Application of aggressive space mapping (ASM) to the automated design of differential-mode wideband bandpass filters with common-mode suppression

This paper is focused on the automated design of microstrip differential-mode (balanced) wideband bandpass filters based on mirrored stepped impedance resonators (SIRs) coupled through admittance inverters. The central metallic patches of the mirrored SIRs introduce common-mode transmission zeros, useful for the suppression of that mode in the differential filter pass band. The main relevant and novel aspect of this paper is the implementation of an unattended filter design algorithm, able to automatically provide the filter layout of these differential filters satisfying predefined specifications. The optimization algorithm is based on a two-step aggressive space mapping (ASM) scheme. The first ASM algorithm provides the optimum filter schematic; namely, it recalculates the lumped elements of the resonators and the electrical lengths of the transmission line sections (impedance inverters), in order to compensate the bandwidth reduction related to the limited functionality of the inverters. Once the filter schematic providing the required specifications (optimum schematic) is determined, the second ASM automatically generates the filter layout. The two-step ASM algorithm is validated through the design of an order-5 common-mode suppressed balanced Chebyshev bandpass filter with 40% fractional bandwidth (corresponding to 43.96% -3dB fractional bandwidth) centered at f₀ = 2.4 GHz. Filter design is achieved following a completely unattended scheme, and the response satisfies the specifications to a very good approximation.