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Multi-frequency radars, which are employed in a plurality of emerging applications, exploit the echoes for different bands that come from illuminated targets. A compact multi-function architecture that makes use of direct sampling could be a preferable choice for these new-generation radar systems, with RF front-end design a first step in their development. Here, within the framework of multi-frequency radars, an analytical tool for simultaneously acquiring the signal bands by using the minimum sub-Nyquist sampling frequency is addressed. Although the dual-band radar scenario is exhaustively analyzed, keys to understand the acquisition of more-than-two-band systems are also provided. Moreover, as the core of the associated RF architecture to carry out arbitrary multi-band signal pre-selections, a new class of microwave planar multi-passband filters based on stepped-impedance-line signal-interference concepts is reported. Also, for validation, the construction and testing of a microstrip spectrally-asymmetrical dual-passband filter prototype for a specific example of minimum-sampling dual-frequency radar RF front-end are shown.