Design of Microwave Dual-Band Rat-Race Couplers in Printed-Circuit Board and GIPD Technologies

In this paper, the dual-band π-section is proposed to design three types of dual-band rat-race couplers. Except for the experimental verification by the printed-circuit board process, the miniaturized dual-band coupler is also implemented in the glass integrated passive device (GIPD) technology to reveal the practical value. The π-section is realized by a section of the transmission line (TL) with two reactive shunt elements at the bilateral ends. The design formulas of the 90° and 270 ˆ π-sections are derived, and their corresponding design figures are then plotted for simplifying design procedures. Moreover, the reactive shunt elements of the 90 ˆ π-section can be implemented by series LC resonators, while those of the 270 ˆ π-section can be realized by series or parallel LC resonators. As six 90 ˆ π-sections are integrated to form a dual-band coupler, the band ratio, which is defined as the center frequency of the second passband dividing by that of the first passband, covers from 1.38 to 3.16. By setting the band ratio to 2 and constructing the dual-band rat-race coupler by three 90 ˆ π-sections and one 270 ˆ π-section with parallel LC resonators, two sets of the adjacent series and parallel resonators can cancel each other to reduce the power loss. Consequently, this dual-band coupler uses only two series LC resonators. In addition, the proposed dual-band coupler can be miniaturized by replacing microstrips with the complementary-conducting-strip TLs. It only occupies a 10% circuit size of the conventional one at the center frequency of the first passband. This miniaturized approach is also demonstrated in the GIPD technology.