Triple-Band Marchand Balun Filter Using Coupled-Line Admmittance Inverter Technique

This study presents the first reported design and implementation of triple-band Marchand balun filter. The novel balun filter is constructed from the short-circuited coupled line with a triple-band resonator to demonstrate the triple-band admittance inverter characteristic. To improve the amplitude and phase responses within three passbands, the compensation techniques for phase-angle, impedance-matching, and susceptance-slope discrepancies are employed in the balun filter design. Four types of triple-band balun filters with significantly different bandwidths and passband separations are designed and the results are consistent with the specifications. The design procedure for the triple-band Marchand balun filter is established to obtain the amplitude and phase balances in three operating bands with arbitrarily controlled center frequencies and fractional bandwidths. The proposed Marchand balun filter realized by the microstrip coupled-line sections and the defected ground structure stubs is measured and a good agreement among equivalent-circuit calculation, full-wave simulation, and measurement is observed, demonstrating the validity and versatility of the proposed design methodology and balun filter configuration.