High selectivity and CM suppression wideband balanced BPF

In this letter, a novel wideband balanced bandpass filter (BPF) with high selectivity and common-mode (CM) suppression is constructed with slotline resonator. When the intersection points of the microstrip feedline and slotline resonator are close to the midpoint of the slotline resonator, the coupling between the second mode and microstrip feedline can be suppressed. Making use of this feature, the wideband single-ended filter with wide upper-stopband performance is designed using the slotline resonators and two quarter-wavelength microstrip resonators paralleled to the slotline resonator with tapped-line coupling at their short-circuited ends. Further, high-selectivity fifth-order wideband single-ended filter and corresponding balanced BPF are designed with parallel coupling structures between two slotline resonators. The fifth-order wideband balanced filter is designed and fabricated, and good agreement between measurements and simulations is obtained. The measured CM suppressions are better than 54 dB within the whole differential-mode passband. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2998–3003, 2014