Enhanced microstrip filter design with a uniform dielectric overlay for suppressing the second harmonic response

Parallel-coupled microstrip filters are designed to suppress spurious response at twice the passband frequency (2f_o) with a uniform dielectric overlay. The overlay dielectric is used to equalize the modal phase velocities of each coupled stage. Based on the method, we have a large degree of freedom in choosing thickness and permittivity of the overlay dielectric. The image impedances of all the coupled stages in such a filter need adjusting to complete the filter synthesis. Two filters are fabricated and measured results show a good agreement with the simulation. A suppression of at least 40 dB to the spurious responses at 2f_o is achieved.