A 250 GHz Low-Loss Inline Waveguide Bandpass Filter Using Bandstop Resonator Pairs

This paper presents an inline terahertz (THz) waveguide bandpass filter (BPF) based on the bandstop resonator pairs. It is implemented by E-plane stubs. The proposed resonators operate at the lower and upper stopbands of the designed BPF to provide sharp roll-off and out-of-band rejection. Both stopbands can be controlled independently and cascaded in parallel from source to load without mutual interference. The unloaded quality factors of E-plane stub resonators determine the bandwidths of both stopbands, which are equal to the spurious-free frequency ranges of the lower and upper stopbands, respectively. Since the proposed BPF design adopts bandstop resonator pairs resonating at the rejection band, it will achieve a lower transmission loss than the conventional BPFs with resonators at the passband. A 5
^th
-order BPF with a minimum measured insertion loss of 0.2 dB and 11% 1-dB bandwidth at 250 GHz is fabricated. The measurement results verify that multiple transmission zeroes (TZs) are achieved at the lower and upper stop bands, respectively.