Design and Optimization of a Compact Superconducting Quadruplexer at VHF-Band With an Accurate Equivalent Circuit Model

This paper presents the modeling and optimization of a manifold-coupled superconducting quadruplexer at VHF-band. The quadruplexer consists of four individually designed channel filters with a bandwidth of 400 kHz centered at 216, 224, 232, and 240 MHz, respectively, connected to a common manifold. The complex interactions between the channel filters make the design and optimization of the quadruplexer difficult and even impossible. An equivalent circuit for the manifold and the capacitive external couplings of the four channel filters in the quadruplexer is presented. The parameters of the equivalent circuit are accurately extracted from electromagnetic simulations. Afterward, the equivalent circuit is optimized to compensate for the interactions between the channel filters and obtain a good common-port return loss and channel transfer characteristics. Thereafter, the design of the individual channel filters is improved according to the optimized parameters of the equivalent circuit. The superconducting quadruplexer is successfully designed and fabricated. The measurements show high performance and match well with the simulations. The experimental insertion losses of the four channels are less than 0.45 dB, the center frequency discrepancies of the four channels are less than 40 kHz, the common-port return loss is greater than 16.3 dB, and the out-of-band rejection is higher than 90 dB. Moreover, a high isolation greater than 75 dB between the channels is achieved.

I. Introduction

Multiplexers are essential components in multiband and multiservice wireless communications systems. They commonly use filters to split input signals into certain channels according to their frequencies. High-temperature superconducting (HTS) filters have many advantages such as very low insertion loss, steep band edges, and high out-of-band rejection compared with conventional ones [1]–[3]. Multiplexers comprising HTS filters with superior performance have been developed for many wireless communications systems [4]–[9].