A radio polarimeter spectrometer
Creators: name: Hall, PJ
Publisher: name: University Of Tasmania
Year: 2023

description: For several years it has been apparent that a requirement in radioastronomy is a device capable of providing high time resolution polarization spectra of a wide bandwidth. This thesis describes such an instrument. The new polarimeter spectrometer is based on two surface acoustic wave (SAW) spectrum analysers operating in a coherent (phase preserving) mode. The method of spectrum analysis is a high sensitivity one, the analysers being more akin to filterbank types than to swept-frequency instruments. A wideband analog multiplier has been developed, permitting the spectrum analyser outputs to be correlated directly to yield cross-spectral or polarization data. As well, the usual problem with SAW analysers - the high output data rate - has been overcome with the development of a fast digital spectrum integrator. This device allows the SAW intermediate-frequency processor to be interfaced to a general purpose computer, resulting in a real-time signal processing system with great versatility. The prototype SAW processor produces polarization spectra over a bandwidth of about 30 MHz with frequency and time resolutions of 650 kHz and 2.4 uS. When interfaced to a typical computer, Stokes polarization spectra with a time resolution of about 150 uS are available. After allowing for non-continuous operation of the prototype spectrum analysers, the radiometer efficiency of the complete processing system exceeds 70%. Most techniques used in the development of the first polarimeter spectrometer are applicable to larger scale instruments. Although primarily the report on a project in applied signal processing, this thesis contains three other main sections. Firstly, an extensive and up-to-date review of SAW spectrum analysis and associated techniques is included. Secondly, the results of a series of numerical simulations are given. The simulations relate to the performance of post-detection digital integrators when quantizing envelope-detected, inverse bandwidth noise. In the third major section of the thesis, data from initial scientific observations conducted using the new instrumentation are presented. The SAW and digital processors, together with other items of hardware constructed in the course of the research project, were used in conjunction with the 14 m steerable radio telescope at the Llanherne (Hobart) observatory. As well as confirming laboratory radiometer efficiency measurements, the first astronomical data from the new system have been used to demonstrate changes in the rotation and dispersion measures of the Vela pulsar, PSR 0833-45. The rotation measure change is in general agreement with a trend deduced by earlier workers. The change in dispersion measure is only the second confirmed for any pulsar, the first changes being reported more than 10 years ago for the Crab nebula pulsar. The present variation is over 100 times the order of the Crab pulsar changes and amounts to more than 1% of the Vela pulsar dispersion measure reported by investigators in 1970. The new data support the hypothesis of relative motion between PSR 0833-45 and a magnetized filament in the Vela supernova remnant.