A Phase Tunable Hybrid Coupler With Enhanced Bandwidth

The majority of the previous works on tunable coupler concentrates on the tunability of the frequency and amplitude. The capability to control the phase characteristics draws little attention. However, the tunability in phase becomes more and more important, as the flexible control in phase affects the performance significantly in the modern wireless communication systems. Regarding this, several phase tunable couplers were proposed but with narrow bandwidth. In this article, a novel phase-tunable coupler with enhanced bandwidth is proposed. It is constructed by introducing two varactor loaded coupled line sections between two branch line sections. By varying the capacitances loaded on and between the coupled lines, the continuously tunable phase difference can be achieved between two output ports without affecting the equal power division characteristics. The analysis of the ideal model and detailed circuit has been conducted to obtain the design formulas and guidelines. For demonstration, a phase tunable coupler operating at 1.8 GHz was designed, fabricated, and measured. It exhibits a tunable phase difference from 60 degrees to 120 degrees by varying the two biasing voltages. Meanwhile, the equal power division, good return loss, and high isolation are still maintained. The desired characteristics have been implemented over a wide bandwidth from 1.6 GHz to 2.0 GHz.