A wideband balanced filter with 3/4λ, stepped-impedance resonator

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.