A Balanced-to-Unbalanced Filtering Power Divider with Frequency-Dependent Coupled Resonators

This paper proposes a third-order balanced-to-unbalanced filtering power divider (FPD) based on a frequency-dependent coupled resonator filter structure. Analytical design formulas for key parameters are derived using odd-/even-mode analysis and filter design theory. The proposed FPD achieves compact size and good passband selectivity due to two transmission zeros introduced by the frequency-dependent coupled resonators. A prototype of the FPD is fabricated on a Taconic RF-35A2 substrate. Simulation results show that the fractional bandwidth with a return loss better than 20 dB is 4%, while the measured one is 2%. The minimum insertion loss is 1 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{dB}$</tex> in simulation and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${1. 4}\ \text{dB}$</tex> in measurement. The simulated 3dB fractional bandwidth is 5.4%, whereas the measured one is 5.2%. Both simulated and measured results confirm that the common-mode rejection exceeds <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$23.5\ \text{dB}$</tex> in the passband, with a rectangular coefficient of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${2 0 -} \text{dB} {/ 3 -} \text{dB}$</tex> equal to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${1. 5}$</tex>.