A Coupled Line-Based Coupler With Simultaneously Tunable Phase and Frequency

The tunability in phase characteristics becomes more and more important in the modern wireless communication system. However, the existing phase tunable coupler structures suffer from the narrow bandwidth and limited tunable range due to the much more complex design requirements. The further deterioration in performance occurs when additional tunability in operating frequency is considered. In this paper, a new simultaneously phase- and frequency- tunable coupler is proposed with the enhancement in both bandwidth and phase tunable range. Two tunable capacitors are loaded on the two terminals of a coupled line section to realize a novel tunable transmission line. This tunable element is introduced between two quarter-wavelength coupled line sections to implement the simultaneous tunability in both phase and operating frequency. For demonstration purpose, a highly flexible 3 dB coupler was designed, fabricated and measured. For a fixed operating frequency of 2.4 GHz, the coupler can provide a continuously tunable phase difference from 30° to 150° within a 10% bandwidth. For a standard phase difference of 90°, the operating frequency of the same coupler can be varied from 2.0 GHz to 2.75 GHz. Good performance in return loss, isolation, and output amplitude imbalance is achieved for all these states.