Tunable absorptive bandpass filter with ultra-wide reflectionless range

This article presents the scheme of a tunable absorptive bandpass filter with an ultra-wide reflectionless range, which is composed of a tunable reflective BPF and a tunable reflective bandstop filter. A lowpass prototype for the absorptive bandpass filter with reflectionless characteristics at all frequencies is proposed, which is transformed into a bandpass circuit model using impedance converters while maintaining the reflectionless characteristics over the whole frequencies. In addition, parallel coupled lines are chosen to realize the absorptive filter, and varactor diodes are introduced to achieve the tunable characteristics. The proposed filter has been fabricated and the measurements show a tuning range of 38.7 % (from 1.02 GHz to 1.51 GHz) and a reflectionless range of over 20:1 (from 0.1 GHz to 2 GHz).