A novel and effective architecture of tunable multiband balanced bandstop filter (MBBSF) is introduced for the first time in this paper. Each symmetrical bisection of the proposed branch line structure consists of K series cascaded tunable N-band sections to realize a reconfigurable K-th order N-band response in differential mode (DM) operation. The main advantage lies on the fact that all these N bands can be tuned simultaneously or each band independently. Moreover, it maintains a high common mode rejection ratio (CMRR) for all the tuning states by incorporating open stubs in the symmetrical plane of the balanced structure. To validate the proposed topology, a balanced dualband tunable BSF is designed where the two DM stopbands tune in the range of 1.16 GHz-1.29 GHz and 1.6 GHz-1.76 GHz, respectively. The lower and the upper bands maintain a constant absolute bandwidth (ABW) of 115 MHz and 135 MHz, respectively, and stopband rejection is better than 20 dB for each band. The fabricated prototype occupies an area of 0.31λg2, and the experimental results show a good agreement with the simulation results.
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