Compact UWB Filter with High Selectivity and a Deep Notched Band

A compact ultra-wideband (UWB) bandpass filter with high selectivity and deep notched band attenuation is presented in this letter. The main structure of this filter is a balun-based coplanar waveguide (CPW)-microstrip CPW transition. This structure has UWB bandpass characteristic (2.85– 11GHz) and a transmission zero at its lower transition band. To achieve a transmission zero at its upper transition band, some complementary split ring resonators (CSRR) are added in the ground of microstrip. Therefore, this filter, whose skirt factor is 89%, presents high selectivity. Then, a notched band is created by short-ended stubs for 5.5GHz WLAN. Owing to the stepped impedance characteristic of these stubs, this filter achieves −41 dB deep notch in its S21. Besides, the size of the whole filter is only 0.38λg ∗ 0.45λg . The simulated and measured results agree well with each other.