Microstrip Balanced Bandpass Filter With Compact Size, Extended-Stopband and Common-Mode Noise Suppression

In this letter, a novel balanced bandpass filter (BPF) with its simple design procedure are proposed. The filter is based on a novel balanced resonator which is composed by two closed-loop transmission lines of characteristic impedance Z(CL) = 50 Omega and electrical length of 53.13 degrees and one transmission line of characteristic impedance Z(HI) = 100 Omega and electrical length of 53.13 degrees. Also the differential-mode (DM) and common-mode (CM) equivalent circuits of the resonator are given for design purposes. A prototype with measured DM passband at 1.029 GHz and fractional bandwidth (FBW) of 9.833% is presented. The prototype exhibits compact size (0.149 lambda(g) x 0.228 lambda(g)), DM stopband with a 40 dB suppression level broadened up to 5.633 f(od) and CM suppression level better than -40 dB from 0 to over 3.7 f(od) and better than -30 dB from 0 to 5.91 f(od).