A Low-loss Compact X-band Superconducting Phase Shifter

We have developed a low-loss compact superconducting phase shifter for X-band applications. By cascading a discrete variable phase shifter and a continuous variable phase shifter, a total phase shift of 360 degrees was obtained. Both the discrete and variable phase shifter consist of an asymmetrical branch-line coupler, 2 impedance transformers, 2 to 4 varactor diodes and a DC bias feeder. By controlling the bias voltage of the varactor diodes, the asymmetrical branch-line couplers exhibit a nonlinear phase response. As a result, the impedance transformers located between the varactor diodes and the branch-line coupler can be made small, which ensured a compact design. Simulation predicted an insertion loss of less than 1.0 dB and a maximum phase shift of 360 degrees or more. To verify the performance, the proposed superconducting phase shifter was fabricated using a high-Tc superconducting material called YBCO. Measurements performed at X-band show an insertion loss of less than 1.7 dB and a layout size of 0.75 lambda(g) x lambda(g), which confirms the design procedure of a low- loss compact X band phase shifter.