Compact High Selectivity Uwb Filter Using Composite Cpw-Microstrip Structure

A novel ultra-wideband (UWB) bandpass filter with high selectivity and compact size is proposed in this letter. The major architecture of this filter is a coplanar waveguide (CPW)-microstrip-CPW transition from broadband balun mechanism. This structure has UWB bandpass characteristic and a transmission zero at its lower transition band. And some complementary split ring resonators (CSRR) are added in the ground of microstrip. A transmission zero located in upper transition band of filter is introduced by these CSRRs. Besides, the size of filter is only 0.25 lambda(g) x 0.36 lambda(g) (lambda(g) is the guided wavelength of 6.85 GHz). Finally, a filter prototype is fabricated and measured. The measured results show that its 3 dB passband covers 2.9 to 11 GHz. And two transmission zeros are located at 2.45 GHz and 11.45 GHz, which means the proposed filter presents high selectivity of 0.9 skirt factor.