It is demonstrated that spurious passband suppression in stepped impedance lowpass filters can be achieved by simply patterning appropriate geometries in the low impedance sections. These patterns consist of concentric rings with splits etched in opposite sides, that behave as sub-wavelength resonators electrically coupled to the line. By tuning ring dimensions, signal propagation is inhibited at the frequencies of interest, and spurious passbands are rejected. The main advantages of the technique are: (i) no additional cascaded stages are needed, (ii) no extra area is added, (iii) the passband is virtually unaltered and (iv) the conventional design methodology of the filter holds. A prototype device fabricated in CPW technology exhibits spurious suppression levels above 30 dBs.
García-García, J., Bonache, J., Falcone, F., Baena, J. D., Martín, F., Gil, I., Lopetegi, T., Laso, M. A. G., Marcotegui, A., Marqués, R., & Sorolla, M. (2004). Stepped-impedance lowpass filters with spurious passband suppression. Electronics Letters, 40, 881-883. https://doi.org/10.1049/el:20040560