Synthesis of Continuously Tunable Diplexers With Decoupling Network

Abstract

This article proposes a synthesis method for continuously tunable diplexers based on an all-resonator decoupling topology. In coupled-resonator diplexers, strong interchannel interference often leads to severe passband distortion, making independent channel frequency tuning highly challenging. To overcome this limitation without resorting to complex external matching or power-distribution networks, a decoupling strategy is introduced by incorporating proper cross-couplings between channels. Based on a three-port network admittance analysis, mathematical conditions for effectively eliminating interchannel interference are derived. The diplexer synthesis process with these conditions imposed is detailed, guiding the design of diplexers with decoupled channels. To validate the proposed method, this article presents the synthesis examples of the diplexers with and without transmission zero, as well as with asymmetric topologies. Additionally, a continuously tunable diplexer prototype is designed and fabricated. Experimental results demonstrate that, while the Rx channel is fixed, the Tx channel achieves wide-range continuous and independent frequency tuning. Moreover, the Tx band can be contiguous with the Rx band. Throughout the tuning process, the return losses within the Rx and Tx passbands are well maintained, thereby validating the effectiveness of the proposed decoupling strategy.