Multi-Tunable Quasi-Circulator with Co-Designed Bandpass Filter and Switching-Off Functionality

This paper reports on the RF design and practical validation of a multi-tunable quasi-circulator with co-designed bandpass filtering and switching-off functionality (QCFS). The QCFS concept is based on two tunable unilateral frequency-selective stages (UFSs), which are integrated with a Wilkinson divider and microstrip resonators. The UFSs in combination with Wilkinson divider facilitates fully-directional signal transmission in each of the Tx/Rx paths of the QCFS. Microstrip resonators and resonant stages are added to facilitate highly-selective bandpass filtering (BPF) and create transmission zeros (TZs). By tuning their resonances, center frequency and bandwidth tunability can be achieved. An intrinsic-off function is also facilitated by moving the TZs at the center frequency of the passband. For experimental validation purposes, a QCFS was designed, manufactured, and measured at L band. In its direction of propagation, it exhibits a three-pole/two-TZ frequency-selective response with center frequency tuning between 1.4 to 1.8 GHz and insertion loss (IL) ranging from 1.6 to 4.3 dB, fractional bandwidth tuning from 7.5% to 11.3% with IL ranging from 2.5 to 4.3 dB, and RF switching with more than 30 dB isolation. In the reverse direction, it facilitates an isolation between 25-39 dB within the entire center frequency tuning range.