A Novel Compact Balanced-to-Unbalanced Low-Temperature Co-Fired Ceramic Bandpass Filter With Three Coupled Lines Configuration

This paper proposes a novel compact balanced-to-unbalanced bandpass filter. Firstly, a pre-design circuit is presented, which is composed of an inductive coupled-line bandpass filter and an out-of-phase capacitive coupled-line bandpass filter. A novel compact circuit with three coupled lines configuration, derived from the pre-design circuit, is then proposed for miniaturizing the balanced-to-unbalanced bandpass filter. A 2.4-GHz multilayer ceramic chip type balanced-to-unbalanced bandpass filter with a size of 2.0 mm times 1.2 mm times 0.7 mm is developed to validate the feasibility of the proposed structure. The filter is designed by using circuit simulation, as well as full-wave electromagnetic simulation softwares, and fabricated by the use of low-temperature co-fired ceramic 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 3deg, and the maximum in-band magnitude imbalance is less than 0.32 dB.