A Compact Frequency- and Power-Dividing Ratio-Tunable Quadrature Coupler

This paper presents a compact quadrature coupler with a tunable frequency and power-dividing ratio. Wide tunable frequency and power-dividing ratio are achieved by using the novel tunable unit instead of the transmission line sections in a traditional varactor-based quadrature coupler. Closed-form equations are derived for design parameters. For verification, a quadrature coupler is designed based on the given parameters, which demonstrate the tunable frequency of 2.0 GHz to 6.0 GHz and the tunable power-dividing ratio of -20 dB to 7.2 dB at 3 GHz. Finally, a microstrip tunable quadrature coupler is fabricated and measured. The measurements agree well with simulations. Under the frequency-tunable state, the measured 3-dB working frequency of this coupler can be continuously adjusted from 2.0 GHz to 4.7 GHz. And during the adjustment process, the return loss and isolation are always >15 dB. In the power-dividing ratio-tunable state, the adjustment range of the measured power-dividing ratio is from -14 to 6.0 dB at 3 GHz. Moreover, the return loss and isolation are always maintained at >20 dB.