Design of a Microstrip Filtering Balun With a Wide Stopband

Abstract

A microstrip filtering balun with a wide stopband is presented. Its filtering function is realized by four meandering stepped impedance resonators (SIRs). Except for an identical fundamentalmode resonant frequency, the SIRs have different high-order resonant frequencies. Thus, the parasitic passbands are suppressed, and a wide stopband is achieved. The unbalance-to-balance transition is accomplished by introducing two coupling output ports at two symmetry positions of the output SIR. The voltages at symmetrical positions have equal magnitudes and opposite phases, thus, signals coupled from the symmetrical positions also have equal magnitudes and opposite phases, i.e., balanced output signals are achieved. A more general design approach is discussed in detail, and the proposed approach is similar to the design method of the conventional filter except that a small modification is made. Additionally, two kinds of external coupling structures, microstrip coupling lines and tapped line, are compared in terms of stopband performance. The comparison shows that better stopband performance is observed when utilizing the microstrip coupling lines as the external coupling structure. A filtering balun with central frequency of 2.4 GHz and Chebyshev frequency response is designed, fabricated and measured. The measured results give a reasonable agreement with the simulated ones, which verifies the effectiveness of the filtering balun.

Using Open Stubs and Tuning the Width of Output Microstrip Lines of a Balun Diplexer to Obtain the Matching Output Impendence With Rf Output Transceiver

“…Compared with conventional single-ended circuits, balanced circuits can easily meet and even exceed requirements. Balunselectrical devices that convert between a balanced and unbalanced signal and thereby enable the transition from unbalanced circuits to balanced ones -are used as interface devices [1][2][3]. Baluns can take many forms and may include devices that transform impedances.…”