An ultra-wideband (UWB) bandpass filter with complementary split ring resonator for coupling improvement

This paper presents a novel, compact & low cost ultra-wideband (UWB) bandpass filter (BPF) using folded coupled stepped impedance resonator (SIR) pairs and a complementary split ring resonator (CSRR). Based on the advantage of symmetry of the structure, the odd-even mode decoupling scheme is applied for the analysis of different resonant modes which is generated by the coupled SIR pairs. The overall coupling of the proposed BPF is made stronger by placing a modified CSRR unit cell in the ground plane and it is confirmed through circuit analysis, coupling architecture & field distribution analysis. The proposed UWB filter provides 108% measured 3 dB fractional bandwidth (FBW) & insertion loss 1.6-2.1 dB in the passband (2.6-10 GHz) with a minimum flat group delay of 0.2 ns. The proposed UWB BPF is fabricated using commercially available FR4 substrate (εr=4.4 , h = 0.8 mm) and a good agreement is found between the simulated and measured results.