Automated design of balanced wideband bandpass filters based on mirrored stepped impedance resonators (SIRs) and interdigital capacitors

This paper presents small balanced bandpass filters exhibiting wide differential-mode pass bands and high common-mode suppression. The filters are implemented in microstrip technology and their topology consists of multisection mirrored stepped impedance resonators (SIRs) alternating with mirrored interdigital capacitors. The mirrored SIRs provide the required common-mode transmission zeros to achieve effective rejection of that mode in the region of interest, i.e. the differential-mode pass band. An automated design method for such filters, based on aggressive space mapping, is reported. The method uses the equivalent circuit model of both the mirrored SIRs and the interdigital capacitors, and filter synthesis is based on a quasi-Newton iterative algorithm where parameter extraction is the key aspect. The automated design approach is illustrated through an order-3 filter, where it is demonstrated that the filter topology is generated from the specifications. As compared with previous balanced filters based on mirrored SIRs coupled through admittance inverters, the proposed filters of this work are smaller and the design method is simplified, since bandwidth compensation due to the narrowband functionality of the inverters is avoided.

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