Active Impedance Profile Technique for Size Reduction, Loss Compensation and Selectivity Improvement of Planar Passive Filters

A novel general design technique is presented for lossless highly selective and compact planar filter structures. We illustrate the approach for a classical second-order passive interdigitated filter assisted by two one-port MMICs which first role is to compensate for losses through the negative real part of the equivalent impedance. In comparison to the fully passive equivalent structure, an adequate imaginary part profile of the active impedances leads to reduce the resonator lengths by a factor 3, and to increase the selectivity of the response by a factor 10 (0.5% bandwidth) while smoothing the geometrical constraints. Simulated and measured results are given at 1.56 GHz.