LTCC Multilayered Helical Filters With a Mixed Electric and Magnetic Coupling Structure

In this paper, compact multilayered helical resonators and various coupled resonator pairs with a mixed electric and magnetic coupling structure were developed. A multilayer structure is suitable for miniaturization and enhances mutual coupling. A pair of transmission zeros (TZs) can be created using mixed coupling between two coupled helical resonators, achieving superior frequency selectivity. An equivalent circuit model was proposed to analyze the creation of TZs. The planar helical structure provides design flexibility in control of TZs. Eight coupled resonator pairs were proposed for synthesizing various filter responses. An experimental second-order helical filter was fabricated using low-temperature cofired ceramic technology. The measurement results of this second-order filter showed the center frequency to be 0.976 GHz, with two TZs located at 0.8 and 1.4 GHz. The insertion loss was lower than 1.96 dB, and the return loss was 25 dB at f₀. The size of this filter was only 5.7 mm × 3.4 mm (0.051 λ_g × 0.031 λ_g), which is highly compact compared with filters from other published studies. In addition, the proposed coupled pairs have potential in the construction of high-order filters with superior frequency selectivity by cascading multiple coupled helical pairs.