Compact stripline lowpass filters with very wide stopband

This paper presents two new approaches for extending the stopband performance of planar quasi-lumped stripline lowpass filters. First, a countermeasure is introduced against rectangular waveguide modes which are able to propagate in a fully shielded planar filter assembly. Incorporating this technique in the traditional high-low-impedance filter, a better stopband performance can be achieved, at the expense of only slightly increased complexity of the filter geometry. The second part of the paper proposes a trade-off for the capacitance-to-inductance ratio that has to be realized by a certain lowpass specification. By changing the terminating impedances of the filter, this ratio can be shifted in favor of the type of element which is more easily realized in a certain design technology.