A Tunable Reflection/Transmission Coefficient Circuit Using a 45° Hybrid Coupler With Two Orthogonal Variables

Abstract

This paper presents a tunable reflection/transmission coefficient (TRTC) circuit based on a 45° hybrid coupler that is potentially useful for leakage cancelation purposes. The analysis of the proposed TRTC circuit shows that the circuit can cover any reflection/transmission coefficient of less than 0.5 on the Smith chart by adjusting only two variable resistors. Meanwhile, the real and imaginary parts of the reflection/transmission coefficient can be tuned independently, facilitating the use of this circuit in feedback systems. To implement the 45° hybrid coupler used in the proposed TRTC circuit, a new wideband planar topology for arbitrary phase and amplitude hybrid coupler is introduced. The circuit is analyzed and closed-form derivations for designing the proposed hybrid coupler are extracted. A prototype of the proposed 45° hybrid and TRTC circuits are fabricated and measured on 1.6-mm FR-4 PCBs at 1 GHz. The measurement results show a fractional bandwidth of 50% for the proposed 45° hybrid and a coverage radius of larger than 0.3 on the Smith chart in a frequency range of 0.8-1.2 GHz for the proposed TRTC circuit, which are well matched to the simulation results.