Wideband Bandpass Filters With Ultrawide Stopband Based on Cascade Multiple Square Ring Elements

A wideband bandpass filter (BPF) with ultrawide out-of-band rejection capability is proposed in this article. A new design concept is constructed with the employment of cascading ring resonators and a lumped element filtering network. The center frequency (f₀) is located at 2.1 GHz from 0.9 to 3.4GHz with a fractional bandwidth (FBW) of 119% that can simultaneously cover the telecommunications networks of 2G, 3G, 4G, and 5G frequency bands. The 12 transmission zeros (TZs) within the5.2f₀frequency range have been achieved due to the superposition of signals of multiple transmission paths and the virtual short effect of shorted stubs, and upper sideband rejection is up to 40 GHz (19f₀). Meanwhile, the proposed BPF realized the minimum insertion loss of 0.11 dB, the group delay in the passband within 0.6 ns, and the overall circuit only occupies0.21λ g × 0.18λ g. The measured results match well with the simulation so as to prove the feasibility of the design method.