Octo-Band Bandpass Filter Based on Unequal-Width Cruciform Connected Resonator with Sharp Skirt

This brief proposes a novel octo-band bandpass filter (BPF) utilizing an unequal-width cruciform connected resonator (UCR) with open-and short-stub. In the UCR, each section featuring different characteristic admittances and electrical length helps obtain multi-poles and -zeros. The multi-mode resonator (MMR), based on two UCRs and one parallel short-stub connected in-between them, features 16 poles and 8 zeros with high tuning flexibility. In addition, the short-stub is constructed by Y-shaped short-stubs with unequal characteristic admittances connected in parallel, which enhances pole and zero tunablity further. The octo-band bandpass filter, utilizing a simple structure, obtains a high tuning flexibility of bandwidths and a low insertion-loss. To the best of the authors’ knowledge, it is the first time that the octo-band bandpass filter is implemented. The overall size of the BPF is as small as 0.1λg × 0.24λg. The proposed filter also features sharp skirt and deep isolation between passbands.