A compact UWB bandpass filter with ultra narrow notched band and competitive attenuation slope

A compact ultra-wideband (UWB) bandpass filter with a notched band is proposed using the microstrip/coplanar waveguide (CPW) composition structure and quarter wavelength (λ/4) embedded CPW resonator in this paper. Basic quasi-elliptic function response and multi-mode UWB operation are achieved simultaneously through a λ/4 CPW detached-mode resonator (DMR) and broadside-coupled transitions from microstrip to CPW. To avoid the interferences such as WLAN signals in the UWB passband, two identical λ/4 CPW resonators embedded in the DMR are employed to obtain a narrow notched band. To further improve the selectivity at the lower passband edge, two microstrip short-stubs are introduced. These short-stubs could provide an additional observable transmission zero at 1.86 GHz, which leads to enhanced roll-off characteristic at the lower end of the passband. As such, the proposed filter exhibits a quasi-elliptic function response with 10 dB notched band fractional bandwidth (FBW) smaller than 2.26%. Meanwhile, a competitive attenuation slope with 94.34 dB/GHz in the lower and 67.92 dB/GHz in the upper transition bands can be easily achieved, respectively.