Balanced octave-tunable absorptive bandstop filter

This paper presents, for the first time, a balanced octave-tunable absorptive bandstop filter. The balanced (differential) operation is realized by coupling two transmission lines with an opposite polarity to each of two high-Q tunable Substrate Integrated Waveguide (SIW) cavity resonators. The filter is based on the 90^o hybrid-coupler absorptive bandstop filter topology enabling high levels of rejection over a wide tuning range. Both simulated and measured results are shown. The total size of the proposed filter is 0.39λg × 0.36λg (λg at 1.57 GHz), and it is tunable from 1.57–3.18 GHz with 102% tuning range. Over the full range, the measured 10-dB fractional bandwidth ranges from 1–2.4%, and the attenuation level is better than 47 dB.