A Wideband CPS Bandpass Filter with Ultra-wide Upper Stopband Using Stepped-Impedance Rat-Race Hybrid Couplers

A wideband coplanar strip (CPS) bandpass filter using two 3-dB stepped-impedance rat-race hybrid couplers is proposed in this paper. The stepped-impedance structure results in both ultra-wide upper stopband and size reduction. The low impedance and high impedance CPS lines are realized using the interdigital structure for low impedance and conventional structure for high impedance, respectively. The ideal phase inverter has been implemented with hybrid CPS/interdigital CPS structure. Experimental results show that this 2-GHz fifth order filter depicts a 10-dB return loss bandwidth of 100% and upper stopband rejection levels of better than -40dB up to 17.5 GHz. This proposed filter achieves wide passband and ultra-wide stopband performance simultaneously. This filter is fabricated using CPS on 15-mil Al₂O₃ substrate with a dielectric constant of 9.8. The measured results agree well with the simulated results.