Synthesis and Implementation of LTCC Bandpass Filter With Harmonic Suppression

Abstract

In this paper, we utilize the coupling matrices to design a compact low-temperature cofired ceramic (LTCC) bandpass filter with wide stopband and high selectivity. One coupling matrix at the fundamental frequency is synthesized to obtain required passband responses. The other one at the third harmonic is manipulated to realize wide stopband. Two transmission zeros are appointed at both sides of the passband to enhance the skirt selectivity. They are generated by multiple cross-coupling paths, which are easy to realize in the multilayer LTCC structure. To reduce the circuit size, two coupled quarter-wavelength resonators are folded vertically and horizontally on different layers. An experimental filter is designed for verification. High skirt selectivity and wide stopband are observed in the measurement. Due to the LTCC techniques, the proposed circuit has a compact size of $2~\text {mm}\times 1.7~\text {mm}\times 2$ mm or $0.042\lambda _{g}\times 0.035\lambda _{g}\times 0.042\lambda _{g}$ . It is suitable for system-in-package applications.