A Compact 1.08–5.94 GHz Balun With 1○/0.07 dB Phase-/Amplitude-Imbalances Using Reverse Series Paths

In this paper, a compact broadband balun with low phase-/amplitude-imbalances based on reverse series paths is proposed. Such reverse series paths can generate a differential characteristic with slight phase- and amplitude-imbalances within a wideband. To achieve the reverse series paths in a planar technology, the hybrid coupler is introduced, which is composed of a substrate integrated coaxial line (SICL) and a quasi-microstrip line (QML). Besides, multi-stage matching lines are embedded in combined transmission lines to improve the operation bandwidth and miniaturize the circuit size. By properly optimizing dimensions of SICLs and QMLs, the balun achieves ultrawide bandwidth and good in-band balance, simultaneously. To verify the mechanisms mentioned above, a prototype balun is fabricated and measured. The passband is from 1.08 to 5.94 GHz with a 138% fractional bandwidth (FBW), while the minimum insertion loss is 0.283 dB, excluding 3-dB power splitting loss). The in-band phase- and amplitude-imbalances are less than 1
^○
and 0.07 dB, respectively. Meanwhile, the core size of balun is about 0.32
λg
× 0.27
λg
, where λ
g
is the microstrip guided wavelength at the center operation frequency.