Modified Wideband Marchand Balun With Tunable Power Division Ratio and Constant Phase

A balun with a wide tunable power division ratio and constant phase across a wide frequency band is presented. The proposed design is a modification of the traditional Marchand balun with a varactor and series resistor loaded at the central point of the two connected pairs of coupled lines. The varactor is used to control the power division ratio at two output ports, whereas the resistor is used to stabilize the power division across a wide operational bandwidth. The achievable power division ratio and differential phase are investigated for different band ranges. A prototype is designed, fabricated, and tested. The results indicate a tuning power ratio range from 1:1 to 3.5:1 across 80% fractional bandwidth (1.2-2.8 GHz) with more than 10 dB return loss and less than 9° differential phase.