A novel ultra-wideband (UWB) ring resonator bandpass filter with improved in-band and out-of-band performance

This letter proposes a novel ultra-wideband (UWB) bandpass filter (BPF) using a ring resonator with improved in band and out-of-band performances. A ring resonator is structured to equally allocate its first three resonant frequencies in the 3.47-10.47-GHz UWB band. Two capacitive-ended interdigital coupled lines are then made up to assign their transmission zero towards the fourth-order resonant frequency, finally suppressing the first spurious passband. Moreover, a two- stage UWB BPF is designed to obtain improved out-of-band performances.