This paper introduces a balanced (differential) multiband reconfigurable (tunable) bandstop filter (BSF) using all lumped elements. The main features of the design include its ultra-compact size as well as flexibility to control any frequency band independently in terms of both center frequency and absolute bandwidth (ABW). In the proposed structure, the corresponding non-resonating nodes (NRN) of the symmetrical bisections are connected to N number of LC π-circuits (N-band cell) through capacitors. Again, in each symmetrical bisection, K number of NRNs are series cascaded through LC π-circuits. This results in a Kth order N-band stopband (notch) response in differential mode (DM) operation whereas provides a passband response when excited by a common mode (CM) signal. Reconfiguration of any DM stopband is obtained by using tunable capacitors for the corresponding LC π-circuit in each N-band cell and also, for its coupling capacitors to the NRNs. To validate the proposed topology, a dualband differential tunable BSF is designed and fabricated where both DM stopbands are controlled independently in the range of 1.16 GHz-1.32 GHz. Also, the bandwidth of each band is varied independently by 20-50 MHz without affecting the other band. At any tuning state, DM stopband rejection for each band is found to be ≥19 dB, resulting in a minimum CMRR value of 19 dB. The fabricated prototype occupies an area of 0.13λg×0.04λg (21 mm×7 mm) where λg is the guided wavelength at the center frequency of the entire spectral range, and the experimental results show a good agreement with the simulation results.
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