Planar Reconfigurable Balanced Rat-Race Coupler With Improved Amplitude Imbalance Performance and Common-Mode Noise Absorption

In this article, the design method for absorptive balanced rat-race couplers with reconfigurable power-dividing ratio is proposed for the first time. First, the differential-mode (DM) and common-mode (CM) port equivalent impedances of the eight-port balanced network is derived by the even-odd mode method under the mixed-mode isolation and matching conditions. The power-dividing ratio is tuned by changing the admittance ratio of the network components, which greatly extends the amplitude imbalance bandwidth. Then, a nonabsorptive reconfigurable balanced rat-race coupler that consists of transmission lines with adjustable characteristic impedance is proposed. Furthermore, based on the derived DM and CM port equivalent impedances, an absorptive network is designed. Finally, the absorptive reconfigurable balanced rat-race coupler is achieved by attaching the absorptive network at the balanced ports of the designed nonabsorptive reconfigurable balanced rat-race coupler. For verification, the 2-GHz nonabsorptive and absorptive reconfigurable balanced rat-race couplers are designed and measured. The measured results agree well with the simulations. For the nonabsorptive coupler, the measured power-dividing ratio is tuned from -8.5 to 8.9 dB, while the 15-dB DM matching and isolation bandwidth is greater than 370 MHz. For the absorptive coupler, the measured power-dividing ratio is tuned from -8.3 to 9.0 dB, while the 15-dB DM matching and isolation bandwidth and 15-dB CM matching bandwidth are greater than 450 and 1150 MHz, respectively. The measured 0.5-dB amplitude imbalance bandwidths of the nonabsorptive and absorptive couplers are 6%-14.5% and 7%-15.6%, respectively.