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/sub 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/sub o/ is achieved.

I. Introduction

High-Performance bandpass filters are usually required in the RF front end of a wireless communication system. One of the most common methods for implementing a planar filter is to use parallel-coupled microstrip filters [1], [2]. The traditional design of parallel-coupled line filters, however, suffers from the spurious response at twice the design frequency [2]–[6], which not only degrades passband symmetry but also deteriorates rejection levels in upper stopband. It is due to that phase constants of the even and odd modes of each coupled stage are not identical.