Recent progress in planar microwave filters

This paper describes the recent development of advanced planar microwave filters for RF/microwave applications. These include cascaded quadruplet (CQ) filters, filters with asymmetrical transmission zeros at finite frequencies, and miniature slow-wave filters. An 8-pole microstrip filter of this type has been designed, fabricated and tested. The filter exhibits two pairs of attenuation poles at finite frequencies, which result in a high selectivity. Microstrip filters using patch or line resonators and having finite-frequency transmission zeros on either sides of passband are demonstrated. A miniature high-temperature superconducting (HTS) bandpass filter using novel slow-wave half-wavelength coplanar waveguide (CPW) resonators has been developed. The experimental results are in good agreement with the simulated responses.