Quasi-dual-mode microstrip spiral filters using first and second harmonic resonances

The range of bandwidths that can be implemented using spiral microstrip resonators has been extended using a structure with two patterned metal layers plus a ground plane, which also enhances compactness. Each microstrip section can be regarded as a dual-mode half-wave resonator employing the fundamental and second harmonic or as two coupled quarter-wave resonators. A fourth-order and an eighth-order filter are presented, both with 50% fractional bandwidth and 600-MHz center frequency, using basic printed-circuit-board fabrication. The eighth-order filter has an active area of approximately 45 mm×18 mm, and a linewidth of 0.5 mm. The topology, suitably scaled for higher frequency, may be suited for low-temperature co-fired ceramic filters.