A new quasi-elliptic UWB BPF with a sharp transition band using 3/4-wavelength SIRs

A new ultra-wideband (UWB) bandpass filter composed of two parallel coupled stepped impedance resonators (SIRs) is proposed. Each resonator is tapped with stepped-impedance open stubs at its center providing a very sharp transition band and two uniform open stubs at the junctions of the low- and high-impedance sections to slide the upper resonant modes. Two interdigital parallel coupled lines are used to enhance couplings at input and output of the circuit. A planar UWB bandpass filter with a quasi-elliptic function response and a fractional bandwidth of about 110% (from 3.1GHz to 10.6GHz) is designed and simulated by Ansoft HFSS EM simulator. Simulated response of the filter shows an insertion loss of 0.6 dB, a return loss of 15 dB and a flat group delay of 0.4 ns in the passband.