Very Sharp Roll-Off Ultrawide Stopband Low-Pass Filter Using Modified Flag Resonator

In this paper, a low-pass filter (LPF) using modified flag resonators to achieve sharp roll-off and an ultrawide stopband up to 18 times the cutoff frequency (f
_c
) is presented. To obtain a sharp roll-off, the inductor value of the prototype LC resonator is increased, and two series prototype LC resonators are considered. To realize compact size, the flagpoles of resonators are meandered. On the other hand, the flag resonators with coupling effect as flag-suppressing cells are designed and utilized in the suppressing structure. The implemented LPF with 3-dB cutoff frequency of 1.94 GHz demonstrates a very sharp roll-off rate of 525 dB/GHz, i.e., transition band of 0.07 GHz and ultrawide stopband from 2.01 to 37.5 GHz. The size of the proposed filter is only 0.165λg × 0.067λg, where λg is the waveguide length at the cutoff frequency. The proposed LPF is designed and fabricated. The measured results demonstrate good agreement with simulation results.