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In this letter, signal-interference filtering sections shaped by two in-parallel stepped-impedance transmission lines are applied to frequency-asymmetrical microstrip filter design. When realizing single-passband filters, a different selectivity for the lower and upper stopband can be generated, making them appropriate for duplexing devices. In the case of double-passband filters, fully-asymmetrical dual bands in terms of bandwidth, cutoff slopes and class of filtering transfer function can be shaped. Examples of design curves to adjust the performances of the synthesized signal-interference section filtering profile are given, e.g., bandwidth or in-band ripple level for Chebyshev-type functions. To show practical viability, a duplexer and a spectrally-asymmetrical dual-passband filter circuit are also built and characterized.