This Letter presents a compact microstrip diplexer. The high-frequency signal path is constructed by two electrically-coupling half-wavelength resonators, its external coupling adopts the tapped line structure. The low-frequency signal path is formed by two magnetically-coupling quarter-wavelength resonators, its external coupling is accomplished by microstrip coupling lines, which are formed from the tapped line structure and resonator of the high-frequency signal paths. The novel combination of the two signal path removes the need of T-junction in conventional diplexers. Meantime, the coupling strength of the two pass bands at the input port can be separately designed. A diplexer with Chebyshev frequency response at both signal paths with the central frequencies of 1.8/2.4 GHz is designed, fabricated and measured. The measured results give a good agreement with the simulated ones, which verifies the effectiveness of the diplexer.

A microstrip filter structure suitable for implementation of multi-order dual-band bandpass filter is presented. Two passbands have separate signal paths and the structures are assembled together novelly. The independently controlled external and inter-resonator coupling can be simultaneously achieved. The design is based on the conventional filter design method. To take account of the effect between two passbands, the extraction of the external quality factor in one passband considers the effect of the other passbands and the direct source-load coupling introduced by the upper band structure is equivalent to an impedance inverter in the lower passband. As an example, a fourth-order dual-band filter for WLAN applications is designed and fabricated. The measured results have a good agreement with the simulated ones, which verifies effectiveness of the design.Index Terms-Bandpass filter (BPF), dual-passband, multiorder, multipath, stepped impedance resonator (SIR).

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