Nonlinear Microwave Measurement Architectures for Wideband Device Characterization

Licentiatavhandling, 2015

The global surge for ubiquitous mobile communication requiring high speed and high capacity cellular networks has resulted in a golden age for the development of wireless technology. Modern cellular standards employ complex modulation formats with wider signal bandwidths to cope with the growing demand. The increasing complexity of wireless systems, however, directly translates into a need for more detailed characterization using advanced measurement setups. Furthermore, new semiconductor material technologies such as gallium nitride (GaN) are utilized to enable higher performance in microwave circuits. As GaN-based devices tend to suffer from slow and fast dispersion phenomena, relevant wideband characterization is needed. The main focus of this thesis is on the implementation of a wideband two-port nonlinear measurement setup, consisting of a real-time oscilloscope (RTO) with 4~GHz bandwidth as a measurement receiver. The large instantaneous bandwidth enables new types of measurement scenarios to be carried out. The measurement setup was demonstrated by its use as an active load-pull setup, where the efficiency of a GaN high electron mobility transistor (HEMT) was improved by tuning the load impedances at intermodulation frequencies. Moreover, due to nonidealities in the microwave front-end of the oscilloscope, a correction technique was developed to improve signal fidelity of the acquired waveforms in the system over a large dynamic range. Furthermore, device characterization was carried out on in-house processed GaN HEMTs, and investigations on how dispersive effects manifest on various DC and RF performance aspects were made. As concluded from the measurements, dispersive phenomena are extremely dynamic and cause complex device behavior. These results express the need for wideband characterization, enabled by the proposed measurement setup, to further understand dispersion and its influence on system performance. The results in this thesis enables future research activities within wideband device characterization, which is essential for the design of high performance microwave circuits for next generation wireless systems.