Cross-coupled UWB bandpass filter with dual-mode stepped impedance resonator

In this paper, a new three-mode ultra wideband (UWB) bandpass filter (BPF) with compact size and highly selectivity is proposed using dual-mode stepped impedance resonator (SIR). The resonator, configured by two sections big impedance ratio transmission lines, generates two operating modes in the desired band and the even-mode frequency can be tuned effectively by adjusting the length and width of the low impedance line if keeping the perimeter unchanged. In order to obtain a good in-band and high selectivity performance, a cross-coupled feed structure is applied to generate a reflection zero in passband and two transmission zeros at the lower and upper stopband. By properly optimizing the structure by HFSS, an UWB BPF with 1dB fractional bandwidth 56% from 3.1 to 5.7 GHz, group delay between 0.25 and 0.45ns, and return loss better than 15dB is presented.