Analysis and design of new single-to-balanced multicoupled line bandpass filters using low-temperature co-fired ceramic technology

This paper presents the design procedure and performance of a new single-to-balanced multicoupled line bandpass filter structure. The proposed filter is composed of a multicoupled line of electric length much shorter than λ/8 along with shunt capacitors loaded at suitable positions. By a proper design of ground terminations for the multicoupled line, the proposed filter is simultaneously equipped with the functionality of a bandpass filter, a balun, and an impedance transformer. The bandpass characteristic can be easily developed to higher order for better selectivity. The graph-transformation method for coupled-line analysis is adopted to make the design procedure efficient and intuitive. To validate the design procedure and feasibility of proposed filter for mobile applications, two design examples with different filter order, impedance transformation ratio, fractional bandwidth and center frequency have been implemented in chip type by using the low temperature co-fired ceramic technology. The second-order design is realized in a chip size of 2012, while the third-order one is realized in a chip size of 2612. Moreover, an additional transmission zero in the upper stopband can be achieved and controlled flexibly by adjusting the outer printed circuit board layout with minimum effect on passband performance. Fabrication and measurement of these designs show that compact sizes and good agreements between measured and simulated results can be obtained, which demonstrate their suitability in modern mobile communication applications.