A broadband balun with complex impedance transformation and high isolation

The design of baluns with wide bandwidth, transformation between complex source and load impedances, and high isolation is presented in this paper. These features can be accomplished simultaneously by employing double-stub couple lines (DSCL). The differential characteristics are established by only exciting the odd-mode operation of the balun, and suppressing its even-mode operation. For the odd-mode operation, the structure of the DSCL acts as a multi-stub structure and contributes to bandwidth enhancement. An additional resistor added to the DSCL between the output ports improves their isolation and contributes to the impedance transformation. An experiment was prepared for the operations of the proposed design at 2.8 GHz with Z_S = 35 − j10 Ω and Z_L = 60 + j10 Ω. A measured wide bandwidth of 35% was obtained under the criterions of |S₁₁| < −10 dB, (⁶ S₃₁ − ⁶ S₂₁) < 180^◦ ± 5^◦, and (|S₃₁| − |S₂₁|) < 1 dB.