Compact miniaturized stepped-impedance low pass filters with a broad stopband

Stepped-impedance low-pass filters (LPFs) are designed to have a miniaturized area and good rejection in wide stopband. The low-Z sections are implemented by microstrips loaded with periodic open-circuit stubs. Adjacent low-Z sections have a strong capacitive coupling through an interdigitated configuration. Equivalent circuit model for the interdigital capacitor is established for CAD purpose. In the stopband of the LPF, through adjusting the capacitance value, two or three transmission zeros can be realized. To enhance the circuit performance in the stopband, a higher-order filter is made by cascading two third-order LPFs with different cutoff frequencies. These filters present not only a broad stopband but also good insertion loss in the passband. Measured results of four fabricated circuits show good agreement with data obtained by circuit analysis and simulation by commercial 3D EM software package.