Microstrip trisection filters with quasi-elliptic and flat group delay responses

Based on stepped-impedance resonators (SIRs), a trisection filter is proposed to create a pair of real or imaginary zeros. Such zeros are useful to implement quasi-elliptic or flat group-delay response. Y-parameters are formulated for deriving the equivalent circuit. It is found that the sign of Y₁₃ above and below the center frequency can be opposite, leading to a pair of zeros. Two more zeros are created by tapped-line input/output. Measurements are conducted to confirm the theory.