Balanced coupled-resonator bandpass filters using multisection resonators for common-mode suppression and stopband extension

Novel fourth-order balanced coupled-resonator bandpass filters are proposed-using suitably designed half-wavelength (lambda/2) multisection resonators for common-mode suppression. By properly designing the input/output (I/O) resonators associated with the filter composed of four bi-section resonators, a balanced filt er with good common-mode suppression is realized, but its rejection bandwidth is rather limited. To widen the rejection bandwidth, the I/O bi-section resonators are replaced by the tri-section ones so that a balanced filter with good common-mode suppression and wide rejection bandwidth may be realized by suitably arranging the composed bi-/tri-section resonators. Specifically, a stopband-extended balanced filter with good common-mode suppression (>50 dB) within the differential-mode passband is implemented and its stopbands are also extended up to 5 f(0)(d) with a rejection level of 30 dB, where f(0)(d) is the center frequency in differential-mode operation.