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 Ω and electrical length of 53.13° and one transmission line of characteristic impedance Z_HI = 100 Ω and electrical length of 53.13°. 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 λ_g × 0.228 λ_g), DM stopband with a 40 dB suppression level broadened up to 5.633 f_0d and CM suppression level better than -40 dB from 0 to over 3.7 f_0d and better than -30 dB from 0 to 5.91 f_0d.