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Narrowband microwave filters exhibiting transmission zeros in the response (either imaginary and/or complex) can be typically designed with cross-coupled or extracted pole topologies. These configurations have advantages and disadvantages that must be carefully considered during the choice of a specific topology. In some cases, however, it may be convenient to choose a mixed topology, i.e., part of the structure with an extracted-pole and part with a cross-coupled configuration. Unfortunately, even if several well-established techniques are available in the literature for the cross-coupled or extracted-pole synthesis taken separately, nothing can be found for the design of mixed topology filters (apart direct optimization). In this study, two solutions for the synthesis of a low-pass prototype with a mixed topology are presented, which noticeably increases the design flexibility of narrowband microwave filters. Several examples are presented for illustrating the proposed techniques for the synthesis of mixed-topology prototype filters. A test filter, synthesized with the second method introduced and fabricated in coaxial technology, has given an experimental validation of the proposed technique.