Coupled-line sections are well-known and broadly used components in microwave electronics. Their advantages are wide operational bandwidth and small size and both are important with respect to the constant development of communication systems and devices, which incorporate many communication standards in a single device. The size reduction is not only justified by the desire to offer communication devices as handheld and pocket-size, but it strictly affects the cost of circuit production. Within the project a methodology for design of components for realization of radio-communication circuits will be developed with respect to coupled-line directional couplers, and networks consisting of coupled-line directional couplers. The project focuses on chip-scale design, and all the components consisting of coupled-lines will be designed in the selected integrated circuit fabrication technologies.
Typically in the design of microwave circuits as directional couplers branch-line devices are utilized, but their main disadvantages are narrow operational bandwidth and large size. The operational bandwidth of such devices can be enhanced by designing multisection couplers but the size of such devices is even larger. The advantage of branch-line couplers is their very easy topology, thus can be easily designed and fabricated. On the other hand the coupled-line directional couplers feature much reduced size but the design of such devices is more complex, and form the fabrication point of view they are more demanding. In particular to design tight-coupled directional couplers it is required to utilize at least two different metallization layers with a thin dielectric separation. The challenge is to fit these components into the fabrication technologies without compromising their performance. Further functional blocks for communication systems with the use of the developed couplers will be designed as a proof of concept.
Research project objectives
The project aims at investigation on designing coupled transmission-line sections taking into account constraints that are inherent to monolithic technologies. The focus will be put on achieving superior performance of the designed sections and to investigate wheatear it is possible to design compact monolithic directional couplers featuring bandwidths exceeding one frequency decade.
The main goal of the project is to research the possibility of coupled-line sections’ design that have good return losses and good isolation properties, and at the same time feature the required coupling between excited and coupled lines. In general coupled-line geometries that will be considered within the project can be complex, consisting of a number of appropriately inter-connected conductors in inhomogeneous dielectric medium. The problem of such coupled-line section realization has been research and experimentally investigated in the techniques of strip-transmission lines for both geometrically symmetrical and asymmetrical conductors. It has been shown that the principal condition of ideal coupled-line section realization to be fulfilled in the case symmetrical conductors is the equalization of phase velocities of the waves propagating in the structure. Alternatively, it can be formulated as the equalization of effective dielectric constants for coupled line geometry under even and odd excitations. The more general condition for ideal coupled-line realization has been derived for the case of geometrically asymmetrical conductors for which it is required that the capacitive and inductive coupling coefficients are equalized. Additionally, in both cases an additional impedance condition needs to be fulfilled to obtain good return losses measured for the assumed reference impedance.
The project aims at developing methods for the design of coupled lines which allow to equalize coupling coefficients under the limitations specified by the chosen monolithic technology. The hypothesis that will be verified within the project is that new compensation methods together with the design techniques would allow for achieving similar performance in terms of directivity, return losses and bandwidth of coupled-line sections and directional couplers realized in monolithic technologies comparing to such networks designed in well-developed PCB technology. The second hypothesis to be verified and the final goal of the project is to investigate, whether the utilization of the design quasi-ideal coupled-line sections together with novel circuit solutions and miniaturization techniques would allow for the development of MMIC directional couplers that feature bandwidths exceeding one frequency decade despite the technological limitations encountered at the commercially available processes.