The design of a two-element radio interferometer using satellite TV equipment

Master Thesis


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This research presents the design of a two-element radio interferometer capable of performing complex correlation. With the development of sophisticated radio astronomy instruments, particularly in South Africa, there is a need to develop an affordable educational instrument which can be used to demonstrate the fundamental concepts of radio interferometry to university students. The mass production of satellite TV equipment has resulted in relatively sensitive radio frequency (RF) equipment such as parabolic reflector dishes and low-noise block down-converters (LNBs) being available at significantly reduced costs. This served as the front-end of the interferometer which was used to observe the sun between 10.70 GHz - 12.75 GHz (RF). The LNB then down-converted these to an intermediate frequency (IF) between 0.95 GHz - 2.15 GHz. The LNBs were modified to make use of a common 25 MHz reference, which ensured that the observed fringes were only as a result of the source's geometric time delay. A power detector was also designed since the adding interferometer architecture was chosen. This power detector included the Analog Devices LT 5534 power detector integrated circuit (IC) and a Teensy 3.6 microcontroller. The calibrated power detector could detect signals as weak as - 60 dBm and showed less than 21 mV error in output for input signals in the range [- 50 dBm, -30 dBm]. The modified LNBs experienced issues, in particular the presence of a spurious LO signal, which distorted initial observations of the sun. This was resolved by the design and manufacture of narrowband hairpin filters and quarterwavelength stub filters which were used to isolate the IF band between 1.05 GHz - 1.15 GHz (corresponding RF between 10.80 GHz - 10.90 GHz). This also improved the interferometer's resolution. A series of filter-integrated Wilkinson power dividers and branchline couplers were designed to filter and further separate signals into in-phase and quadrature-phase (I-Q) components - these were required for complex correlation. The integrated quarter-wavelength stub filter and Wilkinson power divider achieved a maximum amplitude imbalance of 0.13 dB and phase imbalance of 0.9◦ between output ports. The integrated quarter-wavelength stub filter and branchline coupler achieved a maximum amplitude imbalance of 0.13 dB and phase imbalance of 91.1◦ between output ports. These results closely agreed with the simulated performance. First light was observed on the 5th December 2020 when the sun was successfully detected using the coherent two-element interferometer along a 1.1 m baseline. Other tests included using the observed fringe phase to verify the physical baseline. A theoretical baseline of 1.11 m was calculated for a physical baseline of 1.3 m indicating an error of less than 0.2 m. The sun's fringe frequency and amplitude was also observed for varying baselines - the sun was resolved along a 3 m baseline. Finally, full-system observations of the sun were conducted. These included observing the sun's cosine and sine fringes, which indicated that the analogue complex correlator was operating correctly. Thus, the primary goal of this project had been fulfilled. Specifically, developing a low-cost, educational two-element radio interferometer capable of detecting the sun.