RF IV waveform engineering applied to VSWR\ud sweeps and RF stress testing
This thesis looks at how the Radio Frequency (RF) waveform\ud measurement and engineering techniques developed for Power Amplifier\ud (PA) design can be used to investigate RF reliability. Within this area two\ud major themes are concentrated on – firstly the effect of a load impedance\ud mismatch and secondly an investigation into using the RF IV waveform\ud measurement system for RF stress testing.\ud The initial aim for this work was to investigate the potential for removing\ud the output protection isolator from a PA. It was seen that in doing so there is\ud the potential to cause an impedance mismatch, which results in a portion of\ud the power produced being reflected back. It was shown that the conditions\ud that could be presented to a device as a result of an impedance mismatch\ud can be found by performing a Voltage Standing Wave Ratio (VSWR) sweep.\ud The results of the worst possible case scenario VSWR sweep, when all of\ud the power is reflected back, can be split into three regions. One of high RF\ud drain voltage swings, one of high RF drain currents and a transition region of\ud simultaneously high RF drain currents and voltage swings. Each of these\ud regions presents different operating conditions to the device, and in turn\ud different stresses.\ud The second part of this thesis concentrates on an investigation into\ud Gallium Nitride (GaN) Heterostructure Field Effect Transistor (HFET)\ud reliability, specifically if the RF waveform measurement system can be used\ud to provide detailed information about the state of the device during RF stress\ud testing. A stress testing procedure was developed to allow this, which\ud featured both DC and RF characterisation measurements before and after\ud every stress period. It was shown that the measurements made during the\ud characterisation stages only gives a representation of the degradation seen\ud in the same measurements during the RF stress period.
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