Compact single-to-balanced bandpass filter

Abstract

A compact single-to-balanced bandpass filter is proposed in this invention. Firstly, a pre-design circuit is presented, which is composed of an inductive coupled-line (ICL) bandpass filter and an out-of-phase capacitive coupled-line (CCL) bandpass filter. A novel compact circuit with three coupled lines configuration, derived from the pre-design circuit, is then proposed for miniaturizing the single-to-balanced bandpass filter. In order to verify the feasibility of the proposed structure, a 2.4 GHz multilayer ceramic chip type single-to-balanced bandpass filter with size of 2.0 mm×1.2 mm×0.7 mm is developed. The filter is designed by using circuit simulation as well as full-wave electromagnetic (EM) simulation softwares, and fabricated by the use of low-temperature co-fire ceramic (LTCC) technology. The measured results agree quite well with the simulated. According to the measurement results, the maximum insertion loss is 1.65 dB, the maximum in-band phase imbalance is within 3 degrees, and the maximum in-band magnitude imbalance is less than 0.32 dB.