Wideband Bandpass Filtering Balun With Perfect In-Band Matching and Isolation

Abstract

In this brief, a wideband balun with bandpass filtering response is synthesized based on even-/odd-mode analysis. To obtain the desired filtering selectivity, the transfer function of the proposed balun is derived and forced to be the same as the ${n}$ th-order Chebyshev-transfer function. Thus, with specified parameters (impedance ratio, bandwidth, and in-band return loss), the filtering characteristics of bandpass baluns can be flexibly controlled. Considering favorable in-band output matching and isolation, multiple pairs of resistors are added to the circuit. Finally, complete design procedures are provided. To verify, a 5th-order balun filter at the center frequency of 3 GHz is fabricated and measured. The simulation and measurement are in agreement, proving the correctness of the theory.