Analysis and design of single-to-balanced combline bandpass filters with two independently controllable transmission zeros in LTCC technology

In this paper, the analysis and design of single-to-balanced combline bandpass filter are presented. The cross-coupled effect is introduced to create two transmission zeros that can be located independently in either the upper or lower stopband. Notably, the effect of nonadjacent line coupling on the filter response is properly addressed, and an efficient way to compensate it is proposed. Also, the issue of a frequency-dependent J-inverter in bandpass filter design is also well treated. The proposed filter can be implemented using the low-temperature co-fired ceramic (LTCC) process to achieve very compact circuit size, in which the combline line lengths are as small as λ15. Specifically, two third-order single-to-balanced combline bandpass filters in LTCC are implemented. Good balance performance, low loss, good selectivity, and compact size are achieved.