Novel balanced coupled-line bandpass filters, using suitable balanced coupled-line sections and quarter-wavelength resonators, are proposed. For design purposes, the differentialand common-mode equivalent half-circuits are established. Based on these circuits, a better balanced filter structure is implemented so that the desired differential-mode response may be realized and the level of common-mode noise may be minimized simultaneously. Besides, a suitable capacitive or inductive cross-coupled effect is introduced so as to create two transmission zeros for improving the filter selectivity; however, it also enhances the signal imbalance and degrades the common-mode rejection. In this study, various second-and fourth-order balanced filters are implemented to discuss the associated differential-mode responses and the signal-imbalance phenomena resulted from the cross-coupled effect. Specifically, the fourth-order filter with a common-mode rejection ratio of 40 dB within the passband is demonstrated and examined.

A novel fourth-order balanced bandpass filter is proposed based on the half-wavelength ( 2) stepped-impedance resonators (SIRs). By properly adjusting the parameters of each SIR, the proposed filter may be made compact and its stopband may also be extended simultaneously. Specifically, a balanced filter with acceptable common-mode rejection is implemented with its differential-mode and common-mode stopbands extended up to 5.5 0 , where 0 is the center frequency of differential-mode passband.

Novel fourth-order balanced coupled-resonator bandpass filters are proposed using suitably designed half-wavelength ( 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 filter 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 0 with a rejection level of 30 dB, where 0 is the center frequency in differential-mode operation.

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A novel patch-via-spiral resonator based on the dual-metal-plane configuration is proposed and examined. With the microstrip patch on the top plane serving as a capacitor and linking to the quasi-lumped spiral inductor on the bottom plane through a connecting via, the proposed dual-plane resonator structure located on the opposite sides of the single substrate may form a miniaturized one in the printed-circuit board fabrication. By suitably combining the proposed patch-via-spiral resonators, useful coupled-resonator pairs may be constructed to simultaneously provide electric and magnetic couplings. Based on these coupled-resonator pairs, a second-order bandpass filter with multiple transmission zeros is realized without requiring either the cross-coupled path or the source-load coupling. For design purpose, the equivalent-circuit model is also derived and verified. In this study, a fourth-order patch-via-spiral bandpass filter with both good passband selectivity and miniaturized size of only 22.14 mm 5.08 mm (i.e., 0 188 0 0 043 0 ) is implemented, where 0 denotes the guided wavelength of the 50-microstrip line at center frequency.