Ultra-pure digital detection of polarization for radio astronomy applications

Abstract

Polarization is a key measurement within radio astronomy because its state can give insight as to the physical conditions of the source and the medium the radiation has traveled through. In radio astronomy receivers, the device used to separate the polarization is known as an Ortho-Mode Transducer (OMT). The OMT is a passive component that separates signals into two orthogonal components. Using available millimeter-wave analog technology, wideband heterodyne receiver systems generally obtain polarization isolation ratios of 15dB to 20dB, insufficient for modern astronomical applications. That low polarization isolation ratio is due to the presence of leakage from one polarization contaminating the other, which is known as cross-polarization. In order to improve polarization isolation and reduce cross-polarization, a new approach called Digital OMT has been proposed, based on passing one given functionality from the analog domain to the digital domain using digital signal processing techniques. Digital techniques were studied to improve OMT performance, which led to the design of a polarimeter based on the Fast Fourier Transform (FFT). The polarimeter was implemented on a Field Programmable Gate Array (FPGA), including the implementation of a Ku-band analog front-end to characterize the design and measure the synthesized polarization. This thesis introduces a prototype of a DOMT created for real-time processing and continuous integration, suitable for radio astronomy observations. The configuration used is based on the architecture of a compact four-port OMT. The outputs are downconverted using mixers and then directly digitalized using four 8bit ADC sampling at 1.2 GSPS. A 180° hybrid was implemented on the FPGA together with a set of calibration vectors, with the purpose of compensating the amplitude and phase imbalance typical of analog radio astronomy front-ends. This calibrated Digital OMT exhibits a polarization isolation ratio of more than 40 dB for the entire RF bandwidth, representing an improvement factor of 100 over its analog counterpart.