|dc.description.abstract||Nowadays portable wireless communication devices are being widely adopted into daily life, with ever-greater demands on more functionalities, higher performance, and
lower cost in smaller and lighter formats. Especially, the demand for high-performance and miniaturized passive components such as filters, baluns, and matching circuits continues to grow since these passive components usually dominate the circuit area of RF transceiver. In this study, we proposed two kinds of single-to-balanced multicoupled line bandpass filters that integrated above three functions in a simple circuit. In this way, one can reduce the circuit area as well as component count effectively.
The first design is composed of a multicoupled line of electric length as small as λg/24 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, several 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 (LTCC). The second-order design is realized in a chip size of 2.0 mm × 1.2 mm (2012), while the third-order one is realized in a chip size of 2.6 mm × 1.2 mm (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.
The second design is also composed of multicoupled line with loaded capacitors. Besides, the cross-coupled effect is introduced to create two transmission zeros that can be located independently in either the upper or lower stopband. The effect of non-adjacent 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 well treated. The proposed filter can be implemented using the LTCC process to achieve very compact circuit size, in which the combline line length is as small as λg/29. Two design examples implemented in LTCC demonstrate the controllability of transmission zeros, good selectivity, and compactness.
The proposed multi-functional bandpass filters have the advantages of compact size, high integration level, good selectivity, and simple circuit topology. With the increasing demands on highly integrated, multifunctional, miniaturized, and high-performance RF front-end modules, we believe that the proposed single-to-balanced bandpass filters are highly suitable for modern mobile communication applications.