High selectivity and common-mode suppression wideband balanced bandpass filter using slotline resonator

Novel wideband balanced bandpass filter 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 fundamental mode of the slotline resonator and microstrip feedline can be maximised, whereas the second mode can be suppressed. Making use of this feature, the third-order wideband filter with low insertion loss, wide upper-stopband and flat group delay is designed by using the slotline resonator and two quarter-wavelength microstrip resonators paralleled to the slotline resonator with tapped-line coupling at their short-circuited ends. Based on the third-order single-ended filter structure, a novel balanced filter with a 1 dB fractional bandwidth up to 72% is designed with low insertion loss and enhanced CM suppression. To improve the selectivity, the fifth-order wideband balanced filter with wide upper-stopband is designed with parallel coupling structure between two slotline resonators. The corresponding CM suppression is better than 42 dB within the whole differential mode passband. The wideband balanced filter is designed and fabricated, and good agreement between measurements and simulations is obtained.