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Wideband and high-power nonlinear measurement system for the characterisation of GaN amplifiers

Alimohammadi, Yashar 2021. Wideband and high-power nonlinear measurement system for the characterisation of GaN amplifiers. PhD Thesis, Cardiff University.
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Recent RF applications and cellular networks architecture require to use high-power PAs, therefore it is critical to characterise their behaviour under appropriate operating condition with an accurate, reliable, fast, and flexible measurement system. As highpower PAs are typically operated in the pulsed mode, measurement system should be able to apply pulsed excitation and perform different types of characterisation under pulsed condition. Integrating pulsed capability into a measurement system to characterise large-signal behaviour of the device is relatively expensive and rigid in terms of instrumentation. Moreover, the inaccessible software that is used by the traditional systems has made them a vendor-defined system, where their application cannot be amended to scope specific phenomena. This is contrary to the need for a flexible pulsed system that can be extended and modified according to user preferences. This thesis presents a high-speed pulsed measurement system that maintains flexibility, upgradability, accuracy, expanded power, and bandwidth ranges. The system is configured around NI modules to apply DC and RF signals and analyse them by employing vector signal transceiver (VST) with up to 1 GHz bandwidth. Due to employing programable VSTs, and accessibility of measured raw samples, different types of measurements can be performed, and small and large-signal behaviour of the device can be analysed. The aforementioned features of the developed measurement system assist to analyse the time-domain behaviour of the device and characterise the physical phenomenon such as thermal, traps effect on large-signal behaviour of the device. As an application of developed high-power measurement system and operating it under wideband RF stimulus, the trapping effect on the large-signal behaviour of the PA is investigated. Time-domain behaviour represents significant variation in the input and output of the device. These variation leads to change the optimum load impedance of the device in time-domain, which decreases the performance of the device up to 8% and mostly linked to the traps of the device. moreover, linearity of the device in time-domain is analysed and intermodulation distortion levels of the device are extracted in time-domain at various timeslots of RF pulses. Interesting results are achieved by applying various drain-lag levels to the device, which shows linearity of the device increases up to 5 dB in back-off operation regime. In this work, for the first time, a nonlinear behavioural model is provided for the surface traps of GaN device. Developed measurement system utilise Cardiff behavioural model during load-pull measurements to decrease the number of load impedances to find the optimum impedance of the device and increase the speed of process. By applying gate-lag levels and pre-charging the surface traps of the device before conducting active load-pull measurements, relation between the Cardiff behavioural model and gate-lag levels of the device is investigated and a new model is achieved by using quadratic function to incorporate the gate-lag effect into the Cardiff behavioural model. Furthermore Cardiff behavioural model variation in timedomain is also achieved at different gate-lag levels and clarifies pre charging traps can reduce the variation in time-domain.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Load-pull measurements, RF measurement techniques, Pulse measurements, Trapping effect, GaN Power amplifier characterisation , Cardiff model
Date of First Compliant Deposit: 10 February 2022
Last Modified: 14 Feb 2022 14:13

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