High power mm-wave tuneable reflective terminations

Ghozati, Seyed ORCID: https://orcid.org/0000-0002-6014-5223 2024. High power mm-wave tuneable reflective terminations. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

A Ka-band single pole double throw (SPDT) switch capable of operating at power levels up to 3 W, along with its corresponding behavioural model, was developed to demonstrate the application of tunable reflective terminations within an orthogonal load modulated balanced amplifier (OLMBA) architecture, enabling the dynamic selection of reactive loads. To determine the most suitable switching cell for the target frequency band and operating conditions, the investigation began with an analysis of high electron mobility transistor (HEMT)-structured GaN-on-SiC with 150 nm and 120 nm gate length technologies across different device peripheries, achieved by varying gate width and number of gate fingers. Standard small- and large-signal characterisation were conducted to assess the frequency response and power-handling capabilities. Additionally, load-pull (LP) measurements were performed to capture the device behaviour under non-50 Ω conditions, providing insights into realistic operational scenarios. The acquired data was used to develop the first Cardiff behavioural model for microwave switches, paving the way for further refinements on future models and optimisations of the tunable reflective termination application. Experimental findings highlight the significant impact of device periphery scaling on power-handling performance, with larger periphery devices exhibiting minimal gain compression at elevated power levels. Moreover, isolation characteristics in the off-state were found to be predominantly influenced by the applied bias voltage, with higher voltages delivering enhanced isolation. Subsequently, the Ka-band SPDT was designed and developed, incorporating a previously shortlisted and characterised HEMT device. During the characterisation of the switching cells, it was observed that in the OFF state, the impedance deviates significantly,shifting from a high impedance condition (approaching an open circuit) at lower frequencies to approximately (≈ (0.121 − j0.655)Z0) near 30.5 GHz. To mitigate this degradation in performance at higher frequencies, an impedance transformation technique was implemented in the passive network design, ensuring improved frequency response. The fabricated SPDT exhibited an insertion loss of -1.8 dB and isolation exceeding -25 dB, while maintaining linearity up to 3 W at the centre frequency of 30.5 GHz. These performance metrics demonstrate the designed SPDT’s viability for conventional applications, such as transceiver (TxRx) switching. Finally, the designed SPDT was characterised through LP measurements to obtain the necessary data for the final phase of this study—the development of a behavioural model for the tunable reflective termination. The model was further enhanced to provide high accuracy with minimised data through an experimentally structured phase-distributed technique. This model was intended for implementation in computer-aided design (CAD) software, such as advanced design systems (ADS), to facilitate the efficient and effective design of the OLMBA architecture.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Engineering
Last Modified:	04 Dec 2025 09:55
URI:	https://orca.cardiff.ac.uk/id/eprint/182844

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