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In this work, we present a frequency transformation based dual-band filter design approach using scatteringbased real frequency technique. In the design process, conventional low-pass to band-pass frequency transformation isintegrated with the simplified real frequency technique to construct double pass-band filters. The approach isparticularly advantageous for designing matching filters between different termination resistances. As an alternative toa direct low-pass to dual-band frequency transformation, the use of dual-band mapping on a normalized band-passprototype is investigated for efficient control of the passbands. Application of the proposed approaches for dual-bandfilters is presented by comparative design examples.