency-variant triplet topology is given in detail. A fifth-order, four-zero-point, dual-pass band filter synthesis procedure is given to demonstrate the effectiveness of the synthesis method.