Tunable transmission-line transformer with two varactors and its application for Wilkinson power divider

Abstract A tunable transmission-line transformer (TLT) is newly introduced in this paper. It consists of two transmission lines (TLs), one parallel varactor and one series varactor. The design formulas are newly derived in closed-form, and its design procedure is also summarized in a flowchart. Because two TLs provide different characteristic impedances and electrical lengths, the tunable TLT can be designed much more flexible with different phase responses. By using this TLT topology, a novel Wilkinson power divider circuit could provide a wide tunable frequency range. The simulated and experimental S -parameters match very well. It is also proved that, although the phase responses of the proposed TLT are shifted from 77.22° to 90.72° in the tunable range 0.5–1.3 GHz, its Wilkinson power divider application is still working well, and S 22 , S 33 and S 32 are still lower than – 20 dB in the total tunable range.