Iterative Synthesis of Dual-Band Filters With Separately Assignable Operating Bands Using Bandpass Polynomials and Prototypes

This letter introduces an iterative synthesis approach in the bandpass domain, tailored for dual-band filters that can assign the order, bandwidth, center frequency, and return loss level for each operating band individually. The synthesis commences by iteratively deriving a set of dual-band characteristic polynomials within the bandpass frequency range, which possess all real coefficients. Notably, an additional pair of transmission zeros (TZs) is introduced, supplementing the prescribed ones to ensure an equal-ripple response in each passband. Subsequently, a practical in-line bandpass prototype is constructed based on the predetermined polynomials, which requires no approximation and is robust for wideband situations. The proposed theory is validated by several illustrative examples with dual wide bandwidths, far interval distance, and different faraway responses.