A Wideband Balanced Filter With 3/4λ Stepped-Impedance Resonator

摘要

For the purpose of improving the in-band performance of differential-mode (DM) passband and the design freedom of common-mode (CM) stopband, a balanced filter using branch line structure is presented in this paper. Prototype-based on Optimum Distributed Highpass Filter, a five-stage bandpass filter with three branch lines is firstly designed. Then to form the 3λ/4 stepped-impedance resonators (SIRs), two sections of λ/2 open-circuited stubs are additionally attached on the horizontal symmetrical line of the first and third branch line respectively. While excited by DM or CM signal, this filer is considered as a bandpass filter (BPF) in the former case or a bandstop filter (BSF) in the latter case. Finally, measured and simulated results are in agreement with each other pretty well, indicating that favorable DM transmission with 3dB fractional bandwidth of 57% is over acceptable CM suppression with fractional bandwidth of 128% more than 15dB.