FPGA-controlled Outphasing Modulator Using A Power Combining Antenna

Abstract

Coupling between antenna elements is often an undesired characteristic in antenna arrays, due to its distortion of radiation patterns and degradation of beamforming accuracy. However, if this coupling could be exploited in outphasing through load modulation, it has the potential to improve total system efficiency. This project aimed to design a high-efficiency outphasing system for communication applications, consisting of two power amplifiers (PAs) and a power combining antenna array. To evaluate how different load impedances affect PA performance, a custom active vector-receiver load-pull setup was constructed and calibrated using an FPGA board. This setup enabled precise control over the presented load impedance to the PAs and was critical for measuring power added efficiency and output power under dynamic loading conditions. The resulting load-pull data informed the selection of ideal 2-port S-parameters for the power combiner. Based on these, a two-element patch antenna array was designed to serve as the combiner. Separate matching networks were implemented to match the antenna ports to the desired S-parameters. Two types of matching networks were considered: using lumped components and using microstrip lines. Using lumped components, the desired S-parameters were able to be matched quite well, yielding an error of around 9% whereas the microstrip line network was unable to provide sufficient matching in its current configuration. Using the PA load-pull data and the S-parameters of the designed antenna, the performance of the outphasing system was tested through simulations in both MATLAB and CST. The results confirmed that outphasing was achievable with this setup, where amplitude modulation was realized by steering the antenna beam away from the receiver. Additionally, the simulations indicated that implementing an appropriate matching network could enhance the overall system efficiency.