Design of Rat-Race Couplers with Lumped Elements on Multilayer Structure

With the demand of miniaturized products, size reduction of elements has become a trend in communication systems. This thesis takes rat-race coupler which is part of 180° hybrid coupler as the main research topic. Combining the advantage of lumped elements – low spurious problem and predictable response – with multilayer PCB process, circuit layouts can be integrated vertically to realize a miniaturized rat-race coupler. Two types of rat-race coupler design are proposed in this thesis – a single passband design and a dual passband design. Both of them are designed at 2.4 GHz, which belongs to ISM band, as the main center frequency.he single passband rat-race couplers design uses which use high-pass and low-pass lumped model to replace the transmission lines. At first, a simplified narrow band structure is realized on a 2-layer print circuit board. Responses of ideal and actual lumped elements are verified with simulation and measurement, good results on area reduction and spurious suppression can be observed. A broader bandwidth structure is then achieved by increasing the order of the 270° transmission line section. This structure is designed and fabricated with low temperature co-fired ceramics (LTCC) process. A 1.37% area compared to conventional rat-race coupler and a 50% fractional bandwidth were obtained.he dual passband rat-race couplers design adopts a new kind of dual passband lumped circuits. A pair of dual rat-race coupler responses can be constructed by utilizing the phase reversion characteristics before and after resonance, thereby achieving the dual passband feature. A narrow band structure is designed with 1st order lumped circuit model on a 2-layer print circuit board, which takes 4 GHz as the second passband center frequency to verify the feasibility of a second passband below its spurious passband. Finally, an enhanced bandwidth structure designed by increasing the order of lumped circuit model is also simulated, which could be applied to other systems.