Incorporation of Tapped Input/Output in Microstrip Bandpass Filter Design with a Uniform Dielectric Overlay for Suppression of Spurious Responses

A uniform dielectric overlay is incorporated in design of microstrip bandpass filters for suppressing spurious response at twice the design frequency (2fo). The dielectric overlay is found to have a wide range of thickness in which the even- and odd-mode phase velocities of each coupled stage are equalized. Since all the coupled stages have various image impedances, tapped input/output is incorporated into the filter synthesis to release the difficulty in root-searching process. Synthesis method with coupled stages of arbitrary respective image impedances is then formulated. Two circuits are fabricated and measured to demonstrate the proposed approach. Measured results show good agreement with simulation response.