A Compact UWB Bandpass Filter with High Selectivity and Dual Notched-Band

A novel compact-sized ultra-wideband (UWB) bandpass filter (BPF) is proposed in this paper. The proposed BPF is highly selective and is able to eliminate WLAN signals from 5.15-5.35 GHz, and downlink of X-band satellite communication signals from 7.25-7.75 GHz. Generally, a multiple-mode resonator (MMR), comprised of a U-shaped line, with two high impedance stubs connected to it, and one stepped impedance resonator placed in the center of the U-shaped line is used to generate five resonate modes in the desired band (range of 3.1-10.6 GHz). Two transmission zeroes (TZs) are also placed to improve the bandage steepness. One of these TZs is with a lower cutoff frequency and the other TZ is with a higher cutoff frequency. A C-shaped open-circuited stub without adding any circuit size is parasitically coupled to the inner part of the U-shaped stub and can provide a narrow notched band at any desired frequency with appropriate bandwidth through the dimension of the parasitic element and its distance to the MMR. This C-shaped stub is used to reject WLAN signals. Furthermore, UWB passband is fully covered using asymmetric feed lines with defected interdigital coupled-lines. At the same time, another notched-band is implemented to reject X-band signals. Slots in the ground plane and input/output ports are etched to increase coupling and improve the in-band performance. The proposed BPF in the study covers UWB span completely and eliminate two bands of unwanted WLAN and X-band signals, with good linearity in the passband. A prototype filter was fabricated and its performance was measured to validate the simulation results.