Wideband Filtering Balun with Wide Stopband and Imbalance Compensation Using Hybrid DGS with Float Strip-Line

In this paper, a wideband filtering balun with wide stopband and imbalance compensation is proposed based on hybrid defected ground structure (DGS) with float stripline. The proposed dumbbell-shaped DGS could introduce multi resonances for a wideband operation. Then, the multi-section stepped-impedance microstrip feed-lines on the top- and bottomlayers can effectively extend the stopband bandwidth. Besides, a float strip-line is located in the middle of DGS, which could achieve phase- and amplitude-compensation of the balun. To verify the mechanisms mentioned above, a compact 0.46-3.3 GHz balun with wide upper stopband is implemented and fabricated. The measured upper stopband is expanded to 5.6 f
0
with attenuation level higher than 23 dB. Moreover, the core circuit-size of the balun is 0.38 λ
g
× 0.30 λ
g
, where λ
g
is the microstrip guided wavelength at the center frequency of 1.88 GHz.