Design of compact microstrip triple-passband quasi-elliptic filters with multi-order spurious mode suppression

An efficient method for designing multi-band filters with compact size, high selectivity, and multi-order spurious mode suppression has been proposed in this paper. The design concept is to add some extra coupled resonator sections in a single-circuit filter to increase the degrees of freedom in extracting coupling coefficients between resonators and, therefore, the filter is capable of realizing a multi-passband response. In addition, by choosing the constitutive resonators with staggered higher-order resonant frequencies over the stopband, a broad upper stopband can be obtained. To verify the proposed concept, a compact tri-band quasi-elliptic filter has been designed and fabricated with microstrip technology. The measured results are in good agreement with the simulated predictions. The measured results show that the tri-band filter has a good stopband rejection better than 20 dB up to more than 18 GHz (9f₁).