A class of balanced dual-band bandpass filters (BPFs) with planar transversal-signal-interference coupled-line sections is reported. In their building balanced dual-band BPF stage under differential-mode excitation, a second-order quasi-elliptic-type dual-band bandpass filtering transfer function is obtained. Specifically, from the transversal interaction among their two open-ended and virtually-short-ended half-wavelength coupled-line paths, sharp-rejection differential-mode dual passbands with several out-of-band transmission zeros at both sides are realized. To attain high common-mode suppression levels within the differential-mode passbands, two open-ended line segments are connected at the symmetry plane of the devised balanced dual-band BPF stage. Moreover, higher-order schemes based on in-series-cascaded multi-stage designs to further increase differential-mode selectivity and in-band common-mode rejection are illustrated. The operational principles and parametric-analysis design rules of the engineered transversal-coupled-line-based balanced dual-band BPF approach are detailed. Additionally, for a rigorous interpretation of their zero/pole characteristics, a digital-modeling framework is applied to them to connect RF balanced filters with their discrete-time versions. For practical-validation purposes, a microstrip prototype of two-stage/fourth-order balanced dual-band BPF is built and tested. It exhibits measured differential-mode dual passbands with center frequencies of 1.464 GHz and 2.294 GHz, 3-dB fractional bandwidths of 8.74% and 9.68%, and in-band common-mode rejection levels above 23.16 dB and 31.36 dB, respectively.

A separately-designable diplexer with multiple transmission zeroes (TZs) using common stub-loaded stepped impedance resonator (SIR) is proposed. The common stub-loaded SIR operating in third harmonic (f3) and fifth harmonic (f5) is used for designing the two diplexer channels. The stub is loaded at the voltage-null point of f3 of the SIR. It can separately control f5 but has no effect on f3 so that the two channels can be separately designed. Meanwhile, the input port is tap-connected to the common stub-loaded SIR, which necessarily produces a TZ between f3 and f5, existing in both channel filtering responses. By properly choosing coupling schemes of the two channels, more TZs are realized at the desired locations. Thanks to the generation of the multiple TZs, both passband selectivity and isolation between the two channels are improved significantly. For demonstration, a diplexer operating at 2.22 and 2.95 GHz is designed, fabricated, and measured. The simulated and measured results are presented, showing good agreement.