The Design of A Dual Ultra-Wide Bandpass Filter Using L-Shaped Step Impedance Resonators

To inhibit interference from WLAN in UWB front-ends, it is required to provide an effective band-stop filter with 5 to 6 GHz rejection band.

Several typical configurations for providing rejection bands such as defect ground structure (DGS) [1], photonic band gap (PBG) [2], split ring resonator (SRR) [3], and slot resonator [4] are employed. One of these above proposed methods suffers either degradation of passband performance or complexity of fabrication.

A simple and effective approach for generating narrow rejection band is to excite the critical coupling from main transmission structure to the resonators. Conventionally, half-wavelength resonators in different shapes are used for this purpose through electric coupling, magnetic coupling, or mixed coupling.

For compactness considerations, open-circuited half-wavelength resonators with electric coupling can be replaced by hairpin-shaped half-wavelength resonators with magnetic coupling [5]. To obtain the maximum coupling, the resonators must be positioned in the proper location along the transmission line [6].

In some cases, the resonators using mixed coupling may further miniature the structure [7], [8]. In this letter, step impedance L-shaped resonators are required to provide mixed coupling mechanism for trapping the energy of spectrum ranged from 5- to 6-GHz. They are integrated with a multiple mode resonator (MMR). Finally, the high performance and compact dual ultra-wide bandpass filter is proposed.