Optimized wideband differential-mode bandpass filters with broad stopband and common-mode suppression based on multi-section stepped impedance resonators and interdigital capacitors

This paper presents optimized balanced microstrip filters, in terms of size and design simplicity, by combining multi-section mirrored stepped impedance resonators (SIRs) and interdigital capacitors. The mirrored SIRs provide the necessary transmission zeros (for both the differential- and common-mode responses) to simultaneously achieve a broad stopband for the differential mode and efficient common mode suppression in the region of interest (differential-mode pass band) and beyond. The interdigital structures actually act as series resonators and allow for the design of filters with smaller size (as compared to other implementations based on quarter wavelength admittance inverters) and, most important, reduce the complexity of the automated (and unattended) design algorithm, based on aggressive space mapping (ASM). The design of an order-3 balanced quasi-Chebyshev filter with central frequency f0 = 1.5 GHz, fractional bandwidth FBW = 43% and 0.15 dB ripple, is reported as an illustrative example.