Compact Microstrip UWB Dual-Band Bandpass Filter with Tunable Rejection Band

In this paper, coupled open stub circuits are applied for providing band notch in filter design. Even- and odd-mode excitations for a coupled line can be represented as the equivalent capacitance networks. The rejection bandwidth of the designed filter can be controlled by the mutual capacitance Cm . An ultra-wideband (UWB) bandpass filter with 5 GHz to 6 GHz rejection band is proposed. To inhibit the signals ranged from 5 to 6 GHz, two pairs of stepped-impedance coupled open stubs are implemented on the multiple-mode resonator (MMR) The designed UWB filter has two transmission bands. The first passband from 2.8 GHz to 5 GHz has less than 0.8 dB magnitude of insertion loss and greater than –15 dB return loss. The second passband within 6 GHz and 10.6 GHz has less than 1.5 dB magnitude of insertion loss and greater than 15 dB return loss. The rejection at 5.5 GHz is better than 50 dB. This filter can be integrated in UWB radio systems and efficiently enhance the interference immunity from WLAN.