Radio astronomy instrumentation for the AVN

Thesis

The African VLBI network (AVN) is a proposed network of VLBI capable radio telescopes to be situated in the African countries that will later host antenna elements of the Square Kilometre Array (SKA) radio telescope. This network was proposed to help develop human capital in the African partner countries in preparation for the SKA. Once completed the network will consist of telescopes implemented both by converting decommissioned telecommunication ground stations, and new purpose built radio telescopes. This thesis presents the design and development of a potential cryogenic radio receiver for one of the new purpose built AVN elements. Since the telescope is still under development, I will present my contribution towards this design effort.

The front end of the receiver features a corrugated wide-flare angle feedhorn design compatible with the SKA antenna optics for operation over 4.6 to 8.5 GHz bandwidth and a scaled version to cover 8.3 to 15.3 GHz. The design process is presented along with how the simulated performance compares with potential SKA feeds. Measurements of the prototype's beam pattern are also presented.

A novel 3.3:1 bandwidth, 3-probe triple-ridge transducer (TRT) was designed and prototyped. The simulated and measurement results agree and confirm the broad-band spurious-free characteristics of the TRT. The derived analytic expressions for extracting linear orthogonal polarisations is presented, along with the experimental results.

For receiver gain stabilisation, a new technique based on cross-correlation whilst only using a single receiver chain was explored. Experimental results confirm gain stabilisation using this technique is possible without noticeable degradation in the receiver noise temperature.

A brief description of the cryostat housing the LNAs and TRT is presented. The radio frequency (RF) Analogue signal processing architecture is based on quadrature sampling. This required the design and prototyping of the RF bandpass filter, anti-aliasing lowpass filter and mixer chip circuit board. For the receiver backend, an RF system on chip (RFSoC) FPGA based digital signal processor is proposed. This will provide the reconfigurability needed to switch between single dish and VLBI observations. Finally, the thesis closes with a presentation on the expected radio telescope system temperature and it's implications on the sensitivity and mapping speed.

Authors: Banda, D

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Instrumentation