The latest discoveries in the field of astronomy have been associated with the development of extremely sophisticated instruments. With regards to radio-astronomy, instrumentation has evolved to higher processing data rates and continuous performance improvement, in the analog and digital domain. Developing, maintaining, and using such kinds of instruments, requires understanding complex processes which involve plenty of subtle details. The above has inspired the engineering and astronomical communities to design low-cost instruments, which can be easily replicated by the non-specialist or highly skilled personnel who possess elementary technical background. The goal of this work is to provide the means to build an affordable tool for teaching radiometry sciences. In order to take a step further this way, a design of a basic interferometer (two elements) is introduced, intended to turn into a handy tool for learning the basic principles behind the interferometry technique and radiometry sciences.
One of the pedagogical experiences using this tool is the measurement of the sun’s angular diameter. Using these two Ku band receptors, we aim to capture the solar radiation in the 11-12[GHz] frequency range, the power variations at the earth spin, with a proper phase-lock of the receptors will generate a cross-correlation power oscillation where we can obtain an approximation of the angular sun’s diameter. Variables of interest in this calculation are the declination of the sun (which depends on the capture date and location), and the relation between maximal and minimal power within a fringe cycle.
The research started with a study and modification of \ac{COTS} \acp{LNBF}, where it was found that \acp{LNBF} can be altered for working with an external reference signal, allowing to have several \acp{LNBF} in phase. The tests, after the modification of the underlying structure of the downconversion stage in receptors, confirmed the availability to have coherence between receivers. The modification process acts inside the \ac{PLL} stage by removing the internal 27[MHz] quartz reference signal and injecting an external source, which is compliant with the voltage levels accepted by \acp{LNBF}. Although \acp{LNBF} can down-convert Ku band signals to L band (950[MHz] to 2150[MHz]), they are not able to be directly captured with selected instrument (\ac{ROACH} board). Therefore, an analysis was made to work on the third Nyquist zone for \ac{ROACH}’s \ac{ADC} in the range of 800[Mhz] to 1200[MHz] by including special filters. The digitized signal is then processed by the \ac{ROACH} board, where the correlation function is applied to the inputs of two different \acp{LNBF}’ received signal. The work was complemented with the creation of RF circuits, programming of \ac{GUI} in Python, interacting with \ac{KATCP} between the major works.
The experiment consisted in configure \acp{LNBF} to point its beam at the space where the sun obtains its maximum elevation through the day, the instrument was prepared in the morning, and the control software collected cross-correlation data and power measure such as the \ac{PSD} along the entire day. The collected and computed data confirmed that coherent signals were valid and a reading of the power and the period of the cross-correlation power allowed to estimate the angular diameter of the sun.
After theory was validated with the \ac{ROACH} board, cheaper solutions were made with a B200 board, and a concept test was shown with an Arduino board for checking feasibility to perform sun diameter measurements using low-cost equipment.
The work concludes that basic experiments can be made with basic and cheap instruments that bring science to people with tight budget requirements, fostering the study of science and engineering concepts related to radio-astronomy.