Design of dual-bandpass filters using stepped-impedance circular ring resonator

A stepped-impedance circular ring resonator (SICRR) with two pairs of stubs is proposed for the design of a dual-bandpass filter. This quad-mode resonator is applied to produce two passbands. It is found that the centre frequencies and bandwidths of the two passbands can be tuned by varying the lengths of the stubs. Simulation results show that the defined impedance ratio K and the electrical length ratio α of between the SICRR segments can affect the frequency separation between the two passbands. A prototype filter centred at 3.4 and 5.56 GHz is designed and fabricated. Good agreement between the simulated and measured results was achieved for the SICRR filter.