Design of compact notched UWB filter using coupled external stepped-impedance resonator

A compact ultra-wideband (UWB) bandpass filter (BPF) with a controllable notch band is proposed and implemented for the application in UWB wireless communication system. In terms of a recently-proposed filter synthesis theory, a short-circuited-stub-loaded multi-mode resonator based UWB filter is designed to create an ultra-wide passband, within which a narrow notch (rejection) band is introduced by an external coupled stepped-impedance resonator (SIR). The location of the notch band is controlled by the proper selection of the high-low impedance ratio and electric length of the SIR. A simulative investigation exhibits that the external SIR is able to create narrower notch band than other conventional techniques, such as open-circuited stubs, spur-lines and embedded inline stubs. In addition, since the distortion of the filter characteristics, introduced by the external SIR at non-resonance frequencies, can be neglected, the proposed UWB notch filter can be implemented by the design of UWB BPF and external SIR respectively. As an example, an FCC UWB filter (BW: 3.1~10.6 GHz) with a WLAN notch band was designed, simulated and fabricated. A very good agreement between the measured and predicted filter characteristics validated the proposed filter prototype and corresponding design theory.