Highly Reconfigurable Quadrature Coupler With Ideal Impedance Matching and Port Isolation

This paper proposes a varactor-based coupler structure that allows for reconfigurable operating frequency, power-dividing ratio, and coupler to/from crossover responses. Particularly, this coupler can theoretically realize perfect impedance matching and port isolation at all tuning states. The coupler analysis, design equations, and design graphs were conducted, given, and discussed in detail. For demonstration purposes, two prototypes were experimentally developed and characterized. The first quadrature coupler demonstrates a frequency-tuning range from 0.632 to 1.5 GHz with a constant power-dividing ratio of -3 dB, <2.3 degrees phase imbalance, >20-dB isolation, and >14-dB return loss. In the second prototype, for the phase error <6.7 degrees, isolation and return loss >19 dB, the power-dividing ratio was tuned from -6.4 to 20.5 dB at 1 GHz, yielding a 26.9-dB tuning range and crossover characteristics at low bias voltages. Measured and simulated results are in good agreement.