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The ever-growing field of wireless communication requires efficient radio frequency transceivers with tunable absorptive bandstop filters that can prevent the interference of out-of-band and reflected signals. This paper presents 4-pole tunable absorptive bandstop filters using folded coupled-lines with an inductor. A 4-pole filter transfer function is achieved to obtain an absorptive bandstop filter coupling diagram with 45&#x00B0; electrical-length transmission-line source/load coupling. The folded-loaded coupled-lines with an inductor are proposed to obtain the control for the required coupling coefficients based on the filter transfer functions. The required quality factor extracted through the transfer function is accomplished using a distributed resonator loaded with asymmetrical varactors without auxiliary circuits, such as an impedance-matching network. Two types of filters were designed and built on a substrate with <inline-formula> <tex-math notation="LaTeX">$\varepsilon _{\mathrm {r}} = 3.42$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$h = 30$ </tex-math></inline-formula> mil. The center frequency tuning was performed using silicon varactors. The first filter shows a return loss of 16.2&#x2013;38 dB and a rejection level of 41&#x2013;48 dB in the tuning range 0.83&#x2013;1.09 GHz. The second filter results in a measured return loss of 14&#x2013;34 dB and a rejection level of 40.1&#x2013;51 dB at 1.16&#x2013;1.47 GHz. 4-pole tunable absorptive bandstop filters with high attenuation and reflectionless responses were achieved which have applications in cognitive radios and carrier aggregation systems.

4-Pole Tunable Absorptive Bandstop Filters Using Folded Coupled-Lines With an Inductor