Design of a Fourth-Order Dual-Band Bandpass Filter With Independently Controlled External and Inter-Resonator Coupling

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.