High CM suppression balanced BPF with two CM transmission zeros

A new compact, low insertion loss, and wide stopband balanced bandpass filter (BPF) with two common-mode (CM) transmission zeros within the differential-mode (DM) passband is designed in this article. The DM filtering topology is formed by two quarter-wavelength resonators and a short stub loaded source-load coupling structure which can generate new transmission zeros to improve the selectivity and suppress the harmonic to widen the stopband. The center-loaded step-impedance open stub can be applied to misalign two CM fundamental resonant frequencies, meanwhile adjust one CM transmission zero to the DM passband. To enhance the CM suppression within the DM passband, another CM transmission zero is created by the folded stub loaded in the 50- input feedline which creates the first DM transmission zero located at 2 f0d ( f0d is the center frequency of DM passband) and has little effect on the DM in-band and lower-stopband performance. Finally, a high CM suppression balanced BPF prototype for WLAN application is designed and fabricated. The simulated and measured results show a good agreement. (c) 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:135-141, 2014