Super High-Selectivity Fifth-Order Bandpass Filter with Twelve Transmission Zeros

A fifth-order bandpass filter (BPF) with super high selectivity using three pairs of coupled lines and two open stubs is proposed. Twelve transmission zeros (TZs) from 0 to 2f_0 (f_0 denotes center frequency of the passband) and five transmission poles (TPs) in the passband can be obtained to realize good out-of-band suppression and sharp roll-off skirts. For demonstration, a simple BPF prototype centered at 2.04 GHz is designed, fabricated with measured 3-dB fractional bandwidth of 18% and very high transition band roll-off rates of over 567 dB/GHz. Good agreement between the simulations and measurements validates the design method.