An absorptive reconfigurable bandstop filter (BSF) with compact size and ultra-wide frequency tuning range using distributed lossy resonators is presented. In each reconfigurable bandstop resonator, a varactor and a PIN diode are utilized as the control and absorption devices. When the PIN diodes are in off and on states, the upper and lower frequency tuning ranges of stopbands can be obtained, respectively. Therefore, the ultra-wide total frequency tuning range which is the combination of the upper and lower frequency tuning ranges can be realized. Meanwhile, the stopband frequency and bandwidth tuning can be independently controlled by bias voltages. The resistances in the varactors and PIN diodes can dissipate the electromagnetic power and thus result in absorptive stopband without using extra absorptive circuits. The stopband suppression level and stopband absorption ratio are proportional to the number of distributed lossy resonators. For demonstration, an absorptive reconfigurable BSF prototype using six pairs of distributed lossy resonators is designed and fabricated. The measured total frequency tuning range is 3.03–6.39 GHz (71%) with the suppression level of 20 dB, while the frequency tuning range with the suppression level of 10 dB is 2.04–6.39 GHz (103%).

The design of mixed-technology quasi-reflectionless planar bandpass filters (BPFs), bandstop filters (BSFs), and multi-band filters is reported. The proposed quasi-reflectionless filter architectures comprise a main filtering section that determines the power transmission response (bandpass, bandstop, or multi-band type) of the overall circuit network and auxiliary sections that absorb the reflected radio-frequency (RF) signal energy. By loading the input and output ports of the main filtering section with auxiliary filtering sections that exhibit a complementary transfer function with regard to the main one, a symmetric quasi-reflectionless behavior can be obtained at both accesses of the overall filter. The operating principles of the proposed filter concept are shown through synthesized first-order BPF and BSF designs. Selectivity-increase techniques are also described. They are based on: (i) cascading in-series multiple first-order stages and (ii) increasing the order of the filtering sections. Moreover, the RF design of quasi-reflectionless multi-band BPFs and BSFs is discussed. A hybrid integration scheme in which microstrip-type and lumped-elements are effectively combined within the filter volume is investigated for size miniaturization purposes. For experimental validation purposes, two quasi-reflectionless BPF prototypes (one- and two-stage architectures) centered at 2 GHz and a second-order BSF prototype centered at 1 GHz were designed, manufactured, and measured.