A Compact Tunable Directional Coupler with Continuously Tuned Differential Phase

A tunable directional coupler with outputs that have continuously tuned phase difference and constant magnitude is presented. The initial design is based on a 3-dB branch-line coupler with two arms having variable electrical lengths. To realize the variable-length lines, a novel concept of tunable phase shifting unit, which includes a pair of inductor-varactor loaded coupled lines, is proposed. By controlling the shifting phase of the two arms, the differential phase (i.e., the phase difference between the two output ports) can be tuned continuously. Explicit relation between the objective differential phase of the device and the required shifting phase of those units is analyzed and explained. To validate the design, a prototype is built, simulated, and tested. The experimental and predicted results agree well and show that the device can realize arbitrary and continuously tunable differential phase from 45° to 135°. The overall size of the design is only 0.18λ
_g
× 0.24λ
_g
, which is extremely compact compared with using a cascaded coupler-phase shifters and is thus suitable for miniaturized wireless systems.